Second generation mobile systems, GSM and DECT in Europe, IS-95 in the US, were originally designed essentially for voice and short messages services dedicated to person to person communications. 2.5 G evolutions, without modification of the air interface, have introduced data services with bit rates of tens of kbit/s paving the way to communications more appropriate to smart devices. Simultaneously, the first wireless LANs have been devoted to connections between machines, either wireless Ethernet as Hiperlan or IEEE 802.11, or wired substitution as Bluetooth. The number of foreseeable smart devices have been at least an order of magnitude greater than the number of human beings; the necessary capacity in terms of information rates for the links between Smart devices is a major challenge for the near future. The 3G UMTS, based on a new radio interface, will provide a bit rate of hundreds of kbit/s enabling new multimedia services connecting more sophisticated smart devices. The future WLAN will also offer higher bit rates. The 4G, which generates now a lot of prospective works, can be seen as the convergence of these two kinds of systems toward a global system optimising the radio resources depending on the environment. The air interface will play a major role to take into account an important growth of traffic as the radio spectrum is a scarce natural resource which unlike wired systems is not extensible. As in any radio problem, an air interface has to be designed according to the radio channel characteristics “ spectrum efficiency, capacity, available frequency bands and quality of service will be at the core of the considerations.