GSM and CDMA Report

CHAPTER-I INTRODUCTION TO GSM 1.1 Introduction

GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATION) is the world‟s most widely deployed and fastest growing digital cellular standard. Currently here are over 200 million GSM subscribers world-wide - two-thirds two- thirds of the world‟s digital mobile population - and this figure is increasing by nearly four new users per second. GSM covers every continent, being the technology of choice for over 360 operators in more than 137 countries. But this is only the beginning of the wireless revolution. The industry predicts that there will be nearly 600 million GSM customers by 2003. Internet use has grown at an almost parallel exponential rate to GSM subscriptions, fuelling the growth of GSM and its development towards 3

rd

Generation multimedia services. GSM is leading the way in this evolution,

moving far beyond the traditional mobile telephone offering to embrace a whole range of services that provide unrivalled benefits to users, operators and investors alike. GSM‟s been able to grow, develop and adapt to meet changing user demands across a wide range of markets around the world. This is due to the unique co-operation of manufacturers, operators and standards bodies world-wide, working together to develop and market GSM. As a result, since its first commercial rollout GSM has led to the development of a global multi-vendor market, stimulating competition, cost reduction and technological advances based on open standards. International roaming is a key feature unique to GSM, which allows users to stay in touch wherever they travel. Users can also access data services such as the unique SMS (Short Message Service), fax transmission and e-mail access on the move. GSM has also enabled regulators to successfully introduce competition in mobile services worldwide. GSM offers voice and data services to users on the move world-wide. Pre-paid services have also been a key driver behind GSM‟s success, opening up mobile services to new subscriber groups in established and developing markets alike. In many cases, pre-paid subscriptions have overtaken traditional subscriptions, as users recognize the flexibility and control that pre-paid services can offer. Mobile users are already heavily exploiting the data services 1

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available on GSM. Business users are sending faxes and accessing e-mail facilities via their GSM terminals as they travel. Business and private users are exploiting SMS for quick communication and to receive information such as stock quotes, sports headlines and news bulletins. As GSM develops towards 3rd Generation services, new opportunities will continue to make GSM the most attractive choice. SIM cards - the thumbnail or credit card-sized module that contains subscription related data as well as security management and personal telephone books - allow users to change handsets to get one with the latest design or one with additional features. The SIM has proven to be a driver of quality and innovation from both manufacturers and operators, which have benefited customers and the entire industry. SIM cards have also proved to be invaluable for future evolution into 3rd Generation services, where special protocols and tool kits offer possibilities to download and run applications and/or features on the handset or card with embedded personal data for management and security. In addition, operators and service providers can easily and cost-effectively upgrade feature-sets or capabilities by issuing new cards to users without affecting handsets. 1.2 GSM Standards

The system specifications for GSM networks are: 

Frequency Band:

Uplink:

890MHz-915Mhz

Downlink: 935MHZ-960MHz The GSM system is originally specified to operate in the 900MHz band, so even before a commercially viable system could be in place governments of various countries were told to reserve the frequency band for GSM. The frequencies are arranged into pairs so that unique sets can be defined. There are 125 channels in GSM 900, however only 124 are used the first pair are not used, as it is employed as a Guard Band. Shown below is the GSM frequency band, although this is not the only band over which GSM operates it also operate in the 1800Mhz and 1900Mhz band also.

















Figure 1.1 GSM Uplink and Downlink

At this moment it would be important to mention here the need to use a lower frequency for uplink. The reason is, since this carries the information from the MS to the BTS over the Air Interface, using a higher frequency means higher attenuation. Secondly to compensate for the attenuation we need to send the signal at a higher power, which consumes more battery, power and leading to a smaller talk time. 

Duplex Distance: 45MHz

This is the standard distance between the uplink and the downlink frequencies. This is not constant for all versions of GSM however the separation between the uplink & downlink bands is constant to 20kHz. 

