Measurement-Based Transmission
Schemes for Network MIMO
Krishna C. Garikipati
Kang G. Shin
MobiHoc 2014 Introduction Traditional Wireless
•  Multi-antenna Access Points (APs)
•  Mobile Users (STAs)
•  More APs => More capacity?
netMIMO
•  Central controller, Backhaul
•  Channel (CSI) feedback
•  Precoding => No interference
•  Linear increase in capacity! MobiHoc 2014 Controller"
Backhaul"
AP2"
AP1"
netMIMO "
STA1" STA2"
STA3"
CSI
Feedback"
STA4"
2 Challenges & Solutions Backhaul: Real-time, Predictable
ü  Baseband over fiber (e.g. CPRI[1])
Synchronization: Frequency, Time
ü  Over-the-air using master/slave AP model[2]
Performance loss: Interference caused by
imperfect channel(CSI); CSI feedback overhead[3,4]
[1] Common Public Radio Interface (CPRITM)
[2] JMB: Scaling Wireless Capacity with User Demands, SIGCOMM ‘12
[3] Rethinking network MIMO: Cost of CSIT, performance analysis, and architecture comparisons, ITA ‘10
[4] Multiuser MIMO Achievable Rates With Downlink Training and Channel State Feedback, IEEE Transactions on Information
Theory, 2010
MobiHoc 2014 3 Problem of Interference CSI aging
Measured"
•  Time-varying wireless channel
Channel
Magnitude
•  Measured =/= observed
•  Imperfect Zero-forcing with
measured CSI causes interference
•  CSI Delay (increases w/ scaling)
"
• 
• 
Duration of acquiring feedback
Transmission duration during which
CSI remains constant
Observed"
Frequency"
Controller"
Backhaul"
AP2"
AP1"
netMIMO "
How to mitigate interference in
large netMIMO setups with
mobile channels?
MobiHoc 2014 STA1" STA2"
STA3"
CSI
Feedback"
STA4"
4 Our work Transmission schemes
•  Non-heuristic feedback protocol
•  Adaptive transmission adjustment
NDP2"
NDP1"
CSI"
CSI"
Measurement-based approach
Adapt"
•  Adapts to STA interference
•  Analysis backed by theory
Implementation
•  Fully synchronized testbed
•  Real-world wireless channels
Measure"
Clock Buffer !
clock cable!
D13!
Twisted-pair !
cable!
D12!
AP1!
MobiHoc 2014 DATA"
AP2!
AP3!
5 Background
MobiHoc 2014 6 netMIMO Model •  M APs with total Nt antennas
•  Set of STAs, K = {1, 2 . . . K}, each with Nr antennas
˜ k, right
•  Estimated Channel matrix (Nr ⇥ Nt) of STA k is H
singular matrix Vk and strongest vector vk (CSI)
•  Precoding matrix: W = ( † ) 1 where = [v1 v2 . . . vK ]
•  Precoding weights w˜1 , w˜2 , . . . , w˜K applied to antennas
•  Data symbols, x1 , . . . , xK, with avg. power P/K, observed
channel Hk, received signal is
interference
zX }|
{
y k = Hk w
˜ k xk +
Hk w
˜ i xi
"
˜
Zero if H
k = Hk
i6=k
•  Avg. Interference power
XP
Ik =
|Hk w
˜ i |2
K
i6=k
MobiHoc 2014 7 Protocol Design SIFS
…! NDP CSI-STA …!
"
NDP1"
M"
1"
…! Sync"
SIFS"
SIFS"
…! netMIMO DATA
CSI-STAK" netMIMO DATA" ACK"
Training" Feedback"
Data"
"
ACK"
…!
