Wireless Communication Systems

Wireless Communication Systems

@CS.NCTU

Lecture 14: Full-Duplex Communications

Instructor: Kate Ching-Ju Lin ( 林靖茹 )

1

Outline

• What’s full-duplex

• Self-Interference Cancellation

• Full-duplex and Half-duplex Co-existence

• Full-duplex relaying

2

What is Duplex?

• Simplex

• Half-duplex

• Full-duplex

How Half-duplex Works?

• Time-division half-duplex

• Frequency-devision half-duplex

Co-Channel (In-band) Full- duplex

• The transmitted signals will be an interference of the received signals!

• But, we know what we are transmitting

 Cancel it!

Very strong self-interference (~70dB for

802.11)

Benefits beyond 2x Gain

• Can solve some fundamental problems

⎻Hidden terminal

⎻Primary detection for cognitive radios

⎻Network congestion and WLAN fairness

⎻Excessive latency in multihop wireless

6

Mitigating Hidden Terminal

7

• Current network have hidden terminals

⎻CSMA/CA cannot solve this

⎻Schemes like RTS/CTS introduce significant overhead

• Full-duplex solves hidden terminals

⎻Since both slides transmit at the same time, no hidden terminals exist

X

Primary Detection in Whitespaces Primary Detection in Whitespaces

Secondary transmitters should sense for primary transmissions before channel use

Time

56

Primary TX (Wireless Mics)

Secondary TX (Whitespace AP)

Primary sensing

Primary TX (Wireless Mics)

Secondary TX (Whitespace AP)

Traditional nodes may still interfere during transmissions

Interference

8

Primary Detection in Whitespaces

Secondary transmitters should sense for primary transmissions before channel use

Time

57

Primary sensing

Primary TX (Wireless Mics)

Secondary TX (Whitespace AP)

Full-duplex nodes can sense and send at the same time

Primary sensing

Primary TX (Wireless Mics)

Secondary TX (Whitespace AP)

9

Primary Detection in Whitespaces

Network Congestion and Fairness

Network Congestion and WLAN Fairness

Without full-duplex:

•

Downlink Throughput = 1/n Uplink Throughput = (n-1)/n

58 10

Network Congestion and WLAN Fairness

Without full-duplex:

•

Downlink Throughput = 1/n Uplink Throughput = (n-1)/n

59

With full-duplex:

•

Downlink Throughput = 1 Uplink Throughput = 1 ₁₁

Network Congestion and Fairness

Reducing Round-Trip Time

Long delivery and round-trip times in multi- hop networks

Solution: Wormhole routing

N1 N2 N3 N4

N1

N2

N3

N4

N1

N2

N3

N4

Time Time

Half-duplex

Time

Full-duplex

Reducing Round-Trip Times

12 60

Outline

• What’s full-duplex

• Self-Interference Cancellation

• Full-duplex and Half-duplex Co-existence

• Full-duplex relaying

13

Self-Interference Cancellation

Y = Hx + H

x

+ n

H_serlf H

Wanted signals Unwanted

self-interference

Challenge1: self-interference is much stronger than wanted signals, i.e.,|H

|

≫|H|

Challenge 2: hard to learn real H

Self-Interference Cancellation

• Analog interference cancellation

⎻RF cancellation (~25dB reduction)

⎻Active

• Digital interference cancellation

⎻Baseband cancellation (~15dB reduction)

⎻Active

• Antenna cancellation

⎻Passive

What Makes Cancellation Non-Ideal?

• Transmitter and receiver phase noise

• LNA (low-noise amplifier) and Mixer noise figure

• Tx/Rx nonlinearity

• ADC quantization error

• Self-interference channel

16

Noise figure (NF) is the measure of degradation of SNR caused by

components in a RF chain

Analog Cancellation

• Why important?

