UMTS GSM RF Funda Interview Questions Msr

Compare UMTS to CDMA 2000 (if from predominantly CDMA background)

Compare UMTS to GSM (if from predominantly GSM background)

What do you understand by the term UTRAN (analogous to BSS)? What are the two main UTRAN elements and what is their respective function?

UTRAN (UMTS Terrestrial Radio Access Network) is composed as following:

Radio Network Subsystem (RNS): Sub network within the UTRAN. It comprises RNC and NodeB. The main network elements are:

Radio Network Controller (RNC): Owns and control radio resources in its domain. Service access point for the all UTRAN provided services to the CN. (analogous to BSC at a high level)

Node B: Converts data flows and participates in radio resource management. It is the Base Station with the radio equipment (analogous to BTS at a high level).

Radio Network Controller (RNC)

• Comparable to Base Station Controller in GSM

• Responsible for L2 processing of user data

• Responsible for Radio Resource Management

• Key RNC Functions:

• Closed loop power control

• Handover control

• Admission control

• Code allocation

• Packet scheduling

• Macro diversity combining/splitting over number of Node Bs

Node B

• Standardization term (normally called as Base Station)

• Comparable to Base Transceiver Station in GSM

• Responsible for Air Interface Layer 1

What are the key Node B Functions?

• Modulation and spreading

• RF Processing

• Inner-loop power control

• Rate matching

• Macro diversity combining/splitting inside Node B

What are the key RANAP functions?

• Radio Access Bearer (between UE-CN)

• RAB Set-UP

• RAB Modification

• Clearing RAB

• Iu Bearer Release

• SRNC Relocation

• Paging Commands

What are the key benefits of a WCDMA system?

• Multiple Access: Since every user has a unique code sequence, multiple users can simultaneously use the same frequency – spectral efficiency

• The Processing Gain adds mitigation to noise & interference → frequency reuse of 1 (i.e. a frequency is reused in every cell/sector)

• Many users on the same carrier → interference diversity that improves system capacity

• With the wideband signal, the different propagation paths of the radio signal can be resolved at high accuracy → diversity against fast fading (Rake receiver)

• Low Probability of Interception: Due to its low power density, the spread-spectrum signal is difficult to intercept by hostile listener without the code sequence

What are the basic requirements for spreading codes?

Requirements for the spreading codes:

• Good auto-correlation properties. For separating different paths.

• Good cross-correlation properties. For separating different channels.

Explain the differences between scrambling codes and channelization codes. Where and how are they used in UMTS? What is channelization code for? What is scrambling code?

Downlink Multi-cell Operation

Uplink Multi-cell Operation

The channelization code is used to separate the channels coming from one transmitter (UE or Cell)

The scrambling code is used to separate the transmitters that are transmitting on the same frequency i.e. UE (UL) and cells (DL)

What is the method of slot synchronization?

The UE synchronizes to the start of the Primary Sync Code, which is broadcast in the first 256 chips of every time slot

What is the method of frame synchronization?

Decoding the slot timing, the UE decodes 15 consecutive Secondary Sync Code bursts, which occur in the same time period as the Primary Sync Code bursts. The pattern observed every 15 consecutive SSC bursts conveys the start of the 15-slot cycle that is the frame

How is the scrambling code of the cell identified?

The decoding of the SSC pattern also provides definitive identification of the SCG (1 of 64). After that, the UE correlates each code in the group with the CPICH.

What is “processing gain”?

Processing Gain (PG): Ratio of transmitted bandwidth to data signal bandwidth

PG = Wc / Wb ≈ Rc / Rb = Tb / Tc

Can you give some example values of processing gain?

What is the Processing Gain, in dB, if a UMTS system utilizing a chip rate of 3840 kbps is used with the following user rates?

Rate(kbps) PG (dB) PG (dB) 9.6 400 26.0206 26

12.2 314.7541 24.97971 25

64 60 17.78151 18

128 30 14.77121 15

384 10 10 10

What is handover?

Handover is the automatic transfer of the subscriber from one cell to another during the call process, without causing any hindrance to the call. There are two main aspects to this: the necessity to find a dedicated mode in the next cell as the mobile is on call, and the switching process being fast enough so as not to drop that call. So, how does the handover actually take place? There are many processes that can be used, but the one most used is based on power measurements. When a mobile is at the interface of two cells, the BSS measures the power that is received by the base stations of the two cells, and then the one that satisfies the criteria of enough power and least interference is selected. This kind of handover being directly related to power control, it provides an opportunity to improve the efficiency of use of the spectrum.

The connection quality has to be maintained as the UE moves between cells. This is the purpose of handover function.

What are the different types of handoffs in a WCDMA system, and explain each of them.

• Inter-Radio Access Technology (IRAT) Handover

o Handover from a WCDMA system to GSM/GPRS. This is a form of hard handover

o Traffic and Control Channels are Disconnected and must be Reconnected

o This functions preserves signal quality on dedicated channels for circuit switched services when the UE is moving from a WCDMA network to a GSM network and vice versa.

• Inter-RAT Cell Change

o This function preserves signal quality on common and dedicated channels for packet switched services when UE is moving from WCDMA to GPRS and vice versa.

o It is either network initiated (for dedicated channels) or UE initiated (for common channels).

o No resources are reserved in the target cell before cell change is executed.

• Inter-frequency Handover (IFHO)

o When the UE must change WCDMA carrier frequency during the handover. This is a form of hard handover

o Traffic and Control Channels are Disconnected and must be Reconnected

During IRAT, IRAT Cell Change and IFHO, the UE has only one radio link active at a time. During hard handover or cell change the connection is broken off for a short period (between removal of old radio link and establishment of the new).

• Soft Handover

o During Handover, the UE has concurrent traffic connections with multiple RBSs.

o Handover is less noticeable

• Softer Handover

o Similar to soft handover but between two cells of the same RBS

o Handover is simplified since timing sectors have identical timing

What are “soft handover” and “softer handover”? Why is SHO essential in a WCDMA system?

WCDMA systems must use SHO to reduce interference caused by near-far problems resulting in from UEs at cell borders. SHO allows UEs to be power controlled by both base stations, which eliminates the excessive interference.

If there was only HHO, then a UE may cause excessive interference to the handover destination cell (in the interval prior to the execution of the HHO). During this time, the UE is still under power control from the originating cell, but has excessive power for the destination cell.

In the DL, in soft (as well as softer) handover, maximum ratio combining is used but in the UL the RNC selects and chooses the best frame during soft handover whereas in softer handover the radio links are coherently combined (maximum ratio combining) in the RBS.

What is the essential power control “rule” during SHO?

During SHO, the UE responds to TPC bits from all RBSs in the active set. The rule is that the UE will only increase power when all RBSs send power up commands; otherwise, the UE will decrease power.

What are the advantages and disadvantages of SHO? Does SHO increase overall system capacity?

SHO consumes more downlink resources and results in DL interference. It may reduce capacity when the system is heavily loaded.

What is fundamentally limits the UL/DL capacity of a WCDMA system? Which link is more limited in practice?

UL capacity: Interference between UE transmissions. There could be also some limitations regarding radio variations.

DL capacity: The total RF power budget, which serves all UEs within the cell. In some systems, interference between RBS (cell) transmissions becomes dominant.

In the design of the cell, the UL and DL should be balanced, but dependent on the situation either the UL or the DL can be limiting. The capacity can be UL limited if too many UEs accessing the cell but the DL power is enough. The capacity can be DL limiting if the UEs are close to the cell border or have a high power need so that the RF DL power budget is reached.

What is the fundamental difference between IS-95/cdma2000 and WCDMA handover?

In IS-95/cdma2000, all RBSs are time-synchronized to GPS. This assures that all UE will receive the same frame timing from all RBSs in the active handover set (small time offsets due to propagation delay are compensated in the RAKE receiver).

In WCDMA, since there are no explicit frame synchronization between cells, the UE may receive highly different frame timing from the cells in the active set.

What is the fundamental problem with soft handover when BSS are not precisely synchronized? How does WCDMA resolve this problem?

Without frame synchronization, the RAKE receiver cannot coherently combine data bits from two cells. This includes data bits and power control bits. Without coherent combination at the data-bit level, RAKE reception in meaningless.

The UE measures the relative time offset between two cells by measuring the received phase of the pilot channels (CPICH) from each cell. The relative time offset is reported to

the RNC via a signaling message. The RNC then adjusts the DPDCH/DPCCH timing of all cells in the active set to compensate for this delay.

Why is there “handover gain” for CDMA? The soft handover phenomenon gives an additional gain against the fast fading that takes place in the network. Due to the soft handover phenomenon, mobile connectivity is to a base station that gives a better signal quality. Thus due to macro diversity combinations, the soft handover gain has a positive impact on the base station.

What is “compressed mode”? How is IFHO triggered? How is inter-system HO triggered?

IFHO is based on coverage (Ec/No or RSCP) and UETxPower

ISHO is based on coverage (Ec/No or RSCP)

Start compressed mode measurements

Ec/No below a threshold (event 2d, cell level) OR RSCP below a threshold (event 2d, cell level) OR UE TX power above a threshold (event 6a, RNC level)

All evaluated in parallel

Stop compressed mode measurements

Ec/No above a threshold (event 2f, relative to 2d) AND RSCP above a threshold (event 2f, relative to 2d) AND UE TX power below a threshold (event 6b, RNC level)

All evaluated in parallel

Trigger IRAT handover/cell change

Ec/No/RSCP(depending on the what triggered 2d) below a threshold (relative to 2d) AND GSM RSSI above a threshold (event 3a, RNC level)

Trigger inter-frequency handover

Ec/No/RSCP(depending on the what triggered 2d) below a threshold (relative to 2d) AND non-used frequency RSCP AND non-used frequency Ec/No above a threshold (event 2b, RNC level)

IRAT HO & IFHO

Describe UE States and Transitions

IRAT HO & IFHO Summary

IRAT and Inter-frequency measurements are done in compressed mode.Parameter hoType decides which type of measurements and HO evaluation (IRAT or IFHO) to perform

IFHO measurement will start if hoType = [IFHO_preferred] for atleast one cell in the ASIRAT measurement will start if hoType = [GSM_preferred] for at least one cell in the AS and no cell in the AS has hoType = [IFHO_preferred]

Three event types to control handoverstarting measurementstopping measurementtrigger handover

The events can be triggered by RSCP, Ec/No, UE TX power (used freq) and RSCP, Ec/No, GSM RSSI (non-used freq)

UE States & Transitions in U3.01 (Lucent)

List the soft/softer HO events. What are the events 1a, 1b, 1c, 2a…?

