4G Wireless Access based on Wideband OFDM
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Transcript of 4G Wireless Access based on Wideband OFDM
The Evolution of TDMA to 3G & 4G Wireless Systems
Nelson Sollenberger
AT&T Labs-Research
Wireless Systems Research Division
AT&T Wireless Services
• TDMA
– European GSM over 250 million
– North American TDMA ~ 50 million
– Japanese PDC ~ 50 million
• CDMA
– North American CDMA ~ 60 million (including S. Korea)
AT&T serves over 14 million subscribers with digitalTDMA technology and some remaining analog technology,and provides packet data service with CDPD technology
Other TDMA operators - Rogers AT&T - Cingular (SBC & BellSouth) - throughout Mexico, Central & South America
Cellular Telephony Handsets
Nokia5160
EricssonPD 328
MotorolaStarTAC®ST7790 Phone
Nokia8860
Various TDMA phones available today
TDMA parameters
• 30 KHz channels (like analog & CDPD)
• 20 msec speech frames
• 24.3 kbaud symbol rate
• 3 time-slots/users
• 7.4 kbps ACELP speech coding
• 1/2-rate channel coding on important bits interleaved over 2 bursts in 40 msec
• Differential pi/4-QPSK modulation
TDMA Capacity Roadmap
Reuse N = 7 N = 5 N = 4
Dual band base •Operation at 800 or 1900 MHz.
Calls can be set up on either frequency band and handed between them to manage traffic
•Additional spectrum at 1900 MHz adds directly to capacity of cell
Smart Antennas •Base station antennas systems that
use digital signal processing to cancel interference
2000 2001 2002
Base Station Power Control •Base stations only transmit power required to
reach mobile with adequate signal quality resulting in lower interference
Dynamic Channel Assignment •Network automatically assigns radio frequencies
to cell sites for more efficient utilization of frequencies
Discontinuous Transmission•Mobiles transmit only during when user is speaking.
Lowers interference in the system and increases talk time
IS-136 Smart Antenna Test Bed
•Reuse of 3/9 to 4/12, instead of 7/21, approximately 2x capacity
•Two dual polarization uplink antennas, downlink multibeam antenna with 4 - 30° beams
•Shared linear power amplifier unit with Butler matrices
•Real-time downlink power control with beam tracking
Wireless Data Terminals
Nokia 9110
3COMPalm VII
Nokia3G visionSierra PCMCIA
CDPD Modem
The new Ericsson R380 phone, which features wireless data functions
WIRELESS COMPUTING
WIRELESSGROWTH
INTERNETGROWTH
RF & DIGITALTECHNOLOGY
MOBILESOFTWARE
- web access- e-mail- file transfer- location services- streaming audio & video
datarate
1 M
384 k
64 k
9.6 k IS-136
IS-136+
EDGE
WidebandOFDM
Macrocellular Wireless Data Evolution& AT&T’s Roadmap
CDPDGSM
IS-95
GPRS
IS-95+
WCDMA
1995 2000 2005
PDC
5 M
HDR
EDGE TechnologyEnhanced Data-rates for Global Evolution
• Evolutionary path to 3G services for GSM and TDMA operators
• Builds on General Packet Radio Service (GPRS) air interface and networks
• Phase 1 (Release’99 & 2002 deployment) supports best effort packet data at speeds up to about 384 kbps
• Phase 2 (Release’2000 & 2003 deployment) will add Voice over IP capability
GPRS Airlink• General Packet Radio Service (GPRS)
• Same GMSK modulation as GSM
• 4 channel coding modes
• Packet-mode supporting up to about 144 kbps
• Flexible time slot allocation (1-8)
• Radio resources shared dynamically between speech and data services
• Independent uplink and downlink resource allocation
EDGE Airlink• Extends GPRS packet data with adaptive
modulation/coding
• 2x spectral efficiency of GPRS for best effort data
• 8-PSK/GMSK at 271 ksps in 200 KHz RF channels supports 8.8 to 59.2 kbps per time slot
• Supports peak rates over 384 kbps
• Requires linear amplifiers with < 3 dB peak to average power ratio using linearized GMSK pulses
• Initial deployment with less than 2x 1 MHz using 1/3 reuse with EDGE Compact as a complementary data service
GPRS Networks• consists of packet wireless access network and IP-based backbone
