-
8/8/2019 NPD GTL RF
1/42
CDMA Technologies
for Cellular Phone System
May 20th, 2010
Shailendra Singh
GTL Limited
-
8/8/2019 NPD GTL RF
2/42
Contents
Introduction Spread Spectrum Technology DS-CDMA
Spreading Codes Features of CDMA RAKE Receiver Power Control Frequency Allocation
Soft Handoff Conclusion
-
8/8/2019 NPD GTL RF
3/42
Introduction:Overview of Cellular
systems
-
8/8/2019 NPD GTL RF
4/42
Evolution of CellularSystems
1st.Generation
(1980s)Analog
NMT CT0
TACS CT1
AMPS
3rd. Generation
(2000s)
2nd. Generation
(1990s)
Digital
GSM DECT
DCS1800
CT2
PDC PHS
IS-54
IS-95
IS-136
UP-PCS
IMT-2000
CDMA2000
W-CDMA
-
8/8/2019 NPD GTL RF
5/42
Requirements for 3G mobile systems
High Capacity Tolerance for interference Privacy Tolerance for fading Ability to various data rate
transmission
Flexible QoS
-
8/8/2019 NPD GTL RF
6/42
Transmission Techniques
Traffic channels:differentusers are assigned uniquecode and transmitted overthe same frequency band,for example, WCDMA andCDMA2000
Traffic channels: different frequencybands are allocated to different users,forexample, AMPS and TACS
Traffic channels: different timeslots are allocated to differentusers, for example, DAMPS andGSM
Freque
ncy
Time
Power
Frequ
ency
Time
Power
Freque
ncy
Time
Power
FDMA
TDMA
CDMA
User
User
User
User
User
User
Introduction
-
8/8/2019 NPD GTL RF
7/42
Duplex & Multiple Access
Methods
-
8/8/2019 NPD GTL RF
8/42
Duplex Methods of RadioLinks
Mobile Station
Base Station
Forward link
Reverse link
-
8/8/2019 NPD GTL RF
9/42
Frequency Division Duplex(FDD)
Forward link frequency and reverse linkfrequency is different
In each link, signals are continuouslytransmitted in parallel.
Mobile Station
Base Station
Forward link (F1)
Reverse link (F2)
-
8/8/2019 NPD GTL RF
10/42
Time Division Duplex (TDD)
Forward link frequency and reverse linkfrequency is the 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)
-
8/8/2019 NPD GTL RF
11/42
Example of FDD systems
Transmitter
Receiver
BPF: Band Pass Filter
BPF
BPF
Transmitter
Receiver
BPF
BPF
F1
F2 F1
F2
Mobile Station Base Station
-
8/8/2019 NPD GTL RF
12/42
Example of TDD Systems
Transmitter
Receiver
BPF: Band Pass Filter
BPF
Transmitter
Receiver
BPF
F1 F1
Mobile Station Base Station
Synchronous Switches
-
8/8/2019 NPD GTL RF
13/42
Multiple AccessMethods
Mobile Station
Base Station
Mobile StationMobile Station
Mobile Station
Forward link
Reverse link
-
8/8/2019 NPD GTL RF
14/42
FDMA Overview
A A
B B
C CFrequency
Time
f2
f1
f
0
-
8/8/2019 NPD GTL RF
15/42
TDMA Overview
C B A C B A C B A C B A
C
A
B
Time
f0
Frequ
ency
-
8/8/2019 NPD GTL RF
16/42
What is CDMA ?
