3g cdma-qualcom

Jan 5, 2000
<3rd generation CDMA wireless systems>
<Avneesh Agrawal, Qualcomm>
www.TempusTelcosys.com

Jan 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 provide
some insight into the level of optimization that went into designing the
physical layer of the next generation wireless systems.

Jan 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 specific
IMT2000 guidelines defined by the ITU.
• Envisioned as a single Global standard allowing seamless roaming
across the world.
– Market is expected to be fragmented amongst several competing
standards.
– 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.

Jan 5, 2000
80 81 95 99 00 0201 03 04
Indicative timeline of
commercial launch
92
IS-41
The Big Picture
AMPS
GSM
HCSD
GPRS
EDGE II
EDGE
CDMA-95
CDMA-DS
FDD
CDMA-MC1x
CDMA-MC3x
CDMA-TDD
CDMA-95B
GSM-MAP
IP
IS95 HDR

Jan 5, 2000
IS95 Forward Link (BS to mobile)
Walsh 0
Pilot Channel
(all 0's)
User m Forward
Traffic Channel
Rate Set 1
Walsh m
Mux
Add CRC
Add 8 tail bits
Conv Code
Rate 1/2
K = 9
Symbol
Repetition
Block
Interleaver
Long Code
Generator
Decimator Decimator
42 bit Long Code
1.2288 Mcps
19.2 ksps 800 Hz
8.6 kbps
4.0 kbps
2.0 kbps
0.8 kbps
9.6 kbps
4.8 kbps
2.4 kbps
1.2 kbps
19.2 ksps
9.6 ksps
4.8 ksps
2.4 ksps
19.2 ksps
Power Control
Bits
800 bps
Walsh j
Mux
Add CRC
Add 8 tail bits
Conv Code
Rate 1/2
K = 9
Symbol
Repetition
and Puncture
Block
Interleaver
Long Code
Generator
Decimator Decimator
42 bit Long Code
1.2288 Mcps
19.2 ksps 800 Hz
13.35 kbps
6.25 kbps
2.75 kbps
1.05 kbps
14.4 kbps
7.2 kbps
3.6 kbps
1.8 kbps
28.8 ksps
14.4 ksps
7.2 ksps
3.6 ksps
19.2 ksps
Power Control
Bits
800 bps
User j Forward
Traffic Channel
Rate Set 2
A
A
A
Sync Channel
Paging Channel (x n)
Other users traffic channel
A

Jan 5, 2000
IS95 Forward Link (contd.)
A s(t)Σ
Baseband
Filter X
Baseband Filter X
I-channel PN sequence
1.2288 Mcps
Q-channel PN sequence
1.2288 Mcps
cos (2π f t)
sin (2π f t)

Jan 5, 2000
IS95 Reverse Link (Mobile to Base Station)
Add CRC
Add 8 tail bits
Conv Code
Rate 1/3
K = 9
OR
Rate 1/2
K=9
Symbol
Repetition
Block
Interleaver
8.6 kbps
4.0 kbps
2.0 kbps
0.8 kbps
9.6 kbps
4.8 kbps
2.4 kbps
1.2 kbps
28.8 ksps
14.4 ksps
7.2 ksps
3.6 ksps
28.8 ksps
14.4 kbps
7.2 kbps
3.6 kbps
1.8 kbps
13.35 kbps
6.25 kbps
2.75 kbps
1.05 kbps
64-ary
Orthogonal
Modulator
Data Burst
Randomizer
Long Code
Generator
B
1.2288 Mcps
1.2288 Mcps

Jan 5, 2000
IS95 Reverse Link (contd.)
s(t)B
Baseband
Filter X
I-channel PN sequence
1.2288 Mcps
Q-channel PN sequence
1.2288 Mcps
cos (2π f t)
Baseband Filter X
Σ
sin (2π f t)
D
1/2 PN Chip
Delay = 406.9 ns

Jan 5, 2000
3G CDMA

Jan 5, 2000
3G Standards
• Focus on 2 systems
– WCDMA FDD and CDMA2000
– Expected to be the dominant 3G standards, although IS95 HDR is gaining
popularity.
– 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

Jan 5, 2000
WCDMA Forward Link
S/P Cch ,1
DPDCH1/DPCCH
S/P Cch ,2
DPDCH2
S/P Cch ,N
DPDCHN
Σ
Σ
.
.
.
.
.
.
.
.
.
.
*j
I+jQ
I
Q
Cscramb
OVSF Codes = BitReverse(Walsh Codes)
Root Raised Cosine
Filter (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 Code
PN sequence

Jan 5, 2000
WCDMA Forward Link
Tx.
Antenna 2
M
U
X
Data
Tx.
Antenna 1
Channelization code and
long scrambling code C ,
spreading length = M
Ant1
Ant2
Ant1
Ant2
TPC
TFI
Pilot
M
U
X
Channel
Encoder
Interleaver
STTD
Encoder
Rate
Matching
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)
Data2
Ndata2 bits
DPDCH
TFCI
NTFCI bits
Pilot
Npilot bits
Data1
Ndata1 bits
DPDCH DPCCH DPCCH

Jan 5, 2000
WCDMA Reverse Link
Pilot
Npilot bits
TPC
NTPC bits
Data
Ndata 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
DPCCH
FBI
NFBI bits
TFCI
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

Jan 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 interference
statistics, 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 2
Mbps.
– Only a single user (in good channel conditions) / sector can be supported at
these rates, i.e. high rate service is not going to be cheap !

