Cooperative Communications in Random Access Networks - Adaptive Signal Processing in Wireless Communications - page 330

On the other hand, if r n + k ∉ I [ n ], then it will amplify and forward the signal with
amplifying gain equal to c [ n + k ], which is determined by the energy constraint at each
node. The signal received at the destination is given by

y [ nk hnkcnk += +⋅ +⋅ ++. ] [ ] [ ] z [] nnk v [ ]

rd r

With amplify-and-forward relaying, the signal can also be decomposed into a signal
component and a noise component, and the channel coefficient of the signal will be
equal to the product of the source-to-relay and the relay-to-destination channels. There-
fore, the detection methods given in (14.14) and (14.15) are equally applicable for this
case.
By fully exploiting the signals that were received during collision, the system through-
put may increase dramatically due to significant improvements in bandwidth ineffi-
ciency. Significant gains in energy efficiency may also be observed since all signals are
combined for detection at the receiver. This concept can be exploited in ad hoc networks
as described in the following section.

11.6 Asynchronous Cooperation in Multihop

Ad Hoc Networks

In ad hoc wireless networks, users communicate over multihop relaying paths with no
predetermined network infrastructure. In fact, multihop relaying is basically a form of
cooperative communications but does not exploit diversity-combining techniques at the
destinations. That is, as the messages are transmitted over a multihop route, each user in
the route utilizes for detection only the signal coming from the single closest transmit-
ter, even though the messages transmitted by all users in the same route are identical.
The signals received from other users are essentially treated as interference. This can be
improved upon by considering diversity combining at the destination, but must be done
without the strict synchronization requirements that are prohibitive in large networks.
For network broadcasting applications, the opportunistic large arrays (OLA) system
was proposed in [9, 19] to exploit the advantages of signal combining in cooperative
multihop networks. The key idea is to treat the mixture of signals that arrive at unsyn-
chronized time instants as an artificial multipath signal and utilize existing techniques
such as the RAKE receiver or equalizers to resolve the multipath signal [20].
Consider a network broadcasting scenario where we have one source node transmit-
ting a message to all other nodes in the network through multihop transmissions. At
the beginning of the process, the source node first broadcasts its message through the
wireless medium to all other users. Each user that is able to reliably decode the mes-
sage will then retransmit the same message, which is again heard by all other nodes
in the network. The users that have not yet transmitted will then take the mixture of
signals received from their upstream nodes and decode the message using standard
RAKE receivers or equalizers [9, 20]. Given that the transmission energy is sufficiently
high, these operations will trigger an avalanche of signals propagating through the net-
work. The intermediate nodes can either transmit a repetition of the decoded message