Physical-layer Network Coding: Prototyping and Application

Physical-layer Network Coding: Prototyping and Application

Lu Lu30th August 2013

Network Coding Innovation Technology Workshop

• 1. Background of Physical-layer Network Coding (PNC)

• 2. Real-time PNC Prototype– Demo Video– PNC Realization Challenges

• 3. PNC in Non-relay Setting– Experimental Setup– Performance Evaluation

• 4. Conclusion

Outlines

2PNC Prototyping & Application

What is PNC?

• Traditional view in wireless networking: interference is bad.

• PNC turns things around by exploiting network coding (NC) performed by nature.

• When electromagnetic waves superimpose, they add, a form of NC.

• Benefits of PNC: – boost throughput


Simplest Set-up: Two-Hop Relay Network

• System Model: 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 node R?

A B

R


Traditional Scheduling (TS)

• Transmissions non-overlapping in time

A R BPA PA

PB PB

Time slot 1

Time slot 3

Time slot 2

Time slot 4


Straightforward Network Coding (SNC)

• Transmissions by nodes A and B still non-overlapping• Relay R uses one time slot to broadcast

RB AP P P RA BP P P

R A BP P P A R B

PA PB

Time slot 1

Time slot 3

Time slot 2

R A BP P P


Physical-layer Network Coding (PNC)

• Transmissions by nodes A and B are simultaneous!

RB AP P P RA BP P P

R A BP P P A R B

PA PB

Time slot 1 Time slot 2

R A BP P P





• 4. Conclusion and Future Works

Outlines


Specifics

• Frequency-domain PNC (FPNC) for TWRC

–Build on OFDM technology as used in Wi-Fi

–First PNC implementation in 2012

–First real-time PNC implementation in 2013 • Support “real” application in real-time through

API9PNC Prototyping & Application

PNC Implementation: Wireless• Frequency-Domain PNC (FPNC) in GNU Radio testbed

A

BR


Demo Video: Real-Time File Exchange with PNC

• A short video followed by a demo

See demo video in http://www.youtube.com/watch?v=HmRBm_IIBQQ


http://www.youtube.com/watch?v=HmRBm_IIBQQ

PNC Realization Challenges• Asynchrony

– Signals from nodes A and B may arrive at the relay R with symbol and carrier-phase misalignments

– Solution: PNC with OFDM (FPNC)

• Channel Estimation– Relay needs to estimate two channels based on

simultaneous signals– Solution: FPNC Frame Format Design


Frame format for RPNC

lts lts data0

0 data

Node A

Node B

80 samples 128 samples

320 samples

Cyclic prefix16 samples

2 long training symbols

0

lts lts 0

CP

CP

0CP tsAtsA

CP tsBtsB0

80 samples

User Detection Channel Estimation

Data Totally Overlap


PNC Realization Challenges• Channel-decoding and Network Coding (CNC)

– Tradeoff between optimality and simplicity – Solution: Opt for simplicity; adopt 802.11

convolutional code and XOR-CD CNC. Reduced constellation approach

• ARQ for Retransmission– End-to-end ARQ or Relay-assisted ARQ– Solution: Opt for simplicity; adopt end-to-end

ARQ


Single-user Channel Decoder and PNC Channel Decoder in Overall RPNC System

1,2,...[ ]

ky k

Two Users

Single User Decoder

Binary Viterbi Decoder

User Detector

SU Soft Demodulator

PNC SoftDemodulator


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

x k 1,2,...

[ ] [ ]A B ms m s m

1,2,...or [ ]B m

s m 1,2,...

or [ ]B kx k

1,2,...[ ]A m

s m

1,2,...[ ]A k

x k

PNC Decoder

One User

PNC Decoder

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

log( [ ] | [ ] 1)

B

B

P y k x k

P y k x k


Reduced Constellation Approach to Computing Soft Information on XOR bit

real

Imag

A Bh h

A Bh h

A Bh h

A Bh h

y

0Ah

Bh

Mapping four constellation points to two Constellation points for log

likelihood computation

• BPSK for both nodes A and B• Between two points of the same XOR, choose the one with the shorter Euclidean distance

Received sample with noise


PNC Realization Challenges• Long Latency between USRP-PC

– The turn-around time for USRP-PC may be long and unpredictable

– Solution: Burst Transmission Mode


Burst Transmission Mode in RPNC

f2

f1 Tx End node B

Tx End node A

1AX

1BX 2

BX

2AX

BMX

AMX… …

… …

Rx End node B

Rx End node A

A BMX … …

1A BX … …

Tx Relay node R

Beacon

M packets for FDD PNC (Burst 1)

