Communications through High Delay Spread x Bandwidth (HDB) Channels: Opportunities and Challenges M. Emami, F. Lee and A. Paulraj Stanford University October 18, 2004 AIM Workshop on Time-Reversal Communications in Richly Scattering Environments
Mar 27, 2015
Communications through High Delay Spread x Bandwidth (HDB) Channels:
Opportunities and Challenges
M. Emami, F. Lee and A. Paulraj
Stanford University
October 18, 2004
AIM Workshop on Time-Reversal Communications in Richly Scattering Environments
Agenda
What is a HDB Channel and the “TR” Effect Experimental Data Characterization of Spatial Focusing Communications in HDB Channels Single User
Capacity Equalization
Multi User Capacity Equalization
Concluding Remarks
Rich Channel - HDSBW
What is a HDB Channel?
Delay
AmplitudeAmplitude
Delay
Amplitude
Delay
High Delay Spread Sparse Channel
Few resolved pathsFew resolved paths
Low Delay Spread
Many resolved paths
High Delay Spread Rich Channel
HDB Metric
The TR effect depends on the number of significant resolvable taps (N) in the channel response
Typically, N > 30 represents a good HDB channel
Time Reversal (TR) Experiment
x(t) = s(t) h*(-t)
s(t)
r(t) = s(t) h*(-t) h(t)
Tx Rxx(t) h(t) r(t)
Step 2
Tx h(t) Rx(t)
h(t)
Step 1
TR Effects
Spatial focusing Temporal focusing Channel hardening
Normalized Magnitude
Num
ber
of O
ccur
renc
es
Original Channel
After TR
Magnitude PDF of One Tap
Agenda
What is a HDB Channel and the “TR” Effect Experimental Data Characterization of Spatial Focusing Communications in HDB Channels Single User
Capacity Equalization
Multi User Capacity Equalization
Concluding Remarks
Experimental Evidence for TR Effects
Indoor (Intel/Stanford) Large office space with cubicles (40 x 60 yards) Bandwidth 2 to 8 GHz (UWB) Channel measured with fixed Tx and Rx in a grid
of .5m x .5m at (approx.) every 3 cm Outdoor (Nokia)
Bandwidth 100 MHz Underwater Acoustics
Indoor Wireless: Spatial Focusing Effect
NLOS Data
Distance in WavelengthP
ower
LOS Data
Distance in Wavelength
Pow
er
Spatial power profile strongly localized at intended receiver location
Indoor Wireless: Temporal Focusing Effect
Tap Index
Nor
mal
ized
Mag
nitu
de
Impulse Response after TR
Temporal power profile at intended receiver strongly localized in time
Side lobes double channel length
Tap Index
Channel Impulse Response
Nor
mal
ized
Mag
nitu
de
Outdoor Wireless: Temporal Focusing Effect
Tap Index
Nor
mal
ized
Mag
nitu
de
Impulse Response after TR
Tap Index
Channel Impulse Response
Nor
mal
ized
Mag
nitu
de
N 17 for this case
Underwater Acoustics
Distance
High N Low N
Tim
e (µ
s)
Agenda
What is a HDB Channel and the “TR” Effect Experimental Data Characterization of Spatial Focusing Communications in HDB Channels Single User
Capacity Equalization
Multi User Capacity Equalization
Concluding Remarks
Characterizing Spatial Focusing
Single Ring (SR) Model h(τ,R) is the channel from Tx to r = R r=0 represents center of circle
r=0Tx
rmd
N i.i.d. uniformly distributed scatterers
rM
1
2
Spatial Focusing Statistics
Space-time (S-T) random field generated by a one shot TR pulse offers multiple characterization
Influencing parameters N - HDB metric λ - wavelength BW - bandwidth Δθ = θ2 -θ1 (receive angle spread)
Define E{(R )} = [max {s(, R)}]2
where s(, R) = h*(-, 0) h(, R)
Spatial Focusing Statistics - Metrics
Long range spatial focusing:
3-dB contour of (R ) around Rx (Ga and Gx are the range and cross-range widths of contour)
)0()(lim ηηp || RR
5.0)(/)(
5.0)(/)(
0u
0u
DxxD
DaaD
G
G
One-Shot Results: Single Tx AntennaD
ista
nce
in W
avel
engt
h
Ga
Gx
N = 1
N = 100
Typical one-shot realizations of (R ) around target point
Distance in Wavelength
One-Shot Results: 5 Tx Antennas
Typical one-shot realizations of (R ) around target point
Distance in Wavelength
N=1 N = 100
Dis
tanc
e in
Wav
elen
gth
Spatial Focalization: E{(R)}
Distance in Wavelength Pulse Bandwidth (MHz)
Peak
Pow
er (
dB)
S-T Focalization: Empirical Relationships for SR Model
5.12.0
6.035.04.0
2sin
228.0
TRMS MBppE
105.0
3.12.01.0
2sin
262.0
TRMSa MBG
43
2
sin
162.0
43
2sin
217.0
105.0
2.01.0
105.0
2.12.01.0
TRMS
TRMS
x
MB
MB
G
Agenda
What is a HDB Channel and the “TR” Effect Experimental Data Communications in HDB Channels Single User
Capacity Equalization
Multi User Capacity Equalization
Concluding Remarks
What is a HDB Communication System?
