Where are the Relay Capacity Gains in Cellular Systems?ctw2010.ieee-ctw.org/mon/Heath.pdf · multi-hop relay base station user ii user i strong link transmission cell Two-way half

The Wireless Networking and Communications Group

Where are the Relay Capacity Gains inCellular Systems?

Robert W. Heath Jr.Steven Peters, Kien Truong, and Ali Yazdan-Panah

The University of Texas at AustinWireless Networking and Communications Group

http://www.ece.utexas.edu/~rheath

* Funded by a gift from Huawei



Introduction to Relays

Relays assist communication

May be fixed (infrastructure), mobile (bus) or cooperative (other users)

Single antenna, multiple antennas, multiple relays, multiple users

Main purpose of relays

Coverage (large-scale effects), diversity (small-scale) [?], and capacity [??]

2

relay

base station

user ii

user i

strong linkstrong link

transmission cell

relay

base station

user ii

user i


transmission cell

relay

base station

user ii

user i


transmission cell

relay

base station

user ii

user i


transmission cell

relay

base station

user ii

user i


transmission cell

Types of Relays

3

One-wayfull duplex

relay

base station

user ii

user i


transmission cell

One-wayhalf duplex

relay

base station

user ii

user i


transmission cell

1st time-slot2nd time-slot

One-waymulti-hop

relay

base station

user ii

user i


transmission cell

Two-wayhalf duplex

relay

base station

user ii

user i


transmission cell

Relay Operation

4

1

Multi-User Bidirectional Communications:

Schemes for the Shared Relay Concept

Ali Y. Panah§†

and Robert W. Heath, Jr.†

[email protected] and [email protected]

The University of Texas at Austin

The Department of Electrical and Computer Engineering

Wireless Networking Communications Group

1 University Station C0803, Austin, TX, USA 78712− 0240

r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.

• Four-phase one-way DF relaying

• Three-phase two-way DSF relaying

• Two-phase two-way ASF relaying

§Corresponding author.

†This work was supported by a gift from Huawei Technologies Ltd.

May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

DeMOD Detect MOD

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

1



Ali Y. Panah§†







r

r = γr

r = Q (r)

r = Γr

E{|r|2} ≤ Pr

Abstract

abstract here.






May 6, 2010 DRAFT

Compress

Amplify-and-forward

Relay scales RX signal

Seems practical; less interest in standards

Decode-and-forward

Relay decodes RX signal

Very practical

Compress-and-forward

Relay compresses RX sig

Receiver decodes with partial information

Less industry interest

Expected Gains of Relays

The maximum capacity gain over the direct transmission

As , bps/Hz (DF is the best)

As , bps/Hz (CF is the best)

If , bps/Hz

5

relay

base station

user ii

user i


transmission cell

1

d 1-d

Linear system: full-duplex relay, no fading & same transmit powers [KraEtAl05]

d→ 0d→ 1

d = 0.5 α ∈ [2, 5)

Rrelay −Rdirect = log(1 + k P )− log(1 + P )→ log(k) � ∆R∞ as P →∞

∆R∞ = 2

∆R∞ = 1

∆R∞ = log(1 + 2α) ≈ α path-loss exponent

Capacity gains of relays are modest (there is diversity as well)

relay

base station

user ii

user i


transmission cell

relay

base station

user ii

user i


transmission cellCellular Systems Interference Limited

Relay deployments 7% indoor coverage improv. [DoppEtAl08]

30% improvement in outage SINR w/ fast PC [SreEtAl02]

Better uplnk coverage, 10% rate improvement 3GPP [IrmDie08]

6% rate improvement downlink LTE-A [LinEtAl09]

6

relay

base station

user ii

user i


transmission cell

Lots of interference

With interference, capacity gains seem to reduce further

Purpose of this Talk

(1) Discuss performance of different relaying strategies

(2) Suggest some solutions to make relays better

Compare with no relay & coordinated transmission (via DPC)

7

Simulation Framework

Channel

IEEE 802.16j Type E channel model

46 dBm base station transmit power

37 dBm relay transmit power

24 dBm mobile transmit power

Configuration

3 cells, 6 sectors per cell, 1 antenna UE

Compute throughput on ring

Switch from direct to relay link as move from BTS [ring based]

8

Interfering Sectors

0 100 200 300 400 500 600 700 800 9000

0.5

1

1.5

2

2.5

3

3.5

4

MS distance from BS (m)

1 us

er a

chie

vabl

e ra

te (b

ps/H

z)

Direct transmissionBS CoordinationSingle−antenna relay

0 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7

MS distance to BS (m)

1 us

er a

chie

vabl

e ra

te (b

ps/H

z)

Direct transmissionBS Coordination

Relay Performance

9

No gain from a single relay (ouch!!!)

