Wireless Ad Hoc and Sensor Networks

Background on Networking 23identifying the new base station and the voice and control signal channelsthat need to be associated with the new base station.There are two kinds of handoff strategies that are usually employed. Theyare hard handoff and soft handoff. In a hard handoff procedure, the MSCassigns different radio channels to service the user in the event of handoff.The IS-95 CDMA system provides a more efficient handoff, called softhandoff, which cannot be provided with other wireless systems. In softhandoff, there will not be any physical change in the assigned channel, buta different base station handles the radio communication task. An MSCactually decides the servicing base station depending upon the receivedsignals from a user at several neighboring base stations. Soft handoff effectivelyreduces the inadvertent dropping of calls during handoff.1.3.4 Near–Far ProblemAll the signals in CDMA systems are transmitted on the same frequencyband at the same time. When all the mobiles are transmitting to the basestation at the same power level, then the power of a nearby (unwanted)mobile arriving at the listening base station overwhelms the signal froma distant (wanted) mobile. This is the primary hurdle in implementingthe CDMA cellular systems and is defined as the near–far problem(Mohammed 1993). Efficient power control schemes are useful to combatthis near–far problem.1.3.5 CDMA Power ControlThe limit on the system performance and the number of simultaneoususers available with CDMA is a function of the system’s ability to overcomethe near–far problem. To achieve the maximum number of simultaneoususers, the transmitter power of each mobile should be controlledsuch that its signal arrives at the cell site with the minimum required SIR.If the signal power of all mobile transmitters within an area covered bya cell site are controlled, then the total signal power received at the cellsite from all mobiles will be equal to the average received power times thenumber of mobiles operating in the region of coverage. A trade-off mustbe made; if a strong signal from a mobile user arrives at the cell site alongwith a weak signal from another user, the weak user will be dropped. Ifthe received power from a mobile user is significantly high, the performancemay be acceptable, but it will add undesired interference to all otherusers in the cell, wasting energy and causing a drop in network capacity.Efficient power control schemes can be employed to satisfy this trade-off.Chapter 5 presents an overview of power control in cellular networks anddescribes in detail how to design a distributed power control (DPC)scheme. Next we provide an overview of mobile ad hoc networks.