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Recent Advances in Full-Duplex Relaying

Taneli Riihonen

Department of Signal Processing and Acoustics Center of Excellence in Smart Radios and Wireless Research Aalto University School of Electrical Engineering, Finland

Session B2, April 24, 2013 XXXIII Finnish URSI Convention on Radio Science

Presenter: Taneli Riihonen

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• Master of Science, Helsinki University of Technology (TKK), Finland, 2006

⊲ Received the McKinsey Award for the best graduating student

(only one among all 1007 M.Sc. degrees completed at TKK during that year)

⊲ Currently wrapping up D.Sc. thesis at Aalto University

• Productive (co-)author in scientific publications

⊲ 15/38 published journal/conference papers, some under review

• Dedicated (co-)supervisor for younger students

⊲ 8 M.Sc. theses completed, 1 currently in progress ⊲ 2 D.Sc. theses in progress (and collaboration with many others as a co-author)

• Diligent and punctual reviewing service for the community

⊲ Regularly since 2008: so far ∼ 200 papers (∼ 1/1 journals/confs.) ⊲ Exemplary Reviewer 2012 for IEEE Communications Letters

• Looking for a postdoc position abroad to grow academically and personally

Agenda

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• Overview of the presenter’s work on full-duplex relaying in

2008–2011 which constitutes ∼ 1/3 of his upcoming dissertation

• Tutorial to essential aspects that need to be considered when

introducing full-duplex operation into multihop relaying systems

• The basis for seminal research: loopback self-interference!

⊲ Mitigation techniques and evaluation of their performance ⊲ The feasibility of full-duplex relaying in the presence of

residual self-interference, i.e., comparison to half duplex

⊲ Merging full duplex with MIMO and OFDM techniques

• The results were originally published in multiple conference

and journal papers [1]–[12] (see the next two slides)

References (published in 2009)

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[1] T. Riihonen, S. Werner, and R. Wichman, “Comparison of full-duplex and half-duplex modes with a fixed amplify-and-forward relay,” in Proc. IEEE Wireless Communications and Networking Conference, Apr. 2009. [2] T. Riihonen, S. Werner, R. Wichman, and J. H¨ am¨ al¨ ainen, “Outage probabilities in infrastructure-based single-frequency relay links,” in Proc. IEEE Wireless Communications and Networking Conference, Apr. 2009. [3] T. Riihonen, S. Werner, and R. Wichman, “Optimized gain control for single-frequency relaying with loop interference,” IEEE Transactions on Wireless Communications, vol. 8,

• no. 6, pp. 2801–2806, Jun. 2009.

[4] T. Riihonen, S. Werner, R. Wichman, and E. Zacarias B., “On the feasibility of full-duplex relaying in the presence of loop interference,” in Proc. 10th IEEE Workshop on Signal Processing Advances in Wireless Communications, Jun. 2009. [5] T. Riihonen, K. Haneda, S. Werner, and R. Wichman, “SINR analysis of full-duplex OFDM repeaters,” in Proc. 20th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Sep. 2009. [6] T. Riihonen, S. Werner, and R. Wichman, “Spatial loop interference suppression in full-duplex MIMO relays,” in Proc. 43rd Annual Asilomar Conference on Signals, Systems, and Computers, Nov. 2009.

References (published in 2010–2011)

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[7] T. Riihonen, S. Werner, and R. Wichman, “Rate-interference trade-off between duplex modes in decode-and-forward relaying,” in Proc. 21st IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Sep. 2010. [8] T. Riihonen, S. Werner, and R. Wichman, “Residual self-interference in full-duplex MIMO relays after null-space projection and cancellation,” in Proc. 44th Annual Asilomar Conference on Signals, Systems, and Computers, Nov. 2010. [9] T. Riihonen, A. Balakrishnan, K. Haneda, S. Wyne, S. Werner, and R. Wichman, “Optimal eigenbeamforming for suppressing self-interference in full-duplex MIMO relays,” in Proc. 45th Annual Conference on Information Sciences and Systems, Mar. 2011. [10] T. Riihonen, S. Werner, and R. Wichman, “Hybrid full-duplex/half-duplex relaying with transmit power adaptation,” IEEE Transactions on Wireless Communications, vol. 10, no. 9,

• pp. 3074–3085, Sep. 2011.

