Spatial Multiplexing - · PDF file–space Öspace–division multiplexing (SDM) or spatial multiplexing • different bits from different antennae • requires independent channels

MIMO Communications with Applications to (B)3G and 4G Systems ─ Spatial Multiplexing

© M. Juntti et al., University of Oulu, Dept. Electrical and Inform. Eng., Centre for Wireless Communications (CWC) 1

Spatial Multiplexing

Tutorial ─ MIMO Communications with Applications to (B)3G and 4G Systems

Markku Juntti & Juha Ylitalo

Contents1. Introduction to spatial multiplexing2. Layered architectures3. Spatial multiplexing in 3G systems: PARC4. Summary and Conclusions

References



1. Introduction to SpatialMultiplexing

• The basic concept of multiplexing: divide (multiplex) transmit a data stream several branches and transmitvia several (independent) channels in – time time–division multiplexing (TDM)– frequency frequency–division multiplexing (FDM)

• typical example: orthogonal FDM (OFDM)– space space–division multiplexing (SDM) or spatial

multiplexing• different bits from different antennae• requires independent channels

– code code–division multiplexing (CDM)• applied in 3G systems.



Spatial Multiplexing Idea

• Several different data bits are transmitted via several independent (spatial) channels.

zF1*

zFN*

Rx

zF1s

zFNsOutput

s(n)

Signal to betransmitted

S/P

s1

s2

Feedback: zF1 , ... , zFN



Characteristics

• No bandwidth expansion.• Space–time equalization needed in the receiver.

– Conventionally: no. Rx antennae ≥ no. Tx antennae.

• The data streams can be separated by the equalizer, if fading processes of the spatial channels are (nearly) independent.Actual multiple–input multiple–output channel with capacity linearly increasing the number of antennae or more precisely independent spatial channels.

• Alternative to spatial diversity: multiplexing–diversity trade–off is under intensive study.



Linear Dispersion Coding

• Linear dispersion coding (LDC) offers a framework to combine spatial multiplexing and transmit diversity.

• Code design consists of finding optimum dispersion matrices.

Serialto

parallel

M1

MN

Codewordcalculation

TM

ns1



2. Layered Space–TimeArchitectures

• Bell Labs Layered Space–Time (BLAST) architecture was one of the first spatial multiplexing systems.– Called also layered space–time (LST).

• Detection originally based on linear and decision–feedback equalization, i.e., nulling and cancellation.

Modulo-Mshift of layer-antenna

S/P

Layer 1 (mod/code) ANT 1

M data streamsLayer 2

(mod/code) ANT 2

Layer M(mod/code)

ANT M



Vertical LST (V–LST)

• Basic scheme with no coding involved.

S/P

Layer 1 Mod ANT 1

Layer 2 Mod ANT 2

Layer MMod

ANT M



Horizontal LST (H–LST)

S/P

ANT 1

ANT 2

Layer 1 Enc & mod

Layer 2 Enc & mod

• Coding included.

S/P

ANT 1

ANT 2

Layer 1 Mod

Layer 2 Mod

Layer MMod

ANT M

Encoder

Layer MEnc & mod

ANT M



Diagonal LST (D–LST)

• Coding and spatial interleaving included.• Spatial interleaving to improve performance via

spatial diversity.

Spatial interleaving

S/P

Layer 1 Enc & mod ANT 1

Layer 2 Enc & mod ANT 2

Layer MEnc & mod ANT M



Nulling and Cancellation

MMSEdetectionforremainingantennawith highestSINR

Despread 1

Despread 10

Detect andreconstructsignals forcancellation

Collect andmuxsubstreams

Demap,deinter-leave,decode

V–BLAST



Nulling and Cancellation Order in D–LST

ANT 1

ANT 2

Modulo-M shift of layer-antenna ANT 3

0 t 2t 3t 4t 5t 6t 7t 8t

ANT 4

d

c

b

a

a

d

c

b

c

b

a

d

b

a

d

c

d

c

b

a

a

d

c

b

c

b

a

d

b

a

d

c

cancelledAlready detected

Detect now

Detect later

nulled

Diagonal layer a (e.g one coding

block)a

Layer 1 (mod/code)

Layer 2 (mod/code)

Layer 4 (mod/code)

= N x 4-element vectors



Space–Time Equalization

• The optimum receiver for LST transmissions is maximum a posteriori (MAP) equalizer similarly to the intersymbol interference (ISI) or multiple–access interference (MAI) channels.

