6-weeks industrial training report ON

GSM Basics &Switching Operations

IN Partial fulfillment forThe award of degree in

B.Tech (ECE)

Submitted byName:- BasitAli

Univ.rollno:-1151506

Submitted to:-Mr. Sanjeev Chopra

ECE- DEPARTMENT

RIMT-MAHARAJA AGGRASEN ENGINEERING COLLEGE

MANDI GOBINDGARH (PUNJAB)

ACKNOWLEDGEMENT

The successful completion of any task would be incomplete without accomplishing the people who

made it all possible and whose constant guidance and encouragement secured us the success.”

This seems to be a fitting moment for me to express my heartfelt gratitude towards all those who

helped me tirelessly and patiently in my training work.

To begin with, I express my sincere thanks to Mr. Tauseef Ahmad (manager

planning) for allowing me to avail all the available amenities in the division. He kept faith in me

and made me an active member of my team. I am thankful to him for sharing his vast resource of

knowledge and experience with me.

Last but not the least I would like to express my heartfelt thanks to my teammates,

who with their thought provoking views, veracity and whole hearted co-operation supported me

throughout the duration of the training.

Basit Ali

PREFACE

With the ongoing telecom revolution where innovations are taking place at the blink of

the eye, it is impossible to keep pace with the emerging trends. Excellence is an attitude that

whole of the human race is born with. It is the environment that makes sure that whether the result

of this attitude is visible or otherwise. A well planned, properly executed and evaluated industrial

training helps a lot in inculcating a professional attitude.

During this period, I got the real, firsthand experience for working in the

actual environment. Most of the theoretical knowledge that has been gained during the course of

their studies is put to test here. I had the opportunity to have a real experience on many ventures,

which increased my sphere of knowledge to a great extent.

This report is a brief summarization of the project work and tasks that I have completed till

now. Report includes company profile and the ongoing ventures of the company. The first part of the

report contains the overview of the company. Then is the introduction to my project which gives the

overview of my project.

INDEX

` Topic NamePage No.

1. Industry Profile 6

2.Modular Description 10

3. Detailed Analysis ` 12

Chapter 1-Introduction to GSM 12

Chapter 2-Channels on Air Interface 23

Chapter 3-Cell Planning 32

Chapter 4-Frequency Planning 37

Chapter 5-Radio Parameter Optimization 42

4. Conclusion 50

5. Bibliography and References 51

Industry Profile

Bharti Airtel Limited, commonly known as Airtel, is an Indian multinational telecommunications services

company headquartered in New Delhi, India. It operates in 20 countries across South Asia, Africa, and

the Channel Islands. Airtel has a GSM network in all countries in which it operates,

providing 2G, 3G and 4G services depending upon the country of operation. Airtel is the world's third

largest mobile telecommunications company by subscribers, with over 275 million subscribers across 20

countries as of July 2013. It is the largest cellular service provider in India, with 192.22 million subscribers

as of August 2013.] Airtel is the third largest in-country mobile operator by subscriber base, behind China

Mobile and China Unicom.

Airtel is the largest provider of mobile telephony and second largest provider of fixed telephony in India,

and is also a provider of broadband andsubscription television services. It offers its telecom services under

the "airtel" brand, and is headed by Sunil Bharti Mittal. Bharti Airtel is the first Indian telecom service

provider to achieve Cisco Gold Certification. It also acts as a carrier for national and international long

distance communication services. The company has a submarine cable landing station at Chennai, which

connects the submarine cable connecting Chennaiand Singapore.

Airtel is credited with pioneering the business strategy of outsourcing all of its business operations except

marketing, sales and finance and building the 'minutes factory' model of low cost and high volumes. The

strategy has since been copied by several operators.[  Its network—base stations, microwave links, etc.—

is maintained by Ericsson and Nokia Siemens Network whereas IT support is provided by IBM,[11] and

transmission towers are maintained by another company (Bharti Infratel Ltd. in India). Ericsson agreed for

the first time to be paid by the minute for installation and maintenance of their equipment rather than being

paid up front, which allowed Airtel to provide low call rates of  1/minute (US$0.02/minute).[13] During the

last financial year (2009–10), Bharti negotiated for its strategic partner Alcatel-Lucent to manage the

network infrastructure for the tele-media business. On 31 May 2012, Bharti Airtel awarded the three-year

contract to Alcatel-Lucent for setting up an Internet Protocol access network (mobile backhaul) across the

country. This would help consumers access internet at faster speed and high quality internet browsing on

mobile handsets.

 operating in the L band around 1500 MHz which allows the use of electronically steerable antennas mounted atop the aircraft fuselage and encased within a fiberglass, RF-transparent radome that have a low profile compared to systems operating in the Ku  band  or Ka  band  which today still require mechanically steerable antennas with a significantly higher profile. Thus drag and fuel costs are reduced llowing economical operation even on smaller aircraft like business or regional jets. Inmarsat's SwiftBroadband system covers much of the planet except for the polar regions above −82 and below +82 degrees latitude and currently provides symmetric data rates of

up to 432 kbit/s per channel dependent on signal quality and overall load on the satellite's spotbeam serving the corresponding geographical area. Currently the Thales SDU can bond two channels resulting in a maximum bandwidth of 864 kbit/s.

90% of the onboard equipment can be used with any radio link, for example other satellite networks or a direct air-to-ground link. There is also a clear upgrade path from SwiftBroadband to Inmarsat's Global Xpress system, a constellation of three Ka  band  satellites which will come on stream in 2014-2015 and will globally provide downlink bandwidths of up to 50 Mbit/s. OnAir was appointed as distribution partner for Inmarsat's Global Xpress service in November 2011.

Onboard Server

A server installed onboard manages the satellite connection and routes the data traffic while also compressing and decompressing all data transmitted, including GSM phone calls that are recoded using the AMR codec which reduces bandwidth while maintaining a voice quality virtually indistinguishable from the native GSM codec.

Wi-Fi Network

Broadband Internet access (Internet OnAir) is provided by Wi-Fi access points. In order to access OnAir's Wi-Fi Internet service (Internet OnAir) passengers need to bring a Wi-Fi compatible device. Access can usually be purchased on board.

GSM Network

A picocell operating according to the GSM-1800 standard provides a GSM network (Mobile OnAir) enabling voice telephony, SMS and narrowband Internet access (GPRS). The GSM signal is distributed by a leaky line antenna, essentially a coaxial cable with a slotted shielding through which RF signals are radiated. This coaxial cable is installed above the ceiling panels along the whole aircraft cabin and provides a uniform linear coverage of the aircraft cabin at very low radiation power levels. In order to prevent handsets from connecting to terrestrial networks which would lead to high transmission power levels a so-called network control unit (NCU) installed onboard generates a broadband noise floor which is being emitted through the existing leaky line antenna masking reception of terrestrial mobile networks within the aircraft. These measures ensure that handsets can only connect to the onboard GSM network and will then operate with the lowest possible transmission power level (GSM-1800 power control level 15, nominal output power of 0 dBm) which results in significantly lower radiation levels than those experienced on average when using a mobile phone with terrestrial networks on the ground. The GSM network is being realized based on Monaco Telecom's core network. It uses the MCC / MNC tuple 901-15 assigned to SITA, one of OnAir's two owners, in March 2005.

Passenger Equipment Compatibility

OnAir's inflight cellphone service (Mobile OnAir) requires a mobile phone supporting the GSM-1800 standard, also called DCS (Digital Cellular Service), which most modern GSM phones support as well as a SIM card from a network operator having a roaming agreement with Monaco Telcom. So-called quad-band handsets always support GSM-1800 and so are compatible with Mobile OnAir.

IFE Connectivity

The system can also provide IP-based connectivity to existing in-flight entertainment systems which allows news content to be fed in and messaging services as well as Internet access to be offered on in-seat units.

