HIAST-Ayman Alsawah Lecture on Multiple-Antenna Techniques in Advanced Mobile Systems v10
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Transcript of HIAST-Ayman Alsawah Lecture on Multiple-Antenna Techniques in Advanced Mobile Systems v10
Multiple-Antenna Techniques
Communication Systems Master PROGRAM
Advanced Mobile Communication
2013-2014
Ayman [email protected]
Lecture 03-v10
Higher Institute of Applied Sciences & Technology
HIAST – Advanced Mobile Communication Ayman Alsawah, 2013/2014 2
Multiple-Antenna (MIMO) in the “Big Picture”
Narrow-Band FDMA-TDMA (200 KHz), GMSK
Multiple time-slots/user, packet-switching
8PSK, Adaptive Modulation & Coding
WCDMA (5 MHz), QPSK/BPSK, Freq. full-reuse, fast power control
Carrier Aggregation (to 40 MHz), 16/64 QAM, HARQ, MIMO
OFDM, 4x4 MIMO, All-IP
Uo to 5 x 20 MHz, 8x4 MIMO
Why Multiple Antennas?Exploit spatial dimension to: Enhance received S(I)NR (Spatial diversity/Diversity Gain or Beamforming/Array Gain) Enhance bit rate (Spatial Multiplexing/Multiplexing Gain)
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WiFi APIEEE 802.11n (2007)IEEE 802.11ac (2012)
Multiple-Antenna Configurations
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S = SingleM = Multiple
I = InputO = Output
SU-MIMO versus MU-MIMO
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Single-UserMIMO
Multi-UserMIMO
LTE Rel’8 /DL (2008)Wifi 802.11ac /DL (2013)
Fading problem (Flat fading)
Fading of Rx power causes: - degradation in BER if the Bit Rate is fixed- limitation in Bit Rate if the BER is fixed
Time
Average Rx pwr
Min required pwr(Rx sensitivity)
Rx
Pow
er (
dB
m) Fading margin
Deep fade (target BER violation)
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(dB)
BPSKUncodedFlat Rayleigh fading
Coherence time ≥ Tb
Coherent detection
(AWGN)
Erro
r p
rob
abili
tyExample of performance over flat fading
Solutions?
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1-Time diversity via Coding & Interleaving
code word bit Error burst
Block fading model(Approximation)
A B C D E F G H INon-Interleaved:
A D G B E H C F IInterleaved:
Interleaving depth
tR
x P
wr Tc
Deep fade
Tc’
After deinterleaving, isolated errors have “more chance” to be corrected
Example: GSM
• Coded speech packet interleaved over 8 bursts• 1 user-assigned burst every frame of ~5 ms Packet interleaved
on 40 ms• @900 MHz, 120 km/h:
fd = 100 Hz
Tc = 10 ms
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A B C D E F G H IDeinterleaved:
2- Frequency diversity via Freq. Hopping
Example: GSM
• Slow-FH ~200 hop/s
(Optional feature)
• Frame ≈ 4.6 ms(8 user bursts)• Typical Urban:
τRMS ≈ 1 µs
Bc = 1/(5τRMS)
= 200 KHzTime (ms)
Pow
er g
ain
(d
B)
900 MHz, 3 Km/h, Rayleigh flat fading
Frequencies much be spaced by more than the coherence
bandwidth Bc
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2- Frequency diversity via OFDM
|H(f)|
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3- Spatial diversity via Multiple Antennas
Time
Pow
er g
ain
(d
B)
Ant 1
Ant 2
Tx Rx
• For uniform surrounding scatterers:uncorrelated power gainsif antenna spacing = λ/2
• In practice: spacing λ
Example: GSM900
• 2 Rx antenna @BTS
• λ = 30 cm• Separation = 2-3 m
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4- Polarization diversity
+45°Rx
-45°Rx
Methods for exploiting “Rx Diversity” intime, frequency, space, polarization, …?
DECT Handset(1900 MHz)
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H
V
4- Polarization diversity in GSM
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DuplexingFilter
+45° -45°
Main Diversity
To CombinerFrom Tx PA
Rx Diversity: mathematical model
s = transmitted symbol (M-QAM in general) with normalized average power E[|s|2] = 1 & symbol period Ts = 1
L = number of diversity brancheshk = baseband complex gain on antenna k, invariant during 1 symbol{hk} are iid zero-mean complex circular Gaussian random processes
|hk| is Rayleigh distributed with E[|hk|2] = 1 (normalized power gain)
|hk|2 (power gain) is exponentially distributed
{nk} are iid zero-mean complex circular WGN processes with E[|nk|2]=N0
rk = hk . s + nk , k = 1,…,L.
