12 Chapter 2 2.0 Training Experiences 2.1 Radio Access Network Having considered the radio network communication, to enhance the performance and efficiency of the mobile Environment, it has been evolving into 4 generations and furthers more. The evolution can be named as follow 1G- Analog 2G- GSM (Global System for Mobile Communication) 2.5G- GPRS (General Packet Radio Access) 2.75- EDGE (Enhanced Data rates for GSM Evolution) 3G- UMTS (Universal Mobile Telecommunication System) 3.5G-HSDPA (High-Speed Downlink Packet Access) 3.75G- HSPA+ (High Speed Packet Access) 4G- LTE (Long Term Evolution) 1G -Analog 2G GSM 3G 4G WCDMA OFDMA TDMA/FDMA 2.5 GPRS 2.75 EDGE 3.5 HSDPA 3.75 HSPA Figure 2.1: Radio network
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
12
Chapter 2
2.0 Training Experiences
2.1 Radio Access Network
Having considered the radio network communication, to enhance the performance and
efficiency of the mobile
Environment, it has been evolving into 4 generations and furthers more. The evolution can be
named as follow
1G- Analog
2G- GSM (Global System for Mobile Communication)
2.5G- GPRS (General Packet Radio Access)
2.75- EDGE (Enhanced Data rates for GSM Evolution)
3G- UMTS (Universal Mobile Telecommunication System)
3.5G-HSDPA (High-Speed Downlink Packet Access)
3.75G- HSPA+ (High Speed Packet Access)
4G- LTE (Long Term Evolution)
1G -Analog
2G GSM 3G 4G
WCDMA OFDMA
TDMA/FDMA
2.5 GPRS 2.75 EDGE 3.5 HSDPA 3.75 HSPA
Figure 2.1: Radio network
13
2.1.1. What Is Radio Access Network (RAN)
Keep an eye on to the mobile telecommunication system; Radio Access Network (RAN) is
key sector of the system, which implements radio access technology as the underlying
physical connection method for a radio based communication network. Conceptually, RAN
resides between mobile phone and connection with its core.
Types of Radio Access Network (RAN)
GSM Radio Access Network
EDGE packet service with GSM Radio Access Network
UMTS GSM Radio Access Network
LTE high speed and low latency Radio Access Network
GSM Radio Access Network
Having considered the GSM architecture, the circled area in the figure called as GRAN
which links the mobile station with Network Switching Subsystem (NSS) or core and MSC.
GRAN comprises with Mobile Station (MS), BTS and BSC.
Base Station Subsystem Network Switching Subsystem
Figure 2.2 :2G Architecture
14
Mobile Station (MS)
Having considered the GSM network the initial point, mobile station (MS), which grants the
access for the mobile subscriber to connect GSM network through air interface, comprises
with Subscriber Identity Module (SIM). Being attached the International Mobile station
Equipment Identity (IMEI), Mobile Termination conducts several operations such as speech
encoding and decoding, radio transmission and handover, Error detection and correction,
signaling and access to the SIM.
Base Transceiver Station (BTS)
To communicate between Mobile Station and operator network, it composes several physical
components such as transceivers (TRX) which control transmission and reception of signals
to or from higher network singletons such as Base Station Controller (BSC), power amplifiers
which use to amplify signals through antennas, duplexers which use to separate transmitting
and receiving signals, combiners which use to combine several TRXs to send out from a
single antenna to reduce number of antennas, alarm extension system which uses to gather
status of units in the BTS and extends them to operation and maintenance (O&M) monitoring
stations, and antenna which is an external part of the BTS.
Dialog has been using Huawei and Ericsson BTSs.
Base Station Controller (BSC)
Having the ability to control several BTSs, BSC communicates with the higher level, MSC,
to keep up the information overflow which impacts the MSC efficiency. As key functions
BSC controls and manages the timeslot allocation implemented by the BTSs, handles inter
cell handovers within BSS area, perform basic switching functions, handles paging data, and
identifies the mobile stations within the cell and informs MSC/VLR.
Mobile Switching Center (MSC)
Having known as the centerpiece of Network Switching Subsystem (NSS), MSC acts as the
interface between radio part and fixed part of the GSM network. The difference between
fixed line switching center and MSC can be distinguished as the ability of handling mobility
of subscriber by storing and updating the location of mobile station in Visitor Location
Registry (VLR) and Home Location Registry (HLR). Being controlled several Base Station
Subsystems (BSS), MSC associates with switching functions such as routing SMS and voice
15
calls, call setup and release. The A-interface is known as the interface between BSC and
MSC which is connected using fiber cables
Home Location Registry (HLR)
The database which stores permanent data about subscribers such as service profile, location
information, and activity status, it stores the information about the subscription whenever
customer buys a subscription in the form of SIM.
