In-building Coverage CDMA and TDMA

Now let's deal with another issue involving CDMA and TDMA. In-building coverage is something that many people talk about, but few people properly understand. Although CDMA has a slight edge in this department, due to a marginally greater tolerance for weak signals, all the technologies fair about the same. This is because the few dB advantage CDMA has is often "used up" when the provider detunes the sites to take advantage of this process gain.

So, while a CDMA phone might be able to produce a reasonable call with a signal level of -106 dBm, whereas a GSM phone might need -99 dBm to provide the same level of service, does this mean that CDMA networks will always have a 7 dB advantage? If all things were equal, then yes, but they aren't equal.

As I mentioned earlier, channel pollution is a big issue with CDMA networks and to keep channel pollution to a minimum in urban environments a CDMA provider needs to keep site overlap to a minimum. Subsequently, a CDMA network engineer will use that 7 dB advantage to his advantage by de-tuning the network accordingly. This means that CDMA users will frequently see markedly lower signal levels indoors than a GSM user will, but in the end it all works out about the same.

The most important aspect to in-building coverage is the proximity of the nearest site. When a site is located just outside of a building it can penetrate just about any building material. When a site is much further away however, the signals have a much harder time of getting past the walls of a structure.

When it comes to distance, remember that signals are subject to the "distance squared law". This means that signals decrease by the square of the distance. A site at 0.25 kilometers away will have 4 times the signal strength of a site at 0.50 kilometers away, and 16 times that of a site 1.0 kilometers away. Distance squared however is the rate of signal reduction in free space. Recent studies have shown that terrestrial communications are usually subject to rates as high as "Distance cubed", or even "Distance to the 4th". If the latter is true, then a site 1.0 kilometers away will actually be 256 times weaker than a site 0.25 kilometers away.

In-building penetration is therefore less a technology issue than it is an implementation issue. Service providers who have sites close to the buildings you commonly visit will inevitably look better than those who don't. Never use someone else's in-building experiences unless you expect to go in the same buildings as they do. You cannot make useful generalizations about in-building coverage based upon one person's experience.

CDMA does have 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).