Spatial-Frequency Signal Alignment for Opportunistic Transmission

In this paper, we investigate opportunistic transmission based on signal alignment for multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) cognitive radio (CR) systems, where both primary and secondary users have multiple antennas. In the studied scenario, primary users are transmitting based on OFDM and secondary users generate interference-free transmission to the primary users by using both the spatial and frequency space leftover by the primary system. Moreover, inter- and intra-interference among secondary users are considered as well. The number of symbols that can be transmitted by each secondary user satisfying all interference-free constraints is obtained. Precoding matrix design will be discussed for perfectly- and partially-aligned transmission. Two chordal-distance based schemes, called as exhaustive search algorithm and heuristic algorithm, will be developed for each case. Compared to the traditional time-division multiple-access (TDMA) scheme, the proposed scheme can support more interference-free symbols to be transmitted by secondary users simultaneously.