Wireless Communication GSM and CDMA Wireless Networks

Ravi S

Table Of ContentsDifferences between............................................................................................................3the GSM and CDMA Wireless Networks...........................................................................3

Abstract............................................................................................................................3

Introduction......................................................................................................................3The Mobile Station..........................................................................................................5The Base Transceiver Station..........................................................................................5The Base Station Controller.............................................................................................5The Mobile Switching Center..........................................................................................5The Location Registers....................................................................................................5Historical View of GSM and CDMA..............................................................................6Classification of CDMA..................................................................................................7

Table 2 – CDMA Era...................................................................................................8Comparison of Technologies.........................................................................................10

Frequency Division Multiple Access (FDMA):........................................................11Time Division to Multiple Access (TDMA):............................................................11Code Division Multiple Access (CDMA):................................................................11

Network Architecture....................................................................................................11Mobile Station:..........................................................................................................13Cell Design................................................................................................................14Base Station Sub-System (BSS):...............................................................................15

Radio Interface Differences...........................................................................................15Uplink and Downlink differences:.............................................................................15

Logical Channel differences..........................................................................................17Call Processing..............................................................................................................19Evolution to 3G..............................................................................................................20Conclusion:....................................................................................................................23

Differences between

the GSM and CDMA Wireless Networks

Abstract

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GSM and CDMA have been the two leading commercial wireless technologies that are being used all over the world. This paper presents to the readers the key differences between the two technologies1. The various topics in which this paper presents the difference are:

Radio Spectrum Usage Network architecture differences Radio channel differences Call Processing Evolution to 3G Network capacity differences Deployment

Introduction

This section presents the basic wireless network architecture and lays the foundation for the readers to understand the later sections of this paper.Though this paper concentrates on the differences between these networks, but the basic network architecture for both these networks is same.The diagram below presents the general architecture of a wireless network.

1: This paper concentrates mostly on the differences in the BSS.

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Figure 1: General Architecture of Wireless Networks

The Mobile Station

The Mobile Station (MS) is the user equipment in Wireless Networks.. Production of Mobile Stations is done by many different manufacturers, and there will almost always be a wide range of different Mobile Stations in a mobile network. Therefore the specifications specify the workings of the MS in great detail.

The Base Transceiver Station

The Base Transceiver Station (BTS) is the entity corresponding to one site communicating with the Mobile Stations. Usually, the BTS will have an antenna with several TRXs (radio transceivers) that each communicates on radio frequency. The link-level signaling on the radio-channels is interpreted in the BTS, whereas most of the higher-level signaling is forwarded to the BSC and MSC

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The Base Station Controller

Each Base Station Controller (BSC) control the magnitude of several hundred BTSs. The BSC takes care of a number of different procedures regarding call setup, location update and handover for each MS. The handover control procedures will come especially into focus in this thesis. It is the BSC that decides when handover is necessary. This is accomplished by analyzing the measurement results that are sent from the MS during a call and ordering the MS to perform handover if this is necessary. The continuous analyzing of measurements from many MSs requires considerable computational power. This put strong constraints on the design of the BSC.

The Mobile Switching Center

The Mobile Switching Center is a normal ISDN-switch with extended functionality to handle mobile subscribers. The basic function of the MSC is to switch speech and data connections between BSCs, other MSCs, other Wireless networks and external non-mobile-networks. The MSC also handles a number of functions associated with mobile subscribers, among others registration, location updating and handover. There will normally exist only a few BSCs per MSC, due to the large number of BTSs connected to the BSC. The MSC and BSCs are connected via the highly standardized A-interface. However, due to the lack of standardization on Operation and Management protocols, network providers usually choose BSCs, MSCs and Location Registers from one manufacturer.

The Location Registers

With each MSC, there is associated a Visitors Location Register (VLR). The VLR can be associated with one or several MSCs. The VLR stores data about all customers who are roaming withing the location area of that MSC. This data is updated with the location update procedure initiated from the MS through the MSC, or directly from the subscriber Home Location Register (HLR). The HLR is the home register of the subscriber. Subscription information, allowed services, authentication information and localization of the subscriber are at all times stored in the HLR. This information may be obtained by the VLR/MSC when necessary. When the subscriber roams into the location area of another VLR/MSC, the HLR is updated. At mobile terminated calls, the HLR is interrogated to find which MSC the MS is registered with. Because the HLR is a centralized database that need to be accessed during every call setup and data transmission in the GSM network, this entity need to have a very large data transmission capacity suggests a scheme for distributing the data in the HLR in order to reduce the load.

