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Rajesh GuptaUniversity of California, Irvine
Mani SrivastavaUCLA
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•1868•
LET THE R E BE
LIG H T
Copyright 1997 Rajesh Gupta & Mani Srivastava
ICCAD 1997 Tutorial
Design Technology for BuildingWireless Systems
2
● 35-60% annual growth in PCS users● By 2000, one in three phones will be
mobile (42% in US)● Nordic countries: 10 mobile phones
being added for every wireline phone● Japan: number of users doubled from
10M to 21M from March to october 1996● 600M mobile phone users by 2001● $17B in PCS license auctions● 300% growth in wireless data from 1995
to 1997
Big demand for portable computers:
● 2m ($290M) in 1988 to 74M ($54B) in 1998● 20% of all computers sold are laptops
Phenomenal Growth in Wireless Voice & Data Services
-
3
“Anytime Anywhere Anyform” Information Systems
Fax & email on the beach
PCS & MultimediaMessaging on the road
Multimedia wireless LANs & PBXsin offices, schools, hospitals, homes
UCLA
mani
Networked sensors everywhere
Wireless Sensors
4
● Battery technology is a key hurdle - no Moore’s Law here!
Battery Rechargeable? Gravimetric Density (Wh/lb) Volumetric Density (Wh/l)
alkaline-MnO2 (typical AA) NO 65.8 347
silver oxide NO 60 500
Li/MnO2 NO 105 550
zinc air NO 140 1150
NiCd YES 23 125
Li-Polymer YES 65-90 300-415
Size & Battery Life are Critical in Wireless Devices
Year
Nom
inal
Cap
acit
y
65 70 75 80 85 90 950
10
20
30
40
NiCd
NiMH
(Wat
t-h
ours
/ lb
)
-
5
Where does the Battery Power go?
● Typical laptop: 30% display, 30% CPU + memory, 30% rest● Wireless devices: increasing communication & multimedia processing
Low power VLSI are a key to wireless
Laptop CellularPhone
Laptop +WirelessAdapter
PersonalWirelessTerminal
Microprocessor 1-4 W 1-4 W
Memory 1 W 1 W
Logic 2 W 2 W 0.3 W
Hard Disk 1 W 1 W
Display 2-6 W 2-6 W 0.185 W
Programmable DSP 0.5 W
RF Transceiver 2 / 4 W 0.6 / 1.8 W 0.6 / 1.8 W
Commn. Processing 2.5 W 2.5 W
Sound/Audio I/O ? ? 0.085 W
6
● Increasing integration of communication & multimedia systemcomponents due to advances in semiconductor technology & circuits
- RF CMOS circuits- MEMS structures
RF components, display
● Relentless digitization continues- high speed digital circuits & A/D converters
IF and even RF processing in digital domaindirect conversion techniques
- complex communication algorithms favor digital implementation- increasing CPU MIPS make even a “software radio” possible
A wireless-system-on-a-chip is becoming possible
Wireless Systems Design: Key Driving Forces
-
7
Building a Wireless System on a Chip
RF & IF Transceiver
Baseband Processing
CustomASICLogic
AlgorithmAccelerationCoprocessors
DSP Core
RAM/ROM
WirelessNetwork Protocol
ProcessorRAMROM
DRAM
Network/Host/Peripheral Interface
(Microcontroller)
ApplicationProcessor
RAM/ROMDRAM
8
Challenge to VLSI & CAD
RF & IF Transceiver
Baseband Processing
CustomASICLogic
AlgorithmAccelerationCoprocessors
DSP Core
RAM/ROM
WirelessNetwork Protocol
ProcessorRAMROM
DRAM
Network/Host/Peripheral Interface
analog circuits that minimizespecial analog process steps
maximize digital andminimize analog computation
reusable communication &
energy efficient embeddedsoftware synthesis
multimedia modules
(Microcontroller)
low cost & low powerprotocol processor cores
Computer with Radios
ApplicationProcessor
RAM/ROMDRAM
-
9
Present basics of wireless systems,and VLSI design issues, techniques, and tools
for building integrated wireless systems
This tutorial will NOT describe:- detailed CAD algorithms for solving system design problems- theory of radio and communication systems design- detailed architecture of any wireless communication systems
Tutorial Goals
10
● Introduction to Wireless Communication Systems- system and medium characteristics- technological evolution in the design of wireless communication systems
● Wireless Systems Design- digital communications: modulation, coding, multiple access- example designs
● VLSI Circuits for Wireless Systems- micro-architecture for wireless systems-on-a-chip- direct-conversion for digital communications using VLSI
● Design technology for Wireless Systems- design entry, validation, and analysis tools
● Pre-designed Core Blocks and IP Issues for Wireless● Future Outlook and Conclusions
Tutorial Outline
-
Part 1:
Introduction to Wireless Communication Systems
12
Wireless Spectrum
104 106 108 1010 1012 1014 1016 1018 1020 1022 1024Frequency in Hz
Radio
IR UV
LightX-Ray Cosmic
RaysLFMF VHF UHF
HF
46 49 824-849 869-894 902-928 1850-1990 2400-2483
Cordless(CT-1)
Cellular(AMPS, IS-136,
IS-95)
ISM PCS ISM
Frequency in MHz
5.15 - 5.35 & 5.725 - 5.825 GHz
U-NII
-
13
Diversity of Applications in Wireless Communications
● Multimega bits/sec throughput for robust, reliable multimedia networkingover wide range of environments.
Cellular: GSM, IS95, IS54, PDC,
Wireless Data: Mobitex, CDPD, pACT, GPS
Information
Environment
0.01
100.0
Content (Mbps)
10.0
1.0
0.1
OutdoorsIndoors
Office Building Stationary Walking Vehicular
Cordless:
WirelessLAN: IEEE 802.11
WirelessATM
Mobile Wireless Multimedia
DECT, PHS, PACS, WLL
Vid
eo te
leco
nfer
enci
ng
Inte
ract
ive
Dat
aVo
ice
Low
Dat
aR
ate
14
● Wireless- limited bandwidth, high latency- variable link quality (noise, disconnections, other users)- heterogeneous air interfaces- easier snooping necessitates encryption
● Mobility- user and terminal location dynamically changes- speed of terminal mobility impacts wireless bandwidth- easier spoofing necessitate authentication
● Portability- limited battery capacity, computing, and storage- small dimensions
Characteristics of Wireless Systems
moresignalprocessing
moreprotocol
higherenergyefficiency
processing
-
15
Time Varying Wireless Environment
● Available wireless resource undergoes dramatic & rapid changes- multipath reflection, doppler fading, frequency collisions
● Rapid signal fades & distortions as the receiver moves- e.g. noise-like Rayleigh Fading when multipath signals are summed
R
DS
D
No LOS!