Beacon

Packet Gap

Rx Relay node R

1A BX A B

MX … …

1A BY … …2

A BY A BMY

RandomDelay

2A BX

A BMX

2A BX

2A BX

1A BX

FixedDelay

FixedDelay

Uplink

Downlink


Normalized throughput of PNC and TS

5 10 15 20 250

0.5

1

1.5

2

SNR (dB)

Nor

mal

ized

Thr

oug

hpu

t

PNCTS

PNC can double the throughput




• 3. PNC in Non-relay Setting– Network Coding Multiple Access (NCMA)– Performance Evaluation

• 4. Conclusion and Future Works

Outlines


PNC in Non-Relay Setting?

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

• Does PNC have a role to play?

A B

Wireless LAN

Access Point Access Point Internet

A B

C

D


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

• Nodes A and B send to AP simultaneously• AP uses three decoders to separately decode packet A,

packet B, and packet A B• Eight possible events:

– Packets A, B, and A B decoded– Packets A and B decoded– …– Packet A B decoded– None decoded

A AP B


PHY-Layer Decoders of NCMA

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

log( [ ] | [ ] 1)

B

B

P y k x k

P y k x k


1,2,...[ ]

ky k

Two Users

Single User

User Detector

SU Soft Demodulator

PNC SoftDemodulator

MUD SoftDemodulator

Binary Viterbi Decoder or B

iC 1,2,...or [ ]B k

x k

AiC 1,2,...

[ ]A kx k


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

x k A B

i iC C


1,2,...[ ]A k

x k


1,2,...[ ]B k

x k

AiC

BiC

Alternatives for MUD Decoding

1,2,...[ ]

ky k

Two Users

Single User Decoder


User Detector

SU Soft Demodulator

PNC SoftDemodulator


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

x k A B

i iC C

RMUD 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

PNC Decoder

MUD Decoder

RMUD Decoder

SIC Decoder

Option 1

Option 2

AiC

BiC

One User


MUD Decoder

NCMA: PHY-layer Bridging

3BC


Are Lone XOR Packets Useful?

Complementary XOR

Lone XOR

• Do lone XOR packets have a role to play?


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

1 1 2 2Are XOR packets, { , , ..., }, useful?A B A B A BN NC C C C C C


NCMA: MAC-Layer Bridging with L = 3Example: Decoding , based on and , with 3B A A BM M M M L

1

2

3

4

5

1AC

2A BC

3BC

4AC

5AC

2AC

3AC

PacketIndex

A BEq AEq BEq

4BC4

A BC

1

2

3

4

5

1AC

2A BC

3BC

4AC 4

BC

5AC

PacketIndex

A BEq AEq BEq

4A BC

PacketIndex

A BEq

1

2

3

4

5

AEq BEq

1AC

2A BC

3BC

4AC

5AC

2AC

3AC

2BC

4BC4

A BC

PacketIndex

A BEq

1

2

3

4

5

AEq BEq

1AC

2A BC

3BC

4AC

5AC

2AC

3AC

2BC

1BC

4BC

5BC

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)

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

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


Solo XOR packets

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

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


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

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

9

14

5

3

26

7

8

AP

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?

Strategy 1: P2 and P4Strategy 2: P3 and P5

9

14

5

3

26

7

8

AP

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

NCMA: Overall Summary

• First venture into non-relay setting for PNC• PNC may have a role to play in the multiple access

scenario– for simplification of decoder design– for jumbo messages

A AP B





• 4. Conclusion

Outlines


• There has been a lot of theoretical work on PNC• Relatively few experimental investigations• RPNC: The first real-time PNC prototype• NCMA: PNC can be applied in a non-relay

setting to boost system throughput• Future: apply PNC and NCMA to commercial

wireless networks: cellular (e.g., LTE-A) and WLAN

Conclusions


• “Implementation of Physical-layer Network Coding,” in ICC ’12 and Phycom, Mar. 2013.

• “Real-time Implementation of Physical-layer Network Coding,” in SRIF ’13, an ACM SIGCOMM Workshop.

• “Network-coded Multiple Access,” Technical Report, http://arxiv.org/abs/1307.1514.

To Probe Further


http://arxiv.org/abs/1307.1514

Physical-layer Network Coding: Prototyping and Application

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