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Wireless networks  gsm cdma Wireless networks gsm cdma Document Transcript

  • Wireless CommunicationGSM and CDMA Wireless Networks Ravi S
  • Table Of ContentsDifferences between ............................................................................................................3the GSM and CDMA Wireless Networks............................................................................3 Abstract............................................................................................................................3 Introduction......................................................................................................................3 The Mobile Station .........................................................................................................4 The Base Transceiver Station .........................................................................................4 The Base Station Controller ............................................................................................5 The Mobile Switching Center .........................................................................................5 The Location Registers ...................................................................................................5 Historical View of GSM and CDMA..............................................................................6 Classification of CDMA..................................................................................................7 Table 2 – CDMA Era...................................................................................................8 Comparison of Technologies.........................................................................................10 Frequency Division Multiple Access (FDMA):........................................................10 Time Division to Multiple Access (TDMA):.............................................................10 Code Division Multiple Access (CDMA):................................................................11 Network Architecture.....................................................................................................11 Mobile Station:...........................................................................................................12 Cell Design.................................................................................................................13 Base Station Sub-System (BSS):...............................................................................14 Radio Interface Differences...........................................................................................14 Uplink and Downlink differences:.............................................................................14 Logical Channel differences..........................................................................................16 Call Processing ..............................................................................................................18 Evolution to 3G..............................................................................................................19 Conclusion:....................................................................................................................22 2
  • Differences between the GSM and CDMA Wireless NetworksAbstractGSM and CDMA have been the two leading commercial wireless technologies that arebeing used all over the world. This paper presents to the readers the key differencesbetween the two technologies1. The various topics in which this paper presents thedifference are: • Radio Spectrum Usage • Network architecture differences • Radio channel differences • Call Processing • Evolution to 3G • Network capacity differences • DeploymentIntroductionThis section presents the basic wireless network architecture and lays the foundation forthe readers to understand the later sections of this paper.Though this paper concentrates on the differences between these networks, but the basicnetwork 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. 3
  • Figure 1: General Architecture of Wireless NetworksThe Mobile StationThe Mobile Station (MS) is the user equipment in Wireless Networks.. Production ofMobile Stations is done by many different manufacturers, and there will almost always bea wide range of different Mobile Stations in a mobile network. Therefore thespecifications specify the workings of the MS in great detail.The Base Transceiver StationThe Base Transceiver Station (BTS) is the entity corresponding to one sitecommunicating with the Mobile Stations. Usually, the BTS will have an antenna withseveral 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 thehigher-level signaling is forwarded to the BSC and MSC 4
  • The Base Station ControllerEach Base Station Controller (BSC) control the magnitude of several hundred BTSs. TheBSC takes care of a number of different procedures regarding call setup, location updateand handover for each MS. The handover control procedures will come especially intofocus in this thesis. It is the BSC that decides when handover is necessary. This isaccomplished by analyzing the measurement results that are sent from the MS during acall and ordering the MS to perform handover if this is necessary. The continuousanalyzing of measurements from many MSs requires considerable computational power.This put strong constraints on the design of the BSC.The Mobile Switching CenterThe Mobile Switching Center is a normal ISDN-switch with extended functionality tohandle mobile subscribers. The basic function of the MSC is to switch speech and dataconnections between BSCs, other MSCs, other Wireless networks and external non-mobile-networks. The MSC also handles