• shares mobility databases with circuit voice services and adds new packet switching nodes (SGSN & GGSN)
• will support GPRS, EDGE & WCDMA airlinks
• provides an access to packet data networks
– Internet
– X.25
• provides services to different mobile classes ranging from 1-slot to 8-slot capable
• radio resources shared dynamically between speech and data services
Compact vs Classic• Classic
– 4/12 reuse– continuous downlinks on first 12 carriers– 2.4 MHz x2 minimum spectrum
• Compact– 1/3 reuse in space– frame synchronized base stations– reuse of 4 in time for control channels– partial loading for traffic channels– discontinuous downlinks– 600 KHz x2 minimum spectrum
EDGE Channel Coding and Frame Structure
464 bits1 data block
Convolutional CodingRate = 1/3Length = 7
Puncture Interleave
Burst N
Burst N+1
Burst N+2
Burst N+3 Burst Format
8PSK Modulate
1392 bits 1392 bits
348 bits/burst
348 bits468.75 bits
156.25 symbols/slot
0 1 2 3 4 5 6 7
8 Time Slots
1 Time Slot = 576.92 µs
Tail symbols
3
Data symbols
58
Tail symbols
3
Data symbols
58
Training symbols
26
Guard symbols
8.25
Modulation: 8PSK, 3 bits/symbolSymbol rate: 270.833 kspsPayload/burst: 348 bitsGross bit rate/time slot: 69.6 kbps - overhead = 59.2 kbps user data
20 msec frame with 4 time-slots for each of 8 bearers
EDGE Modulation, Channel Coding & Bit Rates
Scheme Modulation Maximum
rate [kb/s]
Code Rate Family
MCS-9 59.2 1.0 A
MCS-8 54.4 0.92 A
MCS-7 44.8 0.76 B
MCS-6 29.6 0.49 A
MCS-5
8PSK
22.4 0.37 B
MCS-4 17.6 1.0 C
MCS-3 14.8 0.80 A
MCS-2 11.2 0.66 B
MCS-1
GMSK
8.8 0.53 C
EDGE Link Throughput
9
EDGE Compact System Performance
0102030405060708090
100
0 10 20 30 40 50 60 70
Probability throughput < = X per timeslot
X (kb/s)
26 users/sector at 3.5 kbps average load per user
0102030405060708090
100
0 1000 2000 3000 4000 5000
Probability packet delay < = X
X (msec)
% %
0
50
100
150
200
250
300
9 18 27 36 45
single-slot
Multi-slot
Average User Throughput (kb/s)
EDGE Classic Multi-slot Gain
Ave. # of users per sector
EDGE Evolution
• Best effort IP packet data on EDGE
• Voice over IP on EDGE circuit bearers
• Network based intelligent resource assignment
• Smart antennas & adaptive antennas
• Downlink speeds at several Mbps based on wideband OFDM and/or multiple virtual channels
0
5
10
15
20
25
30
35
40
45
50
55
Baseline Enhanced
Nor
mal
ized
voi
ce c
apac
ity
(Erl
ang/
Sit
e/M
Hz)
GSM IS-136 EGPRS/GMSK/F EGPRS/8PSK/H
30 29
50
35
11
7
20
10
7.2 MHz Spectrum
* 1/3 reuse* no shadow fading change due to mobility*Signal-based power control is assumed for baseline EGRPS*SINR-based power control & LI-DCA assumed for enhanced
VoIP over EDGE Bearer Performance• Focused on GMSK full-rate & 8PSK half-rate EDGE channels with
dedicated MAC & random frequency hopping for 7.4 kbps voice coding
*This assumes 30 mph vehicle speed for micro fading* SINR-based power control with adaptive target
Aggressive frequency re-use High spectrum efficiency
Increased co-channel interference
Downlink Switched Beam Antenna
SIGNALOUTPUT
INTERFERENCE
SIGNAL
SIGNALOUTPUT
BEAMFORMERWEIGHTS
Uplink Adaptive Antenna
SIGNAL
INTERFERENCE
BE
AM
FO
RM
ER
BEAMSELECT
Smart antennas provide substantial interference suppression for enhanced performance
Smart Antennas for EDGE• Key enhancement technique to improve system capacity and user experience• Leverage Smart Antennas currently in development/deployment for IS-136 & GSM
EDGE Smart Antenna Processing
Dual Diversity Receiver Using DDFSE for Joint ISI and CCI Suppression
Deinter-leaver
ViterbiDecoder
Soft Output
OutputData
Receiver
Feed-forwardFilter
Symbol Timingand Recovery
DDFSEEqualizer
EqualizerTraining
Rx Rx Filter
Rx Rx FilterFeed-forward
Filter
• Simulation results show a 15 to 30 dBimprovement in S/I with 2 receive antennas
• Real-time EDGE Test Bed supports laboratory and field tests to demonstrate improved performance
Jack WintersHanks ZengAshutosh Dixit
EDGE 2-Branch Smart Antenna PerformanceLaboratory Tests
