CDMA VS TDMA

TDMA stands for "Time Division Multiple Access", while CDMA stands for "Code Division Multiple Access". Both technologies are intended to achieve the same goal, but by using different methods. Each strives to better utilize the radio spectrum by allowing multiple users to share the same physical channel.

TDMA chops up the channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in the channel. Only one person uses the channel at a time and for very short bursts and leaving it right after, thus giving time for others to use it in the same way...

CDMA on the other hand, lets everyone transmit at the same time using a special type of digital modulation called "Spread Spectrum". This form of modulation takes the user's stream of bits and splatters them across a very wide channel in a pseudo-random fashion. The "pseudo" part is very important here, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order.

Imagine a room full of people, all trying to carry on one-on-one conversations. In 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 doesn't cause any real problems.

Most cellphones decide when there is no inteligent data to transmit, trying to minimize battery consumption during calls by keeping the transmission of unnecessary data to a minimum. The phone decides whether or not you are presently speaking, or if the sound it hears is just background noise. If the phone determines that there is no intelligent data to transmit, it blanks the audio and reduces the transmitter duty cycle (in the case of TDMA) or the number of transmitted bits (in the case of CDMA).


C D M A

One of the terms you'll hear in conjunction with CDMA is "Soft Handoff". A handoff occurs in any cellular system when your call switches from one cell site to another as you travel. In all other technologies, this handoff occurs when the network informs your phone of the new channel to which it must switch. The phone then stops receiving and transmitting on the old channel, and commences transmitting and receiving on the new channel.

In CDMA, every phone and every site are on the SAME frequency. In order to begin listening to a new site, the phone only needs to change the pseudo-random sequence it uses to decode the desired data from the jumble of bits sent for everyone else. While a call is in progress, the network chooses two or more alternate sites that it feels are handoff candidates. It simultaneously broadcasts a copy of your call on each of these sites. Your phone can then pick and choose between the different sources for your call, and move between them whenever it feels like it. It can even combine the data received from two different sites to ease the transition from one to the other.

This arrangement therefore puts the phone in almost complete control of the handoff process. Such an arrangement should ensure that there is always a new site primed and ready to take over the call at a moment's notice. In theory, this should put an end to dropped calls and audio interruptions during the handoff process. In practice it works quite well, but dropped calls are still a fact of life in a mobile environment. However, CDMA rarely drops a call due to a failed handoff.

A big problem facing CDMA systems is channel pollution. This occurs when signals from too many base stations are present at the subscriber's phone, but none are dominant. When this situation occurs, audio quality degrades rapidly, even when signals seem otherwise very strong. Pollution occurs frequently in densely populated urban environments where service providers must build many sites in close proximity. Channel pollution can also result from massive multipath problems caused by many tall buildings. Taming pollution is a tuning and system design issue. It is up to the service provider to reduce this phenomenon as much as possible.

CDMA has a very high "spectral efficiency". It can accommodate more users per MHz of bandwidth than any other technology. Unlike other technologies, in which the capacity is fixed and easily computed, CDMA has what is known as "Soft Capacity". You can always add just one more caller to a CDMA channel, but once you get past a certain point, you begin to pollute the channel such that it becomes difficult to retrieve an error-free data stream for any of the participants.


T D M A

There are three different flavors of TDMA in the PCS market. Each of these technologies implements TDMA in a slightly different way. The most complex implementation is GSM. It overlays the basic TDMA principles with many innovations that reduce the potential problems inherent in the system.

To reduce the effects of co-channel interference, multipath, and fading, the GSM network can use something known as "Frequency Hopping". This means that your call literally jumps from one channel to another at fairly short intervals. By doing this, the likelihood of a given RF problem is randomized, and the effects are far less noticeable to the end user. Frequency Hopping is always available, but not mandated. This means that your GSM provider may or may not use it.

IS-136 is another form for TDMA, and it is this implementation that people generically refer to as TDMA. I personally wish they wouldn't do this, since it confuses the issue. It makes it sound as though IS-136 is the only TDMA technology. IS-136 is probably the crudest implementation of TDMA. It will suffer from various maladies far more easily than GSM, but it does have one unique feature that compensate for its crudeness. It is the only technology that integrates with existing analog systems. While CDMA can provide handoffs from digital to analog, there is no way to send the call back to digital. In IS-136 you can go both ways at any time.