Carrier Separation: 200kHz

Figure 1.2 Carrier Separation

In GSM we have uplink and downlink carriers. These individual carriers are separated

















900 they start form 1-124. Henceforth any mention of channels will be done using their ARFCN. The figure below shows the carriers. Each carrier frequency is then divided according to time using a TDMA scheme. Each of the carrier frequencies is divided into a 120ms multiframe. A multiframe is made up of 26 frames. Two of these frames are used for control purposes, while the remaining 24 frames are used for traffic as shown:

Figure 1.3 TDMA Multiframe 

Modulation: Gaussian Minimum Phase shift Keying(GMSK)

The modulation method used in GSM had to be very specific, according to the needs of communication and also to cater for the anomalies in the radio interface. However this would be taken in detail in a later section of this report. 

Transmission Rate: 270kbps



Access Method: Time Division Multiple Access(TDMA)

TDMA is used in GSM in conjunction with FDMA to allow voice communication. The 200Khz channel is divided into 8 slots and each slot represents a call. However the whole channel is available to the caller, were a caller gets particular time duration in a round robin fashion to proceed with his call as described in the figure below.

Figure 1.4 TDMA Frame

















In modern landline telephone systems, digital coding is used. The electrical variations induced into the microphone are sampled and each sample is then converted into a digital code. The voice waveform is then sampled at a rate of 8 kHz. Each sample is then converted into an 8 bit binary number representing 256 distinct values. Since we sample 8000 times per second and each sample is 8 binary bits, we have a bit-rate of: 8kHz * 8 bits = 64kbps This bitrate is unrealistic to transmit across a radio network since interference will likely ruin the transmitted waveform. In GSM, speech encoding works to compress the speech waveform into a sample that results in a lower bitrate using RPE-LPC. The actual process will be discussed later in the section, where the journey from speech to radio waves is considered. 

Over The Channel Bit Rate: 22.8kbps



Slow Frequency Hopping: 217hops/second

GSM can use slow frequency hopping where the mobile station and the base station transmit each TDMA frame on a different carrier frequency. A form of slow frequency hopping is used by GSM to help combat the multipath burst errors characteristic of cellular environments. Each base station has its own pattern for hopping from one carrier frequency to another from slot to slot, with mobiles using that base station following suit. This frequency hopping also reduces the incidence of co-channel interference between clusters of cells. The frequency-hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading is dependent on carrier frequency, slow frequency hopping help mitigate the problem. Frequency hopping is an option for each individual cell and a base station is not required to support this feature. 

Synchronisation Compensation: 223sec



Equalisation Limit: Up to 16sec time dispersion Typical Base Station Transmit Power: 320W

















1.3



GSM Bands E-GSM:

This represents an extension of the lower end of the two sub blocks by 10MHz

adding 50 More ARFCN‟s to the Primary GSM (P-GSM). (P-GSM). Uplink Frequency:

880MHz-915MHz

Downlink Frequency: 925MHz-960MHz 

DCS 1800: At a late stage in GSM development the existing technology was modified to

meet the need for PCN networks. This involves changes to the radio interface, which moves spectrum allocation up to around 1.8Ghz. More spectrum is available in this frequency range, for two sub-blocks of 75Mhz with duplex spacing of 95Mhz, giving a total of 374 carriers. Uplink Frequency:

1710Mhz-1785Mhz 1710Mhz-1785Mh z

Downlink Frequency: 1805Mhz-1880Mhz 

PCS 1900: Used in the USA. The FCC has split the designated spectrum into six duplex

blocks. The USA has been divided into 51 Major Trading Areas and 493 Basic Trading Areas. An MTA is broadly equivalent in size to a state, whilst a BTA approximates to a large city. Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz blocks. 1.4 Features of GSM

The GSM services are grouped into three categories: 1. Teleservices (TS) 2. Bearer services (BS) 3. Supplementary services (SS) 1.4.1 Teleservices

