Network Coding Innovation Technology Workshop
Physical-layer Network Coding:
Prototyping and Application
Lu Lu
30th August 2013
Outlines
• 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
PNC Prototyping & Application
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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
PNC Prototyping & Application
3
Simplest Set-up: Two-Hop Relay Network
R
B
A
• 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?
PNC Prototyping & Application
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Traditional Scheduling (TS)
PA
PA
A
R
PB
B
PB
Time slot 1
Time slot 2
Time slot 3
Time slot 4
• Transmissions non-overlapping in time
PNC Prototyping & Application
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Straightforward Network Coding (SNC)
PB  PR  PA
PA
PB
A
PR  PA  PB
R
PR  PA  PB
Time slot 1
PA  PR  PB
B
Time slot 2
Time slot 3
• Transmissions by nodes A and B still non-overlapping
• Relay R uses one time slot to broadcast
PNC Prototyping & Application
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Physical-layer Network Coding (PNC)
PB  PR  PA
PA
PB
A
PR  PA  PB
R
PA  PR  PB
PR  PA  PB
Time slot 1
B
Time slot 2
• Transmissions by nodes A and B are simultaneous!
PNC Prototyping & Application
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Outlines
• 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 and Future Works
PNC Prototyping & Application
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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
API
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PNC Implementation: Wireless
• Frequency-Domain PNC (FPNC) in GNU Radio testbed
A
R
B
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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
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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
PNC Prototyping & Application
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Frame format for RPNC
Data Totally
Overlap
320 samples
80
samples
Node A
Node B
CP tsA
80
samples
tsA
0
2 long training symbols
128 samples
0
CP tsB
CP
lts
tsB
lts
0
0
0
CP
lts
data
lts
0
Cyclic prefix
16 samples
User Detection
Channel Estimation
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data
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
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Single-user Channel Decoder and PNC
Channel Decoder in Overall RPNC System
Two Users
 y[k ]k 1,2,...
PNC Soft
Demodulator
x A B [k ]k 1,2,...
Binary Viterbi
Decoder
PNC Decoder
User
Detector
One User
s A [m]  sB [m]m 1,2,...
SU Soft
Demodulator
x A [k ]k 1,2,...
 or x [k ]
B
k 1,2,...
PNC Decoder

Binary Viterbi
Decoder
s A [m]m 1,2,...
 or s [m]
B
m 1,2,...
Single User Decoder
 P ( y[k ] | xB [k ]  1) 
log

 , k = 1, 2,… )
Soft Information (Log Likelihood Ratio:
P
(
y
[
k
]
|
x
[
k
]


1)
B


PNC Prototyping & Application
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
Reduced Constellation Approach to
Computing Soft Information on XOR bit
Received sample with noise
Mapping four constellation points
to two Constellation points for log
y
likelihood computation
Imag
hA  hB
hB
hA  hB
0
hA
hA  hB
hA  hB
real
• BPSK for both nodes A
and B
• Between two points of
the same XOR, choose the
one with the shorter
Euclidean distance
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PNC Realization Challenges
• Long Latency between USRP-PC
– The turn-around time for USRP-PC may be long
and unpredictable
– Solution: Burst Transmission Mode
PNC Prototyping & Application
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Burst Transmission Mode in RPNC
Packet
Gap
Uplink
Fixed
Delay
Tx End
node A
X 1A
X 2A
……
X MA
……
X MB
f1
X 2B
X 1B
Tx End
node B
Y1A B
Rx Relay
node R
Y2A B
……
YMA B
Fixed
Delay
M packets for FDD PNC (Burst 1)
Tx Relay
node R
X 1A B
Bea
con
Rx End
node B
……
X MA B
Bea
con
f2
Downlink
Rx End
node A
X 2A B
Random
Delay
X 1A B
X 2A B
……
X MA B
X 1A B
X 2A B
……
X MA B
PNC Prototyping & Application
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Normalized throughput of PNC and TS
Normalized Throughput
2
PNC
TS
1.5
PNC can double
the throughput
1
0.5
0
5
10
15
SNR (dB)
PNC Prototyping & Application
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25
19
Outlines
• 1. Background of Physical-layer Network Coding
(PNC)
• 2. Real-time PNC Prototype
– Demo Video
– PNC Realization Challenges
• 3. PNC in Non-relay Setting
– Network Coding Multiple Access (NCMA)
– Performance Evaluation
• 4. Conclusion and Future Works
PNC Prototyping & Application
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PNC in Non-Relay Setting?
C
Access Point
Access
Point
A
D
Internet
B
A
Wireless LAN
B
• Access point wants to get both Message A and
Message B, not just their XOR.
• Does PNC have a role to play?
PNC Prototyping & Application
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PNC In Non-Relay Network:
Network Coding Multiple Access (NCMA)
A
AP
B
• 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
PNC Prototyping & Application
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PHY-Layer Decoders of NCMA
x A [k ]k 1,2,...
MUD Soft
Demodulator
Two Users
 y[k ]k 1,2,...
PNC Soft
Demodulator
SU Soft
User
Demodulator
Detector
Single User
xB [k ]k 1,2,...
x A B [k ]k 1,2,...
x A [k ]k 1,2,...
 or x [k ]
B
k 1,2,...
Binary Viterbi
Decoder
Binary Viterbi
Decoder
Binary Viterbi
Decoder

