Controlling the Peak-to-Average Power Ratio in the Downlink of WiMAX Systems

It is well known that one of the main implementation issues in multicarrier modulation schemes, like OFDM, is their high peak-to-average power ratio (PAPR). Power amplifiers that are employed in the transmitters of communications systems are highly non-linear devices, which present a saturation level, and typically show a higher power efficiency close to saturation. Therefore, the high peak amplitudes common in OFDM may cause severe non-linear effects, such as signal clipping, which result in out-of-band radiation and signal constellation distortion. Several techniques are proposed in the literature in order to reduce the PAPR of OFDM signals, such as clipping with additive or multiplicative windowing, use of virtual subcarriers, active constellation extension and partial transmit sequences, among others. In this contribution we demonstrate the use of some of these techniques in a WiMAX wireless system, based on the IEEE 802.16e, focusing on techniques that can be employed with no or little modifications to existing standards. The PAPR effects are investigated in terms of several different performance metrics, namely the adjacent channel power ratio (ACPR), the total degradation and data throughput, and we see that through the proper combination of different techniques significant performance gains can be achieved.