  Opportunistic Routing (OR) is a novel routing technique for wireless mesh
networks that exploits the broadcast nature of the wireless medium. OR combines
frames from multiple receivers and therefore creates a form of Spatial
Diversity, called MAC Diversity. The gain from OR is especially high in
networks where the majority of links has a high packet loss probability. The
updated IEEE 802.11n standard improves the physical layer with the ability to
use multiple transmit and receive antennas, i.e. Multiple-Input and
Multiple-Output (MIMO), and therefore already offers spatial diversity on the
physical layer, i.e. called Physical Diversity, which improves the reliability
of a wireless link by reducing its error rate. In this paper we quantify the
gain from MAC diversity as utilized by OR in the presence of PHY diversity as
provided by a MIMO system like 802.11n. We experimented with an IEEE 802.11n
indoor testbed and analyzed the nature of packet losses. Our experiment results
show negligible MAC diversity gains for both interference-prone 2.4 GHz and
interference-free 5 GHz channels when using 802.11n. This is different to the
observations made with single antenna systems based on 802.11b/g, as well as in
initial studies with 802.11n.
