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Page 1 Jan 5, 2000 <3rd generation CDMA wireless systems> <Avneesh Agrawal, Qualcomm>
Page 1Jan 5, 2000
<3rd generation CDMA wireless systems>
<Avneesh Agrawal, Qualcomm>
Page 2Jan 5, 2000
Overview• What is 3G ?
• A brief overview of IS95
• Key design choices for CDMA 3G systems.– Bandwidth
– Modulation
– Coding
– Power Control
– Transmit Diversity
– Base Station Synchronization.
– Acquisition
– Beam Forming
– Multi-user detection
– Peak To Average Power
• Objective is not to provide detailed justifications, but instead providesome insight into the level of optimization that went into designing thephysical layer of the next generation wireless systems.
Page 3Jan 5, 2000
What is 3G ?
• A loosely defined term referring to next generation wireless systems.– Analog was 1G. GSM/IS95 were 2G. Next is 3G.
• Used interchangeably with IMT2000 although there are some specificIMT2000 guidelines defined by the ITU.
• Envisioned as a single Global standard allowing seamless roamingacross the world.
– Market is expected to be fragmented amongst several competingstandards.
– Mostly dominated by Direct Sequence CDMA.
– Marketed as Global 3G CDMA implying a single unified standard. In reality,Global 3G comprises of 3 modes :
» Multi-carrier CDMA FDD
» Direct Spread CDMA FDD
» Direct Spread CDMA TDD
– There are others : IS95 HDR, EDGE, etc.
Page 4Jan 5, 2000
80 81 95 99 00 0201 03 04
Indicative timeline ofcommercial launch
92
IS-41
The Big Picture
AMPS
GSMHCSD
GPRS
EDGE IIEDGE
CDMA-95
CDMA-DSFDD
CDMA-MC1x
CDMA-MC3x
CDMA-TDD
CDMA-95B
GSM-MAP
IP
IS95 HDR
Page 5Jan 5, 2000
IS95 Forward Link (BS to mobile)
Walsh 0
Pilot Channel(all 0's)
User m ForwardTraffic Channel
Rate Set 1
Walsh m
Mux
Add CRCAdd 8 tail bits
Conv CodeRate 1/2
K = 9
SymbolRepetition
BlockInterleaver
Long CodeGenerator
Decimator Decimator
42 bit Long Code
1.2288 Mcps
19.2 ksps 800 Hz
8.6 kbps4.0 kbps2.0 kbps0.8 kbps
9.6 kbps4.8 kbps2.4 kbps1.2 kbps
19.2 ksps9.6 ksps4.8 ksps2.4 ksps
19.2 ksps
Power ControlBits
800 bps
Walsh j
Mux
Add CRCAdd 8 tail bits
Conv CodeRate 1/2
K = 9
SymbolRepetition
and Puncture
BlockInterleaver
Long CodeGenerator
Decimator Decimator
42 bit Long Code
1.2288 Mcps
19.2 ksps 800 Hz
13.35 kbps6.25 kbps2.75 kbps1.05 kbps
14.4 kbps7.2 kbps3.6 kbps1.8 kbps
28.8 ksps14.4 ksps7.2 ksps3.6 ksps
19.2 ksps
Power ControlBits
800 bps
User j ForwardTraffic Channel
Rate Set 2
A
A
A
Sync ChannelPaging Channel (x n)
Other users traffic channel
A
Page 6Jan 5, 2000
IS95 Forward Link (contd.)
