MIMO On-Frequency Repeater with Self-Interference Cancellation and Mitigation
Peter Larsson, Mikael
Prytz Ericsson Research
ADHOC’
08, 7/5-2008
Peter Larsson, Mikael Prytz 2008-04-282
Outline
IntroductionMIMO On-Frequency Repeater
–
Basic idea–
Design
MIMO Self-Interference CancellationMIMO Self-Interference Mitigation
Performance potentialLow rank channel assumptionImpact of channel estimation errorConclusion
Peter Larsson, Mikael Prytz 2008-04-283
The communication problem and some options
How to improve cellular range-rate, system capacity, battery use time … performance?
Smaller cells DAS Split path in multiple hops
–
Power–
Antennas –
Bandwidth–
Parallelism (e.g. MIMO) )(Γ= fRateNIGGGP AntAntCh
+⋅⋅⋅
=Γ 21
αDDGCh 1)( ∝
( ) )(221)2( DGDDG ChCh ⋅=∝ αα
–
Channel gainsShorter com. distanceSplitting thepath in ≥2 hops
Peter Larsson, Mikael Prytz 2008-04-284
Duplex loss in traditional relaying
Relay(s)TX RX(s)Link 1 Link 2
BS
RS1
MS
Resource 1 Resource 2
RSK
R1 s1 R2 s2Freq. 1. Time
Alternating between transmission and reception
⇒ Reduced capacity (duplex loss)
Peter Larsson, Mikael Prytz 2008-04-285
Repeaters (Amplify and Forward)
BS MS
Resource 1 Resource 2FTRRS1
BS MS
Resource 1 Resource 1OFRRS1
FTR:Frequency translating repeaterOFR:On-frequency repeater
Relay(s)TX RX(s)Link 1 Link 2
Peter Larsson, Mikael Prytz 2008-04-286
OFR basics
OFR–
RX and TX on same freq.
–
Out-in isolation through donor + coverage antennas, directivity, separation, “walls”
–
Isolation through Self-IC
Self-IC–
≈
35 dB extra isolation.
–
≈
15 dB backoff for low- ripple
Peter Larsson, Mikael Prytz 2008-04-287
WCDMA OFR
SOTA WCDMA OFR–
Inter-chip-interference
–
Complex receivers for good performance (non-
rake)–
Direct signal not-
efficiently exploited (chips can not interfere constructively)
–
Does not enable efficient cooperating RNs and BS (chips can not interfere constructively)
BS
RS1
MS
Resource 1 Resource 1
Resource 1
Delay = 5 μs
≈0.26 μs
hr (n)∗
hd (n)∗
Peter Larsson, Mikael Prytz 2008-04-288
OFDM-OFR
OFDM inherently lend itself to OFR –
Sufficient long CP!
–
Allows the opportunity for direct and relayed signal(s) to constructively interfere with each other, without ISI.
Relay(s)Transmitter Receiver(s)Link 1 Link 2
BS
RS1
RSV
MS
Resource 1 Resource 1s(n)CP
s(n)CP
s(n)CP
s(n)CP
h0 (n)*
h1 (n)*
hV (n)*
Delay=TRS
Delay=TRS
PDSRSCP TTTT ++>
Peter Larsson, Mikael Prytz 2008-04-289
More Antennas...
Traditional (repeaters and) OFRs use single RX + TX antennas
We consider Multi-antenna OFR
–
Why?
OFR OFR
H0
H1 H2
Peter Larsson, Mikael Prytz 2008-04-2810
Antenna aspects: SM-MIMO
Traditional cellular systems uses MIMO based comm. methodsTraditional repeater collapses the channel rank to one
–
Key-hole effect!Graceful rate degradation with distance from BS suggest to use MIMO end-to-end
MIMO- OFRTX RX
Range from BS
Cha
nnel
cap
acity
Desired (MIMO)
Un-desired (SISO)
(MIMO)
Repeater position
Peter Larsson, Mikael Prytz 2008-04-2811
OFR
Antenna aspects: Interf. cancelation
Problem–
Interference caused by other BSs, MSs, RSs should not be forwarded
Solution–
Use multiple antennas for interference cancelation
Issues–
Frequency selective processing may be needed!
