Diana B. Llacza Sosaya Digital Communications Chosun University

2 15.1The Challenges of Communicating over Fading Channels 15.2 Mobile Radio Propagation Large-Scale Fading Small-Scale Fading 15.3 Signal Time-Spreading View in the Time-Delay Domain View in the Frequency Domain 15.4 Time Variance of the Channel caused by Motion 15.5 Mitigating the Degradation Effects of Fading 15.6 Summary of the Key Parameters Characterizing Fading Channels 15.7 Applications 15.8 Conclusion

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4 Additive white Gaussian Noise (AWGN) channel, with statistically independent Gaussian noise, samples corrupting data samples free of intersymbol interference (ISI). Source of performance degradation: Thermal noise at the receiver. External noise and Interference received by the antenna. Introduction of bandlimiting filters Filtering in the transmitter usually serves to satisfy some regulatory requirement on spectral containment. Due to bandlimiting and phase-distortion properties of filters, special signal design and equalization techniques may be required to mitigate the filter-induced ISI. Signal attenuation vs Distance behaves as if propagation takes place over ideal Free Space.

5 d  distance between transmitter and receiver  wavelength of the propagation signal Scintillation  describe the fading caused by physical changes in the propagation medium. Region free of all objects between the transmit and receive antenna that might absorb or reflect radio frequency (RF) energy. Free Space Free Space Path loss A signal can travel over multiple reflective paths. It can cause multipath fading or scintillation, due its fluctuations in the received signal’s amplitude, phase, and angle of arrival. Multipath Propagation

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7 FADING Large Scale Fading Small Scale Fading Attenuation with distance Variation about mean Time spreading of the signal Time variance of channel Flat Fading Frequency Selective Fading Fast Fading Slow Fading Time delay Domain Frequency Domain Flat Fading Frequency Selective Fading Doppler shift Domain Time Domain Fast Fading Slow Fading

8 Large-Scale Fading Average signal power attenuation or the path loss due to motion over large areas. This fading is affected by prominent terrain contours. The estimation of path loss is in function of distance and described in terms of: Mean-path loss (nth-power law) Log-normally distributed variation about the mean Small-Scale Fading Dramatic changes in signal amplitude and phase, due to small changes in the spatial positioning. The two small scale mechanisms are examined in two domains: time and frequency Time-spreading of the signal (signal dispersion): are categorized as frequency selective or flat (frequency nonselective). Time-variant behavior on the channel: both are categorized as fast fading or slow fading.

9 There are 3 basic mechanism impacting Signal Propagation in a Mobile Communication System: Reflection: A propagation electromagnetic wave impinges on a smooth surface with very large dimensions relative to the RF signal. Diffraction: Propagation path is obstructed by a dense body, causing secondary waves to be formed behind the obstructing body with large dimensions relative to the RF signal. It’s termed Shadowing. Scattering: Radio waves impinge on a large rouge surface causing the energy to be spread out or reflected in all directions.

10 Show contributions that must be considered when estimating path loss: Mean path loss as function of distance due to large-scale fading. Near worst case variations about the mean path loss or large-scale fading margin (6-10dB) Near worst case Rayleigh or small scale fading margin (20-30dB).

11 In fading environment, the transmitted signal is: m(t)  large-scale-fading component or local mean or log-normal fading  small-scale-fading component or multipath or Rayleigh fading Relationship between multiplicative factor α (t) and large-scale-fading component m(t): The signal power received is a function of the multiplicative factor. The typical antenna displacement between adjacent signal-strength nulls due to small-scale fading is approximately a half wavelength.

12 Relationship between multiplicative factor α (t) and large-scale-fading component m(t): The large-scale fading has been removed in order to view the small- scale fading referred to some average constant power. The log-normal fading is relative slow varying function of position. Rayleigh fading is relatively fast varying function of position.

