Cooperative Communications in Random Access Networks - Adaptive Signal Processing in Wireless Communications - page 332

Firing instant

Received phase Rest phase

Received signal

Time

Delay spread

FIgure 11.11 Signal received by active nodes.

is the average delay of the signals received at user i . With the symbol duration T s ≈ Δτ,
the rate R s ≈ 1/Δτ is achieved without intersymbol interference. Nevertheless, a higher
rate is achievable with the use of equalizers at the destination, which is shown in [20]. An
example of the signature waveform is depicted in Figure 11.11.
The signal received at each user can be divided into two phases: the receive phase and
the rest phase. In the receive phase , the user first accumulates the signal transmitted by
its upstream nodes until the signal energy is sufficient to perform reliable detection. At
this instant, the symbol is detected and retransmitted to other users in the network. This
instant is called the iring instant . Once a user has transmitted, it will then shut down
until the downstream signals fade away. This period of time is called the rest phase and is
used to prevent an infinite feedback, which may cause the system to become unstable.
The operations described above can be demonstrated with a simple four-node exam-
ple, as shown in Figure 11.12 . Assume node 1 is the source node and nodes 2-4 are the
relays. We can see from the figure that node 2 will be the first to receive the signal from
node 1. By the time user 2 accumulates sufficient signal energy for reliable detection, it
will immediately retransmit the packet to nodes 3 and 4. By shutting down the receivers
in the rest phase, the nodes that transmitted earlier in the symbol period will not receive
signals from their downstream nodes.
Two main properties of OLA allow it to improve over the conventional multihop
broadcasting scheme. The first property is its ability to combine the signals from all
relays, including those that were transmitted from nodes that are located far away and
were originally omitted in conventional networks. This enables a tremendous amount
of energy savings, as detailed in [19]. Second, the simultaneously transmitting signals,
which were originally treated as interference, are also combined for detection. Not only
does this increase the energy efficiency, but it also reduces the network congestion and
significantly decreases the broadcasting delay [see 9 ]. A specific equalizer design that
performs the signal combining is proposed by Wei, Gofeckel, and Valenti in [20], where a
random delay at the users is also proposed to increase the delay diversity of the system.
OLA provides a simple and efficient way to achieve broadcast communications in
wireless networks. The system scales naturally to a large network since users are only
required to follow a simple local rule. The integrate-and-fire mechanism at each user
emulates the behavior of many biological networks, such as the flashing of fireflies, the
firing of neurons, or even highway traffic patterns. These examples show that complex
large-scale behaviors can result from the interaction between simple local mechanisms.