lecture’analog’network’coding’ - kom.aau.dkkom.aau.dk/~del/summerschool_15/analognc.pdf ·...
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Lecture Analog Network Coding
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Network Coding for Wireless Networks
Conven8onal relaying
4 8me slots 3 sinks
Use of network coding
3 8me slots 3 sinks
Use of analog network coding 2 8me slots 2 sinks
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Physical Layer Network Coding
• Presented by [Zhang et al 2006]
• First, simple example: no fading
• Let us look at bandpass signals
• How to generate s3(t) ?
s1(t) s2(t)
s3(t)
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Physical Layer Network Coding How to generate s3(t) ?
• Amplify and forward? • Decode and forward? Ini8al approach: decode and forward
Example with BPSK: say
Note that there are 3 possible values of : • ”-‐2” and ”2” correspond to • ”0” corresponds to
s1(t) s2(t)
s3(t)
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Physical Layer Network Coding Example with BPSK:
Let us generate s3(t) (hint: XOR-‐like opera8on)
If , then If , then
Alice and Bob receive as standard BPSK modula8on Then, XOR bit by bit with the sent packet
s1(t) s2(t)
s3(t)
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Analog Network Coding
Alice Bob Relay
1st step – coding in the air
BPSK Example
What if we A&F?
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Analog Network Coding
Alice Bob Relay
1st step – coding in the air – e.g. 1/1
BPSK Example
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Analog Network Coding
Alice Bob Relay
1st step – coding in the air – e.g. 0/0
BPSK Example
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Analog Network Coding
Alice Bob Relay
1st step – coding in the air – e.g. 1/0
BPSK Example
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Analog Network Coding
Alice Bob Relay
1st step – coding in the air – e.g. 0/1
BPSK Example
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Analog Network Coding
Alice Bob Relay
2nd step -‐relay
BPSK Example
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Analog Network Coding
Alice Alice Alice
decoding
BPSK Example
Rx‘ed
Sent Sent
Rx‘ed
Rx‘ed
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Analog Network Coding What were our assump8ons so far? • No fading there is amplitude + phase distor8on • Perfect sync • Perfect detec8on of a collision • Perfect knowledge of packet used for decoding at Alice and Bob
• The ”right” packets interfere (MAC / Network impact)
How to make it prac8cal? [Kad et al 2007] Analog network coding [Gollakota et al 2008] ZigZag decoding (different problem, similar intui8on)
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Analog Network Coding Key intui5on: exploit asynchrony [Kad et al 2007]
Alice
Bob
No overlap No overlap
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Analog Network Coding Areas with no overlap allow us to address some of the key challenges
Alice
Bob
No overlap No overlap
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Analog Network Coding
Alice
Bob
Construct „header“ and „footer“ for each packet • Pilot sequence: channel es8ma8on
• ID of sender+des8na8on+sequence number of the packet: ac8ve session and to determine which packet was used
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Analog Network Coding
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ZigZag Decoding • Draws from the same intui8on as the above problem • Difference: • More general sedng • A node can use it to recover several interfering signals (no knowledge required on its end)
• We need to receive n collisions of n packets to recover
• Where is it useful? • Hidden terminal problem • In high SNR, to boost overall data rate from mul8ple sources to a single receiver [ParandehGheibi et al 2010]
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ZigZag Decoding: Basic Idea
Again: asynchrony
• Chunk 1 of bits from user A from 1st collision is decoded successfully
• Thus, can subtract it from 2nd collision to decode Chunk 2 of bits of user B
Once Chunk 2 is free, can use to free Chunk 3, and so on
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ZigZag Decoding: Single Hop Analysis Work in [ParandehGheibi et al 2010]
Tx 1
Tx 2
Tx n
x Rx
p
p
p
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ZigZag Decoding: Single Hop Analysis Work in [ParandehGheibi et al 2010]
First result: Mean 8me to deliver one packet each
With zigzag:
Perfect scheduler (no collisions):
Tx 1
Tx 2
Tx n
x Rx
p
p
p
p = ½, n = 3 ZZ: 4+ 10/21 PS: 6
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ZigZag Decoding: Single Hop Analysis Work in [ParandehGheibi et al 2010]
Second result: Stable throughput increases
Tx 1
Tx 2
Tx n
x Rx
p
p
p λn
λ2
λ1
λ2
λ1
1-‐p
1-‐p
λ2
λ1
1-‐p
1-‐p p(1-‐p)
p(1-‐p) Region PS
Region ZZ