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MIMO Systems and Spatial Diversity

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MIMO Systems and Spatial Diversity MIMO systems and Spatial Diversity. Dmitriy Shutin [email protected]. Signal Processing and Speech Communication Laboratory www.spsc.tugraz.at Graz University of Technology MIMO systems andSpatial Diversity. – p.1/36 Content of the talk PART I. Wireless communication and MIMO systems. Wireless channels. Part II. Spatial Diversity MIMO systems andSpatial Diversity. – p.2/36 Outline – PART I. PART I. What is MIMO Multiple antennas in communication Channel and channel models. Channel and signal models. MIMO systems andSpatial Diversity. – p.3/36 PART II. MIMO systems andSpatial Diversity. – p.4/36 Outline – PART II. What is diversity? Diversity schemes. What do we actually win. Implementing diversity – IID channels RX diversity(SIMO). Alamouti scheme and TX diversity (MISO). RX-TX diversity (MIMO). Extended channels. MIMO systems andSpatial Diversity. – p.5/36 What is diversity? Diversity – Multiple independent look at the same transmitted signal. MIMO systems andSpatial Diversity. – p.6/36 What is diversity? Fading impairs wireless link. Use multiple antennas to create alternative transmission branches. −60 1st receiver antenna 2nd receiver antenna −80 −100 −120 Power variations, dB −140 −160 216 218 220 222 224 226 distance, λ MIMO systems andSpatial Diversity. – p.7/36 What is diversity? If p = P (signal is in fade) Having M independent branches results P (all M branches are in fade) = pM p There are Frequency diversity Time diversity Space diversity MIMO systems andSpatial Diversity. – p.8/36 Diversity schemes. Space diversity: RX/TX has multiple antennas, spaced d meters apart such that d > Dc Frequency diversity: signal is transmitted over two carrier frequencies, spaced ∆f, such that 1 ∆f > Bc ∼ RMS delay spread Time diversity: the same signal is re-transmitted with a delay ∆t such that 1 ∆t > Tc = BD MIMO systems andSpatial Diversity. – p.9/36 Diversity Gain To exploit diversity we need: Combine multiple branches in some way. Make sure branches are independent. Separation > Bc; Tc; Dc Signal model for each branch: Es yi = his + ni; n = 1; : : : ; M r M hi – flat-fading channel coefficient; Es – symbol energy; ni – additive noise. MIMO systems andSpatial Diversity. – p.10/36 Diversity Gain, cont’d To combine branches – Maximum Ratio Combining: M ∗ z = hi yi i=1 X Each branch increases SNR M 1 2 Es η = hi = α ρ M j j · N0 · i=1 ! X M 1 2 Es α = hi – branches gain, ρ = – Single-branch SNR M j j N0 i=1 ! X MIMO systems andSpatial Diversity. – p.11/36 Diversity Gain, cont’d Assuming ML detection, averaged probability of the symbol error can be upper-bounded as 2 −M ρdmin P e N e ≤ 4M ! Ne - number of nearest neighbors 2 dmin – minimum separation of the symbol constellation. Es ρ = N0 – averaged SNR at the receiver antenna. MIMO systems andSpatial Diversity. – p.12/36 Diversity Gain, cont’d Effect of diversity on the SER performance in fading channels. Diversity affects the slope of the SER-SNR curve. MIMO systems andSpatial Diversity. – p.13/36 Coding gain vs diversity gain Approximate P e can be expressed c P e M ≈ (γcρ) c – modulation/channel constant γc 1 – coding gain M ≥– diversity order MIMO systems andSpatial Diversity. – p.14/36 Spatial vs Time/Frequency diversity Spatial diversity No additional bandwidth required Increase of average SNR is possible Additional array gain is possible Time/Frequency diversity Time/frequency is sacrificed No array gain. Averaged receive SNR remains as that for AWGN channel. MIMO systems andSpatial Diversity. – p.15/36 Implementing Spacial diversity Receiver diversity – SIMO Transmit diversity– MISO RX/TX diversity – MIMO MIMO systems andSpatial Diversity. – p.16/36 Receive antenna diversity Let us assume flat fading T y = pEshs + n; h = [h1; : : : ; hMR ] Maximum Ratio Combining at the receiver H H H z = h y = pEsh hs + h n MRC assumes perfect channel knowledge at RX. MIMO systems andSpatial Diversity. – p.17/36 Receive antenna diversity, cont’d Averaged probability of symbol error is given as (at high SNR regime and rich scattering) 2 −MR ρdmin P e N e ≥ 4 ! Averaged SNR at the receiver η = MR ρ · Array gain is MR (10 log10 MR [dB]) MIMO systems andSpatial Diversity. – p.18/36 Receive antenna diversity - performance Can be