Time Dispersion Parameters

Time Dispersion ParametersWhen a signal is transmitted, this signal enkindle suffer a distortion caused by reflections and scattered propagation paths in the radio line of credit, and these phenomenons cause that an identical signal arrives at different times at its destination. These different times ar due that to the signal arrives via multiple paths and in different incident angles. The time difference between the arrival flash of the first multipath component and the last one is called stay spread.In order to match different multipath channels and to develop some general design guidelines for wireless systems, some parameters are used to quantify the multipath channel. Some of these multipath parameters are the mean additional delay, rms delay spread, and maximum excess delay, and can be determined from a power delay profile. However, the mean excess delay and the rms delay spread are frequently used to quantify the time dispersive properties of wide band multipath channel s.Mean Excess DelayThe Mean Excess Delay is the first moment of the power delay profile (PDP) and is delineate byRMS Delay SpreadThe root-mean-square (RMS) delay spread is probably the most important single measure for the delay time extent of a multipath radio channel. This parameter calculates the standard deviation value of the delay of reflections, weighted proportional to the energy in the reflected waves. This parameter can be considered like the square root of the second central moment of the power delay profile and is defined byWe must take into friendship that these delay are measured relative to the first detectable signal arriving at the receiver at = 0, and their equations do not rely on the rank(a) power level of P(),but only the relative amplitudes of the multipath components within P().Maximum Excess Delay (X dB)The maximum excess delay (X dB) of the power delay profile is defined as the time delay value after which the multipath energy falls to X dB below the maxi mum multipath energy (not necesarily belonging to the first arriving component). It is in addition called excess delay spread, but in all cases must be specified with a threshold that relates the multipath noise floor to the maximum received multipath component.The value of these time dispersion parameters also depend on the noise threshold used to process P(), and if this noise is set too low, then the noise forget be processed as multipath and thus causing the parameters to be higher.EnvironmentCoherence BandwidthCoherence bandwidth is a statistical measure of the wave of frequencies over which the channel can be considered flat.If we define Coherence Bandwidth (BC) as the range of frequencies over which the frequency cor coitus is above 0.9, thenIf we define Coherence Bandwidth as the range of frequencies over which the frequency correlation is above 0.5, thenThe coherence bandwidth of the channel gives a good indication about the frequency variations of the channel in relatio n to the bandwidth of the transmitted signal. We can have two different cases, depending on this bandwidth. If a signal with a bandwidth larger than Bc is transmitted through the channel, it will be subject to frequency selective distortion. The channel will be, in this case, referred to as a frequency selective fading channel. However, if the signal transmitted has a bandwidth good less than Bc, it will experience amplitude attenuation only with no distortion since the channel characteristics will be the same all over the spectrum of the signal. In this case the channel is referred to as a frequency non-selective (flat) fading channel.