a number of functions associated with mobilesubscribers, among others registration, location updating and handover. There willnormally exist only a few BSCs per MSC, due to the large number of BTSs connected tothe 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 onemanufacturer.The Location RegistersWith each MSC, there is associated a Visitors Location Register (VLR). The VLR can beassociated with one or several MSCs. The VLR stores data about all customers who areroaming withing the location area of that MSC. This data is updated with the locationupdate procedure initiated from the MS through the MSC, or directly from the subscriberHome Location Register (HLR). The HLR is the home register of the subscriber.Subscription information, allowed services, authentication information and localization ofthe subscriber are at all times stored in the HLR. This information may be obtained by theVLR/MSC when necessary. When the subscriber roams into the location area of anotherVLR/MSC, the HLR is updated. At mobile terminated calls, the HLR is interrogated tofind which MSC the MS is registered with. Because the HLR is a centralized databasethat need to be accessed during every call setup and data transmission in the GSMnetwork, this entity need to have a very large data transmission capacity suggests ascheme 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 (MobileApplication Part) of the Signalling System 7. The MAP is defined in and will be furtherdiscussed in 5
  • Historical View of GSM and CDMAGSMThe 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, theGSM 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 coder 200 – KHz carrier bandwidth 14.4 Kbp data service Slow 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 CEPT 1984 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 signed 1988 GSM becomes an ETSI technical committee 1990 GSM phase 1 specifications frozen (drafted 1987 – 1990) GSM1800 standardisation begins 1991 GSM1800 specifications are frozen 1992 GSM900 commercial operation starts 1992 GSM phase 2+ development starts 1995 GSM submitted as a PCS technology candidate to the United States 1995 PCS1900 standard adopted in the United States 1996 Enhanced full rate (EFR) speech codec standard ready 1996 14.4-Kbps standard ready GSM1900 commercial operation starts 1997 HSCSD standard ready GSM cordless system (home base station) standardisation started EDGE standardisation started 1998 GPRS standard ready WCDMA selected as the third generation air interface 6
  • Classification of CDMAi) based on the modulation method CDMA : direct sequence (DS) CDMA : frequency hopping (FH) CDMA : time hopping (TH) Frequency Direct sequence Frequency hopping Time hopping 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 7
  • Table 2 – CDMA EraPioneer Era 1949 John Pierce : time hopping spread spectrum 1949 Claude Shannon and Robert Pierce : basic ideas of CDMA 1950 De Rosa-Rogoff : direct sequence spread spectrum 1956 Price and Green : antimultipath “RAKE” patent 1961 Magnuski : near-far problem 1970s Several developments for military field and navigation systems Narrowband CDMA Era 1978 Cooper and Nettleton : cellular application of spread spectrum 1980s Investigation of narrowband CDMA techniques for cellular applications 1986 Formulation of optimum multiuser detection by Verdu 1993 IS-95 standardWideband CDMA Era 1995 - Europe : FRAMES FMA2 Japan : Core-A WCDMA USA : cdma2000 Korea : TTA I, TTA II 2000s Commercialization of wideband CDMA systemsTable 3 shows the technical parameters of second generation systems. All thesesystems are frequency division duplex (FDD) systems. They transmit and receivein different frequency bands. Time division duplex (TDD). The actual data rateavailable in commercial systems is usually much smaller. In 1998 GSM supports14.4 Kbps, IS-95 9.6 Kbps, IS-136 9.6Kbps and PDC 9.6 Kbps. 8
  • Table 3 – Second Generation Digital Systems GSM IS-136 IS-95 PDC Multiple access TDMA TDMA CDMA TDMA Modulation GMSKa π/4-DQPSKb QPSK/0-QPSKc π/4-DQPSK Coherent π/4- DQPSK Coherent 8-PSK Carrier spacing 200 kHz 30 kHz 1.25 MHz 25 kHz Carrier bit rate 270.833 Kbps 48.6 Kbps (π/4-PSK 1.2288 Mchip/sd 42 Kbps and π/4-DQPSK) 72.9 Kbps (8-PSK) Frame length 4.615 ms 40 ms 20 ms 20 ms Slots per frame 8/16 6 1 3/6 Frequency band 880-915 / 935-960 824-849 / 869-894 824-849/869-894 810-826 / (uplink/ 1720-1785 / 1930-1990 / 1930-1990 / 940-956 downlink) 1805-1880 1850-1910 1850-1910 1429-1453/ (MHz) 1930-1990 / 1477-1501 1850-1910 Speech codec RPE-LTPe 13 Kbps VSELPf 8 Kbps QCELP 8 Kbps VCELP Half rate 6.5 Kbps IS-641-A: 7.4 Kbps CELP 8 Kbps 6.7 Kbps Enhanced full rate (ACELP)g CELP 13 Kbps (EFR) 12.2 kbps US1: 12.2 Kbps (ACELP) Maximum HSCSD:115.2 Kbps IS-136+: 43.2 Kbps IS95A:14.4 Kbps 28.8 Kbps possible data GPRS : 115.2 – IS95B:115.2 Kbps rate 182.4 Kbps (depending on the coding) Frequency Yes No N/A No hopping Handover Hard Hard Soft Harda 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 9
  • Comparison of TechnologiesFrequency Division Multiple Access (FDMA):The frequency spectrum is divided into number of narrow band channels. These channelsare assigned to users. Therefore, users transmit in their assigned frequency range. This isthe assigned dynamically. The frequency range can be reassigned once the call iscompleted. 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, inTDMA, the same channel is assigned to multiple users. The available time is divided intoa 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 beaccommodated in a channel. The frequency assignment, along with the assigned timeslot, serves as a channel identifier. This technology is used in GSM. 10
  • 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 ArchitectureThis section presents the differences between the GSM and CDMA networkarchitectures.The diagram below shows the GSM network architecture: 11