EDGE MCS-5 with Interference Suppression in aTypical Urban Environment
Blo
ck
Err
or
Ra
te
Signal-to-Interference Ratio (dB)
20 dB SNR
Laboratory results show a 15 to 30 dB improvement in S/I with 2 receive antennas
Improvement with Terminal Diversity and Interference Suppression: User Experience
010
2030
405060
7080
90100
0 10 20 30 40 50 60 70
No Diversity
SimpleDiversityInterferenceSuppression
Prob. (throughput <=X) (%)
X (kb/s)
Typical user throughput increased from 30 to 45 kbps per time-slot
Prototype Dual AntennaHandset
External Whip
Internal Patch
Multi-cell EDGE Compact Simulation- 1/3 reuse- 18 users per sector- 3.5 kbps average load per user
• spectrum - 500 MHz to 3 GHz• 3G EDGE/WCDMA network for uplink, downlink, control and signalling
• 4G WOFDM high speed downlink “a wireless cable modem”
• Complement to EDGE/UMTS
• High peak data rates (up to 10 Mb/s) in a 5 MHz channel
4G Wireless: One View
Path Loss and Fading Challenge
Delay Spread
Rayleigh Fading
Path Loss
rapid fading of 20 to 30 dB(power varies by 100 to 1000 timesin level at rates of about 100 times per second)
path loss up to ~ 150 dB(that is a 1 followed by 15 zeroes)
Reflected signalsarrive spread outover 5 to 20microsecond
Cellular Interference Challenge
0.001
0.01
0.1
1
-5 0 5 10 15 20 25
1|3 reuse
2|6 reuse
3|9 reuse
4|12 reuse
7/21 reuse
Signal to Interference ratio in dB
Cum
ulat
ive
Pro
babi
lity
Each base station is equippedwith three 120 degree directionalantennas to reduce interference& improve capacity
AT&T Labs-Research Work on 4G
• Smart antennas
• Multiple-Input-Multiple-Output Systems
• Space-Time Coding
• Dynamic Packet Assignment
• Wideband OFDM
MIMO Radio Channel Measurements
• Multiple antennas at both the base station and terminal can significantly increase data rates with sufficient multipath
• Ability to separate signals from closely spaced antennas has been demonstrated indoors and in AT&T-Lucent IS-136 field trial
• Lucent has demonstrated 26 bps/Hz in 30 kHz channel with 8 Tx and 12 Rx antennas indoors
• AT&T has performed measurements on 4 Tx by 4 Rx antenna configurations in full mobile & outdoor to indoor environments
MIMO Channel Measurement System
Transmitter
• 4 antennas mounted on a laptop
• 4 coherent 1 Watt 1900 MHz transmitters with synchronous waveform generator
Receive System
• Dual-polarized slant 45° PCS antennas separated by10 feet and fixed multibeam antenna with 4 - 30° beams
• 4 coherent 1900 MHz receivers with real-time baseband processing using 4 TI TMS320C40 DSPs
MIMO Measured Channel CapacityPotential Capacity Relative to a Single Antenna System
• Capacity increase close to 4 times that of a single antenna is possible with 4 transmit and 4 receive antennas
• Capacity for pedestrians is similar to mobile users
Performance Measure
• Complex channel measurement: H = [ H ij] for the ith transmit and jth receive antenna
• Capacity (instantaneous and averaged over 1 second) for 4 TX by 4 RX:
C = log2(det[I + (/4)H†H]) = log2(1 + (/4)i)
where is the total signal-to-noise ratio per antenna and
i is the ith eigenvalue of H†H
• To eliminate the effect of shadow fading, the capacity is normalized to the average capacity with a single antenna:
Cn = log2(1 + (/4)i) / (1/16) log2(1 + Hij)
Multiple Input Multiple Output Wireless• RX diversity - HF, terrestrial microwave, cellular….• TX frequency offset diversity & simulcasting for paging - 70’s• Adaptive array processing in military systems• TX diversity - 80’s
– frequency offset (channel decoding combining)– delay (equalizer combining)
• Optimum combining for cellular (multipath channels) - 80’s• Space-division multiple access - 80’s & 90’s
– angle-of-arrival based– multi-path based (supports co-location & multi-channels per user)
• MIMO - 80’s & 90’s– Multiple spatial channels using adaptive antenna arrays– BLAST - successive interference cancellation combined with coding– Space-Time coding
Space-Time Coding
How do you enhance TX delay diversity ( a repetition code)?