Customers

OnAir's customers which have been publicly announced are:

Airlines:

 Air France - Trial in an A318 - 2007-2008  TAP Portugal - Trial in an A319 - 2008, A330 and A350 (announced at Aircraft

Interiors 2012)  Qantas - A380 (late 2008). In service since March 2012  British Midland Airways - Trial in an A320 - 2009  Ryanair - Boeing 737 - Service started Feb-2009, ended March 2010[4]

 Shenzhen Airlines - Boeing 737 and A320 - Mid-2009  Wataniya Airways - A320 - Service started Jan. 2009  Air Blue - A320 - Mid-2009  Kingfisher Airlines - A330 and A340 - Mid-2009  Royal Jordanian - A320 (2008) and A340 (2009)  Jazeera Airways - A320 - 2009  Oman Air - A330 - 2009 -Oman Air was the first to take OnAir’s integrated GSM and

inflight wifi service  TAM Airlines - A320 (2008) First aircraft launched in October 2010. Expansion to

another 26 aircraft announced in February 2011, all to be operational by the end of 2011.  British Airways - A318 (only from City Airport to JFK) - Mid 2009  Air Asia - A320 - Mid 2009  Hong Kong Airlines - A330 - Service launched in March 2012  Qatar Airways - A320 (End 2009) - Boeing 787 (May 2010)  Saudi Arabian Airlines - A330 - Mid 2010  Emirates (airline) - use OnAir across a fleet of 90 A380 airliners, with roll-out of

service that began in Autumn 2011.[5]

 Egypt Air - A330 (2010)  Libyan Airlines - A320 & A330 (2010)  Afriqiyah Airways - A319, A320 & A330 (2010)  Singapore Airlines - A320 & A330 (2010) - plans to use OnAir across its fleet of

A380, B777, A340-500 – starting first half of 2012. Announced in Oct. 2010  Aeroflot – A320, A330 (2010)  Air New Zealand – A320 (2011)  AZAL Azerbaijan Airlines – Airbus and Boeing (2012)  Etihad Airways – A320, A330-300 (2012)  Thai Airways – A330 and A380 (2012)  All Nippon Airlines – B777 and B767 (2012)  Cebu Pacific Air – A330 and A320 in a second phase(2012)

Cruise ship operators:

 Hapag-Lloyd - MS EUROPA (2010)  Hapag-Lloyd - MS Bremen (2011), MS Hanseatic (2011)

Private jet operators and VIP:

 Comlux the Aviation Group - A320 Prestige (2010) & A319 CJ (2011)

 Dasnair - Falcon 7X (2011)

 Solar Impulse - Solar Impulse HB-SIA (2010)

Modular Description

The major modules covered during six weeks training are mentioned below.

A. Introduction to GSM

GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe technologies for second generation (or "2G") digital cellular networks. GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz bands. Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones to use. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio timeslots (or eight burst periods) are grouped into a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 Kbit/s, and the frame duration is 615 ms. The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

The network is structured into a number of discrete sections:

Mobile Station consisting of the Mobile Unit and the SIM. The Base Station Subsystem (the base stations and their controllers). The Network and Switching Subsystem (the part of the network most similar to a fixed

network). This is sometimes also just called the core network. The Operations support system (OSS) for maintenance of the network.

B. Channels on Air Interface.In GSM, two types of channels are defined namely Physical channels and Logical channels.

Physical channels: A physical channel is defined by its position in the RF frequency-time plane. There are: 124 duplex RF frequency channels for GSM-900

374 duplex RF frequency channels for GSM-1800Each RF frequency channel provides eight time slots.

Logical channels: A logical channel describes the kind of information to be transmitted on the physical channel (time slot).The basic logical channels in GSM are: Traffic channels (TCH)

Control channels (CCH).Call Management: This lesson describes the procedures which are performed during the different phases of a call in a GSM network: Call establishment

Call in active phase Call release

The procedures are explained using call scenarios of the following basic call types: Mobile-to-land call Land-to-mobile call Mobile-to-mobile call

C. Site Planning

To ensure coverage and to avoid interference, every network needs cell planning. The various steps involved in cell planning process are: Initial Cell Planning Nominal Cell Plan Surveys System Design Implementation System Tuning System Growth.

D. Frequency Planning (Hopping)

The Frequency Hopping function permits the dynamic switching of radio links fromone carrier frequency to another. Frequency Hopping changes the frequency usedby a radio link every new TDMA frame in a regular pattern. Frequency Hopping isa GSM feature which can be enabled or disabled on a per cell basis.There are two types of hopping used: Baseband Hopping. Synthesizer Hopping.

E. Radio Parameter Optimization

The number of resources (channels) in a network is limited. So there is always a requirement of functions that are able to cope up with the demand and share these resources dynamically between all demands. These functions are basically performed by the BSC and the MS. The Various functions which provide radio resource management are: Power control. Handover. Discontinuous transmission. Call re-establishment. Frequency hopping.

Detailed Analysis

Chapter 1: Introduction to GSM

Fig 1.1 GSM Reference Model

A Public Land Mobile Network (PLMN), as represented by the GSM reference includes the following system entities:

1.1 Mobile-services Switching Center (MSC)1.2 Home Location Register (HLR)1.3 Visitor Location Register (VLR)1.4 Equipment Identity Register (EIR)1.5 Authentication Center (AUC)1.6 Base Station System (BSS)

a) Base Station Controller (BSC)b) Base Transceiver Station (BTS)

1.7 Mobile Station(MS)a) SIMb) Mobile Equipment

1.8 Operation and Maintenance Center (OMC)

1.1. Mobile Services Switching Center (MSC)

Mobile-services Switching Center (MSC) performs the switching functions for allmobile stations located in the geographic area covered by its assigned BSSs.Functions performed include interfacing with the Public Switched TelephoneNetwork (PSTN) as well as with the other MSCs and other system entities, suchas the HLR, in the PLMN.Functions of the MSC include: Call handling that copes with mobile nature of subscribers (e.g., paging) Management of required logical radio-link channel during calls Management of MSC-BSS signaling protocol Handling location registration and ensuring interworking between Mobile Station and VLR Control of inter-BSS and inter-MSC handovers Acting as a gateway MSC to interrogate the HLR Exchange of signaling information with other system entities Standard functions of a local exchange switch in the fixed network (Example: charging).

Fig 1.1 MSC

1.2. Home Location Register (HLR)

The Home Location Register (HLR) contains the identities of mobile subscribers(Called International Mobile Subscriber Identities or IMSIs), their serviceparameters and their location information.In summary, the HLR contains: Identity of mobile subscriber ISDN directory number of mobile station Subscription information on tele services and bearer services Service restrictions (if any) Supplementary services Location information for call routing.

1.3. Visitor Location Register (VLR)

The Visitor Location Register (VLR) contains the subscriber parameters andlocation information for all mobile subscribers currently located in thegeographical area (i.e., cells) controlled by that VLR.In summary, the VLR contains: Identity of mobile subscriber. Any temporary mobile subscriber identity. ISDN directory number of mobile. A directory number to route calls to a roaming station. Location area where the mobile station is registered. Copy of (part of) the subscriber data from the HLR.

1.4. Equipment Identity Register (EIR)

The Equipment Identity Register (EIR) is accessed during the equipmentvalidation procedure when a mobile station accesses the system. It contains theidentity of mobile station equipment (called International Mobile StationEquipment Identity or IMEI) which may be valid, suspect, or known to befraudulent.This contains: White or Valid list - List of valid MS equipment identities. Grey or Monitored list - List of suspected mobiles under observation. Black or prohibited list - List of mobiles for which service is barred.

1.5. Authentication Center (AUC)

The Authentication Center (AUC): Contains subscriber authentication data called Authentication Keys (Ki). Generates security related parameters needed to authorize service using Ki. Generates unique data pattern called a Cipher Key (Kc) needed for

encrypting user speech and data.

