Gugrajah_Yuvaan_ Ramesh_2003.pdf
Gugrajah_Yuvaan_ Ramesh_2003.pdf
Gugrajah_Yuvaan_ Ramesh_2003.pdf
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Evaluation ofNetwork Blocking Probability<br />
4.6. Erlang Capacity of CDMA Cellular Systems<br />
Chapter 4<br />
The Erlang-B formula, can be used to evaluate conventional multiple access systems,<br />
such as those employing Frequency Division Multiple Access (FDMA) and Time<br />
Division Multiple Access (TDMA). In networks using FDMA and TDMA, traffic<br />
channels are allocated to users as long as there are channels available. When all<br />
channels are being used, arriving traffic is blocked until a channel becomes free at<br />
the end of a current call. In a Code Division Multiple Access (CDMA) system,<br />
however, the number of channels available to mobile users on the reverse link is not<br />
fixed. The reverse link (or uplink) is the link from the mobile user to the base station<br />
and instead of access being restricted to the number of channels, its restriction is<br />
based on the amount of interference and noise present at the base station [Viterbi93],<br />
[Gilhousen91].<br />
[Viterbi93] defines blocking to occur when the users within the cell of interest and in<br />
other cells introduce an interference density so great that it exceeds the background<br />
noise level by an amount that is taken to be lOdB. The total interference at a base<br />
station receiver is the sum of the intracell interference, intercell interference, and the<br />
thermal noise. The intracell interference includes interference from all mobiles<br />
within the cell of interest while the intercell interference includes interference from<br />
mobiles in other cells. The thermal noise is the product of the spread bandwidth W<br />
and the one-sided spectral noise density No. For a call to be admitted, the total<br />
interference must be less than the acceptable interference, IoW, where la is the<br />
maximum total acceptable interference density,<br />
with the following assumptions:<br />
Intracell into + Intercell into + NoW::; IoW<br />
(4-33)<br />
(i) The number of active calls is a Poisson random variable with mean )Jp<br />
where Ais the call arrival rate and lip is the mean call holding time.<br />
(ii) Each mobile involved in a call actively transmits with probability 'Y.<br />
4-15
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