the wireless revolution: a signal processing perspective vince poor ([email protected]) federal...
TRANSCRIPT
THE WIRELESS REVOLUTION:A Signal Processing Perspective
Vince Poor
Federal Communications CommissionMay 29, 2001
May 29, 2001 - The Wireless Revolution
OUTLINE
• The Role of Signal Processing in Wireless
• Some Recent Signal Processing Advances– Space-time Multiuser Detection
– Turbo Multiuser Detection
– Quantum Multiuser Detection
• Conclusion
May 29, 2001 - The Wireless Revolution
THE ROLE OF SIGNAL PROCESSING IN WIRELESS
May 29, 2001 - The Wireless Revolution
Motivating Factors
• Telecommunications is a $1012/yr. business
• c. 2005: wireless > wireline
• > 109 subscribers worldwide
• Explosive growth in wireless services
• Use of a public resource (the radio spectrum)
• Convergence with the Internet
The Role of Signal Processing in Wireless
Wireless Applications
• Mobile telephony/data/multimedia (3G)
• Nomadic computing
• Wireless LANs
• Bluetooth (piconets)
• Wireless local loop
• Wireless Internet/m-commerce
The Role of Signal Processing in Wireless
Wireless is Rapidly Overtaking Wireline
The Role of Signal Processing in Wireless
Source:The EconomistSept. 18-24, 1999
Traffic Increasingly Consists of Data
Source: http://www.qualcomm.com
The Role of Signal Processing in Wireless
Demand Growing Exponentially
The Role of Signal Processing in Wireless
Source: CTIA
- As of 05/01/01, there were 114,546,113, in U.S., according to www.wow-com.com - Every 2.25 secs., a new subscriber signs up for cellular in U.S.
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Mobile Subscriptions as a %of all telephone Subscriptions
Source: ITU
Mobile PhonesSubscribers per 100 inhabitants, 1998
The Role of Signal Processing in Wireless
There’s Plenty of Room to Grow - I
Mobile PhonesMarket Penetration, 2000
The Role of Signal Processing in Wireless
There’s Plenty of Room to Grow - II
76% 72%67%
58%50%
46%39%
7%
0%
10%
20%
30%
40%
50%
60%
70%
80%
Courtesy of: Tom Sugrue (FCC)
Wireless Challenges
• High data rate (multimedia traffic)
• Networking (seamless connectivity)
• Resource allocation (quality of service - QoS)
• Manifold physical impairments
• Mobility (rapidly changing physical channel)
• Portability (battery life)
• Privacy/security (encryption)
The Role of Signal Processing in Wireless
Wireless Channels
• Fading: Wireless channels behave like linear systems
whose gain depends on time, frequency and space.
• Limited Bandwidth (data rate, dispersion)
• Dynamism (random access, mobility)
• Limited Power (on at least one end)
• Interference (multiple-access, co-channel)
The Role of Signal Processing in Wireless
Not Growing Exponentially
• Spectrum - 3G auctions!
• Battery power
• Terminal size
The Role of Signal Processing in Wireless
Moore’s and “Eveready”’s Laws
Courtesy of: Ravi Subramanian (MorphICs)
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Battery Capacity(i.e. Eveready’s Law)
Signal Processor Performance (~Moore’s Law)
The Role of Signal Processing in Wireless
Signal Processing to the Rescue
• Source Compression• Transmitter Diversity (Fading Countermeasures):
– Spread-spectrum: CDMA, OFDM (frequency selectivity)– Temporal error-control coding (time selectivity)– Space-time coding (angle selectivity)
• Advanced Receiver Techniques:– Interference suppression (multiuser detection - MUD)– Multipath combining & space-time processing– Equalization– Channel estimation
• Improved Micro-lithography (phase-shifting masks)
The Role of Signal Processing in Wireless
Advances in ASIC Technology
Courtesy of: Andy Viterbi
Microns
.8
.5
.35.25
.18
Time 1991 Future199819971995
The Role of Signal Processing in Wireless
5/30/00: 25 nm gate announced with optical lithography using phase-shifting masks (T. Kailath, et al.).
Fleming Valve 1910
Helical Transformer 1919
Marconi Crystal Receiver 1919 DeForest Tubular Audion
1916
Signal Processing for Wireless (v 1.0)
The Role of Signal Processing in Wireless
SOME RECENT SIGNAL PROCESSING ADVANCES
• Introduction
• Space-time Multiuser Detection (3G)
• Turbo Multiuser Detection (2.5G)
• Quantum Multiuser Detection (?G)
May 29, 2001 - The Wireless Revolution
INTRODUCTION
Some Recent Signal Processing Advances
First, A Few Words About MUD • Multiuser detection (MUD) refers to data detection in
a non-orthogonal multiplex; it’s of interest, e.g., in– CDMA channels – TDMA channels with channel imperfections– DSL with crosstalk
• MUD can potentially increase the capacity (e.g., bits-per-chip) of interference-limited systems significantly
• MUD comes in various flavors – Optimal (max-likelihood, MAP)
– Linear (decorrelator, MMSE)
– Nonlinear interference cancellation
Some Recent Signal Processing Advances
Some Recent Developments • The basic idea of MUD is to exploit (rather than
ignore) cross-correlations among signals to improve data detection. Recent developments in this area:
• Space-Time MUD – Joint exploitation of spatial and temporal structure.