• 1a Add

• 1b Delete

• 1c Replace

• 1d Change of best cell

Radio Link addition (event 1a)

Delete threshold

CELL A

CELL B

CELL C

Time

Quality

Add threshold

ASmax = 2 CELL A in AS CELL A & B in AS CELL B&Cin AS CELL B in AS

TTT

replace

TTT

delete

Events1a add1b delete1c replace1d change of

best cell

Change of best cell

TTT

add

TTT Time To Trigger

SHO Algorithm (Ericsson)

If neighbour cell is within reportingRange1a to the strongest cell it is added to the Active set

Radio Link deletion (event 1b)

If cell in active set is reportingRange1b below the strongest cell it is removed from active set

Radio Link replacement (event 1c)

If neighbour cell is within reportingRange1c to the weakest cell it replaces the weakest cell in the Active set

Change of best cell (event 1d)

If any cell in the monitored set becomes stronger than the current cell best cell in the active set

List the two major functions of the pilot channel.

CPICH Common Pilot Channel (DL) :

• Coherent phase reference for the DL channels and sends the scrambling code of the cell

• It also used for cell selections and reselections as well as HO measurements.

What is the main task of the UE in handover/ Describe handover process? 1. The UE measures on CPICHs, a CPICH enters the reporting range. An event is

triggered. 2. The UE sends a report to the RNC for evaluation 3. The UE gets a message to add/remove/replace a cell for the AS 4. The cell is added/deleted/removed from the AS

If a candidate pilot (CPICH2) is below the best server (CPICH1) at most by (AS_Th - AS_Th_Hyst/2 - CIO2 ) for a period of 'Time-to-trigger' and the active set is not full,

SHO Algorithm (Lucent)

Event 1A is triggered and measurements are reported to RNC. Radio Link Addition (1A) is performed.

If Active Set is full and a new candidate pilot (CPICH3) is better than the worst existing pilot in the active set (CPICH1) at least by (As_Rep_Hyst/2 - CIO3 + CIO1) for a period equal to 'Time-to-trigger', Event 1C is triggered. The best cell outside active set (CPICH3) is added and the worst cell in the active set (CPICH1) is removed. Combined radio link addition and removal (1C) is performed. It's assumed here that the Active Set Size is 2.

If a pilot in active set (CPICH3) is below the best server (CPICH2) at least by (As_Th + As_Th_Hyst/2 - CIO3) for a period equal to 'Time-to-trigger', Event 1B is triggered and the worst pilot (CPICH3) is removed from the active set. Radio link removal (1B) is performed.

What is the handover window size? Do you use absolute threshold or relative threshold?

Add window and drop window are relative thresholds.

Describe the differences between active, monitored and detected set.

Compare UMTS & IS-95B/CDMA 2000 SHO process

What are the factors effecting UMTS SHO Performance?

• Larger HO area costs more resources – higher transmit power & capacity usage

• Excessive coverage overlap can be removed by site optimization

• By adjusting the add remove threshold it is possible to control the handover area

o Ericsson recommends Add: 3dB and Drop: 5dB

Factors effecting UMTS Soft and Softer HO Performance

ParametersAddition, Drop & Replacement windowsAddition, Drop & Replacement timersMaximum Active Set SizeCPICH Ec/No Filter Coefficient Active Set Weighting Coefficient

Other factorsNetwork topology (macro or micro sites)Placement of antennasRadio environment

UMTS and IS-95B/2000 SHO Algorithm Comparison

SimilaritiesBoth cdma2000 and UMTS consider the effectiveness of connection legs. Dynamic thresholds are used to decide the actual effect by adding (dropping) legs to (from) the active set.A common pilot is used to as the measurement for handover triggersDynamic add/drop thresholds are usedTimer is used for drop-leg trigger (to avoid the ping-pong effect)

DifferencesIn cdma2000, the aggregate Ec/Io is used to calculate the add/drop thresholds. In UMTS, the best pilot is used to calculate the add/drop thresholdsBesides the timer for the drop-leg trigger, in UMTS, the timer is also used for the add-leg trigger and swap-leg triggerIn UMTS, between the active set stage and the remaining set stage, there is only the monitored set stage which is similar to the neighbor set stage. In cdma2000, the candidate set stage is inserted between the active set stage and the neighbor (monitored) set stage. The pilots in the candidate set stage have the same search priority as the pilots in the active set stage.

o more restrictive settings (e.g. 2dB/ 4dB) results in better performance in DL limited scenarios

o less restrictive settings (e.g. 4dB/ 6dB) yield better performance in UL limited scenarios

What information is contained in a measurement control message from RNC to UE?

• Measurement type: Intrafrequency or inter-RAT

• Measurement identity number

• Measurement command

• Measurement objects

• Measurement quantity (Ec/No)

• Measurement reporting criteria

• Measurement reporting mode (///support AM)

List the idle mode tasks. What kind of services do we have in the idle mode?

• PLMN selection and reselection

• Cell selection and reselection

• Location registration

• Paging

• Reading of system information

Limited services, normal services, and operator related services

How is cell selection criteria defined?

Squal = Qqualmeas – qQualmin > 0

Srxlev = Qrxlevmeas – qRxLevMin – Pcompensation > 0

What does the UE do when it is “camped normally” state?

It performs inter- and intrafrequency and interRAT measuremrents, monitor broadcast information and paging info

When is cell reselection triggered?

• When the cell on which it is camping is no longer suitable

• When the UE, in camped normally state, has found a better neighboring cell than the cell on which it is currently camping

• When the UE is in limited service state on an acceptable cell

List 3 types of registration updating.

Normal registration, Periodic registration, IMSI attach/detach

Which groups are logical channels divided into?

The logical channels describe the type of data to be sent and are divided into control and traffic channels.

Which groups are transport channels divided into?

The transport channels are defined by how and with what characteristics the data is transferred. Transport channels are divided into common and dedicated channels.

List some functions provided by the physical layer

The physical layer offers functions such as CRC, FEC, interleaving, channelization coding, multiplexing, scrambling coding, filtering and modulation.

What three channels are always aligned with the RBS frame timing?

CPICH, SCH, P-CCPCH

Explain the terms soft capacity, soft blocking and hard blocking.

An analog base station blocks calls when there is no channel available. This form of blocking is called hard blocking. However, another blocking condition exists for a CDMA base station. Unlike AMPS and TDMA, CDMA does not impose a definite limit on blocking. As the number of users increases in a CDMA system, the level of interference increases as well, and this increase in interference negatively affects the quality of service. Because all users share the same RF spectrum, the interference increase contributes to a higher FER and a higher drop-call rate. In this case, the blocking is soft because the number of users can be increased if the service provider is willing to tolerate a higher level of interference and a lower quality of service. Soft blocking is a characteristic of the CDMA system. Therefore, two blocking scenarios exist for a CDMA base station: • There may be plenty of channels available at a base station, but since

there are many users in the same cell already, the interference level is such that adding an additional user would increase the interference above a predetermined threshold. The call is thus denied. This is the soft-blocking scenario.

• A call may have an excellent quality, but if there is no channel available at the base station, the call is still blocked. This is the hard-blocking scenario.

We consider the soft blocking condition on the reverse link since this link is often the limiting link in terms of capacity. Soft capacity can be assumed to be number of users in the same cell. Unit is Erlang.

What is the purpose of power control in WCDMA? Explain various power control strategies in UL and down link.

Why is fast power control used in WCDMA? How fast is the power control? Is fast power control required on the DL, UL, both?

Power control is necessary in any spread spectrum system to ensure that each user transmits the right amount of power to maintain the connection quality while at the same time causing as little interference as possible to other users. For optimum performance the power control must be fast so that the variations, caused by the rapidly changing radio environment, can be followed. Fast power control is more

Mechanisms of Power Control

Open Loop power controlFor initial power setting of MS

Across the air interface

Inner (Fast Closed) loop power controlMitigates fast fading rate 1.5 kbpsOn UL and DLUses a fixed quality target set in MS/BS

Outer loop power control:Compensates changes in environmentAdjust the SIR target to achieve the required FER/BER/BLERDepends on: MS speed available, multipath diversityIn the soft handover comes after frame selection

Need for Power Control

Adjusts the transmission power so that the received quality (SIR,BLER,..) is sufficiently good from a user’s point of view

Removes near far effectMitigates fadingCompensates changes in propagation conditions

At a system levelDecrease interference from other users

Downlink: In downlink the power control must keep the signal at minimal required level in order to decrease the interference to users in other cells

Increase capacity of the systemUplink: Power control in uplink must make signal powers from different users nearly equal in order to maximize the total capacity in the cell

important in the UL than in the DL since there are more UEs causing interference in the UL (interference limited) than it is cells in the DL.

If fast power control is not required for DL transmission, why is it implemented both in CDMA 2000 and WCDMA?

Power control of the RBS improves interference conditions and optimizes the DL power budget. In IS-95A, power control of the BS is very slow (only as a result of link and FER monitoring). In IS-95B, power control of the BS was implemented on a frame-by-frame basis using a feedback bit from UE. For 3G CDMA systems, fast power control (800 Hz for CDMA2000 and 1500 Hz for WCDMA) is implemented in both UE and BS.

What is the purpose of open loop control? How is it implemented?

In order to prevent unnecessary interference during initial random access, the UE may estimate the minimum required transmit power by measuring the received power from RBS on the CPICH, and taking into account the RBS CPICH transmit power as recorded from monitoring the BCH.

Open Loop Power Control

What is the purpose of outer loop power control?

What is the purpose of inner loop power control?

• In the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target.

o SIR ≥ SIR-target ⇒ Decrease power

o SIR < SIR-target ⇒ Increase power

• The UE transmitter is capable of changing the output power with a step size of 1, 2 and 3 dB, in the slot immediately after the TPC_cmd can be derived. Inner loop power control frequency is 1500Hz.

• The transmit power of the downlink channels is determined by the network. The power control step size can take four values: 0.5, 1, 1.5 or 2 dB. It is mandatory for UTRAN to support step size of 1 dB, while support of other step sizes is optional. The UE generates TPC commands to control the network transmit power and send them in the TPC field of the uplink DPCCH. Upon receiving the TPC commands UTRAN adjusts its downlink DPCCH/DPDCH power accordingly.

Outer Loop Power Control

Is used to maintain the quality of communication at the level ofbearer service quality requirement, while using as low power as possible.

The uplink outer loop power control is responsible for setting a target SIR in the Node B for each individual uplink inner loop power control. This target SIR is updated for each UE according to the estimated uplink quality (BLock Error Rate, Bit Error Rate) for each Radio Resource Control connection.

