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GSM Versus CDMA Technology
Mar 12, 2003 11:32 PM 228407 Views
(Updated Aug 03, 2003 11:38 AM)

The battle between GSM and CDMA technologies in India is hotting up! While the initial tilt was towards the GSM with players like Bharti, Hutch, Spice, BPL, Essar and BSNL preferring it, the TATAs and the 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. The GSM is built around the TDMA, or the Time Division Multiple Access concept.

Basically, the two technologies address differently the same fundamental problem of mobile communication, how to divide the finite frequency of airwaves between multiple users at the same time, or, how to make more than one person to carry on a conversation on the same frequency without causing mutual interference.

In common analogy, imagine a room full of people, all trying to carry on one-on-one conversations. In GSM(TDMA), 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 the listeners understand any language other than that of the individual to whom they are listening, the background din does not cause any real problems.

In technical jargon, GSM(TDMA) does it by chopping up the channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in a round-robin fashion. In reality, only one person is actually using the channel at any given moment, but he only uses it for short bursts. CDMA on the other hand, uses a special type of digital modulation called Spread Spectrum, which takes the user's voice stream bits and splatters them across a very wide channel in pseudo-random fashion. The receiver undoes the randomization in order to collect the bits together in a coherent order.

GSM(TDMA) started getting used in mobile communication early in the mid-1980's. A lot of time, energy and money has been spent upon improving the quality of the GSM(TDMA) technology, and because of these efforts, GSM(TDMA) won over as the standard of mobile communication in most developed nations, particularly in Europe, and as on date 500 million mobile customers all over the world use GSM(TDMA). Services like mobile banking, ticket booking, info services are today exclusively available on GSM(TDMA) networks only. A GSM(TDMA) mobile has a 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.

CDMA has its roots in pre-World War II America. In 1940, a Hollywood actor turned inventor, Hedy Lamarr, along with George Antheil, co-patented a way for torpedoes to be controlled; the U.S. Navy at that time discarded their work as architecturally unfeasible. But in 1957, engineers at Sylvania Electronic Systems Division, Buffalo, New York, took it up, and used it to secure communications for the U.S. during the 1962 Cuban Missile Crisis. After remaining classified for a long time, the CDMA technology was finally declassified in the mid-1980's. Only in 1995 CDMA was, for the first time, used for mobile communication in the U.S.A. Today, the CDMA customer base hovers around 80 million, concentrated mainly in South Korea, North America, Australia, Taiwan and parts of China. Infact the very entry of CDMA into non-U.S. countries is the direct result of politics by the U.S. giant, Qualcomm which put its weight behind the CDMA technology. For example, Beijing clearly linked the entry of CDMA into the country to its US WTO deal. A CDMA phone does not have a SIM card, and therefore you have to stick to the phone you have been provided with.

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 users. 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(TDMA). Being the standard for mobile communication in very few countries, CDMA also cannot offer international roaming, a large disadvantage.

Ideally, the GSM(TDMA) technology's talk-range from a tower is 35 kms in comparision with CDMA's 110 kms, and the power output of a GSM(TDMA)phone is 2W, in comparision with CDMA phone's 200 mW i.e., CDMA implies lesser radiation hazard. But the talk time is generally higher in a GSM(TDMA) phone due to its pulse nature of transmission, in comparision 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(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 any be deterioration in voice quality due to traffic. In addition, GSM(TDMA) network is also equipped with Frequency-Hopping, i.e., when a lower frequency is cluttered, the mobile phone effortlessly jumps to a higher frequency(e.g., from 900 MHz to 1800MHz). GSM(TDMA) technology also employs the EFR(Enhanced Frequency Rate) add-on, which improves the voice quality greatly.

If a GSM(TDMA)phone determines that there is no intelligent data to transmit, it blanks out the audio, but to keep the listener comfortable, it inserts what is known as Comfort Noise, which mimics the volume and structure of the real background noise. This fake background noise assures the caller that the connection is alive and well. On the otherhand, in CDMA phones, background noise can be effectively suppressed even while the user is talking, so that Comfort Noise, as it is, is unnecessary.

CDMA technology facilitates a Soft Handoff, i.e., when a mobile phone has to choose between two cells, and then shift from one of them to another as you travel, the transition is very smooth. In GSM(TDMA), the handoff is a Hard Handoff, i.e., the phone first stops receiving and transmitting on the old channel, and then commences transmitting and receiving on the new channel. Therefore, if you are making a call during a handoff, the call needs to be dropped.

One of the main problems facing CDMA technology these days is channel pollution, and signal deterioration inside buildings. But CDMA really comes into its elements when you are out in the countryside with few sites covering large expanses of land. CDMA also has a very high data transmission rate, from 153.6 to 614 kbps. Hence the Reliance India Mobile's claim to supply internet at 144 kbps speed.

But the GSM(TDMA), which 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 the EDGE.

The final conclusion is that it is not so much over technology that a customer decides which network to adopt. It is basically performance! Whichever mobile communication provider puts in better efforts in giving a higher performance, the subscriber is bound to follow that path!

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