_Article_  
**Performance Comparison of Massive MIMO System**  
**with Orthogonal and Nonorthogonal Multiple Access**  
**for Uplink in 5G Systems**  
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definition are time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), and orthogonal frequency division multiple access (OFDMA). OMA can accommodate only a single user within the same time/frequency resource block to cancel any user interferences (for example, TDMA, FDMA, and OFDMA; but CDMA allows multiple users to be served within the same resource block using a unique spreading sequence for each user in order to separate them) [17]. OMA also offers satisfactory performance regarding the inter-user interference (IUI), but it can be overlooked if IUI can be kept under normal standards using other domains [15; 18]. By contrast, NOMA supports multiple users that can be served in the same time and frequency resource block, within the same cell, multiplexed in the power domain or code domain, typically used to suppress IUI [4; 7]. The code domain NOMA schemes perform user multiplexing in code domain using a unique spreading sequence for each user with sparse, low-density, and low intercorrelation properties. As long as the orthogonality property is not available, these schemes offer grant-free access, but IUI is present; thus, at least from these aspects, these schemes can be perceived as an involution if compared with CDMA [15; 16]. A couple of best-known code domain NOMA schemes are low-density signature (LDS) sequence-based CDMA [16] and interleave division multiple access (IDMA) [19]. Other closely related schemes are lattice partition multiple access (LPMA) [20], sparse code multiple access (SCMA) [21; 22], multiuser shared access (MUSA) [23], resource spread multiple access (RSMA) [24], and pattern division multiple access (PDMA) [25; 26]. In [27], the authors make a comparison between IDMA, RSMA, PDMA, and MUSA, taking into account the multiplexing domain, user overload, receiver type, receiver complexity, and grant-free transmission (each user can choose freely its spreading code), reaching the conclusion that MUSA offers a favorable balance between performance and complexity--high user overload and low implementation complexity, and it provides flexibility in grant-free transmissions. Although orthogonal multiplexing offers a satisfactory performance regarding the throughput level in packet-domain services with a simple design of the receiver [28], future improvements are required and accomplished by nonorthogonal multiplexing, achieving a better efficiency of the system, achieving the capacity region of the downlink broadcast channel [29; 30]. Note that NOMA can be applied also on the uplink channel [29; 30; 31] and together with OMA achieves the capacity region, but NOMA offers an enhancement regarding the trade-off between the capacity of the system and the user-fairness which is important in a cellular system when the channel conditions are different for different users because of the near-far effect [32]. In this paper, we are going to study the performance of an uplink system that employs a NOMA scheme, namely MUSA, in conjunction with massive multiple input multiple output (MIMO) techniques. In an uplink MUSA system, the symbols of each user are being spread, using a complex spreading sequence, before transmitting by means of \"shared access\", basically meaning a superposition process. Multiple spreading sequences form a pool from which a user can randomly choose one, but for the same user, different spreading sequences can be used, also randomly picked, for each transmitted symbol.