Novel Metric-Based PAPR Reduction Schemes for MC-CDMA Systems

This paper proposes the use of a metric-based peak-to-average power ratio (PAPR) reduction method and the joint spreading code allocation and subcarrier scrambling (JSCASS) scheme in downlink multicarrier code-division multiple-access (MC-CDMA) systems. Subcarrier scrambling is employed in MC-CDMA systems to introduce additional disturbances to the correlated patterns among subcarriers. In particular, the variance of the time-domain signal power is derived from an approximate closed-form expression. Given that PAPR values are monotonically related to the proposed metric values, the PAPR of a downlink MC-CDMA signal can be efficiently reduced by assigning the lowest metric value to the spreading code allocation and subcarrier scrambling pattern. The proposed JSCASS scheme adopts the cross entropy (CE) method to jointly optimize the spreading code allocation and scrambling pattern. To reduce implementation cost, a sequential spreading code allocation and subcarrier scrambling (SSCASS) method is proposed by optimizing the spreading code allocation with the tree-search method (TSM) and by determining the scrambling pattern with the CE method. Simulation results show that the performance gap between the proposed JSCASS and SSCASS schemes is within 0.2 dB. Moreover, both schemes outperform the conventional method, which simply allocates spreading codes by 6 dB when the number of subcarriers is 512.