Tuesday, September 28, 2004



1. School of Engineering & Technology, Bharathidasan University

2. J.J. College of Engineering & Technology, Trichy.

In 1989, Global System for Mobile communication’s (GSM) responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and Phase-I of the GSM specifications were published in 1990. Commercial service of GSM service was started in mid 1991.

GSM uses a variation of Time Division Multiple Access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA,GSM and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. GSM is a form of multiplexing, which divides the available bandwidth among the different channels. The digital nature of GSM allows, both synchronous and asynchronous data to be transported as a bearer service to or from an ISDN terminal. The data rates supported by GSM are 300bps, 600bps, 1200bps, 2400bps and 9600 bps. The most basic teleservice supported by GSM is telephony. A unique feature of GSM compared to older analog system is the Short Message Service (SMS).

Services like mobile banking, ticket booking, info services are today exclusively available on GSM (TDMA) networks only. A GSM (TDMA) mobile has a Subscriber Identity Module (SIM) card, which provides more functionality and is convenient (eg, change your phone, but keep your phone numbers and settings). Above all, you can take a GSM (TDMA) phone to virtually anywhere in the world and keep talking. Though GSM (TDMA) will not accommodate more than a finite number of users (the user will get the Network Busy message, if this number is exceeded) there won’t be any deterioration in voice quality due to traffic. In addition GSM (TDMA) is also equipped with Frequency-Hopping i.e., when a lower frequency is cluttered, the mobile phone effortlessly jumps to a higher frequency (eg, 900 MHz to 1800 MHz). GSM (TDMA) technologies also employ the Enhanced Frequency Rate (EFR) add-on, which improves the voice quality greatly.

Code Division Multiple Access (CDMA), is a digital wireless technology that was pioneered and developed by Qualcomm. CDMA commercially introduced in 1995. Code Division Multiple Access (CDMA) is “spread spectrum’ technology which means that it spreads the information contained in particular signal of interest, over a much greater bandwidth than the original signal. A CDMA call starts with a rate of 9600 bits per second (9.6 kilobits per second). This is then spread to a transmitted rate of about 1.23 Megabits per second. Spreading means that digital codes are applied to the data bits associated with users in a cell. The goal of the spread spectrum is a substantial increase in bandwidth of an information-bearing signal, far beyond that needed for basic communication. CDMA uses a unique code to distinguish each different call, which enables many more people to share the airways at the same time without static, cross-talk or interference.

CDMA is a form of multiplexing (access to the same resource will be given to more than one user), which allows the use of a particular frequency for a number of signals, optimizing the use of available bandwidth.

With CDMA, unique digital codes, rather than separate Radio Frequencies (RF) or channels are used to differentiate subscribers. The codes are shared by both mobile station (cellular phone) and the base station (The base station connects the mobile to the Mobile Switching Centre [MSC]), and are called pseudo-random code frequencies. Since each user is separated by a unique code, all users can share the same frequency band (range of radio spectrum). This gives many unique advantages to the CDMA technique over other Radio Frequency (RF) technique in cellular communication.

CDMA is typically done through Wireless in Local Loop (WiLL / WLL) systems where only the last mile is wireless (instead of copper wires). In CDMA, you are stuck with one WLL mobile operator, because the WLL handset is programmed and locked to work with the service provider who sells it to you. It does not have a SIM card. It is expensive and time-consuming to reprogram the handset in case you want to change your service provider. In CDMA, unfortunately your handset being stolen/ lost/ damaged, you cannot be reconnected immediately. You would have to procure a new handset, get it reprogrammed. Since, CDMA is a new technology, the network is not setup to provide as many facilities as GSM(TDMA). CDMA is being the standard for mobile communication in very few countries, it cannot offer international roaming, which is a large disadvantage. CDMA technology has a Soft Accommodation feature, i.e., when the number of users of the network goes up, the voice quality progressively gets poorer.

In GSM (TDMA) technology’s talk-range from a tower is 35 kms in comparison with CMDA’s 110 kms, and the power output of a GMS (TDMA) phone is 2W in comparison with CDMA phone’s 200mW, i.e., CDMA implies lesser radiation hazard. For a city like Chennai (typical Indian city), the GSM grids requires approximately 130 base stations to cover the city. WiLL/ WLL requires only about 10 base stations in Chennai as they beam all frequencies received by everyone. Because CDMA uses the entire frequency spectrum and hence can have the broadcast happen at very high signal strength spread over the radius.

The number of channels (users) that can be allocated in a given bandwidth is comparatively higher for CDMA than for GSM. The cost of setting up a CDMA network is also comparatively less than the GSM network. Due to these advantages, there is high probability that CDMA technology will dominate the future of mobile communication.

The following parameters may help you to take a decision based on the importance of the following in your usage pattern. (1)Mobility (i.e.,between different circles), (2) Short Message Servicing, (3) Frequency of handset change (or the need to change handset) and (4) Your expenses on mobile. Incase you are a heavy user on first three parameters, then go for GSM and wait for CDMA to grow up. If the last parameter dominates your mobile life and compromise on the first three you can go for CDMA.


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