【正文】 an delay τ , the root mean square(RMS)delay spread τRMS and the maximum delay τmax are characteristic parameters of the delay power density mean delay isWhereFigure 12 Timevariant channel impulse response and channel transfer function with frequencyselective fading is the power of path RMS delay spread is defined as Similarly, the Doppler power density spectrum S(fD)can be defined that characterizes the time variance of the mobile radio channel and gives the average power of the channel output as a function of the Doppler frequency frequency dispersive properties of multipath channels are most monly quantified by the maximum occurring Doppler frequency fDmax and the Doppler spread Doppler spread is the bandwidth of the Doppler power density spectrum and can take on values up to two times |fDmax|, .， Fade Statistics The statistics of the fading process characterize the channel and are of importance for channel model parameter simple and often used approach is obtained from the assumption that there is a large number of scatterers in the channel that contribute to the signal at the receiver application of the central limit theorem leads to a plexvalued Gaussian process for the channel impulse the absence of line of sight(LOS)or a dominant ponent, the process is magnitude of the corresponding channel transfer functionis a random variable, for brevity denoted by a, with a Rayleigh distribution given byWhereis the average phase is uniformly distributed in the interval [0, 2π].In the case that the multipath channel contains a LOS or dominant ponent in addition to the randomly moving scatterers, the channel impulse response can no longer be modeled as the assumption of a plexvalued Gaussian process for the channel impulse response, the magnitude a of the channel transfer function has a Rice distribution given byThe Rice factor KRice is determined by the ratio of the power of the dominant path to thepower of the scattered is the zeroorder modified Bessel function of first phase is uniformly distributed in the interval [0, 2π].(ISI)and InterChannel Interference(ICI)The delay spread can cause intersymbol interference(ISI)when adjacent data symbols overlap and interfere with each other due to different delays on different propagation number of interfering symbols in a singlecarrier modulated system is given byFor high data rate applications with very short symbol duration Td τmax, the effect of ISI and, with that, the receiver plexity can increase effect of ISI can be counteracted by different measures such as time or frequency domain spread spectrum systems, rake receivers with several arms are used to reduce the effect of ISI by exploiting the multipath diversity such that individual arms are adapted to different propagation the duration of the transmitted symbol is significantly larger than the maximum delay Td τmax, the channel produces a negligible amount of effect is exploited with multicarrier transmission where the duration per transmitted symbol increases with the number of subcarriers Nc and, hence, the amount of ISI number of interfering symbols in a multicarrier modulated system is given byResidual ISI can be eliminated by the use of a guard interval(see Section ).The maximum Doppler spread in mobile radio applications using singlecarrier modulation is typically much less than the distance between adjacent channels, such that the effect of interference on adjacent channels due to Doppler spread is not a problem for singlecarrier modulated multicarrier modulated systems, the subchannel spacing Fs can bee quite small, such that Doppler effects can cause significant long as all subcarriers are affected by a mon Doppler shift fD, this Doppler shift can be pensated for in the receiver and ICI can be , if Doppler spread in the order of several percent of the subcarrier spacing occurs, ICI may degrade the system performance avoid performance degradations due to ICI or more plex receivers with ICI equalization, the subcarrier spacing Fs should be chosen assuch that the effects due to Doppler spread can be neglected(see Chapter 4).This approach corresponds with the philosophy of OFDM described in Section and is followed in current OFDMbased wireless , if a multicarrier system design is chosen such that the Doppler spread is in the order of the subcarrier spacing or higher, a rake receiver in the frequency domain can be used [22].With the frequency domain rake receiver each branch of the rake resolves a different Doppler of Discrete Multipath Channel Models Various discrete multipath channel models for indoor and outdoor cellular systems with different cell sizes have been channel models define the statistics of the 5 discrete propagation overview of widely used discrete multipath channel models is given in the 207 [8]: The COST 207 channel models specify four outdoor macro cell propagation scenarios by continuous, exponentially decreasing delay power density of these power density spectra by discrete taps are given by using up to 12 for settings with 6 taps are listed in Table this table for several propagation environments the corresponding path delay and power profiles are terrain causes the longest classical Doppler spectrum with uniformly distributed angles of arrival of the paths can be used for all taps for , different Doppler spectra are defined for the individual taps in [8].The COST 207 channel models are based on channel measurements with a bandwidth of 8–10 MHz in the 900MHz band used for 2G systems such as 231 [9] and COST 259 [10]: These COST actions which are the continuation of COST 207 extend the channel characterization to DCS 1800, DECT, HIPERLAN and UMTS channels, taking into account macro, micro, and pico cell models with spatial resolution have been defined in COST spatial ponent is introduced by the definition of several clusters with local scatterers, which are located in a circle around the base