Flat fading complex baseband model (Narrow-band or per-OFDM-subcarrier)
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Rx Diversity: SNR & CSI
rk = hk . s + nk , k = 1,…,L.
γk = |hk|2 . E[|s|2]/E[|nk|
2] =|hk|2/N0Instantaneous SNR:
Average SNR: γk, ave = E[|hk|2]/N0 = 1/N0 γk, AWGN
Instantaneous Ebno: (Eb /N0)k =|hk|2/(N0 log2 M),
Average Ebno:
M = Modulation order
(Eb /N0)k, ave = 1/(N0 log2 M) (per branch)
Received signal:
CSI Rx = subset of {{mag(h1), …, mag(hL)}, {arg(h1), …, arg(hL)}}
CSI Tx = none!
Channel State Information:
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Rx Diversity: 1-Antenna Selection
Select the highest power gain branch (max |hk|
2)
Suitable for non-coherent detection where fading phases are not needed
CSI Rx = {mag(hk)}Used on LTE UL with 2 antennas
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Rx Diversity: 2-Antenna Switching
Switch to the max power gain antenna when the current one falls below a given threshold
threshold
Ant. Sw.
Better solutions?
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CSI Rx = {mag(hk)}
Simplified hardware at the price of degraded error performance compared to “Antenna Selection”
Rx Diversity: 3-Equal-Gain Combining
z = r1 exp(-j arg(h1)) + r2 exp(-j arg(h2))
= (|h1| + |h2|) . s + [ n1 exp(-j arg(h1)) + n2 exp(-j arg(h2)) ]
rk = hk . s + nk , k = 1,2.
Instant. SNR (= Instant. EbNo for BPSK):
γEGC = (|h1|+|h2|)2 E[|s|2]/E[|n1 exp(-j arg(h1))+n2 exp(-j arg(h2))|2]= (|h1|+|h2|)2 / (2N0)
BPSK error proba.: Pe, EGC = E[Q((2γEGC)1/2)] (1/N0) -L
Expectation w.r.t. (h1, h2) joint pdf
CSI Rx = {arg(hk)}
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EGC is optimum when branches’ SINR’s have similar values
Rx Diversity: 4-Max. Ratio Combining
z = r1 h1* + r2 h2
*
= (|h1|2 + |h2|2) . s + [ n1 h1* + n2 h2
* ]
rk = hk . s + nk , k = 1,2.
Instant. SNR (= Instant. EbNo for BPSK):
γMRC = (|h1|2+|h2|2)2 E[|s|2]/E[|n1 h1* + n2 h2
*|2]= (|h1|2+|h2|2)2 / [(|h1|2+|h2|2)N0] = (|h1|2+|h2|2) /N0
= |h1|2/N0 +|h2|2 /N0 (sum of branches’ SNRs)
BPSK error proba.:
CSI Rx = {hk}
Pe, MRC = E[Q((2γMRC)1/2)] (1/N0) -L
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MRC is THE optimum combining method, equivalent to a spatial matched filter
Bit
err
or
pro
bab
ility
BPSK performance over Rayleigh Flat Fading
Other methods for exploiting multiple antennas?
Eb/N0 (dB)Average SNR per branch
Diversity Gain
Slope increase
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Reminder: Antenna Radiation Pattern
Tx power = PT
Uniform radiation intensity = PT /4 (W/strad)
Isotropic Antenna Directive Antenna
Radiation intensity = R(, )
Tx power = PT
Gain(, ) = 10 log10 D(, ) is measured in dBi (dB relative to isotropic antenna)
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Vertical Cut
Maximum gain = 2.15 dBi
Example: Half-wave dipole antenna
Horizontal Cut
How to synthesize more complex directive patterns?
Antenna pattern is Tx/Rx reciprocal
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Array antennas
Array antennas allows to control the radiation pattern by suitably arrangingantenna elements and adjusting the amplitude and phase of the signalreceived from/fed to each element, we talk about “Beamforming”
WiFi AP.
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Adaptive beamforming
Var
. Gai
ns
Var
. ph
ases
Applications?
RF beamforming
Tx case
Main beam steering Remote electrical tilting Interference reduction …
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App. 2: BS electrical down-tilting
Max Gain = 15 - 20 dBi
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App. 3: Switched beam
CSI @ BS:DL: CSI Tx = direction of userUL: CSI Rx = direction of user
1
2
3
4
Direction of Arrival(DoA)
Active beam selection
Index
Codebook
Weights
Array Gain
= Average SNR enhancement due to radiation focusing in the direction of user, w.r.t.
average SNR of single antenna
Multi-beam is also possible
No diversity gain
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App. 4: SIR maximization
Desired user
Interfering user
Directions of Arrivals
Beam synthesis
Weights
What if baseband complex CSI was available instead of DoA?
