MIMO Techniques in 3G LTE

8/3/2019 MIMO Techniques in 3G LTE

http://slidepdf.com/reader/full/mimo-techniques-in-3g-lte 1/8

3G Evolution

Chapter:

66u -an enna ec n ques

Vanja Plicanic

Department of Electrical and Information Technology

. . .

3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 1

Outline

•Introduction

•Multi-antenna confi urations

•Multi-antenna techniques

•Multiple receiver antennas, SIMO

,

•Multiple antennas at both RX and TX, MIMO


Introduction

Multi-antenna systems

Multi-antenna techniques

Smart antennas

Multiple antennas at the receiver and/or transmitter

+

Smart signal processing

+


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




Antenna configurations cont.

- Configuration of the antennas is decided by the requirement on the antenna

mutual coupling and correlation (low/high)

- Thus, configuration decided by choice of

- spatial distance between the antennas

Low mutual coupling and correlation when:

BS: >10 wavelengths (due to small AoA in macro cell, shorter distance in micro cells)

.

- polarization directions of the antennas

Antennas with different polarizations for both BS and MS gives lower mutual

coupling and correlation.

3/26/2009 3G Evolution - H SPA and LTE for Mobile Broadband

Antenna configurations cont.

- However, the case of MS at low frequencies < 900 MHz

=> 0.5 wavelengths is large distance for low frequencies

~ .

=> Polarization diversity hard to implement due to antenna + chassis radiation,

difficult to rotate chassis wave-mode

3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband

Multi-antenna techniques cont.

Why? How ?

- To improve system capacity (more users per cell),

better link reliabilityDIVERSITY

- To improve coverage (possibility for larger cells)BEAM-FORMING

- To achieve higher data rates per user,

higher spectral efficiencySPATIAL MULTIPLEXING


Figures above Courtesy of Ericsson

Multi-antenna techniques cont.

DIVERSITY

- Antennas at receiver and/or transmitter

- Mitigates fading in the radio channel

- Low mutual cou lin re uired

BEAM-FORMING

- Antennas 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


Figures above Courtesy of Ericsson



Multiple receiver antennas, SIMO

Smart signal processing techniques:

- RX diversity

- Receive beam-forming

-



RX diversity-

• mitigate fading

• suppress specific interferer

Linear receiver antenna combining

- All information is ex loited b combinin co ies of the si nal 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)

-

- Maximum Ratio Combining (MRC)

- Interference Rejection Combining (IRC)

- Minimum Mean Square Error (MMSE)



RX diversity

Linear receiver antenna combining in:

- o se m e case:


- n er erence m e sys em:



-

- Minimum Mean Square Error (MMSE)


RX diversity

Maximum Ratio Combining (MRC)

- mp u e an p ase we g ng- 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.

- Diversity gain and array gain

- For noise limited environments





RX diversityRX diversity

Interference Rejection Combining (IRC)Interference Rejection Combining (IRC)

-

- Uplink intra-cell interference suppression, Spatial Division Multiple Access (SDMA)

- Able to suppress Nr-1 interferers, however large noise increment after combining

-

- 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.



Adaptive space-time processing

- Frequency selective channel

- Linear time domain filtering/equalization, linear processing to signals received at different, - ,

- Linear receive antenna combining, linear processing to signals received at different

antennas



Receive beam-forming

- Switched beam antennas

- Antenna array that can form pattern beams pointing in certain discrete direction

- switching selects the “best” beam for down conversion and post processing, goal tomaximize the SNR

- simple implementation, since only one signal to post process

- limited flexibility, since only fixed directions

- Amplitude and phase weights

MRC => a receiver beam with maximum gain NR in the direction of the

target signal

=>

target signal



Receive beam-forming


Figures above Courtesy of jackwinters.com



Multiple transmit antennas


- TX diversity

- Transmit beam-forming



TX diversity

- Does not require channel knowledge at the receiver -

- Delay/Temporal diversity

- Cyclic-delay diversity

- Space time/frequency coding (STBC/STFC)



TX diversity

Delay/Temporal diversity

-

=> signals received at different times are uncorrelated

=> delay diversity already there and can be extracted in advanced receivers

ex.

- 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



TX diversity

Cyclic-delay diversity

- pp es cyc c s ns ea o near e ays





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 arra ain onl diversit ,

- Space-time trellis to provide full diversity, array gain and coding gain



TX Diversity

Space-frequency block coding (SFBC)

- Space-frequency Transmit Diversity (SFTD)



Transmit beam-forming

- Requires channel knowledge

- Antenna configurations with high mutual coupling

- Small antenna distances- Different phase shifts applied to steer

the direction of the beam

- “CLASSICAL BEAM-FORMING”

- High array gain, no diversity gain

- Antenna configurations with low mutual coupling-

- 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 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


Multiple antennas at both RX and TX


- Spatial multiplexing

- Pre-coder based spatial multiplexing





Spatial multiplexing

Background:- SIMO and MISO

Low SNR => capacity increase ~ SNR increase (NTxNR)

High SNR => capacity increase ~ log2(SNR)

- Spatial multiplexing

us, capac y ncrease ~ mn T, R



Spatial multiplexing



Pre-coder based s atial multi lexin

- If SNR low => beam-forming better than spatial multiplexing

- = =- L T

Pre-coding=> “orthogonalizes” parallel streams, better signal isolation

- If NL< NT

=> -

- Depending on the channel information pre-coder code-books chosen



Some SM detection techniques

- Maximum-Likelihood ML

- ayere space me arc ec ures- Successive Interference Cancellation (SIC)

- Single and Multi-codeword Transmission

- er n e nna a e on r o




Chapter summary

Multi-antenna techniques

SIMO Diversity gain and array gain

MISO Diversity gain and/or array gain

SIMO Diversity gain and array gain

MISO Diversity gain and/or array gain

Multiplexing gain

Link reliability

Spectral efficiency

Diversity gain

Diversity gain and array gainCoverage

Link reliability


References

[1] Dahlman E. et al., 3G evolution-HSPA and LTE for Mobile Broadband, 2nd edition,

Elsevier, UK 2008

[2] Paulraj A. et al., Introduction to Space-Time Wireless Communications, Cambridge,

UK 20033 Molisch A.F. Wireless Communications IEEE Press Wile & Sons US 2006 . ., , , ,


MIMO Techniques in 3G LTE

Documents