[11] T. Riihonen, S. Werner, and R. Wichman, “Transmit power optimization for multiantenna decode-and-forward relays with loopback self-interference from full-duplex operation,” in

• Proc. 45th Annual Asilomar Conference on Signals, Systems, and Computers, Nov. 2011.

[12] T. Riihonen, S. Werner, and R. Wichman, “Mitigation of loopback self-interference in full-duplex MIMO relays,” IEEE Transactions on Signal Processing, vol. 59, no. 12, pp. 5983–5993, Dec. 2011.

Introduction

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Old Terminology

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half/full-duplex link

• Recommendation ITU-R V.662-2 (1993), or Wikipedia:

half duplex — “Designating or pertaining to a method of operation in which information can be transmitted in either direction, but not simultaneously, between two points.” full duplex — “Designating or pertaining to a mode of operation by which information can be transmitted in both directions simultaneously between two points.”

• Ambiguity problems

⊲ What is the level of abstraction, e.g., considered OSI layer? ⊲ May the two directions use different transmission media? ⊲ What if communication involves more than two points?

... and even ITU itself characterizes the terms as “deprecated”!

New Terminology

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half/full-duplex point

• Herein, we shall adopt the following revised definitions:

half duplex — “Designating or pertaining to a mode of operation by which information can be transmitted to and from a point in two directions, but not simultaneously on the same physical channel.” full duplex — “Designating or pertaining to a mode of operation by which information can be transmitted to and from a point in two directions simultaneously on the same physical channel.”

• Unambiguous and suitable for discussing modern topics

⊲ Focus on the operation mode of any transceiver instead of

bidirectional communication between exactly two points

⊲ Physical-layer perspective creates a link to spectral efficiency

... and it is not only me who already understands the terms like this

Hot Emerging Topic: Full-Duplex Wireless

Taneli Riihonen Recent Advances in Full-Duplex Relaying – 9 / 56

• Systems where some node(s) operate in the full-duplex mode
• Sometimes descriptively referred to as single-frequency

“simultaneous transmit and receive” (STAR)

• Progressive physical/link-layer frequency-reuse concept

= up to double spectral efficiency at a system level, if the significant technical problem of self-interference is tackled

• Transmission and reception should use the band for the same

amount of time to make the most of full duplex

⊲ (a)symmetry of traffic pattern, i.e.,

requested rates in the two simultaneous directions

⊲ (a)symmetry of channel quality, i.e.,

achieved rates in the two simultaneous directions

Full-Duplex Radio Transceivers

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Full-duplex transceiver Full-duplex transceiver Full-duplex transceiver Full-duplex transceiver

• Basic building blocks

for more complex networks

• The benefits go beyond

the physical layer!

⊲ e.g., simultaneous

spectrum sensing and transmission

• Will single-array (or -antenna)

full-duplex transceivers be viable some day?

⊲ Our study is not limited

to the dual-array case although it is assumed

Full-Duplex Communication Scenarios

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Source Destination Downlink user Uplink user Relay Terminal 1 Terminal 2 Access point

1) Multihop relay link

• Symmetric traffic
• Asymmetric channels
• Direct link may be useful

2) Bidirectional communication link between two terminals

• Asymmetric traffic (typically)
• Symmetric channels (roughly)

3) Simultaneous down- and uplink for two half-duplex users

• Asymmetric traffic
• Asymmetric channels
• Inter-user interference!

Full-Duplex Relaying

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Source Destination Relay

• Multihop relay link

⊲ Symmetric traffic ⊲ Asymmetric channels ⊲ Direct link may be useful

Agenda

• Tutorial to essential aspects that need to be considered when

introducing full-duplex operation into multihop relaying systems

• The basis for seminal research: loopback self-interference!

⊲ Mitigation techniques and evaluation of their performance ⊲ The feasibility of full-duplex relaying in the presence of

residual self-interference, i.e., comparison to half duplex

⊲ Merging full duplex with MIMO and OFDM techniques

Full-Duplex Relaying

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Relaying

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Source Relay Destination

• The general purpose of a relay node is to forward signals

from a source transmitter to a destination receiver

⊲ Other network topologies are also possible,

e.g., with multiple hops or parallel relays

⊲ Common protocols: amplify-and-forward (AF),

decode-and-forward (DF)

