What is CDMA?CDMA In A Nutshell
CDMA stands for Code Division Multiple Access. What does than mean? Essentially is describes the technique used to jam lots of people talking at once onto a small band of frequencies.
GSM uses a technique called TDMA (Time Division Multiple Access).
AMPS (the analogue network) uses FDMA (Frequency Division Multiple Access). Essentially FDMA simply chops the band of frequencies up into channels. One conversation takes two channels. One channel is used to carry voice to the mobile from the tower and another carries voice from the mobile to the tower.
GSM also has channels but heaps less. One GSM channel is as big as about 8 (the precise number is 200/25) AMPS channels. But GSM squeezes 8 conversations in to one channel. It does this by allowing each telephone to use the channel in bursts. This accounts for the buzzing noise you can hear when a GSM phone comes near a radio or a wireline telephone. The GSM phone switches its transmitter on for a short period of time and then off again.
CDMA has no channels. All conversations are effectively on top of each other. The reason it works is because the frequency carrying the voice within the band is changing at high speed in a computer generated random pattern. The chances of any two conversations being on the same frequency at the same time and tracking each other the next instant the phone's computer changes frequency is tiny.
This high speed computer controlled frequency hopping is called code division (or in technical jargon "spread spectrum"). CDMA can jam in more conversations in the same band than other technologies. This is good for telecommunications companies because they can pay less at a spectrum auction for a small quantity of frequencies and still have lots of capacity to handle many conversations. It also means less infrastructure to support the same subscriber base (isn't that what they said about GSM?)
Channels, as used in the analogue network, have an inherent problem associated with them that makes them inefficient. In order to prevent interference between adjacent channels there must be a "guardband". The guardband is a tiny amount of spectrum that must not be used by either adjacent channel. In the AMPS band there are 800 or so channels. That's 799 or so tiny bits of spectrum that are wasted.
GSM is better than AMPS because there are only 125 channels. However, GSM has a peculiar problem in Australia. Since 8 conversations can be carried in one channel and the sharing is done by each phone transmitting in turn 213 time every second. This causes a distance restriction. Essentially transmitting at the speed of light, once you go over 35 kilometers from a tower the phone begins to slip into an adjacent time slot. There are 213 x 8 time slots every second i.e. 1/1704th of a second to do the round trip of 70 kilometers (35Km to the phone and 35Km back again).
CDMA has no channels and no time slots. When a call is active on a CDMA phone the transmitter is on continously (however that's a battery time problem, which we'll see when I review the phones!). So like an analogue phone, there's no buzz associated with these phones.
The CDMA folks say their phones can talk to three towers at one time! AMPS and GSM can only talk to a single tower. That means fading and dropouts are less of a problem when more than one tower can hear you. This can't be done with GSM or AMPS.
With AMPS and GSM when you're driving down the road and your phone needs to "handover" to the next tower there's a scenario (a dreaded one) where that tower already has the maximum number of people talking. Guess what happens you try to talk to it as well. You get cut off, right. Not so with CDMA. This is the so called soft failure mode. Everybody using the overloaded tower just hears a bit more noise (higher bit error rate BER, GSM ping-pongs, I've never heard CDMA first hand). That means less dropouts with CDMA.
Distance restrictions or lack of them is a key feature in Australia. CDMA goes for miles just like good old-fashioned AMPS does.
So what's bad about CDMA?
Well the first thing is there isn't much CDMA in use anywhere in the world. Korea is held up as the biggest example. The US networks are finding it difficult to convince people to give up their trusty AMPS phones with less than a million subscribers in January 98. In late news CDG (CDMA Development Group) claims there were 3.23 million US subscribers on June 15, 1998. CDG claims there are 8.75 million in Asia.
This raises the question of how big can a CDMA network get, since there are no big ones yet. Another issue is also whether there is enough handsets available?
Page 1 | 2 | 3 | 4 | 5 | 6
Home
