Wireless Network Design: Optimization Models and Solution ...

30 Dinesh Rajan
nel can support a peak data rate of R, then with random multiple access using the
Aloha protocol, the maximum effective data rate is only 1 2eR, or approximately 18%
of data rate. However, for lightly loaded systems Aloha is a simple strategy that
offers reasonable performance. An excellent historical perspective of Aloha written
by the original inventor including recent application areas is given in [2].
Several variations of Aloha including slotted-Aloha have also been proposed
which improve the performance of the basic scheme [7]. In a slotted Aloha system,
the entire time is discretized into time-slots of fixed duration and nodes are
allowed to transmit packets only at the beginning of a time-slot. Further, each node
that has a packet to transmit will only transmit it with a finite probability q at the
beginning of the next time-slot. These changes to Aloha protocol, reduces the number
of collisions and increases the overall throughput. The maximum data rate of a
slotted Aloha system can be shown to equal 1 e R.
Many recent multiple access (MA) strategies are based on the idea of carrier
sense multiple access (CSMA) [7, 25]. Several modifications of CSMA have been
proposed in the literature. In a CSMA scheme, each node listens to the medium
to see if the medium is free or if there exists any other signal in the medium. If
the wireless medium is believed to be free then the node transmits its data after
waiting for a random amount of time. If the medium is sensed as being busy, then
Fig. 2.12 Performance of hard decision and soft decision decoding of convolutional codes over an
AWGN channel. The rate of the codes are 1 2 , 1 3 , and 1 4 .