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CDMA Technologies
for Cellular Phone System
July 7th, 2004
Takashi INOUE
KDDI R&D Laboratories Inc.
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Contents
Introduction
Spread Spectrum Technology
DS-CDMA
Spreading Codes Features of CDMA
RAKE Receiver
Power Control
Frequency Allocation
Soft Handoff Conclusion
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Introduction:
Overview of Cellular systems
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Evolution of Cellular Systems
1st.Generation
(1980s)Analog
NMT CT0
TACS CT1AMPS
3rd. Generation
(2000s)
2nd. Generation
(1990s)
Digital
GSM DECT
DCS1800 CT2
PDC PHS
IS-54
IS-95
IS-136UP-PCS
IMT-2000
CDMA2000
W-CDMA
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Major Operators of Cellular Phone Services
in Japan
Operator2G 3G
FrequencyRemarks for 3G handset
KDDI/au800 MHz(1.5GHz
For Tu-Ka)
800MHz
2GHz
backward compatibility
with 2G (cdmaOne)
NTT DoCoMo800 MHz1.5GHz
2GHzW-CDMA singleW-CDMA/PDC Dual
J-Phone(vodaphone)
1.5 GHz 2GHzW-CDMA singleW-CDMA/GSM Dual
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Japans Cellular Market Growth History
0
10,000,000
20,000,000
30,000,000
40,000,000
50,000,000
60,000,000
70,000,000
80,000,000
1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002
CDMA2000 1x ,W-CDMA /6,093KcdmaOne /7,757KPDC/60,517K(NTT DoCoMo,KDDI, Tu-Ka,J-phone)Analog / 0(endof Srv.)
end of Mar. 2003
Total No . of Sub scrib ers: 74,368K
(end of each fiscal year)
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Growth of 3G Mobile Subscribers
in Japan
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
J-phone
FOMA
CDMA2000 1x
The number of subscribers
surpassed 7 million
in March, 2003
Oct-01 Nov-01 Dec-01 Jan-02 Feb-02 Mar-02 Apr-02 May-02 Jun-02 Jul-02 Aug-02 Sep-02 Oct-02 Nov-02 Dec-02 Jan-03 Feb-03 Mar-03
334 696 1151 1644 2142 2652 3293 3897 4673 5312 5891 6805
11 14 27 43 56 89 106 112 115 127 134 136 142 149 152 154 191 330
1 4 9 25
No.ofSubs
.(x1,
000)
CDMA2000 1x
FOMA
J-phone
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Requirements for 3G mobile systems
High Capacity
Tolerance for interference
Privacy Tolerance for fading
Ability to various data rate transmission
Flexible QoS
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IMT-2000 systems approved by ITU-R
Popular
name
Access
method
Body of
Technical
Spec
production
IMT-DS IMT-MC IMT-TC IMT-SC IMT-FT
(Direct Sequence) (Multi Carrier) (Time Code) (Single Carrier) (Frequency Time)
W-CDMA CDMA2000
UTRA-TDD
UWC-136 DECT
CDMA-FDD CDMA-TDDCDMA-FDD TDMA-TDDTDMA-FDD
3GPP(FDD 3GPP2 3GPP(TDD)CWTS
IS-136 DECT
ESTIESTI TIA
TTA
T1
CWTS
ARIB/TTC ARIB/TTC
CWTS
TTA
ESTI
TTA
T1
CWTS
TIA
Approved in 2000 as ITU-R M.1457
TD-CDMA
TD-SCDMA
Organization
Partners
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Duplex & Multiple Access Methods
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Duplex Methods of Radio Links
Mobile Station
Base Station
Forward link
Reverse link
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Frequency Division Duplex (FDD)
Forward link frequency and reverse link frequency isdifferent
In each link, signals are continuously transmitted in
parallel.
Mobile Station
Base Station
Forward link (F1)
Reverse link (F2)
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Time Division Duplex (TDD)
Forward link frequency and reverse link frequency isthe same.
In each link, signals are incontinuously transmitted
by turns just like a ping-pong.
Mobile Station
Base Station
Forward link (F1)
Reverse link (F1)
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Example of FDD systems
Transmitter
Receiver
BPF: Band Pass Filter
BPF
BPF
Transmitter
Receiver
BPF
BPF
F1
F2 F1
F2
Mobile Station Base Station
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Example of TDD Systems
Transmitter
Receiver
BPF: Band Pass Filter
BPF
Transmitter
Receiver
BPF
F1 F1
Mobile Station Base Station
Synchronous Switches
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Multiple Access Methods
Mobile Station
Base Station
Mobile StationMobile Station
Mobile Station
Forward link
Reverse link
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FDMA Overview
A A
B B
C C
Time
f2
f1
f0
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TDMA Overview
C B A C B A C B A C B A
C
A
B
Time
f0
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What is CDMA ?