Interf.
* Evidently, all antenna schemes, (MIMO,IC…) eats of the same cake. Tradeoff!
Peter Larsson, Mikael Prytz 2008-04-2812
Antenna aspects: Interf. mitigation
Problem–
Interference caused to other BSs, MSs, RSs should be avoided
Solution–
Use multiple antennas for interference mitigation to exposed (fixed) nodes
Issues–
Frequency selective processing
Interf.
OFR
Peter Larsson, Mikael Prytz 2008-04-2813
Self-interference cancellation MIMO-OFR
s vector signal (SM-MIMO)H0 = Repeater output to input matrix channelW is a matrix selected for self-interference cancellation
–
W=AH0
C
H0
W
ΣA CΣn
H1 s -
Peter Larsson, Mikael Prytz 2008-04-2814
Self-interference mitigation MIMO-OFR
Problem–
Isolation insufficient?
Constraints / Objectives–
Keep ETE channel rank
–
Adapt A and C based on H1
and H2
Assumption–
H0
has low rankSolutions
–
Adapt A and C to reduce feedback
Null space projection
( )( )( )( )H
H
Nn
Nnsvd
::,
::,],,[ 0
UA
VCHVSU
=
==
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎦
⎤⎢⎣
⎡
100100
00000000
100010 1σ
[ ] [ ]=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎦
⎤
⎢⎢⎣
⎡32
3
2
11
3213
2
00000000
vvvvv
uuuuu
H
H
H
H
H σ
⎥⎦
⎤⎢⎣
⎡0000
Peter Larsson, Mikael Prytz 2008-04-2815
Considered repeater and relay cases
TX RX
RS
Resource 1
Res. 1
TX RX
RS
Res. 1
Res. 2
TX RX
RS Res. 2Res. 1 Res. 1 Res. 1
1-Phase OFR 2-Phase AF 2-Phase DF
BwAxr +=
)))((det(log21 1
2−+= HHC BBRAARI wx
⎥⎦
⎤⎢⎣
⎡= (1)(2)
(D)
GHHHA
⎥⎦
⎤⎢⎣
⎡=
IGH000I
B (2)
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
=),2(
),1(
),1(
rx
rs
rx
www
w
NPP
RXIG =( ) 2
wH
RX NtrNPP σ+= HH
{ }CEC =
{ } IxxRx NPE H == { } IwwRw
2w
HE σ==
BwAxr +=
(1)(2)(D) GHHHA +=
[ ]IGHB (2)=
⎥⎦
⎤⎢⎣
⎡= ),1(
),1(
rs
rx
www
wxHr += )1(
))(det(log21 1)1()1(
2−+= wx RHRHI
HC
)1(H )2(H
)(DH)1(H )2(H
)(DH)1(H )2(H
)))((det(log 12
−+= HHC BBRAARI wx
NPP
RXIG =( ) 2
wH
RX NtrNPP σ+= HH
{ }CEC =
{ } IxxRx NPE H == { } IwwRw
2w
HE σ==
),0( 1)1( GΝ=H ),0( 1
)2( GΝ=HAssuming that:
{ }CEC =
{ } IxxRx NPE H == { } IwwRw
2w
HE σ==
and
P. Herhold, E. Zimmermann, G. Fettweis, On the performance of cooperative amplify-and-forward relay networks, in: 5th Int. ITG Conf. on Source and Channel Coding, Erlangen, Germany, 2004.
