CDMA Technologies for Cellular Phone System

1
CDMA Technologies for Cellular Phone System
2
Contents

• Introduction
• Spread Spectrum Technology
• DS-CDMA
• Spreading Codes
• Features of CDMA
• RAKE Receiver
• Power Control
• Frequency Allocation
• Soft Handoff
• Conclusion

3
Introduction Overview of Cellular systems
4
Evolution of Cellular Systems
3rd. Generation (2000s)
2nd. Generation (1990s)
IMT-2000 CDMA2000 W-CDMA
1st.Generation (1980s)
Digital
GSM DECT DCS1800 CT2 PDC
PHS IS-54 IS-95 IS-136 UP-PCS
Analog
NMT CT0 TACS CT1 AMPS
5
Japans Cellular Subscriber Growth Record
No. of Subscribers (Million)
/Nov.
end December of Year
6
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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Duplex Multiple Access Methods
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Duplex Methods of Radio Links
Base Station
Forward link
Reverse link
Mobile Station
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Frequency Division Duplex (FDD)

• Forward link frequency and reverse link frequency
  is different
• In each link, signals are continuously
  transmitted in parallel.

Forward link (F1)
Reverse link (F2)
Base Station
Mobile Station
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Time Division Duplex (TDD)

• Forward link frequency and reverse link frequency
  is the same.
• In each link, signals are in continuously
  transmitted by turns just like a ping-pong.

Forward link (F1)
Reverse link (F1)
Base Station
Mobile Station
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Example of FDD systems
Mobile Station
Base Station
Transmitter
BPF
Transmitter
BPF
F1
F2
Receiver
BPF
Receiver
BPF
F1
F2
BPF Band Pass Filter
12
Example of TDD Systems
Mobile Station
Base Station
Transmitter
Transmitter
BPF
BPF
F1
F1
Receiver
Receiver
Synchronous Switches
BPF Band Pass Filter
13
Multiple Access Methods
Base Station
Forward link
Reverse link
Mobile Station
Mobile Station
Mobile Station
Mobile Station
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FDMA Overview
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TDMA Overview
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What is CDMA ?
spread spectrum
Sender
Receiver
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Summary of Multiple Access
FDMA
power
TDMA
frequency
time
power
CDMA
frequency
time
power
frequency
time
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Spread Spectrum Technology
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How to spread spectrum...
Direct Sequence (DS)
user data
data rate
Modulation (primary modulation)
Spreading (secondary modulation)
Tx
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Demodulating DS Signals (1/2)
If you know the correct spreading sequence (code)
,
received signal
spreading sequence (spreading code)
gathering energy !
000 101 011 110
Accumulate for one bit duration
11111111
00000000
00000000
Demodulated data
0
0
1
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Demodulating DS Signals (2/2)
If you dont know the correct spreading sequence
(code)
Power Density
received signal
01001011
10110100
10110100
spreading sequence (spreading code)
Radio Frequency
Accumulate for one bit duration
10110100
10110100
10110100
No data can be detected
Demodulated data
-
-
-
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Feature of SS
Privacy, Security
Power density of SS-signals would be lower than
the noise density.


transmitted SS-signal
received signal
de-modulator
Noise
Other system cannot recognize the existence of
communication, because of signal behind the noise.
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DS-CDMA
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DS-CDMA System Overview (Forward link)
CDMA is a multiple spread spectrum.
Freq.
Freq.
BPF
Data A
BPF
Despreader
Data A
MS-A
Code A
Code A
Freq.
Freq.
Freq.
Freq.
BPF
Despreader
Data B
BPF
Data B
MS-B
Code B
Code B

BS

Difference between each communication path is
only the spreading code
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DS-CDMA System Overview (Reverse Link)
CDMA is a multiple spread spectrum.
Freq.
Freq.
BPF
Data A
BPF
Despreader
Data A
Code A
Code A
MS-A
Freq.
Freq.
Freq.
Freq.
BPF
Despreader
Data B
BPF
Data B
Code B
Code B
MS-B