LOS
16
Simplified View of a Digital Radio Link
SourceCoder
ChannelCoder Modulator
MultipleAccess
PowerAmplifier
carrier fc
SourceDecoder
ChannelDecoder
DemodulatorMultipleAccess
RFFilter
carrier fc
& Equalizer
antenna
antenna
Sou
rces
Des
tin
atio
ns
transmittedsymbol stream
Multiplex
Demultiplex
received (corrupted)symbol stream
SourceCoder
SourceDecoder
RADIOCHANNEL“Highly variable b/w”
“Random & Noisy”
“Limited b/w”
“Spurious disconnections”
-
17
● Line of Sight (LOS)- free space
● Reflection (with Transmittance and Absorption)- radio wave impinges on an object >> λ (30 cm @ 1 GHz)- surface of earth, walls, buildings, atmospheric layers- if perfect (lossless) dielectric object, then zero absorption- if perfect conductor, then 100% reflection- reflection a function of material, polarization, frequency, angle
● Diffraction- radio path obstructed by an impenetrable surface with edges- secondary waves “bend” around the obstacle (Huygen’s principle)- explains how RF energy can travel even without LOS, a.k.a “shadowing”
● Scattering (diffusion)- when medium has large number of objects < λ (30 cm @ 1 GHz)- similar principles as diffraction, energy reradiated in many directions- rough surfaces, small objects (e.g. foliage, lamp posts, street signs)
Pr PtGtGrλ2( ) 4π( )2d2L( )⁄=
Propagation of Radio Waves
18
● Assume average power (in dB) decreases proportional to log of distance
● Path-loss exponent, n, depends on propagation environment
● Problem: “Environment clutter” may differ at two locations at same d● Measurements show that at a given path loss has a normal distribution
- is a zero-mean Gaussian r.v. (in dB) with standard deviation (in dB)- says how “good” the model is
Environment nFree Space 2
Urban area cellular radio 2.7 - 3.5Shadowed urban cellular radio 3 to 5
In-building LOS 1.6 to 1.8Obstructed in building 4 to 6Obstructed in factories 2 to 3
PL d( ) PL d0( ) 10ndd0-----
log+=
d
PL d( ) PL d0( ) 10ndd0-----
log Xσ+ +=
Xσ σσ
Log-normal Shadowing Path Loss Model
-
19
● Maximum separation distance vs. transmitted power (with fixed BW)Given:
- cellular phone with 0.6W transmit power- unity gain antenna, 900 MHz carrier frequency- SNR must be at least 25 dB for proper reception- receiver BW is B = 30 KHz, and noise figure F = 10 dB
What will be the maximum distance?
Solution:N = -174 dBm + 10 log 30000 + 10 dB = -119 dBmFor SNR > 25 dB, we must have Pr > (-119+25) = -94 dBmPt = 0.6W = 27.78 dBmThis allows path loss PL(d) = Pt - Pr < 122 dBλ = c/f = 1/3 mAssuming d0 = 1 km, PL(d0) = 91.5 dBFor free space, n = 2, so that: 122 > 91.5 + 10*2*log(d/(1 km))or, d < 33.5 kmSimilarly, for shadowed urban with n = 4, 122 > 91.5 + 10*2*log(d/(1 km))or, d < 5.8 km
Example Link Budget Calculation
20
Small-Scale Fading
● Fading manifests itself in three ways1. time dispersion caused by different delays limits transmission rate
- replicas of signals with different delays (reflection, diffraction etc.)2. rapid changes in signal strength (up to 30-40 dB) over small ∆x
-
21
● Received signal a sum of contributions from different directions- random phases make the sum behave as noise (Rayleigh Fading)- “fades”: intervals of increased BER, or reduced channel capacity
Error Bursts due to Raleigh Flat Fading
In FadeGoodBER = 10-5 BER = 10-1
● Function of speed of mobile as well as other objects, e.g.,- a 50 kmph car in 900 MHz band: 1 ms long >20dB fade every 100 ms- a 2 kmph pedestrian in 900 Mhz band: 25 ms long >20dB fade every 2.5s
● Also, a function of frequency, and fade depth
● Diversity techniques help- multiple antennas, multiple frequencies
22
● “Frequency selective fading” results in inter-symbol interference
- e.g. GSM has a bit period of 3.69 µs, or a rate of 270 kbps
● Data rate can be improved by “equalization”- equalizer is a signal processing function (filter)
cancels the inter-symbol interferenceusually implemented at baseband or IF in a receiver
- must be adaptive since channel is unknown & time varyingtraining, tracking, and re-training during data transmission
● GSM example- with its equalizer, GSM can tolerate up to 15 µs of delay spread- otherwise, with 15 µs of delay spread, GSM would be limited to 7 kbps
maximum data rate without significant errors 0.1delay spread------------------------------=
Data Rate Limitation in Frequency Selective Fading
-
23
● Increase transmitter power- counters flat fading, but costly and greatly reduces battery life
● (Adaptive) Equalization- compensates for intersymbol interference
● Antenna or space diversity for “multipath”- usually, two (or more) receiving antennas, separated by λ/2- selection diversity vs. scanning diversity vs. combining diversity- “adaptive antenna arrays” or “smart antennas”
● Forward error correction- transmit redundant data bits - “coding gain” provides “fading margin”- not very effective in slowly varying channels or long fades
● Automatic Repeat Request (ARQ) protocols- retransmission protocol for blocks of data (e.g. packets) in error- stop-and-wait, go-back-N, selective-repeat etc.
