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EE 6331, Spring, 2009 Advanced Telecommunication

EE 6331, Spring, 2009 Advanced Telecommunication. Zhu Han Department of Electrical and Computer Engineering Class 8 Feb. 12 th , 2009. Outline. Review Small Scale Fading Small-scale Multipath Propagation Dopler Effect Small-scale Multipath Measurements Four types of Fading Slow Fading

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EE 6331, Spring, 2009 Advanced Telecommunication

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  1. EE 6331, Spring, 2009Advanced Telecommunication Zhu Han Department of Electrical and Computer Engineering Class 8 Feb. 12th, 2009

  2. Outline • Review • Small Scale Fading • Small-scale Multipath Propagation • Dopler Effect • Small-scale Multipath Measurements • Four types of Fading • Slow Fading • Fast Fading • Flat Fading • Frequency Selective Fading • Introduction to shape factors • An indoor model ECE6331 Spring 2009

  3. Multipathradio propagation in urban areas ECE6331 Spring 2009

  4. Determining the impulse response of a channel • Transmit a narrowband pulse into the channel • Measure replicas of the pulse that traverse different paths between transmitter and receiver ECE6331 Spring 2009

  5. Doppler Shift ECE6331 Spring 2009

  6. Comparison of the BER for a fadingand non-fading channel ECE6331 Spring 2009

  7. Parameters of Mobile Multipath Channels Time Dispersion Parameters Grossly quantifies the multipath channel Determined from Power Delay Profile (average over different time, a function of delay) Parameters include Mean Access Delay RMS Delay Spread Excess Delay Spread (X dB) Coherence Bandwidth Doppler Spread and Coherence Time ECE6331 Spring 2009

  8. Power Delay Profiles Power delay profiles are generally represented as plots of relative received power as a function of excess delay with respect to a fixed time delay reference. Power delay profiles are found by averaging instantaneous power delay profile measurements over a local area. Are measured by channel sounding techniques Plots of relative received power as a function of excess delay They are found by averaging intantenous power delay measurements over a local area Local area: no greater than 6m outdoor Local area: no greater than 2m indoor Samples taken at l/4 meters approximately For 450MHz – 6 GHz frequency range. ECE6331 Spring 2009

  9. Impulse Response Model of a Multipath Channel ECE6331 Spring 2009

  10. PDP Outdoor ECE6331 Spring 2009

  11. PDP Indoor ECE6331 Spring 2009

  12. Time Dispersion Parameters The mean excess delay, rms delay spread, and excess delay spread (X dB) are multipath channel parameters that can be determined form a power delay profile. The mean excess delay is the first moment of the power delay profile and is defined as The rms delay spread is the square root of the second central moment of the power delay profile, where Typical values of rms delay spread are on the order of microseconds in outdoor mobile radio channel and on the order of nanoseconds in indoor radio channel Example 5.4 ECE6331 Spring 2009

  13. Maximum Excess Delay (X dB) Maximum Excess Delay (X dB):Defined as the time delay value after which the multipath energy falls to X dB below the maximum multipath energy (not necesarily belongingto the first arriving component). It is also called excess delay spread. The maximum excess delay is defined as (x - 0), where 0 is the first arriving signal and x is the maximum delay at which a multipath component is within X dB of the strongest arriving multipath signal. The value of x is sometimes called the excess delay spread of a power delay profile. In practice, values depend on the choice of noise threshold used to process P(). The noise threshold is used to differentiate between multipath components and thermal noise. Noise Thresholds The values of time dispersion parameters also depend on the noise threshold (the level of power below which the signal is considered as noise). If noise threshold is set too low, then the noise will be processed as multipath and thus causing the parameters to be higher. ECE6331 Spring 2009

  14. RMS Delay Spread ECE6331 Spring 2009

  15. Effect of delay spread ECE6331 Spring 2009

  16. Effect on error rate ECE6331 Spring 2009

  17. Coherent bandwidth Analogous to the delay spread parameters in the time domain, coherence bandwidth is used to characterize the channel in the frequency domain. Coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered flat. Two sinusoids with frequency separation greater than Bc are affected quite differently by the channel. f1 Receiver f2 Multipath Channel Frequency Separation: |f1-f2| ECE6331 Spring 2009

  18. Coherence Bandwidth Frequency correlation between two sinusoids: 0 <= Cr1, r2 <= 1. Coherence bandwidth is the range of frequencies over which two frequency components have a strong potential for amplitude correlation.  is rms delay spread If correlation is above 0.9, then If correlation is above 0.5, then This is called 50% coherence bandwidth Example 5.5 ECE6331 Spring 2009

