Optimum Bit Loading for Cognitive Relaying

Cognitive relaying is a recent communication paradigm that increases network coverage and improves spectral efficiency by allowing unlicensed (secondary) users to concurrently operate with licensed (primary) users. The major contribution of this paper is in proposing a bit allocation algorithm to be used in a cognitive relay network. The dynamic spectrum access in an OFDM-based cognitive relay network may lead to unwanted interference to the primary user. Assuming knowledge of the instantaneous channel gains for all links in the network and the primary user band, the proposed algorithm meets the twofold challenge of achieving optimum secondary system throughput while ensuring minimum interference to the primary. In its first pass, the proposed algorithm achieves optimum number of bits per subcarrier. However, in the process of achieving integer granularity for practical constellation sizing, the interference constraint may be violated. A second pass of the algorithm tackles this condition by executing a interference-based greedy bit removal. Simulation results are provided, which are indicative of the fact that the proposed algorithm exhibits enhanced performance for both primary and secondary users in the cognitive radio network.