  Cooperative techniques have been shown to significantly improve the
performance of wireless systems. Despite being a mature technology in single
communication link scenarios, their implementation in wider, and practical,
networks poses several challenges which have not been fully identified and
understood so far. In this two-part paper, the implementation of cooperative
communications in non-centralized ad hoc networks with sensing-based channel
access is extensively discussed. Both analysis and simulation are employed to
provide a clear understanding of the mutual influence between the link layer
contention mechanism and collaborative protocols. Part I of this work focuses
on reactive cooperation, in which relaying is triggered by packet delivery
failure events, while Part II addresses proactive approaches, preemptively
initiated by the source based on channel state information. Results show that
sensing-based channel access significantly hampers the effectiveness of
cooperation by biasing the spatial distribution of available relays, and by
inducing a level of spatial and temporal correlation of the interference that
diminishes the diversity improvement on which cooperative gains are founded.
Moreover, the efficiency reduction entailed by several practical protocol
issues related to carrier sense multiple access which are typically neglected
in the literature is thoroughly investigated.
