Differences between
the GSM and CDMA Wireless Networks
PIR IDREES AZAD
Table Of Contents
Differences between
the GSM and CDMA Wireless Networks
Abstract
Introduction
The Mobile Station
The Base Transceiver Station
The Base Station Controller
The Mobile Switching Center
The Location Registers
Historical View of GSM and CDMA
Classification of CDMA
Table 2 – CDMA Era
Comparison of Technologies
Frequency Division Multiple Access (FDMA):
Time Division to Multiple Access (TDMA):
Code Division Multiple Access (CDMA):
Network Architecture
Mobile Station:
Cell Design
Base Station Sub-System (BSS):
Radio Interface Differences
Uplink and Downlink differences:
Logical Channel differences
Call Processing
Evolution to 3G
Conclusion:
Differences between
the GSM and CDMA Wireless Networks
Abstract
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.
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
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 MobileSwitchingCenter 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
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 interfaceGSM Phase 2+
8 channels per carrierAdaptive multirate coder
200 – KHz carrier bandwidth14.4 Kbp data service
Slow frequency hoppingGeneral 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 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
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
Table 2 – CDMA Era
Pioneer Era
1949 / John Pierce : time hopping spread spectrum1949 / 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 spectrum1980s / Investigation of narrowband CDMA techniques for cellular applications
1986 / Formulation of optimum multiuser detection by Verdu
1993 / IS-95 standard
Wideband CDMA Era
1995 - / Europe : FRAMES FMA2Japan : Core-A
USA : cdma2000
Korea : TTA I, TTA II
2000s / Commercialization of wideband CDMA systemsTable 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.
Table 3 – Second Generation Digital Systems
GSM / IS-136 / IS-95 / PDCMultiple access / TDMA / TDMA / CDMA / TDMA
Modulation / GMSKa / /4-DQPSKb
Coherent /4-
DQPSK
Coherent 8-PSK / QPSK/0-QPSKc / /4-DQPSK
Carrier spacing / 200 kHz / 30 kHz / 1.25 MHz / 25 kHz
Carrier 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 ms
Slots per frame / 8/16 / 6 / 1 / 3/6
Frequency band (uplink/
downlink)
(MHz) / 880-915 / 935-960
1720-1785 /
1805-1880
1930-1990 /
1850-1910 / 824-849 / 869-894
1930-1990 /
1850-1910 / 824-849/869-894
1930-1990 /
1850-1910 / 810-826 /
940-956
1429-1453/
1477-1501
Speech codec / RPE-LTPe 13 Kbps
Half rate 6.5 Kbps
Enhanced full rate
(EFR) 12.2 kbps / VSELPf 8 Kbps
IS-641-A: 7.4 Kbps
(ACELP)g
US1: 12.2 Kbps
(ACELP) / QCELP 8 Kbps
CELP 8 Kbps
CELP 13 Kbps / VCELP
6.7 Kbps
Maximum possible data rate / HSCSD:115.2 Kbps
GPRS : 115.2 –
182.4 Kbps
(depending on the coding) / IS-136+: 43.2 Kbps / IS95A:14.4 Kbps
IS95B:115.2 Kbps / 28.8 Kbps
Frequency hopping / Yes / No / N/A / No
Handover / Hard / Hard / Soft / Hard
a Gaussian minimum shift keying
b Differential quadrature phase shift keying
c Offset QPSK
d A “chip” is used to denote a spread symbol in DS-CDMA systems
e Regular pulse excited long term prediction
f Vector sum excited linear predictive
g Algebraic code excited linear predictive
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.
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:
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.
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.
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.
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 215- (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.
Long Code
There 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 differences
Both 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.