• Full-duplex relays exploit STAR such that source–relay and

relay–destination links share one physical channel

⊲ can be more sophisticated than simple on-channel repeaters

Direct Link

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Source Relay Destination

Potential direct link

• Two different applications for relays:

a) coverage extension where the relay is deployed because the direct link is weak b) diversity improvement where transmission from both the relay and the source is strong (on average) at the destination

• The former is more potential application for full-duplex relays

⊲ Half-duplex relaying can offer maximum diversity gain ⊲ Rate/SNR gain of full-duplex relaying becomes marginal

with a strong direct link: simple switching works well

Inherent Symmetry: Advantage for Full Duplex

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Source Full-duplex relay Destination Source Half-duplex relay Destination

• r
• Full duplex can ideally render up to double spectral efficiency

when compared to conventional half-duplex operation

⊲ Largest gains are achieved when simultaneous transmissions

• ccupy the channel for the same amount of time
• Relay links are good candidates for adopting the full-duplex mode

because their traffic pattern is inherently symmetric:

⊲ Equal requested source–relay and relay–destination data rates

to avoid data overflow or underflow in the relay

⊲ Unequal achieved data rates due to channel imbalance

Mitigation of Loopback Self-interference

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Mitigation of Loopback Self-interference (Refs)

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• The following discussion mainly originates from

[12] T. Riihonen, S. Werner, and R. Wichman, “Mitigation of loopback self-interference in full-duplex MIMO relays,” IEEE Transactions on Signal Processing, vol. 59, no. 12, pp. 5983–5993, Dec. 2011.

• Related results are available also in conference papers:

[6], [8], [9], [11]

• Measurement data on prototype antenna arrays by courtesy of

colleagues from Department of Radio Science and Engineering:

[H+] K. Haneda, E. Kahra, S. Wyne, C. Icheln, and P . Vainikainen, “Measurement of loop-back interference channels for

• utdoor-to-indoor full-duplex radio relays,” in Proc. 4th European

Conference on Antennas and Propagation, Apr. 2010.

Loopback Self-interference

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Full-duplex relay

Loopback self-interference

• Full-duplex operation is possible only after tackling a significant

technical challenge: unavoidable self-interference

⊲ Huge difference in power levels (interference vs. desired signal)

• Full duplex is adopted first for fixed infrastructure nodes and

later (maybe) for small portable, or even handheld, radios

⊲ Initially, the concept of full-duplex relaying is different from

cooperative communication among mobile nodes where time-division half-duplex operation is the baseline assumption

• Next: self-interference mitigation techniques

Passive Physical Isolation

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Full-duplex relay

Loopback self-interference

• State-of-the-art devices require two separate antenna arrays:
• ne for receiving and the other for transmitting

⊲ Mainly antenna design and placement problems:

directivity, back-to-back coupling, distance, obstacles

⊲ But using two arrays is useful for relaying in general since the

source and the destination are located at different directions

• In (future?) single-array devices, all physical isolation is provided

by a circulator: mainly an electronics design problem

• Next: measured physical isolation with prototype antenna arrays

Experimental Antenna Arrays for Full-Duplex MIMO Relay∗

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• Design goals:
• 1. Compact size but high isolation
• 2. 2.6GHz ± 100MHz operation band
• 3. Multiple Rx and Tx antenna elements
• Building and measuring 4 × 4 array prototype

∗Further details are provided in [H+]:

• K. Haneda et al., “Measurement of loop-back interference channels for outdoor-to-indoor full-duplex radio relays,” April 2010.

Channel Measurement Campaign for Outdoor-to-Indoor Relaying Scenarios∗

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Compact array configuration

• Arrays attached side-by-side (2cm)
• Small box like a Wi-Fi router
• Several positions next to windows

Separate array configuration

• Four Tx antenna orientations
• LOS: Tx in the same room as Rx
• NLOS: Tx in the adjacent corridor

∗Further details are provided in [H+]:

• K. Haneda et al., “Measurement of loop-back interference channels for outdoor-to-indoor full-duplex radio relays,” April 2010.

