adaptive code allocation for interference exploitation on the downlink of mc-cdma systems

Emad Alsusa & Christos Masouros

Dept. of Electrical & Electronic Engineering

University of Manchester

Adaptive Code Allocation for Interference Exploitation on the Downlink of MC-CDMA

Systems

Principles of the Proposed Method

For PSK Modulation, Interference can be Separated to Constructive and Destructive

Interference Depends on Users Crosscorrelations as well as the instantaneous Data

By Reallocating the Codes According to the Current Data, the Crosscorrelations and hence Interference Amongst Users can be Manipulated

By Exploiting Constructive Interference the Effective SINR can be Increased and Performance can be Improved Without the Need to Increase Transmitted per-User Power

MC-CDMA Downlink Employing post-Equalization (K users)

Received Signal at the u-th Mobile Unit (MU) at the i-th symbol period:

Decision Variable:

1,

K

H

iu iu u uu iu ku ik iuk k u

x x nd C

r1,

( ) Hiu iu u k

K

u u uu iu iuku ikk k u

a x a x n

d r CE

1, ( ) H

iu iu u k

K

u u uu iu iuku ikk k u

a x a x n

d r CE

= [ · · ] + iu i u ur x A C H N

=( )· Hpq p q q q C H E C

Constructive - Destructive Interference Separation

User-to-User Constructive MAI:

0iu k ik kux a x

Cumulative Constructive MAI:

1,

0K

iu k ik kuk k u

ax x

1

-1

-1

1

c1 axis

c2 axis

x2 . c2

x1 . c1

r = x1c1 + x2c2

c1 axis

c2 axis

1

1

-1

-1

x2 . c2

x1 . c1

r = x1c1 + x2c2

d1

d2 1

1

-1

-1

c1 axis

c2 axis

x2 . c2

x1 . c1

r = x1c1 + x2c2

d2

d1

d2

d1

a b c

Constructive - Destructive Interference Separation

Instantaneous per Symbol Effective SINR:

constructivee

destructive

S MAISINR

MAI N

Im

ReRe

Im

1-1

BPSK QPSK 8PSK

1+i

1-i

-1+i

-1-i

a b c

Decision Variables Distributions for pc=8 Different Allocation Patterns for K=5, L=16

x1 x2 x3 x4 x5

1 -1 1 1 -1

allocation pattern No. d1 d2 d3 d4 d5

s=1 0.75 -1 0.5 0.75 -1

s=2 0.5 -1.75 -0.5 1.25 -2

s=3 2.5 -2.75 2.25 3 -0.5

s=4 0.5 -1 0.75 0.75 -1

s=5 0.75 -0.5 -0.25 0 -1

s=6 2.25 -0.5 3.25 2.5 -2.5

s=7 -0.25 -0.5 0.75 0 -1

s=8 1.25 -2 0.5 -0.5 -1.75

b

x 1 x 2 x 3 x 4 x 5

- 1 - 1 1 - 1 -1 allocation

pattern No. d 1 d 2 d 3 d 4 d 5

s =1 - 0.25 0 0.5 - 0.25 - 1

s =2 - 1 - 1.25 1 - 0.25 - 0.5

s =3 - 0.5 0.25 0.25 0 - 0.5

s =4 - 3 - 1. 7 5 3.25 - 2.75 - 2. 2 5

s =5 0.25 0 0.25 - 0.5 - 0.5

s =6 - 0.25 - 1 0.5 0 - 0.25

s =7 - 2.75 - 3 3.25 - 2.5 - 1.5

s =8 0.75 - 1.5 1 - 1 - 1.25

a

Code-to-User Allocation (CUA) Technique (1/9)

