Cellular Concept- Part 01 Udhay Prakash Jntuh

7/30/2019 Cellular Concept- Part 01 Udhay Prakash Jntuh

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Cellular Concept-System Design FundamentalsPart 01

Udhay Prakash

Lecturer, ECE Dept., [email protected]


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Concepts under consideration Introduction to cellular systems

Frequency Reuse Scheme

Channel Assignment Strategies

Handoff Strategies and associated practicalconsiderations

Interference and system capacity

Channel planning Trunking and grade of service

Techniques to improve coverage and capacity

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Wireless networks Vs. fixed networksHigher loss-rates due to interference

emissions of, e.g., engines, lightning

Restrictive regulations of frequencies frequencies have to be coordinated, useful frequencies are almost all occupied

Low transmission rates local some Mbit/s, regional currently, e.g., 9.6kbit/s with GSMHigher delays, higher jitter

connection setup time with GSM in the second range, several hundred milliseconds for otherwireless systems

Lower security, simpler active attacking radio interface accessible for everyone, base station can be simulated, thus attracting calls

from mobile phones

Always shared medium secure access mechanisms important


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Wireless & Mobility

Wireless: Freedom of movement

No loss of connectivity

Increase in productivity

Mobility:

user mobility: users communicate (wireless) anytime, anywhere, with anyone

device portability: devices can be connected anytime, anywhere to the network.

Wireless vs. Mobile Examples stationary computer

notebook in a hotel room with Ethernet cable wireless LANs in historic buildings Personal Digital Assistant (PDA)

The demand for mobile communication creates the need for integration of wirelessnetworks into existing fixed networks. Eg: Mobile IP extension of IP, internetworking of GSM and ISDN


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Effects of device portabilityPower consumption

limited computing power, low quality displays, small disks due to limited batterycapacity

CPU: power consumption

Loss of data

higher probability, has to be included in advance into the design (e.g., defects,theft)

Limited user interfaces compromise between size of fingers and portability

integration of character/voice recognition, abstract symbols

Limited memory limited value of mass memories with moving parts

flash-memory or ? as alternative


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Wireless systems: overview of the

developmentcellular phones satellites

wireless LANcordless

phones

1992:GSM

1994:

DCS 1800

2001:IMT-2000

1987:

CT1+

1982:Inmarsat-A

1992:Inmarsat-B

Inmarsat-M

1998:

Iridium

1989:

CT 2

1991:

DECT 199x:proprietary

1997:

IEEE 802.11

1999:

802.11b, Bluetooth

1988:

Inmarsat-C

analogue

digital

1991:

D-AMPS

1991:

CDMA

1981:

NMT 450

1986:

NMT 900

1980:

CT0

1984:

CT1

1983:

AMPS

1993:

PDC

4G fourth generation: when and how?

2000:

GPRS2000:

IEEE 802.11a

200?:

Fourth Generation

(Internet based)


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Areas of researchWireless Communication

transmission quality (bandwidth, error rate, delay) modulation, coding, interference media access, regulations ...

Mobility location dependent services location transparency quality of service support (delay, jitter, security) ...

Portability power consumption limited computing power, sizes of display, ... usability ...


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Wireless Communication Evolution Demonstration of long distance wireless communication using

electromagnetic waves by Marconi, started an era of wireless

communications. Invention ofsolid state devices (vacuum tubes, transistors,..) led to

the manufacturing of compact handheld devices suitable for

mobility.

Thereby, the era ofmobile communication started.



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Wireless Communication EvolutionMilestone chronicle

First version of a mobile radio telephone being used in 1924. Source: www.bell-labs.com/technology/wireless/earlyservice.html

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Evolution of Mobile radio system Guglielmo Marconi invented wireless telegraph in 1896.

Communication by encoding alphanumeric characters inanalog signal.

Sent telegraphic signals across the Atlantic Ocean.

Motorola, in association with Bell Systems, operated thefirst commercial mobile telephone service MobileTelephone System (MTS) in 1946.