Average Physical Isolation

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• For compact array configuration,

measured isolation is 36.2dB

• For separate array configuration,

isolation is directly proportional to antenna separation (2–3dB/m) 1 2 3 4

dRR

Rrx Rtx

2 4 6 8 10 12 50 55 60 65 70 75 80 85 Orientation 1 Orientation 2 Orientation 3 Orientation 4 line-of-sight (LOS) non-line-of-sight (NLOS)

E{Ptx/PI} [dB] dRR [m]

• 20dB isolation from window glass for separate array configuration
• Mere physical isolation may not be sufficient which gives

motivation for active mitigation by signal processing

Objective for Active Mitigation

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−20 −10 10 20 30 40 50 60 70 80 0.01 0.1 0.5 (ǫH,ǫt) Natural isolation TDC (0.02,0.02) NSP (0.02,0.02) TDC (0,0.02) NSP (0,0.02) TDC (0.02,0) NSP (0.02,0) Half duplex

BER E{PI

• natural} [dB]

PI

• man-made

∆PI

The target use case for full-duplex relaying with mitigation

bit-error rate in a DF relay vs. physical isolation

• Transparent minimization of self-interference: the relay protocol

can operate as in the half-duplex mode but at double symbol rate

⊲ Mitigation becomes separated from the protocol design

and the schemes are applicable with all kinds of protocols

Active Mitigation

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Time-domain cancellation:

R

filter

Spatial-domain suppression:

R

filter filter

• Two main techniques for active self-interference mitigation

⊲ Cancellation: time-domain filtering in feedback path ⊲ Suppression: spatial-domain filtering in feedforward path

• Both schemes could ideally eliminate all self-interference
• Cancellation is a rather straightforward task while

suppression can be implemented in various ways

Imperfect Side Information

Taneli Riihonen Recent Advances in Full-Duplex Relaying – 26 / 56

R

filter noise error

• In practice, self-interference cannot be perfectly eliminated

⊲ Channel estimation error in filter design ⊲ Transmit-side noise due to non-ideal electronics

(the actual transmitted signal is not known)

• Sufficient physical isolation and analog pre-cancellation are also

required to cope with limited dynamic range at the receive side

Spatial-Domain Suppression

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R

filter filter noise error error

• Next: evaluating the main variations of suppression

⊲ antenna selection (AS) ⊲ beam selection (BS) ⊲ null-space projection (NSP) ⊲ minimum mean square error (MMSE) filtering

• In some cases, it may be beneficial to combine time-domain

cancellation with spatial-domain suppression

Antenna vs. Beam Selection

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2 4 6 8 10 12 14 16 0.01 0.1 1 ˆ Nrx × ˆ Ntx: E{∆PI} [dB] 3 × 4: 0.94 2 × 4: 1.88 3 × 3: 2.20 1 × 4: 3.25 2 × 3: 3.63 2 × 2: 5.75 1 × 3: 6.18 1 × 2: 10.05 1 × 1: 22.63

F∆PI(x) x [dB]

antenna selection (AS)

2 4 6 8 10 12 14 16 0.01 0.1 1 ˆ Nrx × ˆ Ntx: E{∆PI} [dB] 3 × 4: 3.06 2 × 4: 7.37 3 × 3: 7.86 1 × 4: 21.81 2 × 3: 23.57

F∆PI(x) x [dB]

beam selection (BS)

• Ideal side information; four receive and transmit antennas
• AS improves isolation significantly only in the single-stream case

⊲ BS is reduced to null-space projection (NSP) and eliminates

self-interference completely if less than five streams are used

Rank of Loopback Channel

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2 4 6 8 10 12 14 16 0.01 0.1 1 Nrx × Ntx (rk{HLI}): E{∆PI} [dB] AS, 3 × 4 (3): 1.29 AS, 3 × 4 (2): 1.62 AS, 4 × 4 (4): 2.20 AS, 4 × 4 (3): 2.57 BS, 3 × 4 (3): 5.04 BS, 3 × 4 (2): 8.12 BS, 4 × 4 (4): 7.86 BS, 4 × 4 (3): 11.87

F∆PI(x) x [dB]

ideal side information; three receive and transmit antennas

• Spatial-domain suppression can benefit from low channel rank

⊲ Beam selection (BS) directs the self-interference energy

to the weakest eigenmodes which include the null space

• Time-domain cancellation (not shown) would not be affected at all

Imperfect Side Information

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0.01 0.1 1 10 5 10 15 20 25 30 35 40 45 ˆ Nrx × ˆ Ntx (rk{HLI}) NSP, 3 × 4 (1) BS, 3 × 4 (2) BS, 3 × 4 (3) BS, 3 × 4 (4) NSP, 3 × 3 (1) NSP, 3 × 3 (2) BS, 3 × 3 (3) BS, 3 × 3 (4) NSP, 2 × 4 (1) NSP, 2 × 4 (2) BS, 2 × 4 (3) BS, 2 × 4 (4) TDC, 4 × 4 (1–4)