Sender
Receiver
Code A
A
Code B
B
AB
AB
CB
C
A
Code A
AB
C
Time
Frequenc
y
BC
B
A
Base-bandSpectrum
Radio Spectrumspreadspectrum
-
8/8/2019 NPD GTL RF
17/42
Summary of Multiple Access
FDMA
TDM
A
CDMA
time
time
time
pow
er
pow
er
pow
er
frequ
en
cy
frequ
en
cy
frequen
cy
-
8/8/2019 NPD GTL RF
18/42
Spread Spectrum
Technology
-
8/8/2019 NPD GTL RF
19/42
How to spread spectrum...Direct Sequence (DS)
Modulation
(primary modulation)
Modulation
(primary modulation)
userdata
Spr
eading
(secondar
ymodulati
on)
Sp
reading
(seconda
r ymodulation)
Tx
Base-bandFrequency
Power
Density
RadioFrequency
Pow
er
Density
TIME
data rate
10110100
spreading sequence(spreading code)
-
8/8/2019 NPD GTL RF
20/42
Demodulating DS Signals(1/2)
ou know the correct spreading sequence (code) ,
RadioFrequency
Power
Density
receivedsignal
spreading sequence
(spreading code)
you can findthespreadingtimingwhich givesthemaximumdetectedpower, and
Accumulate for
one bit duration
Accumulate forone bit duration
Demodulateddata
Base-bandFrequency
gatheringenergy !
10110100
1011010010110100 10110100
TIME
0100101110110100 10110100
0 01
1111111100000000 00000000
-
8/8/2019 NPD GTL RF
21/42
Demodulating DS Signals(2/2)
dont know the correct spreading sequence (code)
Base-bandFrequency
receivedsignal
spreading sequence(spreading code)
you cannotfind thespreadingtimingwithoutcorrectspreadingcode, and
Accumulate for
one bit duration
Accumulate for
one bit duration
Demodulateddata
RadioFrequency
Power
Density
010101010101010101010101
101010101010101010101010
TIME
0100101110110100 10110100
No data can bedetected
- --
1011010010110100 10110100
-
8/8/2019 NPD GTL RF
22/42
Feature of SSPrivacy, Security
RadioFrequency
Power
Density
er density of SS-signals could be lower than the noise density.
transmitted SS-signal
Noise
Power
Density
RadioFrequency
Noise
received signal de-modulato
r
de-
modulato
r
Base-bandFrequency
Power
DensityWith incorrect code
(or carrierfrequency),SS-signal itselfcannot be detected.
They cannot perceive the existence ofcommunication, because of signal
behind the noise.
With correct code(and carrier frequency),data can be detected.
Base-bandFrequency
Power
Density
-
8/8/2019 NPD GTL RF
23/42
DS-CDMA
-
8/8/2019 NPD GTL RF
24/42
Freq.Freq.
BPFDespread
er
Code B
Freq.Freq.
BPFDespread
er
Code A
DS-CDMA System Overview(Forward link)
CDMA is a multiple spread spectrum.
Difference between each communication path is only the spreadingcode
Data B
Code B
BPF
Freq.Freq.
Data A
Code A
BPF
Freq.Freq.
MS-A
MS-B
BS
Data A
Data B
-
8/8/2019 NPD GTL RF
25/42
Freq.Freq.
BPFDespread
er
Code B
Freq.Freq.
BPFDespread
er
Code A
DS-CDMA System Overview
(Reverse Link)
DS-CDMA System Overview
(Reverse Link)
CDMA is a multiple spread spectrum.
Difference between each communication path is only the spreadingcode
Data B
Code B
BPF
Freq.Freq.
Data A
Code A
BPF
Freq.Freq.
MS-B
MS-A
BS
Data A
Data B
-
8/8/2019 NPD GTL RF
26/42
Spreading Code
-
8/8/2019 NPD GTL RF
27/42
Cross-CorrelationCross-Correlation
Cross-Correlationbetween Code A and Code B = 6/16
Self-Correlationfor each code is 1.
one data bit duration
Spreading Code A
1 0 10 1 1 0 0 10 1 0 1 0 0 1
one data bit duration
Spreading Code A
1 0 01 1 1 0 0 10 1 0 1 0 0 1
Spreading Code A
1 0 01 1 1 0 0 10 1 0 1 0 0 1
0 0 00 0 0 0 0 00 0 0 0 0 0 0
Spreading Code B
1 0 01 1 0 0 1 11 0 0 1 0 1 1
0 0 00 0 1 0 1 01 1 0 0 0 1 0
-
8/8/2019 NPD GTL RF
28/42
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
-
8/8/2019 NPD GTL RF
29/42
Features of CDMA
o e ropagat on
-
8/8/2019 NPD GTL RF
30/42
o e ropagat onEnvironment Multi-
path Fading
The peaks and bottoms ofreceived power appear, inproportion to Doppler frequency.
