network-coded multiple access

Network-Coded Multiple Access

Presented by Soung Liew (Joint Work with Lu Lu, Lizhao You, Shenghao Yang)

Institute of Network Coding

The Chinese University of Hong Kong

March 2014 Soung Liew, CUHK 1

March 2014 2

Background on PNC

First proposed in Mobicom 06 [1]

A subfield of Network Coding building up momentum in recent years:

- Google scholar citations: 1000+ (annual growth ~300)- A main topic in special journal issues and numerous conferences.- A main topic in an upcoming IEEE JSAC issue on wireless network

coding- Half-day workshop in IEEE WNCN 2013: New Advances for PNC- Invited talks

[1] S. Zhang, S. C. Liew, P. P. Lam, “Physical-Layer Network Coding,” ACM Mobicom 2006 .Similar idea in [2] P. Popovski and H. Yomo, “The anti-packets can increase the achievable throughput of a wireless multi-hop

network,” ICC 2006.

Soung Liew, CUHK

March 2014

Simplest Set-up: Two-Hop Linear Network

Two-way Relay Channel (TWRC) No direct channel between nodes A and B.Half duplex: nodes cannot transmit and receive at the

same time.What is the minimum number of time slots needed

for nodes A and B to exchange one packet via relay R?

A R B

SA SB

Soung Liew, CUHK 3

PNC

March 2014

B R AS S S A R BS S S

R A BS S S A R B

SA SB

Time slot 1 Time slot 2

R A BS S S

Transmissions by nodes A and B are simultaneous!

Soung Liew, CUHK 4

March 2014

PNC for Multiple Access (Non-Relay Setting)?

Access Point InternetRelay

(a) (b)

A B A B

C

D

Access point wants to get both Message A and Message B, not just their XOR

Does PNC have a role to play?

Relay Network

Soung Liew, CUHK 5

Wireless LAN

Access Point

PNC In Non-Relay Network: Network Coding Multiple Access (NCMA)

Nodes A and B send CA and CB to AP simultaneously AP tries to decode packets CA , CB, and


A AP B

AC

A BC C

BC

1,2,...[ ]

ky k

Two Users

Single User

Binary Viterbi Decoder

User Detector

SU Soft Demodulator

PNC SoftDemodulator


1,2,...[ ]A B k

x k A B

i iC C

MUD SoftDemodulator


1,2,...[ ]A k

x k


1,2,...[ ]B k

x k

AiC

BiC

or BiC 1,2,...

or [ ]B kx k

AiC 1,2,...

[ ]A kx k

PHY-Layer Decoders of NCMA

March 2014

Soft Information (Log Likelihood Ratio: , k=1,2,… )

Soung Liew, CUHK 7

( [ ] | [ ] 1)log

( [ ] | [ ] 1)B

B

P y k x k

P y k x k

PHY-layer Bridging

March 2014

3BC

Soung Liew, CUHK 8

Are Lone XOR Packets Useful?

March 2014

Complementary XOR

Lone XOR

Soung Liew, CUHK 9

• Do lone XOR packets have a role to play?

March 2014

MAC-Layer Erasure Code + PHY-Layer Channel Code

1 2

1 2

Message Packets { , ,..., }

Message Packets { , ,..., }

A A A AN

B B B BN

M C C C

M C C C

1 2

If RS code is used, as soon as AP decodes any of

packets { , ,..., }, it can obtain .

Similarly for .

A A A AN

B

L

C C C M

M

Soung Liew, CUHK 10

MAC-Layer Bridging with L = 3

1

2

3

4

5

Erasuredecoding andreconstruct A

Network decoding B using XOR information

Erasuredecoding andreconstruct B

Example: Decoding , based on and , with 3B A A BM M M M L

1AC

2A BC

3BC

4AC 4

BC

5AC

1

2

3

4

5

1AC

2A BC

3BC

4AC

5AC

2AC

3AC

PacketIndex

A BEq

1

2

3

4

5

AEq BEq

1AC

2A BC

3BC

4AC

5AC

2AC

3AC

2BC

PacketIndex

A BEq AEq BEq PacketIndex

A BEq AEq BEq

PacketIndex

A BEq

1

2

3

4

5

AEq BEq

1AC

2A BC

3BC

4AC

5AC

2AC

3AC

2BC

1BC

4BC

4BC 4

BC

5BC

4A BC

4A BC

4A BC

4A BC


Experiments: Layout of Indoor Environment for 9 USRP N210 Nodes

9

14

5

3

26

7

8

AP

Institute of Network Coding (INC)


PHY-Layer Packet Decoding Statistics(Balanced Power Case)

AB: Both A and B decodedAX|BX: A and XOR decoded or B and XOR decodedA|B: Either only A or only B decodedX: Only XOR decoded