ǫH E{∆PI} [dB]

channel estimation error

0.01 0.1 1 10 5 10 15 20 25 30 35 40 Nrx × Ntx (rk{HLI}) NSP, 3 × 4 (1) BS, 3 × 4 (2) BS, 3 × 4 (3) NSP, 4 × 3 (1) BS, 4 × 3 (2) BS, 4 × 3 (3) NSP, 4 × 4 (2) BS, 4 × 4 (3) BS, 4 × 4 (4) TDC, 3 × 3 (1–3)

ǫt E{∆PI} [dB]

transmit-side noise

• Additional isolation from BS is limited with ideal side information

⊲ Imperfect side information determines the additional isolation

achieved with NSP or time-domain cancellation (TDC)

• NSP can be made immune to transmit-side noise

Cancellation vs. Suppression

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8 16 24 32 40 48 56 64 0.01 0.1 1 (rk{HLI}): E{∆PI} [dB] BS, (3): 5.04 BS, (2): 8.15 MMSE, (3): 8.77 MMSE, (2): 14.76 TDC, (1): 25.20 TDC, (2): 25.30 TDC, (3): 25.34 MMSE, (1): 40.20 NSP, (1): 40.52

F∆PI(x) x [dB]

minimum MSE filtering

8 16 24 32 40 48 56 64 0.01 0.1 1 (rk{HLI}): E{∆PI} [dB] BS, (4): 7.80 BS, (3): 11.56 TDC, (1): 25.25 TDC, (2): 25.32 TDC, (3): 25.35 TDC, (4): 25.36 NSP, (2): 29.67 both, (4): 34.49 both, (3): 34.49 both, (2): 34.49 both, (1): 36.81 NSP, (1): 40.51

F∆PI(x) x [dB]

combined time/spatial-domain mitigation

• Loopback channel rank defines which scheme is preferable
• The combination of TDC and suppression offers better

performance than either alone, except when rank-deficient loopback channel enables the usage of NSP

Transmit Power Adaptation

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Transmit Power Adaptation (Refs)

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• The following discussion mainly originates from

[3] T. Riihonen, S. Werner, and R. Wichman, “Optimized gain control for single-frequency relaying with loop interference,” IEEE Transactions on Wireless Communications, vol. 8, no. 6,

• pp. 2801–2806, Jun. 2009.

[10] T. Riihonen, S. Werner, and R. Wichman, “Hybrid full-duplex/half-duplex relaying with transmit power adaptation,” IEEE Transactions on Wireless Communications, vol. 10, no. 9,

• pp. 3074–3085, Sep. 2011.
• Related results are available also in conference papers:

[4], [5], [7], [11]

Transmit Power Adaptation

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Source Full-duplex relay

Residual leakage

Destination power control

• In practice, there will always be residual self-interference

after applying all means of mitigation

• Fortunately, transmit power adaptation can still exploit

the channel imbalance caused by residual interference

⊲ In principle, the relay should appropriately lower its own

transmit power if the first hop is the bottleneck of the system

• Win–win solution: energy savings can be achieved

while performance is also optimized

Example with Amplify-and-Forward Protocol

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1 2 3 4 5 6 7 8 9 10 5 10 15 20 25

β2 [linear] γ [linear]

Varying β2, fixed |hLI|2 With β2

• pt, varying |hLI|2

With β2

tar, varying |hLI|2

With β2

max, varying |hLI|2

|hLI|2 = 0 |hLI|2 ≈ −29.86dB |hLI|2 = −25dB |hLI|2 = −20dB |hLI|2 = −15dB |hLI|2 = −10dB

end-to-end SINR vs. relay gain

• The end-to-end signal-to-interference and noise ratio (SINR) starts

to decrease when increasing relay gain beyond the optimal point

⊲ Relay should use its maximum allowed transmit power

• nly in the case of low residual self-interference

Full Duplex vs. Half Duplex

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Full Duplex vs. Half Duplex (Refs)

Taneli Riihonen Recent Advances in Full-Duplex Relaying – 37 / 56

• The following discussion mainly originates from

[10] T. Riihonen, S. Werner, and R. Wichman, “Hybrid full-duplex/half-duplex relaying with transmit power adaptation,” IEEE Transactions on Wireless Communications, vol. 10, no. 9,

