01 cdma concept
TRANSCRIPT
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CDMA Concept
Dilshad SDE RTTC TVM
CDMA: Code Division Multiple Access
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Objectives
Evolution of CDMA
Duplex Radio channel
Multiple Access: CDMA, TDMA, FDMA
spread spectrum modulation
CDMA codes: Walsh code, short PN and long PN
Handoff
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2Mbps153.6kbps
CDMA
1x
CDMA
1xEV-DV
CDMA
IS-95
CDMA
1xEV-DO
CDMA
3x 5x
2G 2.5G 3G
9.6kbps
CDMA Technology Evolution
Code Division Multiple Access uses direct sequence spread-spectrum technology to
transmit a number of independent conversations across one or multiple 1.2288MHz
segments of the radio spectrum.
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Duplex Radio Channel
Duplex channel made of two 1.25MHz-wide bands1. BTS to MS ( DOWNLINK)
2. MS to BTS (UPLINK)
45 or 80 MHz
CDMA CHANNELCDMA
ReverseChannel 1.25 MHz
CDMA
ForwardChannel 1.25 MHz
Carrier Frequency
In 800 MHz Cellular these two duplex 1.25 MHz bands are 45 MHz apartIn 1900 MHz they are 80 MHz apart
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MS
Radio Channel Duplexing Technique
BTS
UPLINK
MOBILE
TRANSMIT
DOWNLINK
BTS TRANSMIT
1.25 MHz
1.25 MHz
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800MHz
Uplink: 824- 849 MHz
Downlink: 869- 894 MHz
824 824+25
824+45 824+45+25
1850 1850+60
1850+80 1850+80+60
CDMA Frequency Bands
1900MHz
Uplink: 1850- 1910 MHz
Downlink: 1930- 1990 MHz
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800MHz Uplink =825+0.03N
Downlink =870+0.03N
CDMA Frequency Calculation
N = FA (Frequency allocation)
1900MHz Uplink =1850+0.05N
Downlink =1930+0.05N
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Multiple Access
Each p air of users
enjoy s a dedicated,pr ivate circui t throug h
the transmiss ion
medium , unaware that
the other u sers exist .
Multiple Access: Simultaneous private use of a transmissionmedium by multiple, independent users.
Transmission
Medium
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Multiple Access
FDMA---Different user use different
frequency
TDMA---Different user use different
time slot of one frequency
CDMA---Different user use same
frequency at the same time,but with
different spre ding code
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CDMA Is a Spread-Spectrum System
Traditional technologies try to squeeze the signal into theminimum required bandwidth
Spread SpectrumTRADITIONAL COMMUNICATIONS SYSTEM
SlowInformation
Sent
TX
SlowInformationRecovered
RX
NarrowbandSignal
CDMASPREAD SPECTRUM SYSTEM
FastSpreadingSequence
SlowInformation
Sent
TX
SlowInformation
Recovered
RX
FastSpreadingSequence
Wideband Signal
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0 1 10 1Spreading
1 0
1 0 1 0
Spread Process
Digital Signal Spreading Signal
Spreading Code
1 0 0
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0 1 010 01 1 1 1 001 01 0De-spreading
1 0 1 0
Integrator1 0
De-spread Process
Out put data
De spreading Code
Input spreaded Signal
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Spread Spectrum PrinciplesSHANON Formula
C=B*log2(1+S/N)
Where,
C is capacity of channel, b/s
B is signal bandwidth, Hz
S is average power for signal
N is average power for noise
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Signal Spectrum Before SS
f
Sf
f0
Signal
ff0
Signal Spectrum after SS
Sf
Signal
Signal Spectrum After Decoding
f
Sf
f0
Signal
Noise
f
Signal Spectrum Before Decoding
Sf
f0
Signal
Noise
Spectrum Variation of Spread & De-spread
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SpreadingSequence
A
SpreadingSequence
B
SpreadingSequence
C
InputData
X
X+A X+A+B
ORIGINATING SITE
SpreadingSequence
C
SpreadingSequence
B
SpreadingSequence
A
RecoveredData
X
X+A+B X+A
DESTINATION
CDMA Spreading Principle
Using Multiple Codes
X+A+B+C
Spread-Spectrum Streams
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CDMA CODE CHANNEL
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CDMA Code Channel
All CDMA users separated based on their
digital code, rather than divide the
spectrum based on frequency or time.
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Walsh Code: Forward Code Channels
64 Forward code Channel 4 BTS to MS
traffic channels +overhead channels.