1.6. Base Station Subsystem (BSS)

Characteristics of the Base Station System (BSS) are: The BSS is responsible for communicating with mobile stations in cell areas One BSC controls one or more BTSs and can perform inter-BTS and intra- BTS handovers The BTS serves one or more cells in the cellular network and contains one

or more TRXs (Transceivers or radio units). The TRX serves full duplex communications to the MS. In the GSM network implementation of Lucent Technologies the BSC

includes the TRAU (Transcoder/Rate Adapter Unit). The TRAU adapts thetransmission bit rate of the A-interface (64 Kbit/s) to the Abis-interface (16Kbit/s).

(a) Base Station Controller(BSC)

The BSC provides the control for the BSS.Any operational information required by the BTS will be received via the BSC. Likewise any information required about the BTS (by the OMC for example) will be obtained by the BSC.The BSC incorporates a digital switching matrix, which it uses to connect the radiochannels on the air interface with the terrestrial circuits from the MSC.The BSC switching matrix also allows the BSC to perform “handovers” between radiochannels on BTSs, under its control, without involving the MSC.

Fig 1.2 Base Station Subsystem

(b) Base Transceiver Station (BTS)

The BTS provides the air interface connection with the MS. It also has a limited amount ofcontrol functionality which reduces the amount of traffic passing between the BTS andBSC. Each BTS will support 1 or more cells.

Fig 1.3 Base Transceiver Station

Fig 1.4 Functions of BSC and BTS

1.7. Mobile Station(MS)

The Mobile Station (MS) represents the terminal equipment used by the wirelesssubscriber supported by the GSM Wireless system. The MS consists of twoentities, each with its own identity:(a) Subscriber Identity Module (SIM).(b) Mobile equipment (ME).

(a)SIM: The SIM may be a removable module. A subscriber with an appropriate SIM canaccess the system using various mobile equipments. The equipment identity is not linked to a particular subscriber. Validity checks made on the MS equipment are performed independently of the authentication checks made on the MS.

Fig1.5. SIM

The SIM is a smart card which plugs into the ME and contains information about the MS subscriber hence the name Subscriber Identity Module.

The SIM contains several pieces of information: International Mobile Subscriber Identity (IMSI)This number identifies the MS subscriber. It is only transmitted over the air duringinitialization.

Temporary Mobile Subscriber Identity (TMSI)This number identifies the subscriber, it is periodically changed by the systemmanagement to protect the subscriber from being identified by someoneattempting to monitor the radio interface.

Location Area Identity (LAI)Identifies the current location of the subscriber.

Subscriber Authentication Key (Ki)This is used to authenticate the SIM card.

Mobile Station International Services Digital Network (MSISDN)This is the telephone number of the mobile subscriber. It is comprised of a countrycode, a network code and a subscriber number.

Most of the data contained within the SIM is protected against reading (Ki) or alterations(IMSI). Some of the parameters (LAI) will be continuously updated to reflect the currentlocation of the subscriber.The SIM card, and the high degree of inbuilt system security, provide protection of thesubscriber’s information and protection of networks against fraudulent access. SIMcards are designed to be difficult to duplicate. The SIM can be protected by use ofPersonal Identity Number (PIN) password, similar to bank/credit charge cards, to preventunauthorized use of the card. The SIM is capable of storing additional information such as accumulated call charges. This information will be accessible to the customer via handset/keyboard key entry. The SIM also executes the Authentication Algorithm.

(b) Mobile Equipment(ME): The ME is the only part of the GSM network which the subscriber will really see. There are three main types of ME, these are listed below: Vehicle MountedThese devices are mounted in a vehicle and the antenna is physically mounted onthe outside of the vehicle.

Portable Mobile UnitThis equipment can be handheld when in operation, but the antenna is notconnected to the handset of the unit.

Hand portable UnitThis equipment comprises of a small telephone handset not much bigger than acalculator. The antenna is be connected to the handset.

The ME is capable of operating at a certain maximum power output dependent on itstype and use. These mobile types have distinct features which must be known by the network, for example their maximum transmission power and the services they support. The ME is therefore identified by means of a classmark. The classmark is sent by the ME in itsinitial message. The following pieces of information are held in the class mark: Revision Level –Identifies the phase of the GSM specifications that the mobile complies with.

RF Power Capability –The maximum power the MS is able to transmit, used for power control andhandover preparation. This information is held in the mobile power class number.

Ciphering Algorithm –Indicates which ciphering algorithm is implemented in the MS. There is only onealgorithm (A5) in GSM phase 1, but GSM phase 2 specifies different algorithms(A5/0–A5/7).

Frequency Capability –Indicates the frequency bands the MS can receive and transmit on. Currently allGSM MSs use one frequency band, in the future this band will be extended but notall MSs will be capable of using it.

Short Message Capability –Indicates whether the MS is able to receive short messages.

Fig 1.6 Mobile Unit

1.8. Operations and Maintenance Center (OMC)

The operations and maintenance system provides the capability to manage the GSMnetwork remotely. This area of the GSM network is not currently tightly specified by the GSM specifications, it is left to the network provider to decide what capabilities they wish it to have. The Operations and Maintenance System comprises of two parts:

(a) Network Management Center(NMC)The Network Management Centre (NMC) has a view of the entire PLMN and isresponsible for the management of the network as a whole. The NMC resides at the topof the hierarchy and provides global network management.

(b) Operation and Maintenance Center(OMC)The Operations and Maintenance Centre (OMC) is a centralized facility that supports theday to day management of a cellular network as well as providing a database for longterm network engineering and planning tools. An OMC manages a certain area of thePLMN thus giving regionalized network management.

1.9. GSM Interfaces

For the connection of the different nodes in the GSM network, different interfacesare defined in the GSM specifications. The GSM interfaces used are: Air interface or Um-interface

The Air Interface is the interface between the BTS (Base TransceiverStation) and the MS (Mobile Station). The air interface is required forsupporting: Universal use of any compatible mobile station in a GSM network A maximum spectral efficiency.

Abis-interfaceThe Abis-interface is the interface between the BSC (Base StationController) and the BTS. The interface comprises traffic and controlchannels. Functions implemented at the Abis-interface are: Voice-data traffic exchange Signaling exchange between the BSC and the BTS Transporting synchronization information from the BSC to the BTS.

A-interfaceThe A-interface is the interface between the BSC and the MSC.

Proprietary M-interfaceIn the GSM network implementation, the BSC includes the TRAU

(Transcoder/Rate Adapter Unit). The TRAU adapts the transmission bit rate of the A- interface (64 kbit/s) to the Abis-interface (16kbit/s).The interface between the physical BSC and the TRAU is known as the M-interface.

Fig 1.7 Different Interfaces

1.10 GSM SPECIFICATION:-

1)Duplex Distance:-

It is the minimum distance between Uplink and Downlink.In case of 900MHzDD=935-890=45MHzIn case of 1800MHzDD=1805-1710=95MHz

2)Channel Bandwidth:-

It is the difference between the two channels.In case of 900MHzCB=915-890=25MHzIn case of 1800MHzCB=1785-1710=75MHz3)Carrier Bandwidth:-

It is applicable to 900MHz case.The formula to calculate carrier bandwidth is 890+0.2*n for Uplink 935+0.2*n for Downlink

where n=Number of frequency or channels

For UplinkN=1 =>890+0.2*1=890.2 Difference of 200 KHz means thisN=2 =>890+0.2*2=890.4 much of data can be transmitted N=3 =>890+0.2*3=890.6 on this bandwidth.

For DownlinkN=1 =>935+0.2*1=935.2N=2 =>935+0.2*2=935.4Means the capacity of carrier is 200 KHz.

4)Total Number Of Channels:-

It is defined as the ratio of Channel Bandwidth and Carrier Bandwidth. TNOC=Channel Bandwidth Carrier Bandwidth =25MHz =125 Channels=124+1 where 1 200 KHz is Guard Band of 100 KHz. TNOC= Channel Bandwidth Carrier Bandwidth =75MHz =375 Channels=374+1 where 1 200 KHz is Guard Band of 100 KHz.