Sender Receiver
Code A
A
Code B
B
AB
AB
CB
C
A
Code A
AB
C
Time
BC
B
A
Base-band Spectrum Radio Spectrum
spread spectrum
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Summary of Multiple Access
FDMA
TDMA
CDMA
power
power
pow
er
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Spread Spectrum Technology
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How to spread spectrum...
Direct Sequence (DS)
Modulation
(primary modulation)
user data
Sp
reading
(seconda
rymodulation)
Tx
Base-band
Frequency
Power
Density
Radio
Frequency
Power
Density
TIME
data rate
10110100
spreading sequence
(spreading code)
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Demodulating DS Signals (1/2)
If you know the correct spreading sequence (code) ,
Radio
Frequency
Power
Density
received signal
spreading sequence
(spreading code)
you can find thespreading timingwhich gives themaximum detectedpower, and
Accumulate forone bit duration
Demodulated data
Base-band
Frequency
gathering energy !
10110100
1011010010110100 10110100
TIME
0100101110110100 10110100
0 01
1111111100000000 00000000
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Demodulating DS Signals (2/2)
If you dont know the correct spreading sequence (code)
Base-band
Frequency
received signal
spreading sequence
(spreading code)
you cannot findthe spreadingtimingwithout correctspreading code,and
Accumulate for
one bit duration
Demodulated data
RadioFrequency
Power
Density
01010101 01010101 01010101
10101010 10101010 10101010
TIME
0100101110110100 10110100
No data can be detected
- --
1011010010110100 10110100
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Feature of SS
Privacy, Security
RadioFrequency
Power
Density
Power density of SS-signals could be lower than the noise density.
transmitted SS-signal
Noise
Power
Density
RadioFrequency
Noise
received signal de-
modulator
Base-band
Frequency
Power
DensityWith incorrect code
(or carrier frequency),SS-signal itselfcannot be detected.
They cannot perceive the existence of communication,because of signal behind the noise.
With correct code(and carrier frequency),data can be detected.
Base-bandFrequency
Power
Density
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DS-CDMA
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Freq.Freq.
BPFDespreader
Code B
Freq.Freq.
BPFDespreader
Code A
DS-CDMA System Overview
(Forward link)
CDMA is a multiple spread spectrum.
Difference between each communication path is only the spreading code
Data B
Code B
BPF
Freq.Freq.
Data A
Code A
BPF
Freq.Freq.
MS-A
MS-B
BS
Data A
Data B
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Freq.Freq.
BPFDespreader
Code B
Freq.Freq.
BPFDespreader
Code A
DS-CDMA System Overview
(Reverse Link)
CDMA is a multiple spread spectrum.
Difference between each communication path is only the spreading code
Data B
Code B
BPF
Freq.Freq.
Data A
Code A
BPF
Freq.Freq.
MS-B
MS-A
BS
Data A
Data B
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Spreading Code
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Cross-Correlation
Cross-Correlation
between Code A and Code B = 6/16
Self-Correlation
for each code is 1.
one data bit duration
Spreading Code A
1 0 10 1 1 0 0 10 10 1 0 0 1
one data bit duration
Spreading Code A
1 0 01 1 1 0 0 10 10 1 0 0 1
Spreading Code A
1 0 01 1 1 0 0 10 10 1 0 0 1
0 0 00 0 0 0 0 00 00 0 0 0 0
Spreading Code B
1 0 0
1 1 0 0 1 11 00 1 0 1 1
0 0 00 0 1 0 1 01 10 0 0 1 0
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Preferable Codes
In order to minimize mutual interference in DS-CDMA ,
the spreading codes
with less cross-correlation should be chosen.
Synchronous DS-CDMA :Orthogonal Codes are appropriate. (Walsh code etc.)
Asynchronous DS-CDMA : Pseudo-random Noise (PN) codes / Maximum sequence
Gold codes
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Multiplexing using Walsh Code
Co
d
e
fo
r0
0
Codefor01
Codefor10
Codefor11
Data
Modulator
Codefor01
Codefor10
Codefor11
0dtT
Select
maximumvalue
Codefor00
0dtT
0dtT
0dtT
Demodulator
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Synchronous DS-CDMA
Forward Link(Down Link)
Synchronous Chip Timing
A
A
Signal for B Station(after re-spreading)
Less Interference for A station
Synchronous CDMA Systems realized in Point to Multi-point System.
e.g., Forward Link (Base Station to Mobile Station) in Mobile Phone.
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Asynchronous DS-CDMA
In asynchronous CDMA system, orthogonal codes have bad cross-correlation.
Reverse Link(Up Link)
BA
Signal for B Station(after re-spreading)
Big Interference
from A station
Asynchronous ChipTiming
Signals from A and B areinterfering each other.
