  Massive multiple-input multiple-output (MIMO) systems have been drawing
considerable interest due to the growing throughput demands on wireless
networks. In the uplink, massive MIMO systems are commonly studied assuming
that each base station (BS) decodes the signals of its user terminals
separately and linearly while treating all interference as noise. Although this
approach provides improved spectral efficiency which scales with the number of
BS antennas in favorable channel conditions, it is generally sub-optimal from
an information-theoretic perspective. In this work we characterize the spectral
efficiency of massive MIMO when the BSs are allowed to jointly decode the
received signals. In particular, we consider four schemes for treating the
interference, and derive the achievable average ergodic rates for both finite
and asymptotic number of antennas for each scheme. Simulation tests of the
proposed methods illustrate their gains in spectral efficiency compared to the
standard approach of separate linear decoding, and show that the standard
approach fails to capture the actual achievable rates of massive MIMO systems,
particularly when the interference is dominant.
