 defined as ratio between the throughput and total power consumption, is an important metric when we talk about 5G [1-3]. With 5G the aim is to improve the SE by three times and EE by ten times simultaneously. In this chapter, we, therefore, consider both SE and EE as a performance metric. We consider a system model, wherein multiple users want to exchange their information with each other in the absence of the direct link between them. E. Sharma (X) Indian Institute of Technology, Roorkee, India e-mail: ekant@ece.iitr.ac.in P. Singh International institute of Information Technology, Bangalore, India e-mail: prem.singh@iiibb.ac.in © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. A. Matin (ed.), A Glimpse Beyond 5G in Wireless Networks, Signals and Communication Technology, https://doi.org/10.1007/978-3-031-13786-0_4 E. Sharma and P. Singh 4.2 Organization of the Chapter First we will cover pre-requisite topics which is important to thoroughly understand this chapter content. This will include massive MIMO; relay protocols, namely, amplify-and-forward (AF) and decode-and-forward (DF), among others; one-way and two-way communication; and difference between half-duplex and full-duplex. This chapter will also have a dedicated section to discuss about energy efficiency, which is an important metric for 5G and beyond technologies. Most conventional works optimize the SE of wireless systems, e.g., by using multiple antennas and cooperative relaying. This improvement, however, comes at the cost of higher energy that is consumed by the additional antennas/nodes. The concept of energy- efficient design, commonly defined as information bits per unit transmit energy, has recently attracted attention. We will next discuss in detail the single-pair and multi-pair half-duplex massive MIMO relaying communication. We will derive SINR expressions for two different precoders maximal-ratio-combiner (MRC)/maximal-ratio-transmission (MRT) and zero-forcing-receiver (ZFR)/zero-forcing-transmitter (ZFT).