Combating the Wireless Channel Problems
24
A Digital Radio Link
SourceCoder
ChannelCoder Modulator
MultipleAccess
PowerAmplifier
carrier fc
SourceDecoder
ChannelDecoder
DemodulatorMultipleAccess
RFFilter
carrier fc
& Equalizer
antenna
antenna
Des
tin
atio
ns
transmittedsymbol stream
Multiplex
Demultiplex
received (corrupted)symbol stream
SourceCoder
SourceDecoder
RADIOCHANNEL“Highly variable b/w”
“Random & Noisy”
“Limited b/w”
“Spurious disconnections”
-
25
Evolution of Mobile & RF Wireless Systems
● First Generation: Analog - Voice- analog modulation- cellular phone (AMPS) with manual roaming- cordless phones- packet radio networks
● Second Generation: Digital - Voice & Data- digital modulation- cellular & PCS phones with seamless roaming, integrated paging
(IS-54, IS-95, IS-136, GSM etc.)- digital cordless, multi-zone cordless, wireless PBXs- wireless data LANs (802.11), MANs (Metricom), WANs (CDPD, ARDIS,
RAM)
● Third Generation: Digital - Multimedia- unified digital wireless access anytime, anywhere- voice, data, images, video, music, sensor etc.
26
● Introduction to Wireless Communication Systems- system and medium characteristics- technological evolution in the design of wireless communication systems
● Wireless Systems Design- digital communications: modulation, coding, multiple access- example designs
● VLSI Circuits for Wireless Systems- micro-architecture for wireless systems-on-a-chip- direct-conversion for digital communications using VLSI
● Design technology for Wireless Systems- design entry, validation, and analysis tools
● Pre-designed Core Blocks and IP Issues for Wireless● Future Outlook and Conclusions
Tutorial Outline
-
Part 2-A:
Wireless Systems Design:
Basics
28
Simplified View of a Digital Radio Link
SourceCoder
ChannelCoder Modulator
MultipleAccess
PowerAmplifier
carrier fc
SourceDecoder
ChannelDecoder
DemodulatorMultipleAccess
RFFilter
carrier fc
& Equalizer
antenna
antenna
Sou
rces
Des
tin
atio
ns
transmittedsymbol stream
Multiplex
Demultiplex
received (corrupted)symbol stream
SourceCoder
SourceDecoder
RADIOCHANNEL“Highly variable b/w”
“Random & Noisy”
“Limited b/w”
“Spurious disconnections”
-
29
● Modulation: maps sequence of “digital symbols” (groups of n bits) tosequence of “analog symbols” (signal waveforms of length TS)
● Demodulation: maps sequence of “corrupted analog symbols” tosequence “digital symbols” - e.g. maximum likelihood decision
Digital Modulation & Demodulation - A “User’s View”
...(0110) (0111) (0000)...
n-bit digital symbol
Set S = {S1, S2,... SM} of M waveforms of length TS
MOD
S1
S2
SM
t=0 t=TS
CHANNELnoise, fading, etc.
DEMOD...(0110) (0111) (0000)...
TS-long analog symbolcorrupted
best effort output
n = floor(log2 M)
e.g. obtained by distinctively modifying the phaseand/or frequency and/or amplitude of a carrier
M=2 is “binary modulation”Otherwise, M-ary modulation
30
● Coherent or Synchronous Detection: process received signal with a localcarrier of the same frequency and phase
● Noncoherent or Envelope Detection: requires no reference wave
Coherent Non-Coherent
Phase-shift keying (PSK) FSK
Frequency-shift keying (FSK) ASK
Amplitude-shift keying (ASK) Differential PSK (DPSK)
Continuous phase modulation (CPM) CPM
Hybrids Hybrids
Commonly Used Digital Modulation Techniques
-
31
● Provides low bit error rates (BER) at low signal-to-noise ratios (SNR)● Occupies minimal bandwidth● Performs well in multipath fading● Performs well in time varying channels (symbol timing jitter)● Low carrier-to-cochannel interference ratio● Low out of band radiation● Low cost and easy to implement● Constant or near-constant “envelope”
- constant: only phase is modulatedmay use efficient non-linear amplifiers
- non-constant: phase and amplitude modulatedmay need inefficient linear amplifiers
No perfect modulation scheme - a matter of trade-offs!
Selecting a Modulation Schemes
32
● Power Efficiency (or, Energy Efficiency)- ratio of signal energy per bit to noise power spectral density required
required at the receiver for a certain BER (e.g. 10-5)
- measures ability to give low BER at low signal power levels- impacts battery life!
● Bandwidth Efficiency- ratio of throughput data rate to bandwidth occupied by modulated signal
- measures ability to accommodate data within a given bandwidth
● Often a trade-off between power and bandwidth efficiencies, e.g.- adding redundancy (FEC) reduces bandwidth efficiency, but reduces the received power required for a given BER- modulation schemes with higher values of decrease but increase
for a given BER
ηP
ηP Eb N0⁄=
ηB
ηB R B⁄ bps/Hz=
M B Eb
Metrics to Evaluate Modulation Schemes
-
33
● At 0.001% BER and a fixed transmission bandwidth:
● BPSK and QPSK has the same energy efficiency but QPSK has two timesmore bandwidth efficiency (bit rate gain factor) than BPSK.
● The drawback of using QPSK is in the poor achievable energy efficiencyin practice => use GMSK to achieve a bandwidth efficiency of 1.25 withBT = 0.3.
a. Relative to BPSK (M=2)
MPowerPenalty
Factora
Bit-RateGain
Factora
EnergyPenaltyFactora
2 1 1 1
4 2 2 1
8 4.7 3 1.56
16 10 4 2.5
32 20.7 5 4.1
64 42 6 7
Choice of a Modulation Scheme
34
● Signal set represents points in a vector space
● Vector space defined by a set of orthonormal (i.e. orthogonal andwith unit energy) basis signals
- is the dimension of the vector space
● Every can be expressed as a linear combination of basis signals
● Example: BPSK signals and
can be represented as:
S s1 t( ) s2 t( ) … sM t( ), , ,{ }=
N M≤φ j t( ) j 1 2 … N, , ,={ }
N
si t( )
s1 t( ) 2Eb Tb⁄ 2π f ct( ) 0 t Tb≤ ≤cos=
s2 t( ) 2Eb Tb⁄ 2π f ct π+( )cos=
φ1 t( ) 2 Tb⁄ 2π f ct( )cos=
s1 t( ) Ebφ1 t( )=
s2 t( ) Eb– φ1 t( )=
A Geometric View of Modulation
-
35
● Geometric representation of is called the Constellation Diagram,e.g. for BPSK:
● Bandwidth occupied by the modulation scheme decreases as thenumber of signal points / dimension increases
- a densely packed modulation scheme is more bandwidth efficient- however, bandwidth increases with dimension
● Probability of bit error is a function of the distance between the closestpoints in the constellation diagram
- a densely packed modulation scheme is less power efficient
S
EbEb–I
Q
N
The Constellation Space
36
● M-ary QAM
● M-ary PSK
I
Q
M=16d2 6
M 1–--------------Es=
d
I
Q
M=4d 2 Es
πM-----sin=
d
Some Examples...