Regular telephony, emergency calls, and voice messaging are within TS. Telephony, the old bidirectional speech calls, is certainly the most popular of all services. An emergency call is a feature that allows the mobile subscriber to contact a nearby emergency service, such as



















1.4.2 Bearer Services

Data services, short message service (SMS), cell broadcast, and local features are within BS. Rates up to 9.6 kbits are supported. With a suitable data terminal or computer connected directly to the mobile apparatus, data may be sent through circuit-switched or packetswitched networks. Short messages containing as many as 160 alphanumeric characters can be transmitted to or from a mobile phone. In this case, a message center is necessary. The broadcast mode (to all subscribers) in a given geographic area may also be used for short messages of up to 93 alphanumeric characters. Some local features of the mobile terminal may be used. These may include, for example, abbreviated dialing, edition of short messages, repetition of failed calls, and others. 1.4.3 Supplementary Services

Some of the SS are as follows: 

Advice of charge. This SS details the cost of a call in progress.



Barring of all outgoing calls. This SS blocks outgoing calls.



Barring of international calls. This SS blocks incoming or outgoing international calls as a whole or only those associated with a specific basic service, as desired.



Barring of roaming calls. This SS blocks all the incoming roaming calls or only those associated with a specific service.



Call forwarding. This SS forwards all incoming calls, or only those associated with a specific basic service, to another directory number. The forwarding may be unconditional or may be performed when the mobile subscriber is busy, when there is no reply, when the mobile subscriber is not reachable, or when there is radio congestion.



















Closed user group. This SS allows a group of subscribers to communicate only among themselves.



Calling number identification presentation/restriction. This SS permits the presentation or restricts the presentation of o f the calling party‟s identification number (or additional address information).



Connected number identification presentation. This SS indicates the phone number that has been reached.



Free phone service. This SS allocates a number to a mobile subscriber, and all calls to that number are free of charge for the calling party.



Malicious call identification. This SS permits the registration of malicious, nuisance, and obscene incoming calls.



Three-party service. This SS permits the establishment of conference calls.

















CHAPTER-II

GSM Network Architecture and GSM Channels 2.1 GSM Network Architecture

GSM network architecture is divided into following three categories: 

Mobile Station (MS): This consists of the mobile telephone, fax machine etc. This is the part of the network that the subscriber will see.



Base Station Subsystem (BSS): This is the part of the network which provides the radio interconnection from the MS to the land-based switching equipment.



Network Switching Subsystem (NSS): This consists of the Mobile services Switching Centre (MSC) and its associated system-control databases and processors together with the required interfaces. This is the part which provides for interconnection between the GSM network and the Public Switched Telephone Network (PSTN).



Network Management System: This enables the network provider to configure and maintain the network from a central location.

2.1.1

Mobile Station (MS)

In GSM there is a difference between the physical equipment and the subscription. The mobile station is piece of equipment, which can be vehicle installed, portable or hand-held. In GSM there is a small unit called the Subscriber Identity Module (SIM), which is a separate physical entity e.g. an IC-card, also called a smart card. SIM and the mobile equipment together make up the mobile station. Without SIM, the MS cannot get access to the GSM

































2.1.2

Base Station Subsystem(BSS)

The GSM Base Station System is the equipment located at a cell site. It comprises combination of digital and RF equipment. The BSS provides the link between the MS and the MSC. BSS communicates with the MS over the digital air interface and with the MSC via 2 Mbit/s links.





















Base Station Controller (BSC): The BSC as its name implies provides the control for the

BSS. The BSC communicates directly with the MSC. The BSC may control single or multiple BTSs. 

Transcoder (TC): The Transcoder is used to compact the signals from the MS so that they

are more efficiently sent over the terrestrial interfaces. Although the transcoder is considered to be a part of the BSS, it is very often located closer to the MSC. The transcoder is used to reduce the rate at which the traffic (voice/data) is transmitted over the air interface. Although the transcoder is part of the BSS, it is often found physically closer to the NSS to allow more efficient use of the terrestrial links.



