 are various multiple-access schemes that have been studied and used in wireless sys- tems in the past decades, which allow the network to share the available radio resources (i.e., time, frequency, code, space, power) among a number of active users in the cell in the down- link and uplink. Fig. 3.29 illustrates the concept of resource sharing in some prominent multiple-access schemes. As mentioned earlier, orthogonal frequency division multiple access (OFDMA) has been a promising MA scheme that has been used in mobile broadband radio access technologies such as NR and LTE. The new radio uses a symmetric OFDMA scheme in the downlink and uplink, whereas LTE uses OFDMA and SC-FDMA as the MA schemes in the downlink and uplink, respectively. In addition, the non-orthogonal concept can be applied to MA scenarios. Sparse code multi- ple access (SCMA), non-orthogonal multiple access (NOMA), and mult-iuser shared access (MUSA) are examples of non-orthogonal multiple access schemes that were studied in 3GPP Rel-16 [23,50-52]. These techniques can superimpose signals from multiple users in the code domain or the power domain to enhance the system-access performance and poten- tially allow asynchronous access in the uplink. NOMA/SCMA/MUSA User K User K User K-1 Users multiplexed in time and User K-1 User K-1 User K frequency User 3 User3 User3 User 2 User1 User2 User 1 Figure 3.29 Illustration of various multiple access concepts [47]. New Radio Access Physical Layer Aspects (Part 1) 351 3.3.1 Orthogonal Frequency Division Multiple Access OFDMA is the multi-user variant of the OFDM scheme where multiple access is achieved by assigning subsets of time-frequency resources to different users, allowing simultaneous data transmission from several users. In OFDMA, the radio resources are 2D regions over time (an integer number of OFDM symbols) and frequency (a number of contiguous or non-contiguous subcarriers). Similar to OFDM, OFDMA employs multiple closely spaced subcarriers that are divided into groups of subcarriers where eac