netMIMO TXOP"
•  Channel/
Transmit
oppportunity
(TXOP)
reservation
•  Master AP
synchronization
2014 MobiHoc •  Null Data Packet
(NDP) sounding
frames
•  Channel
estimation, CSI
computation CSI feedback
•  Beamforming or
channel matrix
•  Quantization,
CSI grouping,
Givens rotation
Precoding
•  Zero-forcing or
Block diagonalization
ACK/Block ACK
8 Protocol Design •  Parameters chosen from
WLAN standard
•  Extension of 802.11ac MUMIMO WLAN protocol
(explicit sounding and
feedback)
•  K = No. of STAs
Feedback duration(each) =
(2K-2) x #BitsperAngle x #Subcarriers
#CSI grouping x MCS0rate
netMIMO parameters CSI Givens Bits per Compression angle 8 Feedback Rate MCS0 AP,STA antennas 2,1 SIFS duration 16us Tx Power 18 dBm NDP duration 30us Bandwidth, 20Mhz, Subcarriers 48 CSI Grouping 2:1 Processing ZF Feedback
Duration
K=4
K=6
K=8 K=10
Each(ms) 0.22 0.29 0.48 0.61 Total(ms) 0.96 1.82 3.96 6.24 (Quadratic increase) "
MobiHoc 2014 9 Wireless Channel •  Different mobility scenarios
•  Stationary channel (stat)
AP"
Minimum disturbances"
STA"
•  Environment disturbances (movE)
Moving objects,
reflectors"
AP"
STA"
•  Device/user mobility (movD)
Moving/walking receiver "
AP"
MobiHoc 2014 STA"
10 Wireless Channel •  Note: Channels in netMIMO
are less coherent due to
diverse AP-STA links
AP1"
AP2"
STA"
•  Measured channel variations
•  MovD > MovE > Stat
MobiHoc 2014 11 CSI Aging •  Signal-to-Interference ratio (SIR)
CSI aging "
in feedback"
Data"
Sync" Training" Feedback"
Effect of Feedback delay
~15dB"
20
Effect of TXOP duration
Stat
MovE
MovD
SIR (dB)
SIR (dB)
25
15
10
5
CSI aging "
in TXOP"
STA5
STA6
STA7
STA8
Location
2
10-STA netMIMO (feedback duration = 6.4ms)
"
22
20
18
16
14
12
10
~6dB"
Stat MovE MovD
TXOP Start
TXOP End
Stat MovE MovD
Stat MovE MovD
TXOP=1ms
TXOP=2.5ms
TXOP=4ms
6-STA netMIMO (feedback duration = 1.8ms)"
Summary: Both feedback delay and length of transmission affect the
interference seen by the STA
MobiHoc 2 014 12 Approach
MobiHoc 2014 13 Two-Phase Training •  Idea: Additional NDP Training sequence
•  Highly mobile STAs trained with NDP2
•  STA partitions: K = K1 [ K2
•  Since T 2 < T 1, STAs in K2 have smaller CSI delay (and
interference ??)
NDP1"
NDP1"
…!NDP
M"
CSI"
STA1"
…!
NDP1"
NDP2"
…!NDP …!
…! NDP …!
M"
CSI"
STAK1"
NDP1"
M"
Feedback delay
for STAs in" K1
1
T =
MobiHoc 2014 CSI"
STAK"
X
i2K2
Ti + T
NDP
netMIMO"
DATA"
CSI"
STAK2"
netMIMO"
DATA"
Feedback delay
"
for STAs in K
2
2
T =
X
i2K2
Ti
14 Two-Phase Training Choosing partitions
•  I¯k = Observed interference of STA k
•  Averaged over subcarriers in single netMIMO packet
•  Objective: min-max interference across STAs
Cannot be solved
since Interference is
not known a priori !
min {max I¯k (K2 )}
K2 ✓K k2K
placement: Take a greedy iterative approach
NDP
by utilizing the observed STA interference from
iteration t!
previous iteration to arrive at current partitions.
At each step, place the STA with maximum
observed interference to be trained with NDP2
unless maximum interference increases … MobiHoc 2014 1!
NDP1!
2!
!
…!
K1
T!
…!
T+1"
NDP2!
K
K2
15 Adaptive TXOP Sizing •  Idea: Each STA has a separate adaptive TXOP duration
•  k = interference threshold of STA k (from MCS)
•  I¯ke = interference at the end of TXOP
•  Lk= TXOP duration of STA k
•  Decrease Lk if I¯ke > k else set to max(L1 , . . . , LK )
…!
MobiHoc 2014 "
Interference > β
k
CSI"
STAK"
netMIMO"
DATA"
ACK"
New TXOP Limit"
netMIMO"
DATA"
ACK"
…!
netMIMO"
DATA"
ACK"
Old TXOP Limit"
TXOP Adjustment: In each iteration, reduce the TXOP
limit of STA that exceeds the interference threshold to
the first time instant in TXOP it exceeds the threshold.
16 Theoretical Analysis Insight 1
•  Independence: Interference at STA is independent of CSI
delays of other STAs
•  Proof: Interference Ik is only dependent on Hk and not on
channels of other STAs
Insight 2
•  Monotonicity: Average interference increases roughly
monotonically with CSI delay
•  Proof: Assume a gaussian error in channel with variance
˜ k + ek
Hk = H
2
k
The average interference becomes (C is constant)
MobiHoc 2014 I¯k ⇡ C
2
k
Does this imply
monotonicity with CSI
delay ?? !
17 Theoretical Analysis Monotonicity
•  Kendall ⌧ rank correlation test
•  Observed: ⌧ > 0.7 (+/-1 indicates monotonically increasing/
decreasing)
Kendall tau coefficients for interference vs CSI delay
with 0.01 significance level"
Example of Interference vs CSI delay "
−25
1
Stat
MovE
MovD
Kendall Coeff
Interference (dBm)
−20
−30
−35
−40
−45
0
MobiHoc 2014 2
4
Time (ms)
6
0.8
0.6
0.4
0.2
0
Stat
MovE
MovD
18 Theoretical Analysis Proposition 1
•  NDP placement algorithm finds the optimal partition within K
iterations
Proposition 2
˜ )2 /K↵
•  If interference threshold k of STA k satisfies k = P (⇤min
k
k
where ↵k is the SIR decoding threshold, and ⇤min
is the
k
˜ k across subcarriers,
minimum largest singular value of H
then the average SIR after TXOP adjustment satisfies
SIRk ↵k.
