Artech House - Voice.over.802.11.pdf - VirtuaLitera

136 Voice over 802.11This phenomenon occurs because waves traveling along different paths may becompletely out of phase when they reach the antenna, thereby canceling eachother. Because signal cancellation is almost never complete, one method of overcomingthis problem is to transmit more power. In an indoor environment,multipath is almost always present and tends to be dynamic (constantly varying).Severe fading due to multipath can result in a signal reduction of morethan 30 dB. It is therefore essential to provide an adequate link margin to overcomethis loss when designing a wireless system. Failure to do so will adverselyaffect reliability. The amount of extra RF power radiated to overcome this phenomenonis referred to as fade margin or system operating margin. The exactamount of fade margin required depends on the desired reliability of the link,but a good rule of thumb for 802.11 protocols is 15 to 20 dB for a 95% confidenceinterval [8].One method of mitigating the effects of multipath is by implementingantenna diversity. Because the cancellation of radio waves is geometry dependent,use of two (or more) antennas separated by at least half of a wavelength candrastically mitigate this problem. On acquisition of a signal, the receiver checkseach antenna and simply selects the antenna with the best signal quality. Thisreduces, but does not eliminate, the required link margin that would otherwisebe needed for a system that does not employ diversity. The downside is thisapproach requires more antennas and a more complicated receiver design.Another method of dealing with the multipath problem is via the use of anadaptive channel equalizer. Adaptive equalization can be used with or withoutantenna diversity.After the signal is received and digitized, it is fed through a series of adaptivedelay stages, which are summed via feedback loops. This technique is particularlyeffective in slowly changing environments such as transmission overtelephone lines, but is more difficult to implement in rapidly changing environmentslike factory floors, offices, and homes where transmitters and receivers aremoving in relation to each other. The main drawback is the impact on systemcost and complexity. Adaptive equalizers can be expensive to implement forbroadband data links.Spread spectrum systems are fairly robust in the presence of multipath.DSSS systems will reject reflected signals, which are significantly delayed relativeto the direct path or strongest signal. This is the same property that allowsmultiple users to share the same bandwidth in CDMA systems. However,802.11’s DSSS does not have enough processing gain and orthogonal spreadingcodes to do this. FHSSs also exhibit some degree of immunity to multipath.Because an FHSS transmitter is continuously changing frequencies, it willalways hop to some frequencies that experience little or no multipath loss. In asevere fading environment, throughput of an FHSS system will be reduced, butit is unlikely that the link will be lost completely. OFDM systems such as