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GSM Vs CDMA

          The battle between GSM and CDMA technologies in India is hooting up. While the initial tilt was towards the GSM with players like Bharti, Hutch, Spice, BPL, Essar and BSNL preferring it. The Tata and Reliance have now entered the market putting their bets on CDMA. GSM stands for Global System for Mobile Communications, and CDMA stands for Code Division Multiple Access.

Today, the two alternatives that face the mobile world is - GSM and CDMA technology. First of all, let’s comprehend the meaning and functionality aspects of these technologies.

          In common analogy, imagine a room full of people, all trying to carry on one-to-one conversation. In GSM (TDMA Time Division Multiple Access), each couple takes turns talking. They keep their turns short by saying only one sentence at a time. As there is never more than one person speaking in the room at any given moment, no one has to worry about being heard over the background din. In CDMA, each couple talk at the same time, but they all use a different language. Because none of them understand any language other than that of the individual to whom they are listening, the background din does not cause any real problems.

 Global Service for Mobile Communications (GSM)

GSM is a digital mobile telephone system that is widely used in Europe and other parts of the world. 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. It operates at either the 900 MHz or 1800 MHz frequency band.

 During the 80s, when the cellular system was introduced, each nation had its own system. As a result, a couple of problems arose. Firstly, the equipment was limited to operate only within the boundaries of a country and secondly, the market for mobile equipment was also limited. In order to overcome these problems, the conference of European Posts and Telecommunications (CEPT) formed the Group Special Mobile (GSM) to create a common European mobile telephone standard.

 The GSM later came to be known as Global Service for Mobile communications (GSM).At the end of 1997, GSM was made available in more than 100 countries. Today it has become the global standard in Europe and Asia. GSM operates in 900-MHz – 1800MHz. If one has to connect to the specific service provider in different countries, GSM-users simply need to switch Subscriber Identification Module (SIM) cards. As per GSM technology, a mobile unit logs on to the network after being switched on. The mobile unit tries to contact a nearby BTS, which then transmits all the frequencies of the neighboring BTS’ to the mobile unit, which identifies the frequency on which the reception is the best and passes on the information to the BTS.

 The BTS then transmits the information to the BSC which has the deciding power as to which BTS should the mobile unit be assigned.

 Code Division Multiple Access (CDMA) 

Developed by Qualcomm for the US Military, Code Division Multiple Access is a system that enables many users to share the same frequency band at the same time. CDMA is a spread spectrum technology whereby multiple users share the same time and frequency allocation in a given band. Each speech signal is modulated (spread) across an entire band. The respective receiver demodulates and interprets the signal using relevant code that is embedded in the signal. The final signal contains only the relevant conversation.

 The CDMA based mobile technology was introduced in India to provide WLL services as the GSM was the choice for full mobility. CDMA based WLL operators were given license of operation as fixed line operators. As CDMA can very well connect large geographical area, WLL operators were given the rights to provide "limited mobility" whereby a WLL operator could allow network coverage to the boundary of a Short Distance Charging Area (SDCA).

 Comparisons:

            While TDMA and FDMA are the types of multiplexing in GSM, CDMA works on spread spectrum. However the bandwidth available is same for both the technologies, which are 800, 1800 and 1900 MHz. Since the bandwidth is the major problem in the modern times the CDMA has a very clear advantage over the GSM in these terms. The number of channels (users) that can be allocated in a given bandwidth is comparatively higher for CDMA than for GSM.

 While GSM cannot add more than a fixed number of subscribers in a cell, the capacity of the system is not rigid in CDMA. The biggest advantage with GSM is its widespread network in Europe and Asia, whereas CDMA is predominant in US and South Korea. The user of GSM is enabled an easy international roaming. As far as SMS, gaming and internet is concerned, both GSM and CDMA score an equal point.

Proponents of CDMA claim high communication security, high carrier efficiency meaning that the network can serve more subscribers at a time, smaller phones, low power requirement, ease of operation for the network operators, and extended reach beneficial to rural areas. CDMA’s detractors say that due to its proprietary nature, all of CDMA’s flaws are not yet known to the engineering community. Also, as CDMA is relatively new, the network is not set up to provide as many facilities as GSM. Being the standard for mobile communication in very few countries, CDMA also cannot offer international roaming, a large disadvantage.         

          The power output of a GSM phone is 2 watts, in comparison with CDMA phone is 200 milliwatts. CDMA implies lesser radiation hazard. But the talk time is generally higher in a GSM phone due to its pulse nature of transmission, in comparison with a CDMA phone which transmits all the time. 

          CDMA technology has a soft accommodation feature, that is, when the number of users of the network goes up, the voice quality progressively gets poorer. Though GSM will not accommodate more than a finite number of users (the user will get Network Busy Message if this number is exceeded), there won’t be deterioration in voice quality due to traffic. In addition, GSM network is also equipped with Frequency Hopping, the mobile phone effortlessly jumps to a higher frequency. GSM technology also employs EFR (Enhanced Frequency Rate) add-on, which improves the voice quality greatly.         

CDMA technology facilitates a soft Hand Off, when a mobile phone has to choose between two cells and the shift from one of them to another as you travel, the transition is very smooth. In GSM, the hand off is a Hard Hand Off, the phone first stops receiving and transmitting on the old channel, and then commences the transmitting and receiving on the new channel. Therefore, if you making a call during hand off, the call needs to be dropped. 

          CDMA also has a very high data transmission rate, from 153.6 to 614 kbps. But GSM can provide only 56 kbps data transmission speed today, is also catching up very fast, and moving towards the next generation protocols, the GPRS and EDGE.

Conclusion:

The final conclusion is that it is not so much over technology that a customer decides which network to adopt. It is basically the Performance. Which ever service provider puts in better efforts in giving higher performance, the subscriber is bound to follow the path. In India, when mobile telephony was introduced, CDMA was an emerging technology. Hence GSM was the only choice. But today’s scenario calls for an evaluation of the alternatives. At present CDMA technology is used for providing WLL (Wireless in Local Loop) for communication in rural areas, where the wired land line telephones could not be provided. 

Prepared by: M.Pandian, Divisional Engineer, CDOT Installation, Salem.

                   E-Mail: [email protected]

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