• pp. 3074–3085, Sep. 2011.
• Related results are available also in conference papers:

[1], [4], [7], [11]

• In articles [2] and [3], our results focus on the full-duplex mode,

but the analysis itself could be also used for comparison purposes

Fundamental Rate–Interference Trade-off

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• Determining the ultimate feasibility of full-duplex relaying

in the presence of residual self-interference. In principle,

⊲ half-duplex relay link: ⊲ full-duplex relay link:

Reduced symbol rate due to two allocated channels Residual self-interference even after mitigation

RHD = 1 2 log2

• 1 + PS

PN

• RFD = log2
• 1 +

PS PI+PN

• Should the system choose to operate with

a) loss of end-to-end symbol rate (half duplex), or b) loss of S(I)NR due to self-interference (full duplex)?

Full- or Half-Duplex (... or Direct Transmission)?

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• Rate–interference trade-off: choosing between

⊲ full-duplex (FD) relaying with residual self-interference

− Direct link treated as interference at the destination − With and without transmit power adaptation

⊲ half-duplex (HD) relaying

− Maximum ratio combining (MRC) for the direct and relayed transmissions at the destination

⊲ direct transmission (DT)

− The same (full) symbol rate as with FD relaying but low channel SNR on average (coverage extension)

• The comparison yields switching boundaries between

the modes according to channel imbalance

Instantaneous Channel State Information

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Let us next consider the case of deterministic (static) channels

• This represents, for example, a snapshot of the system within

channel coherence time in a slow-fading environment

• Instantaneous channel state information (channel SNRs) for

⊲ choosing the proper mode ⊲ transmit power adaptation (with FD) ⊲ maximum ratio combining (with HD)

• Metric for the comparison: instantaneous transmission rate

⊲ The analysis can be completely conducted in terms of

closed-form expressions (see the papers)

Instantaneous Switching Boundaries

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−15 −10 −5 5 10 15 20 25 0.5 1 1.5 2 2.5

ΓLI ΓSD

γLI = γSD + 15 [dB] C [bit/s/Hz]

pR = 1 pR = p∗

R

Full-duplex Half-duplex with MRC Decode-and-forward Amplify-and-forward Direct transmission

instantaneous transmission rates

• Full-duplex (FD) relaying is preferred with low self-interference

⊲ Transmit power adaptation extends the range further

• Pure direct transmission (DT) is preferred with a strong direct link

and MRC gives little benefit for half-duplex (HD) relaying

Direct Transmission vs. Relaying

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−10 −5 5 10 15 20 −10 −5 5 10 15 20

γSR [dB] γRD [dB]

γSD = −6dB γSD = −1dB γSD = 3dB γSD = 6dB

DT FD/HD

Decode-and-forward Amplify-and-forward

switching boundaries

• FD relaying is suitable for the scenario of coverage extension

⊲ When the direct link exists in fortunate fading states,

the relay is not momentarily needed at all

• Simple switching yields also good diversity improvement

Full-Duplex vs. Half-Duplex Relaying

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−10 −5 5 10 15 20 −10 −5 5 10 15 20

γSR [dB] γRD [dB]

γLI = 3dB γLI = 6dB γLI = 9dB

HD FD

Decode-and-forward Amplify-and-forward

without transmit power adaptation

−10 −5 5 10 15 20 −10 −5 5 10 15 20

γSR [dB] γRD [dB]

γLI = 6dB γLI = 9dB γLI = 12dB γLI = 9 d B γLI = 12dB γLI = 15dB

HD FD

Decode-and-forward Amplify-and-forward p∗

R = 1 for AF

with transmit power adaptation

• Instead of adhering to any mode at early design stage, it is

advantageous to implement hybrid full-duplex/half-duplex relaying, i.e., opportunistic switching between the modes, because the rate–interference trade-off favors them alternately during operation

Statistical Channel State Information

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Let us then consider the case of fading channels

• Fixed infrastructure relay node for coverage extension

⊲ Static link between the base station and the relay ⊲ Rayleigh-fading link between the relay and a mobile user

• Statistical channel state information (average channel SNRs) for

⊲ choosing the proper mode ⊲ transmit power adaptation (with FD)