A s(t)Σ
BasebandFilter X
Baseband Filter X
I-channel PN sequence1.2288 Mcps
Q-channel PN sequence1.2288 Mcps
cos (2π f t)
sin (2π f t)
Page 7Jan 5, 2000
IS95 Reverse Link (Mobile to Base Station)
Add CRCAdd 8 tail bits
Conv Code
Rate 1/3K = 9
ORRate 1/2
K=9
SymbolRepetition
BlockInterleaver
8.6 kbps4.0 kbps2.0 kbps0.8 kbps
9.6 kbps4.8 kbps2.4 kbps1.2 kbps
28.8 ksps14.4 ksps7.2 ksps3.6 ksps
28.8 ksps
14.4 kbps7.2 kbps3.6 kbps1.8 kbps
13.35 kbps6.25 kbps2.75 kbps1.05 kbps
64-aryOrthogonalModulator
Data BurstRandomizer
Long CodeGenerator
B
1.2288 Mcps
1.2288 Mcps
Page 8Jan 5, 2000
IS95 Reverse Link (contd.)
s(t)B
BasebandFilter X
I-channel PN sequence1.2288 Mcps
Q-channel PN sequence1.2288 Mcps
cos (2π f t)
Baseband Filter X
Σ
sin (2π f t)
D
1/2 PN ChipDelay = 406.9 ns
Page 9Jan 5, 2000
3G CDMA
Page 10Jan 5, 2000
3G Standards
• Focus on 2 systems– WCDMA FDD and CDMA2000
– Expected to be the dominant 3G standards, although IS95 HDR is gainingpopularity.
– HDR is a data only system.
• WCDMA (CDMA-Direct Sequence)– Strongly pushed by ETSI (Europe) and ARIB (Japan)
– CDMA Air interface (3.84 Mcps), GSM protocol stack.
– NTT DoCoMo (under pressure from IS95 deployment by DDI/IDO in Japan)is targeting initial deployment in Fall, 2001.
• CDMA2000 (CDMA - Multicarrier)– An evolution over IS95
– Two versions : 1x (1.2288 MHz) and 3x ( 3 carriers at 1.2288 MHz each)
• There seems to be little debate on which system has higher capacity(as technically, the two systems are very similar)
• Success depends largely on cost, time to market and political factors.
• Focus of this talk is on Physical Layer
Page 11Jan 5, 2000
WCDMA Forward Link
S/P C ch ,1DPDCH1/DPCCH
S/P C ch ,2DPDCH2
S/P C ch ,NDPDCHN
Σ
Σ
.
.
.
.
.
.
.
.
.
.
*j
I+jQ
I
Q
Cscra mb
OVSF Codes = BitReverse(Walsh Codes)
Root Raised CosineFilter (roll-off = .22)
I
Q
1
1 0
02
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
17
17
16
16
15
15
14
14
13
13
12
12
11
11
10
10
Gold CodePN sequence
Page 12Jan 5, 2000
WCDMA Forward Link
Tx.Antenna 2
MUX
Data
Tx.Antenna 1
Channelization code andlong scrambling code C ,spreading length = M
Ant1
Ant2
Ant1
Ant2
TPC
TFI
Pilot
MUX
ChannelEncoder
Interleaver
STTDEncoder
RateMatching
QPSK symbols
Diversity Pilot
One radio frame, Tf = 10 ms
TPC NTPC bits
Slot #0 Slot #1 Slot #i Slot #14
Tslot = 2560 chips, 10*2k bits (k=0..7)
Data2Ndata2 bits
DPDCH
TFCI NTFCI bits
Pilot Npilot bits
Data1Ndata1 bits
DPDCH DPCCH DPCCH
Page 13Jan 5, 2000
WCDMA Reverse Link
Pilot Npilot bits
TPC NTPC bits
DataNdata bits
Slot #0 Slot #1 Slot #i Slot #14
Tslot = 2560 chips, 10*2k bits (k=0..6)
1 radio frame: Tf = 10 ms
DPDCH
DPCCHFBI
NFBI bitsTFCI
NTFCI bits
Σ
Cch,1
DPDCH1
βd
Cch,3
DPDCH3
βd
Cch,d5
DPDCH5
βd
Channelization codes gain factors
Σ
Cch,2
DPDCH2
βd
Cch,4
DPDCH4
βd
Cch,6
DPDCH6
βd
Cch,0
DPCCH
*j
Cscramb
I+jQ
βc
I
Q
Page 14Jan 5, 2000
Bandwidth
• Both systems support wider bandwidth.