Spatial filteringor
Zero-Forcing Beamforming
CSI @ BS:DL: CSI Tx = direction of usersUL: CSI Rx = direction of users
Array Gain
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Baseband model:
Baseband Beamforming – no interference
On UL:CSI Tx = noneCSI Rx = full
Without noise: Maximize signal power <=> EGC
With noise: Maximize SNR <=> MRC
Prove that:
Exercise:
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User 1
User 2
Baseband model:
Exercise: • Find weights that yield: y = S1 (interference from user 2 is cancelled) under perfect CSI Rx and without noise (Interference Rejection Combining (IRC)). • What is the feasibility condition of this IRC?• In the noisy case, give the expression of both y and the SINR.
On UL:CSI Tx = noneCSI Rx = full
Baseband Beamforming – with interference
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Space-Division Multiple Access (SDMA)
S1
S2
On UL:CSI Tx = noneCSI Rx = full
“Baseband Multi-beam”
Multiplexing Gain
UL data rate is doubled: 2 users transmit simultaneously
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Array Gain & Diversity Gain• Array Gain = Average SNR / Single-Branch average SNR
• Diversity Gain = - log10(Δ Average error proba.) / log10(Δ Single-Branch average SNR)@ Hi SNR
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Multiple Tx Antennas (MISO)
Closed-Loop Techniques: Channel-State Information (CSI) known @ Tx side through Feedback
Tx Diversity: Antenna selection/switching (Feedback = antenna index only)
Precoding (pre-weighting) for in-phase combining @ Rx antenna
Tx RF Beamforming: beam steering, beam switching, multi-beam.
STBC (Space-Time Block Coding)
SFBC (Space-Frequency Block Coding)
Open-Loop Techniques: No CSI @ Tx
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STBC Example: 2x1 Alamouti
CSI Tx = noneCSI Rx = full
S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Sel. Areas Comm., vol. 16, pp. 1451–1458, Oct. 1998.
Space-Time Precoding matrix:
No array gainDiversity gain = 2 (full)No multiplexing gain (full)
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Used in WiFi 802.11n (2008)
2x1 Alamouti Precoding & Decoding
Tx Side:
Rx Side:
(power constraint ignored here)Decoding:
Exercise: Find instantaneous & average decoded SNR
H (orthogonal matrix)
(inverse ~ transpose & conjugate)
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SFBC Example: 2x1 Alamouti
STBC & SFBC can be generalized to more than 2 Tx antennasSee also Alamouti 2x2
Used in UMTS & LTE
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In OFDM-MIMO systems like LTE, time slots can be replaced with subcarriers when implementing Alamouti
MIMO Flat Fading Baseband Model
Pre
cod
er
Dec
od
er
GenerateCSI Tx
GeneratePrecoder
Data in out
T Antennas R Antennas
hRT
n1
n2
nR
x1
x2
xT
y1
y2
yR
hR1
h1T
h11
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MIMO Signal Model
Rayleigh iid model:
AWGN Noise vector: n
Channel matrix:
(R x T)
nxHy Received vector:
RX1 RXT TX1 RX1
(full rank)
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MIMO Spatial MultiplexingSpatial Multiplexing is a Closed-Loop (Full CSI Tx & Rx) MIMOtechnique for increasing data rate (i.e. obtaining a multiplexing gain)
where:
RXT RXR RXT TXT
• U & V are unitary (orthogonal) square matrices (i.e. UUH = IR, VVH = IT)• is a diagonal matrix whose main diagonal is formed of min{T, R} strictly positive real values:
R>T R=T R<T
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- Channel matrix is considered deterministic (known to the receiver) and can be decomposed using SVD = Singular-Value Decomposition:
MIMO: simple & great idea!
Received vector: nxy H VU
Let’s send instead of (data pre-processing):xx V~ x
nxnxy H UV VU~
Now multiply the received vector by (post-processing):HU
nxnxy HHH ~UUU~Uy
y~
has the same distribution as since is unitary
nx ~y
nn HU~ n U
min{T, R} parallel Gaussian channels
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• Case T<R: T data symbols are sent on parallelT received values + (R-T) zeros to be ignored
• Case T≥R: R data symbols + (T-R) dummy zeros are sent on parallelR received values
In all cases: data rate is increased by min{T, R} “Multiplexing Gain”(For the same error performance than SISO and without extra spectrum)
V
n1
n2
nR
UHH=UVHx
xx V~ yH ~Uy
y
x1
x2
xT
~
~
~
y1
y2
yR
~
~
~
T R
Feedback = V
LTE & WiMax (up to 8x4 on DL)WiFi .11n (4 streams)WiFi .11ac (8 streams)
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