Multiple Antennas increase System Capacity
• MIMO (BLAST & space-time coding) techniques increase bit rate and/or quality on a link by creating multiple channels and/or enhancing diversity
• Switched/steered beam antennas for base stations and interference suppression/adaptive antennas for terminals reduce interference, increasing system capacity
OFDM for 4G Wireless
~ 6 kHz
~ 800 tones
~ 5 MHz
• OFDM is being increasingly used in high -speed information transmission systems:
- European HDTV- Digital Audio Broadcast (DAB)- Digital Subscriber Loop (DSL)- IEEE 802.11 Wireless LAN
5 MHz channels~ 6 KHz tones~ 13/26 MHz sample rate2048 FFT size (160 usec OFDM blocks)256/512 sample OFDM block guard timeQPSK & 16-QAM modulation adaptive modulation/coding1 to 2 msec time-slots in 20 to 40 msec frames
Mobile OFDM parameters: ex.
OFDM Characteristics• High peak-to-average power levels• Preservation of orthogonality in severe multi-path• Efficient FFT based receiver structures• Enables efficient TX and RX diversity• Adaptive antenna arrays without joint equalization• Support for adaptive modulation by subcarrier• Frequency diversity• Robust against narrow-band interference• Efficient for simulcasting• Variable/dynamic bandwidth• Used for highest speed applications• Supports dynamic packet access
OFDM Robust Channel Estimation
FFT
FFT
synch wordremovedata
receivedsignals
IFFT FFT...
.
.
.
.
.
.
.
.
.
. . . . . .
data
Estimator 1
Estimator 2
2-branchmaximal-ratiocombining
WOFDM 2-Branch Diversity Performance
0.001
0.01
0.1
1
-1 0 1 2 3 4 5 6 7
CC, k=9CC, k=3RS
Spectrum Efficiency
Efficiency: IS-136 0.04; IS-95 0.07; GSM 0.04
Source: G. J. Pottie, IEEE Personal Communications, pp. 50-67, October 1995
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
5 7.5 10 12.5 15
Synch CDMA
Dynamic ChannelAllocation withPower Control
Dynamic ChannelAllocation
SNR (dB)
Efficiency
Dynamic Packet Assignment
1. Mobile locks to the STRONGEST base
2. Mobile sends measurements of path losses for nearby bases to serving base
3. Serving base forwards measurementsto nearby bases
4. Bases assign channels to all packets/mobiles
5. Bases forward channel assignment info to nearby bases
~ 50 % improvement in performance % improvement in performance
Wideband OFDM Staggered Frame
Frame20 ms
1 2 4 1 2 4 .....
Superframe 80 ms
Superframe 80 ms
Control Slots Control Slots .....
3 3
4 ms
5 Blocks 5 Blocks 5 Blocks
group A group B group C group D
16 resources in 1 msec time-slots
1B 2 B
Sync & data
20 OFDM Blocks
data
5 Blocks
2 B
data
WOFDM Performance with Dynamic Packet Assignment & 5 MHz of Spectrum
0
20
40
60
80
100
120
0 500 1000 1500 2000 2500 3000 3500
MR, No beam-formingIS, No beam-formingMR, Four beams per sectorIS, Four beams per sector
Ave
. Use
r P
acke
t Del
ay (
mse
c)
Throughput per site (kb/s)
OFDM Experimental Program
• Baseband signal processing based on commercial off-the-shelf DSP hardware with some custom designed components• Sony-provided 1900 MHz transceivers• Real-time performance measured through RF channel fading simulator• Phase 1 parameters:
- >384 kb/s end user data rate - 800 kHz downlink bandwidth- GSM-derived clocks (2.166 MHz sample rate with 512 FFT)- 3.467 kbaud - 189 OFDM tones with 4.232 kHz tone spacing- differential detection- Reed-Solomon channel coding
RF A/D FFT
DemodulatorErasure
detectionDecoder
DataIntf
RF A/D FFTOFDM receiver
“Typical Urban” channel
800 kHz
Summary: Key Features of 4G W-OFDM• IP packet data centric• Support for streaming, simulcasting & generic data• Peak downlink rates of 5 to 10 Mbps• Full macro-cellular/metropolitan coverage• Asymmetric with 3G uplinks (EDGE)• Variable bandwidth - 1 to 5 MHz• Adaptive modulation/coding• Smart/adaptive antennas supported• MIMO/BLAST/space-time coding modes• Frame synchronized base stations using GPS• Network assisted dynamic packet assignment