A
B
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Features of CDMA
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Mobile Propagation Environment
Multi-path Fading
The peaks and bottoms of received power appear,in proportion to Doppler frequency.
Base Station (BS)Mobile Station (MS)
multi-path propagation
Path Delay
Power
path-2
path-2
path-3
path-3
path-1
path-1
Time
Power
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Fading in non-CDMA System
Path Delay
Power path-1
path-2
path-3
With low time-resolution,
different signal paths cannot be discriminated.
These signals sometimes strengthen,
and sometimes cancel out each other,
depending on their phase relation.
This is fading.
In this case, signal quality is damaged
when signals cancel out each other.
In other words, signal quality is dominated
by the probability for detected power
to be weaker than minimum required level.
This probability exists with less than two paths.
Time
Power
Detected Power
In non-CDMA system, fading damages signal quality.
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Fading in CDMA System ...
Because CDMA has high time-resolution,
different path delay of CDMA signalscan be discriminated.
Therefore, energy from all paths can be summed
by adjusting their phases and path delays.
This is a principle of RAKE receiver.
Path Delay
Power path-1
path-2
path-3
CDMA
Receiver
CDMA
Receiver
Synchro
nization
Adder
Path Delay
Power
CODE A
with timing ofpath-1
path-1
P
ower
path-1
path-2
path-3
Path Delay
Power
CODE A
with timing ofpath-2
path-2
interference from path-2 and path-3
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Fading in CDMA System (continued)
In CDMA system, multi-path propagation improves
the signal quality by use of RAKE receiver.
Time
Power
Detected Power
RAKEreceiver
Less fluctuation of detected power,because of adding all energy .
Power
path-1
path-2
path-3
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Near-Far Problem
CODE B
CDMA
Transmitter
DATA B
CODE A
CDMA
ReceiverCODE A
CDMA
Transmitter
DATA A
P
Desired Signal Power = P/Lp-a
Interfered Signal Power =
P/Lp-b/(processing gain )
Demodulated DATA
P
Lp-a
Lp-b
When user B is close to the receiver and
user A is far from the receiver,
Lp-a could be much bigger than Lp-b.
In this case, desired signal power is
smaller than the interfered power.
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Power Control...
Time
DetectedPower
from A
from B
When all mobile stations transmit the signals at the same power (MS),
the received levels at the base station are different from each other,
which depend on the distances between BS and MSs.
Moreover, the received level fluctuates quickly due to fading.
In order to maintain the received level at BS, power control technique must be
employed in CDMA systems.
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Power Control (continued)
Open Loop Power Control Closed Loop Power Control
estimating pathloss
calculatingtransmission
power
transmitmeasuring
received power
transmit receive
decidetransmission
power
transmit measuringreceived power
power controlcommand
about 1000 timesper second
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Effect of Power Control
Time
De
tectedPower
from MS Bfrom MS A
Effect of Power Control
Power control is capable of compensating the fading fluctuation.
Received power from all MS are controlled to be equal.
... Near-Far problem is mitigated by the power control.
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Frequency Allocation (1/2)
In FDMA or TDMA,
radio resource is allocated not to interfere among neighbor cells.
f1f2
f3f4
f5f6
f7
cell :
a cell means covered area by one base station.
Neighbor cells cannot use the
same (identical) frequency
band (or time slot).
The left figure shows the
simple cell allocation with
seven bands of frequency.
In actual situation, because of
complicated radio
propagation and irregular cell
allocation, it is not easy to
allocate frequency (or timeslot) appropriately.
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Frequency Allocation (2/2)
In CDMA,identical radio resource can be used among all cells,
because CDMA channels use same frequency simultaneously.
Frequency allocation in CDMA
is not necessary.
In this sense, CDMA cellular
system is easy to be designed.
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Soft Handoff (1/2)
Handoff :
Cellular system tracks mobile stations in order to maintain their communication links.
When mobile station goes to neighbor cell, communication link switches from current cell
to the neighbor cell.
Hard Handoff :
In FDMA or TDMA cellular system, new communication establishes after breaking current
communication at the moment doing handoff. Communication between MS and BS
breaks at the moment switching frequency or time slot.
Hard handoff : connect (new cell B) after break (old cell A)
switching
Cell BCell A
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Soft Handoff (2/2)
Cell
BCell A
Soft handoff : break (old cell A) after connect (new cell B)
transmitting same signal from both BS A and
BS B simultaneously to the MS
Soft Handoff :
In CDMA cellular system, communication does not break even at the moment doing
handoff, because switching frequency or time slot is not required.
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Conclusion
CDMA is based on the spread spectrum
technique which has been used at military
field.
CDMA cellular system is deemed superior tothe FDMA and TDMA cellular systems for the
time being.
Therefore, CDMA technique becomes more
important in radio communication systems.