The second generation of cellular systems came in the beginning of the 90's, they are digital systems and they behave almost identically. These systems include GSM and PDC.
Future systems, also called the third generation, will have to cope with very high demands on voice traffic and data rates much higher than today's systems are able to handle. Furthermore, it must be easy to introduce new services, and the cellular phones must be cheap and consume very little energy. It is likely that they will also have to handle roaming between traditional ground-based cellular systems and satellite-based PCS (Personal Communication Systems).
In a few years we could have this scenario: your car breaks down in a desolate area, you call your local garage and ask for help. You put on your Virtual Reality-helmet, and a repairman can, using real-time audio and video, guide you through the whole repair. The pictures may show what bolt to unscrew, what tools you need in different stages, etc. This puts high demands on the transmitting capacity of the cellular system and is only one example of what kind of situation the third generation of cellular systems would have to handle.