7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 0%

20%

40%

60%

80%

100%

SNR (dB)

Per

cent

age

ABAX|BXA|BXNONE

X

AX|BX

X

AX|BX

PNC decoding works but MUD does not when phases are aligned


-2 -1 0 1 2-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5Constellation Map

XA

XB

XA+X

B

5 6 7 8 90%

20%

40%

60%

80%

100%

SNR (dB)P

erc

enta

ge

ABXABAXBXABXNONE

A BY X X Received Signal Decode Outcomes

PNC does add value when phases are severely misaligned


Decode Outcomes2j

A BY X X e

Received

Signal

5 6 7 8 90%

20%

40%

60%

80%

100%

SNR (dB)P

erc

enta

ge

ABXABAXBXABXNONE

-2 -1 0 1 2-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2


XA

XBej/2

XA+X

Bej/2

Combination of PNC and MUD is effective when phases are moderately misaligned


4j

A BY X X e

Received Signal Decode Outcomes

5 6 7 8 90%

20%

40%

60%

80%

100%

SNR (dB)P

erc

enta

ge

ABXABAXBXABXNONE

-2 -1 0 1 2-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2


XA

XBej/4

XA+X

Bej/4


7.5 8 8.5 9 9.5 10 10.50

0.5

1

1.5

2

SNR (dB)

Thro

ughp

ut (p

acke

ts p

er s

lot)

NCMAMUDSU

Overall Throughputs of Different Schemes for Different SNRs LA = 1.5×LB = 24.

Pairing Strategies


Loc. 2 Loc. 3 Loc. 4 Loc. 5 Sum0

0.5

1

1.5

2

2.5

3

Th

roug

hpu

t

NCMA with (RMUD+SIC)

Strategy 1Strategy 2

Scenario: Four users at locations 2, 3, 4, 5. How to form pairs?

Pair “strong with weak” rather than “strong with strong and weak with weak”

Strategy 1: pair 2 with 3; 4 with 5

Strategy 2: pair 3 with 4; 2 with 5

9

14

5

3

26

7

8

AP

12.3dB 9dB 7dB 7.4dB

Linearly Coupled Codes at MAC Layer

Single-equation System:


0

0

, : Generator matrices of Node A and Node B.

: corresponds to subset of A packets received,

: corresponds t

A AAA AX X A BA BB B

B BB

A B

A AA A

BB B

G CM M

G G G CM M

G C

G G

G G C

G G

o the subset of B packets received,

, : correspond to the subset of lone XOR packets received,

Suppose that and are not full-rank; however, there are 2 rows in .

The

B

X X A BA A B B

A BA B

C

G G G G C

G G L G

8

n the probability of the matrix being full-rank is more than 99.5% if elements of

, are randomly drawn from (2 )A B

G

G G GF

Linearly Coupled Raptor Codes

Near optimal and availability of fast decoding


Channel-coded symbols of user A Channel-coded symbols of user B

Linearly-Coupled Channel-coded symbols from users A and B

Dynamic Equation


Degree Optimization Problem


For some positive integer

Use SIC at low SNR Regime

Suppose an (N,3) RS code


Slot Index

AEq A BEq BEq

1A BC

1AC 1

BC PHY-layer SIC

2AC

3BC

5BC

4BC4

ACCross-layer SIC

Message decoding

1

2

3

4

5

Overall Throughputs of Different Schemes for Different SNRs LA = 1.5×LB = 24.


NCMA: Conclusion

First venture into non-relay (multiple-access) setting for PNC

PNC may have a role to play in the multiple access scenario: To simplify decoder design For delivering jumbo messages

Ongoing NCMA Investigations: Better performance for high and low SNR regime Investigation of linearly-coupled code for MAC layer

of NCMA


The End


Throughputs of Four User Pairs


Pair 1 Pair 2 Pair 3 Pair 40

0.5

1

1.5

2

Th

roug

hpu

t

A, NCMA w. RMUDB, NCMA w. RMUDA, NCMA w. RMUD+SICB, NCMA w. RMUD+SICA, SUB, SU

Overall Throughputs of Different Schemes with RS code parameter LA = 4, 8, 16, 32, and fixed SNR = 9dB.


5 10 15 20 25 300.8

0.9

1

1.1

1.2

1.3

1.4

LB

Thr

ough

put (

pac

kets

per

slo

t)

NCMA w. RMUD Upper BoundNCMA w. RMUD, L

A=1.5L

B

NCMA w. RMUD, LA=L

B

RMUD

March 2014 30

Frequency-Domain PNC (FPNC) in GNU Radio testbed,

PNC Implementation: Wireless

Soung Liew, CUHK

A

BR

network-coded multiple access

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