  We study the network spectral efficiency of decentralized vector multiple
access channels (MACs) when the number of accessible dimensions per transmitter
is strategically limited. Considering each dimension as a frequency band, we
call this limiting process bandwidth limiting (BL). Assuming that each
transmitter maximizes its own data rate by water-filling over the available
frequency bands, we consider two scenarios. In the first scenario, transmitters
use non-intersecting sets of bands (spectral resource partition), and in the
second one, they freely exploit all the available frequency bands (spectral
resource sharing). In the latter case, successive interference cancelation
(SIC) is used. We show the existence of an optimal number of dimensions that a
transmitter must use in order to maximize the network performance measured in
terms of spectral efficiency. We provide a closed form expression for the
optimal number of accessible bands in the first scenario. Such an optimum
point, depends on the number of active transmitters, the number of available
frequency bands and the different signal-to-noise ratios. In the second
scenario, we show that BL does not bring a significant improvement on the
network spectral efficiency, when all transmitters use the same BL policy. For
both scenarios, we provide simulation results to validate our conclusions.
