Interference between nodes is a critical impairment in mobile ad hoc networks
(MANETs). This paper studies the role of multiple antennas in mitigating such
interference. Specifically, a network is studied in which receivers apply
zero-forcing beamforming to cancel the strongest interferers. Assuming a
network with Poisson distributed transmitters and independent Rayleigh fading
channels, the transmission capacity is derived, which gives the maximum number
of successful transmissions per unit area. Mathematical tools from stochastic
geometry are applied to obtain the asymptotic transmission capacity scaling and
characterize the impact of inaccurate channel state information (CSI). It is
shown that, if each node cancels L interferers, the transmission capacity
decreases as the outage probability to the power of 1/(L+1) as the outage
probability vanishes. For fixed outage probability, as L grows, the
transmission capacity increases as L to the power of (1-2/alpha) where alpha is
the path-loss exponent. Moreover, CSI inaccuracy is shown to have no effect on
the transmission capacity scaling as the outage probability vanishes, provided
that the CSI training sequence has an appropriate length, which we derived.
Numerical results suggest that canceling merely one interferer by each node
increases the transmission capacity by an order of magnitude or more, even when
the CSI is imperfect.