On Cubic Metric Reduction in OFDM Systems by Tone Reservation

One of the major drawbacks of multicarrier modulation is the large envelope fluctuations which either requires an inefficient use of high power amplifiers (HPAs) or decreases the system performance. Peak-to-average power ratio (PAPR) is the best known measure of the envelope fluctuations and is widely employed to design multicarrier systems. However, recently, another metric known as cubic metric (CM), is being considered in several multicarrier systems since it can predict HPA power de-rating more accurately. In this paper tone reservation (TR) technique, originally used for PAPR-reduction, is reformulated to reduce CM. We first simplify CM to define the objective function, denoted as objective CM (OCM). Then we demonstrate that OCM is convex and formulate TR for CM-reduction as an unconstrained convex optimization problem. The solution to this problem is found by means of an iterative algorithm which approaches the optimal solution with few steps. In this paper a low-complexity suboptimal algorithm capable of approaching the optimum with sufficient accuracy is also proposed. Simulation results show that for similar computational complexity, the performance of CM-reduction is superior to that of PAPR-reduction both in terms of bit error rate and out-of-band leakage.