Framework for Discrete Rate Transmission in Buffer-Aided Underlay CRN With Direct Path

This paper investigates the performance of a buffered decode-and-forward (DF) relay-based three-node underlay cooperative cognitive relay network (CRN) with a direct path to the destination, assuming that the source and the relay use limited predefined discrete rates. Optimum rules are evolved for joint link and rate selection that maximize throughput while ensuring buffer stability. When two or all three links can provide the same maximum rate, then one of them is picked randomly based on a “coin-toss” with probability chosen to ensure buffer stability. System throughput expressions are derived, with the buffered and direct components evaluated separately. It is seen that the system throughput of the balanced buffer is independent of the coin-toss probabilities that assist in buffer balancing, and also, the buffer balancing condition can be evaluated from the expression of the system throughout itself. The analysis clearly confirms the importance of the buffer. Furthermore, it is observed that considering the direct path is important in underlay cognitive radio. Additionally, a scheme in which the direct link signal is combined with the relayed signal is also considered, and it is demonstrated that it offers improvement in performance when relay-to-destination link is weak.