Random Subcarrier Allocation in OFDM-Based Cognitive Radio Networks

Abstract

This paper investigates the performance of an orthogonal frequency-division\nmultiplexing (OFDM)-based cognitive radio (CR) spectrum sharing communication\nsystem that assumes random allocation and absence of the primary user's (PU)\nchannel occupation information, i.e., no spectrum sensing is employed to\nacquire information about the availability of unused subcarriers. In case of a\nsingle secondary user (SU) in the secondary network, due to the lack of\ninformation of PUs' activities, the SU randomly allocates the subcarriers of\nthe primary network and collide with the PUs' subcarriers with a certain\nprobability. To maintain the quality of service (QoS) requirement of PUs, the\ninterference that SU causes onto PUs is controlled by adjusting SU's transmit\npower below a predefined threshold, referred to as interference temperature. In\nthis work, the average capacity of SU with subcarrier collisions is employed as\nperformance measure to investigate the proposed random allocation scheme for\nboth general and Rayleigh channel fading models. Bounds and scaling laws of\naverage capacity with respect to the number of SU's, PUs' and available\nsubcarriers are derived. In addition, in the presence of multiple SUs, the\nmultiuser diversity gain of SUs assuming an opportunistic scheduling is also\ninvestigated. To avoid the interference at the SUs that might be caused by the\nrandom allocation scheme and obtain the maximum sum rate for SUs based on the\navailable subcarriers, an efficient centralized sequential algorithm based on\nthe opportunistic scheduling and random allocation (utilization) methods is\nproposed to ensure the orthogonality of assigned subcarriers.\n