The downlink outer loop power control is the ability of the UE receiver to converge to required link quality (BLER) set by the network (RNC) in downlink.

Why must a WCDMA receiver use a RAKE receiver?

Multipath reflections will cause unacceptable levels of destructive interference (fading) under normal circumstances. The RAKE receiver helps to solve this problem. The RAKE receiver also provides the means to track and combine data from multiple signals from different transmitters (as used during SHO)

How does a RAKE receiver reduce multipath interference?

The RAKE receiver distinguishes between the multipath components by equalizing the time delay of each component. These components may then be correlated independently and the correlated data can be combined in a constructive, rather than destructive manner.

What is the “searcher” finger used for?

In addition to scanning neighbor cell sites in preparation for HO, the searcher constantly scans available multipath components. This is to keep the tracking fingers updated to the strongest multipath components.

Recap

Open loop PCOpen loop control is used only to set the initial power, and is not used during further transmission (unlike IS-95/IS-2000)

Inner (Closed) loop PCFast PC included in every timeslot (1500 Hz) Measure the actual SIR & compare with SIR-targetSend TPC command in DL or ULSIR ≥ SIR-target ⇒ Decrease powerSIR < SIR-target ⇒ Increase power

Outer loop PCAdjust SIR (EbNo) - target to achieve BLER target

Slower, per TTI (Transmission Timing Interval) basis updatesUL: Jump algorithm

Describe the key steps in WCDMA signal generation

Discuss WCDMA Channels

How much power is assigned to control channels (including CPCH) and traffic channels?

Briefly describe cell search process. What are P-SCH and S-SCH for during cell search?

Why is CRC used?

CRC is used to detect errors. CRC is attached to each block and is used to measure the BLER that is used in the outer loop power control in the RNC.

What types of FEC are used in WCDMA? Under what conditions does FEC work best/fail? What method is used in WCDMA to optimize FEC coder performance?

WCDMA uses convolution coders, rate ½ and 1/3 as well as Turbo coders. Turbo coders combine time diversity with convolutional encoding for near optimal error correction. Turbo decoding is highly complex, suited for mainly data and may require offline processing.

When errors are distributed randomly in time (non-consecutive).

Interleaving.

What is the difference between a “bit”, a “symbol” and a “chip”?

Bit refer to original payload data coming from higher layers to L1. Data symbols refer to the channelization coding (when error protection and detection is added). Chips refer to the rate of the data transfer after channelization coding. The chip rate of WCDMA system is 3.84 Mcps.

Define Eb/No, Eb/Nt, Ec/Nt, Ec/Io, BER and FER

What is Eb/No? How can you improve Eb/No? What is Eb/No target in your design?

W/R is also known as Processing Gain of the system. Eb/No Target: 6 dB Coverage is directly related to link performance. An increase in coverage will demand an increase in the average TX power of a base station in the downlink direction. If the system capacity is downlink-limited, then an increase in coverage will lead to a decrease in capacity. If the system is uplink-limited, then the capacity is not affected. Thus, link performance increase is directly related to the increase in coverage.

There are many ways to improve the coverage. We have already seen the main parameters affecting link performance. Parameters such as block error rate, Eb/No, power control headroom, etc., directly affect the power budget and hence the coverage. Uplink coverage can be improved by decreasing the interference margin or by reducing he base station noise figure, or even by increasing the antenna gain. However, the processing gain and Eb/No are the two major values affecting coverage. So a reduction in Eb/No increases the coverage of the network. This is because, for a lower Eb/No, less power is required for the same performance, so a bigger area can be covered. The performance of Eb/No is dependent on a number of factors such as the bit rates, channel accuracy, SIR algorithms, etc. Uplink coverage becomes an issue at higher bit rates in WCDMA networks, so accurate traffic distribution will play a major role in coverage improvement. However, if the bit rate of the uplink direction can be reduced, the coverage can be improved (as the transmitted power requirement would be less). This is possible only for NRT data (which are less delay-critical) and for voice signals, which have lower bit rates. However, Eb/No cannot be lowered below the requirements of the requested service. Another way to improve the Eb/No ratio is to increase the multipath diversity. Two signals arriving at two antennas instead of one can be combined coherently, while the receiver noise can be combined non-coherently. This technique not only provides better gain, it also gives protection against fast fading etc. Concepts like antenna tilts are also used in WCDMA radio networks to improve the coverage area.

A voice channel with a user data rate of 12200 bps is received in the presence of noise (that can be assumed to be the equivalent of additive white gaussian noise AWGN) such that the received signal to noise ratio is –16 dB. Determine Eb/No at the output of the receiver. Eb/No =

What is Ec/Io? How can you improve Ec/Io? What is typical Ec/Io value in your design?

Ec/Io = RSCP-RSSI; Ec/Io Target Voice: -16 dB and Data: -10 dB

What is pilot pollution and how do you overcome it? When multiple sectors are about the same strength, and there is no dominant server, we call this “pilot pollution”. To get acceptable reception during pilot pollution, the mobile must be using a majority of the sectors it is hearing. This causes too much soft

handoff and consumes capacity. Overcoming pilot pollution requires careful coverage control of the related sectors. Pilot pollution is a type of co-channel interference in CDMA systems caused when the pilot code from a distant cell or base station is powerful enough to create an interference problem. Soft handovers are an essential part of WCDMA network functionality. Taking full advantage of SHOs requires careful setting of parameters. In addition, state of the network affects the performance of SHO algorithm. Pilot pollution, i.e., too many hearable pilots, can make it difficult for SHO algorithm to perform properly, and DL transmit power as well as signaling capacity can be wasted for poor decisions and unnecessary active set updates.

Why is I/Q modulation used instead of simple BPSK?

I/Q modulation allows two data streams to be sent on one RF carrier at the same BER compared to Eb/No performance for BPSK. I and Q are orthogonal so QPSK corresponds to two BPSK transmissions.

Why is a raised cosine filter used prior to I/Q modulator?

The raised cosine filter provides spectral attenuation with controlled ringing. The ringing period is carefully chosen so that ringing will equal zero at all data sampling instants in the future. This minimizes ISI.

What fundamentally limits the UL and DL capacity of a WCDMA system? In practice which link is more limited and why?

Basically it is the interference between UE transmissions. There could also be some limitations regarding radio variations. The total RF power budget, which serves all UEs within the cell. In some systems, interference between RBS (cell) transmissions becomes dominant.

In the design of the cell, the UL and the DL should be balanced, but dependent on the situation either the UL or DL can be limiting. The capacity can be UL limited if there are many UEs accessing the cell but the DL power is enough. The capacity can be DL limited if the UEs are close to the cell border or have a high power need so that the RF DL power budget is reached.

In scrambling code planning, how do you group the codes (how many groups and how many codes in each group)? Do you assign sister cells (cells on same site) with codes from the same code group or from different code group? Why?

Briefly describe admission control.

• If the air interface load increases excessively, the coverage area is reduced below the planned values, and the quality of service of the existing connections can not be guaranteed

• Admission control is to be executed whenever a new radio bearer is to be established or modified

• The admission control algorithm predicts the load after the establishment of the new bearer for both uplink and downlink

• Admission Control accepts the establishment of the new bearer if the predicted uplink and downlink loads are below the planned values

• Typical load indicators load are:

o Uplink : total received wideband power

o Downlink: total base station transmission power

• Accurate predictions of the load increase is highly important to maximize the cell capacity while maintaining network stability

Briefly describe load control. • Load Control ensures that the system is not overloaded and remains stable • Load Control continuously updates the load information of the cells controlled by

the RNC and provides this information to PS and AC algorithms • If overload is encountered, the Load Control functionality returns the system quickly

and controllably back to the target load • Node B actions:

o Downlink fast load control: deny downlink power-up commands received from the UEs

o Uplink fast load control: reduce the uplink EbNo target used by the uplink fast power control

• RNC actions:

o Reduce the throughput of packet data traffic o Handover to another WCDMA carrier o Decrease bit rate of real time UEs, e.g. AMR speech codec o Drop low priority calls in a controlled manner

In your design process, what kinds of plots do you usually use to determine if you have met design targets?

Briefly describe Monte Carlo simulation. What is the difference between static analysis and Monte Carlo simulation? Traditionally, TDMA/FDMA network planning used static analysis and calculated the margins for a tuned propagation model to allow for interference. However, since the level of intra-cell and inter-cell interference varies between cells, this approach gave misleading results in early networks. Therefore, in CDMA networks, it is often better to use other methods to predict coverage and cell capacity. An alternative approach has developed based around simulating networks using Monte Carlo algorithms and ASSET3g uses this approach as it provides a good balance between accuracy and usability. During the simulation: • A large number of randomised snapshots are taken of network performance

for different user equipment (UE) or terminals over time. In these snapshots, the UEs are in statistically determined positions and generated independently for each snapshot.

• The number of terminals in an active session in a pixel is determined using a Poisson distribution with a mean given by the number of terminals in the traffic array. This means that the total number of terminals in a snapshot is Poisson distributed and so it will vary from snapshot to snapshot.

• These snapshots are then used in calculations to obtain statistically valid measurements giving an estimate of the mean network performance. An advantage of using the static Monte Carlo simulation approach is that it takes less time than dynamic simulation (where you look at mobiles moving through the network) but is more accurate than straightforward static analysis. Repeated static (and dynamic) simulation may prove valuable for detailed optimisation of site configurations, problem areas and radio resource management algorithms.

Static analysis uses a deterministic algorithm to analyse the performance of the network. Repeating an analysis gives the same result. The static analysis algorithm

generates results quickly by not considering as many effects as the Monte Carlo simulator. In addition, unlike the simulator, it can be used before traffic rasters have been created. If traffic rasters are available, they can be used to improve the results. Static simulation is a method where the performance of the network is analysed over various instances in time or "snapshots", where User Equipment (UEs) are in statistically determined places. The ability of each terminal to make its connection to the network is calculated through an iterative process. Various failure mechanisms are considered: • Maximum mobile power • Maximum Node B power reached • No available channels • Low pilot SIR or Ec/Io if using CDMA2000 or TD-SCDMA The performance of the network is then analysed from the results of the snapshots carried out. Monte Carlo analysis, as used in 3G, is a form of static simulation. Repeating an analysis may give different results.

Dynamic simulation is a type of 3rd Generation planning method that simulates User Equipment (UE) moving through the network in successive timesteps. A mobile list is generated and solved for the first timestep. The simulation may consider time to be split into: • Chip periods • Bit periods • Timesteps (SNR considered) • Successive timesteps are then simulated and are dependent upon the results of the previous timeslot. New mobiles are simulated coming into the network and terminating their calls.