Analog frequency modulation High power BS tower to cover 50 miles radius

Inefficient (12kHz spectrum for a voice connection)



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Evolution of Mobile radio system Improved mobile telephone system (IMTS) developed in

1960. Full duplex services and direct-dialling

23 FM channels with BW reduced to 25-30KHz

Cellular Concept Exploits the attenuation of radio signal with distance to

achieve frequency reuse.

Originally proposed by D.H. Ring in 1947.

Bell Labs began work on cellular telephone system in the late1960s.



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Evolution of Mobile radio system First cellular system:

1979, Nippon Telephone and Telegraph Company (NTT) in Japan 600 FM duplex channels, 25 KHz channel bandwidth, 800 MHz

frequency range

In Europe: 1981, The Normadic Telephone system (NMT 450) 200 FM duplex channels, 25 KHz channel bandwidth, 450 MHz

frequency range

In USA:

1983, The Advanced Mobile Phone System (AMPS) 666 FM duplex channels, 30 KHz channel bandwidth, 800 MHz

frequency range



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Limitations ofearly mobile radio systems

Uses a single, high powered transmitter with an antennamounted on a tall tower, to achieve a large coveragearea.

Inefficient spectrum utilization. Very few calls blankets the complete bandwidth

available for a city.



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Cellular concept Replacing a single, high power transmitter (large cell) with

many low power transmitters (small cells).

Implements space division multiplex: base station covers a

certain transmission area (cell)

Mobile stations communicate only via the base station

Cell sizes from some 100 m in cities to, e.g., 35 km on thecountry side (GSM) - even less for higher frequencies.



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Cellular Structure Pros:

higher capacity, higher number of users

less transmission power needed

more robust, decentralized

base station deals with interference, transmission area etc.locally

Cons: fixed network needed for the base stations handover (changing from one cell to another) necessary

interference with other cells



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Cellular system Two types: circuit switched & Packet switched

Circuit switched systems- each traffic channel is dedicated to a user until its cell is

terminated. There are two variants of circuit switched systems

Analog Systems

Digital Systems

Packet-switched cellular systems.



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Circuit switched cellular systemAnalog System Digital System



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Packet Switched Cellular System

Node B: The name for base station in GSM.

RNC (Radio Network Controller) analogous to BSC. SGSN (Service GPRS Support Node) analogous to MSC/VLR.

GGSN (Gateway GPRS Support Node): Point of interface with external packet data

networks.



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Cellular Spectrum*A band

10 MHz

333 channels30khz

B band

10 MHz

333 channels30khz

825 835 845

870 880 890

824 846.5 849

869

A band

10 MHz

333 channels30khz

B band

10 MHz

333 channels30khz

Phone Transmit

Base Transmit

891.5 894

Aband

Aband

Aband

Aband

Bband

Bband

1 MHz33 chan

1.5 MHz50 chan

2.5 MHz83 chan

1 MHz

33 chan

1.5 MHz

50 chan

2.5 MHz

83 chan

20 MHz Guard


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Cellular System Overview


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Cellular Systems Terms Base Station (BS) includes an antenna, a controller, and

a number of receivers

Mobile telecommunications switching office (MTSO)

connects calls between mobile units Two types of channels available between mobile unit and

BS

Control channels used to exchange information having to dowith setting up and maintaining calls.

Traffic channels carry voice or data connection between users.


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Frequency Reuse It is a frequencyplanning strategy.

Base stations in adjacent cells are assigned channel groups which

contain completely different channels than neighbouring cells. Cell shape for theoretical planning: Square, equilateral triangle,

hexagon.

Among them, hexagon is more suitable, due to its maximal radial

coverage to its edges.



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Cell structure

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Frequency Reuse

Frequency reuse only with a certain distance between the base stations. Cluster No. of Cells sharing the complete assigned bandwidth, using

frequency reuse concept.