Peter Larsson, Mikael Prytz 2008-04-2816
Performance potential
N=1
N=4
No direct signal Direct signal = G1
- 10 dB
0 5 10 15 200
0.5
1
1.5
2
2.5
3
3.5
SNR [dB]
Erg
odic
cha
nnel
cap
acity
[b/H
z/s]
1-phase OFR2-phase AF with comb.2-phase DF
0 5 10 15 200
1
2
3
4
5
6
7
8
9
SNR [dB]
Erg
odic
cha
nnel
cap
acity
[b/H
z/s]
1-phase OFR2-phase AF with comb.2-phase DF
0 5 10 15 200
0.5
1
1.5
2
2.5
3
3.5
SNR [dB]
Erg
odic
cha
nnel
cap
acity
[b/H
z/s]
1-phase OFR2-phase AF with comb.2-phase DF
0 5 10 15 200
2
4
6
8
10
12
SNR [dB]
Erg
odic
cha
nnel
cap
acity
[b/H
z/s]
1-phase OFR2-phase AF with comb.2-phase DF
Peter Larsson, Mikael Prytz 2008-04-2817
Outage probability
N=1
N=4
No direct signal Direct signal = G1
- 10 dB
0 5 10 15 2010
-2
10-1
100
SNR [dB]
Out
age
prob
abili
ty
1-phase OFR2-phase AF with comb.2-phase DF
0 5 10 15 2010
-2
10-1
100
SNR [dB]
Out
age
prob
abili
ty
1-phase OFR2-phase AF with comb.2-phase DF
0 5 10 15 2010
-4
10-3
10-2
10-1
100
SNR [dB]
Out
age
prob
abili
ty
1-phase OFR2-phase AF with comb.2-phase DF
0 5 10 15 2010
-4
10-3
10-2
10-1
100
SNR [dB]
Out
age
prob
abili
ty
1-phase OFR2-phase AF with comb.2-phase DF
P(C < 1 B/Hz/s) P(C < 1 B/Hz/s)
P(C < 4 B/Hz/s) P(C < 4 B/Hz/s)
Peter Larsson, Mikael Prytz 2008-04-2818
Low rank channel assumption ?
A typical repeater feedback channel
–
Identical to LoS-MIMO channel
LoS-MIMO channel can be modeled with DFT matrix with entries
Eigenvalues vs. Distance D (d=λ/2, λ=15 cm, N=4)
Channel rank ≈ 1 at D > 10 m
Rep
eate
rM
IMO
Feedback through a reflex
RX
TX
10-1
100
101
102
0
2
4
6
8
10
12
14
16
Distance [m]
Eig
enva
lues
P. Larsson, "Lattice array receiver and sender for spatially orthonormal
MIMO communication," in Proceedings of the IEEE 61st Vehicular Technology Conference (VTC '05), vol. 1, pp. 192-196, Stockholm, Sweden, May 2005.
Ddj V
ex22
λπ
=
D
Peter Larsson, Mikael Prytz 2008-04-2819
Channel estimation error impact
Additional isolation thanks to the null space projection methodImpact of channel estimation errors?Assumption
–
Complex Gaussian channel estimation error
–
H0
= LoS-MIMO channel at D =10 m
Max eigenvalue determine stability of feedback system
-2 -1.5 -1 -0.5 00
5
10
15
20
25
30
35
40
45
Add
tiona
l rep
eate
r out
put-t
o-in
put i
sola
tion
[dB
]
log 010 ⎟
⎟⎠
⎞⎜⎜⎝
⎛
ErrorHH
( )ErrorH svd HHVSU += 0]~,~,~[
VHUH ~~0
Heff =
)))((max(log20 10 effeigabsI H−=
Peter Larsson, Mikael Prytz 2008-04-2820
Summary & Conclusions
Summary–
MIMO OFR concept presented
1-phase (no duplex loss)MIMO support (keeps the channel rank)
–
Self-interference cancellation idea extended to multiple antenna (MIMO) repeater
–
New method for increased output to input isolation through use of multiple antennas and beamforming
Conclusions–
Performance potential promising at high SNRs
–
Potentially useful at low SNRs
due to low complexity–
Appears robust under influence of channel estimation errors
Peter Larsson, Mikael Prytz 2008-04-2821
MIM
O O
n-Frequency Repeater w
ith Self-Interference C
ancellation and Mitigation
Peter Larsson, M
ikael Prytz Ericsson R
esearch
TXR
X
MIM
OO
FRR
esource 1R
esource 1
Output-to-input
antenna feedback
Fig. 1. MIM
O O
n-Frequency Repeater System
Abstract - In this paper, w
e consider the idea of a MIM
O O
n-F
requency Repeater w
here self interference cancellation and beam
forming
based interference
mitigation
methods
are exploited for increased repeater output to input isolation, and increased E
TE perform
ance. I.