Jan 5, 2000
Modulation
• Reverse Link
– IS95 used 64-ary orthogonal modulation
» This allowed non-coherent demod at the receiver. Coherent demodulation at
receiver was considered risky.
» Peak data rate (I.e. 14.4 kbps) was much lower than the signal bandwidth (1.2288
Mcps).
» Assumed a conventional receiver (I.e single user CDMA receiver) at the base
station. This implies that primary objective is to reduce transmit power at the
mobile.
» So, with this system, objective is to use the best (given the constraints of
implementation 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) orthogonal
block code, followed by a repetition code.

Jan 5, 2000
Modulation
• Forward Link
– IS95 used BPSK because
» More tolerant to phase errors. Performance in fast fading channels was a
concern.
» General view was that IS95 was interference limited and hence more efficient
modulation was not necessary.
• Clearly, increased bandwidth would have allowed more powerful lower rate
codes, and hence could have increased capacity.
• In benign channel conditions (e.g. wireless local loop), the number of
available walsh channels was limiting forward link capacity.

Jan 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 pilot
on the up link more than compensated by improvements in synchronization and
power 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 processing
gain to improve capacity.

Jan 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 data
rate (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 been
introduced.
– 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

Jan 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 on
minimum 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 system
capacity.
– HDR uses rate control instead of power control.

Jan 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 flat
fading environment.

Jan 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 S2
STTD encoder
S1 S2
-S2
*
S1
*
T 2T
0 T 2T
Ant 1
Ant 2
Mobile
Antenna
Path 1
Path j
Ndata
STTD

Jan 5, 2000
Base Station Synchronization
• CDMA systems generally have a frequency reuse factor of 1, and
hence do not require any frequency planning.
• However, they do need to do code planning in order to ensure that they
do not allocate the same PN codes to adjacent base stations.
• In IS95 and CDMA2000, different base stations use a different offset of
the same PN sequence.
– Base stations are synchronized using GPS. Hence, having different offsets
ensures that the PN sequences from different base stations will not coincide
with 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.

Jan 5, 2000
Base Station Synchronization
• Async. Systems cannot use offsets of the same PN sequence for
different base stations and hence we need an efficient way to generate
multiple PN sequences.
• WCDMA uses Gold codes for PN sequences. Gold codes are
constructed 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) for
generating the PN sequence.
• Asynchronous base stations have some problems :
– Initial Acquisition
» Instead of searching for a single PN sequence, with async. Systems, the mobile
has 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, the
initial acquisition window size is 215 chips. For handoff searching, the uncertainty
is much less (= max delay spread)

Jan 5, 2000
Acquisition
• Fast acquisition is very important for a mobile user in a multi-cellular
environment.
– Even more important for CDMA systems where minimizing transmit power
to 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 reduce
acquisition time.

Jan 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. Repeats
every slot (1 slot = 2560 chips)
• SCH2 can be one 16 different sequences. Code length is 256 chips
and 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. 2560
chips) 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.

Jan 5, 2000
WCDMA Synchronization Channel
Primary
SCH
Secondary
SCH
256 chips
2560 chips
One 10 ms SCH radio frame
acs
i,0
acp
acs
i,1
acp
acs
i,14
acp
Slot #0 Slot #1 Slot #14

Jan 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 have
a dedicated pilot for phase reference.
• None of the systems provide a mechanism for the phone to provide the
CSI (Channel State Information) to the transmitter (with the exception
of Feedback Mode Transmit Diversity in WCDMA).
– Beam form on remote scatterers
– Have fixed spot beams for high capacity areas.

Jan 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 lately
there has been little development on that front.
• Biggest problem is designing multi-user receivers with reasonable
complexity for a multi-cellular environment.
• WCDMA standard supports short spreading codes (256 chips as
opposed 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 standards
support.
• In IS95 the downlink was the capacity limiting link. With WCDMA &
CDMA2000, the downlink capacity has been improved, but with
asymmetric data rates, downlink may still be the capacity limiting link.
– Having multi-user receivers on the base station would have little impact on
capacity.

Jan 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 is
discontinued 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.

Jan 5, 2000
Summary
Forward 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.

Jan 5, 2000
Summary
Reverse Link Capacity Improvements
• Coherent Reverse Link
• Improved synchronization and power control because of Reverse Link
Pilot.
• Improved time diversity and power control because of continuous
transmission.
• 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

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