Digital Signal Processing Reference
In-Depth Information
User 2
RTS
User 1
CTS
User 3
FIgure 11.7
Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA).
channel quality. The time duration reserved by the RTS/CTS messages is then adjusted
accordingly. When only a low data rate is achievable between two distant users, the
transmission channel may be occupied for a long period of time but with low channel
utilization, which significantly reduces the system throughput. To improve upon this,
the source can ask a close-by user to relay the message to the destination, assuming
that the source-relay and the relay-destination channels have sufficiently high transmis-
sion rates to reduce the total transmission time.
Cooperation can also be considered with slight modifications to the CSMA/CA pro-
tocol, as illustrated in
Figure 11.8
. In this case, each user is required to maintain a helper
table that records the information about the potential helpers in its vicinity. The table
contains four fields (e.g., see Figure 11.8(a)): the helper ID (i.e.,
MAC address), the time
that a packet from the helper is last received, the rate between the helper and the desti-
nation (i.e.,
R
hd
), and the rate between the source and the helper (i.e.,
R
sh
). Before each
transmission, the user searches this table for the best helper and determines whether
cooperation is actually advantageous in its case.
For example, suppose that the source node (i.e.,
user 1) wants to transmit an
N
-bit
packet to the destination node (i.e.,
user 2), as illustrated in Figure 11.8(b). With
R
sd
being the rate of the source-destination link and
R
sr
,
R
rd
the rate of the source-relay and
relay-destination links, each user can compute the time that it takes for both direct and
cooperative transmissions. Specifically, for direct transmission, the time that it takes is
equal to
N
/
R
sd
, and for cooperative transmissions with relay
r
, the time is equal to
.
N
R
N
R
+
sr
rd
Before each transmission, the source first computes the cooperative transmission time
corresponding to each helper and selects the user with the shortest time as the potential
helper. The source then compares this with the time required for direct transmission
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