5)Carrier Separation:-

The difference between two carriers should be 200Khz in 900 band as well as 1800 band. The length of separation between channels is dependent on the amount of information which is to be transmitted within the channel. The greater the amount of information to transmit ,the greater the amount of separation required.

Chapter 2: Channels in Air Interface

A channel is an air interface and acts as a medium that is used to carry information.

There are two types of channels.

1) PHYSICAL CHANNELS2) LOGICAL CHANNELS

2.1. PHYSICAL CHANNELS:-

Physical channel is a medium over which the information is carried, in the case of a terrestrial interface this would be a cable .Physical channel is used to transmit speech, data or signaling information. There is a technique called Time Division Multiple Access. Time is divided into discrete periods called “timeslots”. The timeslots are arranged in sequence and are conventionally numbered 0 to 7. Each repetition of this sequence is called a “TDMA frame”. Each time slot on a TDMA frame is called a physical channel. Therefore, there are 8 physical channels per carrier frequency in GSM.

Fig 2.1. Timeslots and TDMA Frame

There are 8 physical channels that means 8 subscribers can talk .

2.2. LOGICAL CHANNELS:-

A physical channel may carry different messages, depending on the information that is to be sent. These messages are called Logical Channels. The logical channels consist of the information carried over the physical channel.

There are two types of Logical Channels:-

Fig 2.2 Types of Logical Channels

2.2.1) TRAFFIC CHANNEL:-

The Traffic Channel carries speech or data information. Once call set-up procedure have been completed on the control physical channel, the MS tunes to a traffic channel. It uses the traffic

channel (TCH) logical channel. There are two types of TCH:

a)FULL RATE:-In this one time slot is allotted to 1 subscriber. Therefore a full rate TCH occupies one physical channel.Full Rate is further divided into two types:-

1) NET RATE:- It is defined as the data rate before channel coding i.e.13Kbps.Data – 9.6Kbps and SMS – 4.8Kbps.

2) GROSS RATE:-It is defined as the data rate after channel coding i.e. 22.8Kbps.

b) HALF RATE:-In this one time slot is allotted to 2 subscriber. Therefore a half rate TCH occupies two physical channels.

LOGICAL CHANNEL

TRAFFICCHANNEL

CONTROL CHANNEL

Half Rate is further divided into two types:-

1) NET RATE: - It is defined as the data rate before channel coding i.e.6.5Kbps.Data – 4.8Kbps and SMS – 2.4Kbps

2) GROSS RATE:- It is defined as the data rate after channel coding i.e. 11.4kbps.

Fig 2.3 Traffic Channels

2) CONTROL CHANNEL:-

When an MS is switched on, it searches for a BTS to connect to it. The MS scans the entire frequency band or optionally uses a list containing the allocated carrier frequencies for this operator. When the MS finds the strongest carrier, it must then determine if it is a control channel.

Control Channel is further divided into three types:-

1. Broadcast Control Channel2. Common Control Channel3. Dedicated Control Channel

TRAFFIC CHANNEL

FULL RATE HALF RATE

Fig 2.4 Control Channel

A) BROADCAST CONTROL CHANNEL (BCH):-

When MS finds the strongest carrier, it must then determine if it is a control channel. It does so by searching for a particular logical channel called the Broadcast Control Channel (BCCH). The Broadcast Control Channels are downlink only.

Fig 2.5 Broadcast Control Channel

CONTROLCHANNEL

DEDICATEDCONTROLCHANNEL

COMMONCONTROLCHANNEL

BROADCASTCONTROLCHANNEL

BROADCAST CONTROLCHANNEL

BROADCASTCOMMONCONTROL

CHANNEL

SYNCHRONI-ZATION

CHANNEL

FREQUENCYCORRECTIO

NCHANNEL

1) BROADCAST COMMON CONTROL CHANNEL (BCCH) :-

It gives us information about the Cell Global Identity (CGI).CGI gives following information:- Mobile Country Code (MCC) – For India it is 404 and 405. Mobile Network Code (MNC) – INA-05 Location Area Code (LAC) – 109,110,………. Cell ID – For eg 1212

2) SYNCHRONIZATION CHANNEL (SCH):-

It transmits information about the TDMA frame structure in a cell (eg. Frame number) and the BTS identity i.e. Base Station Identity Code (BSIC).

3) FREQUENCY CORRECTION CHANNEL (FCCH) :-

It transmits a carrier frequency.MS checks which BTS is giving best RX level i.e. -30dbm when we switch on our mobile. Then it will tune to that particular frequency.

B) COMMON CONTROL CHANNEL (CCCH):-When the MS has finished analyzing the information on a BCH, It then has all the information required to work with a network. If the mobile subscriber then wishes to make to receive a call ,the Common Control Channels (CCCH) must be used. The Common Control Channel Group works in both uplink and downlink directions.

Fig 2.6 Common Control Channel

RANDOM ACCESSCONTROL CHANNEL

ACCESS GRANT CONTROL CHANNEL

PAGINGCHANNEL

CELL BROADCAST CONTROL CHANNEL

COMMON CONTROLCHANNEL

1)RANDOM ACCESS CONTROL CHANNEL (RACH)

It receives request from MS for a signaling channel to be used for call set-up.Thus used by the mobile when it requires to gain access to the system. This occurs when the mobile initiates a call or responds to a page.

2)ACCESS GRANT CONTROL CHANNEL (AGCH)

It accepts the request and assigns a signaling channel (SDCCH) to the MS in response to an access message received on the Random Access Channel. The MS will move to the dedicated channel in order to proceed with either a call setup, response to a paging message,Location Area Update or Short Message service .

3)PAGING CHANNEL (PCH)

It is used for transmitting a paging message to indicate an incoming call or short message.The paging message contains the identity number of the mobile subscriber that the network wishes to contact.

4)CELL BROADCAST CONTROL CHANNEL (CBCH)

The location information shown on the cell is through CBCH.It tells from which BTS we are getting the coverage.

C)DEDICATED CONTROL CHANNEL (DCCH)

When the MS and BSS are ready to begin call set-up procedures .For this the MS and BSS use Dedicated Control Channels (DCCH).

1)STAND ALONE COMMON CONTROL CHANNEL (SDCCH)

Following functions are performed by this channel :-a) Signaling is sent after AGCH and before TCH.b) Location Updating is done .c) SMS is also sent on this channel.

2) SLOW ASSOCIATED COMMON CONTROL CHANNEL (SACCH)

It sends measuring reports like:-1) RX level of serving cell and 6 neighboring cell. 2) Quality of the serving cell . 3) Timing Advance

3)FAST ASSOCIATED COMMON CONTROL CHANNEL (FACCH)

Whenever handover takes place, FACCH steals the information of the TCH and sends its own information on it.

Fig 2.7 Summary of Control Channels

Call Management: The steps involved in call management are: Call establishment. Call in active phase. Call release.

The above procedures are explained using the following scenarios: Mobile-to-land call. Land-to-mobile call. Mobile-to-mobile call.

The typical steps involved in a call from mobile phone to a landline are explained below:1. MS transmits a channel request message over the Random Access Channel (RACH).2. Once the BSS receives the Channel Request message, it allocates a Stand-alone Dedicated Control Channel (SDCCH) and forwards this channel assignment information to the MS over the Access Grant channel (AGCH). It is over the SDCCH that the MS will communicate with the BSS and MSC until a traffic channel is assigned.3. The MS transmits a service request message to the BSS over the SDCCH. Included in this message is the MS TMSI and Location Area Identification (LAI). The BSS forwards the service request message to the MSC/VLR.4. The MSC transmits a request to the MS requesting it to respond with its International Mobile Equipment Identity (IMEI).5. The MS upon receiving this request, reads its equipment serial number and returns this value to the MSC.6. The MSC then requests the EIR to check the IMEI for validity. The EIR will first check to see if the IMEI value is within a valid range. If so, it then checks to see if the IMEI is on a suspect or know list of invalid equipment.7. The EIR returns to the MSC the results of the IMEI validation. If the results are negative, the MSC might abort the call or possibly let the call continue but inform the network service provider of the event. In this scenario, we'll assume that the IMEI is valid.8. The MS transmits a call setup request message to the MSC/VLR after it has ciphered the radio channel. Included in this request message are the dialed digits. The MSC, upon receiving the call setup request message, will request the VLR to supply subscriber parameters necessary for handling the call. The VLR will check for call barring conditions, such as the MS being barred from making specific outgoing calls (e.g., international calls), or possibly if some supplementary services are active which prevent the call from being granted. If the VLR determines that the call cannot be processed, the VLR will provide the reason to the MSC. In this scenario, we'll assume that this procedure is successful. The VLR returns a message to the MSC containing the service parameters for the particular subscriber.9. The MSC informs the MS that the call is proceeding.