• Turbo MUD – Joint exploitation of temporal structure induced by channel
coding, and the multi-access channel.
• Quantum MUD – Joint exploitation of quantum measurements and the multi-
access channel.Some Recent Signal Processing Advances
SPACE-TIME MUD
Some Recent Signal Processing Advances
User 1
User 2
User K
r1(t)
r2(t)
rP(t)
Multi-{Access, Antenna, Path} Channel
Space-Time MUD
Non-orthogonal signaling, multipath, fading, dispersion, dynamism, etc.
Single-Antenna Reception
)(1 ts)(1 ib)(1 th)(1 tx
)(2 ts)(2 ib)(2 th)(2 tx
)(tsK)(ibK
)(thK)(txK
---
---
+ +
)(tn
)(tr
Space-Time MUD
• Transmitted signal due to the k-th user:
xk(t) = bk(i)sk(t−iT)i=1
B
∑ , .,,1 Kk L=
[bk(i): data symbol; sk(t): signaling waveform]
• Vector channel (impulse response) of the k-th user:
∑ −==
L
lklklklk tgath
1).()( τδ
[kl: path delay; gkl: path gain; akl: array response]
• Received signal:
∑ +∗==
K
kkk tnthtxtr
1).()()()( σ
Space-Time MA Signal Model
Space-Time MUD
• Log-likelihood function of the received signal r(t):
L({r(t) :−∞<t<∞}b)∝ Ω(b) ≡2Re{bTy}−bTHb
yk(i) = gkl* akl
H r(t)sk(t−iT−τkl)dt−∞
∞
∫l=1
L
∑
• H is a matrix of cross-correlations among the received
signals
• Sufficient statistic {yk(i)}: space-time matched filter output
A Sufficient Statistic: Space-Time Matched Filter Bank
[kl: path delay; gkl: path gain; akl: array response]
Space-Time MUD
Maximum LikelihoodSequence Detection
OR
Iterative InterferenceCancellation
Space-Time Multiuser Receiver
Space-Time MUD
• Maximum likelihood sequence detection maximizes (over b):
Ω(b) =2R{bTy}−bTHb
H ≡
H [0] H[1] L H[Δ]
H[−1] H[0] H [1] L H[Δ]
H [−Δ] L H[0] L H[Δ]
H[−Δ] L H[−1] H[0] H[1]
H[−Δ] L H[−1] H[0]
⎡
⎣
⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢
⎤
⎦
⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥
[: multipath delay spread]• Computational complexity: O(2K)
Optimal Space-Time MUD
Space-Time MUD
y=Hb+σv
[ Decorrelator: sgn(Re {H-1y}); MMSE: sgn(Re {(H+2I)-1y}) ]
– Gauss-Seidel Iteration: (Serial IC)
Problem: Cx=y with C =CL +D+CU
– Jacobi Iteration: (Parallel IC) xm=−D−1(C L +CU )xm−1 +D−1y
xm=−D+CL( )−1CUxm−1 + D+CL( )
−1y
Linear S-T Interference Cancellers
• Computational complexity: O(K mmax)
Solve
Space-Time MUD
Simulation [K = 8; L = 3; P = 3]
Direct-sequencespread-spectrum(16 chips/bit).
Space-Time MUD
– Decision Feedback:
Cholesky Decomposition: C =FHF
ˆ b =sgn(F−Hy−(F−diag(F)ˆ b ))
– Successive Cancellation:
bm=sgny−(C L +CU )bm−1( )=sgny−(H−D)bm−1( )
– EM/SAGE-Based IC: (Interfering symbols are “hidden” data)
Nonlinear S-T Interference Cancellers
– Turbo MUD: - Coded channels (b has constraints).
y=Hb+σv
Space-Time MUD
TURBO MUD
Some Recent Signal Processing Advances
MUD & The Decoding of Error-Control Codes
• Recall: the basic idea of MUD is to exploit cross-correlations among signals to improve data detection.
• Similarly, error-control coding exploits dependencies among channel symbols to improve data detection.
• Turbo MUD is a technique for jointly exploiting these two types of dependencies.
Turbo MUD
• The convolutional code & the multiaccess channel form
a concatenated code.
• Like other concatenated codes, this code can be
efficiently decoded via a turbo-style receiver.
Coded Multiple-Access Channel
Convolutional Encoders
InterleaversMultiaccess
Channel
Information Bits Channel Input Channel Output
Basic Idea of Turbo MUD:
Turbo MUD
r(t) = bk,i(dk)pk(t−iT) +σ n(t)i=1
B
∑k=1
K
∑
Rate-R-Coded Multiaccess Signal Model
Received Signal:
• K = # active users.
• B = # channel symbols per frame
• dk = set of RB data symbols transmitted by user k
• bk(dk) = vector of channel symbols obtained by encoding dk
• pk = rec’d waveform of user k ; 1/T = per-user signaling rate.