Binary Viterbi
Decoder
CiA
CiB
CiA  CiB
CiA
 or CiB 
 P ( y[k ] | xB [k ]  1) 
log
Soft Information (Log Likelihood Ratio: 
 , k = 1, 2,…
P
(
y
[
k
]
|
x
[
k
]


1)
B


PNC Prototyping & Application
)
23
Alternatives for MUD Decoding
 x A [ k ]k  1,2 ,... Binary Viterbi
RMUD Soft
Demodulator
Option 1
Option 2
MUD Decoder
Ci
A
Decoder
 x B [ k ]k  1,2 ,...
B
Binary Viterbi
Decoder
Ci
Ci
A
RMUD Decoder
B
Ci
SIC Decoder
MUD Decoder
Two Users
PNC Soft
Demodulator
 x A  B [ k ]k  1,2 ,...
Ci  Ci
A
Binary Viterbi
Decoder
B
PNC Decoder
 y [ k ]k  1,2 ,... User
Detector
One User
SU Soft
Demodulator
 x A [ k ]k  1,2 ,...
 or  x
[ k ]k  1,2 ,...
B

Binary Viterbi
Decoder
Ci
A
 or C 
B
i
Single User Decoder
PNC Prototyping & Application
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NCMA: PHY-layer Bridging
C3B
PNC Prototyping & Application
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Are Lone XOR Packets Useful?
Complementary XOR
Lone XOR
• Do lone XOR packets have a role to play?
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MAC-Layer Erasure Code
+ PHY-Layer Channel Code
Message M  Packets {C , C ,..., C }
A
A
1
A
2
A
N
Message M B  Packets {C1B , C2B ,..., CNB }
If RS code is used, as soon as AP decodes any L of
packets {C1A , C2A ,..., CNA }, it can obtain M A .
Similarly for M B .
Are XOR packets, {C1A  C1B , C2A  C2B , ..., CNA  CNB }, useful?
PNC Prototyping & Application
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NCMA: MAC-Layer Bridging with L = 3
Example: Decoding M B , based on M A and M A  M B , with L  3
Packet
Index
1
Eq A
Eq A B
Packet
Index
1
Eq A
2
C2A
C3B
3
C3A
C4B
4
C4A
5
C5A
Eq A
C1B
Packet
Index
1
C2B
2
C2A
C3B
3
C3A
C4B
4
C4A
5
C5A
Eq B
C1A
C2A B
2
3
4
C4A
5
C5A
Packet
Index
1
Eq A
2
C2A
3
C3A
4
C4A
5
C5A
C4A B
Eq A B
C1A
C2A B
C4A B
Eq B
C5B
PNC Prototyping & Application
Eq A B
Eq B
C1A
C2A B
C3B
C4A B
C4B
Eq A B
Eq B
C2A B
C2B
C1A
C3B
C4A B
C4B
28
Experiments: Layout of Indoor
Environment for 9 USRP N210 Nodes
4
3
6
1 7
9
AP
2
5
8
Institute of Network Coding (INC)
PNC Prototyping & Application
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PHY-Layer Packet Decoding Statistics
(Balanced Power Case)
Solo XOR
packets
100%
AB
AX|BX
A|B
X
NONE
Percentage
80%
60%
X
X
40%
AX|BX
20%
0%
7
7.5
8
AX|BX
8.5
9
9.5
SNR (dB)
10
10.5
11
11.5
AB: Both A and B decoded
AX|BX: A and XOR decoded or B and XOR decoded
A|B: Either only A or only B decoded
X: Only XOR decoded
PNC Prototyping & Application
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Throughput (packets per slot)
Overall Throughputs of Different Schemes with RS
code parameter LA = 4, 8, 16, 32, and fixed SNR = 9dB.
1.4
1.3
1.2
1.1
NCMA w. RMUD Upper Bound
NCMA w. RMUD, LA=1.5LB
1
NCMA w. RMUD, LA=LB
0.9
RMUD
0.8
5
10
15
20
25
30
LB
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Overall Throughputs of Different Schemes for
Different SNRs LA = 1.5×LB = 24.
2
Throughput (packets per slot)
NCMA
1.5
MUD
SU
1
0.5
0
7.5
8
8.5
9
SNR (dB)
9.5
PNC Prototyping & Application
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10.5
32
Throughputs of Four User Pairs
2
Throughput
1.5
4
A, NCMA w. RMUD
B, NCMA w. RMUD
A, NCMA w. RMUD+SIC
B, NCMA w. RMUD+SIC
A, SU
B, SU
3
6
1 7
9
AP
2
5
8
1
0.5
0
Pair 1 Pair 2 Pair 3 Pair 4
PNC Prototyping & Application
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4
Pairing Strategies
3
6
1 7
9
AP
2
5
8
NCMA with (RMUD+SIC)
3
Throughput
2.5
Strategy 1
Strategy 2
Scenario: Four users at locations 2, 3, 4, 5.
How to form pairs?
2
Strategy 1: P2 and P4
Strategy 2: P3 and P5
1.5
1
0.5
0
Loc. 2 Loc. 3 Loc. 4 Loc. 5
Sum
Pair “strong with weak” rather than “strong with strong and weak with weak”
PNC Prototyping & Application
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NCMA: Overall Summary
A
AP
B
• 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
PNC Prototyping & Application
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Outlines
• 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
PNC Prototyping & Application
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Conclusions
• 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
PNC Prototyping & Application
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PNC Prototyping & Application
38
To Probe Further
• “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.
PNC Prototyping & Application
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