13 Propagation models for both indoor and outdoor radio channels indicate that the mean path loss is proportional to an nth-power of d relative to a reference distance d 0. The value of the path-loss exponent n depends on the frequency, antenna heights and propagation environment. In the free space where signal propagation follows an inverse-square law  n=2 In the presence of a very strong guided wave phenomenon  n<2 When obstructions are present  n>2

14 When the Antenna is traveling, there are multiple scattered paths, each associated with: *time-variant propagation delay *time-variant multiplicative factor The received bandpass signal: Effect of a multipath reflected signal on a desired signal: A reflected signal has a phase delay with respect to a desired signal. The reflected signal has reduced amplitude.

15 Response of a multipath channel to a narrow pulse vs delay as a function of antenna position: Delay Time  time-spreading effect resulting from the fading channel’s nonoptimum impulse response. Transmission Time  related to the antenna’s motion or spatial changes. The response pattern differs in the delay time of the largest signal component, the number of signal copies, magnitudes, and the total received power.

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17 Bello  introduce the simple way to fading phenomemon Wide-sense stationary uncorrelated scattering (WSSUS) model treats signals arriving at a receive antenna with different delays. The channel is effectively WSS in time and frequency domains. Relationships among the channel correlation functions and power density functions:

18 (a) Multipath intensity profile Time delay  referred as excess delay, is the signal’s propagation delay that exceeds the delay of the first signal arrival at the receiver. For the typical wireless channel  receive signal consists of several discrete multipath components causing to exhibit multiple isolated peaks, called as fingers or returns. For some channels as tropospheric  received signal are seen as continuum of multipath components. is a relatively smooth (continuous) function of. For a single transmitted impulse  is the maximum excess delay, after which the multipath signal power falls below some threshold level relative to the strongest component. For an ideal system (zero excess delay)  is the ideal impulse with weight to the total average received signal power.

19 Two different categories: Frequency-selective fading: Excess delay time > symbol time Received multipath components of a symbol extend beyond the symbol’s time duration. Fading degradation is channel-induced ISI  multipath dispersion of the signal yields the same kind of ISI distortion. Mitigating the distortion is possible  because many of the multipath components are resolved by the receiver. Frequency nonselective or flat fading: Received multipath components arrive within the symbol time duration. No channel induced ISI distortion  the signal time spreading doesn’t overlap significantly among neighboring received symbols. There is degradation  components can yield a reduction in SNR. Introduce signal diversity and using error-correction coding  efficient way to improve the received SNR in digital systems.

20 (b) Spaced-frequency Correlation function is the Fourier transform of Represent the correlation between the channel’s response to two signals as frequency difference between the two signals. is the coherence bandwidth Represent a frequency range over which a signal’s frequency components have a strong potential for amplitude correlation, equal gain and linear phase. Two different categories: Frequency-selective : W is the signaling rate or signal bandwidth Fading distortion occurs when a signal’s spectral components are not all affected equally by the channel. Some of the components falling outside the coherence bandwidth will be affected differently.

21 Two different categories: Frequency-selective : Various spectral components will be affected differently. Spectral density vs frequency of a transmitted signal having a bandwidth of W Frequency nonselective or flat fading: All of the signal’s spectral components will be affected by the channel in a similar manner. No introduce channel-induced ISI distortion Performance degradation  due to the loss in SNR whenever the signal is fading..

22 Two different categories: Frequency nonselective or flat fading: All of the signal’s spectral components will be affected by the channel in a similar manner. No introduce channel-induced ISI distortion Performance degradation  due to the loss in SNR whenever the signal is fading. All of the spectral components of the transmitted signal will be affected in the same way..

23 Two different categories: D ue a mobile radio changes its position  there will be times when the received signal experiences frequency-selective distortion even though The null of the channel's frequency transfer function occurs near the band center of the transmitted signal's spectral density  baseband pulse can be mutilated by deprivation of its low-frequency components  the loss is the absence of a reliable pulse peak

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25 What are the two degradation categories that characterize signal time- spreading? Explain them through a comparison.

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