How many iteration and snap shots do you usually run for Monte Carlo simulation?

Based on snapshot calculator which considers service specific traffic, clutter area, clutter type etc.

Could you name a few reasons for failure in Monte Carlo simulation?

Failure Reasons

What do you do when Monte Carlo has noise rise failure?

Identify the sectors causing the failure and mitigate overshoot.

How do you spread traffic in your design? Do you use live traffic or assume same load every where?

Tpc (Traffic per cell), uniform traffic spread based on GSM Best Server coverage.

Is there a parameter to adjust handover margin on a per-adjacency base?

Ensure that you understand the significance of each parameter and write a brief description.

Max Tx Power

Max Tx Power/Connection

Noise Rise

Pilot Power

Orthogonality factor

Soft Handover Window

Primary Common Channel Power

Secondary Common Channel Power

Scrambling Code

Active Set Size

Have you performed optimization activities? Describe in chronological order the steps taken during these activities. What equipment was used? Describe the step taken to troubleshoot a sector that is experiencing a sudden drop in data throughput.

How is QoS realized in UMTS?

Network services are considered as end-to-end. The End-to-End Services used by the Terminal Equipment (TE) is realized using a TE/MT Local Bearer Service, a UMTS Bearer Service and an External Bearer Service. The UMTS Bearer service plays a major role in the end-to-end service provisioning and reflects the capabilities of the UMTS network. It consists of two parts, the Radio Access Bearer (RAB) Service and the Core Network Bearer Service. The RAB represents the RAN part of a UMTS Bearer provided to an End-to-End service between different terminal equipment. WCDMA RAN can provide RAB connections with different characteristics in order to match requirements for different UMTS bearers. In order to achieve an acceptable QoS, QoS parameters particular to the UMTS bearer are mapped onto QoS parameters relevant for the specific behavior of the RAN. Those parameters are then included in the request sent by the CN to WCDMA RAN.

What is the purpose of a RAB?

To provide a connection segment for QoS using the WCDMA RAN for the support of a UMTS bearer service. Iu bearer and radio bearer connection realize a RAB in WCDMA RAN.

What traffic classes are used for real-time applications and non real time applications? Explain the difference between these classes in terms of attributes. Real-time applications: Conversational Class, Streaming Class Non-real time applications: Background Class, Interactive Class. The main distinguishing factor between these QoS classes is how delay sensitive the traffic is: Conversational Class is meant for traffic which is very delay sensitive while Background class is the most delay insensitive traffic class. The Conversational and Streaming class have a guaranteed bitrate, while the interactive and background classes have no guaranteed bitrate. What is your understanding of HSDPA? High Speed Downlink Packet Access (HSDPA) is a packet-based data service in W-CDMA downlink with data transmission up to 8-10 Mbps (and 20 Mbps for MIMO systems) over a 5MHz bandwidth in WCDMA downlink. HSDPA implementations includes Adaptive Modulation and Coding (AMC), Multiple-Input Multiple-Output (MIMO), Hybrid Automatic Request (HARQ), fast cell search, and advanced receiver design.

In 3rd generation partnership project (3GPP) standards, Release 4 specifications provide efficient IP support enabling provision of services through an all-IP core network and Release 5 specifications focus on HSDPA to provide data rates up to approximately 10 Mbps to support packet-based multimedia services. MIMO systems are the work item in Release 6 specifications, which will support even higher data transmission rates up to 20 Mbps. HSDPA is evolved from and backward compatible with Release 99 WCDMA systems.

What are the key enhancements in HSDPA over R99/WCDMA/UMTS?

HSDPA improves on W-CDMA by using different techniques for modulation and coding. It creates a new channel within W-CDMA called HS-DSCH, or high-speed downlink shared channel. That channel performs differently than other channels and allows for faster downlink speeds. It is important to note that the channel is only used for downlink. That means that data is sent from the source to the phone. It isn't possible to

send data from the phone to a source using HSDPA. The channel is shared between all users which lets the radio signals to be used most effectively for the fastest downloads.

HSDPA achieves its performance gains from the following radio features:

• High speed channels shared both in the code and time domains

• Adaptive modulation and coding schemes: Quadrature Phase Shift Keying

(QPSK) and 16QAM (Quadrature Amplitude Modulation).

• Hybrid Automatic Repeat reQuest (HARQ) retransmission protocol.

• Short transmission time interval (TTI)

• Fast packet scheduling controlled by the Medium Access Control - high speed

(MAC-hs) protocol in Node B.

• Fast scheduling How is 16QAM implemented in WCDMA system? Four consecutive chips are mapped onto sixteen different positions on the I and Q modulator. Which 2 algorithms are the bases of the scheduling function? Which one is better? Why?

Round robin and Proportional fair. Round robin does only take into account the delay of the transmissions and is therefore not so effective for an HSDPA system, however it is very fair to the users. Proportional fair takes both the delay of transmission and the quality of the radio link to the different users. This makes this algorithm more effective and creates higher throughput.

Why the ARQ algorithm in HSDPA called Hybrid ARQ?

A normal ARQ algorithm only tells the scheduler that there was a faulty block and request for retransmission. A hybrid ARQ will ask for a retransmission of the faulty block and keep the faulty block in buffer so that it can be combined with the retransmission of the block.

Chase combining and Incremental redundancy is used for combining. Chase combining will send the same block again whereas incremental redundancy will send the rest of the redundant bits that were not sent earlier.

How is power control implemented in HSDPA?

There is no power control on the HSDPA Data channel. Although there is a kind of power control since the rate matching and the modulation will be different depending on the received channel quality of the data channel at the UE. The UE will calculate a suitable transmission format and send that suggestion to the Node B that will decide what rate matching and modulation should be used.

GSM & RF basics

Theory:

Name eight logical channels of GSM protocols

Logical channels-1,2,3) BCH (Broadcast channels):

1.FCH (Frequency correction channel)

2.SCH (Synchronization channel)

3.BCCH (Broadcast control channel)

Logical channels-4,5,6) CCH (Common control channels):

1.PCH (Paging channel)

2.RACH (Random access channel)

3.AGCH (Access Grant channel)

Logical channel-7) SDCCH (Stand Alone Dedicated Control channel)

Logical channel-8) TCH (Traffic channel)

Name five (four) bursts in GSM.

There are four primary types of bursts. Normal Burst, Access burst, Synchronization burst and Frequency correction burst.

• Normal Burst is used to send the traffic channels, stand alone dedicated channels, broadcast control channel, paging channel, access grant channel, slow and fast associated control channels.

• Access Burst which is used to send information on the Random Access Channel (RACH).

• Synchronization Burst is used to send information on the Synchronization Channel (SCH).

• Frequency Correction Burst is used to send information on the Frequency Correction Channel (FCH)

What are the functions of the following channels? BCCH, SDCCH, SACCH?

• BCCH: Broadcast Control Channel is used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Area Identity), BCCH carriers of the neighboring cells, maximum output power allowed in the cell and other broadcast messages like barred cell. (Downlink only)

• SDCCH: Stand Alone Dedicated Control Channel is used for allocating voice channel (TCH) to the mobile (call setup) and Location updating. Also to send Short Text message to Idle Mobile (Uplink & Downlink)

• SACCH: Slow Associated Control Channel is used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Area Identity), BCCH of all the neighbors and TA (Timing Advance); To send Short Text message to Busy Mobile (Downlink) and is also used for sending signal strength & bit error rate measurement of the serving cell and signal strength of the BCCHs of the neighboring cells (Uplink).

Sketch a block diagram of the GSM system overview (include BSS and NSS).

What three components must be added to the diagram in (Theory/Q2) when migrating from GSM to GPRS?

1. SGSN (Serving GPRS Serving Node)

2. GGSN (Gateway GRRS Serving Node)

3. IP Network

What is handover based on? Name at least six (five) conditions.

Handover in GSM is based on the following factors:

1. Quality, RXQUAL too high

2. Received level, RXLEV too low

3. MS BTS distance too large, maximum radius of a GSM cell is about 35 km

4. Better cell, power budget for another cell is more favorable, i.e., path loss is smaller

If the network is strictly noise limited (very low interference), RXLEV HO (or more preferably power budget HO) should be the dominant reason for a HO. In an interference limited network (i.e. urban area) power budget related HO should be the overwhelming HO cause because this guarantees that MS expends as little RF power as possible (assuming that uplink power control is used) thus creating less interference and saving MS battery.

The handover conditions are depicted below:

Insert suitable diagram here

Explain your understanding of GSM power budgets

The Power Budget assessment process may be employed by the network as a criterion in the handover process, by setting a flag in the BSS by O&M command. If the process is employed, every 480 ms, for every connection and for each of allowable 32 adjacent

cells, the BSS shall evaluate the following expression. The Power budget is computed for all cells separately and it is expressed for cell n as:

PBGT(n) = RX_LEV_NCELL(n) – (RXLEV_DL + PWR_C_D)

+ min(MS_TX_PWR_MAX,P) – min(MS_TX_PWR_MAX(n),P)

Here PWR_C_D is the averaged difference between the maximum downlink RF power BS_TXPWR_MAX, and the actual used downlink power due to power control. P is the maximum power capability of the MS.

In a simplified case the downlink power control is not used and MS_TXPWR_MAX is the same for all cells. Power budget can now be written as

PBGT(n) = RX_LEV_NCELL(n) – RXLEV_DL

which portrays the path loss difference between cell n compared to serving cell, if the TX power of both BTS is the same.

In order to initiate a power budget HO to cell n, PBGT(n) must exceed PBGT of the serving cell by at least HOMARGIN(n) which is also defined separately for each cell. HOMARGIN assures that MS will not bounce back and forth between cells due to slow fading or minor user movements. In other words, the condition PBGT(n) > HOMARGIN(n) must be fulfilled in order to initiate a power budget HO to cell n.