The next four steps involve establishing a voice path between the MSC and theMS.10. The MSC allocates a trunk to the BSS currently serving the MS. The MSC sends a message to the BSS supplying it with the trunk number allocated (TN), and requests the BSS to allocate a radio traffic channel (TCH) for the MS.

11. The BSS allocates a radio traffic channel and transmits this assignment to the MS over the SDCCH.12. The MS tunes to the assigned radio traffic channel and transmits an acknowledgment to the BSS.13. The BSS connects the radio traffic channel to the assigned trunk of the MSC. Since a small portion of a radio traffic channel is available for out-of band signaling, the SDCCH is no longer used for signaling between the BSS and MS. The BSS de-allocates the SDCCH. The BSS then transmits a trunk and radio assignment complete message to the MSC.14. The MSC sends a network setup message to the PSTN requesting that a call be setup. Included in the message are the MS dialed digits (DD) and details specifying which trunk should be used for the call.15. The PSTN may involve several switching exchanges before finally reaching the final local exchange responsible for applying the ringing tone to the destination phone. The local exchange will generate the ringing tone over the trunk, or series of trunks (if several intermediate switching exchanges are involved), to the MSC. At this point in time, the MS will hear ringing tone. The PSTN notifies the MSC with a network alerting message when this event occurs.16. The MSC informs the MS that the destination number is being alerted. The MS will hear a ringing tone from the destination local exchange through the establishedvoice path.17. When the destination party goes off-hook, the PSTN will inform the MSC of this event. This event usually triggers the beginning of billing. At this point, the MS will be connected to the destination party.18. The MSC informs the MS that the connection has been established.19. The MS acknowledges the receipt of the connect message.20. The mobile user initiated the release of the call by pressing the “end” button (the button might be labeled with a different term) on the MS. The MS sends a Disconnect message to the MSC.21. The other party (The PSTN party) is notified of the termination of the call by a Release message from the MSC. The end-to-end connection is terminated.22. When the MSC determines that the call has no more reason to exist (no side tasks to complete, e.g charging indication) a Release message is sent to the MS.23. The MS answers back with a Release complete message. At this stage the lower connections are released (unless they are used for something else in parallel).24. The voice trunk between the MSC and the BSS is released.25. The traffic channel is cleared.26. The release of the resources is completed.

Chapter3: Cell Planning

Every cellular network require`s cell planning to ensure coverage and avoid interference.The key terms required are defined below. Radio coverage :-A defined level of the radio signals is received by the mobile station

within this area. Cell:-The area that is covered by a Base Transceiver Station (BTS). Omni cell:-Omni directional cells are served by an antenna which transmits equally in all

directions. Sector cell :-A cell with a uni-directional BTS antenna system. Frequency reuse :-The process of “reusing” the same frequencies within the cellular

network. Grade Of Service (GOS):- The allowed percentage of unsuccessful call set-ups due to

congestion. Site:- The geographical location where the Radio Base Station (RBS) equipment is

housed. 3-Sector site:- A site with three sector cells. Cluster:- Area where all frequency groups are used only once.

Cell Planning Process:To ensure coverage areas and avoid interference, every cellular network requires cell planning. The major activities involved in the cell planning process are shown below.

Fig 3.1 Cell Planning Process

3.1. TRAFFIC AND COVERAGE ANALYSIS (REQUIREMENTS)

Cell planning begins with traffic and coverage analysis. The purpose of this analysis is to prove that there is a need for a cellular network. The analysis should also produce information about the geographical area and the expected capacity (traffic load.) The types of data collected are:

Cost Capacity Coverage Grade Of Service (GOS) Available frequencies Speech quality System growth capability

The traffic demand (i.e. how many subscribers join the system and how much traffic they generate) provides the basis for all cellular network engineering. Geographical distribution of traffic demand can be calculated by the use of demographic data such as:Population distribution

Car usage distribution Income level distribution Land usage data Telephone usage statistics Other factors, like subscription/call charge and price of mobile stations

Traffic CalculationsInput for the traffic calculations is discussed in the section above. Output should be information about how many sites and cells are needed. In order to define the output, the available number of frequencies per cell, as well as the Grade of Service (GOS) must be known.

Frequency re–use

A fundamental principle in the design of cellular systems is the frequency re–use patterns. frequency re–use is defined as the use of radio channels on the same carrier frequency covering geographically different areas. These areas must be separated from one another by a sufficient distance so that any co–channel or adjacent channel interference that may be encountered is not objectionable. Based on the traffic calculations, the cell pattern and frequency plan are worked out not only for the initial network but also to adapt smoothly to the demands of traffic growth.

CARRIER TO INTERFERENCE RATIO

The carrier–to–interference ratio, C/I, is defined as the ratio between the level of the received desired signal to the level of the received undesired signal; see Figure 12-4_ This C/I ratio is dependent on the instantaneous position of the mobile station and is effected by irregular terrain and various shapes, types and numbers of local scatters. Other factors such as antenna type, directionality and height, site elevations and positions and the number of local sources of interference also affect the distribution of the C/I ratio in a system GSM and PCS states C/I is greater than 9dB, when frequency hopping is used. Ericsson recommends C/I is greater than 12dB when frequency hopping is not used.

Fig 3.2 Co- channel Interference

CARRIER TO ADJACENT RATIO

The carrier–to–adjacent ratio, C/A, is defined as the relation in dB in signal strength between the serving and an adjacent frequency, for example 200 kHz apart, as in Figure 12-5 GSM and PCS specifies C/A is greater than -9dB.

Fig 3.3 Adjacent Channel Interference

3.2. NOMINAL CELL PLAN

A nominal cell plan can be produced from the data compiled from the traffic and coverage analysis. The nominal cell plan is a graphical representation of the network and looks like a cell pattern on a map.Nominal cell plans are the first cell plans and form the basis for further planning. Quite often, a nominal cell plan, with one or two examples of coverage predictions, is included in tenders. Nominal cell plans provide the theoretical basis for further planning. Successive planning must take into account the radio propagation properties of the actual environment. Such planning needs measurement techniques and computer-aided analysis tools for radio propagation studies.

3.3. SURVEYS

After a nominal cell plan is completed and coverage and interference predictions are roughly produced, site surveys can be performed.

Site SurveysSite surveys are performed for all proposed site locations. The following must be checked for each site:

Exact location. Space for equipment, including antennas. Cable runs. Power facilities. Contract with owner.

In addition, the radio environment must be checked to ensure that there is no other radio equipment on site that causes Inter modulation problems.

Radio MeasurementsRadio measurements are performed to adjust the parameters used in the planning tool to reality. That is, adjustments are made to meet the specific site climate and terrain requirements. Parameters used in a cold climate will differ from those used in a tropical country, for example. A test transmitter is mounted on a vehicle, and signal strength is measured while driving around the site area. Afterwards, the results from the measurements can be compared to the values the planning tool produces when simulating the same type of transmitter. The planning parameters can then be adjusted to match reality.

3.4. SYSTEM DESIGN

After the planning parameters are adjusted to meet reality, dimensioning of the RBS equipment, BSC and MSC can be adjusted and the final cell plan produced. As the name implies, this plan can then be used for system installation. New coverage and interference predictions are run. A Cell Design Data (CDD) document is completed. It contains cell parameters for each cell.

