  Most existing works on physical-layer (PHY) cooperation (beyond routing)
focus on how to best use a given, static relay network--while wireless networks
are anything but static. In this paper, we pose a different set of questions:
given that we have multiple devices within range, which relay(s) do we use for
PHY cooperation, to maintain a consistent target performance? How can we
efficiently adapt, as network conditions change? And how important is it, in
terms of performance, to adapt? Although adapting to the best path when routing
is a well understood problem, how to do so over PHY cooperation networks is an
open question. Our contributions are: (1) We demonstrate via theoretical
evaluation, a diminishing returns trend as the number of deployed relays
increases. (2) Using a simple algorithm based on network metrics, we
efficiently select the sub-network to use at any given time to maintain a
target reliability. (3) When streaming video from Netflix, we experimentally
show (using measurements from a WARP radio testbed employing DIQIF relaying)
that our adaptive PHY cooperation scheme provides a throughput gain of 2x over
nonadaptive PHY schemes, and a gain of 6x over genie-aided IP-level adaptive
routing.
