3G EvolutionChapter: 6p 6
M lti t t h iMulti-antenna techniquesVanja Plicanic
vanja plicanic@eit lth se
Department of Electrical and Information Technology
3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 1
Outline
•Introduction
•Multi-antenna configurationsg
•Multi-antenna techniques
•Multiple receiver antennas, SIMO
•Multiple transmitter antennas MISO•Multiple transmitter antennas, MISO
•Multiple antennas at both RX and TX, MIMO
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Introduction
Multi-antenna systems
Multi-antenna techniques
Smart antennas
Multiple antennas at the receiver and/or transmitter
+
Smart signal processing
+
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Multi-antenna configurations
Base station (BS) User Equipment (UE),ex. Mobile station (MS)
Single-input single-output
Single-input multiple-output
Multiple-input single-output
Multiple-input single-output
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Antenna configurations cont.
- Configuration of the antennas is decided by the requirement on the antennag y qmutual coupling and correlation (low/high)
- Thus, configuration decided by choice of
- spatial distance between the antennas
Low mutual coupling and correlation when: p g
BS: >10 wavelengths (due to small AoA in macro cell, shorter distance in micro cells)
MS: >0 5 wavelengths (due to wide AoA)MS: >0.5 wavelengths (due to wide AoA)
- polarization directions of the antennas
A t ith diff t l i ti f b th BS d MS i l t lAntennas with different polarizations for both BS and MS gives lower mutual coupling and correlation.
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Antenna configurations cont.
- However, the case of MS at low frequencies < 900 MHzq
=> 0.5 wavelengths is large distance for low frequencies
Common phone size allows for 0 25 wavelengths distance at 850 MHz!Common phone size allows for ~0.25 wavelengths distance at 850 MHz!
=> Polarization diversity hard to implement due to antenna + chassis radiation, difficult to rotate chassis wave-mode
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Multi-antenna techniques cont.
Why? How ?
- To improve system capacity (more users per cell),better link reliability DIVERSITY
- To improve coverage (possibility for larger cells)BEAM-FORMING
- To achieve higher data rates per user,higher spectral efficiency SPATIAL MULTIPLEXING
3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 7Figures above Courtesy of Ericsson
Multi-antenna techniques cont.
DIVERSITY- Antennas at receiver and/or transmitter- Mitigates fading in the radio channel- Low mutual coupling requiredLow mutual coupling required
BEAM-FORMING- Antennas at receiver and/or transmitterAntennas at receiver and/or transmitter - Shaping of antenna beams to maximize gain
in certain direction or suppress specific interferer- Low or high mutual coupling required
SPATIAL MULTIPLEXING- Antennas at both receiver and transmitter- Sending several data streams on multiple parallel
channels- Low mutual coupling required
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p g q
Figures above Courtesy of Ericsson
Multiple receiver antennas, SIMO
Smart signal processing techniques:RX di i- RX diversity
- Receive beam-forming - Adaptive space time processingAdaptive space time processing
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Multiple receiver antennas, SIMO
RX diversity- aims to:aims to:• mitigate fading• suppress specific interferer
Linear receiver antenna combining
- All information is exploited by combining copies of the signal from all the antennas (inAll information is exploited by combining copies of the signal from all the antennas (in comparison to switched/selection diversity)
- Assumes non-time variant channel- Weights the signal copies with corresponding amplitude
and phase correction- Noise limited system:
- Maximum Ratio Combining (MRC)Interference limited system:- Interference limited system:
- Maximum Ratio Combining (MRC)- Interference Rejection Combining (IRC)- Minimum Mean Square Error (MMSE)
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Multiple receiver antennas, SIMO
RX diversity
Linear receiver antenna combining in:
N i li it d- Noise limited case:
- Maximum Ratio Combining (MRC)I t f li it d t- Interference limited system:
- Maximum Ratio Combining (MRC)Interference Rejection Combining (IRC)
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- Interference Rejection Combining (IRC)- Minimum Mean Square Error (MMSE)
Multiple receiver antennas, SIMO
RX diversity
Maximum Ratio Combining (MRC)
A lit d d h i hti- Amplitude and phase weighting - Phase weights- adjustment to assure that signals from two antennas are aligned- Amplitude weights- adjustment of the received signals to correspond to the channels gain, higher
weight for stronger signals. g g g- Diversity gain and array gain- For noise limited environments
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Multiple receiver antennas, SIMO
RX diversityRX diversity
Interference Rejection Combining (IRC)
For interference limited environment
Interference Rejection Combining (IRC)
For interference limited environment- For interference limited environment - Uplink intra-cell interference suppression, Spatial Division Multiple Access (SDMA)- Able to suppress Nr-1 interferers, however large noise increment after combining
- For interference limited environment - Uplink intra-cell interference suppression, Spatial Division Multiple Access (SDMA)- Able to suppress NR-1 interferers, however large noise increment after combining
Minimum Mean Square Error (MMSE)Minimum Mean Square Error (MMSE)
- Weights to minimize the difference between the estimated and transmitted signal.- Weights to minimize the difference between the estimated and transmitted signal.
