7 further compares the BLER performance of the NOMA and OFDMA schemes for six and eight UEs, respectively. LTE turbo code is used for the channel coding and QPSK 1/2 for NOMA QPSK 3/4 is used for OFDMA, the overloading factor is 150%, spectral effi- ciency per user is 0.25 bps/Hz, and the sum-rate is 1.5 bps/Hz. In theory, the multi-user average mutual information analysis suggests that NOMA schemes provide higher capacity relative to OFDMA for given achievable sum-rate. In addition the, coding-based NOMA schemes (SCMA) have some performance advantage over other schemes. When the number of UEs increases the OFDMA system needs to use higher order modulation which would -1.48 OFMDA OFDMA SNR (dB) SNR (dB) Figure 3.37 Comparison of average mutual information and BLER of NOMA and OFDMA schemes [51]. New Radio Access Physical Layer Aspects (Part 1) 369 suffer from performance loss, while NOMA schemes with low order modulation can take advantage of superposition coding for higher overloading factor with slight performance degradation. The BLER performance advantage of NOMA schemes over OFDMA grows with the increase of the number of UEs [51]. 3.4 Duplex Schemes One of the key elements of any radio communication system is the way in which radio communications are maintained in the downlink and uplink. For cellular systems, it is nec- essary to enable simultaneous transmission of data in both directions, which creates a num- ber of constraints on the schemes that may be used to control over the air transmission. As a result, the choice of duplex scheme becomes the basic part of the overall specification for the cellular or any radio communications system. The term duplex refers to the bidirectional communications between two devices. When unpaired spectrum or alternatively the same RF carrier is used for downlink and uplink communications, the transmit/receive functions are time-multiplexed. When paired spectrum or alternatively two RF carriers are used for downlink and uplink communications, the transmit/receive function