Nullspace Releasing for Spatial-Frequency Opportunistic Transmission

In this article, we investigate spatial-frequency opportunistic transmission in multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems, whereby a secondary link uses a nullspace of the primary link to opportunistically transmit its own data without generating interference to the primary link. In a scenario where cooperation between primary and secondary links is allowed, we formulate a throughput maximization problem to determine the optimal nullspace that can be released by the primary link to maximize the throughput of the secondary link without violating its own quality-of-service (QoS) requirement. Three nullspace releasing algorithms are proposed, called exhaustive-search (ESA), small-eigenvalue (SEA), and best-throughput (BTA) algorithms. From the numerical results, the BTA has lower computational complexity than the ESA and provides higher throughput than the SEA, thus offering a good compromise between implementation complexity and achievable throughput.