a schemes is presented. The spectral efficiency of OFDMA is also presented for comparison. In the SISO case, OFDMA outperforms SC-FDMA with an MMSE © 2009 by Taylor & Francis Group, LLC Single-Carrier Transmission for UTRA LTE Uplink BPSK 1/6 CC BPSK 1/3 CC QPSK 1/2 CC QPSK 2/3 IR QPSK 3/4 IR 16QAM 1/2 CC 16QAM 2/3 IR 16QAM 3/4 IR 16QAM 4/5 IR Link adaptation SNR (dB) Figure 6.14 Spectral efficiency of SC-FDMA SISO case with HARQ. receiver. The demodulation for OFDMA is performed in the frequency domain, whereas for SC-FDMA, demodulation is performed in the time domain. A deep fading may affect the whole data symbol in SC-FDMA; thus, the performance of SC-FDMA is worse for a frequency-selective fading channel. For the SIMO case, the SC-FDMA and OFDMA performance is improved due to the diversity gain, and the spectral efficiency gap between OFDMA and SC-FDMA becomes unnoticeable. The MMSE receiver in SIMO with MRC further reduces the noise in comparison to the SISO case, as well as greatly increases the spectral efficiency of SC-FDMA. 6.9 Impact of Frequency Allocation LTE uplink transmission allows a flexible frequency allocation within the chan- nel bandwidth. In the study item phase [6], the 3GPP has defined the local- ized and distributed frequency/subcarrier allocation as shown in Figure 6.16. The distributed case is used to gain frequency diversity, whereas the localized SC-FDMA can be used to simplify subcarrier allocation and benefit from oppor- tunistic scheduling. Lately, the 3GPP has only considered the localized case and not the distributed case because the localized case with frequency hopping can © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology LA SISO-SCFDMA LA SISO-OFDMA LA SIMO-SCFDMA LA SIMO-OFDMA SNR (dB) Figure 6.15 Spectral efficiency for SC-FDMA and OFDMA with two antenna configurations (SISO and SIMO). Localized Transmission Used sub-carriers Sub-carrier 1 2 3 4 index Available Allocated Sub-Carriers Distributed Transmission Used sub-carriers 