More users can be served

Drawback:

Generation of interference

Reduction of users which can be served



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Frequency Reuse Typical Cluster sizes are 4, 7 and 12; with 7 cell reuse

scheme being more popular.

Based on location of base station (BS), cell are

classified as center-excited cells with mainlyomni-directional

antennas, or set ofdirectional antennas, for BS.

corner-excited cells with directional antennas for BS.

f1f2

f3

f2 f1

f1

f2

f3f2

f3f1

f2f1f3f3

f3f3

f3

3 cell cluster

f5f3

f4f1

f3 f2

f6

f7

f2

f4f5

f1f3f5f6

f7f2

f2

7 cell cluster



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Frequency Reuse While tessellating the cells in hexagon geometry, in

frequency planning, the cluster size (N) must satisfythe condition: N=i2+ij+j2.

where i,j are any non-negative integers. To find nearest co-channel neighbours of a

particular cell:1. move i cells along any chain of hexagons, and then

2. turn 60 degrees counter-clockwise and move j cells.



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Frequency Reuse This figure illustrates the method of locating co-

channel cells, in a cellular system with cluster size,N=19, and for i=3 & j=2.



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Channel Assignment Strategies

For efficient utilization of bandwidth. Frequency reuse scheme with increasing capacity and minimizing

interference. Fixed channel assignment:

certain frequencies are assigned to a certain cell problem: different traffic load in different cells. To avoid blocking during overload conditions, follows channel borrowing

strategy.

Dynamic channel assignment: base station chooses frequencies depending on the frequencies already used

in neighbor cells

more capacity in cells with more traffic assignment can also be based on interference measurements. But, requires real-time data on channel occupancy, traffic distribution, and

radiosignal strength indications(RSSI) of all channels on a continuous basis.



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Handoff Also called handover (in Europe).

Handoff - When a mobile moves into a different cell while a

conversation is in progress, the MSC automatically transfers the call

to a new channel belonging to the new base station.

Before the advent of handoff concept, when a mobile user moves

into a different cell, then the mobile shouldgetregistered manually

in the new cell.

Handoff is automatic process, without getting noticed by the user.



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When to handoff? Minimum Usage Signal should be

normally taken as between90 dBm and100 dBm.

Margin, = Pr handoff Pr minimum_usable,cannot be too large or too small.

If is too small insufficient time tocomplete a handoff.

If is too large Unnecessary handoffsburden the MSC.



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Handoff Performance Metrics Cell blocking probability probability of a new call being blocked

Call dropping probability probability that a call is terminated due

to a handoff.

Call completion probability probability that an admitted call is

not dropped before it terminates.

Probability of unsuccessful handoff probability that a handoff is

executed while the reception conditions are inadequate.


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Handoff Performance Metrics Handoff blocking probability probability that a handoff cannot be

successfully completed.

Handoff probability probability that a handoff occurs before call

termination.

Rate of handoffnumber of handoffs per unit time.

Interruption duration duration of time during a handoff in which a

mobile is not connected to either base station.

Handoff delay distance the mobile moves from the point at which the

handoff should occur to the point at which it does occur.


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Handoff Strategies 1G analog cellular systems

signal strength measurements are made by BSs & supervised by MSC.

Each BS constantly monitors signal strengths of all of its reverse voice

channels to determine relative location of each mobile user w.r.t BS.

Locator receiver ( a spare receiver) in each BS, is used to scan and determine

signal strengths of mobile users which are in neighbouring cells.

This receiver is controlled by the MSC and is used to monitor the signal strength

of users in neighbouring cells which appear to be in need of handoff and reports

all RSSI values to the MSC.



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Handoff Strategies

2G cellular systems Mobile Assisted Handoff (MAHO) takes place.

every MS measures the received power from surrounding BSs and

continually reports the results of these measurements to the serving BS.

Handoff is initiated when the power received from adjacent BS is

greater than that of serving BS.

Mainly suited for microcell environment, where handoffs are initiated

more frequently.