The third TDMA available is called iDEN, its a Motorola technology that no other company seems to participate in. Only Motorola makes iDEN phones, and only Motorola makes iDEN infrastructure equipment. iDEN performs reasonably well, but its chosen CODEC is not quite as good as those on GSM or CDMA.

Each of the three TDMA technologies uses a different CODEC. GSM sports a CODEC called EFR (Enhanced Full Rate) that is the best sounding one available in the PCS world. IS-136 used to sound horrible, but in the fall of 1997 they replaced their old CODEC with a new one. This new CODEC sounds much better than the old, but it doesn't quite match the GSM and CDMA entries.

TDMA systems still rely on the switch to determine when to perform a handoff. Unlike the old analog system however, the switch does not do this in a vacuum. The TDMA handset constantly monitors the signals coming from other sites, and it reports this information to the switch without the caller being aware of it. The switch then uses this information to make better handoff choices at more appropriate times.

Perhaps the most annoying aspect of TDMA system to some people is the obviousness of handoffs. Some people don't tend to hear them, and I can only envy those individuals. Those of us who are sensitive to the slight interruptions caused by handoffs will probably find GSM the most frustrating. It's handoffs are by far the most messy. When handoffs occur infrequently (such as when we are stationary or in areas with few sites), they really don't present a problem at all. However, when they occur very frequently (while travelling in an area with a huge number of sites) they can become annoying.

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Channel capacity in a TDMA system is fixed and indisputable. Each channel carries a finite number of "slots", and you can never accommodate a new caller once each of those slots is filled. Spectral efficiency varies from one technology to another, but computing a precise number is still a contentious issue. For example, GSM provides 8 slots in a channel 200 kHz wide, while IS-136 provides 3 slots in a channel only 30 kHz wide. GSM therefore consumes 25 kHz per user, while IS-136 consumes only 10 kHz per user.

One would be sorely tempted to proclaim that IS-136 has 2.5 times the capacity of GSM. In a one-cell system this is certainly true, but once we start deploying multiple cells and channel reuse, the situation becomes more complex. Due to GSM's better error management and frequency hopping, the interference of a co-channel site is greatly reduced. This allows frequencies to be reused at closer range without a degradation in the overall quality of the service.

Capacity is measured in "calls per cell per MHz". An IS-136 system using N=7 reuse (this means you have 7 different sets of frequencies to spread out around town) the figure is 7.0. In GSM we get figures of 5.0 for N=4 and 6.6 for N=3. It was hoped that IS-136 could use tighter reuse than N=7, but its inability to cope with interference made this impossible.

Computing this figure for CDMA requires that certain assumptions are made. Formulas have been devised, and using very optimistic assumptions, CDMA can provide a whopping 45 users per cell per MHz. However, when using more pessimistic (and perhaps more realistic) assumptions, the value is 12. That still gives CDMA an almost 2:1 advantage over the TDMA competition.

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CDMA has one peculiarity concerning in-building penetration that does not affect TDMA. When the number of users on a channel goes up, the general level of signal pollution goes up in tandem. To compensate for this the CDMA system directs each phone to transmit with slightly more power. However, if a phone is already at its limit (such as might be the case inside a building) it cannot do anything to "keep up with the pack". This condition is known as "the shrinking coverage phenomenon" or "site breathing". During slow periods of the day you might find coverage inside a specific building quite good. During rush hour however, you might find it exceedingly poor (or non-existent).


Final:

CDMA really comes into its element when you are out in the countryside with few sites covering large expanses of land. Under these conditions CDMA provides extremely stable audio with few frame errors to mess things up. This is because Channel Pollution is almost unknown in these situations. Under similar conditions TDMA suffers too readily from interference and it will often blank the audio. Many people who use CDMA systems in sparsely populated areas have given this technology extremely high marks.

TDMA systems also have great difficulties in open regions just outside densely populated areas. In this situation your phone is exposed to signals coming from countless sites in the densely populated areas, but there are no dominant signals from a close-by site. CDMA can suffer under these conditions too (due to channel pollution), but not quite so badly. Valleys don't present a big problem for TDMA, but high ground is a killer. You can experience choppiness in the audio even when your signal indicator is reading 2 or 3 bars.