Visitor Location Registry (VLR)
Having integrated with MSC, VLR keeps information relating to the subscribers with in the
particular MSC and associates with few location areas. When mobile station registers in a
new location area, the subscriber‟s details are copied from HLR into the VLR for that area.
If the subscriber subsequently moves into a location area controlled by a different VLR, the
details are deleted from the old VLR and added to the new VLR. When a call setup is needed,
without accessing HLR, by using the local copy of subscriber details the efficiency is
acquired.
Authentication Centre (AUC)
The security related information such as copy of the secret key stored in each subscriber‟s
SIM card which uses for authentication and encryption of the radio channel are kept in the
database of AUC.
Core
The data sent from BSC is transmitted to core while voice is sent to MSC. Core has two main
parts GGSN (Gateway GPRS Support Node) where Control Plane (CP) data is sent and
SGSN (Serving GPRS support node) where User Plane (UP) data is sent. The transmission is
done through Gb interface.
EDGE with GSM Radio Access Network
Though GERAN is essentially the same as GRAN, it specifies the inclusion of EDGE packet
radio services.
16
UMTS GSM Radio Access Network
Having regarded the 3G architecture, the circled area in the figure is known as the UTRAN
which comprises User Equipment (UE), Node B and Radio Network Controller (RNC). It
differs from the 2G network slightly such as the BTS is named as Node B, BSC is named as
RNC, Abis and interfaces are named as IUB and IU-CS interfaces respectively.
RNC
The Radio Network Controller (or RNC) is a governing element in the UMTS radio access
network (UTRAN) and is responsible for controlling the Node Bs that are connected to it.
The RNC carries out radio resource management, some of the mobility
management functions and is the point where encryption is done before user data is sent to
and from the mobile. The RNC connects to the Circuit Switched Core Network through
Media Gateway (MGW) and to the SGSN (Serving GPRS Support Node) in the Packet
Switched Core Network.
SGSN
The Serving GPRS Support Node (SGSN) is a main component of the GPRS network,
which handles all packet switched data within the network, e.g. the mobility management
and authentication of the users.
Figure 2.3 : 3G Architecture
17
MGW
A media gateway is a translation device or service that converts media streams between
disparate telecommunications technologies (2G,3G)
LTE Radio Access Network
Functions of eNodeB
Cell control and MME pool support
Mobility control
Control and User Plane security
Shared Channel handling
Segmentation/Concatenation Measurements and reporting
Figure 2.4 : 4G Architecture
18
Functions of MME (Mobility Management Entity) The MME is in charge of all the Control plane functions related to subscriber and session management.
Idle mode UE (User Equipment) tracking Process
Paging Process.
Bearer activation/deactivation process
Core Network (CN) node relocation at time of intra-LTE handover
Security key management
SGW
Serving Gateway is handling the user data transmission between eNodeB and PGW. It is
responsible to initiate the connection with PGW during the initial UE attachment process.
Core network structure
Figure 2.5: Core network structure
19
2.1.2 Frequency Allocation for GSM
GSM 900
In order to the frequency allocation provided by TRCSL for GSM 900 communication 880 –
960 MHz frequency range, is granted for all operators in Sri Lanka. With 35 MHz guard
band, the total bandwidth is bisected to 2 parts for uplink and downlink. Dialog has
purchased two 7.5 MHz bands separately for uplink and downlink communication.
Figure 2.6: Frequency allocations for GSM 900
GSM 1800
In order to the frequency allocation provided by TRCSL for GSM 1800, which is known as
Digital Cellular System (DCS) communication 1710 – 1880 MHz frequency range, is granted
for all operators in Sri Lanka. With 20 MHz guard band, the total bandwidth is bisected to 2
parts for uplink and downlink. Dialog has purchased two 15 MHz bands separately for uplink
and downlink communication
As DCS operates in a higher frequency than GSM 900, the path loss is higher in DCS to lead
its smaller coverage area in comparison to GSM 900. Since the one sector comprises with
both DCS & GSM 900, GSM 900 signals travel longer than DCS. Therefore from the
network, it gives priority to select DCS when both DCS & GSM 900 receive to mobile
station, increasing the capacity in GSM for mobile stations which are out of DCS coverage
area.
Figure 2.7: Frequency allocations for GSM 1800
20
Frequency hopping
According to interferences explained above we want use huge frequency channels to
overcome that problem. But according to limited radio frequency band we can‟t use that
much of channels. So frequency hopping is use to save limited available frequency band. In
that case,
The BCCH channel frequency is fixed.
Change other TRXs frequencies randomly. To do that they allocate two parameters
to each cell called HSN (Hopping Sequence Number) & MAIO (Mobile Allocation
Index Offset). If that two parameters are allocate correctly there is no available in
adjacent or Co channels between neighbor cells.
