Beamforming for Interference Mitigation in TDD System
IEEE 802.16 Presentation Submission Template (Rev. 9)
Document Number:
IEEE S802.16m-08/653r2
Date Submitted:
July 16, 2008
Source:
Wendy C Wong, Shilpa Talwar
Voice:
408-765 2429
Intel Corporation
E-mail:
wendy.c.wong@intel.com
2200 Mission College Blvd,
Santa Clara, CA 95054
*<http://standards.ieee.org/faqs/affiliationFAQ.html>
Venue: IEEE 802.16m-08/024 Call for Comments and Contributions on Project 802.16m System Description Document (SDD), on the topic of
“Interference mitigation”.
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IEEE C802.16m-08/653r2
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Discussion and approval.
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Beamforming for Interference Mitigation
in TDD System
Outline
– Motivation
– Beamforming with Nulling
• Cell-edge classification
• Resource allocation for cell edge users
• Distributed scheduling
–
–
–
–
Simulation results
Conclusions
Standards impact
Proposed SDD text
Motivation for DL Transmit Beamforming with
nulling
– 802.16m requirements
• 2x gain in cell-edge user throughput
– DL is usually interference limited for cell edge users
– DL Transmit beamforming with nulling can improve cell-edge
capacity by
• by significantly reducing the interference from neighboring BSs and
increase SINR.
– Beamforming with active interference mitigation is proposed
• 2x2 BF nulling increases average cell edge system throughput rate by
37% relative to 2x2 STBC/SM
• 4x2 BF with nulling increases average cell edge system throughput rate by
97% relative to 2x2 STBC/SM
4
Beamforming with IM Concept: Why
does it work?
• BS1 forms beam towards
BS1
towards
MS1forms
& nullbeam
towards
MS2
MS1 & null towards MS2;
RRM
Manager
BS2 forms beam towards
MS2 & null towards MS1
MS1
BS1
MS2
BS2
•
BS1 & BS2 serve ‘spatially’ independent MS1 and MS2 simultaneously using
same resource;
•
BS1 assist BS2 by forming a null to its MS2 while BS2 assist BS1 by forming a
null to its MS1;
•
If BS1 enables BF with nulling while BS2 does not. MS2 will experience good
SINR while MS1 will not.
•
Resources shall be set aside for all BSs to apply BF with nulling in order to help
all cell edge users.
5
Beamforming with Nulling: Implementation
• 3 steps
1. BS selects cell-edge users that can benefit from BF
with nulling
• Example: MS1, MS2
2. RRM unit determines resources to serve cell edge
users only
• Inter BS co-ordination
3. Independent scheduling performed by all BSs to
serve their cell edge users
• Special UL Sounding
Step 1: Cell-edge Classification
Goal: BS selects cell-edge users that can benefit from
BF with nulling
– Multiple ways to identify cell-edge users
– One example: use CINR or RSSI caused by neighbor BS as
metric
– Measurement reports by MS can be sent to serving BS in
MOB_SCN_REP of 802.16e
– BS decide if MS is cell edge user on metric value and knows
number of significant interfering BSs from the
MOB_SCN_REP
Step 2: Resource allocation to cell edge users
• Goal: allocate resources for cell edge users only. When
number of interferers exceeds system nulling capacity, FFR
can be used to reduce interferers.
• All BS report number of its cell edge users and maximum
number of BS interferers to RRM unit over backhaul;
• RRM unit allocate resources to serving cell edge user only
and send resource allocation info back to BSs over backhaul;
• When system nulling capacity is not exceeded, all BSs share same
resources for cell edge users.
• Under rare circumstances when the system nulling capacity is
exceeded, use FFR as to reduce interferer number.
Step 3: Independent scheduling
Goal: Each BS schedule its cell edge MSs over the
allocated resources independently.
– Each BS schedule its cell edge MSs over the allocated
resources independently using whatever scheduler it prefers;
– However, to form the proper nulls to the cell edge users of
other BSs, BS needs to know the scheduling decision of
neighboring BSs.
– For BSs to inform each other of their scheduling decision,
special UL sounding is used.
Special UL Sounding
Special Uplink sounding region to infer scheduling
decision made by other BSs and form beamforming
weight for data transmission.
• BSs that allocate transmission to its cell edge users in resource block (RB) n
will require those MSs to perform UL sounding in sounding block (SB) n.
• Each sounding region in the UL will have the same interference environment
as the corresponding resource block in the DL.
• Each BS can use sounding region to perform channel measurements and
form covariance matrix for beamforming weight calculation.
MS_1_1 will perform UL sounding in SR 1 SR
MS_2_3 will perform UL sounding in SR 1
MS_3_2 will perform UL sounding in SR 1 SR
1
2
SR 3
UL sounding
Region A
RB 1
RB 2
RB 3
UL subframe n
DL subframe n
DL subframe n+1
BS 1 will transmit to MS_1_1 in RB 1
BS 2 will transmit to MS_2_3 in RB 1
BS 3 will transmit to MS_3_2 in RB 1
DL data
transmission
region
Simulation Results
Performance Improvement
–
–
–
–
–
Use 7 cells network deployment with 3 sectors and 5 cell edge users per sector;
This result is for mobile with travelling at 3kmph.
Simulation parameters compliant to EVM document;
CDF of average BS throughput rate and average SS throughput rate over at least
10 trials with 300 frames per trial are presented. Some trials have less than 300
frames.
A comparison of average BS throughput rate per trial for center 3 sectors:
average BS throughput
% increase relative to
System Mode
rate (Mbps)
baseline
Baseline, 2x2,
STBC/SM
5.6394
0
IM BF with nulling,
2x2
7.7451
37.34
IM BF with nulling,
4x2
11.1253
97.28
Conclusions
– Downlink BF with nulling is a simple, effective
technique to reduce BS interference for cell edge
users operating in an interference limited
environment.
– Can be used for low to moderate mobility user
– FFR can be used to enhance BF performance when
system nulling capacity is exceeded
Standard impact
• There are two standard impacts
1.Information exchange to/from RRM unit for
information regarding resources allocated to
cell edge users;
2.Special UL sounding to infer scheduling
decision of interfering BSs and calculate
beamforming weights;
Proposed SDD text - 1
Insertion into 8.1 The IEEE 802.16m Protocol
Structure
The inserted text is marked as italics:
Interference Management block perform functions
to manage the inter-cell/sector interference. The
operations may include:
– MAC layer operation
• Interference measurement/assessment report sent via MAC signaling
• Interference mitigation by resource allocation for beamforming with
nulling, scheduling and flexible frequency reuse
Proposed SDD text - 2
Insertion into 11 Physical Layer
Insert the following text into SDD Section 10 – Physical Layer [3]
11.x. UL sounding
A special UL sounding region is needed for DL transmit beamforming with nulling. This special UL
sounding region will enable all BSs to infer scheduling information of neighboring BSs and form
proper beamforming weights during DL data transmission to cell edge users .
11.x.y Special UL sounding region
Special UL sounding region is divided into N sounding blocks where N is the number of resource
blocks allocated to cell edge users. BSs that allocate transmission to cell edge users in resource
block n shall require those cell edge users to perform UL sounding in sounding block n as
depicted in Figure xyz. Hence, each sounding region in the UL will have the same interference
environment as the corresponding resource block in the DL.
MS_1_1 will perform UL sounding in SR 1 SR
MS_2_3 will perform UL sounding in SR 1
MS_3_2 will perform UL sounding in SR 1 SR
1
2
SR 3
UL sounding
Region A
RB 1
RB 2
RB 3
UL subframe n
DL subframe n
DL subframe n+1
BS 1 will transmit to MS_1_1 in RB 1
BS 2 will transmit to MS_2_3 in RB 1
BS 3 will transmit to MS_3_2 in RB 1
DL data
transmission
region
Backup
TDD : UL Receive/DL Transmit beamforming at BS:
system layout
UL received signal at BS p with M receive
antennae from MS_p_m is:

H
x (k )  H p _ p _ m (k ) wMS
_ p _ m _ UL ( k ) s p _ m ( k ) 
–
–
–

x (k ) =
–
H p _ p _ m (k )
–
H

H
(
k
)
w
(
k
)
s
(
k
)

n
(k )
n p _ q _ n MS _ q _ n _UL q _ n
q, pq
k = subcarrier index;
received vector at BS, Mx1;
Sp_m(k) = QAM sybmol of MS_p_m;
channel response from MS_p_m
to BS p, MxN;
H
wMS
_ p _ m _ UL (k )weight
vector applied to signal at
MS_p_m, Nx1
– Sq_n = QAM symbol of interfering user
MS_q_n;
–
channel response from MS
interferer MS_q_n to BS p;
– n(k ) noise with 0 mean and variance s2I;
H p_q_n
Hq_q_n BS q
Hp_q_n
BS
p
MS_q_n
Hq_p_m
Hp_p_m
MS_p_m
TDD : UL signal processing and UL Tx BF weight
calculation at MS/BS
• With proper pilot design in the UL, the BS can measure Hp_p_m
for each of its cell edge user.
• Using SVD, we have H p _ p _ m  UV H .
• Hence, the BS can request MS_p_m to set its UL transmit

w
v
weight to be
where v1 (Nx1) is the right singular
vector corresponding to the largest singular value of Hp_p_m.
• The actual weight vector which is 2x1 can be calculated by the
MS from its DL channel measurement.
1 0
H
w

