An Adaptive Subcarrier Sharing Scheme for OFDM-Based Cooperative Cognitive Radios

Incorporation of orthogonal frequency division multiplexing (OFDM) in cooperative cognitive radio network facilitates subcarrier sharing to achieve spatial diversity with opportunistic spectrum access. In addition, adaptive modulation has been adopted widely in wireless communication to improve spectral efficiency. Use of adaptive modulation for cooperative cognitive relaying transmission to maximize throughput under bit error rate (BER) constraint is an open issue. In this paper, we propose an adaptive subcarrier sharing scheme for OFDM-based cooperative cognitive radio system, wherein cognitive (secondary) system helps the primary system to achieve its target rate of communication in exchange for opportunistic spectrum sharing. Secondary transmitter uses adaptive mode of transmission to relay the primary signal with higher throughput while maintaining the BER constraint of primary system. At primary receiver, a BER-based selection combining scheme is employed to combine the signals received in two phases. Closed-form analytical expressions for BER and outage probability of primary and secondary system for a Rayleigh flat fading channel have been derived. Results show that the outage probability with the proposed scheme (for dissimilar modulation) outperforms direct transmission and conventional maximal ratio combining scheme (for similar modulation).

I. Introduction

The next generation wireless standards, for instance fifth generation (5G) cellular networks will further transform the way people access and communicate information. However, each new access technology puts additional burden on the already constrained spectrum resources. As a consequence, there is an acute need for alternative spectrum access technique which will increase the spectral efficiency and capacity of wireless communication systems. Recently, cognitive radio [1], [2] has emerged as a promising solution to alleviate the problem of spectrum scarcity by allowing two wireless systems to operate over the same frequency band albeit with different priorities. Cognitive radio achieve this through the use of opportunistic spectrum sharing (OSS) [3] wherein secondary user (unlicensed) is allowed to opportunistically share the spectrum of primary user (licensed) without causing any adverse impact on primary user transmission. Practical implementation of OSS for Long Term Evolution-Advanced (LTE-A) standard with carrier aggregation technique has been proposed in [4]. Furthermore, LTE in unlicensed band (LTE-U) has physical layer topology to do OSS in underutilized Wi-Fi bands, specifically in 5 GHz band [5], [6]. Here OSS plays a significant role in improving the spectrum use in Wi-Fi bands and provide new paradigm in co-existence of LTE and Wi-Fi technologies.