Is there a difference between digital cellular technology and digital PCS?
The similarity is that all modern radio-telephone service is, for the most part, cellular radio. A base station and an antenna provides coverage within a small geographical area called a cell. Networking many of those stations, called cell sites, allows roaming between the individual cells. This wireless wide area network lets a carrier provide coverage to a city or a town. Cooperative agreements with other carriers allow people to roam outside their normal coverage area. Most notably, cellular employs frequency reuse. The same set of frequencies are used over and over within a given area, to make the most economical use of the radio spectrum. No matter how the radio works, be it PCS or conventional cellular, no matter the enabling transmission technology, say TDMA or CDMA, it is all cellular radio, since distributed cell sites, employing frequency reuse, make up a wireless network.
(Just go here and it will all be explained :-) Cellular telephone basics)
PCS generally refers to cellular radio at higher frequencies. PCS is "all digital" compared to conventional cellular which is a hybrid of analog and digital routines. Being purely digital, though, is not necessarily a good thing. Quite often the all digital services lack coverage since more base stations are required. Audio quality can suffer terribly, with analog still superior. As wireless evolves the different services may operate on high and low frequencies as needed, thus blurring the defining lines between the technologies.
AT&T's IS-136 service, for example, an all digital evolution of conventional cellular, can operate on high and low frequency radio bands as needed. Depending on what the local carrier offers, it even changes its operating method, dropping back to analog cellular service where digital service isn't provided. Thus, IS-136 provides many "PCS like" services where it can, and in areas where it can't, it at least lets you talk and complete calls. I am a fan of this system.
GSM or PCS, besides being all digital, and designed in Europe with no thought of being compatible with conventional cellular systems, does differ from standard cellular in the way it makes and sends calls. The difference is in the thousands of details. But in the end it is all cellular radio.