The communication between MSC, VLR and HLR is done using the MAP (Mobile Application Part) of the Signalling System 7. The MAP is defined in and will be further discussed in

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Historical View of GSM and CDMA

GSM

The first step towards GSM was the allocation of a common frequency band in 1978, twice 25 MHz, at around 900 MHz for mobile communication in Europe. In 1990, the GSM specifications for 900 MHz were frozen. In 1990 it was decided that GSM 1800

GSM radio interface GSM Phase 2+8 channels per carrier Adaptive multirate coder200 – KHz carrier bandwidth 14.4 Kbp data serviceSlow frequency hopping General pocket radio service

Enhanced data rates using optimised modulation (EDGE)

Table 1 shows the time schedule of GSM.

Table 1 – GSM Development Time Schedule

1982 Groupe Special Mobile established within CEPT1984 Several proposals for GSM multiple access : wideband TDMA,

narrowband TDMA, DS-CDMA, hybrid CDMA/FDMA, narrowband FDMA

1986 Eight prototype systems tested in CNET laboratories in France Permanent nucleus is set up

1987 Basic transmission principles selected : 8-slot TDMA, 200-kHz carrier spacing, frequency hopping

1987 MoU signed1988 GSM becomes an ETSI technical committee1990 GSM phase 1 specifications frozen (drafted 1987 – 1990)

GSM1800 standardisation begins1991 GSM1800 specifications are frozen1992 GSM900 commercial operation starts1992 GSM phase 2+ development starts1995 GSM submitted as a PCS technology candidate to the United States1995 PCS1900 standard adopted in the United States1996 Enhanced full rate (EFR) speech codec standard ready1996 14.4-Kbps standard ready

GSM1900 commercial operation starts1997 HSCSD standard ready

GSM cordless system (home base station) standardisation startedEDGE standardisation started

1998 GPRS standard readyWCDMA selected as the third generation air interface

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Classification of CDMA

i) based on the modulation method

CDMA : direct sequence (DS)CDMA : frequency hopping (FH)CDMA : time hopping (TH)

Frequency

Time

[1] In DS-CDMA, spectrum is spread by multiplying the information signal with a pseudo-noise sequence, resulting in a wideband signal.

[2] In FH-CDMA. In the frequency hopping spread spectrum, a pseudo-noise sequence defines the instantaneous transmission frequency. The bandwidth at each moment is small, but the total bandwidth over, for example, a symbol period is large. Frequency hopping can either be fast (several hops over one symbol) or slow (several symbols transmitted during one hop).

[3] In TH-CDMA, in the time hopping spread spectrum, a pseudo-noise sequence defines the transmission moment.

CDMA era, as shown in table 2

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Direct sequence

Frequency hoppingTime hopping

Table 2 – CDMA Era

Pioneer Era1949 John Pierce : time hopping spread spectrum1949 Claude Shannon and Robert Pierce : basic ideas of CDMA1950 De Rosa-Rogoff : direct sequence spread spectrum1956 Price and Green : antimultipath “RAKE” patent1961 Magnuski : near-far problem1970s Several developments for military field and navigation systems

Narrowband CDMA Era1978 Cooper and Nettleton : cellular application of spread spectrum1980s Investigation of narrowband CDMA techniques for cellular applications1986 Formulation of optimum multiuser detection by Verdu1993 IS-95 standard

Wideband CDMA Era1995 - Europe : FRAMES FMA2

Japan : Core-AUSA : cdma2000Korea : TTA I, TTA II

2000s Commercialization of wideband CDMA systems

Table 3 shows the technical parameters of second generation systems. All these systems are frequency division duplex (FDD) systems. They transmit and receive in different frequency bands. Time division duplex (TDD). The actual data rate available in commercial systems is usually much smaller. In 1998 GSM supports 14.4 Kbps, IS-95 9.6 Kbps, IS-136 9.6Kbps and PDC 9.6 Kbps.

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WCDMA

Table 3 – Second Generation Digital Systems

GSM IS-136 IS-95 PDCMultiple access TDMA TDMA CDMA TDMAModulation GMSKa /4-DQPSKb

Coherent /4-DQPSKCoherent 8-PSK

QPSK/0-QPSKc /4-DQPSK

Carrier spacing 200 kHz 30 kHz 1.25 MHz 25 kHzCarrier bit rate 270.833 Kbps 48.6 Kbps (/4-PSK

and /4-DQPSK) 72.9 Kbps (8-PSK)

1.2288 Mchip/sd 42 Kbps

Frame length 4.615 ms 40 ms 20 ms 20 msSlots per frame 8/16 6 1 3/6Frequency band (uplink/downlink) (MHz)

