Enhancing air interface characteristics is a prerequisite to offering higher capacities and allow the growth of smart devices links. In this respect, important evolutions are presently investigated.
Multiple access combinations of CDMA and OFDM in order to increase the spectrum efficiency: multi-carrier-direct-sequence CDMA (MC-DS-CDMA) where data are spread with the same code on several sub- carriers , multi-carrier CDMA (MC-CDMA) where each sub-carrier is associated to a ˜chip of the spreading code, Multi-Tone CDMA (MT-CDMA) where the spreading code is the same for each sub-carrier.
Ultra Wide Band Transmission (UWB) using very short pulses position-modulated, multiple access being is obtained by time- hopping in different time slots ( converse of frequency-hopping) not to improve the spectrum efficiency itself but to simplify the access to the channel and the deployment (no licenses?)
MIMO diversity (multiple input, multiple output ) or spatio-temporal diversity: it is based on multiple antennas at the transmit and receive sides in a propagation medium favourable to multipath, creating several distinct propagation channels and giving a new degree of freedom. If the signals transmitted on each transmit antenna are independent (BLAST) they can be recovered at the receive side by appropriate channel estimation and digital processing. Another solution introduces a time dimension by using FEC codes before spreading the signal on several antennas. Information theory has shown that the Shannon capacity could grow linearly with the number of antennas.
Multi-users detection : Multiple-access signal are never fully orthogonal because of distortions introduced by the channel. Resulting interferences added to noise will reduce the performance and the spectrum efficiency. The principle is to consider these interferences not as random noise as in an usual receiver but as other signals to be detected and subtracted from the useful signal. Processing is rather complicate but affordable in a base station.
Software Defined Radio : its rationale is found in the context of radio networks interoperability and air interfaces for seamless links. Experience has shown that it was not sensible to expect a universal standard which would be technically intractable due to the variety of situations. From a radio point of view, the terminal will be adaptatively reconfigured in different frequency bands and for different combinations of access/modulation/coding, an improvement of multi-modes terminal with pre-programmed processing. One of the key technical issues is the analog to digital converter which must be as close as possible of the receive antenna. High resolution 16-bi,t 500 Msample/s, good dynamic range and sufficient bandwidth, 100 MHz, are the important characteristics to be satisfied.