  19. Example For a multipath channel, s is given as 1.37ms. The 50% coherence bandwidth is given as: 1/5s = 146kHz. This means that, for a good transmission from a transmitter to a receiver, the range of transmission frequency (channel bandwidth) should not exceed 146kHz, so that all frequencies in this band experience the same channel characteristics. Equalizers are needed in order to use transmission frequencies that are separated larger than this value. This coherence bandwidth is enough for an AMPS channel (30kHz band needed for a channel), but is not enough for a GSM channel (200kHz needed per channel). ECE6331 Spring 2009

  20. Coherence Time Delay spread and Coherence bandwidth describe the time dispersive nature of the channel in a local area. They don’t offer information about the time varying nature of the channel caused by relative motion of transmitter and receiver. Doppler Spread and Coherence time are parameters which describe the time varying nature of the channel in a small-scale region. ECE6331 Spring 2009

  21. Doppler Spread Measure of spectral broadening caused by motion, the time rate of change of the mobile radio channel, and is defined as the range of frequencies over which the received Doppler spectrum is essentially non-zero. We know how to compute Doppler shift: fd Doppler spread, BD, is defined as the maximum Doppler shift: fm = v/l If the baseband signal bandwidth is much less than BD then effect of Doppler spread is negligible at the receiver. ECE6331 Spring 2009

  22. Coherence Time Coherence time is the time duration over which the channel impulse response is essentially invariant. If the symbol period of the baseband signal (reciprocal of the baseband signal bandwidth) is greater the coherence time, than the signal will distort, since channel will change during the transmission of the signal . TS Coherence time (TC) is defined as: TC f2 f1 Dt=t2 - t1 t1 t2 ECE6331 Spring 2009

  23. Coherence Time Coherence time is also defined as: Coherence time definition implies that two signals arriving with a time separation greater than TC are affected differently by the channel. Coherence time Tc is the time domain dual of Doppler spread and is used to characterize the time varying nature of the frequency dispersive-ness of the channel in the time domain. If the coherence time is defined as the time over which the time correlation function is above 0.5, then the coherence time is approximately, where Example 5.6 ECE6331 Spring 2009

  24. Types of Small-scale Fading ECE6331 Spring 2009

  25. Flat Fading Occurs when symbol period of the transmitted signal is much larger than the Delay Spread of the channel Bandwidth of the applied signal is narrow. If Bs Bc , and Ts Flat fading May cause deep fades. require 20 or 30 dB more power to achieve low BER during times of deep fades. Increase the transmit power to combat this situation. The spectral characteristics of the transmitted signals are preserved at the receiver, however the strength of the received signal changes with time. Flat fading channels are known as amplitude varying channels or narrow-band channels. Radio channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of the transmitted signal. It is the most common type of fading described in the technical literature. ECE6331 Spring 2009

  26. Flat Fading h(t,t) r(t) s(t) t << TS 0 t TS+t TS 0 0 Occurs when: BS << BC and TS >> st BC: Coherence bandwidthBS: Signal bandwidth TS: Symbol periodst: Delay Spread ECE6331 Spring 2009

  27. Frequency Selective Fading Occurs when channel multipath delay spread is greater than the symbol period. Symbols face time dispersion Channel induces Intersymbol Interference (ISI) Bandwidth of the signal s(t) is wider than the channel impulse response. Radio channel has a constant gain and linear phase response over a bandwidth which is smaller than the bandwidth of the transmitted signal. Frequency selective fading is due to time dispersion of the transmitted symbols within the channel. Thus the channel induces inter-symbol-interference. Statistical impulse response model and computer generated impulse responses are used for analyzing frequency selective small-scale fading. Frequency selective fading channels are known as wideband channels since the BW of the signal is wider than the BW of the channel impulse response. As time varies, the channel varies in gain and amplitude across the spectrum of s(t), resulting in time varying distortion in the received signal r(t). If Bs Bc , and 0.1Ts Frequency selective fading ECE6331 Spring 2009

  28. Frequency Selective Fading h(t,t) r(t) s(t) t >>TS TS 0 TS+t t TS 0 0 Causes distortion of the received baseband signal Causes Inter-Symbol Interference (ISI) Occurs when: BS> BC and TS<st TS<st As a rule of thumb: ECE6331 Spring 2009

