7.7 Conclusions

Multicarrier systems have generated increased interest over the last years, fuelled by a large demand for frequency allocation resulting in a crowded spectrum, as well as a large number of users requiring simultaneous access. This book proposes a new technique called the hybrid approach. This involves the amalgamation of OFDM/CDMA/SFH with a view to improving the number of users and yielding high bit rates based on 16-QAM modulation. It has essentially been developed for the 60-GHz frequency, but it is equally applicable at any other frequency, provided the necessary bandwidth is available.

This proposal pertains to the downlink as well as to the uplink, the only difference being that for the synchronous downlink we can apply orthogonal Walsh-Hadamard sequences leading to the well-known user separation for MC-CDMA systems. On the other hand, in the asynchronous uplink scenario, pseudonoise sequences are used with the drawback of high MAI.

This research has presented and discussed the performance of such a system in AWGN and fading channels, assuming perfect estimation and realistic estimation. We have concluded that the hybrid approach using 16-QAM provides high bit rates with an acceptable BER performance and have proposed a new synchronization scheme for such a system. This scheme is based on an algorithm. In our case, this algorithm is based on fuzzy logic. Simulations have been performed using this algorithm in AWGN as well as a Rician channel and have proved that this technique is excellent for tracking both timing and frequency offsets.