• Suboptimal receivers include:– linear equalizers– interference cancellation (IC)– iterative (turbo) receivers– sphere detectors.

• Space–time equalization is under intensive study.



3. Spatial Multiplexing in 3G Systems: PARC

• Current MIMO proposals (e.g., code reuse, code reuse with STTD (DSTTD)):– Node B transmits with the same rate on each antenna (or antenna

pair) depending on UE feedback and spatial channel realization

If the transmitter can adjust the antenna rates independently, alayered receiver architecture (MMSE with successive cancellation) can approach Shannon capacity. [Varanasi, Guess 1998] [Chung, Huang, Lozano 2001]

• Per-antenna rate control (PARC) for HSDPA MIMO:– Node B adjusts antenna rates independently depending on UE

feedback and spatial channel realization. – Receiver consists of MMSE linear transformation followed by

interference cancellation based on decoded bits.



PARC: Transmission technique

• PARC MIMO transmission example:

DEMUX

...

Spreading Code 1

Spreading Code 2

Spreading Code 10

ScramblingCode

ScramblingCode

...

...

...

Highspeeddatastream

Ant 116QAMrate 1/2

Ant 4QPSKrate 3/4

...



PARC: Receiver technique

• Proposed MIMO receiver for PARC transmission– MMSE linear transformation followed by interference cancellation

based on decoded bits.– Coding gain results in performance improvement over pre-decoding

interference cancellation receiver.– This architecture can also be applied to conventional code reuse

transmission.

MuxMMSEdetectionforremainingantennawith highestSINR

Despread 1

Despread 10

Reconstructsignals forcancellation

Collectandmux

Detect,demap,deinter-leave,decode

PARC



4. Summary and Conclusions

• MIMO techniques allow high user data rates• Advanced receivers have to be applied for MIMO

systems• Additional receiver diversity beneficial• Feedback of CSI may lead to complex systems• MIMO performance has to be evaluated at system

level



References1. G. J. Foschini, “Layered space-time architecture for wireless communication in a fading environment

when using multi-element antennas,” Bell Labs Tech. J, vol. 1, no. 2, pp. 41–59, 1996.

2. G. J. Foschini, G. D. Golden, R. A. Valenzuela, and P. W. Wolniansky, “Simplified processing for high spectral efficiency wireless communication employing multi-element arrays,” IEEE J. Select. Areas Commun., vol. 17, no. 11, pp. 1841–1852, Nov. 1999.

3. G. D. Golden, C. J. Foschini, R. A. Valenzuela, and P. W. Wolniansky, “Detection algorithm and initial laboratory results using V-BLAST space-time communication architecture,” Electronics Letters, vol. 35, -no. 1, pp. 14–16, Jan. 1999.

4. B. M. Hochwald and S. ten Brink, “Achieving near-capacity on a multiple-antenna channel,” IEEE Trans. Commun., vol. 51, no. 3, pp. 389–399, Mar. 2003.

5. B. Vucetic & J. Yuan, Space–Time Coding. John Wiley and Sons, 2003. ISBN 0-470-84757-3

6. P. W. Wolniansky, G. J. Foschini, G. D. Golden, and R. A. Valenzuela, “V-BLAST: An architecture for realizing very high data rates over the rich-scattering wireless channel,” in International Symposium on Signals, Systems, and Electronics (ISSSE), 1998, pp. 295–300.

7. L. Zheng and D. N. C. Tse, “Diversity and multiplexing: A fundamental tradeoff in multiple-antenna channels,” IEEE Trans. Inform. Theory, vol. 49, no. 5, pp. 1073–1096, May 2003.

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Spatial Multiplexing - · PDF file–space Öspace–division multiplexing (SDM) or spatial multiplexing • different bits from different antennae • requires independent channels