• If selection diversity is chosen for the system, MS _ p _ m _UL 0 or 1
• The BS gets to decide which MS antenna to use for its UL
transmission.
H
MS _ p _ m _ UL
1
TDD: UL signal processing and UL Tx BF weight
calculation at BS

• With receive beamformer weight vector as wp _ m (Mx1) to combine
signals received at M antennas:
 
sˆ p _ m  w pH_ m x

H
 w pH_ m ( H p _ p _ m ) wMS
_ p _ m _ UL s p _ m 
H
H
H 
w
(
H
)
w
s

w
n p _ m p _ q _ n MS _ q _ n _UL q _ n p _ m n
q, pq
Equation 1
• The weight vector derived using MMSE with interference nulling or MRC
H

H
is 
wp _ m  Rxx1H p _ p _ m wMS
,
w

H
w
_ p _ m _ UL
p_m
p _ p _ m MS _ p _ m _ UL
Equation 2
• Assuming data and noise are uncorrelated, we have
 
Rxx  E ( x H x )
H

H
Rxx  s s2 H p _ p _ m wMS
w
_ p _ m _ UL MS _ p _ m _ UL H p _ p _ m 
s s2
Equation 3
H

H
w
w
n p _ q _ n MS _ q _ n _UL MS _ q _ n _UL H pH_ q _ n  s 2 I
q, pq
TDD: DL Tx beamforming at BS
• At the BS, apply BF weights calculated from UL signal
to DL transmit signal as
H
 wp _ m
z 
sp_ p_m
wp _ m
Equation 4
• At the MS with N Rx antennae, the received signal is



xMS _ p _ m  H pDL_ p _ m z T  (int erference)  n
• where H pDL_ p _ m  H Tp _ p _ m due to TDD and Hp_p_m is the
channel matrix describing the channel from MS to its
serving BS.
TDD: DL signal processing at MS receive
• If the same number of antennae at the MS is used for its UL
H
transmit and DL receive, the weight wMS
(1xN)
_ p _ m _ DL
applied at the MSis
H
sˆ p _ m  wMS
_ p _ m _ DL xMS _ p _ m
H
wMS
_ p _ m _ DL

H
T
 wMS _ p _ m _ DL H p _ p _ m  


T
H
 v1T  wMS
_ p _ m _ UL


Equation 5
T
H

wp _ m
H

 w

s
n
 ( Interference) 
H
p_ p_m
MS _ p _ m _ DL

wp _ m

• There are cases where the MS DL will use a different weight
T
H
H


w

w
.
from the uplink. In this case MS _ p _ m _ DL
MS _ p _ m _ UL
• One example is to use MMSE at the MS DL receive to
calculate the MS BF weight.
Simulation Setup
•
•
Compliant to simulation parameters setup in EVM document;
Some parameters highlight are
# of cells
7
Channel model
eITU/-PedB
# of sector per cell
3
mobile speed (kmph)
0/3
# of cell center user
0
Number of BS antenna
2 or 4
# of cell edge user
5
BS antenna gain (dBi)
Cell radius (m)
866
frequency reuse
1
17
# of MS antenna
2
channel bandwidth (MHz)
10
scheduling
PF
MS noise figure (dB)
baseline setup
2x2, STBC/SM
PHY abstraction model
MI
traffic model
best-effort
HARQ
synchronous with retransmission
maximum of 4, chase
combining
MCS
no repetition,
QPSK/16QAM/64QAM with
CTC
Channel estimation
Perfect channel with delay
Tx power of BS (dBm)
Antenna azimuth
Permutation
BS cable loss (dB)
46
3 sector antenna defined in
3GPP
2x3 AMC defined in 16e, full
loaded
2
7
CDF of average BS throughput rate with cell edge
users only
CDF of average user throughput rate
MCS distribution PDF
Overhead calculation and adjusted data rate increase
If we take into account of special UL sounding overhead,
the adjusted data rate increase will be:
Size of
scheduling
quantum in
time slot
Number of
scheduling
quanta per
DL per 5ms
frame
size of sounding
symbols in
time slot
% overhead
to add
Adjusted
Throughput
improvement
for 2x2
Adjusted
Throughput
improvement
for 4x2
1
6
1.00
16.67
31.71
83.14
2
3
1.00
16.67
31.71
83.14
3
2
1.00
16.67
31.71
83.14
4
1
1.00
25.00
29.60
77.60
5
1
1.00
20.00
30.83
80.83
6
1
1.00
16.67
31.71
83.14
7
1
1.00
14.29
32.38
84.88