A set of 64 mathematical codes to differentiate the 64
forward code channels. The codes are called Walsh Codes
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Walsh Codes
64 Sequences, each 64 bit long
Each Walsh Code is Orthogonalto all other
Walsh Codes
Two same-length binary strings are
orthogonal if the result of XORing them has
the same number of 0s as 1s
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Walsh function is formed by orthogonal square matrix.It is just
composed of 0 and 1.
creation of Walsh code
0110
1100
1010
0000
10
000
Hn Hn
H2n = ___
Hn Hn
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Orthogonal code
PARALLEL
XOR: all 0s
Correlation: 100%(100% match)
ORTHOGONAL
XOR: half 0s, half 1s
Correlation: 0%(50% match, 50% no-match)
ANTI-PARALLEL
XOR: all 1s
Correlation:100%(100% no-match)
#23
#23
(#23)
#23
#23
#59
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Discriminating Among BTS : short PN code
All BTS having same frequency.
A Mobile Station discriminate between different Sectors of different Base
Stations using short PN code.
These code can be used in 512 different ways. which can be used to identify a
particular sector.
215 bits long
A B
Short PN
Code 1
Short PN
Code 2
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CDMA Spread Code
Walsh Codes
Short PNSequences
Long PNSequences
Type ofSequence
User identity within cells signal- forward traffic
Distinguish Cells & Sectors
Distinguishusers & reverse traffic
Link Function
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Objectives
-- The forward channel
Pilot ;Sync ; Paging and Traffic
-- The reverse channelAccess; Traffic
-- CDMA Call Processing
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IS-95 CDMA Channels
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Pilot Channel
Function: obtains timing information
pilot signal strength used for perform soft handoffs
Characteristic:
transmitted constantly by the base station
use Walsh code 0, (W0)
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Sync Channel
Once a strong pilot channel is located,the mobile
station listen and acquire the corresponding syncchannel for system informat ion.
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Sync Channel
Carries a data stream of essential system identification and
parameter information used by mobiles during system
acquisition stage
Bit rate is 1200 bps
(Acquired Pilot)
Sync Channel
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Paging Channels
There is one paging channel per sector per CDMA carrier
The Paging Channel uses Walsh code 1 up to 7,unusedpaging channels can be used as forward traffic channel
Paging Channel
Used by the base station to
transmit system overhead informationand mobile station-specific messages.
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Reverse Traffic Channels
TO send:
Voice traffic from the subscriber
Response to commands/queries from the base station
Requests to the base station
Reverse Traffic Channel
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CDMA Handoffs
Duringa Call
Idle Handoff
Soft Handoff
Softer Handoff
Hard handoff
While in the
Idle State
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CDMA Idle Handoff
PN 104
PN 108
A
104 108
>3dB
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CDMA Soft Handoff
Soft Handoff:Make-before-break
Cell Site
B
Cell Site
BCell Site
A
CDMA
Cell Site
A
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Rake receiver
a set of four or more receivers (or fingers).
One of the receivers constantly searches for
different multipaths and helps to direct the other
three fingers to lock onto strong multipath signals.
Allows combined reception of up to three different
paths.
Provides both diversity.
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Correlator 1
Correlator 2
Correlator 3
Searcher
Correlator
c
o
m
b
i
n
e
r
CDMA mobile rake receiver
Input from
antenna
Output
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Speech Coding
Speech coding algorithms (digital compression) arenecessary to increase cellular system capacity.
Coding must also ensure reasonable fidelity,
A-to-D
C
O
N
V
E
R
TE
R
64 Kbps
V
O
C
O
D
E
R
Codebook
Instruction
8Kbps
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Variable Rate Vocoding
CDMA uses a superior Variable Rate Vocoder Full rate during speech
Low rates in speech pauses
Increased capacity
Voice, signaling, and user secondary data may be mixed inCDMA frames
VOCODER Internal structure.
Codebook
Pitch
Filter
Format
Filter
Coded Result Feed-back
20ms Sample
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Variable Rate Vocoder
BTS BSC MSC
Analog voice
Variable RatePCM
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Power control
Need effective power control, because of
near-far problem,
fading & varying path loss.
The system requires fast closed loop power control.
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O l l
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Open loop power control
in the reverse link (Mobile to Base)
When the mobile is turned on, it locks on tothe pilot, paging and synch channel.
MS adjust its transmit power according to the
changes in its received power from the BTS
Mobile BTS
Reverse Open Loop
Power Control
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Closed loop power control
1. Closed loop power control in reverse link (Mobile to Base)
After the traffic channel is assigned, the power control shifts to closed
loop control
BTS measure received power once every 1.25ms (800 Hz).
BTS send Power control bit in the next 1.25 ms cycle to increase or
decrease the power by 1db.
Mobile BTS
Signal StrengthMeasurement
Setpoint
orReverse Closed Loop
Power Control
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Questions ???
conclude
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Thank you