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Chapter 5. Radio Wave > Channel Impulse Response Model

5.4. Channel Impulse Response Model

We have examined the general transmission characteristics of indoor radio frequency channels in terms of signal losses. To utilize the transmission potential of the channel fully, an optimized transceiver architecture is required. The choice and design of the transceiver architecture including signal modulation and detection methods depend on the impulse response and the noise environment of the channel. The impulse response of an indoor radio frequency channel is useful at determining how fast the signaling rate could be with or without using certain types of channel equalization techniques. The channel impulse response can be exactly calculated if the reflection mechanism is relatively simple. Such a simple channel impulse response is the combination of the direct received signal and a limited number of reflections from a few walls. However, measurements show that the reflection mechanism of an indoor radio channel usually is more complicated because of reflections from walls and many other household objects. Therefore, the statistical modeling approach has been taken for the indoor radio environment. The statistical indoor radio channel model is based on extensive measurements of indoor radio environment at different locations. Through data analysis, statistical parameters have been derived depending on separation distance and whether the radio wave is directly received. We call the directly received case line of sight (LOS) and the indirectly received case obstructed (OBS).

The measurement of the indoor radio channel can be carried out in the time domain or in the frequency domain. To measure the indoor radio channel impulse response in the time domain, we need to send a short time duration pulse (simulating an impulse) at one point and synchronously receive the pulse at another point. To represent the channel accurately, the sampling rate needs to be fast (at least twice the interested bandwidth). At gigahertz frequency bands, the direct sampling of the radio frequency channel at a high enough resolution presents a challenge to existing measurement equipment. In practice, the measurement of the indoor radio frequency channel is usually carried out in the frequency domain. The channel attenuation at each frequency is measured step by step to cover the whole spectrum. The time domain information is obtained through offline Fourier transforms. The step frequency should be chosen such that the Fourier-transform time domain response is long enough to cover all possible reflections. For example, a 1-MHz step frequency results in a time domain duration of 1 microsecond (µs). Since only the power attenuation level is measured, the Fourier-transform time domain information represents the squared magnitudes along with different time delays. The amplitude of the impulse response can be obtained by taking the square root. However, the phase information is still missing. This is the shortcoming of the frequency measurement method. A method of random phase has been used to supplement the information gathered from these Fourier-transform measurements when a complete channel impulse response is desired.


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