Wireless Ad Hoc and Sensor Networks

22 Wireless Ad Hoc and Sensor Networksthe total RF bandwidth, W. The SNR at the base station receiver can beEbrepresented in terms of given by (Rappaport 1999)N0EbS/R W/R==N0 ( N−1)( S/ W)N − 1(1.9)Equation 1.9 does not take into account the background thermal noise,and spread bandwidth. To take this noise into consideration, Ebcan beN0expressed asEbNW/R=( N− ) + ( η/ S)0 1(1.10)where η is the background thermal noise. The number of users that canaccess the system is thus given by simplifying Equation 1.10 asEbN0W/R= 1+SN − 1− ( η/ )(1.11)where W/R is called the processing gain. The background noise determinesthe cell radius for a given transmitter power.1.3.2 Channel AssignmentSeveral channel assignment strategies have been developed to achievecertain objectives like increased capacity and channel reuse. In a fixedchannel assignment strategy, each cell is allocated a predetermined set ofchannels. A new user requesting to call is provided with a channel in the setof unused channels available. If no unused channel is available, the callmay be blocked. To minimize call blocking, borrowing strategies are usedto assign channels, if available, from neighboring cells to serve the newuser. These borrowing processes are supervised by a mobile switchingcenter (MSC). Another kind of channel assignment is dynamic channelassignment. Here a serving base station requests a channel from MSCevery time a call is made. The channel is then allocated depending uponseveral constraints which increase the channel reuse.1.3.3 Handoff StrategiesWhen a mobile moves into a different cell while a conversation is in progress,the MSC automatically transfers the call to a new channel belonging tothe new base station. This handoff operation requires the knowledge for