Contents I. Introduction
The increase in demand for delay-sensitive applications with symmetric data rate requirements such as wireless gaming, video telephony, and voice-over-IP, has led to the adoption of orthogonal frequency division multiple access (OFDMA) as the access scheme for 4G cellular networks, and has fostered the need for efficient resource allocation in OFDMA systems. OFDMA is used in both the uplink and downlink of WiMAX systems [1]. One of the major advantages of OFDMA is its immunity to inter-symbol interference and frequency selective fading [2]. With OFDM as a modulation scheme, it is straightforward to exploit frequency selectivity of the multipath channel. This has in fact been exploited for sum-throughput maximization in the downlink (e.g., [3], [4]) and uplink (e.g., [5], [6]) of OFDMA. However, in spite the numerous advantages of OFDM and OFDMA, their major disadvantage is that their waveforms have high peak to average power ratio (PAPR). High PAPR is problematic for uplink transmission where the mobile transmission power is usually limited. For PAPR reduction, 3GPP long term evolution (LTE) is based on using single carrier frequency division multiple access (SCFDMA) transmission with cyclic prefix in the uplink where frequency domain generation of the signal by a DFT precoding followed by an IFFT structure was assumed [7]. SCFDMA is a modified form of OFDMA. As in OFDMA, the transmitters in SCFDMA use different orthogonal frequencies (subcarriers) to transmit information symbols. However, they transmit the subcarriers sequentially, rather than in parallel [8], as shown in Fig. 1. Relative to OFDMA, SCFDMA reduces considerably the envelope fluctuations in the transmitted waveform. Therefore, SCFDMA signals have inherently lower PAPR than OFDMA signals [8].