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Multiple receiver antennas, SIMO
Adaptive space-time processing
- Frequency selective channel- Linear time domain filtering/equalization, linear processing to signals received at different
times (MRC Zero forcing MMSE)times (MRC, Zero-forcing, MMSE)- Linear receive antenna combining, linear processing to signals received at different
antennas
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Multiple receiver antennas, SIMO
Receive beam-forming
- Switched beam antennas- Antenna array that can form pattern beams pointing in certain discrete direction
i hi l h “b ” b f d i d i l- switching selects the “best” beam for down conversion and post processing, goal to maximize the SNR
- simple implementation, since only one signal to post process- limited flexibility, since only fixed directionsy, y
- Amplitude and phase weights
MRC => a receiver beam with maximum gain NR in the direction of the target signal
IRC => a receiver beam with high attenuation in the direction of theIRC > a receiver beam with high attenuation in the direction of the target signal
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Multiple receiver antennas, SIMO
Receive beam-forming
Switched antenna array Adaptive antenna arraySwitched antenna array Adaptive antenna array
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Figures above Courtesy of jackwinters.com
Multiple transmit antennas
Smart signal processing techniques:- TX diversity - Transmit beam-forming
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Multiple transmit antennas
TX diversity
- Does not require channel knowledge at the receiver- Techniques:Techniques:
- Delay/Temporal diversity- Cyclic-delay diversity- Space time/frequency coding (STBC/STFC)
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Multiple transmit antennas
TX diversity
Delay/Temporal diversity
- Time variant channel- Time variant channel
=> signals received at different times are uncorrelated=> delay diversity already there and can be extracted in advanced receivers
(ex GRAKE)(ex. GRAKE)
- Time in-variant channel
=> create artificial time dispersion (frequency selectivity)
=> transmit identical signals with different delays from different antennas
- Delay diversity usually implemented by forward error correction, ARQ, repetition coding etc.- Delay diversity invisible to mobile terminal since it is just additional time dispersion handled by the
receiver
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Multiple transmit antennas
TX diversity
Cyclic-delay diversity
A li li hift i t d f li d l- Applies cyclic shift instead of linear delays
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Multiple transmit antennas
TX Diversity
Space-time block coding (STBC)
- Sending same but differently coded information on each of the antennas, ex. Alamouti scheme
- used in 3G WCDMA standard as Space Time Transmit Diversity (STTD)- Orthogonal STBC => full rate=1, full diversity gain only for two antennas- No array gain, only diversityNo array gain, only diversity- Space-time trellis to provide full diversity, array gain and coding gain
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Multiple transmit antennas
TX Diversity
Space-frequency block coding (SFBC)
- Space-frequency Transmit Diversity (SFTD)
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Multiple transmit antennas
Transmit beam-forming
- Requires channel knowledge
- Antenna configurations with high mutual couplingS- Small antenna distances
- Different phase shifts applied to steer the direction of the beam
- “CLASSICAL BEAM-FORMING” Hi h i di i i- High array gain, no diversity gain
- Antenna configurations with low mutual coupling- Large antenna distances or different polarizationLarge antenna distances or different polarization- Different gain and phase shifts to steer the direction of the beam - Pre-coding decided from:
- Channel feedback from mobile terminal average downlink estimate, ex. FDD- Recommendation from mobile terminalRecommendation from mobile terminal
- Pre-coding for non-frequency-selective fading and white noise- Maximum Ratio Transmission- instant channel estimate, “fast beam-forming”- diversity gain and array gain
- Pre-coding for frequency-selective fading not possible, NB! OFDM time invariant sub-channels
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Multiple antennas at both RX and TX
Smart signal processing techniques:- Spatial multiplexing- Pre-coder based spatial multiplexing
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Multiple antennas at both RX and TX
Spatial multiplexing
Background:- SIMO and MISOSIMO and MISO
Low SNR => capacity increase ~ SNR increase (NTxNR)High SNR => capacity increase ~ log2(SNR)
- Spatial multiplexing
Th it i i {N N }Thus, capacity increase ~ min {NT, NR}
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Multiple antennas at both RX and TX
Spatial multiplexing
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Multiple antennas at both RX and TX
Pre-coder based spatial multiplexingPre coder based spatial multiplexing
- If SNR low => beam-forming better than spatial multiplexing- If N =number of multiplexed streams = N- If NL=number of multiplexed streams = NT
Pre-coding=> “orthogonalizes” parallel streams, better signal isolation- If NL< NT
=> combination of beam-forming and spatial multiplexing used=> combination of beam-forming and spatial multiplexing used
- Depending on the channel information pre-coder code-books chosen
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Multiple antennas at both RX and TX
Some SM detection techniques
- Maximum-Likelihood MLLa ered space time architect res (BLAST)- Layered space time architectures (BLAST)
- Successive Interference Cancellation (SIC)- Single and Multi-codeword Transmission
P A t R t C t l (PARC)- Per Antenna Rate Control (PARC)
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Chapter summary
Multi-antenna techniques
SIMO Diversity gain and array gainMISO Diversity gain and/or array gain MIMO Diversity gain and/or array gain and/or multiplexing gain
SIMO Diversity gain and array gainMISO Diversity gain and/or array gain MIMO Diversity gain and/or array gain and/or multiplexing gainMIMO Diversity gain and/or array gain and/or multiplexing gainMIMO Diversity gain and/or array gain and/or multiplexing gain
Multiplexing gain
Link reliability
Spectral efficiency
Diversity gain
Diversity gain and array gain Coverage
Link reliability
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References
[1] Dahlman E. et al., 3G evolution-HSPA and LTE for Mobile Broadband, 2nd edition,El i UK 2008Elsevier, UK 2008
[2] Paulraj A. et al., Introduction to Space-Time Wireless Communications, Cambridge, UK 2003
[3] Molisch A.F., Wireless Communications, IEEE Press, Wiley & Sons, US 2006[3] Molisch A.F., Wireless Communications, IEEE Press, Wiley & Sons, US 2006
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