  This paper investigates the performance of MIMO ad hoc networks that employ
transmit diversity, as delivered by the Alamouti scheme, and/or spatial
multiplexing, according to the Vertical Bell Labs Layered Space-Time system
(V-BLAST). Both techniques are implemented in a discrete-event network
simulator by focusing on their overall effect on the resulting
signal-to-interference-plus-noise ratio (SINR) at the intended receiver. Unlike
previous works that have studied fully-connected scenarios or have assumed
simple abstractions to represent MIMO behavior, this paper evaluates MIMO ad
hoc networks that are not fully connected by taking into account the effects of
multiple antennas on the clear channel assessment (CCA) mechanism of CSMA-like
medium access control (MAC) protocols. In addition to presenting a performance
evaluation of ad hoc networks operating according to each individual MIMO
scheme, this paper proposes simple modifications to the IEEE 802.11 DCF MAC to
allow the joint operation of both MIMO techniques. Hence, each pair of nodes is
allowed to select the best MIMO configuration for the impending data transfer.
The joint operation is based on three operation modes that are selected based
on the estimated SINR at the intended receiver and its comparision with a set
of threshold values. The performance of ad hoc networks operating with the
joint MIMO scheme is compared with their operation using each individual MIMO
scheme and the standard SISO IEEE 802.11. Performance results are presented
based on MAC-level throughput per node, delay, and fairness under saturated
traffic conditions.
