Wireless Ad Hoc and Sensor Networks
Wireless Ad Hoc and Sensor Networks Wireless Ad Hoc and Sensor Networks
Background on Networking 21Multisectorized antenna: The directional antennas receive signals fromonly a fraction of the current users, thus leading to the reductionof interference.Discontinuous transmission mode: Another way of increasing CDMAcapacity is to operate the antenna in discontinuous transmission(DTX) mode. In DTX, the transmitter is turned off during theperiods of silence in speech. It has been observed that voice signalshave a duty factor of about 3/8 in landline networks and 1/2for mobile systems, where background noise and vibration cantrigger voice activity detectors. Thus, the average capacity of aCDMA system can be increased by a factor inversely proportionalto the duty factor.Effective power control schemes: An effective power control schemewould choose an appropriate power value that causes decreasedinterference but still maintains the power required for transmission.A good power control scheme would help accommodate anincreased number of users in the network, thereby increasingsystem capacity. Higher spatial reuse can be obtained because thecochannel interference is reduced to a minimum with efficientpower control.1.3.1.6 Evaluation of Capacity of a Cellular CDMA NetworkThe cellular network consists of a large number of mobile users communicatingwith a base station within a cell. The cell-site transmitter consistsof a linear combiner, which adds the spread signals of the individual usersand also uses a weighting factor for each signal for forward-link powercontrol purposes. For a single-cell system under consideration, theseweighting factors can be assumed to be equal. A pilot signal is alsoincluded in the cell-site transmitter and is used by each mobile to set itsown power control for the reverse link. For a single-cell system withpower control, all the signals on the reverse channel are received at thesame power level at the base station. Let the number of users be N. Then,each demodulator at the cell site receives a composite waveform containingthe desired signal of power S and (N − 1) interfering users, each ofwhich has power, S. Thus the SNR isS 1SNR = =( N−1)S N−1(1.8)In addition to SNR, bit energy-to-noise ratio is an important parameterin communication systems. It is obtained by dividing the signal powerby the baseband information bit rate, R, and the interference power by
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
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22 <strong>Wireless</strong> <strong>Ad</strong> <strong>Hoc</strong> <strong>and</strong> <strong>Sensor</strong> <strong>Networks</strong>the total RF b<strong>and</strong>width, 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,<strong>and</strong> spread b<strong>and</strong>width. 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 <strong>and</strong> 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 H<strong>and</strong>off 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 h<strong>and</strong>off operation requires the knowledge for
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