Parameters/OSS

What parameter to change BS output power?

bs tx pwr max (PMAX): With this parameter you identify the maximum transmission power of the BTS as an attenuation from the peak power of the TRX; Range: 0..30 (dB) with a step size of 2 dB; Default: 0 dB

bs tx pwr min (PMIN): With this parameter you identify the minimum transmission power of the BTS as an attenuation from the peak power of the TRX; Range: 0..30 (dB) with a step size of 2 dB; Default: 30 dB

HO parameters (nx/px, pbgt)

MS speed threshold Nx (STN): The parameter is used in threshold comparison between the averaged MS speed and the upper and lower speed thresholds. Nx is the total number of averages to be taken into account before the decision is possible. This parameter is common to both lower and upper speed thresholds; Range: 1..32; Default: 6

MS speed threshold Px (STP): The parameter is used in threshold comparison between the averaged MS speed and the upper and lower speed thresholds. Px is the number of averages out of total averages that have to be lower/higher than the threshold, before a handover due to MS speed is possible. This parameter is common to both lower and upper speed thresholds; Range: 1..32; Default: 3

HO margin pbgt (PMRG): With this parameter you define a threshold in the power budget process. The handover margin prevents repeated handover between adjacent cells; Range: -24..63 (dB) (1 dB step size); Default: 6

The Power budget is computed for all cells separately and it is expressed for cell n as:

PBGT(n) = RX_LEV_NCELL(n) – (RXLEV_DL + PWR_C_D) + min(MS_TX_PWR_MAX,P) – min(MS_TX_PWR_MAX(n),P)

Here PWR_C_D is the averaged difference between the maximum downlink RF power BS_TXPWR_MAX, and the actual used downlink power due to power control. P is the maximum power capability of the MS.

In a simplified case the downlink power control is not used and MS_TXPWR_MAX is the same for all cells. Power budget can now be written as

PBGT(n) = RX_LEV_NCELL(n) – RXLEV_DL

which portrays the path loss difference between cell n compared to serving cell, if the TX power of both BTS is the same. In order to initiate a power budget HO to cell n, PBGT(n) must exceed PBGT of the serving cell by at least HOMARGIN(n) which is also defined separately for each cell. HOMARGIN assures that MS will not bounce back and forth between cells due to slow fading or minor user movements. In other words, the condition

PBGT(n) > HOMARGIN(n) must be fulfilled in order to initiate a power budget HO to cell n.

RNW experience

MML(ZEEI, ZEOL)

CM Planner

How to implement massive parameter changes

Dual Band Paramters (NBL)

Number of ignored transcoder failures (ITCF): With this parameter you define how many successive remote transcoder failures the BSC may receive before it releases the call; Range: 0..5; Default: 0

Radio Link Timeout(RLT)

With this parameter you define the maximum value of the radio link counter expressed in SACCH blocks; Range: 4..64, with step size of 4; Default: 0

What is “pole capacity”? For AMR 12.2kbps voice what is the theoretical pole capacity? In your experience with vendor what is their proposed pole capacity?

What is little “FRE” What is the typical value for an evenly loaded network?

Optimization

Which KPIs to monitor?

Typically we monitor the following:

• CSSR (Call Setup Success Rate)

• DCR (Dropped Call Rate)

• HO fail & success rate

• TCH assignment success rate

• TCCF (Traffic Channel Confirmation Failure)

• TCH block rate

• SDCCH drop rate

• SDCCH block rate

• Traffic (MOU, BBH traffic, Hourly traffic etc.)

What is blocking rate?

The blocking rate is defined as the probability that a call attempt cannot be handled successfully due to unavailability of the radio resources (i.e. unavailability of free

circuits or other switching equipments). Blocking rate: ≤�2% is good, 10% is bad service.

What is grade of service?

The Grade of Service (GOS) is defined as the probability of a lost call. This includes not only the blocking rate, but also the reliability of the system. GOS is dictated by license conditions and quality of the competing network.

What is call success rate?

The call success rate is often expressed as the proportion of calls connected and held for two minutes within the declared coverage area (this is 90% at launch of service and 98% for mature networks). Calls may fail due to coverage holes, interference, congestion, network problems, handover failure or equipment failure.

What is dropped call rate?

The dropped call rate is defined as the probability that a call will be lost prior to completion due to handover failure, coverage holes, interference or congestion (typical values of 2% to 5% as defined by the operator).

What to do for high DCR?

The dropped call causes can be drilled down as shown below. The resolution of the individual issue(s) for the rouge cells can help to reduce the dropped calls.

The following parameters/events/statistics to be analyzed for the rogue cells (having high dropped call rate):

• Drop after TCH assignment

• TCH Radio fail

DROPPED CALL

CAUSES

RF issues Non-RF issues

Poor coverage Interference HO Failure

Internal External

Adjacent channel interferenceCo-channel interference

Missingneighbours

Resourceblocking

Typically hardware relatedissues

TCH blockingBackhaulcongestion

DROPPED CALL

CAUSES

RF issues Non-RF issues

Poor coverage Interference HO Failure

Internal External

Adjacent channel interferenceCo-channel interference

Missingneighbours

Resourceblocking

Typically hardware relatedissues

TCH blockingBackhaulcongestion

• TCH Non-Radio fail, TCH_Transcoder fail

• HO fails, HO fails due to power budget, level, quality

• Uplink Interference bands

Resolution:

• Physical optimization viz. re-orientation of antenna, tilt adjustment etc. to improve coverage and thus improve S/N ratio

• Parameter optimization viz. power adjustment, lowering HO margin to push the call faster from the affected cell towards a better one, reducing the measurement interval shall trigger a faster quality HO for the rouge cells, adjusting the power control window, BCCH frequency and/or Non-BCCH frequency modification to avoid interference, missing neighbors to be defined etc.

• Implementing Dual Rate (full rate & half rate) TCH, AMR (adaptive multi rate) and TRX addition, offloading traffic for congested cells to reduce TCH blocking and thus HO failure.

• Hardware issues to be diagnosed and resolved

What to do for high call setup failure?

Successful call = Successful Origination + Termination calls

Call setup success rate (CSSR) = (Successful orig + term calls) / (Total orig + term seizures)

Call setup failure rate = (1 – CSSR) %

= ( Orig + term calls failed) / (Total ori + term seizures)

The call failure causes can be drilled down as shown below. The resolution of the individual issue(s) for the rouge cells can help to reduce the dropped calls.

Resolution:

• Physical optimization viz. re-orientation of antenna, tilt adjustment etc. to improve coverage and thus improve S/N ratio, or adjust the link budget parameters to result in proper path balance

• BCCH frequency and/or Non-BCCH frequency modification to avoid interference

Failed Calls(orig. & term.)

CAUSES

RF issues Non-RF issues

Poor coverage,Improper path balance Blocking Interference

TCH blocking

SDCCH blocking

CP (call processing failure)

Internal External

Co-channel interference

Adjacent channel interference

Failed Calls(orig. & term.)

CAUSES

RF issues Non-RF issues

Poor coverage,Improper path balance Blocking Interference

TCH blocking

SDCCH blocking

CP (call processing failure)

Internal External

Co-channel interference

Adjacent channel interference

• Implementing Dual Rate (full rate & half rate) TCH, AMR (adaptive multi rate) and TRX addition, offloading traffic for congested cells to reduce TCH blocking, additional SDCCH to be defined for high SDCCH blocking etc.

• Hardware issues to be diagnosed and resolved

What is frequency hopping?

Frequency hopping is the dynamic switching of radio links from one carrier frequency to another. Frequency hopping changes the frequency used by a radio link every new TDMA frame in a regular pattern.

What are the advantages of frequency hopping?

• To improve the transmission quality for slow moving mobiles (frequency diversity)

• To reduce the overall interference in the network (interference diversity).

• To increases capacity (tightens frequency reuse).

Why hopping?

Advantages of frequency hopping: 1. frequency diversity and 2. Interference averaging

1. Frequency diversity: In cellular urban environment, multipath propagation exists in most cases. Due to Rayleigh fading, short-term variations in received level are frequently observed. This mainly affects stationary or quasi-stationary mobiles. For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious. Frequency Hopping is able to take the advantage due to frequency selective nature of fading to decrease the number of errors and at the same time they are temporally spread. As a result, the decoding and de-interleaving processes can more effectively remove bit errors caused by bursts received whilst on fading frequencies (errors will be randomly distributed instead of having long bursts of errors). This increase in effectiveness leads to a transmission quality improvement of the same proportion.

Frame Erasure Rate reduces due to 6 dB to 8 dB gain.

Number of reports with rxqual 6 and 7 reduce.

Reported values of rxlev are more concentrated around mean.

2. Interference averaging: Interference Averaging means spreading raw bit errors (BER caused by the interference) in order to have random distribution of errors instead of having burst of errors, and therefore, enhance the effectiveness of decoding and de-interleaving process to cope with the BER and lead to better value of FER.

With hopping, the set of interfering calls will be continually changing and the effect is that all the calls experience average quality rather than extreme situations of either good or bad quality. All the calls suffer from controlled interference but only for short and distant periods of time, not for all the duration of the call.

For the same capacity, Frequency Hopping improves quality and for a given average quality Frequency Hopping makes possible increase in capacity.

When more than 3 % of the reports have rxqual of 6 or 7 then voice quality disturbances start to appear.

Gains (reduction in the C/I value needed to satisfy the quality requirements involved in the criterion) from hopping relative to fixed frequency operation can be achieved.

1/3 interference: 1 dB gain

i.e. if 1 out of 3 frequencies are experiencing a continuous interference a gain of 1 dB in C/I requirement is obtained.

Similarly,

1/4 interference: 4 dB gain

1/5 interference: 6 dB gain

2/4 interference: 0 dB gain

2/5 interference: 4 dB gain

The effective gain obtained with Frequency Hopping is due to the fact that the interference effect is minimized and it is easier to keep it under control.

Explain sequence generation

Each call has its time slots transmitted in sequence across a defined set of hopping frequencies. The sequence is derived from an algorithm (according to GSM recommendation 05.02). Frequency hopping occurs between time slots: a mobile station transmits or receives on a fixed frequency during one time slot, then changes frequency before the time slot on the next TDMA frame. The total number of available hopping sequences is 64 multiplied by the number of hopping frequencies (64xN). Hopping sequences are described per channel by two network parameters: HSN and MAIO.

HSN HSN (Hopping Sequence Number): defines a number that is fed into the frequency hopping algorithm to generate the hopping sequence. Values can be 0 to 63. Value 0 defines cyclic hopping; all other values generate a random sequence.

MAIO MAIO (Mobile Allocation Index Offset): defines the starting frequency, or offset, the transmission will start on within a hopping sequence. The value can be 0 to N-1 where N is the number of allocated frequencies.

What are the rules for using HSN and MAIO?

To fight the interference with frequency hopping, use the following rules:

• Two channels with the same HSN but different MAIO never use the same frequency at the same time.

• Two channels using the same frequency list and the same time slot, but with a different and non-zero HSN, will interfere in 1/Nth of bursts, as if the sequences were chosen randomly.

• Channels in the same cell using the same hopping frequency set should have the same HSN, and different MAIO, to avoid co-channel interference within the cell.

• If random hopping is used, each channel in distant cells using the same frequency set should have a different HSN; this optimizes the benefits of interference diversity.

