Towards All-digital mmWave Massive MIMO: Designing around Nonlinearities
The small carrier wavelengths at mmWave frequencies enable a large number of antenna elements to be packed into a relatively small form factor. While existing implementations employ RF beamforming, it is now becoming possible to realize fully digital beamforming, with each antenna interfaced to a separate RF chain. This opens up the possibility of supporting multiuser MIMO, with the number of simultaneous users scales linearly with the number of antenna elements.
In this paper, we investigate the impact of two fundamental hardware challenges in supporting such a fully digital architecture: the large bandwidth limits the available precision of analog-to-digital conversion, and the massive number of RF chains at mmWave frequencies constrains area and power consumption, which motivates relaxing the specifications on RF nonlinearities such as the IIP3. We provide guidelines on ADC precision and RF specifications for a multiuser MIMO uplink using a linear MMSE receiver, with nominal parameters corresponding to outdoor picocells operating at a data rate of 10 Gbps per user, and a carrier frequency of 140 GHz. Specifically, in nonlinearity-limited scenarios, we show that the output SNR of a user is proportional directly to the intrinsic SNR due to the nonlinearity self-noise and inversely to the system load factor.