BSS Configurations



















BSC may control several BTSs, the maximum number of BTSs which may be controlled by one BSC is not specified by GSM. BTSs and BSC may be either located at the same cell site “co“co-located”, or located at different sites “Remote”. In reality most BTSs will be remote, as there are many more more BTSs than BSCs in a network. Another BSS configuration is the “Daisy Chain”. A BTS need not to communicate directly with the BSC which controls it, it can be connected to the BTS rather than all the way to BSC. Problems may arise when chaining BTSs, due to the transmission delay through the chain, the length of the chain therefore should be kept sufficiently short to prevent the round trip speech delay becoming too long. 2.1.2.1 Base Transceiver Station

BTS is the hardware component in the GSM architecture. It provides an interface between Mobile Station (MS) and Base Station Subsystem (BSS). The BTS provides radio channels (RF carriers) for a specific RF coverage area. The radio channel is the communication link between the MSs within an RF coverage area and the BSS. The BTS also has a limited amount of control functionality which reduces the amount of traffic between the BTS and BSC.





















Transfers signaling information to and from different mobile stations (MS).

2.1.2.3 Transcoder

Transcoder is required to convert the speech or data output from MSC(64 kbit/s PCM) into the form specified by GSM specifications for the transmission over the air interface, that is, between BSS and MS (64 kbit/s to 16 kbit/s and vice versa).The 64 kbit/s Pulse Code Modulation (PCM) circuits from MSC, if transmitted on the air interface without modification, would occupy an excessive amount of radio bandwidth. This would use the available radio spectrum inefficiently. The required is therefore reduced by processing the 64 kbit/s circuits so that the amount of information required to transmit digitized voice falls to a gross rate of 16 kbit/s. The transcoding function may be located at the MSC, BSC or BTS. 2.1.3 Network Switching Subsystem

The Network Switching System includes the main switching functions of the GSM network. It also contains the databases required for subscriber data and mobility management. Its main function

is

to

manage

communications

between

the

GSM

network

and

other

telecommunications networks. Various components of the Network Switching System are



















their activities are inextricably linked with those of the switch as it connects speech and data calls to and from the MSs. 2.1.3.1 Mobile Switching Centre

MSC is included in the GSM operation for call-switching. Its overall purpose is similar to that of any telephone exchange. MSC carries out several different functions depending upon its position in the network. When MSC provides interface between the PSTN and the BSSs BSSs in the GSM network, it will be known as a Gateway MSC. In this position it will provide the switching required for all MS originated or terminated traffic. Functions carried out by MSC are written below: Call Processing Includes control of data/voice call setup, inter-BSS and inter-MSC handovers and control of



mobility management (subscriber validation and location). 

Operations and Maintenance Support

– machine Includes database management, traffic metering and measurement, and a man man – machine interface.



















2.1.3.3 Authentication Centre

The Authentication Centre provides security information to the network, so that we can verify the SIM cards (authentication between the mobile station and VLR and cipher the information transmitted in the air interface between the mobile station and Base Transceiver Station (BTS)). The Authentication Centre supports the VLR‟s work by issuing so so-called -called authentication triplets upon request. . It will normally be co-located with the Home Location Register (HLR) as it will be required to continuously access and update, as necessary, the system subscriber records. The AUC/HLR centre can be co-located with the MSC or located remote from the MSC. The authentication process will usually take place each time the subscriber “initializes” on the system. 2.1.3.4 Equipment Identity Register

Similar to Authentication Control, the Equipment Identity Register is used for security reasons. But while the Authentication Control provides information for verifying the SIM cards, the EIR is responsible for checking IMEI (checking the validity of the mobile equipment). When performed, the mobile station is requested to provide the International Mobile Equipment Identity (IMEI) number. This number consists of type approval code,





















2.1.3.5 Visitor Location Register (VLR)

VLR is a database which contains information about the subscribers currently being in the service area of the MSC/VLR, such as: 

Identification numbers of the subscribers.