  29. Fast Fading Due to Doppler Spread Rate of change of thechannel characteristics is larger than theRate of change of thetransmitted signal The channel changes during a symbol period. The channel changes because of receiver motion. Coherence time of the channel is smaller than the symbol period of the transmitter signal It causes frequency dispersion due to Doppler spread and leads to distortion. Note that, when a channel is specified as a fast or slow fading channel, it does not specify whether the channel is flat or frequency selective A flat, fast fading channel  the amplitude of the delta function varies faster than the rate of change of the transmitted baseband signal. A frequency selective, fast fading channel  the amplitudes, phases, and time delays of any one of the multipath components varies faster than the rate of change of the transmitted baseband signal. BS: Bandwidth of the signalBD: Doppler Spread TS: Symbol PeriodTC: Coherence Bandwidth Occurs when: BS < BD and TS>TC ECE6331 Spring 2009

  30. Slow Fading Due to Doppler Spread Rate of change of thechannel characteristics is much smaller than the rate of change of thetransmitted signal Occurs when: BS >> BD and TS<< TC BS: Bandwidth of the signalBD: Doppler Spread TS: Symbol PeriodTC: Coherence Bandwidth ECE6331 Spring 2009

  31. Different Types of Fading With Respect To SYMBOL PERIOD TS Flat Fast Fading Flat Slow Fading Symbol Period of Transmitting Signal st Frequency Selective Fast Fading Frequency Selective Slow Fading TC TS Transmitted Symbol Period ECE6331 Spring 2009

  32. Different Types of Fading With Respect To BASEBAND SIGNAL BANDWIDTH BS Frequency Selective Fast Fading Frequency Selective Slow Fading Transmitted Baseband Signal Bandwidth BC Flat Fast Fading Flat Slow Fading BD BS Transmitted Baseband Signal Bandwidth ECE6331 Spring 2009

  33. Introduction to shape factors Multipath shape factors Quantify distribution of non-omnidirectional multipath wave. Direction of arrival Three factors Angular spread, angular constriction, azimuthal direction of matimum fading Four second order small scale fading statistics Level-crossing rate Average fade duration Autocovariance Coherence distance ECE6331 Spring 2009

  34. Multipath Shape Factors: Angular spread Complex Fourier coefficient Angular spread Λ is a measure of how multipath concentrates about a Azimuthal direction of arrival F0, overall power over different angle F1, first order angle gain 0 denotes extreme case of multipath components. Multipath from only one direction 1 denotes no clean bias in the angular distribution of received power. Multipath from all directions ECE6331 Spring 2009

  35. Multipath Shape Factors: Angular Constrictionand Azimuthal Direction of Maximum Fading Angular constriction is a measure of how multipath concentrates about two Azimuthal directions. 0 denotes no clear bias in arrival of two directions. 1 denotes arrival of exactly two directions. Azimuthal Direction of Maximum Fading Θmax Θmax corresponds to the direction in which mobile radio will move in order to experience maximum fading rate. ECE6331 Spring 2009

  36. Angular Distribution of power ECE6331 Spring 2009

  37. Angular Spread predicts correlation distances ECE6331 Spring 2009

  38. Angular Spread predicts correlation distances ECE6331 Spring 2009

  39. Saleh and Valenzuela Indoor Model Measured same-floor indoor characteristics Found that, with a fixed receiver, indoor channel is very slowly time-varying RMS delay spread: mean 25ns, max 50ns Maximal delay spread 100ns-200ns With no LOS, path loss varied over 60dB range and obeyed log distance power law, 3 > n > 4 Model assumes a structure and models correlated multipath components. Multipath model Multipath components arrive in clusters, follow Poisson distribution. Clusters relate to building structures. Within cluster, individual components also follow Poisson distribution. Cluster components relate to reflecting objects near the TX or RX. Amplitudes of components are independent Rayleigh variables, decay exponentially with cluster delay and with intra-cluster delay ECE6331 Spring 2009

  40. SIRCIM and SMRCIM indoor/outdoor Models These models were developed by Rappaport and seidel SIRCIM is a computer program , that generates small scale indoor channel response measurements. The most salient feature of the model is that it produces multipath channel conditions that are very realistic since they are based on real world measurements and may thus be used for meaningful system design in factories and office buildings These programs are very useful and poplar and are used in over 100 institutions. Model can measure individual multipath fading and small scale receiver spacing. Multipath delay inside the building was found to be 40ns to 800ns. Mean multipath delay ranged from 30-300 ns. Arriving multipath component has a Gaussian distribution. Average number of multipath components range from 9 to 36 ECE6331 Spring 2009

  41. SIRCIM and SMRCIM indoor/outdoor Models SIRCIM Model Based on measurements at 1300MHz in 5 factory and other buildings Model power-delay profile as a piecewise function ECE6331 Spring 2009

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