BS-RS interference limited BS-RS interference limited

Downlink (DL) Uplink (UL)

Half duplex relay, ignore direct link, DF, optimum time sharing

Add Multiple Antennas

First phase: Receive MMSE RX receive filter

Second phase: TX beamforming (BF) at relays

MRT (max. ratio trans.): maximizing desired signal power to its own user

ZF (zero-forcing): minimizing sum of leakage powers to other users

SLNR (signal-to-leakage-plus-noise ratio): balancing MRT & ZF (like MMSE)10

leakage signals

desired signal

relay

base station

user ii

user i


transmission cell

relay

base station

user ii

user i


transmission cell

relay

base station

user ii

user i


transmission cell

One-Way Relay with Antennas

Using multiple antennas at relays provide significant gains

Better than direct trans./single-ant. relays from half of cell radius to cell-edge

Interference cancellation is good only for DL cell-edge users

11

Downlink (DL) Uplink (UL)

Enhanced relay functionality reduces gap to full coordination

0 100 200 300 400 500 600 700 800 9000

0.5

1

1.5

2

2.5

3

3.5

4


1 u

se

r a

ch

ieva

ble

ra

te (

bp

s/H

z)

Single antenna + Direct

BS coordination

MRT 2 antennas + Direct

ZF 2 antennas + Direct

SLNR 2 antennas + Direct







0 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7


1 u

se

r a

ch

ieva

ble

ra

te (

bp

s/H

z)

Single antenna + Direct

BS coordination










Shared Relaying

Multi-antenna relay placed at cell intersection [PetEtAl09]

Relay shared among multiple base stations

Inter-cell interference removed through MU-MIMO techniques

Performs a decode and forward operation

12

0 100 200 300 400 500 600 700 800 9000

0.5

1

1.5

2

2.5

3

3.5

4

MS distance to BS (m)

1 us

er a

chie

vabl

e ra

te (b

ps/H

z)

3−Antenna Shared Relay + Direct4−Antenna Shared Relay + Direct5−Antenna Shared Relay + Direct6−Antenna Shared Relay + DirectBS CoordinationDirect transmission

Shared Relay Performance

13

UplinkDownlink

Enhanced relay functionality reduces gap to full coordination

0 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7

MS distance from BS

1 us

er a

chie

vabl

e ra

te (b

ps/H

z)

3−Antenna Shared Relay + Direct4−Antenna Shared Relay + Direct5−Antenna Shared Relay + Direct6−Antenna Shared Relay + DirectBS CoordinationDirect transmission

Two-Way Shared Relay

14

inter-sector interf. other MS interf.

4

at the signal level to construct

ti = si1

�1 + γ

2+ xi1

�1− γ

2, −1 ≤ γ ≤ 1, i = 1, 2, 3. (7)

Next, to spatially separate the BS-MS pairs, the relay assigns unique beamforming vectors wi to each

ti. The transmitted vector from the relay is tR =√

Pr�3

i=1 witi =√

PrWt, where W �= [w1,w2,w3]

with tr(WW∗) = 1, t �= [t1, t2, t3]T , and Pr is the total average power from the relay terminal. Assuming

reciprocity in the channels, the received signal in first sector of the ith BS is

yi = h∗i1tR + ni, (8)

where ni ∼ CN (0, N0) is AWGN. Similarly, at the ith MS

zi = g∗i1tR + vi, (9)

where vi ∼ CN (0, N0) is AWGN. Viewing these signals in corresponding pairs we define the 2 × 1

vector di�= [yi, zi]T so that

di = [hi1 gi1]∗tR + [ni, vi]T

=�

PrFiWt + ni

=�

PrFiwiti +�

Pr

�

j �=i

Fiwjtj + ni, (10)

where Fi�= [hi1 gi1]∗ is a composite BS-MS channel for the ith cell and ni ∼ CN (0, N0I2). To enforce

spatial separation in (10), i.e. cancel the interference from other BS-MS pairs, we set the following

constraint on the beamforming vectors Fiwj = 02, ∀j �= i. By defining the 4 × M matrix Fi�=

[F∗1 · · · F∗i−1 F∗i+1 · · · F∗3]∗, the beamforming vectors may be obtained from a “block diagonalization”

constraint Fiwi = 04, i = 1, 2, 3. Denote the SVD of Fi as Ui[Σi 04×M ]V∗i , where Vi = [V(1)

i V(0)i ]

and Ui are unitary matrices, Σi is a 4 × 4 diagonal matrix with nonzero elements and the columns of

V(1)i are the corresponding right singular vectors. The M×(M−4) matrix V(0)

i represents the null-space

of Fi which for M = 5 consist of a single column vector that may be chosen for the beamforming vector

wi (with normalization by√

3 to preserve the power constraint since ||V(0)i ||22 = 1).