Security information for authentication of the SIM card and for pipelining.



Services that the subscriber can use.

The VLR carries out location registrations and updates. It means that when a mobile station comes to a new MSC/VLR serving area, it must register itself in the VLR, in other words perform a location update. A mobile station must always be registered in a VLR in order to





















2.1.3.7 Echo Canceller (EC) An EC is used on the PSTN side of the MSC for all voice circuits. Echo control is required at

the switch because the inherent GSM system delay can cause an unacceptable echo condition, even on short distance PSTN circuit connections. The total round trip delay introduced by the GSM system (the cumulative delay caused by call processing, speech encoding and decoding etc.) is approximately 180 ms. This might not be apparent to the MS subscriber, but for the inclusion of a 2-wire to 4-wire hybrid transformer in the circuit. This is required at the land party‟s local switch because the standard telephone connection is 2-wire. The transformer causes the echo. This does not affect the land subscriber.





















2.1.4.1 Network Management Center





















The OMC should support the following functions as per ITS-TS recommendations: 

Fault management



Configuration management



Performance management

These functions cover the whole of the GSM network elements from the level of individual BTSs, up to MSCs and HLRs. 

Fault Management

The purpose of fault management is to ensure smooth operation of the network and rapid correction of any kind of problems that are detected. Fault management provides network























Abis: This is the internal interface linking the BSC and a BTS, and it has not been

standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS. 

A: The A interface is between the BSS and the MSC. The A interface manages the

allocation of suitable radio resources to the MS‟s and mobility management. 

B: The B interface between the MSC and the VLR uses the MAP/B protocol. Most

MSC‟s are associated with a VLR, making the B interface "internal". Whenever the Whenever the MSC needs access to data regarding an MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.













































2.3.1 Physical Channels

A single GSM RF carrier can support up to eight MS subscribers simultaneously. The diagram opposite shows how this is accomplished. Each channel occupies the carrier for one eighth of

























Full-Rate Data Channel for 9600 bps (TCH/F9.6)

The full-rate traffic data channel carries user data which is sent at 9600 bps.With additional forward error correction coding applied by the GSM standard, the 9600 bps























2.3.2.2.1 Broadcast Channels (BCH)

The broadcast channel operates on the forward link of ARCN within the cell and transmits data only in the first time slot (TS 0) of certain GSM frames. There are three types of BCH as

























Access Grant Channel (AGCH)

The AGCH is used by the base station to provide forward link communication to the mobile to operate in a particular physical channel with a dedicated control channel. The

















































































































































































































































































































































































































































































































4.3.7Power Supply Unit

Fig. 4.7 Power Supply Unit

PSUs rectify or convert incoming power to the regulated +24 V DC system power and are available in two versions: AC (200.250 V) and DC (−48 V). This means the RBS can be powered directly from the mains or from an existing −48 V DC power supply. No PSU is required when the RBS is powered from a +24 V DC power supply. PSUs are connected in parallel at the secondary side and can be configured with N+1 redundancy. When using a battery backup system, an additional PSU is recommended to reduce batterycharging times. An unused PSU can of course also be used for this purpose. Number of units: 0-3 Four configuration types are available to choose from depending on the way the transmitter signals are combined: combined, uncombined, mixed (combined and uncombined), and TCC/4WRD. 

Combined mode means that the GSM carriers are subject to hybrid combining inside

the DRU. The output power is 3.5 dB lower than in uncombined mode. The advantage is

















urban areas. Combined capacity is added in steps of two carriers per sector. Configurations based on combined mode are sometimes referred to as capacity mode. Uncombined mode means that the GSM carriers are not subject to any combining. The



output power is 3.5 dB higher than in combined mode. The drawback is that uncombined mode requires more feeders and antennas than combined mode, which is why it is normally used for coverage in rural, road, and suburban environments. Uncombined capacity is added in steps of two carriers that can be in either one or two sectors. Configurations based on uncombined mode are sometimes referred to as coverage mode.