Common control channels

In order to ease initial synchronization acquisition, the following common channels must use a fixed frequency:

• FCCH (Frequency Correction Channel)

• SCH (Synchronization Channel)

• BCCH (Broadcast Control Channel)

• PAGCH (Paging Access Grant Channel)

• RACH (Random Access Channel)

Frequency hopping therefore can not be applied to these channels. Common channel extension sets (CCCH) must use the same fixed frequency as the primary group. This avoids the need to transmit their frequency organization description on the BCCH.

Traffic channels on the rest of the time slots in BCCH transceivers can hop. Only the common channels cannot hop.

What are the different kinds of hopping? What are the differences?

1. Base Band Frequency (BBH): This is accomplished by routing the traffic channel data through fixed frequency TRXs via the TDM highway on a timeslot basis. In this case, the TRX would have fixed tuned transmitters combined either in low loss tuned combiners or hybrid combiners.

TRX always transmits fixed frequency.

The information for every call is moved among the available TRXs on a per burst basis. (Burst of 577 µs)

Call hops between same timeslots of all TRXs.

Processing (coding and interleaving) is done by digital part associated with TRX on which call was initially assigned.

For uplink – call is always processed by TRX on which the call was initially assigned.

Number of TRXs needed is equal to the number of frequencies in the hopping sequence.

BCCH frequency can be included in the hopping sequence.

Power control does not apply to BCCH or bursts transmitting BCCH frequency.

BCCH, timeslot 0 will never hop.

Any timeslot with CCCH will never hop.

Timeslot carrying all SDCCHs can hop.

2. Synthesizer Frequency Hopping (SFH): This is accomplished by high speed switching of the transmit and receive frequency synthesizers of the individual TRXs. As a result of dynamic nature of the transmit frequency, broadband (hybrid) combining of the transmitters is necessary.

TRX changes transmitting frequency every burst.

Call stays on the same TRX where it started.

Remote tune combiners (RTC) are not allowed.

Number of TRXs is not related to number of frequencies in hopping sequence.

BCCH TRX will never hop. It either carries traffic in timeslots 1 to 7 or it transmits dummy bursts.

Transmission and reception is done on the same timeslot and same TRX.

What is the primary limitation of baseband hopping?

The primary limitation of baseband hopping is that the number of hopping frequencies is limited to the number of transceivers in the cell.

What is the primary limitation of synthesizer hopping?

Synthesizer hopping can only be performed when hybrid combiners are used. Be aware that hybrid combiners cause high losses.

What are the hopping parameters and brief their functions?

Frequency Hopping Parameters:

Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop over. Maximum of 63 frequencies can be defined in the MA list.

Hopping Sequence Number (HSN): Determines the hopping order used in the cell. It is possible to assign 64 different HSNs. Setting HSN = 0 provides cyclic hopping sequence and HSN = 1 to 63 provide various pseudorandom hopping sequences.

Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence, which frequency the mobile starts to transmit on. The value of MAIO ranges between 0 to (N-1) where N is the number of frequencies defined in the MA list. MAIO is set on per carrier basis.

Frequency Hopping Indicator (FHI): Defines a hopping system, made up by an associated set of frequencies (MA) to hop over and sequence of hopping (HSN). The value of FHI varies between 0 to 3. It is possible to define all 4 FHIs in a single cell.

GSM has defined an algorithm for deciding hopping sequence. The algorithm is used to generate Mobile Allocation Index (MAI) for a given set of parameters.

ARFCN: absolute radio frequency channel number

MA: mobile allocation frequencies.

MAIO: Mobile allocation offset (0 to N-1), where N is the number of frequencies defined in MA.

HSN: Hopping sequence number (0-63)

T1: Super frame number (0-2047)

T2: TCH multiframe number (0-25)

T3: Signaling multiframe number (0-50)

This algorithm generates a pseudorandom sequence of MAIs. MAI along with MAIO and MA will decide the actual ARFCN to be used for the burst.

What do you mean by Full rate/Half rate traffic?

A full rate TCH uses all allocated time slots (TSs) and twenty six TSs in a multiframe. In half rate traffic, the speech coders cut down the bit rate from the present 13kbps to 6.5 kbps. There are two-fold advantages for half rate:

a) The result will be that 50% of the allocated TSs will be needed for one call and the other 50% will be idle. The idle frame from a full rate channel can then be used for SACCH signaling for the second mobile. One mobile can also be allocated two half rate channels and use one for speech and the other for data.

b) Two mobiles will be able to use the same physical channel for their calls which doubles the traffic capacity.

But it should be remembered that the quality in half rate traffic will not be that much good as in full rate.

What is AMR?

GSM/AMR product is a complete implementation of the Global System for Mobile (GSM) Adaptive Multi-Rate (AMR) voice coder, and is fully compliant with the ETSI GSM 06.90 specification.

The Adaptive Multi Rate (AMR) speech codec is the default speech codec for GSM 2+ and WCDMA third generation wireless systems. GSM-AMR is based on Algebraic CELP (ACELP) and operates at eight bit rates ranging from 4.75 kbps to 12.2 kbps. It is developed to preserve high speech quality under a wide range of transmission conditions. The AMR codec is designed with a voice activity detector (VAD) and generation of comfort noise (CNG) parameters to reduce the number of transmitted bits and packets during silence periods.

Technical Specification:

• Performs compression and decompression of all 8 specified data rates, ranging from 4.75 kbps to 12.2 kbps

• Includes support for frame substitution and muting (GSM 06.91)

• Includes support for comfort noise generation (GSM 06.92)

• Includes support for DTX - Discontinuous Transmission (GSM 06.93)

• Includes support for VAD - Voice Activity Detection (GSM 06.94)

• Supports a C callable API for initialization / encoding and decoding

• Supports Multi channel Capability and thus supports full rate to half rate conversion and vice versa (if necessary) during handover of a voice call.

What is the difference between HO Level and HO Interference?

HO Level: The handoff takes place on the basis of the signal level difference between the serving cell and the adjacent cells. The difference is calculated both in the uplink and downlink.

Downlink: The averaged values of signal strength downlink are measured for triggering the handover process.

Uplink: The averaged values of signal strength uplink are measured for triggering the handover process.

HO Interference: This is an intracell handoff which takes place on account of the uplink or downlink interference.

Uplink: The averaged values of interference downlink are measured for triggering the handover process.

Downlink: The averaged values of interference uplink are measured for triggering the handover process.

What is meant by frequency re-use in GSM? What is meant by 3/9, 4/12, 7/21 frequency re-use patterns?

A cluster is a group of cells in which all available carriers have been used once (and only once). Since the same carriers are used in cells in neighboring clusters, interference may become a problem. The frequency re-use distance (i.e. the distance between two sites using the same carrier) must be kept as large as possible to help prevent interference. At the same time, the distance must be kept as small as possible from a capacity point of view. Cellular systems are often interference-limited rather than signal-strength-limited. Re-using the carrier frequencies according to well-proven re-use patterns, neither co-channel interference nor adjacent channel interference should become a problem. This is true if the cells have homogenous propagation properties for the radio waves.

The re-use patterns recommended for GSM are the 3/9, 4/12, 7/21.

3/9 means that there are three three-sector sites supporting nine cells

4/12: there are four three-sector sites supporting twelve cells

7/21: seven three-sector sites supporting twenty-one cells

What are ARFCN, BSIC, LAC, LAI?

ARFCN: Within the spectrum allocated for cellular mobile communications, the radio channels are identified by

ARFCN (Absolute Radio Frequency Channel No.). The channel number is associated with both the uplink and downlink radio channels. Within the GSM900 spectrum ARFCN 1 to 124 are used. In GSM1800, ARFCN 512 to 885 are used and in GSM1900, ARFCN 512 to 810 are used. The frequency corresponding to the channel no. is calculated by the following relation:

fuplink = [Uplink start freq.] + [Channel bandwidth X ARFCN]

fdownlink = [Downlink start freq.] + [Channel bandwidth X ARFCN]

fdownlink - fuplink = Duplex gap

Thus, for GSM900, ARFCN 123 corresponds to fuplink = 890 + 123 x 0.2 = 914.6 MHz,& fdownlink = fuplink + 45 MHz (channel b/w) = 959.6 MHz

BSIC: BSIC (Base Station Identity Code) makes it possible for a mobile station to distinguish between different neighboring base stations. BSIC consists of: BSIC = NCC + BCC

NCC (Network Color Code) is a 3 bit number that identified the PLMN. NCC is primarily used to distinguish between operators on each side of a border.

BCC (Base Station Color Code) is also a 3 bit number that identifies the Base Station to help distinguish between BTS using the same BCCH frequencies.

LAC: Location Area Code, the maximum length is 16 bits, enabling 65,536 different location areas to be defined in one PLMN.

LAI: Location Area Identity is related to “Paging” and “Location updating” of mobile subscribers. LAI = MCC + MNC + LAC where MCC is mobile country code, MNC is mobile network code and LAC is location area code.

What is meant by “Combined BCCH”?

Normal (non-combined) mode: In TS-0 the broadcast channels – BCCH, FCCH, SCH, and the common control channel – CCCH are repeated over and over again in a specific pattern. The cycle is 51 TS long. On TS-1 of the same carrier, eight SDCCHs can share the same physical channel (A SDCCH/8 channel). A SACCH is allocated to each SDCCH. These are labeled in the same manner as the SDCCH i.e., SACCH/1 to SACCH/8.

Combined BCCH: BCCH, FCCH, SCH, CCCH and SDCCH are all combined on the same channel, TS-0. Here SDCCH/4 channels and the corresponding SACCH/4 channels can exist at maximum. The separate SDCCH/8 channel can still be reserved at TS-1.

What is a TDMA frame, 26-TDMA frame multiframe, 51-TDMA frame multiframe, superframe and multiframe?

One TDMA frame consists of 8 time slots (TS). Duration = 4.615 ms.

26 TDMA frame multiframe is used to carry TCH, SACCH and FACCH. Duration = 26 x 4.615 = 120 ms.

51 TDMA frame multiframe is used to carry BCCH, CCCH, SDCCH and SACCH. Duration = 51 x 4.615 = 235 ms.

A Superframe consists of 51 or 26 multiframes and a Hyperframe consists of 2048 superframes. 1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames (3 hrs. 28 min. 53 sec. 760 microsec.)

What do you understand by “directed retry”?

Directed retry is a procedure which is used in call setup phase in assigning a traffic channel to a mobile station from a cell (no matter if macro or micro) other than the serving cell, in situations when the first attempt fails due to tch congestion allowing the mobile subscriber to make a second attempt at gaining access. Directed retry is an handover from SDCCH to other cell's TCH.

