 with many active antenna elements that can serve a large number of user terminals. One should avoid specify- ing a certain ratio Nantennas/Nuser since it depends on a variety of conditions such as the system performance metric, propagation environment, and coherence block length. 586 Chapter 4 The massive MIMO gains do not require high-precision hardware; in fact, lower hardware precision can be handled compared to other systems since additive distortions are sup- pressed in the processing. Another reason for the robustness is that massive MIMO can achieve high spectral efficiencies by transmitting low-order modulations to a multitude of terminals, while contemporary systems require high-precision hardware to support transmis- sion of high-order modulations to a few terminals [16,52]. In an OFDM system, resource allocation means that the time-frequency resources are divided between the terminals to satisfy user-specific performance constraints, finding the best subcarriers for each terminal, and overcoming the small-scale fading effects by power control. Frequency-selective resource allocation can provide significant improvements when there are large variations in channel quality over the subcarriers, but it is also demanding in terms of channel estimation and computational overhead since the decisions depend on the small-scale fading, which varies in time. If the same resource allocation concepts were applied to massive MIMO systems, with tens of terminals at each of the thousands of sub- carriers, the system complexity would have been excessively prohibitive. However, the channel hardening effect in massive MIMO means that the channel variations are negligible over the frequency-domain and mainly depend on large-scale fading in the time domain, which typically varies much slower than small-scale fading, making the conventional resource allocation concepts unnecessary for massive MIMO. The available bandwidth can be simultaneously allocated to each active terminal, and the power control decisions are made joint