A communication system that exploits the “TR effect” to improve performance factors.
The transmitter uses a pre-filter derived from the time reversed channel for transmission to the intended receiver.
Demod. /Decode
h()Encode /Mod.
))(( * hf
Tx Rx
Important Questions for HDB Communications
How is capacity affected by HDB channels in single and multi-user scenarios?
What are the key communication problems? Equalization for ISI Channel coding Can spatial focusing be preserved Are there any “LPI” or CCI reduction effects Design tradeoffs
Agenda
What is a HDB Channel and the “TR” Effect Experimental Data Communications in HDB Channels Single User
Capacity Equalization
Multi User Capacity Equalization
Concluding Remarks
Capacity of Single User HDB Channels
Capacity of a communication channel determines maximum rate of transmission per channel use.
HDB channels are frequency selective fading channels. They will suffer a capacity penalty w.r.t. AWGN channels at high SNR.
Optimum approach to maximizing capacity is water-filling (WF). TR is close to but not true WF.
Effect of HDB Channels on Capacity
TRfor )(
)(h
hp
)(tx )(u )(h )(ty
)(tn
)(p
TR rate:
Max. achievable rate:
d
h
HITR 22
4
2 ||||
|)(|1log
2
1
d
HEI
PdEWF 2
2
2)(
|)(|)(1log
2
1max
Tx power spectral density
Water-Filling
In order to obtain IWF , the input energy must satisfy the water-filling solution:
d
HI
HE WF 2
2
22
2 |)(|log
2
1
|)(|)(
2
2
|)(|
H
Capacity: TR vs. WF
Ergrodic capacity of TR is near optimal at low SNR Outage capacity decreases with increase in # of taps
Rate (bits/s/Hz)P
roba
bili
ty
Cumulative Distribution
SNR
Ave
rage
Rat
e (b
its/
s/H
z)
50 taps
Equalization Options for HDB Channels
Tx Equalization Rx Equalization
TR None
None LE / DFE / MLSE
LE None
TR LE
THP THP
LE – Linear Equalizer
DFE – Decision Feedback Equalizer
MLSE – Maximum Likelihood Sequence Estimator – Too complex (exponential)
THP – Tomlinson-Harashima Precoding
Tx Eq. Rx Eq.h()
Equalization
HDB = high Inter Symbol Interference Problem Modulation schemes can be used to “mitigate” ISI
problem. e.g. Spread spectrum, OFDM. We discuss Single carrier schemes where the ISI problem
is severest.
TR at Tx – No Receive Processing
This channel has a severe ISI problem.Power of main tap = Power in ISI taps. TR does not solve the ISI problem.
Mitigation: Rate back-off
)(tx )(u )(h )(ty
)(tn
h
h )(
ISI
Rate back-off (RB)
Rate back-off refers to signaling at symbol rate < 1/BW. This effectively sub-samples the channel, reducing the effective ISI while capturing full diversity
Normal Channel after TR Effective Channel with RB = 2
Peak
ISI
ISI vs. Rate back-off for TR
Assuming the channel taps are i.i.d. Gaussian, the ratio of peak to ISI power is related to rate back-off as follows:
Plot of γTR for No Rate back-off (RB = 1)
Theoretical
Intel Indoor Data
NRBISI
PeakTR as
Rx-Only Equalization: LE and DFE
sk'H(z)sk
nk
C(z)
1–B(z)
sk'F(z)H(z)sk
nk
LE
DFE
Performance Complexity
LE Poor
(Noise enhancement)
Time domain: O(n)
Frequency domain: O(log2n)
DFE Close to MLSE at high SNR
(Error propagation negligible)
Time domain: O(2n)
Frequency domain: O(n) + O(log2n)
Tx-Only Equalization: LE
Minimize mean square error (MSE) subject to power constraint:
is the delay of the equalizer and the channel is for removing the bias We investigate
]|[|min 2
1
kk
ggxyE
H
ks ku)(zG )(zH ky
kn
NoiseISI
PeakSNReff
TR vs. Tx-LE: Effect of Rate back-off
Rate back-off improves effective SNR
SN
Ref
f (d
B)
SNRMFB (dB)
RB=25
RB=5RB=2RB=1
Joint Tx & Rx Equalization: TR & LE
ks ku)( 1 zH )(zH ky
kn
)(zC
TR performs near-optimal WF while LE & rate back-off mitigate ISI
For further complexity reduction, only the largest 10 or 20 taps in impulse response after TR and rate back-off are used to design LE
TR & LE: Performance Results
(Full impulse response after TR contains 499 taps)