2.2 Introduction to BTS Operation Division
This division is responsible for maintenance of Base Transceiver Stations (BTS) all
around the country. For the purpose of accuracy and easiness, the country is divided
into regions and BTS maintenance and operations in those regions are conducting
separately in those regions. Alarms and faults of BTSs are informed to relevant
regions by NOC and then the team working in that region takes the necessary actions
to fix those faults and maintain.
During my training I had chances to visit sites for microwave link installments,
VLAN ID configurations, PAT (Preliminary Acceptance Test), Fault corrections, etc.
It is really interesting to see an installation of a microwave link. I also got the
opportunity at several times to climb up to the towers when I training period.
The main objective of this division is to implementation and maintains these network
paths properly. Failure of a main link may cause a number of sites to be down.
Therefore the fault handling and maintenance should be done in an effective manner
in Dialog Transmission division.
Main Functions of a BTS Operation Division
Maintenances – maintaining equipments in BTS cabinets, Antennas and Shelters
Connecting Generators – generators should be connected to important sites
(hubs, mux) which are without in mount generators
and stand by batteries when power failures happened.
21
Connecting Stand by Batteries – connect batteries to sites.
Site Commissions – commissioning newly implemented sites and pass or reject
the PAT
Troubleshooting – the main function, replacing faulty equipments and
troubleshooting the faults when occurred.
2.2.1 GSM Antennas (Sector Antennas)
Figure 2.8 :Basic Structure of BTS site
Figure 2.9: Sector Antennas
22
A sector antenna is a type of directional microwave antenna with a sector-
shaped radiation pattern. There are 3 types of antennas.
Single Band. (Only one band of 900MHz or 1800MHz)
Dual Band (Both 900MHz and 1800MHz bands)
Tri Band. (For 900MHz, 1800MHz and 2100MHz )
Single Band
Having been compatible with one frequency range, to transmit
2G, DCS and 3G signals we need to have separate antennas.
These antennas only have two RF cable ports in bottom side.
Dual Band
Since the dual band antennas are compatible with two
frequency ranges, instead of two single band antennas we can
use only one dual band antenna. These antennas have four RF
cable ports in bottom side for two frequency ranges which are
clearly indicated by using different colors to stop
misunderstanding.
Tri Band
Though tri band antennas are compatible with two frequency
ranges, in contrast to dual band antenna this type has four
ports, both DCS band and 3G band together with GSM band.
Due to that, one Tri band antenna can be used instead of three
single band antennas. These antennas have six RF cable ports
in bottom side.
Figure 2.10: Single Band antennas
Figure 2.11: Dual Band antennas
Figure 2.12: Tri band Antennas
23
These antennas are used for transmit the 3 bands used in mobile communication- 900MHz,
1800MHz and 2100MHz frequencies. 900MHz is for long distance 2G coverage, 1800MHz
is for short distance 2G coverage and 2100MHz is for 3G coverage.
The angle of the antenna can be changed in two different methods.
Mechanical Tilt- In Mechanically changeable tilt, we can change angle with loosen or
tighten at the top of antenna. It has 18 optional angles.
Electrical Tilt- In electrically changeable tilt, antenna transmits waves in relevant
Angle itself. It has 10 optional angles.
It is required to keep antennas in proper angles and directions to reach more efficiency
by direct them towards populated areas, main roads etc.
2.2.2 Micro wave link
Microwave links is establish by placing radio antennas directed to each other called “Line of
Sight”. The antennas transmit in microwave range and separate bands are used to uplink
and downlink. They are Microwave Antenna & ODU (Out Door Unit) and IDU (Indoor
Door Unit) and Feeder Cable (Coaxial Cable)
Figure 2.13: Transmission Equipment‟s
24
2.2.3 Outdoor Unit (ODU)
ODU is the intermediate unit between IF stage and RF stage. ODU receives IF stage
signals from the IDU. IF signals are up converted and feed in to the antenna through a
wave guide. Microwave frequencies are sub banded and spectrum is distributed
among operators. Up link and downlink of the link is maintained by two frequency
bands. The radio units are categorized as upper band unit and lower band unit in the
sub band. There may be placed more than one radio link in a tower. So it is a
convention that using either upper band or lower band radio unit in the same site
ODU used in dialog
Huawei
Ericsson
ZTE
2.2.4 Indoor Unit (IDU)
The IDU contains much of the intelligence of the system. Main functions of the IDU
include Providing the Data interface, Error correction, Modulation and Demodulation,
Alarm status monitoring and Site-to-site communications. An IDU located in an
equipment shelter to interface with the operator interface and is connected to a close
coupled ODU Antenna assembly on the tower by a single coaxial cable.
