LTE MIMO Schemes

MIMO schemes in 3GPPMIMO schemes in 3GPP--LTE LTE (Dec. 2007)(Dec. 2007)

Jacob Scheim, Ph.D.Jacob Scheim, Ph.D.

2(C) 2007 Comsys communication and Signal Processing Ltd. All rights reserved

MIMO schemes in 3GPP-LTE

� Downlink

� Background

� Infrastructure for MIMO over LTE

� SU-MIMO

� MU-MIMO

� Transmit diversity

� Uplink


Background

� Max. code words - 2

� Max. Tx. Antennas – 4� Potentially up to four layers

� Pre-coding� 16 elements, unitary, for SU-MIMO


Pilot patterns for spatial multiplexing

R0

R0

R0

R0

R0

R0

R0

R0

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R2

R2

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R2

On

e ante

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port

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ort

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ante

nna

por

ts

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


Pilot patterns for spatial multiplexing

Antenna port #0

Antenna port #2

Antenna port #3

180

[KHz]

0.5 [msec]

Antenna port #1


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


Multi layer data transmission (SU-MIMO)

Space

Rank 2

MIMO

Rank 4

MIMO


Data/Control partitioning

Increased

<= rate

region

Increased

diversity =>

region

1 msec


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


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


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


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


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


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.)


Transmit diversity

� Summary:

� STBC => SFBC

� For 4 transmit antennas

� Diversity order equals 2

� Unbiased channel estimates


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


� 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


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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