LE only uses largest 20 taps of impulse response after TR
LE only uses largest 10 taps of impulse response after TR
RBRBRB
RBRBRB
RB = Rate-back-off Factor
Joint Tx & Rx Equalization: THP
Modulo operator at transmitter limits average & peak power of xk
Better BER performance than DFE, especially at low SNR, since there is no error propagation
Capacity penalty of 0.255 bits/transmission at high SNR compared to DFE (shaping loss)
H(z)
1–B(z)
modsk
xk
nk
mod sk'F(z)
Effect of HDB on LE & THP
Effect of Equalization on Spatial Focusing
Rx-only equalization: No spatial focusing Tx-only equalization
TR: Shown previously (use as reference)
LE: Similar to TR with a small penalty
1
RBN
Np
Spatial Focusing: Simulation Results
100 i.i.d. Gaussian taps (N=100) We have that for both MMSE and TR
20 40 60 80 100
4
6
8
10
12
14
16
18
Rate Back-off
S to I Ratio vs Rate Back-off
TR
20 40 60 80 100
0
2
4
6
8
10
12
14
16
Rate Back-off
S to I Ratio vs Rate Back-off
MMSE
RBIS
TR vs. Tx-LE: Effect of Multiple Antennas
SN
Ref
f (d
B)
SNRMFB (dB)
Effective SNR increases with # of Tx antennas (MT)
Single-User MIMO Systems
The capacity for a frequency selective MIMO channel is given by:
λi is the energy of space-frequency mode i of the channel
)(1 x
)(2 x
)(Mx
)(H
)(1 ty
)(2 ty
)(tyM
+
+
+
)(tn
MN
ii
is
MN
FS M
E
NC
MN
ii
122 1logmax
1
1
Multi-User Systems
Assumption Each user has 1 antenna Base station (BS) has MT antennas
Key questions What is the effect of HDB on capacity regions? What are the appropriate equalization techniques for
HDB channels?
H()
User 1
User K
. . .BS
. . .
Capacity Regions of Multiple Access Channels
R1
R2
R1
R2
R1
R2
Flat
Single Antenna Multiple Antennas
No ISI
ISI
R1
R2
Flat
Broadcasting Channels
Dirty Paper Coding (DPC)
Examples of practical DPC schemes THP Trellis precoding Flexible precoding Lattice coding
w2nR
sn zn
ŵ(yn)ynxn(w,sn)
interference noise
Tx Equalization for Broacast Channels
kx1 1g 1h ky1+
kn1
1
kx2 2g 2h ky2+
kn2
2
+
TTT
kkkkgg
ggg
xyExyEH
][
]}|[|],|[|max{min
21
2222
2111
1
THP for Broadcast Channels
s1'mod
I - B
HF
sK'modn
x
y1
yK
. .
.
modsk
Element-Wise Operation
Feedback Filter (Triangular)
Channel (Flat or ISI)
Feedforward Filter
Joint (vector/matrix) processing at BS Individual (scalar) processing for each user
THP for Broadcast Channels
Equivalent to VBLAST at Rx No error propagation
Sources of capacity loss relative to optimum DPC Shaping loss induced by modulo operation Symbol-by-symbol encoding
Secure communication possible Difficult for one user to decode other users’ data based on
its own received signal
Performance Example: [2]
2-Tap ISI Channel with Equal Power, # of Users = 4
MT = 4
MT = 5
MT = 6
Simulation
Theoretical Approximation
References
[1] R. Schober and W. H. Gerstacker, “On the Distribution of Zeros of Mobile Channels with Application to GSM/EDGE,” IEEE JSAC, July 2001.
[2] L. U. Choi and R. D. Murch, “ A Pre-BLAST-DFE Technique for the Downlink of Frequency-Selective Fading MIMO Channels,” IEEE Trans. Commun., May 2004.
Publications of TR Group
[1] M. Emami, et al., “Predicted Time Reversal Performance in Wireless Communications Using Channel Measurements,” to appear in IEEE Commun. Letters.
[2] J. Hansen, et al., “Design Approach for a Time Reversal Test Bed for Radio Channels,” Special Session on MIMO Prototyping, 12th European Signal Processing Conference, Sept. 2004.
[3] C. Oestges, et al., “Time Reversal Techniques for Broadband Wireless Communications,” European Microwave Week, Oct. 2004. (Invited Paper)
[4] T. Strohmer, et al., “Application of Time Reversal with MMSE Equalizer to UWB Communications,” to appear in GLOBECOM’04.
[5] M. Emami, et al., “Matched Filtering with Rate Back-off for Low Complexity Communications in Very Large Delay Spread Channels,” to appear in Asilomar Conference on Signals, Systems, and Computers, Nov. 2004.