⇡
MobiHoc 2014 19 Evaluation
MobiHoc 2014 20 Testbed WARP SDR
•  5 APs w/ 2 Tx antennas, 10 STAs w/ 1 Rx antenna
•  2 AP clusters, 3 STA clusters on movable carts
•  WARPLab v7.3, Gigabit Switch
STAs
APs
Signal Processing
•  2.4Ghz, Channel 14
•  20Mhz (oversampling by 2)
•  64 FFT MIMO-OFDM
•  STF, LTF, CFO correction ...
MobiHoc 2014 3
4
2
2
1
1
NetMIMO testbed deployment in our lab"
21 Synchronization Time
•  Within Cylic Prefix (CP) window
•  Trigger through twister-pair cable
•  Trigger delay of AP j is Dj, introduced
offset Oj is such that
Clock Buffer !
D13!
max {Oj + Dj }
min {Oj + Dj }  CP.
1jM
Twisted-pair !
cable!
D12!
AP1!
1jM
clock cable!
AP3!
AP2!
Time and frequency synchronization of 3 APs"
Frequency
•  Shared clocking with master AP
•  RG-174 coax cables, rating 6.6dB/100ft
•  Clock buffer boards ADCLK954
MobiHoc 2014 22 Channel Measurements Prolonged transmission
0.15
•  216
LTF Symbols
Magnitude
samples, half-rate sampling
=> 6.4ms airtime
•  Continuous channel estimation
through LTF symbols
LTF symbol stream used in channel estimation "
0.1
0.05
0
0.1
6.3
6.4
Time (ms)
WARP driver
•  Trace-based evaluation
•  3x to 10x reduction in read
and write latency[5]
Host PC"
PC Trigger"
TX_NODES
WriteIQ"
Host PC"
PC Trigger"
RX_NODES
Tx/Rx"
700μs" 20μs" 100μs"
ReadIQ"
Tx/Rx"
ReadIQ"
≈650μs"
time"
Channel measurement period ≈ 0.8ms"
[5] K. C. Garikipati and K. G. Shin, “Improving Transport Design for WARP SDR Deployments,” in Proc. of the ACM
SIGCOMM Software Radio Implementation Forum (SRIF), 2014
MobiHoc 2014 23 Results: NDP Placement 8-STA netMIMO
MovD(STAs 1--4)
MovD(STAs 5--8)
MovE(STAs 1--4)
MovE(STAs 5--8)
SIR Gain (dB)
12
10
8
6
4
2
0
STA1 STA2 STA3 STA4 STA5 STA6 STA7 STA8
MobiHoc 2014 Observed SIR gains across four experiments"
Max. Interference (dBm)
•  450ms period, netMIMO transmission every 15ms
•  5--10dB gain for mobile (movD) STAs
−10
Default
NDP placement
−15
−20
−25
−30
0
100
200
300
Time (ms)
400
500
Observed maximum interference across 8 STAs"
24 Results: TXOP Adjustment 8-STA netMIMO
•  Decoding errors occur when interference exceeds threshold
•  Between 26%--42% reduction in error rate
100
MovE
MovE w TxAj
MovD
MovD w TxAj
80
Error Rate(%)
Error Rate (%)
100
60
40
60
40
0
0
0
MobiHoc 2014 80
20
20
MovE
MovE w TxAj
MovD
MovD w TxAj
1
2
3
4
5
MCS index
Max TXOP Limit = 2ms"
6
7
0
1
2
3
4
5
MCS index
6
7
Max TXOP Limit = 4ms"
25 Results: Field Test •  Ambient environment with device and people motion
•  Location (1,2) is LOS, Location (3,4) is NLOS
•  More than 15% increase in throughput for 8-STA netMIMO
350 Average !
netMIMO!
300 Throughput!
(Mbps)!
250 200 Default 150 Proposed 100 50 MobiHoc 2014 0 (1,2)"
(3,4)"
4STAs!
(1,2)"
(3,4)"
6STAs!
(1,2)"
(3,4)"
8STAs!
26 Discussion and Conclusion •  Theoretical basis: Channel state across different iterations is
not constant, hence the theorems do not necessarily hold but
serve as guiding principle
•  Channel fading: Our approach is robust as we average
interference across subcarriers
•  Rate adaptation: We have used fixed MCS for adjusting
TXOP duration, which can be improved using rate adaptation
Conclusion: As Network MIMO is closer to reality, it is necessary to
examine the various aspects such as CSI aging. We proposed schemes to
mitigate interference from CSI aging by exploiting the relationship
between Interference and CSI delay. It needs to be futher enhanced with
rate adaptation and interference cancellation.
MobiHoc 2014 27 Thanks
[email protected]
MobiHoc 2014 28 

Improving Transport Design for WARP SDR Deployments

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International SKED - Vanuatu Luxury Holiday Homes

Victor D. Varner - Princeton University