Introduction Future w
ireless comm
unication systems require enhanced
range-rate performance in a cost efficient m
anner. Splitting a radio com
munication path in tw
o or more hops gives
significantly increased path gain for each hop due to the pow
er law
path
loss characteristic.
Various
two-hop
methods have been analyzed, for exam
ple, cooperative relaying [1], cooperative diversity [2], virtual antenna arrays [3], 2-hop relaying [4], and repeaters.
Repeaters could be attractive due to their typically low
er processing com
plexity and HW
requirements relative to
other two-hop schem
es, and the short forwarding delay. A
classical
repeater, like
many
other 2-hop
methods,
is characterized by a so called duplex loss that affects the ETE spectrum
efficiency
adversely. For
the repeater,
this classically corresponds to receiving on one frequency and transm
itting on another. How
ever, a recent trend among
repeater designers is to use on-frequency repeaters (OFR
). Sending and receiving concurrently on the sam
e frequency is possible provided the output-to-input repeater antenna port isolation is larger than the repeater input-to-output gain. Typically the isolation m
argin needs to be 15 dB above the
repeater gain
to m
itigate oscillatory
behavior. V
arious approaches
to ensure
high isolation
include physical
separation (by location or walls) of sender and receiver
repeater antennas,
different receive
transmit
antenna polarizations, and the m
ore recently used method of self-
interference cancellation (self-IC).
Another
trend in
the com
munication
research and
industry comm
unities is the use of multiple antennas for
more efficient com
munication, e.g. spatial
multiplexing
multiple input m
ultiple output (SM-M
IMO
) [6], which has
emerged
as a
key-component
in today’s
wireless
comni
mu
cation system.
II. Proposed m
ethods Since both O
FR and SM
-MIM
O prom
ise enhanced data rates it is interesting to explore a joint design. This is further m
otivated by considering today’s state of the art repeaters, w
hich are of the SISO type, in conjunction w
ith SM-M
IMO
: w
hen the direct signal is to weak a SISO
repeater would act
as a keyhole in the ETE channel, i.e., the maxim
um num
ber of M
IMO
subchannels, or equivalently the ETE channel rank, becom
es one despite multiple antennas at sender and
receiver. In this setup the notion of SM is lost entirely.
Another issue is that O
FRs are practically constrained in
amplification gain due to the lim
ited isolation that can be
achieved. This in turn has an impact on the m
aximum
achievable
comm
unication range.
Now
, M
IMO
uses
multiple antennas at the repeater input and output w
hich can be exploited for R
X and TX
beamform
ing at the repe
HA
-A
WΣ
H1 S
U1
U2
H0 U
2H
2 U2
UΣ
N1
1N
21C
HB
ΣB
HC
2
HD
D
HC
1
Matrix dim
ensionm
in{N
1 ,N2 } ×
min{N
1 ,N2 }
Matrix dim
ensionm
in{N
1 ,N2 } ×
min
{N1 ,N
2 }
Repeater
Fig. 2. M
IMO
On-Frequency R
epeater
ater to im
n particular, for optim
um self-IC
a necessary condition is:
prove isolation and hence also amplification gain.