• Biggest advantage is ability to support higher peak rates.– Although HDR supports the same peak rates in a 1.25 MHz channel.
• Other advantages (increased frequency diversity, better interferencestatistics, etc.) have not been properly quantified.
• The disadvantage is increased design complexity
• WCDMA has a bandwidth of 3.84 Mcps. Big PR effort against IS95 :Wideband vs Narrowband CDMA.
• CDMA2000 1x is the same as IS95. 3x MultiCarrier is 3.6864 Mcps.
• Both WCDMA and CDMA2000 3x MC support data rates around 2Mbps.
– Only a single user (in good channel conditions) / sector can be supported atthese rates, i.e. high rate service is not going to be cheap !
Page 15Jan 5, 2000
Modulation
• Reverse Link– IS95 used 64-ary orthogonal modulation
» This allowed non-coherent demod at the receiver. Coherent demodulation atreceiver was considered risky.
» Peak data rate (I.e. 14.4 kbps) was much lower than the signal bandwidth (1.2288Mcps).
» Assumed a conventional receiver (I.e single user CDMA receiver) at the basestation. This implies that primary objective is to reduce transmit power at themobile.
» So, with this system, objective is to use the best (given the constraints ofimplementation complexity) rate 1/p code, where p = processing gain.
» IS95 used a convolutional code (rate 1/2 or 1/3) followed by a (6,64) orthogonalblock code, followed by a repetition code.
Page 16Jan 5, 2000
Modulation
• Forward Link– IS95 used BPSK because
» More tolerant to phase errors. Performance in fast fading channels was aconcern.
» General view was that IS95 was interference limited and hence more efficientmodulation was not necessary.
• Clearly, increased bandwidth would have allowed more powerful lower ratecodes, and hence could have increased capacity.
• In benign channel conditions (e.g. wireless local loop), the number ofavailable walsh channels was limiting forward link capacity.
Page 17Jan 5, 2000
Modulation
• 3G– WCDMA, CDMA2000 : both are going for QPSK modulation (on both links)
with pilot for phase reference.» Increased capacity, lower rate codes.
» Coherent demodulation not perceived as a problem. In fact, the overhead of piloton the up link more than compensated by improvements in synchronization andpower control.
» Supporting higher data rates. Hence, there is insufficient processing gain for 64-ary orthogonal modulation.
– HDR is using adaptive modulation (upto 16 QAM) and adaptive processinggain to improve capacity.
Page 18Jan 5, 2000
Coding
• IS95– Convolutional codes only.
– Rate 1/2 or 1/3 on uplink, K = 9.
– Rate 1/2 or 3/4 on downlink. The rate 3/4 code is used for the highest datarate (14.4 kbps), and is not a good code …
• 3G– Same conv codes for CDMA2000, WCDMA and HDR, except that the rate
3/4 code has been removed and a rate 1/3 code on the downlink has beenintroduced.
– Turbo Codes for data.» CDMA2000 and WCDMA use the same parallel Concatenated Codes. K = 4, rate
1/3. The turbo interleavers are different
» HDR : Serial Concatenated codes. K = 5, rate 1/2 outer code followed by K = 3,rate 1/2 inner code
Page 19Jan 5, 2000
Power Control
• IS95– Fast Reverse Link Power Control at 800 Hz
– Very slow Forward Power Control» IS95 A forward power control was a few Hz.
» IS95 B increased it to 50 Hz
– Slow Forward Power Control big limitation.
– In order to guarantee voice quality, base station has to put a floor onminimum transmit power.
– Generally, the forward link is the capacity limiting link.
• 3G– CDMA2000 uses 800 Hz for both uplink and downlink.
