  Mobile ad hoc networks (MANETs) are self-configuring wireless networks that
lack permanent infrastructure and are formed among mobile nodes on demand.
Rapid node mobility results in dramatic channel variation, or fading, that
degrades MANET performance. Employing channel state information (CSI) at the
transmitter can improve the throughput of routing and medium access control
(MAC) protocols for mobile ad hoc networks. Several routing algorithms in the
literature explicitly incorporate the fading signal strength into the routing
metric, thus selecting the routes with strong channel conditions. While these
studies show that adaptation to the time-variant channel gain is beneficial in
MANETs, they do not address the effect of the outdated fading CSI at the
transmitter. For realistic mobile node speeds, the channel gain is rapidly
varying, and becomes quickly outdated due the feedback delay. We analyze the
link throughput of joint rate adaptation and adaptive relay selection in the
presence of imperfect CSI. Moreover, for an 802.11 network that employs
geographic opportunistic routing with adaptive rate and relay selection, we
propose a novel method to reduce the effect of the feedback delay at the MAC
layer in the presence of Rayleigh fading. This method exploits channel
reciprocity and fading prediction and does not require significant modification
to the existing 802.11 frame structure. Extensive network simulations
demonstrate that the proposed approach significantly improves the throughput,
delay, and packet delivery ratio for high mobile velocities relative to
previously proposed approaches that employ outdated CSI at the transmitter.