• Metric for the comparison: average transmission rate

⊲ The actual rate expressions can be calculated in a closed form

but switching boundaries and transmit power adaptation need numerical look-up tables (see the papers)

Statistical Switching Boundaries

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5 10 15 20 25 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

Decode-and-forward Amplify-and-forward

¯ γRD [dB] ¯ C [bit/s/Hz]

FD, pR = p∗

R

FD, pR = ¯ p∗

R

Hybrid FD/HD FD, pR = 1 HD FD, pR = p∗

R

FD, pR = ¯ p∗

R

Hybrid FD/HD FD, pR = 1 HD

average transmission rates

• Statistical mode switching and transmit power adaptation yield

rather good performance with much lower signaling overhead

⊲ Hybrid FD/HD relaying (instantaneous switching) gives

the largest gains near statistical switching boundaries

Full-Duplex vs. Half-Duplex Relaying

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5 10 15 20 25 30 5 10 15 20 25 30

¯ γSR [dB] ¯ γRD [dB]

¯ CFD = ¯ CHD 25% 25% 50% 50% 75% AF in DL DF in DL AF in UL DF in UL

without transmit power adaptation

5 10 15 20 25 30 5 10 15 20 25 30

¯ γSR [dB] ¯ γRD [dB]

¯ CFD = ¯ CHD 35% 60% 75% AF in DL DF in DL AF in UL DF in UL

with transmit power adaptation

• Illustrating downlink (DL) vs. uplink (UL) transmission

⊲ self-interference in a mobile channel vs. in a fixed channel

• Rate is significantly improved by choosing the proper mode

which is typically FD when using transmit power adaptation

Conclusion

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Conclusion

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• Wireless full duplex: A progressive frequency-reuse concept!
• Herein: overview of recent work on full-duplex relaying
• Essential aspects that need to be considered when introducing

full-duplex operation into multihop relaying systems

⊲ Loopback self-interference ⊲ Mitigation techniques and evaluation of their performance

− physical isolation − time-domain cancellation − spatial-domain suppression − transmit power adaptation

⊲ Rate–interference tradeoff: the feasibility of full-duplex

relaying in the presence of residual self-interference

• ... and how is all this related to OFDM mentioned in the beginning?

Future Work

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Joint Signal and Interference Processing

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Joint processing Joint processing

• Herein: “transparent” self-interference mitigation schemes

⊲ Any existing relaying protocol could be used ⊲ But the joint design of mitigation and a specific protocol

would probably bring performance gains

• Herein: simple switching between direct transmission and relaying

⊲ Direct link is regarded as interference when using the relay ⊲ The destination could apply signal processing techniques

to separate and constructively combine the superimposed signals from the source and the relay

Extensions to Other Full-Duplex Scenarios

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Source Destination Downlink user Uplink user Relay Terminal 1 Terminal 2 Access point

Full-duplex communication 1) Multihop relay link 2) Bidirectional communication 3) Simultaneous down- and uplink Other potential uses for STAR

• medium access control
• cognitive radios

Generic full-duplex radios

• improved isolation and mitigation

Full-duplex transceivers

Limited Receiver Dynamic Range

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A/D D/A

R

filter filter

• Severe risk of saturating analog-to-digital (A/D) converters

⊲ quantization noise due to limited resolution ⊲ clipping noise which is pronounced with OFDM

• Digital cancellation is useless if dynamic range is not sufficient
• It is difficult and expensive to adapt the response of an analog

filter to match the time- and frequency-selective MIMO channel

Example on Quantization Noise (4-bit A/D)

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Signal of interest Interference signal Sum signal

• ∼1-bit resolution

for the signal of interest

before A/D after A/D after digital cancellation and scaling

• ∼3-bit resolution

for the signal of interest

Example on Clipping Noise (4-bit A/D)

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Signal of interest Interference signal Sum signal

• ∼2-bit clipped resolution

for the signal of interest

before A/D after A/D after digital cancellation and scaling

• ∼3-bit resolution

for the signal of interest

Mitigation in Analog Domain

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A/D D/A

R

filter filter noise

• Self-interference should be minimized before A/D conversion

⊲ Physical isolation is an antenna design problem ⊲ Analog cancellation is an electronics design problem

• Transmit-side beamforming can eliminate the interference

“on-the-air” before it even reaches the receiver front-end

⊲ A digital signal processing problem!

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