Crest Factor Reduction for OFDM-based Wireless Systems
An FPGA-based implementation of the CFR algorithm reduces processing latency while maximizing throughput required to support multiple antennas.
By Deepak Boppana and Lawrence Rigby, Altera Corporation
Orthogonal frequency division multiplexing (OFDM) is widely regarded as the key underlying air interface technology for wireless systems such as WiMAX, 3GPP long term evolution (LTE) and 3GPP2 ultra mobile broadband (UMB). Due to the inherent nature of these technologies, OFDM signals have high peak-to-average power ratio (PAR) that adversely impacts the efficiency of power amplifiers (PAs) used in wireless base stations.
Crest factor reduction (CFR) schemes help reduce PAR and have been widely implemented for code division multiple access (CDMA) systems. But CFR schemes developed primarily for CDMA signals exhibit poor performance when used in conjunction with OFDM signals, given the stringent error vector magnitude (EVM) requirements specified in a standard such as WiMAX.
This article introduces a CFR technique developed primarily for OFDM systems and describes how to efficiently implement the CFR algorithm using field programmable gate arrays (FPGAs). The resulting low latency, high performance design significantly reduces the PAR of the output signal while being standards compliant, leading to improved power amplifier efficiency and reduced cost. Although WiMAX is taken as an example, the topic should be of interest to engineers developing future OFDM based wireless system including LTE, UMB and 4G.
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