better than AWGN due to array gain For BER> 10−5 outperforms AWGN due to array gain. RX diversity extracts full diversity and array gain! MIMO systems andSpatial Diversity. – p.19/36 Transmit antenna diversity Straightforward approach is useless: Signal model (for MT = 2); h1; h2 s (0; 1) N Es y = (h1 + h2)s + n r 2 Equivalently, h (h1 + h2)=p2, and h s (0; 1) ≡ N y = Eshs + n Unlike RX diversity, pre-processingp is needed. MIMO systems andSpatial Diversity. – p.20/36 TX diversity – Alamouti(MISO) Preprocessing without channel knowledge. MIMO systems andSpatial Diversity. – p.21/36 Alamouti scheme, cont’d Channel is constant over two symbols intervals and flat-fading h = [h1; h2] Two received symbols are: Es Es y1 = h1s1 + h2s2 + n1 r 2 r 2 Es ∗ Es ∗ y2 = h1s2 + h2s1 + n2 −r 2 r 2 MIMO systems andSpatial Diversity. – p.22/36 Alamouti scheme, cont’d ∗ T Now, received vector is formed as y = [y1; y2] Es h1 h2 s1 n1 Es H y = ∗ ∗ + ∗ = effs + n 2 h2 h1 s2 n2 2 r " − # " # " # r H 2 MRC gives ( using HeffHeff = h I) k k H Es 2 z = Heff y = h Is + n~ r 2 k k Es 2 which simplifies to zi = h si + n~i. 2 k k q MIMO systems andSpatial Diversity. – p.23/36 Alamouti scheme – Performance High SNR regime gives 2 2 − ρdmin P e N e ≤ 4 2 · ! No array gain: η = ρ Full transmit diversity for MR = 2 How to implement array gain? MIMO systems andSpatial Diversity. – p.24/36 MISO with known channel T Flat-fading channel is given as h = [h1; : : : ; hMT ] Signal at the receiver E y = S hHws + n sMT h where w = pMT khk This scheme is known as transmit MRC. MIMO systems andSpatial Diversity. – p.25/36 Transmit MRC – Performance High SNR regime gives 2 −MT ρdmin P e N e ≤ 4 ! Array gain in rich scattering η = MT ρ · Equivalent to RX diversity. Transmitter should be aware of the channel. MIMO systems andSpatial Diversity. – p.26/36 Alamouti scheme & MIMO Transmit symbols like MISO Alamouti Flat-fading channel matrix h1;1 h1;2 H = "h2;1 h2;2# Receive symbols (MR = 2) ∗ Es s1 Es s2 H 1 H 2 y1 = + n ; y2 = −∗ + n ; r 2 "s2# r 2 " s1 # equivalently... MIMO systems andSpatial Diversity. – p.27/36 Alamouti scheme & MIMO, cont’d ∗ T By defining y = [y1; y2] h1;1 h1;2 2 1 2 2 1 n1 Es 2h ; h ; 3 s y = ∗ ∗ + r 2 h1;2 h1;1 "s2# "n2# 6 ∗ − ∗ 7 6h2 2 h2 17 6 ; − ; 7 4 5 H 2 MRC is formed (using HeffHeff = H I) k kF Es H 2 I z = F s + n~; or, r 2 k k Es H 2 zi = F si + n~i r 2 k k MIMO systems andSpatial Diversity. – p.28/36 Alamouti scheme & MIMO, cont’d High SNR regime gives and rich scattering 2 −MRMT ρdmin P e N e ≤ 4MT ! Diversity order MRMT Array gain η = MRρ Thus, without channel knowledge only array gain is obtained. MIMO systems andSpatial Diversity. – p.29/36 MIMO with channel knowledge If channel known – transmit MRC is used Es y = HH ws + n sMT Receiver forms weighted sum z = gH y Vectors w and g are singular vectors from SVD decomposition of H H = UΣVH; w U; g V 2 2 2 H w and g correspond to the max singular value σmax of This is known as dominant eigenmode transmission – DET MIMO systems andSpatial Diversity. – p.30/36 MIMO with DET – Performance Diversity order MRMT SNR η = E σ2 ρ f maxg 2 max(MT ; MR) E σ ≤ f maxg 2 E σ MT MR f maxg ≤ Channel knowledge increases array gain. MIMO systems andSpatial Diversity. – p.31/36 Summary Configuration Array gain Diversity gain SIMO (Ch.Un.) MR MR SIMO (Ch.Kn.) MR MR MISO (Ch.Un.) 1 MT MISO (Ch.Kn.) MT MT MIMO (Ch.Un.) MR MRMT 2 MIMO (Ch.Kn.) max(MT ; MR) E σmax , MRMT 2 ≤ f g E σ MT MR f maxg ≤ MIMO systems andSpatial Diversity. – p.32/36 Diversity order and channel variability Can be quantified as 1 µvar = pMRMT Link stabilizes as µvar ! 1 MIMO systems andSpatial Diversity. – p.33/36 Diversity order for extended channels Elements in channel matrix H are Correlated Have gain imbalances Fade with Ricean statistics Highest possible diversity rich scattering (IID elements in H) −! Let as consider Alamouti scheme with MT = MR = 2 MIMO systems andSpatial Diversity. – p.34/36 Correlations in H Diversity order decreases to r(R), where R is (4 4) covariance Matrix: × R = E vec(H)vec(H)H f g Rank r(R) = 1 full correlation. −! No spatial diversity. Array gain is present. MIMO systems andSpatial Diversity. – p.35/36 Ricean fading of H Ricean fading occurs when LOS is present. As K link stabilizes. K-factor! 1is given as 2 ALOS K = 2 σdiffuse MIMO systems andSpatial Diversity. – p.36/36.
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