With this solution (10) reduces to di = Fiwiti +ni. The self-interference is manifested in the received

January 28, 2010 DRAFT

4


ti = si1

�1 + γ

2+ xi1

�1− γ

2, −1 ≤ γ ≤ 1, i = 1, 2, 3. (7)



Pr�3

i=1 witi =√




yi = h∗i1tR + ni, (8)


zi = g∗i1tR + vi, (9)




=�

PrFiWt + ni

=�

PrFiwiti +�

Pr

�

j �=i

Fiwjtj + ni, (10)






i V(0)i ]









4


ti = si1

�1 + γ

2+ xi1

�1− γ

2, −1 ≤ γ ≤ 1, i = 1, 2, 3. (7)



Pr�3

i=1 witi =√




yi = h∗i1tR + ni, (8)


zi = g∗i1tR + vi, (9)




=�

PrFiWt + ni

=�

PrFiwiti +�

Pr

�

j �=i

Fiwjtj + ni, (10)






i V(0)i ]









phase I phase II phase III

BSMSRN (shared)

Cellular TopologyConcept

Phase I: Relay decodes BS signals under inter-sector interference

Phase II: Relay decodes MS signals under other MS interference

Phase III: UL/DL power control + spatial orthogonalization via block diag.

A single superposition of UL + DL signals is transmitted by relay

Two-Way Shared Relay Uplink

15

Performance

Performance is reduced due to inter-sector and inter cell MS interference

Direct transmission is preferred unless MS closer to shared relay

Optimal time-share with phase III is also assumed (not shown in equation)

5


signals by substituting for the superposition from (7) into (8) and (9). For example, at the BSs we have

yi =�

Prh∗i1witi + ni

=�

Pr

2h∗i1wi

��1 + γsi1 +

�1− γxi1

�+ ni

=

�Pr(1 + γ)

2h∗i1wisi1

� �� self−interference

+

�Pr(1− γ)

2h∗i1wixi1

� �� desired

+ni, (11)

such that the desired signal from the MS may be detected from yi = yi−�

Pr(1+γ)2 h

∗i1wisi1. The uplink

sum-rate is then

R(III)UL (γ) =

3�

i=1

log2

�1 +

Pr(1− γ)2N0

h∗i1wiw

∗i hi1

�. (12)

Similarly at the MS, detection of the signal from the BS (via the relay) may be obtained from zi =

zi −�

Pr(1−γ)2 g

∗i1wixi1, and the downlink sum-rate is

R(III)DL (γ) =

3�

i=1

log2

�1 +

Pr(1 + γ)2N0

g∗i1wiw

∗i gi1

�(13)

Combining (2), (26), (5), and (25), the uplink and downlink sum-rates as a function of γ are given by

RDL(γ) =13

min{R(I)DL, R(III)

DL }

=13

min{log2 det(IM +Pb

N0 + σ2ζb

HH∗),

3�

i=1

log2

�1 +

Pr(1 + γ)2N0

g∗i1wiw

∗i gi1

�} (14)

RUL(γ) =13

min{R(II)UL , R(III)

UL }

=13

min{log2 det(IM +Pm

N0 + σ2ζm

GG∗),

3�

i=1

log2

�1 +

Pr(1− γ)2N0

h∗i1wiw

∗i hi1

�} (15)

B. Three-phase ASF relaying

A less sophisticated relay may choose not to decode the symbols in phase I and II but instead form

a scaled superposition tR = µdy(I)R + µuy

(II)R to broadcast in the third phase, where µu, µd > 0 are a

scalers chosen such that the average power constraint E{tr(tRt∗R)} = Pr is not violated at the relay. To

allow for a fair comparison with previous relay strategies while satisfying the power constraint for this

scheme we set

µ2d||y

(I)R ||22

µ2d||y

(II)R ||22

=1 + γ

1− γ

Pr = µ2d||y

(I)R ||22 + µ2

u||y(II)R ||22,

(16)


in presence of MS interference w/ block diagonalization

0 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7


1 us

er a

chie

vabl

e ra

te [u

plin

k] (b

ps/H

z)

