LTE_MIMO_schemes.pdf
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Transcript of LTE_MIMO_schemes.pdf
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MIMO schemes in 3GPPMIMO schemes in 3GPP--LTE LTE (Dec. 2007)(Dec. 2007)
Jacob Scheim, Ph.D.Jacob Scheim, Ph.D.
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MIMO schemes in 3GPP-LTE
� Downlink
� Background
� Infrastructure for MIMO over LTE
� SU-MIMO
� MU-MIMO
� Transmit diversity
� Uplink
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Background
� Max. code words - 2
� Max. Tx. Antennas – 4� Potentially up to four layers
� Pre-coding� 16 elements, unitary, for SU-MIMO
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Pilot patterns for spatial multiplexing
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sFour
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nna
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Antenna port 0 Antenna port 1 Antenna port 2 Antenna port 3
Not used for transmission on this antenna port
Reference symbols on this antenna port
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Pilot patterns for spatial multiplexing
Antenna port #0
Antenna port #2
Antenna port #3
180
[KHz]
0.5 [msec]
Antenna port #1
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Comments on reference signal
� Is not affected by applying pre-coding
� Is not affected by applying transmit diversity
� Reference signals density reduces with number of antennas:
� Higher layer MIMO for lower mobility
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Multi layer data transmission (SU-MIMO)
Space
Rank 2
MIMO
Rank 4
MIMO
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Data/Control partitioning
Increased
<= rate
region
Increased
diversity =>
region
1 msec
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Peak rates for LTE-MIMO
� “Back of the envelope” calculations:
� 11x12 – generic tile size (assuming max. control symbols)
� Peak rate:
� 64 QAM - 6 bits/symbols
� Code rate - 1 (no FEC)
� BW - 20 MHz - 110 RBs
� Sub-frames per second - 1000
� Peak rates calculation:
� SISO: 11*12*6*110*1000 =87 Mbps
� MIMO (2x2, 4x2) doubles the rate >150 Mbps
� MIMO (4x4) quadruples the rare= >300 Mbps
� High class UEs category 5 (with 4 Rx antenna)
may exceed 300 Mbps
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Infrastructure
� Pre-coding for what:
� Increasing SNR (“Beam-forming”)
� Decreasing intra-cell interference
� Decreasing inter-cell interference
� Code book design
� Unitary matrices (Householder based)
� 16 matrices for 4x(1:4) scheme
� Feedback:
� Wideband
� Narrowband
� Channel quality indication
� Per layer
� Feedback
� Wideband
� Narrowband
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Comments on MU-MIMO (SDMA)
� Requires standard adaptation for SDMA:
� Optimizing the pre-coding codebook
� Changing the CQI feedback structure
User A
User B User D
User C
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DL – transmit diversity modes
Transmit diversity
� 1 stream to 2 antennae port - Alamouti scheme
� 1 stream to 4 antennae port – Alamouti alike scheme
� Two ‘parallel’ Alamouti schemes
Cyclic delay diversity (CDD)
� Zero/small delay diversity
� Large delay diversity
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Transmit diversity
*
0
*
1
10
xx
xx
−
Origin*:
Antenna
Time*
01
*
10
xx
xx −
Antenna
Freq.
LTE:
Rate 1 codeDiversity order = 2
� Two transmit antennas
� Basic assumptions:
� Two transmit chains
� Channel doesn’t change along the ‘retransmission’
* S.M. Alamouti "A simple transmit diversity technique for wireless communications," IEEE JSAC vol. 16 (8), pp. 1451–1458
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Transmit diversity cont…
*
23
*
32
*
01
*
10
00
00
00
00
xx
xx
xx
xx
−
−
Antenna
Freq.
0123
*
1
*
0
*
3
*
2
*
2
*
3
*
0
*
1
3210
xxxx
xxxx
xxxx
xxxx
−−
−−
−−
Antenna
Time
Rate 1 code
Diversity order = 2 (spatial)
Rate 1 code
Diversity order < 4
� Four transmit antennas
LTE: Alternative*:
* Hamid Jafarkhani "A quasi-orthogonal space–time block code," IEEE Trans. Comm. Vol. 49 (1): pp. 1–4
Unbiased scheme (Ch. Est.) Biased scheme (Ch. Est.)
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Transmit diversity
� Summary:
� STBC => SFBC
� For 4 transmit antennas
� Diversity order equals 2
� Unbiased channel estimates
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Cyclic delay diversity
x0
x1
x2
xN-2
xN-1
FFT Modulator 1st Tx. antenna
FFT Modulator 2nd Tx. antenna
Xd-1
x0
x1
x2
xN-2
xN-1
xd
Xd+1
Xd-2
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� Problem
� The pilots are processed also…
� Solution
� Project the processing backwards – prior the FFT
� CDD requires extra processing for channel estimation
� Pilots remain orthogonal
Cyclic delay diversity
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Uplink
� Requirements � Open loop Tx diversity: SxBC, CDD
� 2x2 MU-MIMO� Virtual MIMO
� 2x2 SU-MIMO
� Standard� Preliminary stages
� The TS 36.211 doesn’t reflect currently any decision taken
� Technically� The same as DL
� Reference signals are orthogonal by time division� Between antennas orthogonal sequences may be used
� Two transmit antennas� A single RF chain?
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