TDMA

Let's move away from CDMA now and have a look at TDMA. Before we can go any further though, I should note that there are actually 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, without a doubt, 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.

iDEN is a proprietary Motorola technology that no other company seems to participate in. Only Motorola makes iDEN phones, and only Motorola makes iDEN infrastructure equipment. Perhaps the company guards its technology on purpose. iDEN was initially deployed as an alternative to standard packet radio systems commonly used by public safety and business users. However, it also provided phone interconnect services that are extinguishable from phone services offered by the other PCS systems, as well as packet data services for web browsing and hooking up your laptop to the Internet.

Finally there is the old IS-136 technology, but this is now an officially dead technology. All of the North American providers who used it (Rogers, Cingular, and AT&T) are abandoning it in favor of GSM. The same is happening in other parts of the world where IS-136 was used. I therefore will not spend much time talking about this variation of TDMA.

Each of these TDMA technologies uses a different CODEC. GSM sports a CODEC called EFR (short for Enhanced Full Rate). This CODEC is arguable 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.