  This paper presents a cluster-based transform domain communication system
(TDCS) to improve spectrum efficiency. Unlike the utilities of clusters in
orthogonal frequency division multiplex (OFDM) systems, the cluster-based TDCS
framework divides entire unoccupied spectrum bins into $L$ clusters, where each
one represents a data steam independently, to achieve $L$ times of spectrum
efficiency compared to that of the traditional one. Among various schemes of
spectrum bin spacing and allocation, the TDCS with random allocation scheme
appears to be an ideal candidate to significantly improve spectrum efficiency
without seriously degrading power efficiency. In multipath fading channel, the
coded TDCS with random allocation scheme achieves robust BER performance due to
a large degree of frequency diversity. Furthermore, our study shows that the
smaller spectrum bin spacing should be configured for the cluster-based TDCS to
achieve higher spectrum efficiency and more robust BER performance.