Directed Retry can be used in both Mobile Originating Calls and in Mobile Terminating Calls. The target cell selection for Directed Retry is made according to the imperative handover criteria so that the criteria itself is not as strict as in normal HO algorithm analysis. The following items are taken into account during the candidate cell selection:

• the Rx signal level compared to the threshold value defined by the parameter RxLevMinCell

• the MS classmark and

• the maximum power level in the cell.

What is the purpose of power control? How power control works in GSM in uplink & downlink?

Power control is used in order to decrease the power consumption of the Mobile Station (in uplink direction) to reach a longer serving time to the Mobile Station. Another reason is to decrease interference in both directions (uplink and downlink) by using as low transmitting power as possible in the Mobile Station and in the BTS at all times. Power control can be used in downlink direction in every TRX, except in a TRX with BCCH, because the BTS has to send data continuously on these frequencies without any Power Control (= full power in that cell. This is needed because the MS is continuously measuring the RX level of the adjacent cell BCCH’s). The Mobile Station can use power control on each frequency continuously, if needed. When using power control, enough margin has to be reserved for Rayleigh fading and it has to be taken into account that handover has always higher priority than power control.

After every SACCH multiframe period, the BSC compares each of the processed measurement results (averages) with the relevant power control thresholds. If the power control (PC) threshold comparison indicates that the MS or the BTS (a radio time slot on a carrier) needs an increase or decrease in RF power, the BSC sends a PC command to the MS/BTS including the new transmission power level of the MS/BTS. When the BSC defines the new transmission power level of the MS, it takes into account both the RF power capability and the revision level of the MS. The BSC may send the PC command simultaneously both to the MS and the BTS or to one of them, that is, the power control for the MS and BTS runs independently. The minimum and maximum MS transmission powers are determined on cell-by-cell basis.

Briefly describe cell search process. What is C1 & C2 parameters and how they participate in cell reselection in idle mode?

One basic idea in the GSM system is that the Mobile Station is always within the cell offering the best coverage. In Dedicated mode this is handled by handovers, but in Idle mode the Mobile has to find the best cell in each area. There is a process for this purpose called Cell Selection, based on C1 or C2 comparison. The idea is that the Mobile compares field strength levels coming from different cells with each other and selects the best one from them. The mobile uses the cellReselectHysteresis (0 ... 14 dB) parameter between cells that belong to different Location Areas in order to avoid the "Ping-Pong" phenomenon, which means that before the mobile changes to a different cell in Idle mode, between different location areas, the field strength level of the cell has to be at least the value of cellReselectHysteresis better than the value of the serving cell. There is no margin between the cells that belong to the same Location Area.

C1 = (A - Max(B,0))

Where A = Received Level Average - p1, B = p2 - Maximum RF Power of the Mobile Station,

where p1 = rxLevelAccessMin, p2 = msTxPowerMaxCCH,

The Mobile takes into account the minimum access level to the cell and the maximum transmitting power allowed to the Mobile in each cell when starting a call.

The rest of the C2 parameters are related to microcellular planning. Parameter penaltyTime (20 ... 640 s) describes the time delay before the final comparison is made between two cells. Parameter temporaryOffset (0 ... 70 dB) describes how much field strength could have been dropped during this penalty time, and parameter cellReselectOffset (0 ... 126 dB) describes an offset to cell reselection. C2 cell reselection is calculated by equation

C2 = C1 + cellReselectOffset - temporaryOffset x H(penaltyTime-T) when penaltyTime¹640

or

C2 = C1 - cellReselectOffset when penaltyTime=640

Where H(x)=1 when x>=0, and H(x)=0 when x<0

What is “idle channel interference”?

The BTS reports the idle channel interference’s also from incomplete measurement periods and active channel interference’s, if they are measured as well. Active channel interference estimation is realized by utilizing idle TDMA frames with TCH/F connections and also the silent periods in mobile transmission during uplink DTX. BTS calculates the interference levels and reports them to the BSC. The reporting is done with RF_RESOURCE_INDICATION message, which originally contained the interference band information for idle channels only. Now the results of an active channel are included in this message when there are enough interference level measurements available.

Measurement for active channel interference level is possible only during the speech connections, not during data connections. If the uplink DTX is not activated, then the active channel interference cannot be measured for half rate calls. However, idle channel interference can be measured from incomplete measurement periods in every case.

Why BCCH carrier is expected to be cleaner than the non-BCCH carriers?

The BCCH carrier carries all the signaling channels such as BCCH, FCCH, SCH, CCCH, SDCCH, SACCH apart from the traffic channels in the downlink, and RACH, TCH in the uplink. The non-BCCH carriers are used solely to carry traffic. The signaling channels specially the BCH ones depict the coverage of the corresponding cell/sector in terms of signal strength and C/I (or quality). A call is originated or terminated by the use of the aforesaid signaling channels. Thus, for maximum accessibility to the network (best CSSR), a very healthy C/I (or quality) is desired and good quality can be achieved if the BCCH carrier is interference-free or clean. The non-BCCH carrier on the other hand is itself equipped with SFH plan for most of the network that helps to achieve the maximum spectrum efficiency with tolerable degradation in quality with loading. Moreover, the non-BCCH carriers accommodate the already established traffic channels and each established call can combat the interference by the virtue of power control. But the BCCH carrier is responsible for establishing an originating or terminating call in the traffic channel, i.e the accessibility to the network.

Design

What factors affect signal propagation?

1. Multipath propagation: Whenever a mobile station is in contact with the GSM network, it is quite rare that there is a direct "line of sight" transmission between the mobile station and the base transceiver station. In the majority of cases, the signals arriving at the mobile station have been reflected from various surfaces. Thus a mobile station (and the base transceiver station) receives the same signal more than once. Depending on the distance that the reflected signals have traveled, they may affect the same information bit or corrupt successive bits. In the worst case an entire burst might get lost. Depending on whether the reflected signal comes from near or far, the effect is slightly different. A reflected signal that has traveled some distance causes "inter symbol interference" whereas near reflections cause "frequency dips".

2. Shadowing: Hills, buildings and other obstacles between antennas cause shadowing (also called Log Normal Fading). Instead of reflecting the signal these obstacles attenuate the signal. Shadowing is generally a problem in the uplink direction, because a Base Transceiver Station transmits information at a much higher power compared to that from the mobile station.

3. Propagation delay: Information is sent in bursts from the mobile station to the Base Transceiver Station (BTS). These bursts have to arrive at the base transceiver station such that they have to map exactly into their allocated time slots. However, the further away the mobile station is from the BTS then the longer it will take for the radio signal to travel over the air interface. This means that if the mobile station or base station transmits a burst only when the time slot appears, then when the burst arrives at the other end, it will cross onto the time domain of the next timeslot, thereby corrupting data from both sources.

In your design process, what kinds of plots do you usually use to determine if you have met design targets?

Prediction tool: Rx level for best cell, C/I or interference plots, Rx level threshold coverage, C/I coverage threshold, Cell overlap, Traffic coverage.

Drive test: Rx level for best cell, Quality plot, BCCH plot

What are the factors considered in GSM link budget and mention the typical values?

Uplink Link Budget

Voice GSM

Voice GSM

Downlink Link Budget

Voice GSM

Voice GSM

Uplink Frequency:

840

MHz

1900

MHz

Downlink Frequency:

885

MHz

1980

MHz

MS Power: 29 dBm 30

dBm

Hatchplate BS Transmitter Power:

44.50

dBm

44.50

dBm

MS Antenna Gain: 0 dBi 0 dBi

Coupling/Combining Losses: 4 dB 1.5 dB

Body Losses: 5 dB 3 dB Computed BTS Output Power:

40.50

dBm

43.00

dBm

BS Antenna Gain: 14 dBi 17 dBi TMA Tx Losses: N/A 0.6 dB

Feeder/Jumper Losses: 3 dB 1 dB

Feeder/Jumper Losses: 3 dB 5.0 dB

TMA Noise Figure: N/A 1.5 dB

BS Antenna Gain: 14 dBi 17 dBi

TMA Gain: N/A 12 dB BS EIRP 51.50

dBm

54.40

dBm

TMA to BS Feeder/Jumper Losses: N/A 4 dB

MS Antenna Gain: 0 dBi 0 dBi

BS Noise Figure: 3 dB 3 dB Body Losses: 5 dB 3 dB

Relevant Noise Figure: 3.0 dB 2.2 dB MS Noise Figure: 8 dB 8 dB

User Bit Rate (Rb):

12.2

kbit/s

12.2

kbit/s

Noise Spectral Density:

-166.0

dBm/Hz

-166.0

dBm/Hz

Chip Rate (RW): 270.8

kbit/s

270.8

kbit/s Required Ec/No: 8 dB 8 dB

Thermal Noise Spectral Density:

-171.0

dBm/Hz

-171.8

dBm/Hz

Computed MS Ref. Sensi.:

-103.7

dBm

-103.7

dBm

Required Ec/No: 5.5 dB 5.5 dB Max DL Free Space Losses:

150.2 dB

155.1 dB

Computed Static BS Ref. Sens.:

-111.2

dBm

-112.0

dBm

Interference Margin: 2.0 dB 2.0 dB

Receive Space Diversity: 3.5 dB 3.5 dB

Worst Case Max Path Losses:

147.7 dB

153.1 dB

Resulting BS Sensitivity:

-114.7

dBm

-115.5

dBm

Max UL Free Space Losses:

149.7 dB

158.5 dB

What do you mean by path-balance and what should be the ideal condition?

In the idle mode, the mobile always prefers to remain with or move to the best serving cell. The best cell is decided on the basis of uplink and downlink path balance in the cells. The path balance depicts how well balanced the coverage is in uplink and downlink. In other words, the more close the uplink path loss and the downlink path loss are, the more balanced the cell/sector is in terms of coverage. For an unbalanced cell or sector the power budget needs to be modified or re-designed.

Path Balance = Uplink path-loss – Downlink path-loss + 110

where Uplink path-loss = Actual Ms Tx power (dBm) – Uplink Rx level (dBm)

& Downlink path-loss = Actual BTS Tx power (dBm) – Downlink Rx level (dBm)

Typically, for a single carrier, Path balance is 105 – 108 & for multiple carriers, Path balance is 101 – 104

Is multipath advantageous to GSM?

GSM works on time division multiplexing scheme in which each user is allowed to hear or speak for a short discrete duration of time (TS = 577 microsec). Thus for a multipath scenario the reflected signal(s) may cause interference to the receiver if they are significantly delayed from the main signal. Also the received signals accommodated within the scheduled TS are the many identical signals which differ only in phase (and to some extent amplitude). It becomes difficult for the receiver to determine which symbol is the correct one. This is called Inter-Symbol Interference (ISI). Hence, multipath is not advantageous to GSM.

