VIEWS: 19 PAGES: 29 POSTED ON: 3/19/2011
Diversity Presented by ENG.: Ahmed Hamza Supervisor: Dr. Mohab Mangoud Outline Introduction. What is diversity? Why? Types of diversity Space diversity. Polarization diversity. Frequency diversity. Time diversity. Receiver diversity system model & combining techniques. Types of combining techniques. Comparative Performance of Diversity Combining Techniques Introduction In a fading environment the received signal power varies randomly over distance or time due to shadowing and/or multipath fading. Transmitter Receiver CON. Transmitter Receiver What is diversity? Why? Diversity – Transmitting and processing multiple copies of the same signal, is an effective way of mitigating fading Diversity-combining uses the fact that independent signal paths have a low probability of experiencing deep fades simultaneously. Prob(all channels are faded) < Prob(one channel is in a fade) Microdiversity: mitigates the effect of multipath fading. Macrodiversity: mitigates the effects of shadowing from buildings and objects. Type of diversity How to create and take advantage of independent paths? Diversity Space Frequency Polarization Time Space diversity Definition Uses multiple transmit or receive antennas separated in distance. Advantages Reduce fading (at receiver) without increase transmitted signal power or bandwidth. Increase SNR. Disadvantages The transmit power must be divided among multiple antennas. the received SNR is the same as single transmit antenna was used. Polarization diversity Definition Using two transmit or receive antennas with different polarization (e.g. vertically and horizontally polarized waves). Disadvantages Uses at most two branches corresponding to two types of polarization. The transmit or received power is divided between two differently polarized antennas. Frequency diversity Definition: Transmit the narrowband signal over multiple carrier frequencies Frequency separation must be greater than the coherence bandwidth of the channel coherence bandwidth = 1/(2πτrms) and it represents the frequency separation of uncorrelated signals For τrms = 10 µs, coh. BW = 16 kHz Pros and Cons: Fewer antenna elements and corresponding circuitry. Requires twice the bandwidth. Multiple mixers for the different carriers. requires additional power to send the signal over multiple frequency bands. Time diversity Definition: transmitting the same signal at different times, with time separation >= 1/fdoppler Can also be realized using interleaving Advantages: not require increased transmit power. Disadvantage: decrease the data rate. Receiver diversity system model & combining How to take advantage of the independent diversity paths to improve the communications link? The out of the combiner is just a weighted sum of the different fading paths or branches Receiver diversity system model & combining The signal output from the combiner equals the original transmitted signal s(t) multiplied by a random complex amplitude term. Sout (t)= S(t) the distribution of SNR is a function of: the number of diversity paths. the fading distribution on each path. combining technique Definitions: עs :the SNR per symbol. Ps :The symbol error probability Pout: the probability that עs falls below a given value corresponding to the maximum allowable Ps Diversity gain: SNR gain given by extracting the same information from two independently fading channels. M : diversity order of the system Types of combining techniques Combining Selection Threshold Maximal ratio Equal gain Selection Combining Definition: the combiner outputs the signal on the branch with the highest SNR Advantages: requires just one receiver co-phasing of multiple branches is not required. Disadvantage: only one branch is used at a time. Performance the average SNR gain increases with M, but not linearly. Performance Threshold Combining Definition: Once a branch is chosen, as long as the SNR on that branch remains above the desired threshold called switch and stay combining (SSC) Advantages: avoids the need for a dedicated receiver on each branch co-phasing of multiple branches is not required. Disadvantage: only one branch is used at a time. Performance SSC with an optimized threshold has the same outage probability as SC. Maximal Ratio Combining Definition: Measure the signal amplitude and phase on each antenna. the output is a weighted sum of all branches. the signals are cophased r1 r2 rM G1 G2 GM Co-Phase and Add rsum Performance r1 r2 rM Consider M diversity branches Co-phase the signal from all branches G1 G2 GM Individually weight and then add all the signals to provide optimal SNR Co-Phase and Add Resulting signal amplitude at the detector will rsum be given by: M rM = ∑ r G i i i = 1 Assuming the same noise power N on all branches, the noise power at the output of the combiner is: M N T = ∑ Gi2 N i =1 SNR at the detector: γM = rM2 / NT Find the Gi such that γM is maximized Optimum Combiner Weight Performance Improvements with MRC Advantage & Disadvantage Advantage: Multi_branch is used at a time. Good performance . Disadvantage: requires knowledge of the time-varying SNR on each branch. co-phasing of multiple branches is required. Equal Gain Combining Definition: co-phases the signals on each branch and then combines them with equal weighting. Same as MRC but with Gi = 1 Performance: Instantaneous SNR after combining The performance gain with EGC is very close to that of MRC Comparative Performance of Diversity Combining Techniques Comparative Performance of Diversity Combining Techniques SC MRC Comparative Performance of Diversity Combining Techniques
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