– WCDMA uses 1500 Hz for both links.
– Improved forward power control has a significant improvement on systemcapacity.
– HDR uses rate control instead of power control.
Page 20Jan 5, 2000
Transmit Diversity
• No transmit diversity for IS95
• CDMA2000 uses 2 forms:– OTD : Orthogonal Transmit Diversity.
» Transmit consecutive symbols on adjacent antennas using orthogonal codes.
– STS : Space Time Spreading» Ant 1 : S1 x W1(t) - S2* x W2(t)
» Ant 2 : S1* x W2(t) + S2 x W1(t)
» W1(t), W2(t) are orthogonal sequences.
• WCDMA supports several forms of Transmit Diversity– STTD : Space Time Transmit Diversity
» Ant 1 : transmit S1 S2 , S1 & S2 are complex symbols
» Ant 2 : transmit -S2* S1*
» For STS & STTD, performance equivalent to two antenna receive diversity in flatfading environment.
Page 21Jan 5, 2000
Transmit Diversity
– Feedback Mode Transmit Diversity» WCDMA provides fast feedback (upto 1500 Hz) mode transmit diversity.
» Allows receiver to control the amplitude and phase of the two antennas.
– Time Switched Transmit Diversity» Signal is transmitted alternately from two antennas using predetermined pattern.
S1 S2STTD encoder
S1 S2
-S2* S1
*
T 2T0 T 2T
Ant 1
Ant 2
MobileAntenna
Path 1
Path j
Ndata
STTD
Page 22Jan 5, 2000
Base Station Synchronization
• CDMA systems generally have a frequency reuse factor of 1, andhence do not require any frequency planning.
• However, they do need to do code planning in order to ensure that theydo not allocate the same PN codes to adjacent base stations.
• In IS95 and CDMA2000, different base stations use a different offset ofthe same PN sequence.
– Base stations are synchronized using GPS. Hence, having different offsetsensures that the PN sequences from different base stations will not coincidewith one another. The offsets are at a minimum of 256 chips apart.
• WCDMA does not require synchronization.– Mostly a political issue as some governments do not want to have their
communications infrastructure rely on a US defense program.
– Once again, this was a big PR effort against IS95 & CDMA2000.
– Most of the initial deployments are expected to be in synchronous mode.
Page 23Jan 5, 2000
Base Station Synchronization
• Async. Systems cannot use offsets of the same PN sequence fordifferent base stations and hence we need an efficient way to generatemultiple PN sequences.
• WCDMA uses Gold codes for PN sequences. Gold codes areconstructed as linear combinations (in GF(2) ) of two m-sequences.
– Cyclic shifts of one sequence with respect to another create different codes.
– IS95 & CDMA2000 use an m-sequence (I.e. maximal length LFSR) forgenerating the PN sequence.
• Asynchronous base stations have some problems :– Initial Acquisition
» Instead of searching for a single PN sequence, with async. Systems, the mobilehas to search for multiple PN sequences.
– Handoff searching.» Every handoff search is like initial acquisition.
» In contrast, for sync. Systems, handoff searching is simpler. E.g. for IS95, theinitial acquisition window size is 215 chips. For handoff searching, the uncertaintyis much less (= max delay spread)
Page 24Jan 5, 2000
Acquisition
• Fast acquisition is very important for a mobile user in a multi-cellularenvironment.
– Even more important for CDMA systems where minimizing transmit powerto close the link is a key determinant of system capacity.
– So, phone should always try to lock onto the strongest pilot.
• CDMA2000 uses a continuous pilot like IS95.
• WCDMA uses a 3 step hierarchical search process to reduceacquisition time.
Page 25Jan 5, 2000
WCDMA Searching
• Total of 512 Gold Codes divided into 64 groups of 8 codes each.
• In addition, there are 2 Synchronization sequences, SCH1 and SCH2.
