module2_5 - Wireless Communication Systems Module 2: Mobile...

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Wireless Communication Systems Module 2: Mobile Radio Propagation Part 5 Main Reference: Rappaport Chapter 5, Sections 5.3, 5.4, 5.5, 5.6, 5.7 Small Scale Fading Estimation of channel frequency response (Sect. 5.3) For the broadband case Bx>>B C , if the transmitted signal has a flat spectrum, then the received signal spectrum is the same as the channel spectrum/frequency response. This is the principle on which small scale multipath channel estimation is based. In practice, the frequency response, or equivalently the impulse response, of a wireless channel needs to be estimated often during a communication session. This is done to enable updating of the receiver’s equalizing filter (which is used to remove channel distortion effects). A common approach to achieve this is to transmit, at regular intervals, a specific (broadband) `training’ signal. Mobile Multipath Channel Parameters (Sect 5.4) Characteristics = time dispersion, frequency dispersion Effects: Time dispersion causes signal distortion. In the case of digital signals, distortion corresponds to intersymbol interference (ISI) … received digital symbol pulses overlap each other in time. Frequnency dispersion corresponds to the distortion or ISI effects to vary with time. Time dispersion parameters Time dispersion related to the length of the impulse response h t ( τ ) (for a fixed time t). Time dispersion = length in time delay τ of channel impulse response h t ( τ29 . Change of notation: τ T. Let P(T k )=|h b (T k )| 2 =power delay profile (in practice, P is the estimated delay profile). Mean excess delay: T AVE = [ Σ {k=1:N}P(T k ) T k ]/ [ Σ{ k=1:N}P(T k )] Delay T k measured relative to first `detectable’ signal arriving at receiver (at delay T 0 ). `Detectable’ means detectable above noise … the signal power must be above a certain threshold as determined by the noise power. Rms delay spread:
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σ T = [ (T 2 ) AVE -(T AVE) 2 ] 1/2 where (T 2 ) AVE = [ Σ {k=1:N}P(T k ) T k 2 ]/ [ Σ{ k=1:N}P(T k )] See Table 5.1 Rappaport: typical measured values for rms delay spread. Maximum excess delay (XdB) T X -T 0 where T X = delay at which a multipath component is within XdB of strongest (not necessarily first) arriving multipath signal. See Figure 5.10 Rappaport. Coherence bandwidth B C = range of frequencies f 0 <= f <= f 0 +B C over which the channel causes minimal distortion to transmitted signal. That is, for f 1 <= f <= f 1 +B C : Magnitude response |H t (f)| = A (flat) ; phase response θ t (f) = b f (linear), where A,b=constants. If a transmitted signal has a bandwidth B X which is larger than B C then the receive signal will be distorted … and the receiver then needs to include an equalizer. Two
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This note was uploaded on 02/04/2012 for the course COMPUTER 101 taught by Professor Ahmed during the Summer '11 term at

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module2_5 - Wireless Communication Systems Module 2: Mobile...

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