BS
Difference between each communication path is
only the spreading code
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Spreading Code
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Cross-Correlation
Spreading Code A
Spreading Code A
one data bit duration
one data bit duration
Spreading Code A
Spreading Code B
000 101 011 110
Cross-Correlation between Code A and Code B 6/16
Self-Correlation for each code is 1.
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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
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Synchronous DS-CDMA
Synchronous CDMA Systems realized in Point to
Multi-point System. e.g., Forward Link (Base
Station to Mobile Station) in Mobile Phone.
Forward Link (Down Link)
Synchronous Chip Timing
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Asynchronous DS-CDMA
Reverse Link (Up Link)
Asynchronous Chip Timing
A B
Big Interference from A station
B
A
Signal for B Station (after re-spreading)
Signals from A and B are interfering each other.
In asynchronous CDMA system, orthogonal codes
produce bad cross-correlation.
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Features of CDMA
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Mobile Propagation Environment ??? Multi-path
Fading
multi-path propagation
Mobile Station (MS)
Base Station (BS)
The peaks and bottoms of received power appear,
in proportion to Doppler frequency.
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Fading in non-CDMA System

• 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.

Detected Power
Power
Required signal level
Time
In non-CDMA system, fading damages signal
quality.
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Fading in CDMA System ...

• As the CDMA system has high time-resolution,
  different path delay of CDMA signals can be
  discriminated.
• Energy from all paths can be summed by adjusting
  their phases and path delays.
• This is a principle of RAKE receiver.

interference produced by path-2 and path-3
CDMA Receiver
CODE A with timing of path-1
Synchronization Adder
CDMA Receiver

CODE A with timing of path-2

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Fading in CDMA System (continued)
In CDMA system, multi-path propagation improves
the signal quality by adopting RAKE receiver.
Detected Power
Power
RAKE receiver
Time
Detected power of CDMA signal will be less
fluctuated by combining all energy
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Near-Far Problem
P
Lp-a
CDMA Transmitter
DATA A
CDMA Receiver
Demodulated DATA
CODE A
P
Lp-b
CODE A
CDMA Transmitter
DATA B

• Desired Signal Power P/Lp-a
• Interfered Signal Power P/Lp-b/G
• G processing gain

CODE 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...

• As the propagation losses between BS and MSs are
  different according to individual communication
  distances, the received levels at the base
  station are different from each other when all
  mobile stations transmit their signals at the
  same power.
• Moreover, the received level fluctuates quickly
  due to fading.
• In order to maintain the strength of received
  signal level at BS, power control technique must
  be employed in CDMA systems.

from A
from B
Detected Power
Time
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Power Control (continued)
Open Loop Power Control
Closed Loop Power Control
?
?
?
transmit
about 1000 times per second
transmit
?
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Effect of Power Control

• 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.

closed loop power control for MS B.
from MS B from MS A
Detected Power
for MS A.
Time
A
B
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Frequency Allocation (1/2)
In FDMA or TDMA, radio resource is allocated not
to be interfered among neighbor cells.

• Neighbor cells cannot use the same (identical)
  frequency band (or time slot).
• The left figure shows the simple cell allocation
  with seven frequency sub-bands.
• In actual situation, it is difficult to allocate
  the frequency (or time slot) appropriately
  because of complicated radio propagation and
  irregular cell allocation.

cell a cell means covered area by one base
station.
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Frequency Allocation (2/2)
In the CDMA system, identical radio resource can
be allocated among all cells as explained in
Introduction.

• Frequency allocation in CDMA is not necessary.
• In this sense, CDMA cellular system is easy to be
  designed.

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Soft Handover (1/2)

• Handover
• 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 Handover
• In FDMA or TDMA cellular system, a new
  communication link is established after breaking
  the current communication link by hard handover.
• Communication between MS and BS instantaneously
  breaks by switching a frequency or a time slot.

Hard handover make connection (new cell B) after
break (old cell A)
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Soft Handover (2/2)

• In CDMA cellular system, communication link keeps
  a connection even in the handover procedure.
  Because the system does not require the frequency
  or time slot switching.

BS A and BS B transmit the same signal to the MS
simultaneously
S
Cell B
Cell A
Soft handover break (old cell A) after make
connection (new cell B)
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Conclusion

• CDMA is based on the spread spectrum technology
  which has been used in military field.
• CDMA cellular system has many advantages compared
  with the FDMA and TDMA cellular systems.
• CDMA system was adopted as the international
  standard for the 3rd generation mobile
  communications.
• The number of CDMA users will dramatically
  increase in the next five years all over the
  world.