3.5. SYSTEM IMPLEMENTATION AND TUNING

After the system has been installed, it is continuously monitored to determine how well it meets demand. This is called system tuning. It involves:

Checking that the final cell plan was implemented successfully. Evaluating customer complaints. Checking that the network performance is acceptable. Changing parameters and taking other measurements, if needed.

Test Mobile Systems (TEMS)

TEMS is a testing tool used to read and control the information sent over the air interface between the base station and the mobile station. It can be used for radio coverage measurements. In addition, TEMS can be used both for field measurements and post processing. TEMS consists of a mobile station with special software, a portable PC and optionally a Global Positioning System (GPS) receiver. The mobile can be used in active and idle mode, and in GSM and PCS networks. The PC is used for presentation, control and measurements storage. The GPS receiver provides exact position of the measurements by utilizing satellites. When satellite signals are shadowed by obstacles, the GPS system switches to dead reckoning. Dead reckoning consists of a speed sensor and a gyro. This provides the position if the satellite signals are lost temporarily. TEMS measurements can be imported to EET via the File and Information Converting System (FICS). This means that measurements can be displayed on the map. Thus, measured handovers can be compared with the predicted cell boundaries, for example. FICS can also download measurements to spreadsheet and word processing packages.

3.6. SYSTEM GROWTHWhen the system needs to be expanded because of an increase in traffic and new subscribers, a new traffic and coverage analysis must be performed. In this case, the user must begin the cell planning process again from the first step. Thus, this should be an ongoing process.

Fig 3.5. TEMS Hardware

Chapter4: Frequency Hopping

The MS/BTS operating in a frequency hopping system are able to Tx/Rx on different frequencies for every TDMA burst (≈ 577µs). GSM recommendation defines the following parameters for a frequency hopping system and they are sent from the BTS to MS in the assignment messages during call setup.

Mobile Allocation (MA): This is the set of frequencies the mobile/BTS are allowed to hop over. Two time-slots on a same transceiver of a cell may be configured to operate on different MA. MA is the subset of the total allocated spectrum for the GSM operator and the maximum number of frequencies in a MA list is limited by GSM recommendation to 6

Mobile Allocation Index Offset (MAIO): This is an integer offset that determines which frequency within the MA will be the operating frequency. If there are N frequencies in the MA list, then MAIO = {0, 1, 2, … N-1}.

Hopping Sequence Number (HSN): This is an integer parameter that determines how the frequencies within the MA list are arranged. There are 64 HSN defined by GSM. HSN = 0 sets a cyclical hopping sequence where the frequencies within the MA list are repeated in a cyclical manner.

HSN = 1 to 63 will provide pseudo random hopping sequence. The pseudo random pattern will repeat itself after every hyper frame, which is equal to 2,715,648 (26x51x2048) TDMA frames or about 3 hours 28 minutes and 54 seconds.

There are 2 ways to implement frequency hopping at a BTS. Synthesizer Frequency Hopping (SFH) Baseband Frequency Hopping (BBH)

4.1. Synthesizer Frequency Hopping (SFH)

The transceiver unit re-tunes to a different operating frequency set (Tx & Rx) on each TDMA burst (≈ 577µs). The re-tuning will follow the sequence explained in the previous section. In theory, there is no restriction on the number of frequencies the transceiver unit can hop on. However, GSM specifications limit the total number to 64 frequencies for a SFH transceiver unit. If the BCCH frequency is included in the MA list, timeslot 1 to 7 of the BCCH carrier will not be able to carry traffic. This is an inherent limitation of SFH and it is recommended that BCCH frequencies should be excluded from the MA list whenever possible. SFH cannot be implemented at a cell that uses narrow-band Tx combiner (e.g. RTC -“Remote Tunable Cavity Combiner”). The reason is SFH requires the hopping carrier and associated Tx combiner to retune to a new frequency every TDMA burst (≈ 577µs). Since RTC re-tuning involves mechanical movement, it is not possible to cope with the speed. As a result, only broadband combiner, e.g. hybrid combiner, can be used at a SFH cell.

Fig 4.1 Synthesizer Frequency Hopping

4.2. Baseband Frequency Hopping (BBH)

In this method, the transceiver unit will always transmit at an assigned frequency. Frequency hopping is done by switching the information frame of one call from one transceiver to another within a cell, per TDMA burst (≈ 577µs). The switching of transceivers will follow the sequence defined in FHI, as explained in previous section. The resultant transmitted signal on the air-interface is identical to SFH. Please note that the uplink path will not use BBH and the transceiver on which the call is established will always receive the uplink signal from the MS. All the processing (e.g. coding, interleaving etc) will be carried out by this transceiver and the processed information will be routed to different transceivers for transmission.When a BBH carrier or time slot is activated or de-activated, other affected carriers or time slots (hop over its frequency) must be reconfigured to include or exclude a channel in the operating frequency list. Intra-cell handover will be attempted to move all the active calls on these time slots to unaffected time slots (e.g. non-hopping or MA list without this frequency) within the cell. Unsuccessful intra-cell handover (e.g no time slots available) will cause the calls to be cleared. In a BBH system, a parameter called “hopping_ins_mode” is used to determine whether a previously inactive carrier would be brought into service as hopping or not, outside of site initialization time (sysgen mode).The number of channels in a MA list must be equal or smaller than the number of BBH carriers in a cell. It is worth noting that the FHI assigned to a timeslot must be in accordance to the MA list of the FHI. For example: In a cell with 3 BBH carriers (namely, A, B & C) and MA = {f1, f2, f3} is defined in FHI 2. If FHI 2 is to be assigned to time slot 2 of carrier A, then time slot 2 of carrier B & C must also be assigned FHI 2, to make the system work.

Fig 4.2 Baseband Frequency Hopping

How Frequency Hopping Improves Quality?

Frequency Hopping can improve the radio air-interface quality of a cellular network in 2 ways: Frequency Diversity. Interference Averaging.

Frequency DiversityQuality is improved in the network by using frequency hopping to alleviate the effects of frequency selective fading that is inherent in radio wave propagation in the GSM 900 band, and especially at frequencies in the GSM1800/1900 band where environmental factors have a great effect on the stability of radio signal levels.Fixed frequency carriers, non-hopping, experience natural signal fading in the radio environment. Generally, fading is not a great problem unless the mobile station is in an area of low signal strength (i.e. indoors or at cell boundaries), or is in an area of no dominant server. In this case, normal Rayleigh fading can cause disruptions to speech by inducing bit error that cannot be corrected, since the receiver is getting too many consecutive corrupted speech bursts over the air interface.In a GSM, once a speech call is allocated to a channel, voice is transmitted over 8 consecutive TDMA frames for every 20 ms of speech. If a speech call is placed on a fixed carrier, non-hopping, then the call is tied to the fading profile of that frequency. So as a call experiences a slow fade the BER becomes a problem and affects call quality. The GSM air interface is designed to handle some degree of BER to counteract a reasonable amount of air interface corruption in the mobile environment.The same call on a frequency hopping system is moving from frequency to frequency every 62 ms, and can take advantage of the different fading profiles of each frequency in the allocated

hopping sequence. The greater the hopping frequencies are spaced, the greater the de-correlation between the fading profile of each frequency and the signal level. Field data shows that when calls are made on a hopping and on a non-hopping carrier, hopping calls have far greater signal stability. Frequency hopping averages out extremes in high signal levels and low signal levels. Field data of calls hopping over as little as four frequencies show a “pyramid” shaped graph of receive signal level with more of the data points near the mean with a smaller standard deviation than the graph for a fixed frequency, non-hopping, call. These are shown in the figures below:

Fig 4.3 Effect of deep fading to TDMA frames

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Effect of deep fading in a hopping system to the TDMA frames. As can be observed, benefit of frequency diversity gained from frequency hopping is significant. Not only the total number of bad frames is reduced, more importantly the occurrence of bad frames in consecutive order is reduced as well. This will improve the speech quality as the lost bits have higher probability to be recovered by the GSM decoding mechanism and hence a lower number of erased speech frames.