TCC/4WRD mode means that two GSM carriers are coherently combined in one GSM



carrier that has almost double the output power of uncombined mode. To achieve a balanced link budget, 4WRD can be used on the uplink together with TMAs, resulting in uplink improvements of 3.6 dB. Configurations based on TCC/4WRD are sometimes referred to as supreme coverage mode. To achieve a balanced link budget, bear in mind the following: 

DRUs provide only one GSM carrier instead of two.



The minimum configuration is two DRUs per cell because four RX paths are needed . using only TCC requires only one DRU.



The minimum antenna system is four feeders and four antenna ports per sector (TCC can be used with 2WRD and only two antennas per sector).





Using 4WRD requires using TMAs.

Mixed mode uses a combination of combined and uncombined modes, as described

above. One DRU can e.g. be used for two sectors, resulting in one uncombined carrier in each of the sectors. The advantage of smart range is that it allows smaller capacity expansions, in steps of two carriers per sector, than in combined mode alone. A sector

















PROCEDURE 1 ACTIVITY: Site survey for BTS, M/W & GSM Installation. STEPS: 

Draw a layout for shelter with present status with labeled space measurement.



Check BTS space availability



Check M/W Antenna height availability



Roxtec entry space for new feeders



Power availability i.e (MCB free or not) if free, note it rating



MMU space is available at transmission rack or not



Feeder Routing space at inner and outer cable ladder



Measure feeder cable ,if cable, and power cable length



Space at EGB IGB



Owner issue



In case of MBC , check type of antenna present



Pole mount is present at given GSM antenna height or not



Make a report and shelter layout

















PROCEDURE 2 ACTIVITY: Installation of a site STEPS: 

Install indoor cable tray, put four l angles to the four edges and fix cable tray with wall mounted channels at the both sides. Connect both trays at both end and door side and rear wall.



Distance 100m from sidewall and 100m below roxtec.



Install Tx rack and BTS cabinet.



– n 150m from sidewall and 30m-50m from backside wall. Tx rack –



Mark holes with marker , drill put fastener, place rack and tight nuts.



Put two l supports on the top of Tx rack, similarly place BTS.



Install DDF as per customer plan.



Connect DDF cable to BTS equipment.



Install the battery bank as per layout plan. Make sure all inter connected strips are tightend.



Place SMPS on top of battery bank and tight it with nut bolts.



Install cable cut for battery bank and SMPS cabling. Connect the RBS power supply cable to the RBs and route it to the SMPS and connect ot to the load port and neutral bar. Then connect the battery i/p cable tray of battery end.



Connect the ac cable from PIU to the SMPS in the MCB.



Route alarm cable from SMPS to DDU and connect according to the need. Route power cable from SMPS to Tx rack.



Do the earthing of each equipments as per points. All to be labeled with printed sticker.



Place krone block 4 inch downward DDU and AMM at 1.5 foot top from Tx rack.



Install fan unit, attach with AMM and connect power cable from DDU. Insert MMU at AMM a lot and give+24v dc.



















Set the degree with the help of compass and set tilt.



Cut the feeder cable and make feeder connectors at both end .tape them properly and route the feeder on VCT and hct from antenna to BTS with the help of clamps.



Provide a proper drip loop near the Roxtec and there should not be any bend less than 120 degree.



Do the feeder earthing near the 1m above the VCT and near the Roxtec.



Similarly place the m/w antenna at near and far end and check its polarization, degree, height.



Route the IF cable from m/w to MMU. Make a loop of 3m if cable and tie it near the m/w for further extension.





Do if earthing same as feeder cable. Now go for alignment.



Turn on the MMU at both ends and feed the frequency, power, RSl at both ends.



Place the multimeter at m/w both ends.



At far end go for horizontal alignment and then for vertical alignment and lock the position with max value.



Repeat the same at near end till the desired value is achieved.



Check for all punch points, if clear go for commissioning.



















REFERENCES

GSM and CDMA Report

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