• {n(t)} = unit AWGN; = noise intensity
Turbo MUD
As before, the vector y of matched-filter outputs:
is sufficient for inferring b1(d1) b2(d2) ... bK(dK) and d1 d2 ... dK.
Sufficient Statistic
yk(i) = r(t)pk(t−iT)dt−∞
∞
∫ , k=1,...K, i =1,...,B
y=Hb+N(0,σ 2H)
(Hn,m= pk(t−iT)pl(t−jT)dt)−∞
∞
∫
Turbo MUD
max[2 ′ y b− ′ b Hb]
Optimal MUD/Decoding
ML Detection (b)/Decoding (d):
MAP Detection/Decoding: maxP(symbolvalue|y)
O(2) - convolutionally encoded symbols, constraint length orthogonal signaling [BCJR, Viterbi algo, etc.]
O(2K) - uncoded symbols, delay spread [MLSD; MAP MUD]
Complexity per Symbol (Assume Binary Symbols):
(Hn,m=0, ∀ |n−m|>KΔ)
(Hn,m=0, ∀ n≠m)
Turbo MUD
Turbo MUD: The Main Idea
• For constraint-length-convolutionally coded transmission on an asynchronous K-user multiaccess channel, optimal detection/decoding has complexity O(2K) [Giallorenzi & Wilson].
• This complexity can be reduced to O(2K) + O(2) via the turbo principle [Moher].
• I.e., iterate between MUD and channel decoding, exchanging soft (channel) symbol information at each iteration.
Turbo MUD
Convolutional Encoders
InterleaversMultiaccess
Channel
Information Bits Channel Input Channel Output
SISOMUD
SISO Decoders
De-Int.Int.
Channel Output
Output Decision Soft-input/soft-output (SISO) Iterative Interleaving removes correlations
{Pdecoder(bk,i y)}
22 +K vs. K2
Multiaccess Channel & Turbo Receiver
{PMUD(bk, i y)}
Turbo MUD
SISO MUD
• To get posterior probabilities from the multiuser detector, we should use MAP MUD.
• MAP MUD is prohibitively complex O(2K) [K = # users]
• This differs from standard turbo decoding, in which the constituent decoders are of similar complexity.
• Many lower complexity approaches: [Alexander et al.; Honig et al., Lu & Wang, Müller & Huber, Naguib & Sheshadri, Reed et al., Schlegel, Tarköy, Wang & Chen, Wang & Poor (COM’99), & others]
Turbo MUD
y=Hb+N(0,σ 2H)
Recall: Low Complexity MUD
Recall the Model:
• MUD fits this model to the observations.
• As noted before, in addition to ML/MAP, there are many low-complexity techniques for doing this; e.g.,– Linear MUD: decorrelator, MMSE, bootstrap (v. efficient
iterative implementation as linear interference cancellers (IC’s))
– Nonlinear IC’s: successive cancellation, multistage, EM/SAGE
• Generally, these don’t allow the computation of the posterior probabilities needed for turbo MUD.
Turbo MUD
Low Complexity SISO MUD
• Conventional MMSE MUD:
• MMSE output desired symbol + Gaussian error
[Poor & Verdú, IT’97]
• From this, posterior probabilities can be estimated
from the MMSE detector output.
• This yields an effective low-complexity SISO MUD.
• MMSE w/ Priors:
≅
ˆ b =sgn{(H+σ 2I)−1y}
(H+σ 2C-1)−1[y−H˜ b ]
Turbo MUD
Simulation Example [K = 4;
Rate-1/2 convolutional code; constraint length 5; 128-long random interleavers
Turbo MUD
QUANTUM MUD
Some Recent Signal Processing Advances
• A basic element of MUD is the matched-filter-bank sufficient statistic.
• With quantum measurements, observation is restricted (uncertainty principles apply).
• In this case, the observation instrument must be designed jointly with the detector.
• Photon counting is usually not optimal.
Quantum MUD
Quantum MUD
Quantum MUD Design Problem
Quantum MUD
A Two-User Quantum Channel
Quantum MUD
Two-User Example: Error Probabilities
Quantum MUD
Conclusion
• The transformation from wireless voice to wireless data is causing exponentially increasing demand for wireless capacity.
• Signal processing is the great enabler: – Source compression– Fading countermeasures/transmitter diversity– Interference suppression/space-time processing – Micro-lithography
• Recent advances:
May 29, 2001 - The Wireless Revolution
Conclusion - Cont’d• MUD exploits signal cross-correlations to substantially improve
data detection.• Space-time MUD
– Combines exploitation of temporal & spatial cross-correlations.• Turbo MUD
– Combines exploitation of cross-correlations introduced by the channel with exploitation of dependence introduced by coding.
• Quantum MUD – Combines exploitation of cross-correlations with the instrument
design for the quantum channels.• Some Open Issues
– Space-time MUD: Hardware implementation– Turbo MUD: Adaptivity, convergence behavior– Quantum MUD: Relevance in applications
May 29, 2001 - The Wireless Revolution
THANK YOU!
May 29, 2001 - The Wireless Revolution