r coding rate (1/2) and higher coding rate (4/5). The HARQ can improve the performance, as can be observed in the low SNR region for each coding rate. The benefit of incre- mental redundancy over chase combining can also be seen, especially for the higher coding rate. As a consequence, the general assumption for this study is to use incre- mental redundancy for coding rates exceeding 1/2 and chase combining for coding rates equal to or less than 1/2. The spectral efficiency with LA including all of the MCSs is shown in Figure 6.14. The combining gain of HARQ can be observed in the smoothing out of the maxi- mum achievable spectral efficiency results, when compared to Figure 6.12. It can also be observed that, at low SNR, most of the spectral efficiency curves are over- lapped. This condition is beneficial when there is an error in the link adaptation mechanism. Although the UE transmits with a suboptimal MCS, it can still obtain the same spectral efficiency due to HARQ. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology 16QAM 1/2 Chase combining 16QAM 1/2 Incr. redundancy 16QAM 4/5 Chase combining 16QAM 4/5 Incr. redudancy SNR (dB) Figure 6.13 Spectral efficiency of SC-FDMA SISO case for chase combining (CC) and incremental redundancy (IR). 6.8 Antenna Configuration Multiple antennas are essential for UTRA LTE in order to reach the targeted peak data rates-in particular, with the aim of improving the performance at the lower SNR operating range. In this work the combination of SC-FDMA with SISO and the single-input, multiple-output (SIMO) antenna configurations has been studied. In LTE, the baseline for uplink is to use one transmit antenna at the UE and two receive antennas with maximal ratio combining (MRC) at the base station [14]. After MRC, the output signal is represented as follows: (6.16) where zl and z1 are the received signals in the frequency domain representation of each antenna branch. In Figure 6.15, the spectral efficiency for both SISO and SIMO antenn