Interference Averaging

Interference protection is probably the biggest improvement that comes as a result of implementing frequency hopping. Calls made on fixed frequency systems may suffer from interference, which has a good chance of not diminishing in the lifetime of a call unless the subscriber changes position or the interfering channel is deactivated. Either co-channel or adjacent channel interference hits fixed frequency calls normally at the cell border. This type of interference is constant to the subscriber in the downlink direction. Usually, interference found at the cell boundary cannot be escaped from unless a handover is made to a “clean” frequency. To avoid interference on fixed frequency systems larger separation between reuse groups is used to lessen the chance of co-channel or adjacent channel interference from degrading call quality. The cost of “loose” reuse schemes to the network is capacity.

How Frequency Hopping Enhances Network Capacity?

In principle, implementation of frequency hopping system will not add extra capacity to the existing network. Frequency hopping when implemented will enable more aggressive frequency re-use pattern that leads to better spectrum efficiency. This enables the network operator to add more transceivers in existing sites while maintaining the network quality. In a congested network with fixed frequency plan, adding transceivers would mean compromising the carrier – interference ratio (C/I), which may lead to unacceptable quality level that may eventually crash the network if pushed to the limit. Thus, frequency hopping is effectively “compressing” the available spectrum to make room for extra capacity, without degrading the average C/I as in a fixed frequency system. In a cellular network, there is always a tradeoff between capacity & quality. Maintaining the current capacity, implementing frequency hopping will improve overall quality. On the other hand, extra capacity could be added by implementing frequency hopping while maintaining the current quality. However, realizing maximum gains in both quality and capacity would not be achievable.

Capacity

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Chapter 5: Radio Parameter Optimization

Radio Resource management is a group of functions concerned with the management of transmission resources on the radio path (Um interface). It must cope with limited radio resources (and the corresponding terrestrial resources) and share them dynamically between all demands.The role of the Radio Resource management is to:1. Establish stable connections between the mobile stations and the BSC(Base Station Controller)2. Maintain them despite user movement for the duration of a call for example3. Release the connections between the mobile stations and the BSC.

The Radio Resource management functions are: Power control Handover Discontinuous transmission Call re-establishment Frequency hopping

Mobile Station Measurements: Downlink signal strength Downlink signal quality

Base Station Measurements: Uplink signal strength Uplink signal quality

5.1. Power Control

Power Control enables the mobile station and/or the BTS to increase or decreasethe transmission power on a per-radio link basis. Power Control is separately performed for the uplink and downlink. In both cases the BSC is responsible for initiating Power Control; the mobile station and the BTS adopt transmit power according to the BSC Power Control commands.While a mobile station is active on a call, it has the responsibility of providing measurement data about the performance of the air-interface to its serving BTS so that the serving BSC can decide if a power control should be performed. Also the serving BTS measures the performance of the air-interface. Whereas the mobile station measures the performance of the downlink, the BTS measures the performance of the uplink.The mobile station measures periodically the performance of the downlink, and sends the measurements in the SACCH (Slow Associated Control Channel) via the serving BTS to the BSC every SACCH multi-frame. This corresponds to the transmission of data every 104 TDMA frames or 480 ms. The base station measures the quality of the uplink. Also, it transfers the measurements in the SACCH to the BSC every 480 ms.

Downlink Measurements

The mobile station measures and reports the following measurements to the BSCregarding the performance of the downlink:

Strength of the signal being received from its serving BTS (in dBm). Quality of the signal being received from its serving BTS (in bit error rate).

Uplink MeasurementsThe BTS measures and reports the following measurements to the BSC regardingthe performance of the uplink:

Strength of the signal being received from the mobile station. Quality of the signal being received from the mobile station.

When the BSC notices that the signal strength of a particular radio link measured on the uplink becomes below the lower pre-defined threshold because the mobile station moves away from the BTS, it sends a Power Control command to the mobile station to increase its transmit power (MS_TXPWR) by a pre-defined step (typically 2 dB). The transmit power of the mobile station can be increased until an maximum defined level is reached.The BSC can also send a Power Control command to the mobile station to reduce its transmit power when it notices that the signal strength measured becomes above the upper pre-defined threshold. The downlink Power Control process is similar to the uplink Power Control process.

Fig 5.1. Power Control Algorithm

Another reason for activating Power Control is an uplink/downlink signal quality measured which is higher or lower than thresholds specified. In fig, the power control algorithm is shown. Power control is implemented whenever the power of either the BTS or The MS exceeds or falls below the threshold values. In the “optimum” area, the area delimited by the different predefinedthresholds, no Power Control actions are taken.

Reasons for Power Control To save the mobile station battery power. To improve the carrier to interference ratio within the cellular network. Reducing power

on the BTS or the mobile station, while keeping similar signal quality received, decreases interference caused on the other calls in the surrounding area.

5.2. Handover Control

Handover is the process of automatically switching a call in progress from one traffic channel to another to neutralize the adverse effects of user movements. The switch can be made either to a TCH within the same cell or in another cell. Usually, handovers take place on the TCH, when the call is in the speech stage.However, in rare cases it may be necessary to perform a handover when, for example, the call is still in build-up stage. In that case the SDCCH will be handed over to another frequency or time slot. This type of handover is more likely to take place during transmission of short messages by the point-to-point Short Message Service (SMS).The handover process will normally only be started if power control is not helpful anymore.To decide if a handover should be performed, the BSC receives measurement data about the performance of the air-interface from its serving BTSs and mobile stations. The BSC uses the same measurements as those used in the power control process.

5.2.1. Handover Objectives and Conditions

When a mobile station is active on a call, the serving BSC has the responsibility of deciding when a handover should be performed. The decision of the BSC is based on the performance measurement data received from the mobile station and the serving BTS. During the performance evaluation, the BSC takes the following objectives for optimizing the handover performance into account:

Maintain a good speech quality. Minimize the number of calls dropped. Maximize the amount of time the mobile station is in the “best” cell. Minimize the number of handovers.

Power Budget HandoverA power budget handover takes place as soon as a better cell with respect to the power budget is available to handle the call. A power budget handover is based on the path loss. The path loss is the differences between the actual transmit power of the BTS and the signal level received by the mobile station. Generally, the mobile station will switch to the BTS with the lowest path loss. By switching to another cell, the necessary power transmitted by both the mobile station and the base station is reduced and the level of interference is decreased.

Distance handoverDistance between the mobile station and the base station can also be a reason forhandover. If the timing advance for the mobile station becomes too big, because itis too far away from the base station, handover has to take place to a cell that iscloser to the mobile station.

Signal level and signal quality handoverOther reasons for handover are the signal level (or signal strength) and the signalquality on either the uplink or the downlink received from the mobile station andthe base station. If the BSC determines that either the signals have too low qualityor too less strength, it can decide to start the handover procedure.

Downlink MeasurementsThe mobile station measures and reports the following measurements to the BSC regarding the performance of the downlink:

Strength of the signal being received from its serving BTS (in dBm). Quality of the signal being received from its serving BTS (in bit error rate). Signal strength of the 6 best neighboring BTS downlink control channels (candidate list).

Uplink MeasurementsThe BTS measures and reports the following measurements to the BSC regarding the performance of the uplink:

Strength of the signal being received from the mobile station. Quality of the signal being received from the mobile station. Distance between the serving BTS and the mobile station (in meters).

As a mobile station moves away from its serving BTS towards the coverage area of neighboring BTSs, the mobile station measurement reports will show a gradual decrease in signal strength from its serving BTS while showing an increase in measured signal strength from one or more neighboring BTSs. It is the responsibility of the serving BSC to analyze the measurement reports from the mobile station and to decide when a handover should be performed. If it is determined that there is a better BTS to serve the call, the serving BSC initiates the handover procedure.