⎻Before digital cancellation, we should avoid saturating the Low Noise Amplifier and ADC

⎻Eg., Tx power = 20 dBm and LNA with a

saturation level -25dB  at least need -45 dB of analog cancellation

• Major drawback

⎻Need to modify the radio circuitry

⎻Should be added after RF down-converter but

before the analog-to-digital converter, usually not accessible

17

Analog Cancellation

• Objective is to achieve exact 0 at the Rx antenna

• Cancellation path = negative of interfering path

• These techniques need analog parts

18

Digital Cancellation

• Cancel interference at baseband

• Conceptually simpler – requires no new

“parts”

• Useless if interference is too strong (ADC bottleneck)

19

How Digital Cancellation Works?

• Assume only Tx is

transmitting  Tx receives self-interference

• Estimate the self-channel

• When Rx starts transmitting

 Tx now receives

• Cancel self-interference by

20

DAC ADC X_tx

Y_tx

Node 1 (Tx)

H_tx,tx

Node2 (Rx) X_rx DAC

H_rx,tx

Digital Cancellation for OFDM

• Cancel for each subcarrier separately

• But, can’t just perform cancellation in the frequency domain  Why

⎻Hard to do iFFT  Cancellation  FFT in real-time

• How can we do digital cancellation for each subcarrier in the time-domain?

⎻See FastForward [Sigcomm’14]

21

Combine RF/Digital Cancellation

22

Tx Rx

DAC ADC

Tx samples

RF canceler Tx signal

Adapter Σ

Rx samples

Analog Cancellation

Digital

Cancellation

Antenna Cancellation

• Separate the antennas such that the two signals become deconstructive

⎻The distance different = λ/2

~30dB self-interference cancellation

combined with analog/digital cancellation

 70 dB

Antenna Cancellation: Block Diagram

24

Tx

RF Frontend Rx

RF Frontend

Digital processor

Power splitter Attenuator

Rx Tx

Tx

Performance

25

-60 -55 -50 -45 -40 -35 -30 -25

0 5 10 15 20 25

RSSI (dBm)

Position of Receive Antenna (cm)

TX1 TX2

Only TX1 Active

Only TX2 Active Both TX1 &

TX2 Active

Antenna Cancellation: Performance

27

Null Position

Impact of Bandwidth

35

fc fc+B fc -B

d d + λ/2

TX1 RX TX2

d2 d2 + λ+B/2

TX1 RX TX2

d1 d1 + λ-B/2

TX1 RX TX2

WiFi (2.4G, 20MHz) => ~0.26mm precision error A λ/2 offs et is precise for one frequency

not for the whole bandwidth

26

Bandwidth v.s. SIC Performance

27

Outline

• What’s full-duplex

• Self-Interference Cancellation

• Full-duplex and Half-duplex Co-existence

• Full-duplex relaying

28

Full-Duplex Radios

• Transmit and receive simultaneously in the same frequency band

• Suppress self-interference (SI)

AP

send receive

self interference

Three-Node Full-Duplex

• Commodity thin clients might only be half-duplex

• Inter-client interference (ICI)

⎻Uplink transmission interferes downlink reception

AP

Alice Bob

downlink uplink

interference

Access Control for 3-Node FD

• ICI might degrade the gain of full-duplex

⎻Appropriate client pairing is required

⎻Always enabling full-duplex may not good due to inter-client interference

⎻Switch adaptively between full-duplex and half- duplex

AP

Rx1 Small ICI

Rx2

Tx1

Tx2 Rx3

Existing Works

• Only allow hidden nodes to enable full- duplex [Sahai et al. 2011]

⎻Favor only part of clients, e.g., hidden nodes

• Pair clients based on historical transmission success probability [Singh et al. 2011]

⎻Statistics takes time and might not be accurate due to channel dynamics

• Schedule all the transmissions based on given traffic patterns [Kim et al. 2013]

⎻Need centralized coordinator and expensive overhead of information collection

Our Proposal: Probabilistic-based MAC

• Flexible adaptation

⎻Adaptively switch between full-duplex and half- duplex

• Fully utilizing of full-duplex gains

⎻Assign a pair of clients a probability of full- duplex access

⎻Find the probabilities so as to maximize the expected overall network throughput