-
37
● Ref.: Wireless Information Networks by Pahlavan & Levesque, 1995
Comparison of Several Modulation Methods
38
Simplified View of a Digital Radio Link
SourceCoder
ChannelCoder Modulator
MultipleAccess
PowerAmplifier
carrier fc
SourceDecoder
ChannelDecoder
DemodulatorMultipleAccess
RFFilter
carrier fc
& Equalizer
antenna
antenna
Sou
rces
Des
tin
atio
ns
transmittedsymbol stream
Multiplex
Demultiplex
received (corrupted)symbol stream
SourceCoder
SourceDecoder
RADIOCHANNEL“Highly variable b/w”
“Random & Noisy”
“Limited b/w”
“Spurious disconnections”
-
39
Multiple Access
● Fundamental problem
How to share the Time-Frequency spaceamong multiple co-located transmitters?
Time
Shared Time-Frequency Subspace
Freq
uen
cy
All
ocat
edS
pec
tru
m
40
Basestation versus Peer-to-Peer Models
Basestation Peer-to-Peer(ad hoc network - fully-connected vs. multihop)(infrastructure - centralized)
-
41
Approaches to Wireless Multiple Access
Sharing of Time-Frequency Space
Static (Fixed) Assignment Demand-based Assignment
Contention-based
Conflict-free
e.g. Time-division &Frequency-division
e.g. Token-passing &Polling
e.g. ALOHA, PRMACarrier-sensing
“Connection Oriented”
“Packet Oriented”
Random Access Scheduled Accesse.g. DQRUMA
Slotted-timevs. Non-slotted time
Controlled RandomAccess
42
Frequency Division Multiple Access (FDMA)
● Assign different frequency bands to individual users or circuits- frequency band (“channel”) assigned on demand to users who request service- no sharing of the frequency bands: idle if not used- usually available spectrum divided into number of “narrowband” channels
symbol time >> average delay spread, little or no equalization required- continuous transmission implies no framing or synchronization bits needed- tight RF filtering to minimize adjacent band interference- costly bandpass filters at basestation to eliminate spurious radiation- usually combined with FDD for duplexing
f2
f2’ f1
f1’
Time
Freq
uen
cy
f1
f1’
f2
f2’
-
43
Time Division Multiple Access (TDMA)
● Multiple users share frequency band via cyclically repeating “time slots”- “channel” == particular time slot reoccurring every frame of N slots- transmission for any user is non-continuous: buffer-and-burst
digital data & modulation needed, lower battery consumption- adaptive equalization is usually needed due to high symbol rate- larger overhead - synchronization bits for each data burst, guard bits
guard bits for variations in propagation delay and in delay spread- usually combined with either TDD or FDD for duplexing
TDMA/TDD: half the slots in a frame used for uplink, half downlinkTDMA/FDD: identical frames, with skew (why?), on two frequencies
Time
Freq
uen
cy
slot 2
slot 6 slot 1
slot 5
frame i frame i+1frame i-11 2 5 6
Sync Data Guard
44
● GSM handles time dispersion widths up to 18-20 µs... i.e. 5 bits of ISI- transmission bandwidth >> channel coherence bandwidth
● IS-54 handles time dispersion up to 40 µs... i.e. 2 symbols might interfere- less complex equalizer needed than GSM|
● Need equalization indoors at rates > 2 Mbps (DECT is only 1.152 Mbps)
GSM IS-54 DECT PHS
Bit rate 270.8 kbps 48.6 kbps 1.152 Mbps 384 kbps
Carrier spacing (b/w) 200 kHz 30 kHz 1.728 MHz 300 kHz
Time slot duration 0.577 ms 6.7 ms 0.417 ms 0.625 ms
Slots/frame 8 (or 16) 3 (or 6) 12 4
FDD or TDD? FDD FDD TDD TDD
% payload in time slot 73%adaptive equalizertraining overhead
80%adaptive equalizertraining overhead
67%system control
overhead
71%
Modulation GMSK π/4 DQPSK GMSK π/4 DQPSK
Adaptive equalizer required required none none
Some TDMA Systems
-
45
Hybrid FDMA/TDMA
● “Pure” TDMA with single frequency band is undesirable- require tight timing tolerances
● Most TDMA systems actually employ hybrid FDMA/TDMA- multiple carriers with multiple channels per carrier- channel == (frequency band, time slot) tuple- may do “frequency hopping” on a frame-by-frame basis to combat
multipath interference (Time Division Frequency Hopping: TDFH)increases system capacity
(f5, t1)
(f3, t4)(f1, t1)
(f2, t3)
Freq
uen
cy
frame i frame i+1frame i-1
f1f2
f3
f4
f5
f6
t1 t2 t3 t4
46
Code Division Multiple Access (CDMA)
● Multiplexing in the Code Space- multiple transmitters occupy the same frequency-time space- transmissions encoded with codes with very low cross-correlation- receiver retrieves a specific transmission with its corresponding code
● CDMA may be combined with TDMA or FDMA
c1
c3c5
c2
Frequency
Cod
e
-
47
Spread Spectrum Signalling
● Spread Spectrum is the most common CDMA encoding technique- originally developed for military communication systems- “spread” the signal over a much larger bandwidth than the minimum- signal appears pseudo-random with noise like properties- uniform small energy (W/Hz) over a large bandwidth hides the signal
⇒ Note: use of spread-spectrum does not imply use of CDMA● Spreading is done using a unique code● Receiver does the “despreading” by using a time-synchronized
duplicate of the spreading code● Inefficient for a single user, but multiple users can share band● Inherent interference rejection capabilities (e.g. narrowband interferers)● Resistant to multipath effects
- delayed versions appear as uncorrelated noise- can even exploit multipath signals by combining them
● Processing Gain: Gp = Bspread / Bsignal- indicates improvement in signal-to-interference ratio due to spreading
48
What is Spread Spectrum Communication?