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Handoff Strategies-to Determine Instant of Handoff Relative signal strength

Relative signal strength with threshold

Relative signal strength with hysteresis

Relative signal strength with hysteresis and threshold

Prediction techniques


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FOUR Types of Handoff


MSC MSC

BSC BSCBSC

BTS BTS BTSBTS

MS MS MS MS

1

2 3 4


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Prioritizing Handoffs Strategies

Schemes to prioritize handoff requests over call initiation requestswhen allocating voice channels.

1) Guard Channel Concept-

a fractionof the totalavailable channels in a cell is reserved

exclusively for handoff requests. Reduced carried traffic

Efficient spectrum utilization, under dynamic spectrumassignment strategy.

2) Queuing of Handoffs- To reduce the probability of blocking.

Handoff delay time and sizeof the queue are determined from the trafficpattern of the particular service area.



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Practical Handoff ConsiderationsDesigning handoff strategy ofhigh speed vehicular users and thepedestrian users!

Ability to obtain new cell sites, particularly in urban areas, due tovarious geo-techno-political aspects!

Solution::: Umbrella Cell Approach.



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Practical Handoff ConsiderationsCell dragging results when the user has travelled wellbeyond the designed Cell range, and still possessesreceived signal at BS above handoff threshold.

Thus, handoff is not made.

This creates a potential interference and trafficmanagement problem

Particularly occurs in urban areas, where line of sight

(LoS) path exists between MS & BS. Solution: handoff thresholds and radio coverage

parameters must be adjusted carefully.



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Practical Handoff Considerations

1G analog cellular systems consumes 10s to make a handoff. So,should be in the order of 6 to 12dB.

Digital cellular systems takes 1-2s to make a handoff. So, should be in the order of 0 to 6dB.

Newer cellular systems are possessing the ability to make handoffdecisions based on a wide range of metrics other than signalstrength.

Handoff Hard handoff(eg: GSM)

Soft handoff (eg: CDMA)-Due to cluster size, N=1

soft handoff-The ability to select between the instantaneousreceived signals from a variety of BSs.



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Power Control Design issues making it desirable to include dynamic power

control in a cellular system

Received power must be sufficiently above the background noise for

effective communication

Desirable to minimize power in the transmitted signal from the mobile

Reduce co-channel interference, alleviate health concerns, save battery

power

In SS systems using CDMA, its desirable to equalize the received power

level from all mobile units at the BS


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Types of Power Control Open-loop power control

Depends solely on mobile unit.

No feedback from BS.

Not as accurate as closed-loop, but can react quicker to fluctuations in signal

strength.

Closed-loop power control

Adjusts signal strength in reverse channel based on metric of performance. BS makes power adjustment decision and communicates to mobile on control

channel.


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Gain, Loss & Decibels Ratio between two signal power levels is often

measured in decibels (dB):



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mW and dBm

Decibels refers to relative change in magnitude, not absolute values. So, we define the dBW (decibel-Watt) as a reference.

1 Watt of transmission power= 0dBw.

Example: Radio Mirchi 98.3 FM in hyderabad transmits at 100,000 Watts.

Power in dBW=10*log (100,000W/1W)=10*5=50 dBW.

For wireless networks, the dBm (decibel-milliwatt) is more useful.

1mW transmission power==0dBm

10 mW==10 dBm

0.1 mW==-10dBm

802.11b networks have a maximum transmit power of 100 mW== 20 dBm



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Decibel calculations



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References Theodore S. Rappaport, Wirelesss Communications-

Principles and Practice, 2e, PHI, 2002.

William C.Y. Lee, Wireless & Cellular

Telecommunications, 3e, ISBN:0-07-143686-3, TataMcGraw Hills.

T.K. Sarkar, R.J. Mailloux, A.A. Oliner, M. Salazar-palma and D.L. Sengupta, History of wireless, John

Wiley & Sons, Inc.., 2006.

Cellular Concept- Part 01 Udhay Prakash Jntuh

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