• SCH1 is a 256 chip PN code common to all base stations. Repeatsevery slot (1 slot = 2560 chips)
• SCH2 can be one 16 different sequences. Code length is 256 chipsand it is time aligned with SCH1. Sequence length is 15 slots (10 ms).
– Sequence is sub-set of a Reed Solomon Code.
– Comma Free Property. That means, no cyclic shift of a code is a valid code.
– So, receiver can unambiguously determine start of 15 slot sequence.
– 64 different sequences, each representing one code group
• Step 1 : Use 256 chip match filter to determine modulo ‘slot’ (I.e. 2560chips) timing.
• Step 2 : Identify code group and derive frame timing (10 ms timing)
• Step 3 : Exhaustive search against 8 possible codes in a code group.
Page 26Jan 5, 2000
WCDMA Synchronization Channel
PrimarySCH
SecondarySCH
256 chips
2560 chips
One 10 ms SCH radio frame
acsi,0
acp
acsi,1
acp
acsi,14
acp
Slot #0 Slot #1 Slot #14
Page 27Jan 5, 2000
Beam Forming
• IS95 only supports fixed sectorization.
• Beam Forming is considered important for 3G systems.
• All 3G systems (that I am aware of) support beam forming.
• Requirement is simple : Each channel with beam forming should havea dedicated pilot for phase reference.
• None of the systems provide a mechanism for the phone to provide theCSI (Channel State Information) to the transmitter (with the exceptionof Feedback Mode Transmit Diversity in WCDMA).
– Beam form on remote scatterers
– Have fixed spot beams for high capacity areas.
Page 28Jan 5, 2000
Multi-User Detection
• Does not seem to be much interest in multi-user detectors.
• A year ago, NTT was a big proponent of multi-user receivers, but latelythere has been little development on that front.
• Biggest problem is designing multi-user receivers with reasonablecomplexity for a multi-cellular environment.
• WCDMA standard supports short spreading codes (256 chips asopposed to the regular 38400 chips) to aid in multi-user detection.
– With long codes, the correlation matrix of the codes changes every symbol.
• Schemes such as interference cancellation do not require standardssupport.
• In IS95 the downlink was the capacity limiting link. With WCDMA &CDMA2000, the downlink capacity has been improved, but withasymmetric data rates, downlink may still be the capacity limiting link.
– Having multi-user receivers on the base station would have little impact oncapacity.
Page 29Jan 5, 2000
Peak To Average Power
• IS95 uses 2 schemes to reduce Peak To Average Power– Offset QPSK modulation to reduce Peak to Average.
– Constant power transmission. For lower data rates, transmission isdiscontinued for some duration. The Peak to average remains the same;however, peak to average when the Power Amplifier is on is reduced.
• 3G– HPSK (Hybrid Phase Shift Keying)
» c = c1 (w0 + j c2*w1)
» c1 = PN sequence changing at chip rate.
» c2 = PN sequence changing at half the chip rate.
» W0 = { 1 1}; W1 = {1 -1}
» phase transitions less than 90 degrees half the time.
– Continuous transmission => worse peak to average» Compensated by improved power control, time diversity and receiver
synchronization.
Page 30Jan 5, 2000
SummaryForward Link Capacity Improvements
• Fast Forward Power Control
• More spectrally efficient modulation
• Turbo codes and lower rate convolutional codes.
• Transmit diversity
• Dedicated pilots for support of beam forming.
• Support higher peak data rates.
• Protocol improvements to improve packet data transmission.
Page 31Jan 5, 2000
SummaryReverse Link Capacity Improvements
• Coherent Reverse Link
• Improved synchronization and power control because of Reverse LinkPilot.
• Improved time diversity and power control because of continuoustransmission.
• QPSK modulation
• Turbo codes
• Multi-user detection
• Faster Power Control (for WCDMA)
• Improved Access Channel– Reservation based schemes as opposed to slotted Aloha in IS95