Wide BandAnti-jam -> high capacity CDMACombats multipath -> diversityLPI -> PrivacyLPD -> low power density
PGf spread
f bit---------------------=
despread signalspread interference
fdata
Adata
Ai,received
spectral density
frequency
unspread signal
spread signal
interference
fdata
fspread
frequency
spectral density
Adata
Aspread
Ai
frequency
spectral density
TRANSMIT RECEIVESpreading Code run-ning at .f spread
-
49
CDMA Using Direct Sequence (DS) Spread Spectrum
● Spread the narrowband data by multiplying with a wideband pseudo-random code sequence
- bits sampled, or “chipped”, at a higher frequency (e.g. 1.228 Mcps in IS-95)- signal energy is “spread” over a wider frequency (e.g. 1.25MHz in IS-95)- code sequences have little cross-correlation (orthogonal)- code sequences have little correlation with shifted versions of self
● Received signal multiplied by synchronized replica of the code sequence● Energy of each “chip” is accumulated over a full data bit time
X
transmitted signal
PN Sequence (code)
digital data
01101011
X =
01101011
Intended receiver
10110010
X =
Other receiversNoise - can be low pass filtered
Recovered signal
Chip
50
CDMA Using Frequency Hopping Spread Spectrum
● Transmission frequency is periodically changed- available spectrum divided into bands with central frequencies as carriers- sequence of data bursts with time-varying pseudo-random carrier frequencies- time duration between hops is the hop duration or hopping period Th- bandwidth of a frequency band in the hopset is the instantaneous b/w B- bandwidth of spectrum over which hopping occurs is total hopping b/w Wss- processing gain is Wss/B
● Fast frequency hopping: more than one hop during each transmitted symbol● Slow frequency hop: one or more symbols transmitted in a hop
Freq
uen
cy
f1f2
f3
f4
f5
f6
channel #1channel #2
-
51
Contention-based Multiple Access
● Many transmitters access a channel with no or minimal coordination● Transmission in bursts of data● Collisions may happen: need ACK or NACK with retransmission
- delays induced- lower spectral efficiency
● Three categories: random access, scheduled access, hybrid access
Packet B Packet C
Packet A
One PacketTime (τ)
Vulnerable Period (2τ)
Time
Transmitter # 1
Transmitter # 2
52
● Ethernet uses contention-based medium access...
● Following attributes make contention-based multiple access interestingwith wireless:
- “carrier sensing” is much costlier in wireless20-30 µs
- can’t listen while transmittingtherefore cannot detect collisions
- what matters is the collision at a receiver... but the transmitter can’t sense the channel at the receiver!
- effects of spatial distribution of wireless nodeshidden terminal problemexposed terminal problemnear-far problem (capture effect)
Contention-based Multiple Access in Wireless Systems?
-
53
IEEE 802.11 MAC
● Support for multiple PHYs: ISM band DSSS and FHSS, IR @ 1 and 2 Mbps● Efficient medium sharing without overlap restrictions
- multiple networks in same area and channel space- Distributed Coordination Function: using CSMA /CA- based on carrier sense mechanism called Clear Channel Assessment (CCA)
● Robust against interferers (e.g. co-channel interference)- CSMA/CA+ACK for unicast frames with MAC level retransmission
● Protection against Hidden Terminal problem: Virtual Carrier Sense- via parameterized use of RTS/CTS frames with duration information
● Provision for Time Bounded Services via Point Coordination Function● Configurations: ad hoc & distribution system connecting access points● Mobile-controlled hand-offs with registration at new basestation
distribution systemad hoc network
infrastructure network
54
IEEE 802.11 MAC (contd.)
● CSMA/CA: direct access if medium free for > DIFS, else defer & back-off
source
other
DIFS
DATA
DIFS
SIFS
PIFS
defer access
DATAselect slot & decrementback-off as long as idle
contentionwindow
● CSMA/CA + ACK: receiver sends ACK immediately if CRC okay- if no ACK, retransmit frame after a random back-off
source
other
DIFS
DATA
SIFS
defer access
DATAselect slot & decrementback-off as long as idle
contentionwindow
receiver ACK
DIFS
● RTS/CTS with duration: distribute medium reservation information- also used in the defer decision
-
55
● Replace single high power transmitter covering the entire service areawith lots of low power transmitters (basestations) each covering afraction of the service area (cell)
- mobiles in sufficiently distant basestations may be assigned identicalchannel (frequency, time slot, & code)
- system capacity may be increased without adding more spectrum
● Major conceptual breakthrough in spectral congestion & user capacity- required relatively minor technological changes
frequency reuse & co-channel interferencechannel allocationhand-offs
Cellular Systems
Pre-Cellular Post-CellularMSC PSTN
56
Space Division Multiple Access (SDMA)
● Control radiated energy for each user in space- spot beam antennas (sectorized antennas)- different areas served by different antenna beams may use same
frequency (CDMA, TDMA) or different frequencies (FDMA)- in future, adaptive antennas
-
Part 2-B:
Wireless Systems Design:
Standards, Design Issues, and Examples
58
The Un-wired World
Wireless Communications
Amateur Industrial Consumer Business Military/Aero Long-Haul
Automotive Monitoring
Cordless Cellular Paging WPABX WLAN PMR/SMR Mobile Data
- IVHS- GPS
- AMR- Control
Analog Digital Analog Digital
- CT-0- CT-1- CT-300
- DECT- CT-2- PHP- USCT- ISM
- AMPS- ETACS- NMT450- NMT900- NMT-0- Comvik- JTACS
- GSM- IS-54- IS-95- IS-136- RCR-27
- POSCAG- ERMES- SSB
- DECT- CT-2- PHP- USCT- ISM
- 802.11- DECT- HIPerLAN- ISM
- ARDIS- Mobitex- Omnitracs- Cellular/CDPD
ESMR
Conv
- MIRS- TETRA
PCN/PCS
- DCS1800- PHP- LEO
- FPLMTS- UMTS- RACE
- Metricom
-
59
Evolution of PCS Technologies, Systems, and Services
Cellular
Paging
Cordless
Wide Area Data
WLANs
High-tier PCS
Low-tier PCS
WLANs
Satellites?Macro-cellular
Micro-cellular
Messaging
Phone point
PABX
Cordless
Micro-cells
Macro-cells
WLANs
?