Two way + DirectDirectBS Coordination

Break Point (450m)

Downlink Comparison

One-way shared relay is good for cell-edge users

One-way relay with multiple antennas help more-inner users

Two-way shared relay does not help DL transmission

16

Area Spectral Efficiency (ASE)

ConfigurationsASE

(bps/Hz/km2)

Single-antenna relay + direct 6.99

BS coordination 20.20

One-way SLNR (3x) 2 ants + Direct 8.53


One-way shared relay 3 ants + Direct 10.31


Two-way shared relay 6 ants + Direct 6.99

0 100 200 300 400 500 600 700 800 9000.5

1

1.5

2

2.5

3

3.5

4


1 u

ser

achie

vable

rate

(bps/H

z)

Single antenna relay + Direct

BS coordination

Uplink Comparison

Two-way shared relay is good for cell-edge users

One-way relay with multiple antennas help more-inner users

One-way shared relay does not help UL transmission

17


ConfigurationsASE

(bps/Hz/km2)







Two-way shared relay 6 ants + Direct 7.580 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7


1 u

se

r a

ch

ieva

ble

ra

te (

bp

s/H

z)


BS coordination

Uplink / Downlink Sum Comparison

18


ConfigurationsASE

(bps/Hz/km2)







Two-way shared relay 6 ants + Direct 7.28

0 100 200 300 400 500 600 700 800 9000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


1 u

se

r a

ch

ieva

ble

ra

te (

bp

s/H

z)


BS coordination

Two-way shared relay is not as good as one-way shared relay

Where are the Capacity Gains?Not here: Relays that neglect interference

They arguably suck even without interference

Here: Relays that deal with interference

Multiple antennas improve mid-range cell performance

Multiple antenna shared relay improves edge of cell performance

Combination of relay strategies seems very attractive

Future work: Two-way, relay selection, power control

19

References[DoppEtAl08] K. Doppler, C. Wijting, and K. Valkealahti, “On the Benefits of Relays in a Metropolitan Area Network”, VTC 2008.

[SreEtAl02] V. Sreng, H. Yanikomeroglu, and. D. Falconer, “Coverage enhancement through two-hop relaying in cellular radio systems,” WNCC 2002.

[IrmDie08] R. Irmer and F. Diehm,”On coverage and capacity of relaying in LTE-advanced in example deployments," PIMRC 2008.

[LinEtAl09] Huang Lin, Daqing Gu, Wenbo Wang, Hongwen Yang , "Capacity analysis of dedicated fixed and mobile relay in LTE-Advanced cellular networks," ICCTA 2009.

[PetEtAl09] S. W. Peters, A. Y. Panah, K. T. Truong, and R. W. Heath, Jr., ``Relaying Architectures for 3GPP LTE-Advanced,'' EURASIP Journal on Advances in Signal Processing, special issue on 3GPP LTE and LTE Advanced, vol. 2009, Article ID 618787, 14 pages, doi:10.1155/2009/618787, 2009.

[KraEtAl05] G. Kramer, M. Gastpar, and P. Gupta, “Cooperative strategies and capacity theorems for relay networks”, IEEE Trans. Info. Theory, no. 9, vol. 51, pp. 3037-3063, Sep. 2005

20

http://ieeexplore.ieee.org.ezproxy.lib.utexas.edu/search/srchabstract.jsp?tp=&arnumber=993387&queryText%3DCoverage+Enhancement+through+Two-hop+Relaying+in+Cellular+Radio+Systems%26openedRefinements%3D*%26searchField%3DSearch+All




http://www.ece.utexas.edu/~peters/

http://www.ece.utexas.edu/~peters/

http://users.ece.utexas.edu/~ayazdanpanah/

http://users.ece.utexas.edu/~ayazdanpanah/

http://users.ece.utexas.edu/~ktruong/

http://users.ece.utexas.edu/~ktruong/



http://users.ece.utexas.edu/~rheath/papers/2009/Eurasip/




Documents

Where are the Relay Capacity Gains in Cellular Systems?ctw2010.ieee-ctw.org/mon/Heath.pdf · multi-hop relay base station user ii user i strong link transmission cell Two-way half