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Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices. It assumes that the power of a signal that has passed through such a transmission medium (also called a communications channel) will vary randomly, or fade, according to a Rayleigh distribution — the radial component of the sum of two uncorrelated Gaussian random variables. It is a reasonable model for tropospheric and ionospheric signal propagation as well as the effect of heavily built-up urban environments on radio signals.•• Rayleigh fading is most applicable when there is no line of sight between the transmitter and receiver. If there is a line of sight, Rician fading is more applicable. The model Rayleigh fading is a reasonable model when there are many objects in the environment that scatter the radio signal before it arrives at the receiver. The central limit theorem holds that, if there is sufficiently much scatter, the channel impulse response will be well-modelled as a Gaussian process irrespective of the distribution of the individual components. If there is no dominant component to the scatter, then such a process will have zero mean and phase evenly distributed between 0 and 2π radians. The power, or envelope, of the channel response will therefore be Rayleigh distributed. Calling this random variable , it will have a probability density function: where . Often, the gain and phase elements of a channel's distortion are conveniently represented as a complex number. In this case, Rayleigh fading is exhibited by the assumption that the real and imaginary parts of the response are modelled by independent and identically distributed zero-mean Gaussian processes so that the amplitude of the response is the sum of two such processes. Applicability
Properties Since it is based on a well-studied distribution with special properties, the Rayleigh distribution lends itself to analysis, and the key features that affect the performance of a wireless network have analytic expressions. Note that the parameters discussed here are for a non-static channel. If a channel is not changing with time, clearly it does not fade and instead remains at some particular level. Separate instances of the channel in this case will be uncorrelated with one another owing to the assumption that each of the scattered components fades independently. Once relative motion is introduced between any of the transmitter, receiver and scatterers, the fading becomes correlated and varying in time. Correlation The normalised autocorrelation function of a Rayleigh faded channel with motion at a constant velocity is a zeroth-order Bessel function of the first kind:• at delay when the maximum doppler shift is . The autocorrelation function of the Rayleigh fading channel shown above with 10Hz maximum Doppler shift is shown in the figure. It is periodic in delay and its envelope decays slowly after the inital zero-crossing. Level crossing rate The level crossing rate is a measure of the rapidity of the fading. It quantifies how often the fading crosses some threshold, usually in the positive-going direction. For Rayleigh fading, the level crossing rate is:• ho e^ where is the maximum Doppler shift and is the threshold level normalised to the root mean square (RMS) signal level: ho = rac. Average fade duration The average fade duration quantifies how long the signal spends below the threshold . For Rayleigh fading, the average fade duration is: . The level crossing rate and average fade duration taken together give a useful means of characterising the severity of the fading over time. For a particular normalised threshold value , the product of the average fade duration and the level crossing rate is a constant and is given by . Doppler power spectral density The Doppler power spectral density of a fading channel describes how much spectral broadening it causes. This shows how a pure frequency e.g. a pure sinusoid, which is an impulse in the frequency domain is spread out across frequency when it passes through the channel. It is the Fourier transform of the time-autocorrelation function. For Rayleigh fading with a vertical receive antenna with equal sensitivity in all directions, this has been shown to be: u) = rac, where is the frequency shift relative to the carrier frequency. This equation is only valid for values of between ; the spectrum is zero outside this range. This spectrum is shown in the figure for a maximum Doppler shift of 10Hz. The 'bowl shape' or 'bathtub shape' is the classic form of this doppler spectrum. Generating Rayleigh fading As described above, a Rayleigh fading channel itself can be modelled by generating the real and imaginary parts of a complex number according to independent normal Gaussian variables. However, it is sometimes the case that it is simply the amplitude fluctuations that are of interest (such as in the figure shown above). There are two main approaches to this. In both cases, the aim is to produce a signal which has the Doppler power spectrum given above and the equivalent autocorrelation properties. Jakes model In his book,• Jakes popularised a model for Rayleigh fading based on summing sinusoids. Let the scatterers be uniformly distributed around a circle at angles with rays emerging from each scatterer. The Doppler shift on ray is and, with such scatterers, the Rayleigh fading of the waveform over time can be modelled as: . Here, and the and are model parameters with usually set to zero, chosen so that there is no cross-correlation between the real and imaginary parts of : and used to generate multiple waveforms. If a single-path channel is being modelled, so that there is only one waveform then can be zero. If a multipath, frequency-selective channel is being modelled so that multiple waveforms are needed, Jakes suggests that uncorrelated waveforms are given by: . In fact, it has been shown that the waveforms are correlated among themselves — they have non-zero cross-correlation — except in special circumstances.• The model is also deterministic (it has no random element to it once the parameters are chosen). A modified Jakes' model• chooses slightly different spacings for the scatterers and scales their waveforms using Walsh-Hadamard sequences to ensure zero cross-correlation. Setting and , results in the following model, usually termed the Dent model or the modified Jakes model: ight)cos. The weighting functions are the th Walsh-Hadamard sequence in . Since these have zero cross-correlation by design, this model results in uncorrelated wavforms. The phases can be initialised randomly and have no effect on the correlation properties. The Jakes' model also popularised the Doppler spectrum associated with Rayleigh fading, and, as a result, this Doppler spectrum is often termed Jakes' spectrum. Filtered white noise Another way to generate a signal with the required Doppler power spectrum is to pass a white Gaussian noise signal through a filter with a frequency response equal to the square-root of the Doppler spectrum required. Although simpler than the models above, and non-deterministic, it presents some implementation questions related to needing high-order filters to approximate the irrational square-root function in the response and sampling the Guassian waveform at an appropriate rate. See also | |||||||||
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