ent to an M - point DFT operation. With a subcarrier mapping matrix N as given in Section 3.3.1, the spectrum of the distributed DFT-precoded OFDMA signal is identical to the IFDMA signal spectrum, thus it corresponds to the same waveform. This is also called DFT-spread OFDM. The two techniques are different implementations of SC-FDMA. The advantage of DFT-precoded OFDMA is in its more flexible structure. While IFDMA imposes a distributed signal structure, DFT-precoded OFDMA allows the use of an appro- priate subcarrier mapping matrix L. Localized variants of implementation or channel- dependent mappings are also possible. A pulse-shaping filter can be further applied in the frequency domain, with a lower complexity than the time-domain filtering. Note that in frequency-selective channel scenarios, interference may occur among the M elements of each data block. This degradation, which is more important in a distributed subcarrier map- ping, also impacts Walsh-Hadamard precoded OFDMA [47] 3.3.3 Non-orthogonal Multiple-Access Schemes Previous generations of cellular standards relied on orthogonal MA, where each time/fre- quency resource block was exclusively assigned to one of the active users to ensure no inter-user interference would occur. In 3GPP NR, synchronous/scheduling-based orthogonal MA continues to play an important role in uplink/downlink transmissions. Non-orthogonal multiple access transmission, which allows multiple users to share the same time/frequency resource, was recently proposed to enhance the system capacity and to accommodate mas- sive connectivity through asynchronous uplink access. Unlike orthogonal MA, multiple Chapter 3 non-orthogonal multiple access users' signals are multiplexed using different power alloca- tion coefficients or different signatures such as codebooks/codewords, sequences, interlea- vers, and preambles. The fundamental theory of non-orthogonal multiple access has been extensively studied in network information theory. The uplink and downlink non-orthogonal multiple acce