1. Create Code Sets 2. Evaluate Code

Sets3. Select Optimum

Code Set4. Spread and

Transmit5. Transmit SI6. Detect SI and

select the correct code

7. Dispread and Detect

Nc codes

Random Shuffling

Code-to-user allocation pattern 1


Code-to-user allocation pattern pc

ˆsR

ˆ1R

pre-estimation

î,sd

Pre-estimation processing


î,1d


î,2d

ˆci,pd

xi

A

Channel Estimation

allocation pattern selection

copt

BS Transmitter Processing

u-th channel

Nu

SUD / MUD SI

detection

u-th MU receiverriu

SI transmission

ôptc

estimation

arg max min ˆs

i,sd

iud

OFDM

ûH

IOFDMPost-

Equalization


Nc codes

Random Shuffling



Code-to-user allocation pattern pc

ˆsR

ˆ1R

pre-estimation

î,sd



î,1d


î,2d

ˆci,pd

xi

A

Channel Estimation

allocation pattern selection

copt

BS Transmitter Processing

estimation

arg max min ˆs

i,sd

ûH

OFDM

SI DATA


Decision Variables pre-Estimation

Code Allocation Selection Criteria

ˆarg max minjs

i,sd̂arg max min

js

i,sd

ˆarg max minjs

i,sd

,1, 2,ˆ ˆ ˆˆ ˆ

iK si s i sd d d i si,s i,s i,sd x AR d e ,1, 2,ˆ ˆ ˆˆ ˆ

iK si s i sd d d i si,s i,s i,sd x AR d e

,1, 2,ˆ ˆ ˆˆ ˆ

iK si s i sd d d i si,s i,s i,sd x AR d e


For Correct Dispreading According to the Updated Codes, Transmission of

SI bits is NecessarySI is Common for all Users

If Code Allocation s=7 is [3, 5, 2, 1, 4] then User k=3 Should Employ Code with Index 2 from the reference set for Correct Dispreading

Control Symbols (SI)

Data bits – Symbol 1

Data bits – Symbol 2

Data bits – Symbol N

FRAMELENGTH

2log ( )s cpN 2log ( )s cpN 2log ( )s cpN

2log ( )s cpN


SUD / MUD SI

detection

u-th MU receiverriu

ˆoptc

iud

Post-Equalization

SI

DATA

• Enhanced Received SINR, Improved Reliability

• Data Detection Very Sensitive to SI Errors


CUA with MRC, EGC, SU- MMSE post-Equalization

•Performance Improvement of an Order of Magnitude Without Increase in Transmitted per-User Power

•Efficiency Reduction to 91% due to Transmission of Side Information (SI)

Number of paths=4, K=20, L=32, pc=16

0 2 4 6 8 10 12 14 16 18

10-4

10-3

10-2

10-1

100

Transmitted Eb/No per user (dB)

BE

R

MRCEGC

MMSE

MRC - CUA

EGC - CUAMMSE - CUA


CUA with EGC post-Equalization and SIC Detection

Number of paths=4, K=20, L=32

0 2 4 6 8 10 12 14 16 18

10-4

10-3

10-2

10-1

100

Transmitted Eb/No per user (dB)

BE

R

EGCEGC - CUA Nc=K

EGC - CUA Nc=32

EGC - SIC

EGC - SIC - CUAEGC - SIC - CUAperfect SI detection

• Limited Improvement for Increased NC

• Performance Loss for Low SNR due to Unreliable SI

Code-to-User Allocation (CUA) Technique (8/9)CUA with EGC

•Limited Improvement for Increased Number of Available Allocation Patterns (pc)

Number of paths=3, K=16, L=16, SNR=7dB

5 10 15 20 25 3010

-4

10-3

10-2

10-1

100

pc

BE

R

MRC

MRC SICEGC

EGC SIC


CUA with pre-decorrelation employing MRC Equalization

•Significant Performance Improvement

•Transmission Efficiency of 32/34=94.2%

Number of paths=3, K=32, L=32, SNR=7dB

0 2 4 6 8 10 12 14 16 18 2010

-4

10-3

10-2

10-1

100

Transmitted Eb/N

0 per user

BE

R

TP-MRCTP-MRC-CUAJT-MRCJT-MRC-CUA

Conclusions In Conventional Systems Energy Inherent in the System is

Wasted due to Data-Code MisalignmentPart of the Existent Interference can be Exploited to

Enhance the Received SINRBy Optimizing the Code Allocation Amongst the Users

with CUA the Constructive Component of Interference can be Maximized Improved Received SINR without Transmitted per-User Energy

IncreaseApplication of CUA can Enhance the Performance of a Number of

Conventional MultiUser Precoding and Detection SchemesThe Dependency on SI Detection Limits the CUA Overall

Performance for Low Transmitted SNRFor High SNR Values Performance Improvement of an Order of

Magnitude is Attained

Thank you

QuestionsCommentsSuggestions

adaptive code allocation for interference exploitation on the downlink of mc-cdma systems

Documents