Fig. 2 shows a generic frequency-dom
ain version of a M
IMO
-OFR
with m
atrix-based self-IC. The vector signal
H1 s, w
here s is the transmitted signal, is received at the
repeater input experiencing additive noise W. Internally, the
vector signal can be tuned by adjusting matrices A
, B, C
, and D
, and the signal is further affected by matrices H
A , HB ,
HC , H
D , which represent (m
inor) internal channel alterations prim
arily due to delays. As seen in Fig. 2 the output signal
U2 couples to the input via the external feedback channel
state matrix H
0 . To avoid oscillations, B is adapted to cancel
the effective feedback signal AH
A H0 U
2 . I12
0C
CA
BC
HH
HA
HB
H=
(1)
Another adaptation aspect is to select A
and C such that
good input-to-output isolation is achieved for all spatially m
ultiplexed signals. In doing so a necessary condition is that the input-to-output rank in the repeater is greater than or equal to the num
ber of spatially multiplexed stream
s. When
H0 has low
rank, which can be assured by an appropriate
antenna array design, A and C
can easily be selected to exploit this property. O
ne approach is to project as much as
possible of the repeater output spatially multiplexed signals
into the null space of HA H
0 HC
2 . Similarly the repeater input
can suppress feedback signal from the output by selecting
receive weight in the null space of H
A H0 H
C2 . Practically,
this is implem
ented by a singular value decomposition
approach where the null space is considered to correspond
to the N-n sm
allest singular values where n is selected such
that the N-n values are L dB
smaller than the strongest
singular value, for a certain limit value L. H
ence, the SVD
operation and selection of A
and C are
) (2)
un
otential construrank increase.
The Ergodic ETE channel capacity is
((
)(
)(
) H
CA
H
Nn
Nn
svd
::,
::,
],
,[
20
UA
VC
HH
HV
SU
= ==
While m
ultiple antennas can be used for SM-M
IMO
and self-interference m
itigation, it is further possible to exploit som
e of the available degrees of freedom to suppress
desired interference and enhance the ETE performance.
The proposed
MIM
O-O
FR
methods
are further
complem
ented with O
FDM
where a sufficiently large cyclic
prefix is selected to allow for relayed signals to arrive near-
concurrently, hence
absorbing any
OFD
M
symbol
ISI caused by path propagation delay differences for repeated and directed signal and allow
ing for pctive
interference and channel III.
Results
⎭ ⎬ ⎫
⎩ ⎨ ⎧+
=H
NE
CH
HI
γlog
where H
is the aggregate ETE channel, γ is the SNR
, and N
is the number of TX
antennas. The final subm
ission will present results for:
i) ETE M
IMO
-OFR
channel capacity compared w
ith com
binations of non-OFR
and SISO operation.
ii) M
IMO
-OFR
output-to-input isolation performance
vs. repeater output-to-input channel estimation error.
iii) ETE channel capacity impact vs. isolation.
IV.
References
[1] A
Sendonaris, “Advanced Techniques for N
ext-Generation W
ireless System
s”, Ph.D. Thesis, A
ugust 1999 [2]
J. N
. Lanem
an, “C
ooperative D
iversity in
Wireless
Netw
orks: A
lgorithms and A
rchitectures,” Ph.D. dissertation, M
assachusetts Institute of Technology, C
ambridge, M
A, A
ug. 2002. [3]
M.
Dohler,
”Virtual
Antenna
Arrays”,
WW
RF2001,
electr. C
onference CD
-RO
M, Paris, France, D
ec. 2001 [4]
IST-4-027756 WIN
NER
II, D3.5.1 v1.0, “R
elaying concepts and supporting actions in the context of C
Gs”
[5] J. C
hun, J. Lee, P. Choi, J. C
. Yun, S. J. Lee, J. H
. Lee, “Smart
antennas for the on-air on-frequency repeater in the 3G m
obile com
munication applications”, pp 376-383.
[6] G
erard J. Foschini (autumn 1996). "Layered space-tim
e architecture for w
ireless comm
unications in a fading environment w
hen using m
ulti-element antennas". B
ell Labs Technical Journal 1: 41–59.
, (3)