?
?
?
PAST PRESENT FUTURE
GrandUnification?
60
AMPS System (First Generation Analog)
● Two 25 MHz bands: 824-849 MHz upstream, 869-894 MHz downstream● Divided into 30 MHz frequency bands - pair needed for a duplex channel● FDD+FDMA: 834 duplex channels● 7-way frequency reuse (18 dB min. signal-to-co-channel interference)● Two types of channels: control and voice channels● Network controlled handoff - MSC becomes a bottleneck● Capacity constraints - 40-50 connections per cell● No on-air privacy, fraud a major problem
MSC(MTSO)
BS
BS
BS
BS
BSBS
MS
HLR VLR AUC
OMC
PSTN
databases
SS7Proprietary
AMPS CommonAir interface
MSC(MTSO)
mobilitymanagement
-
61
GSM System (Second Generation Digital)
● Two 25 MHz bands: 890-915 MHz upstream, 935-960 MHz downstream● Divided into 200 KHz frequency bands - 125 in each direction● FDD+TDMA+FH: 8 slots/4.615 ms frame, 270.833333 kbps raw, 22.8 kbps/user● Frequency hopping to combat multipath problems● Two types of logical channels: traffic channels and control channels● Mobile assisted handoff - BSC reduce the load on MSC● Features: subscriber identity module and on-air privacy● Services: telephone, data or bearer, short messaging
BSC
BSC
MSC(MTSO)
BTS
BTS
BTS
BTS
BTSBTS
MS
HLR VLR AUC
OMC
PSTN
databases SS7A InterfaceAbis Interface
GSM RadioAir interface MSC
(MTSO)
62
● Packet data network overlay on AMPS - same 30 KHz channels● Data packets are sent over unused voice channels● Channel hopping ensures non-interference with voice● Raw data rate is 19.2 kbps Reed-Solomon coded - real rate much less● Broadcast downlink, Data Sense Multiple Access (DSMA) MAC on uplink● Variety of connection-less, connection-oriented, and multipoint services● Reliable and unreliable classes - handling over radio link● In particular, IP (Internet Protocol) datagram connectivity● Mobile controlled handoff, registration at basestation to reduce paging● “Home MD-IS” tunnels incoming traffic to current MD-IS
Cellular Data Packet Network (CDPD)
M-ESMD-BS
MD-BS
MD-IS IS
Data n/w(internet)
M-ES
MD-BS
MD-BS
MD-IS
F-ES
ISmobility
managementconnection-less
router
-
63
Designing Mobile Wireless Multimedia Systems
WIRED NODE
PSTN
modem
ethernettransceiver
• antenna• RF + A/D• digital transmitter/receiver• channel codec• source codec• network protocols
BASE STATION
WIRELESSNODE
ETHERNET
PHONE
http://www.N
http://www.N
64
Generic Mobile & Wireless System Architecture
Radio, IR
Data Link
OS & Middleware
Network
Application & Services
Modulation Schemes
Multiple Access
ReroutingImpact on TCP
Link Error Control
Channel Coding
Channel Allocation
Location Tracking
Disconnection Mgmnt.Power Management
Partitioning
QoS Management
Source Coding & DSP
RF/Optical Circuits
Context Adaptation
-
65
Radio Design Challenges
● High speed digital processing● High performance in Eb/N0● Low complexity● Energy efficient (mW/MSps or nJ/OP)
Algorithm Fixed Point
Digital ModemIC Architecture
RF Front-endArchitecture
Partition
66
Partition between Analog and Digital Processing
Analog RF Analog IF Baseband Digital BasebandSignal Processing
ConverterTransceiver Signal ProcessingAnalog-to-Digital
Digital IFTransceiver
Analog-to-DigitalDigital BasebandSignal Processing
Converter
Analog RF Signal Processing
IF
● Advantagesallows for adaptability with little component replacementsachieves Eb/N0 performance close to optimum (coherent BPSK)parameterizable to provide ease of redesign and upgrade
● Challengesdigital circuits operate at IF signal rate rather than baseband ratedigital implementation can be more complex to minimize loss in Eb/N0
-
67
● Low complexity, high speed, adaptable, and energy efficienttransceiver in a single-chip
A Direct-Sequence Spread-Spectrum Radio Modem
CARRIER
LOOP
CLOCK
LOOP
PN VGA AMPLPF
LPFA/D
BPF
LNAAGC
Spread Data
6RECOVERY RECOVERY
GENERATORTX
POWER CONTROL
FREQ CNTRL
PN
LOOPAcquisition
Decision
To SIR Est. Recv. Data
TX DataCarrier DetectCODE
SELECTPROCESS
GAIN
FREQUENCYSYNTHESIZER
Ack.: C. Chien & R. Jain, UCLA
68
● Challenge: Implement a complete coherent receiver on a single chip
● Circuit Design Issuesfinite wordlengthparameterizabilitycritical path optimizationcomplexity reduction
● System Design Issuesmaintain stability in three feedback loops.
Transceiver Chip Design Issues
-
69
Costas Loop Filter Optimization
0 510 15
20 2530 35
0
10
20
30
40−80
−60
−40
−20
0
20
INPUT Ec/N0= -17 dB
N1
N2
Eb/N
0 (d
B)
Coefficient as powers of two shifts:
Optimization Criteria:min max N1 N2,( )( ), Eb N0⁄ 10 0.5±≥
D
C2 2N2–=
C1 2N1–= C1
C2
N1
N2
5 10 15 20 25 30
5
10
15
20
25
30
10 dB
9 dB 0 dB
-10 dB
Ack.: C. Chien &R. Jain, UCLA
70
IF Wordlength Optimization
Out
put E
b/N
0 (d
B)
IF Input Quantization Size (Bits)0 5 10 15
0
10
20
30
40
IF Input Quantization Size (Bits)
Com
plex
ity In
crea
se (
%)
❥ Minimize IF quantization size reduce complexity and powerdissipation at required throughput.
