With the ever-increasing need for wireless broadband services next generation wire- less systems are being designed to meet three main requirements. These are massive Machine Type Communication, Ultra-Reliable Low-Latency Communication, and enhanced Mobile Broadband. In this thesis, we explore all three areas and show that multi-antenna systems are the framework to meet these demands. We first investigate the age-limited capacity of the Gaussian many channel with total N users, out of which a random subset are active, and a multi-antenna base station. In the setting in which both the number of users, N, and the number of antennas at the BS, M, are allowed to grow large at a fixed ratio ζ = M/N, we derive the achievability bound under maximal ratio combining. Further extensions of the analysis to the zero-forcing receiver as well as imperfect CSI are provided. Using the age of information (AoI) metric as our measure of timeliness we investigate the trade-offs between the AoI and spectral efficiency in the context massive connectivity with large-scale antenna arrays. We find that while the spectral efficiency can be made large, the penalty is an increase in the minimum AoI obtainable. The proposed achievability bound is further compared against recent unsourced random access schemes. One underlying assumption in the previous analysis is the construction of uncoupled arrays. This assumption starts to break down in massive MIMO and therefore in the second part of this thesis, we focus on a physically-consistent single-user MIMO system and model the mutual coupling using multiport networks. Based on this model, in single-carrier frequency-flat channels we show that neglecting the mutual coupling effects leads to inaccurate characterization of the channel and noise correlations. In multi-carrier frequency-selective channels we show that the coupling also increases the number of resolvable channel taps. We therefore develop a linear minimum mean-square error (LMMSE) estimator that calibrates the coupling and optimally estimates the MIMO channel. It is shown that appropriately accounting for mutual coupling through the developed physically consistent model leads to remarkable performance improvements. We demonstrate these gains in a rich-scattering environment using the broadband connected array of slot antennas.