Transcript IEEE C802.16m-08/1471r2

Interference Mitigation using Conjugate Data Repetition for Cell Edge Users
Document Number: IEEE C802.16m-08/1471r2
Date Submitted: 2008-11-06
Source:
Kiran Kuchi, J. Klutto Milleth
CEWiT
Isamu YOSHII
E-mail:
Voice:
[email protected],in, [email protected]
E-mail
[email protected]
Venue:
Dallas, USA. In response to the TGm Call for Contributions and Comments 802.16m-08/040 for Session 58
Topic TGm SDD- Other
Base Contribution: IEEE C802.16m-08/1471r1
Purpose: Presentation associated with comment tgmsdd_Kuchi_Kiran.cmtb
To discuss in TGm for appropriate action. Notice:
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Presentation Outline
• Motivation
• Conjugate Data Repetition (CDR)
– Improving cell edge performance
• Conclusions
• Proposed SDD Text
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Motivation
• Limited Spectrum
– Frequency reuse-1 is the most likely deployment
mode
– Typical urban cell size has 200-300 m radius
– Interference limited in both uplink and downlink
– Very low cell edge data rates
• Provide uniform data rates throughout the cell
• High Spectrum Efficiency
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Interference Mitigation
• Poor cell edge SINR in reuse-1 networks
– Severe co-channel interference (CCI)
– Multiple CCI up to 4-dominant interferers
– Typically cell edge SINR is in the [-6 0] dB range
– Nearly 30% of users in the sector are cell edge
users
• Re-use 1:3 is spectrally inefficient
• Gain of soft re-use is not adequate for cell-edge
users
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Interference Suppression in Legacy
WiMax Systems
• Cell edge users typically have 3-4 dominant interferers
– With 2-Rx antennas, conventional IC algorithms cannot provide adequate
IC gain
• Existing solutions
– For low SINR cell edge users
– Rate ½ QPSK and bit-level data repetition up to 6-times
• Bit level data repetition is spectrally inefficient
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Conjugate Data Repetition (CDR)
•
•
•
BS-1
Signal repetition in time/frequency
Conjugate symbol repetition across
adjacent subcarriers or adjacent
OFDM symbols
All cells synchronously transmit
data in conjugate symbol pairs for
select users
–
–
•
•
The network assigns a CDR frequency
partition in which the RBs from different
BSs overlap
Little or no co-ordination among BSs
MMSE filtering of complex, and
complex-conjugate signals provides
a high IC gain
MMSE IC for each RB independently
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BS-2
F1
X1
F1
Y1
F2
X1*
F2
Y1*
F3
X2
F3
Y2
F4
X2*
F4
Y2*
F5
P1
F5
P2
F6
X3
F6
Y3
F7
X3*
F7
Y3*
F8
X4
F8
Y4
F9
X4*
F9
Y4*
6
Conjugate Data Repetition
•
Basic Idea
– Each BS transmits data in conjugate symbol pairs on a pair of subcarriers
– 1st BS transmits
y1  hx  gxˆ1  n1
( x , x* )
y2  hx*  gxˆ1*  n2
– 2nd BS transmits ( xˆ1 , xˆ1 )
*
– Complex-conjugation on
y2
y2*  h* x  g * xˆ1  n2*
– After complex de-conjugation, the receiver has two copies of signal and interference with
different channel states
•
Signal
( h , h* )
•
Interference
(g , g*)
*
– Receiver jointly filters ( y1 , y2 )
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CDR-MMSE Receiver
•
CDR signal model
M
y  h x d   g l xl  n
 ,
 l 1
Signal
 Noise
Interferen ce
•
CDR-MMSE filtering,
 h1 
h * 
h 1
 h2 
 *
h2 
 g l ,1 
g* 
l ,1
gl   
 g l ,2 
 * 
 g l ,2 
z  wy
w  (1  h * R i1n h ) 1 h * R i1n
•
•
•
In Rayleigh fading channels, conjugation ensures that, the signal and interference
channel vectors h , g1 , g 2 ,.., g M  are linearly independent with probability 1
Linear independence ensures that MMSE provides full IC up to 2N-1 interferers
MMSE filter is applied to each RB independently
–
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High IC gain for each RB
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CDR-MMSE Pilot Processing
•
Use BPSK pilots and preferably pilot-on-pilot mode.
– Step 1: Collect complex, and complex conjugate copies of the received pilots
M
g x
y p  hx p 

l
p ,l
l 1



Pilot tones
 np

Noise
Pilot tones of CCI
– Step 2: Estimate channel coefficients of desired signal using 2D-MMSE
• Assume uniform power-delay-profile
• Knowledge of interferer pilot sequences improves 2D-MMSE
– Step 3: Subtract signal contribution from the received samples
M
e p   g l x p ,l  nˆ p
l 1
– Step 4: Estimate noise-plus-interference covariance matrix