5.2.2 Types of Handover

The type of handover procedure executed depends on what level of switching must be performed in order to move the call from the serving BTS to the new candidate BTS. There are basically four types of handovers:

Internal or intra-BSS handover, which can be:— Intra-cell handover— Inter-cell handover External or inter-BSS handover, which can be:— Intra-MSC handover— Inter-MSC handover

If the serving and candidate BTSs reside within the same BSS, the BSC for the BSS can perform the handover without the involvement of the MSC; thus termed internal or intra-BSS handover. This type of handover can also be sub-divided into intra-cell and inter-cell handovers. An intra-

cell handover is an intra-BSS handover within the same BTS. An inter-cell handover is a handover between different BTSs.

If the serving and candidate BTSs do not reside within the same BSS, then an inter-BSS handover is performed, which requires the MSC to coordinate and switch facilities (handover the call) between the serving BTS and the candidate BTS. This type of handover can also be divided into intra-MSC and inter-MSC handovers.

Various types of handover can be explained using the following example. It illustrates the 4 types of handover, using the example of a system consisting of two MSCs and three BSSs. Also depicted are cell coverage areas with example Cell Global Identification codes for each BSS. Assume that the mobile and land stations are active in a call, the call is being controlled by MSC A, and the mobile station is currently in cell area 234-01-100-51.

5.2.2(a) Intra-BSS, intra cell Handover

For this type of handover, the mobile station is handed over to a different radio channel within the same cell area: 234-01-100-51. This is actually an unusual type of handover, since it is not triggered by poor signal strength (if it was, the candidate base station would be different from the serving base station). A probably cause for this type of handover would be poor signal quality (not strength), possibly due to co-channel interference. For this type of handover, BSC2 would allocate a new radio channel and instruct the mobile station to retune.

5.2.2(b) Intra-BSS, inter cell handover

The mobile station moves from area 234-01-100-51 towards area 234-01-100-52. At some point in time, BSC 2 will determine from the signal strength measurement reports that the base station responsible for cell area 234-01-100-52 can better serve the call. Since the candidate base station is also connected to BSC 2, the handover can be coordinated by BSC 2 without the involvement of MSC A. In this situation, BSC 2 reserves an available radio channel from cell area 234-01-100-52 and instructs the mobile station to retune to the new radio channel. BSC 2 is alsoresponsible for switching the voice path between MSC A and the old radio channel to the new radio channel.

5.2.2(c) Inter-BSS, intra MSC handover

The mobile station moves from area 234-01-100-51 towards area 234-01-100-55. At some point in time, BSC 2 will determine from the signal strength measurement reports that the base station responsible for cell are 234-01-100-55 can better serve the call. BSC 2 will then determine that there are no base stations connected to it that serves area 234-01-100-55 and will request MSC A to arrange the handover to the candidate base station. MSC A will determine that BSC 3 is responsible for cell area 234-01-100-55 and request it to reserve a free radio channel for a handover. MSC A will relay the new radio channel information back to BSC 2. BSC 2 will then ask the mobile to retune to the new channel. At the same time, MSC A will switch voice paths between the Public Switched Telephone Network (PSTN) and the old BSC (BSC 2) to the new BSC.

5.2.2(d) Inter-BSS, inter MSC handover

The mobile station moves from area 234-01-100-51 towards area 234-01-089-21. Similar to previous scenario, BSC 2 will ask MSC A to coordinate the handover to 234-01-089-21. MSC A will determine that it has no base stations under its control responsible for the identified cell area. MSC A then needs to determine which neighboring MSC is responsible for the cell area, in this case MSC B, and will ask it to receive a handover.

Fig 5.2 Example of different types of Handover

5.3. Discontinuous Transmission (DTX)

Discontinuous Transmission (DTX) is a mechanism which allows the radio transmitter to be switched off most of the time during speech pauses. DTX may be applied independently to each direction, so that the control of DTX must take into account two components:

The uplink mode The downlink mode

DTX can be enabled or disabled for the uplink and/or downlink mode on a per-cellbasis.

5.3.1. Discontinuous Transmission Process

DTX inhibits the transmission of the radio signal when not required from an information point of view. In the DTX mode, speech is encoded at 13 kbit/s when the user is effectively speaking, but in a speech pause information is transmitted at a bit rate around 500 bit/s. This low rate flow is sufficient to encode the background noise, which is re-generated to ensure that the listener does not think that the connection is broken (comfort noise).At the transmission side, the voice activity detection function detects whether speech will be transmitted on a particular radio link or not. When it detects that no speech has to be transmitted, transmission will cease after a defined period of time after speech activity has stopped. The transmitter will periodically send a signal called a Silence Indicator Block every certain period of time. The Silence Indicator Block provides the comfort noise level information to the mobile station or BTS.

5.3.2. Reasons for DTX When DTX is applied, actual transmission on the radio path is reduced. This will cause a decrease of the interference level in co-channel cells (using the same frequency). Another advantage will appear when using DTX in the uplink mode: it saves battery power for the mobile station. However, a disadvantage of the DTX mode is that it slightly deteriorates the quality of transmission.

` Fig. 5.3. Discontinuous Transmission

5.4 Call Re-establishment

Call re-establishment enables the mobile station to resume the contact with the cellular network when the connection to a particular BTS is suddenly broken. This may happen because of a brutal propagation loss, due to obstacles such as bridges and tunnels. Call re-establishment is a GSM feature which can be enabled or disabled on a per-cell basis.

5.4.1. Call re-establishment process

After the communication has been lost, the mobile station selects the cell with the highest signal strength from the neighbor cell list. The neighbor cell list contains the cell identifiers to which a handover is allowed. It is kept in the BSC of a particular cell. The list is transferred to the mobile station in the BCCH during the registration phase of a wireless call. The mobile station uses the neighbor cell list by only measuring the signals from the BTSs located in the cells that are on thelist.The selected cell identifier is used to re-establish the connection to that particular cell by following the normal access procedures. Actually, it sends an access request on the RACH (Random Access Channel) of the particular frequency channel.At the moment of time the communication is lost, a timer is initiated in the serving MSC. When the timer expires, it is not possible anymore for the mobile station to re-establish the call. A typical value for the timer is 4 seconds.

Thus to summarize the Call Re-establishment process, Select the cell with the highest signal strength from the neighbor cell list. Re-establish the connection to that cell by following the normal access procedures.

Conclusion

Today's rapidly changing business environment is creating intense competition among corporations. Markets are changing faster now than in any other time in history. Product life cycles are shortening and businesses must compete globally. Recent advances in cellular technology are offering more efficient and reliable wireless transmission of data between remote locations and the central collection points. Wireless telephone technology has been around in various forms since the 1940’s. Today, most of the world uses GSM (Global System for Mobile communications) and/or one variant or another of CDMA (Code Division Multiple Access) technology, both digital cell phone standards. In recent years, the main drive in cellular technology advancement has been in the area of increased bandwidth for data services.

Cellular communication technology gives corporations the ability to extend the bounds of their communications infrastructure to mobile-undeterred users.

Cell planning is the most important part of a cellular network. The cell planning leads to proper utilization and effective use of the available resources. RF Planning is the process of assigning frequencies, transmitter locations and parameters of a wireless communications system to provide sufficient coverage and capacity for the services required (e.g. mobile telephony). The RF plan of a cellular communication system revolves around two principal objectives; Coverage and Capacity Coverage relates to the geographical footprint within the system that has sufficient RF signal strength to provide for a call/data session. Capacity relates to the capability of the system to sustain a given number of subscribers. In the majority of cellular communication systems, both capacity and coverage are interrelated. To improve quality some coverage, capacity has to be sacrificed, while to improve capacity, coverage will have to be sacrificed.

Bibliography and References

1. Cp02 -Motorola Document2. Myworld.aircel.com3. Lucent Documents on GSM4. Wikipedia.com5. BSS Parameters -Alcatel Lucent Document6. Erlang.com