  Extremely large-scale multiple-input multiple-output (XL-MIMO) communications
correspond to systems whose antenna size is so large that conventional
assumptions, such as uniform plane wave (UPW) impingement, are no longer valid.
This paper studies the channel modelling and performance analysis of XL-MIMO
communication based on the generic spherical wavefront propagation model.
First, for the single-user uplink/downlink communication with the optimal
maximum ratio combining/transmission (MRC/MRT), we rigorously derive a new
closed-form expression for the resulting signal-to-noise ratio (SNR), which
includes the conventional SNR expression based on UPW assumption as a special
case. Our result shows that instead of scaling linearly with the base station
(BS) antenna number $M$, the SNR with the more generic spherical wavefront
model increases with $M$ with diminishing return, governed by a new parameter
called angular span. One important finding from our derivation is the necessity
to introduce a new distance criterion, termed critical distance, to complement
the classical Rayleigh distance for separating the near- and far-field
propagation regions. While Rayleigh distance is based on the phase difference
across array elements and hence depends on the electrical size of the antenna,
the critical distance cares about the amplitude/power difference and only
depends on its physical size. We then extend the study to the multi-user
XL-MIMO communication system, for which we demonstrate that inter-user
interference (IUI) can be mitigated not just by angle separation, but also by
distance separation along the same direction. This offers one new degree of
freedom (DoF) for interference suppression with XL-MIMO.