880-915 / 935-9601720-1785 /1805-18801930-1990 /1850-1910

824-849 / 869-8941930-1990 /1850-1910

824-849/869-8941930-1990 /1850-1910

810-826 /940-9561429-1453/1477-1501

Speech codec RPE-LTPe 13 KbpsHalf rate 6.5 KbpsEnhanced full rate(EFR) 12.2 kbps

VSELPf 8 KbpsIS-641-A: 7.4 Kbps(ACELP)g

US1: 12.2 Kbps(ACELP)

QCELP 8 KbpsCELP 8 KbpsCELP 13 Kbps

VCELP6.7 Kbps

Maximum possible data rate

HSCSD:115.2 KbpsGPRS : 115.2 –182.4 Kbps(depending on the coding)

IS-136+: 43.2 Kbps IS95A:14.4 KbpsIS95B:115.2 Kbps

28.8 Kbps

Frequency hopping

Yes No N/A No

Handover Hard Hard Soft Hard

a Gaussian minimum shift keyingb Differential quadrature phase shift keyingc Offset QPSKd A “chip” is used to denote a spread symbol in DS-CDMA systemse Regular pulse excited long term predictionf Vector sum excited linear predictiveg Algebraic code excited linear predictive

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Comparison of Technologies

Frequency Division Multiple Access (FDMA):

The frequency spectrum is divided into number of narrow band channels. These channels are assigned to users. Therefore, users transmit in their assigned frequency range. This is the assigned dynamically. The frequency range can be reassigned once the call is completed. The frequency assigned serves as channel identifier.

Time Division to Multiple Access (TDMA):

As in FDMA, TDMA divides the spectrum into narrow band channels. However, in TDMA, the same channel is assigned to multiple users. The available time is divided into a number of time slots. These slots are assigned to users sharing the same channel. Thus, TDMA provides more spectral efficiency than FDMA. The capacity is increased N times, where N is the number of timeslots within in a channel. Thus, N users can be accommodated in a channel. The frequency assignment, along with the assigned time slot, serves as a channel identifier. This technology is used in GSM.

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Code Division Multiple Access (CDMA):

In CDMA, all users share the wideband spectrum. Each user is spread with a pseudo-random binary sequence. The wide band frequency assignment (common to all users) along with a pseudo-random sequence serves as the channel identifier.

Network Architecture

This section presents the differences between the GSM and CDMA network architectures.

The diagram below shows the GSM network architecture:

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The diagram below shows the IS-95 based CDMA network architecture:

Mobile Station:

GSM:The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

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CDMA:One of the biggest drawbacks of the CDMA mobile stations is the absence of the SIM card. As a result of this, a user’s identity is fixed to a handset.Electronic Serial Number (ESN) uniquely identifies the mobile equipment. ESN is a 32bit number assigned by the mobile station manufacturer.An IMSI and ESN are linked in the operator database to uniquely identify a subscriber.

Cell Design

In CDMA, the same 1.233 MHz wideband channel may be reused in all the cells. Therefore, adjacent cells may use the same frequency; thus the frequency reuse factor is 1. This greatly simplifies the frequency planning.On the other hand in GSM, the frequency assignments in one cell cannot be reused in adjacent cells. Hence, frequency assignments in each cell have to be carefully allocated to avoid interference from adjacent cells.

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Base Station Sub-System (BSS):

An important component of the BSS, which is considered in the canonical GSM architecture as part of the BTS is TRAU, or the Transcoder/Rate Adapter Unit. The TRAU is the equipment in which the GSM specific speech encoder and decoding is carried out, as well as the rate adaptation in the case of data. Although the GMS specifications consider the TRAU as part of the BTS, it can be sited away from the BTS and in many cases it is actually between the BSC and MSC. Having the TRAU as close to MSC saves a lot on the 64kbps link between the BSC and the MSC.

Where as in CDMA , the TRAU is called the Vocoders and they are considered as part of the BSC.

Another key difference in the BSS is that the CDMA BSS gets the time synchronization between the various Network elements using the GPS, where as in GSM is it controlled by the MSC and BSS interface.

Radio Interface Differences

The radio interface in the wireless systems provides the link between the fixed infrastructure of different operators and the mobile station of various manufacturers.

The radio interface serves two main functions: To transport user information, both speech and data – bi-directional. To exchange signaling information between the mobile station and the network.

Uplink and Downlink differences:

The radio link directed from the mobile station to the network is called the uplink. This is also referred to as the reverse link in CDMA networks.The radio link directed from network to the mobile station is called the downlink. This is referred to as the forward link in the CDMA networks.

