CDMA

Now that we have a rudimentary understanding of the two technologies, let's try and examine what advantages they provide. We'll begin with CDMA, since this newer technology has created the greatest "buzz" in the mobile communications industry.

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 it commences transmitting and receiving on the new channel. It goes without saying that this is known as a "Hard Handoff".

In CDMA however, 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 or more 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 the audio quality degrades rapidly, even when the signal 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.

In defense of CDMA however, I should point out that the new EVRC CODEC is far more robust than either of the earlier CODECs. Because of its increased robustness it provides much more consistent audio in the face of high frame error rates. EVRC is an 8 kilobit CODEC that provides audio quality that is almost as good to the older 13 kilobit CODEC. Since CDMA consumes only as much of the "ether" as a user talks, switching everyone to an 8 kilobit CODEC was an inevitable move.

Don't confuse EVRC with the old (and unlamented) 8 kilobit CODEC implemented in the early days of CDMA deployment. That CODEC was simply awful, and very few good things could be said about it. EVRC is a far more advanced compression algorithm that cleans up many of the stability problems inherent in the two older CODECs. The sound reproduction is slightly muddier than the 13 kilobit CODEC, but the improvement in stability makes up for this.

Supporters often cite capacity as one CDMA's biggest assets. Virtually no one disagrees that CDMA has a very high "spectral efficiency". It can accommodate more users per MHz of bandwidth than any other technology. What experts do not agree upon is by how much. 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.

The ultimate capacity of a system is therefore dependent upon where you draw the line. How much degradation is a carrier willing to subject their subscribers to before they admit that they have run out of useable capacity? Even if someone does set a standard error rate at which these calculations are made, it does not mean that you personally will find the service particularly acceptable at that error rate.