 an appropriate CP is appended to compensate for timing misalignments at the receiver. In order to maintain good performance in frequency-selective fading channels, robust forward error correction schemes must be employed. Precoded OFDMA is a variant of OFDMA in which a precoding matrix P is used that spreads the energy of symbols over the subcarriers allocated to the user. Uniform energy distribution is desirable in practice. One of the most well-known precoding matrices is the Walsh-Hadamard matrix P = ( Po PM-1) where the row vectors Pi are orthog- onal Walsh-Hadamard sequences of length M [47]. New Radio Access Physical Layer Aspects (Part 1) 353 3.3.2 Single-Carrier Frequency Division Multiple Access SC-FDMA has been used as the uplink multiple-access scheme in LTE systems. The use of SC-FDMA was motivated by the fact that a single-carrier system with an OFDMA-type multiple-access would combine the advantages of the two techniques, that is, low PAPR and large coverage. The first SC-FDMA concept was interleaved FDMA (IFDMA), which was based on compression and block repetition of the modulated signal in the time domain. It can be theoretically shown that the spectrum of the compressed and K times repeated sig- nal has the same shape as the original signal, with the difference that it presents exactly K - 1 zeros between two data subcarriers. This feature enables us to easily interleave differ- ent users in the frequency domain by applying to each user a specific frequency shift, or equivalently, by multiplying the time-domain sequence by a specific phase ramp. In addi- tion, similar to OFDMA, robustness to inter-cell interference can be achieved by coordinat- ing resource allocation between adjacent cells. The same waveform can be obtained in the frequency domain, if discrete Fourier transform matrix P = [Pk,n]; Pk,n exp(j2nkn/M) used as the precoding matrix in Fig. 3.30, resulting in DFT-precoded OFDMA, which is mathematically identical to IFDMA in a distributed scenario. The precoding operation Ps is equival