An optimization technique for simultaneous reduction of PAPR and out‐of‐band power in NC‐OFDM‐based cognitive radio systems

Noncontiguous orthogonal frequency division multiplexing (NC‐OFDM)‐based cognitive radio (CR) systems achieve highly efficient spectrum utilization by transmitting unlicensed users' data on subcarriers of licensed users’ data when they are free. However, there are two disadvantages to the NC‐OFDM system: out‐of‐band power (OBP) and a high peak‐to‐average power ratio (PAPR). OBP arises due to side lobes of an NC‐OFDM signal in the frequency domain, and it interferes with the spectrum for unlicensed users. A high PAPR occurs due to the inverse fast Fourier transform (IFFT) block used in an NC‐OFDM system, and it induces nonlinear effects in power amplifiers. In this study, we propose an algorithm called “Alternative Projections onto Convex and Non‐Convex Sets” that reduces the OBP and PAPR simultaneously. The alternate projections are performed onto these sets to form an iteration, and it converges to the specified limits of in‐band‐power, peak amplitude, and OBP. Furthermore, simulations show that the bit error rate performance is not degraded while reducing OBP and PAPR.