Figure 2.14: Huawei ODU
Figure 2.15: Huawei IDU
25
Process of installing IDU
First power was switched off from the breakers
Then IDU power cable was connected to the breakers
Next IDU was installed in proper place
Installing the protection grounding cable
Other end of power cable was connected the IDU and then switched on the IDU
Finally configured the IDU
2.2.5Antenna Alignment
The purpose of the alignment of antennas is to ensure that maximum signal strength is
present at both ends of the link. This is achieved by aiming the signal from each antenna
directly at the center of the opposite antenna. Electromagnetic signals consist of lobes as
Shown below. During my training period I participated and have experience of antenna
Alignment project in Hambanthota.
.
During antenna alignment, changed the direction vertically or horizontally meanwhile, used a
multimeter to test the RSS at the receiving end. Then could see the voltage wave was
displayed in multimeter. The peak point of the voltage wave indicates the main lobe position
in the vertical or horizontal direction.
Figure 2.16: Lobes of the microwave signal
26
Alignment Steps
First we have to remove the ODU protective cap of the RSSI (Received Signal
Strength Indication) interface.
Then the voltmeter should be connected to the BNC connector of the RSSI interface.
In the next step the antenna mount (horizontal angle) should be loosen.
Then the antenna is adjusted to the azimuth position for maximum BNC voltage.
To ensure that the voltage observed is the maximum, we have to pan the antenna
Finally. If there are no other values greater than the voltage observed the antenna is
not mistakenly aligned to a side-lobe and hence aligned in the correct manner.
Figure 2.18: Microwave Antenna Alignment
Figure 2.17 :Antenna alignment using voltmeter
27
2.2.6 Operation and Maintenance Software
Transmission unit equipment‟s in all brands come with compatible software for operation and
maintenance. Dialog use this software .This software provides an interface for users to
configure units. One such software is „iManager U2000 Web LCT‟ from Huawei for their
IDU RTN family. This software is installed to the laptop PC which is used to configure the
IDU. This is also used for fault handling. If there is a fault in the system IDU shows an alarm.
When we logged on to the iManager it shows the faulty cards highlighted. Then the type of
the fault can also be found. The most important advantage of software like this is the
graphical user interface. All we have to do is repair or replace the faulty card.
2.2.7 Base Transceiver Station (BTS)
BTS stands for Base Transceiver Station. It is the most important node in the access network.
It directly access to the mobile station and control the radio interface to the MS. Base
Transceiver Station (BTS). It contains components such as antennas, transceivers, and other
components that necessary to serve to the MS s.
A BTS is usually placed in the center of a cell. Its transmitting power defines according to the
size of a cell. Each BTS has in between 1 to 16 transceivers, depending on the density of
users in the cell. Encoding, encrypting, multiplexing, modulating, and feeding the RF signals
to the antenna are done by BTS
Figure 2.19: out door bts cabinet
28
Currently Dialog has BTS cabinets from 2 vendors.
Ericsson
Huawei
ZTE
Functions of a BTS
Provide communication between user and network.
Broadcast/receive to/from MS either signaling and traffic signals.
Modulate/demodulate signals to be broadcasted through air interfaces.
Multiplex the information to be transmitted over each carrier.
Measure the quality of the signaling and traffic signals in downlink and uplink
channels.
2.2.8 VSWR measurements using Site Master
During the training period I had a chance to learn how to measure the VSWR (voltage
standing wave ratio) with a Site Master. It was used to identify the reason for a VSWR alarm
of a Huawei BTS at Negombo-pitipana site.
.
Steps for SWR Test,
Calibrate the Site Master.
Disconnect the jumper cable from the CDU.
Connect it to the Refl Test port of the Site Master.
Set the scale by using AMPLITUDE key.
Observe the trace in the frequency range according to the correct band.
Site Master
CDU jumper Feeder
Antenna Jumper
Figure 2.20 Passive antenna system connected for VSWR Test
29
If the trace does not go over the value 1.30 for any frequency in the range, it is considered as
a normal VSWR level. This test should be done for two jumper cables separately
2.2.9 Distance to Fault (DTF) Test
The purpose of this test is to verify that there are no bad connections or other faults (for
example sharp bends) in the antenna feeder system. Following are the steps to test the DTF.
Connect the jumper cable to the Site Master.
Set the Mode into DTF – SWR.
Set the distance range according to the total length of the feeder system.
Set the scale with the maximum value 1.3.
Wait while the Site Master is calculating.
Observe the waveform and check that no reflections are above 1.05 SWR.
By this test we can identify the location of the fault. Then the fault should be identified and
repaired. However at the end of all, the DTF test should not over take the value 1.03. If there
is any fault in the system it can be easily identified because the Site Master gives the distance
to fault.
Figure 2.21 DTF Test result of a faulty system.
30
2.2.10 Site visits
Mostly I involved that kind of jobs in my training period of dialog .Site visit means visiting to
of the network. ,Nodes (BTS ,Node B) for different reasons .That kind of some reasons are