Improved power loading scheme for orthogonal frequency division multiplexing based cognitive radio

A subcarrier power allocation scheme for orthogonal frequency division multiplexing (OFDM) based cognitive users has been proposed for spectrum sharing with primary users (PU) allowing secondary users (SU) to coexist in same as well as adjacent frequency bands by underlay and interweave approaches respectively. The SU sum capacity of reliable communication has been maximised under the constraints of total power, each sub-channel power and PU aggregate interference limits. Based on the quality of channel gains and spectral distance from PU bands, the powers of subcarriers adjacent to PU bands are suitably controlled till the interference constraint of PUs are satisfied. Controlled subcarrier deactivation achieved by augmenting this `n-adjacent´ power redistribution step to traditional OFDM water-filling gives a powerful tool for efficiently maintaining high SU sum capacity without exceeding PU interference tolerance limit for a wide range of total power budget. Different water levels get assigned for the adjacent and non-adjacent subcarrier groups. In terms of SU capacity and PU interference, the proposed algorithm outperforms some of the existing schemes having almost same complexity as the proposed one. The improvement is significant when the adjacent subcarriers channel gains are relatively `good´ or `bad´ compared with the non-adjacent ones.