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Channels are used in pair for full duplex communications. Thus, GSM uses both uplink and downlink bands of a given spectrum.In other words, a physical channel refers to a pair of frequencies used for a cellular radio talk path. One is used for the cell site to mobile transmission while the other is used for the mobile to the cell site transmission. GMS signal requires channels spacing of 200kHz.

In CDMA two types of PN codes are used for differentiating the forward and the reverse links.

Short Codes

These PN codes are generated with a register length of 15. The length of the code is 2 15-

(32,768) bits. Generated at the rate of 1.2288MHz, these codes repeat every 26.67 msec. Each base station generates a short code with a different offset that identifies the base station.

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Long CodeThere is only one long code, it is defined in the standard, and it is used by all IS-95 and cdma 2000 systems. The long PN code is generated with a register length of 42. Generated at the rate of 1.2288MHz, this code repeats in approximately in 41 days. In the reverse direction, the long code is used for spreading (mobile to the base station) and to uniquely identify each channel. When the mobile needs to uniquely identify itself or a channel using the long code, it applies a long code mask to the long code, which results in a time shifted version of the long code. The receiver applies the same mask to recover the data.

Logical Channel differencesBoth GSM and the CDMA networks have a lot of similarities in the way the logical channels are defined.

In brief both these networks have a Channel, which is used by the mobile to acquire the system. This is called the

Pilot channel in CDMA whereas it is called the FCCH in GSM. A channel used by the mobile to synchronize to the network. This is called Synch

channel in CDMA and in GSM it is called SCH. Channel to transmit the system wide information and also page the mobile for the

termination calls. This in GSM is achieved by two channels called BCCH and PCH, where as in CDMA a single Paging channel does this.

Traffic channels.

The diagrams below shows the logical channel structures of both CDMA and GSM networks.

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The major difference between the GSM and the CDMA logical channels is how they are identified. In GSM each logical channel is pre-assigned to a particular time slot and in CDMA they are identified by a pre-assigned Walsh code.And also in the traffic channel, during the call setup in a GMS the mobile is assigned to a time slot whereas in CDMA a particular Walsh code is assigned.

Call Processing

Both GSM and CDMA networks have similar call setup flows for the origination and the termination of calls and location management.But the major difference is in the CDMA networks, which has both hard handoff and soft handoff whereas GSM networks have only hard handoffs.

Another major difference is how both these networks handle the Near-Far effect.In GSM, during traffic a time slot is allocated for the mobile, when the mobile moves far away from the base station its round trip delay increases and the mobile tends to drift to another user time slot. To avoid this, time advance feature is used in GSM networks.Similarly in CDMA networks, within a cell, mobiles are different radial distances from the base station. If all the mobiles transmit at equal power, the level received at the base station differs from one mobile to another. Mobiles that are nearer are received at significantly high power than the mobiles that are farther away. Because the transmission loss is higher for mobiles farther from the base station, mobiles near to the base station can cause more interference to the mobiles. Introducing power control during the call in the CDMA networks solves this problem.

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Evolution to 3GThe diagram below shows the 3G evolution paths taken by each network.

Here is a brief summary changes for the evolution of each network.

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GSM to GPRS:New additions: Packet core network nodes – SGSN and GGSN.Modifications: BSC hardware and softwareNo Changes: Circuit core network (MSC, HLR, AuC), Air Interface (MS-BTS) and A-Interface (BSC-MSC)The diagram below shows a 2.5 G GSM – GPRS network.

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GSM /GPRS to UMTS:New additions: WCDMA Air Interface (UE-Node B), RAN Interfaces, Iub (Node B – RNC), IuR (RNC- RNC), CN Interface Iu (MSC- RNC & SGSN – RNC)Modifications: MSC and SGSN for Iu Interface.No Changes: Circuit core network (HLR, AuC), Packet Core Network (GGSN)

The diagram below shows the UMTS network.

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IS 95 to CDMA 2000:New additions: Packet core network (PDSN, AAA, HA/FA), New Interface R-P (PDSN – BSS)Modifications: Air Interface (MS-BSS), Network Interface (BSC- MSC)No Changes: Circuit core network (HLR, AC)

The diagram below shows the CDMA2000 network.

Conclusion:This paper tried to capture the technical differences between the world’s two biggest wireless networks – GSM and CDMA. From the practical deployment point of view GMS captured Europe, Asia and Africa where as CDMA has been deployed in the Americas and some parts of Asia like Japan and Korea.

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Reference:

http://www.arcx.com/sites/index.htm GSM Wireless Networks – Nortel Networks Training Division IS –95 Overview – Award Solutions www.gmsworld.com The GSM systems for Mobile Communications – Michel Mouly Introduction to 3G Mobile Communications – Juha Future Mobile networks 3G and beyond - Alan

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