*
ˆ
R
in  E e p e p

• Each RB can be processed independently to obtain high IC gain
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Conjugate Data Repetition
• With N antennas, CDR provides 2N observations
– CDR doubles the number of copies of signal
• Full interference suppression up to 2N-1 interferers
• Typically 3-4 dominant interferers in re-use 1:1 systems
– Use POD, CDR, MMSE for cell edge users
• With 2-receiver antennas CDR-MMSE nulls 3-interferers
• CDR does not require active cooperation
between BSs
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Proposed Downlink RB Structure
for Cell Edge Users
P
P
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
P
P
D
D*
D
D*
D
D*
D
D*
P
P
D
D*
D
D*
D
D*
D
D*
P
P
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
D
D*
P
P
D
D*
D
D*
D
D*
D
D*
P
P
D
D*
D
D*
D
D*
D
D*
• Slot format with CDR in 18 X 6 resource block
• Pilot density with 2-Tx antennas 11.11%
• OL transmit diversity: Phase-Offset-Diversity (a.k.a rank1- precoding)
• CL transmit diversity: single stream MIMO
•For cell edge data users, CDR will be applied in an FFR region as a reuse-2
scheme
•Ensure that DL-MAP overlaps with CDR region to get full IC benefit
•CDR works with both localized, distributed resource allocation schemes
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CDR-POD with MMSE in PED-B
•Simulation Assumptions
•PED-Bchannel
•Localized allocation
•RB size 18x6
•½ QPSK with 360 bits
•2Tx 2Rx
•2D-POD transmit diversity
•Total pilot density 11.11%
•Quasi-orthogonal pilots
•Interference power profile
•[0 -3 -6 -9] dB
•Rest of the interference is
modeled as AWGN
•Conjugate repetition
factor=2
•SNR is defined as signal to rest
of the interference power
•Total SINR=-3 dB with 4-interferers
•Good suppression up to 3-interferers, partial suppression
with4-interferers
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CDF of SINR of Cell Edge Users
CDR implementation
• In a frequency reuse-1 system, the SINR of all the users in a cell are ranked in descending
order. The above CDF is collected for bottom 33% of users.
• All cells in the network synchronously allocate the bottom 33% users to a CDR frequency
partition which is fixed for the entire network
• CDR combined with bit level data repetition can be used for select very low SINR users
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System Level Simulation for Cell Edge Users
CDF of SINR of MMSE Receiver with CDR-POD
•Simulation Assumptions
•PED-A channel
•Localized allocation
•RB size 18x6
•½ QPSK with 360 bits
•2Tx 2Rx
•2D-POD transmit diversity
•Total pilot density 11.11%
•Conjugate repetition factor=2
•Results are obtained with a system
level simulator in which the SINR at
the output of MMSE is estimated
during channel estimation; 400 users
are simulated
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6.0 dB IC gain
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System Level Simulation for Cell Edge Users
Spectrum Efficiency Comparison with MMSE-CDR-POD
•Simulation Assumptions
•PED-A channel
•Localized allocation
•RB size 18x6
•½ QPSK with 360 bits
•2Tx 2Rx
•2D-POD transmit diversity
•Total pilot density 11.11%
•Conjugate repetition factor=2
•MCS selection between R=1/2
QPSK and R=1/2 16-QAM
•Results are obtained with a system
level simulator in which actual link
simulation is run for 1000 frames for
each user. The CDF is obtained by
randomly selecting 400 cell edge
users
•Simulation done with two MCS levels : ½ QPSK and ½ 16-QAM
•Y-axis – Percentage of users whose throughput is less than x-axis (Outage)
•CDR gives significantly higher a spectrum efficiency (SE) than simple repetition
•PHY overheads are not taken into account in SE calculation
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Conclusion
 Due to limited spectrum, frequency re-use 1:1 is the preferred deployment mode
 Very important to improve cell edge performance
 CDR and rank-1 transmission (POD/CL-div) ensures high cell edge performance in
both OL and CL-modes
 CDR and FFR complement each other
 CDR can be implemented along with FFR in a frequency partition
 CDR works with the following resource allocation schemes
 Localized
 Distributed
 PRU based DRU
 Tone based distribution
 Use CDR for both control and cell edge data channels
 Little or no cooperation among base stations
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Proposed SDD Text
• Add section 20.5
• 20.5 Interference mitigation using conjugate
data repetition (CDR)
– Interference mitigation using conjugate data
repetition (CDR) will be supported for cell edge
users for data and/or control channels. CDR
includes simple repetition of modulated data
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