What is spectral efficiency? What are the important factors to achieve it?

Spectrum efficiency is a concept that relates system capacity to the occupied bandwidth. The following factors are important to calculate the spectral efficiency:

• Quality of coverage

• Frequency re-use plan

• Use of cell splitting

• Design for traffic demand

• Allocation of TRXs per sector

• Feedback into the business plan.

In general what are the three main goals for a proper planning?

• Coverage

• Quality in the uplink and downlink direction

• Capacity requirements

What is shadowing?

Shadowing refers to fading of the signal if an obstruction lies in or close to the Line-of-Sight (LOS) path.

Explain Diffraction effect

Shadowing does not always mean that no signal is received behind an obstacle. Radio waves may bend around obstructions to a certain extent. This effect is called diffraction. The diffraction effect depends on the wavelength in relation to the size of the obstacle. The diffraction effect is greater if the wavelength increases.

Explain Fade Margin Concept

The concept of a fade margin is to reserve extra signal power to overcome potential fading. Assume that the mobile radio system needs a signal level of Pr dBm at the receiver, and that the maximum likely fade (loss) is calculated to be Lfade dB. The received signal level of Pr dBm can be ensured by transmitting enough power for a normal received signal level of (Pr + Lfade) dBm.

Explain Shadow effects

The effect of shadowing by obstacles is fading of the received signal. The problems of shadowing are most severe in heavily built-up urban centers. Shadow fading as deep as 20 dB may occur over very short distances, literally from one street to another. The fading effects produced by shadowing are often referred to as slow fading.

What are the counter measure strategies?

Due to the different effects (i.e., propagation loss and multipath propagations effects) that can influence the signal in the mobile environment, different countermeasures have been traditionally considered. The most important ones are:

• Equalization: Equalization is the process of removing time delayed waves to reduce the intersymbol interference effect. Usually, adaptive equalization is used

• Fade margins: The fade margin is an extra power added to the transmission to overcome potential fading of the signal.

• Diversity.: Time (bit interleaving), Frequency (hopping), Antenna (space and polarization +/-45)

What is the purpose of drive tests?

Drive tests are principally applied in both the planning and optimization stage of network development. Other purposes are:

• To provide path loss data for initial site survey work

• To verify the propagation prediction during the initial planning of the network

• To verify the network system parameters

• To verify the performance of the network after changes have been made

• To measure any interference problems such as coverage from neighboring countries

• To locate any RF issues relating to traffic problems such as dropped or blocked calls

• To locate any poor coverage areas.

Discuss about OH Model

The Okumura-Hata model predicts the path loss L between the transmitter and receiver, taking into consideration the frequency, the height of antennas, the propagation distance, and the terrain type. The model predictions correlate reasonably well with measurements, especially in urban and suburban areas. This is not the case in rural areas with irregular terrain. Correction factors are used for irregular terrain types such as: rolling hills, isolated mountains, general slopes (up or down), and mixed land-water paths.

Limitation: The Okumara-Hata formula is limited to a certain range of input parameters and is applicable only over quasi-smooth terrain (height variations ≤�20 m). No obstacles are assumed to be close to the BTS antenna.

What is “noise rise”? What does a higher or lower noise rise mean? What is the relationship between noise rise and network loading?

Noise rise in telecommunication means the significant increase of interference power thus degrading the threshold of a receiver. This degradation is termed as “desensitization”. The receiver threshold is mathematically expressed as

Receiver threshold = Sensitivity + Interference margin (IM) dBm

Sensitivity = 10log(kTB) + NF + S/Nmin dBm where, kTB is the white noise (refer to the “Noise” topic in the ‘System Design Considerations), NF is the noise figure of the receiver and S/Nmin is the minimum signal to noise ratio demand of the receiver. Thus with increase in noise, the interference margin increases and hence the receiver threshold degrades affecting the maximum allowable pathloss and thus shrink the cell coverage radius. A low noise rise is always preferable in any network.

What is Noise figure?

The noise figure F is the ratio between:

The input signal-to-noise ratio generated by thermal noise only

The total output signal-to-noise ratio generated by the thermal noise and the amplifier noise.

In a formula:

Where No = G·Ni+Nampl.

The Si and So are respectively the input and the output signal level.

Typical amplifier noise The typical amplifier noise is 6 dB.

What is Rx Sensitivity? What is the typical value?

The receiver sensitivity is the lowest signal which must be provided at the receiver to make sure the information on the incoming signal is detected in accordance with some predetermined criterion (e.g. bit error rate).

What are the ways of antenna tilting?

Two ways of antenna tilting can be distinguished:

Mechanical tilting

Electrical tilting.

Tilting is possible for omni and sector antennas.

Mechanical downtilt is physically altering the angle of the antenna. By doing so, next to the vertical radiation pattern also the horizontal radiation pattern is affected. Also be aware of the fact that coverage holes can be introduced when too much downtilt is given.

Mechanical tilt: The antenna itself goes through a certain degree of offset (i.e. mechanical tilt) or inclination away from its vertical/normal axis (refer to the diag.). In the process the back and side lobes of the antenna radiation proportionately get distorted with the mechanical tilts provided. Thus if the antenna itself is inclined downward along its vertical axis, the front lobe of the antenna droops down from the horizontal axis, but the back and side lobes get up-tilted from the horizontal axis. Hence, the side and back lobes are often prone to cause interference to other nearby cells or coverage holes in the near vicinity which are not desired.

Electrical tilt is achieved by phasing of the electrical currents in the internal dipole array. Electrical downtilting only changes the size of the illuminated area and not its shape. Only the vertical radiation pattern is affected and not the horizontal.

Electrical tilt: Unlike the mechanical tilt, the electrical tilt is provided by shifting the feed point of the antenna dipole so that the radiation patters in all directions (front, side and back) get equally tilted. The antenna dipole itself remains in its full normal position or non-tilted (refer to the diag.). Thus the side and back lobes are not distorted as in the case of mechanical tilt. Hence in cellular systems electrical tilts are mostly preferred to mechanical tilts.

Some antennas have an adjustment screw at the bottom (Allen). Others have at the side of the antenna a handle with which the electrical downtilt can be adjusted (Kathrein and Allgon). This can be done in steps of 1° in most cases.

What are the side effects of antenna tiliting?

Antenna tilting is not included in the power budget. It will give an extra loss at the edges although gain may increase closer to the BTS

Isolated islands can be reduced (coverage holes), but they can also be introduced.

Why is it necessary to have antenna isolation?

Isolation between transmitter and receiver antennas is required to avoid receiver desensitization. This is a reduction in the receiver sensitivity.

Receiver desensitization is caused by:

• Receiver in-band noise caused by the co-site transmitter (spurious signals)

• Gain reduction of the low-noise amplifier caused by a strong off-channel signal

What are the isolation techniques?

The following table shows Techniques used for isolation:

• Decoupling Adequate antenna spacing

• Filtering Using duplexers, multicouplers and isolators

• Note that the following applies to the isolation methods:

• Antenna spacing can be done horizontally or vertically

• When using filtering strong out-of-band channel noise is attenuated.

What is an Erlang?

Erlang is a dimensionless unit of telephone traffic. One Erlang represents a circuit occupied for one hour.

Consider a speech connection established at 02:00 AM between a GSM subscriber and a BTS. Assuming that the call was maintained for 45 minutes. Determine the amount of traffic in Erlangs transferred over this traffic channel between 02:00 and 03:00, assuming that the BTS handled only that single call. A= 1.(45/60) = 0.75 E

Explain Co-channel interference

Co-channel interference occurs when the interfering channel is on the same frequency channel.

Explain Adjacent channel interference

Adjacent channel interference occurs when the interfering signal is on an adjacent frequency channel (200 kHz up or down).

Explain carrier to interference ratio. What is S/N, or C/I? What is S/N target in your design?

S/N is recognized as Signal to Noise Ratio, or called C/I (Carrier to Interference Ratio). S/N is the ratio of serving sector power to everything else. N is basically the Total Noise + Interference Power.

How much a signal (C) is interfered by an other signal (I) is given by the carrier to interference ratio C/I (dB). The higher the value, the lower the possibility that the signal (C) is influenced by the interferer (I).

Typically a minimum of 9 dB S/N (C/I) fairly combats with co-channel interference in GSM which is considered to be the worst interference inside the allocated spectrum of operation (i.e. in case of internal interference).

What is interference margin?

Besides the C/I values recommended by the ETSI, an extra interference margin should be taken into account for coverage quality.

How can you improve C/I?

The C/I ratio can be increased in a number of different ways:

• Good frequency management chart

• Intelligent frequency assignment

• Selection of a proper channel

• Frequency hopping

• Antenna pattern design

• Tilting of antenna patterns

• Reduction of antenna height

• Power reduction

• Choosing the right cell site.

What are two main problems from collocation?

When considering collocation with another system, a site is defined as an electrical isolation area. Major problems encountered are:

• Interference

• Intermodulation.

What are the reasons of coverage extension?

Some of the reasons for coverage extension:

• Subscribers’ demand to have coverage in a specific area

• Existence of coverage holes

• High losses when the wave penetrates buildings

• High building penetration losses

• Building construction around and within target area.

What are the methods of coverage expansion?

The following methods can be applied to extend overage:

• lower the threshold level of a received signal

• decrease the front-end noise figure F

• engineer the antenna patterns and select proper BTS locations

• increase transmitted power (doubling the power gives a gain of +3 dB)

• increase BTS antenna height (doubling the height may give +6dB gain)

• select proper BTS site locations.

Equipment The following equipment can be installed to extend coverage

• a masthead amplifier

• a diversity receiver

• enhancers or micro/pico cells (to enlarge coverage or to fill in holes)

• a high gain or a directional antenna at BTS.

Explain Coverage Probability The term coverage refers to an area having sufficiently strong signal level on both the uplink and downlink that a user can originate a call with acceptable voice quality in both directions. Coverage requirements are usually stated as: • The need for in building, in car, or outdoor coverage. • The probability of coverage at the cell edge and over the cell area.

Cell area coverage: Refers to the percentage of average useable area by the cell site. This perspective would provide an estimate of successful origination of a call, if the user were to attempt calls at a random positions within the cellboundary.

Cell edge criteria: Gives the threshold of acceptable performance. This perspective will define the cell edge as the distance wherea serving cell site can no longer provide a minimum service reliability. As a result, the cell edge will establish an area within which most of the points should have a probability of coverage greater than threshold.

We use the coverage probability as an important system parameter in network coverage design. In order to give a good reception quality CP has to be 90-99%