Gugrajah_Yuvaan_ Ramesh_2003.pdf

Evaluation ofNetwork Blocking Probability Chapter 4
employing Frequency Division Multiple Access (FDMA) and Time Division
Multiple Access (TDMA), the number of channels available also ?xes the capacity
of the system. This hard capacity contrasts with the so called soft capacity of Code
Division Multiple Access (CDMA) systems which use the same frequency for each
transmitter but whi'ch are distinguished from one another through the use of distinct
orthogonal codes. The spreading codes are however not always perfectly orthogonal
and this results in multiple access interference on a link. A wireless link in a CDMA
system therefore has a threshold signal-to-interference ratio at which the link should
operate. The evaluation of blocking probability in CDMA cellular networks is
frequently based on the Erlang Capacity of the networks [Viterbi93], [Narr99].
CDMA systems offer high spectrum efficiency, multipath resistance, inherent
frequency diversity and interference rejection and' the potential use of advanced
antenna and receiver structures [Viterbi95]. These advantages of CDMA over FDMA
and TDMA systems are the reasons forthe incorporation of CDMA into the blocking
model proposed in Chapter 5.
This chapter is a review of the reduced load method as applied to wired networks,
modelled as loss networks and the use of Erlang Capacity for wireless CDMA
networks to assess the blocking probability under varying load conditions. The
Erlang Loss Formula is presented in Section 4.2 in order to illustrate the
fundamentals behind loss network modelling. The problems associated with the large
state space of a loss network are described in Section 4.3. In Section 4.4 one type of
reduced load approximation, the Erlang Fixed Point Approximation (EFPA) is
described. The fixed-point approximation proposed in [LiuOO], which is the basis for
the model proposed in Chapter 5, is detailed in Section 4.5. In Section 4.6 the
development of the Erlang Capacity for CDMA cellular systems is described based
on the approach proposed by [Gilhousen91] and (Viterbi93].
An ad hoc network can be modelled as a loss network if it is assumed that the routing
protocol chooses its route from a list of possible routes based on the available
resources. Chapter 5 describes the manner in which the reduced load approximation
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