  • 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 cardcalled the Subscriber Identity Module (SIM). The SIM provides personal mobility, so thatthe user can have access to subscribed services irrespective of a specific terminal. Byinserting the SIM card into another GSM terminal, the user is able to receive calls at thatterminal, make calls from that terminal, and receive other subscribed services.The mobile equipment is uniquely identified by the International Mobile EquipmentIdentity (IMEI). The SIM card contains the International Mobile Subscriber Identity(IMSI) used to identify the subscriber to the system, a secret key for authentication, andother information. The IMEI and the IMSI are independent, thereby allowing personalmobility. The SIM card may be protected against unauthorized use by a password orpersonal identity number. 12
  • CDMA:One of the biggest drawbacks of the CDMA mobile stations is the absence of the SIMcard. 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 a32bit number assigned by the mobile station manufacturer.An IMSI and ESN are linked in the operator database to uniquely identify a subscriber.Cell DesignIn 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 is1. This greatly simplifies the frequency planning.On the other hand in GSM, the frequency assignments in one cell cannot be reused inadjacent cells. Hence, frequency assignments in each cell have to be carefully allocated toavoid interference from adjacent cells. 13
  • Base Station Sub-System (BSS):An important component of the BSS, which is considered in the canonical GSMarchitecture as part of the BTS is TRAU, or the Transcoder/Rate Adapter Unit. TheTRAU is the equipment in which the GSM specific speech encoder and decoding iscarried out, as well as the rate adaptation in the case of data. Although the GMSspecifications consider the TRAU as part of the BTS, it can be sited away from the BTSand in many cases it is actually between the BSC and MSC. Having the TRAU as close toMSC 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 ofthe BSC.Another key difference in the BSS is that the CDMA BSS gets the time synchronizationbetween the various Network elements using the GPS, where as in GSM is it controlledby the MSC and BSS interface.Radio Interface DifferencesThe radio interface in the wireless systems provides the link between the fixedinfrastructure 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 isalso referred to as the reverse link in CDMA networks.The radio link directed from network to the mobile station is called the downlink. This isreferred to as the forward link in the CDMA networks. 14
  • Channels are used in pair for full duplex communications. Thus, GSM uses both uplinkand downlink bands of a given spectrum.In other words, a physical channel refers to a pair of frequencies used for a cellular radiotalk path. One is used for the cell site to mobile transmission while the other is used forthe 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 reverselinks.Short CodesThese 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 basestation. 15
  • Long CodeThere is only one long code, it is defined in the standard, and it is used by all IS-95 andcdma 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 thereverse direction, the long code is used for spreading (mobile to the base station) and touniquely identify each channel. When the mobile needs to uniquely identify itself or achannel using the long code, it applies a long code mask to the long code, which resultsin a time shifted version of the long code. The receiver applies the same mask to recoverthe data.Logical Channel differencesBoth GSM and the CDMA networks have a lot of similarities in the way the logicalchannels 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 GSMnetworks. 16
  • 17
  • The major difference between the GSM and the CDMA logical channels is how they areidentified. In GSM each logical channel is pre-assigned to a particular time slot and inCDMA 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 atime slot whereas in CDMA a particular Walsh code is assigned.Call ProcessingBoth GSM and CDMA networks have similar call setup flows for the origination and thetermination of calls and location management.But the major difference is in the CDMA networks, which has both hard handoff and softhandoff 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 faraway from the base station its round trip delay increases and the mobile tends to drift toanother 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 fromthe base station. If all the mobiles transmit at equal power, the level received at the basestation differs from one mobile to another. Mobiles that are nearer are received atsignificantly high power than the mobiles that are farther away. Because the transmissionloss is higher for mobiles farther from the base station, mobiles near to the base stationcan cause more interference to the mobiles. Introducing power control during the call inthe CDMA networks solves this problem. 18
  • 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. 19
  • 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. 20
  • 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. 21
  • 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 biggestwireless networks – GSM and CDMA. From the practical deployment point of viewGMS captured Europe, Asia and Africa where as CDMA has been deployed in theAmericas and some parts of Asia like Japan and Korea. 22
  • 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 23
  • 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 23
  • 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 23