min N( ), Eb N0⁄ 10 0.5±≥
N
DD
FS
N
N
-17 dB
-11 dB
0 dB
10 dB
4 8 12 160
100
200
300Complexity increase in receiverSample rate through the multiplier50.8 MHz sample rate requirement
0
50
100
150M
ultip
lier
Sam
ple
Rat
e (M
Hz)
Ack.: C. Chien &R. Jain, UCLA
-
71
PN-Acquisition: Complexity/Performance Trade-off
Clock
Generation
EnergySlope
Detection
PN-CodeGenerator
❥ 800 Gates
❥ Nc * Nif * 12 Gates + 800
Nc = #chips/bit
Nif = IF Quantization
❥ 10 000 Gates with N c =127 and Nif = 6
ReceivedPN
Timing
N-TapMatched
Filter
Energy
Detection
Clock
Generation PN-CodeGenerator
Match Filter Acquisition
Serial Acquisition
Timing
ReceivedPN
● PN acquisition: correlation between the incoming bits and the P/Nsequence of the desired transmitter
72
A Single-Chip 1.2 Micron CMOS DSSS Radio Modem
❥ Low Complexity -- 51 K Transistors
❥ High Power Efficiency -- 21.7 mJ/MSample
❥ Maximum Chip Rate -- 12.7 Mchips/sec
❥ Scalable Performance -- Data Rates andProcessing Gain: 100, 200, 400, 800 kbps at12, 15, 18, 21 dB, respectively
Performance
DIGITAL IF RECEIVER
-
+
LAT
E P
N
IF S
AM
PLIN
G C
LK
PN
TR
AC
K C
ON
TR
OL
CLOCK RECOVERY
INTEGRATEDUMP I1
INTEGRATEDUMP Q1
IFSIGNAL
DATAOUT
COSTAS LOOP
INTEGRATEDUMP I1
INTEGRATEDUMP Q1
DD
FS
LOOPFILTER
PHASEDETECTOR
INTEGRATEDUMP I2
INTEGRATEDUMP Q2
LOOPFILTER NCO
CHIPDELAY
EA
RLY
PN
50.8 MHz 12.7 MHz
50.8 MHz 12.7 MHz 406.4 MHz
100 kHz -12.7 MHz
PN-ACQUISITIONLOOP
DIFFERENTIALDECODER
(100-800) kHz
DIGITAL BASEBAND TRANSMITTER
DATAINPUT SPREAD
DATA
GOLD CODEGENERATOR
(PNGEN)
DIFFERENTIALENCODER
Ack.: C. Chien & R. Jain, UCLA
-
73
Integration of Radio into a System
CPU
Keyboard
Memory and MassStorage
Camera
DT FrameGrabber
Custom FrameGrabber
ProximRangeLAN2
Adaptive Direct SequenceSpread Spectrum Radio
Video CodecFPGA
RF Front-endDSSS IF modem,Packet Interface,Adaptation Interface,Analog-Digital Conversion
Single-chip DSSSModem IC
Ack.: C. Chien & R. Jain, UCLA
74
Example 1: UCLA’s Wireless Multimedia Node
Wireless
Channel
Serial Data HostInterface Interface
HostCPU
Frame
Buffer 12-bit RGB
16-bit
CompressedData Interface
Host
YUV
Interface
Control
Buffer
Video
Network Interface Chip
Video
Capture
VGA
Controller
VideoCodec
Modem
PC-1
04 B
us
PacketBuffer
-
75
Example 2: Bell Labs’ SWAN Wireless ATM System
FHSS RF XCVR
XCVR Interface
Host Interface
CPU
Perip
he
ral
Inte
rfac
e
FHSS RF XCVR
XCVR Interface
Bus Interface
CPU
Perip
he
ral
Inte
rfac
e
FHSS RF XCVR
XCVR Interface
Bus Interface
CPU
Perip
he
ral
Inte
rfac
e
BASESTATION
FHSS RF XCVR
XCVR Interface
Host Interface
CPUPe
riph
era
lIn
terfa
ce
FAWNFlexibleAdapter
BASESTATION MOBILE END-POINTS
Mobile
SY
ST
EM
S
/W
ForWirelessNetworking
CPU
BACKBONEATM
ADAPTERCARD
To Antenna
Connection Switching
Mobility Management
Drivers for Adapter Cards
MAC
PHY
ATM SWITCH
Notebook
Personal
ET
HE
RW
AR
E
Communicator
PersonalMultimediaTerminal
Lucent
mani
76
FAWN Reconfigurable Wireless Adapter
Dimensions 10.8 cm (W) x 1.9 cm (H)x 11.4 cm (D)
Power Consumptionof FAWN
2.0 W
Power Consumptionof radio transceiver
0.6 W (receive)1.8 W (transmit)
Firmware resources 20 MIPS, 4 MByte
Reconfigurablehardware resources
10000 Gates equivalent
PCMCIA
Dual Port RAM
PCMCIAInterface
PeripheralInterface
RF Modem
ADC
ModemController
UARTSRAM
Control PAL
ARM CPU
to hostprocessor
-
77
● Simple hardware- peripheral card + FAWN adapter
● Multimedia interface- audio, graphics, soft keys, bar code
● Dumb end-point for “network-hosted mobile services”
Example 3: Personal Mobile Terminal
LCD display
Bar code scannerScanner switch
Soft keys
PRESS TO SCAN↓ ↓
SC
AN
NE
R
microphone
Personal Terminal 6808
78
Example 4: Berkeley’s Infopad Project
● Infopad: low power wireless multimedia terminal- no local general purpose processing (“dumb terminal” model)- speech and pen controlled user interface- audio, video, and text/graphics streams to the terminal
● Infonet: network infrastructure for Infopads- based on cell, pad, and type servers
● Medley Gateway: transport & coding of video, audio, & graphics to Infopad● http://infopad.eecs.berkeley.edu/
-
79
Infopad Terminal Architecture
● References:1. http://infopad.eecs.berkeley.edu/research/terminal2. [Narayanaswamy96] Narayanaswamy et. al., “Application and
Network Support for Infopad,” in IEEE Personal Communications, April ‘96
PlesseyDownlink
Radio
ProximUplinkRadio ARM
SubsystemRX/TX
Interface
Low Power Infopad Bus
LCDIF
PENIF
AUDIOIF
VIDEOIF
250 Kbps
1 Mbps
Infopad
ucb
Color
Subsystem mW
Radios 1490
ARM 877 - 2475
Custom H/W 137 - 297
B&W LCD 550 - 3800
Color LCD 3900
Pen Digitizer 150
Codec 50
Voltage Converters 2411
Crystals 75
Test H/W 629
Total 9.9W - 15W
Color
80
Example 5: Xerox PARCTAB
● Extremely portable mobile unit- 7.8x10.5x2.4 cm3, 215 gm, 6.2x4.5 cm2 & 128x64x1 touch screen, 3 buttons- IR communication at 19.2 kbaud with CSMA MAC, PWM modulation- 12 MHz Signetics 87C524/528 CPU, 128K memory