Base Station (BS)Mobile Station (MS)
multi-pathpropagation Path Delay
Power
path-2
path-2path-3
path-3
path-1
path-1
Time
Power
-
8/8/2019 NPD GTL RF
31/42
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 powerto be weaker than minimum required level.
This probability exists with less than two paths.
Time
Power
Detected Power
In non-CDMA system, fading damages signalquality.
-
8/8/2019 NPD GTL RF
32/42
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-1path-2
path-3
CDMAReceiv
er
CDMAReceiv
er
Sy
nc
hroniz
ation
A
dde
r
Path Delay
Power
CODE Awith timing ofpath-1
path-1
Power
path-1
path-2
path-3
Path Delay
Power
CODE Awith timing ofpath-2
path-2
interference from path-2 and path-3
-
8/8/2019 NPD GTL RF
33/42
Fading in CDMA System(continued)
In CDMA system, multi-path propagation improves
the signal quality by use of RAKE receiver.
Time
Power Detected Power
RAKEreceiv
erLess fluctuation ofdetected power,
because of adding allenergy .
Power
path-1
path-2
path-3
-
8/8/2019 NPD GTL RF
34/42
Near-Far Problem
CODE B
CDMATransmitt
er
DATA B
CODE A
CDMAReceiv
erCODE A
CDMATransmitt
er
DATA A
P
Desired Signal Power = P/Lp-a
Interfered Signal Power =
P/Lp-b/(processing gain )
DemodulatedDATA
P
Lp-a
Lp-b
When user B is close to the receiver anduser 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.
-
8/8/2019 NPD GTL RF
35/42
Power Control...
Time
D
etectedPower
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.
-
8/8/2019 NPD GTL RF
36/42
Power Control (continued)
Open Loop Power Control Closed Loop Power Control
estimatingpath loss
calculatingtransmission power
transmit
measuringreceivedpower
transmit receive
decidetransmission power
transmit measuringreceivedpower
powercontrol
command
about 1000times persecond
-
8/8/2019 NPD GTL RF
37/42
Effect of Power Control
Time
DetectedPower
from MS Bfrom MS A
closedl
oop
powerc
ontrol
for
MSB.
forM
SA.
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.
-
8/8/2019 NPD GTL RF
38/42
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 basestation.
Neighbor cells cannot usethe same (identical)frequency band (or timeslot).
The left figure shows thesimple cell allocation withseven bands of frequency.
In actual situation,because of complicatedradio propagation and
irregular cell allocation, itis not easy to allocatefrequency (or time slot)appropriately.
-
8/8/2019 NPD GTL RF
39/42
Frequency Allocation (2/2)
In CDMA,
identical radio resource can be used among all cells,
because CDMA channels use same frequency simultaneously.
Frequency allocation inCDMA is not necessary.
In this sense, CDMA cellularsystem is easy to bedesigned.
-
8/8/2019 NPD GTL RF
40/42
Soft Handoff (1/2)Handoff :
Cellular system tracks mobile stations in order to maintaintheir communication links.
When mobile station goes to neighbor cell, communicationlink switches from current cell to the neighbor cell.
Hard Handoff :
In FDMA or TDMA cellular system, new communicationestablishes after breaking current communication at themoment doing handoff. Communication between MS and BSbreaks at the moment switching frequency or time slot.
Hard handoff : connect (new cell B) after break (old cell A)
switching
Cell B Cell A
-
8/8/2019 NPD GTL RF
41/42
Soft Handoff (2/2)
CellB
Cell A
Soft handoff : break (old cell A) after connect (new cell B)
transmitting same signal fromboth BS A and BS B
simultaneously to the MS
Soft Handoff :
In CDMA cellular system, communication does not breakeven at the moment doing handoff, because switchingfrequency or time slot is not required.
-
8/8/2019 NPD GTL RF
42/42
Conclusion
CDMA is based on the spread spectrumtechnique which has been used atmilitary field.
CDMA cellular system is deemedsuperior to the FDMA and TDMAcellular systems for the time being.
Therefore, CDMA technique becomesmore important in radiocommunication systems.