• Distributed random access

⎻Clients still contend for medium access based on the assigned probability in a distributed way

Candidate Pairing Pairs

• Full-duplex pairs

⎻Only allows those with both clients with non- negligible rates

⎻

• Half-duplex virtual pairs

⎻Let ‘0’ denote the index of a virtual empty node

⎻

• All candidate pairs

⎻

Assign each pair a probability p^(i,j)

Linear Programming Model

Expected total rate

Downlink fairness

Uplink fairness

Sum probability

Probabilistic Contention

• AP selects downlink user i with probability

• Given downlink user i, uplink users adjust its priority by changing its contention window to

AP

Rx1

Rx2

Tx1

Tx2 Rx3

1. AP selects

downlink user first

2. Uplink clients

contend by CSMA/CA

Outline

• What’s full-duplex

• Self-Interference Cancellation

• Full-duplex and Half-duplex Co-existence

• Full-duplex relaying

37

Today’s Wireless Networks

• Ideally, 802.11ac and 802.11n support up to 780 Mb/s and 150 Mb/s, respectively

• In reality, signals experience propagation loss

What Can We Do?

• Increase capacity and coverage using relay

relay

Traditional Half-Duplex Relaying

TX and RX in a time/frequency division manner

direct

Half Duplex relayed buffer or

switch frequency

TX RX

symbol direct 1

relayed symbol 1

symbol

2 time

symbol

2 symbol

n symbol

… n

…

Improve SNR, but also halve the bandwidth

Full-Duplex Relaying!

Simultaneous TX and RX on the same frequency

direct

Full Duplex relayed self-interference

cancellation

TX

symbol direct 1

relayed symbol 1

symbol

2 time

symbol

2 symbol

n

symbol

… n

…

Improve SNR without halving the bandwidth

RX

1. Amplify-and-forward or Construct-and- forward

2. Demodulate-and-forward

relayed

rather decrease the SNR

direct

relayed

combined

rotate before forward

amplify constructively

may amplify destructively

[FastForward, SIGCOMM’14]

direct I

Q

I

Q

combined

[DelayForward, MobiHoc’16]

Pros and Cons of Amplify-and-Forward

✔Negligible processing delay at relay

✘ Also amplifying the noise at the relay

Still decodable with OFDM

noise direct

relayed noise

noise

noise

noise combined CP symbol1

direct

Δt

relayed

CP symbol2 CP symbol1 CP symbol2

S↑

N↑

1. Amplify-and-forward or Construct-and- forward

2. Demodulate-and-forward

relayed

rather decrease the SNR

direct

relayed

combined

rotate before forward

amplify constructively

may amplify destructively

[FastForward, SIGCOMM’14]

direct I

Q

I

Q

combined

11

N_r 01

00 10

x’ x Q

I

denoise at the relay _noise

noise

relayed direct

only amplify the signal

S↑

N

Challenges: Mixed Symbols

• Demodulation takes a much longer time

⎻Receive the whole symbol  FFT  demodulation

 modulation  IFFT

• It’s unlikely to fast forward within a CP interval

Inter-symbol interference at the destination

Need to recover from mixed symbols

CP symbol 1 direct

relayed Δt

CP symbol 2

CP symbol 1 CP symbol 2

• Introduce delay to enable symbol-level alignment

• Structure of combined signals is analogous to convolutional code

How to Ensure Decodability?

reception + processing + delay

x₁ x₂ x₃ x₄ … x_n-1

…

x_n direct

relayed x₁ x₂ x₃ x₄ x_n-1 x_n x₅ x₆

x_n-3 x_n-2

x_i x_i-1 x_i-2 direct +

relayed

combined

 Viterbi-type Decoding

Pros and Cons of Delay-and-Forward

✔Negligible processing delay at relay

✘ Also amplifying the noise at the relay

Still decodable with OFDM

noise direct

relayed noise

noise

noise

noise combined CP symbol1

direct

Δt

relayed

CP symbol2 CP symbol1 CP symbol2

S↑

N↑