● Basestation transceiver (on ceiling of a room nanocell)- IR with variable data rate: 9.6K, 19.2K, 38.4K; CSMA MAC- 38.4K serial link up to 30m with 10 unit daisy chain capability- performs coding/decoding, buffering, link level protocol checks- connected to LAN via serial port of nearby workstations
● Remote host based applications, proxy agents (per tab),and gateways (datagram service to tab)
● http://www.ubiq.com/parctab
Tab Basestation
-
81
Design Trade-offs in Wireless Nodes
● Computation-communication trade-off affects:- terminal cost- service cost
Term
ina
l Co
mp
lexi
ty
Communication Needs &
Laptops
PDAs
Terminals
Comp
utation
Storag
e Palmtops
Notebooks
Infrastructure Dependence
82
● Adaptive process gain improves throughput
● Multipath fading requires equalization
● Bit rate limited by equalizer complexity
Throughput can be improved by physical layer processing
Design Issues
-
83
Adaptive Process Gain Improves Throughput
−15 −10 −5 0 50
20
40
60
80
100
PG = 12 dB
PG = 15 dB
PG = 21 dB
Thr
ough
put (
kbps
)
Signal-to-Interference Ratio (dB)
Desired
Achieved
84
Top
Total Radio Power = 11.87W
Total IF Power = 6.118 W
Total RF Power = 5.75W
Transmit
Receive
Freq. Synth.
AGC
Control
Bottom
Power Reg.
RF Processing: Power Dissipation
-
85
IF/Baseband Processing: Power Dissipation
Analog IF
DSSS
DSSS
Packet InterfaceControl
Top Bottom
Power Regulation
Total Radio Power = 11.87W
Total IF Power = 6.12W
Total RF Power = 5.75 W
Note: Power budget figures includes power dissipation from regulation inefficiencies.
86
Multipath Fading Requires Equalization
0t
102
3
t0 t2 t1 t3
τ
• τ > Its / 10 ⇒ ISI causes degradation in BER and willrequire equalization
• τ is a function of transmit power and cluttering in theenvironment
Dense Foliage Urban Clutter
Transversalequalizer
Linearfeedbackequalizer
TransversalequalizerLinear
feedbackequalizer
Linearfeedbackequalizer
Transversalequalizer
Nointerference
31 taps in transversal equalizer
16 feedforward and 15 feedbacktaps in linear feedback equalizer
γ 1No------- fk
2
k∑=
Pro
babi
lity
of e
rror
SNR, db (10 log γ)
10-1
5
2
10-2
5
5
5
2
2
10
10-3
10-4 0 15 20 25 30 35
MobileWirelessChannel
}
-
87
● Improved performance using MLSE over DFE/FFE
- short training sequence O(100) vs. O(1000) bits
● But, MLSE has high complexity and processing requirements- complexity ∼ O (4 τ Rs M τRs)- e.g. M=2, τ = 3ms, Rs = 2 Mbaud = 2 Mbps
then, complexity ~ 1600 operations ~ 30k gatesprocessing ~ 1600 * 2MHz = 3.2 GOPS
MlSE simulation
Destination-feedbackequalizer
Correct bits
Detected bitsfed back
MlSEbounds
Nointerference
SNR, dB (10 log γ)
Pro
babi
lity
of e
rror
1
10-1
10-2
10-3
10-40 5 10 15 20 25
fed back
Bit Rate Limited by Equalizer Complexity
88
Physical Layer Processing to Improve Throughput
preamble header DATA
throughput = Tpreamble + Theader + Tdata
Tdata
max(throughput) ⇒ min(Tpreamble), min(Theader)
Theader is protocol dependent• TCP/IP header• ATM header• MAC/link layer header
Tpreamble is physical layer dependent• time to acquire / capture packet• settling time of LO frequency
capture-time accumulatesin multihop networks
Aggressive signalprocessing canreduce this!
-
89
Understanding Energy Efficiency
P = α C V2 f
“Continuous”“Event-Driven”
Latency is ImportantOnly Throughput is
Important
Reduce V
e.g., Speech CodingVideo Compression
e.g., X Display Server
(Burst throughput)
Increase h/w andalgorithmic concurrency
Make f low or 0Shutdown when
Disk I/OReduce αCEnergy efficient s/w CommunicationSystem partitioning
inactive
Efficient Circuits & Layouts
90
Voltage-Parallelism Trade-Off for Low Power
● Increased parallelism & reduced voltage can increase energy efficiency- more processors or functional units or pipelining- compiler techniques are the key
● Architectural bottlenecks:- degradation of speed-up- capacitance overhead due to increased communication
1.0 1.5 2.0 2.5 3.0Supply Voltage, V
1.0
3.0
5.0
7.0
Nor
mal
ized
Del
ay
1 2 3 4 5 6 7 8Parallelism, N
1.0
3.0
5.0
7.0 Ideal Speedup
Spe
edup
-
91
● Radios consume a significant fraction of node powerLucent’s WaveLAN: 23 dBm 915MHz radio network interface
transmit = 3Wreceive = 1.48Wsleep = 0.18W
GEC Plessey DE6003: 20 dBm, 2.4GHz radio transceivertransmit = 1.8Wreceive = 0.6Wsleep = 0.05W
Newton PDAactive = 1.2Wsleep = 0.164W
Magic Link PDAactive = 0.7Wsleep = 0.3W
Radios need to be actively managed for low powervia energy efficient wireless link protocols.
Energy Efficiency is not just an Architecture Issue!
92
Low Power Design for Wireless
µProc
DSPs
HDD
MAC Layer
Link Layer
Radio
Display
Protocols
Modem
Protocols
• Hardware has been addressed• Low power CMOS,• Displays,• Hard drives, etc.
• Low power protocols remain