7. Multi-carrier-Based Multiple-Access Techniques

Multiple access, in a mobile environment, implies two-way communication between a base station and multiple surrounding mobile units. Multi-carrier implies communication via a plurality of radio-frequency carriers, normally referred to as subcarriers. In this chapter, we explore multi-carrier-based multiple-access techniques as applicable to 5G NR. First, we review orthogonal frequency division multiplexing (OFDM). OFDM is a multi-carrier technique but not normally by itself a multiple-access one. Its basic structure does, however, lend itself to form the basis of multiple-access techniques. Two such techniques are employed by 5G NR, namely, orthogonal frequency division multiple access (OFDMA) and discrete Fourier transform spread OFDM (DFTS-OFDM). Having reviewed OFDM and some of its implementation issues, OFDMA and DFTS-OFDM are addressed. Next, three other multiple access schemes that were considered for application in 5G NR but ultimately not supported are introduced. Finally, we take a high-level view of nonorthogonal multiple access (NOMA). NOMA can still apply OFDM principles, but in such a way as to have transmitted signals that are nonorthogonal to each other. The motivation to explore the use of NOMA is that it holds the promise of greater network capacity, that is, greater data throughput capability.