Intoduction_to_Mobile_Communications_and_CDMA_technologies.ppt

Mobile Communication Technologies : Evolution of CDMA

2. CDMA?
What does it stand for?
➲Code
➲Division
➲Multiple
➲Access
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3. MULTIPLE ACCESS
NUMBER OF USERS ACCESS AND SHARE
TRANSMISSION MEDIUM &
BANDWIDTH AVAILABLE
FOR COMMUNICATION AT THE SAME TIME.
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4. MULTIPLE ACCESS
•MULTIPLE SIMULTANEOUS TRANSMISSIONS
•SHARING FINITE SPECTRUM AMONG LARGE NO. OF
SIMULTANEOUS USERS
•NO PRE-ASSIGNED CIRCUIT TO ANY USER
•ANY USER CAN ACCESS ANY CIRCUIT AND MAY ACCESS
DIFFERENT CIRCUITS FOR DIFFERENT CALLS
•DEMAND ASSIGNED CIRCUITS ON FIRST COME FIRST SERVE
BASIS
•PRIVACY i.e. ABSENCE OF INTERFERENCE BETWEEN TWO
CONNECTIONS
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5. MULTIPLE ACCESS METHODS
TIME DIVISION MULTIPLE ACCESS (TDMA)
FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)
CODE DIVISION MULTIPLE ACCESS (CDMA)
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6. FDMA
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(1G Cellular Networks)

7. TDMA
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8. FDD & TDMA
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(2G Cellular Networks)

9. CDMA
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A number of communications are combined on the same
frequency channel at the same time, but separated by the ‘codes’

10. SPREAD SPRECTRUM TECHNIQUES
1. Frequency Hopped Spread Spectrum:
Spreadingcanalsobeachievedbyhoppingthenarrowband
informationsignaloverasetoffrequencies.Thetypeofspreadingcan
beclassifiedasfastorslowdependingupontherateofhoppingtothe
rateofinformation.
2.DirectSequenceSpreadSpectrum:
Theinformationsignalisinherentlynarrowband,ontheorderofless
than10KHz.Theenergyfromthisnarrowbandsignalisspreadovera
muchlargerbandwidthbymultiplyingtheinformationsignalbya
widebandspreadingcode.DSStechniqueisusedintheIS-95CDMA
cellularsystem.
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11. CDMA: DS SPREAD SPRECTRUM
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12. DIRECT SEQUENCE SPREAD SPECTRUM TECHNOLOGY
Originally spread spectrum radio technology was
developed for military use to counter the interference by
hostile jamming. The broad spectrum of the transmitted
signal gives rise to “spread spectrum”.
A spectrum signal is generated by modulating the radio
frequency (RF) signal with a spreading code consisting of
different pseudo random binary sequences, which is
inherently resistant to noisy signal environment. Here the
original signal gets spreadedby the modulating spreading
signal
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13. DIRECT SEQUENCE SPREAD SPECTRUM TECHNOLOGY
AnumberofspreadspectrumRFsignalsthusgeneratedsharethe
samefrequencyspectrumandthustheentirebandwidthavailable
inthebandwillbeusedbyeachoftheusersusingsamefrequency
atthesametime.
Onthereceiversideonlythesignalenergywiththeselected
binarysequencecodeisacceptedandinformationcontentis
recovered.Theotherusersignals,whosecodesdonotmatch
contributeonlythenoiseandarenot“de-spread”backin
bandwidth.
Thistransmissionandreceptionofsignalsdifferentiatedby
“codes”usingthesamefrequencysimultaneouslybyanumberof
usersisknownasCodeDivisionMultipleAccess(CDMA).
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14. SPREADING CODES
CDMA systems use different types of spreading code
sequences:
Orthogonal sequences (Walsh codes).
Pseudorandom Noise (PN) sequences.
a) Long codes (2
42
)
b) Short codes (2
15
)
Gold Codes
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15. WALSH CODE
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+1 -1


+1 +1 +1 +1

+1 -1 +1 -1

+1 +1 -1 -1

+1 -1 -1 +1


+1 +1 +1 +1 +1 +1 +1 +1
+1 -1 +1 -1 +1 -1 +1 -1
+1 +1 -1 -1 +1 +1 -1 -1
+1 -1 -1 +1 +1 -1 -1 +1
+1 +1 +1 +1 -1 -1 -1 -1
+1 -1 +1 -1 -1 +1 -1 +1
+1 +1 -1 -1 -1 -1 +1 +1
+1 -1 -1 +1 -1 +1 +1 -1

W
2:
W
4:
W
8:

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CDMA Example
Low-Bandwidth message Signal:
High-Bandwidth Spreading Code:
Mix is a simple multiply
The encoded signal is transmitted…

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To Decode / Receive, take the transmitted signal:
Multiply by the sameSpreading Code:
… to get ...
… which is the original message signal...

18. What if we use the wrong code in the receiver?
… and we get ...
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Take the same signal:
Multiply by the wrongSpreading Code
(for example, shifting the original spreading code one bit to the left):
… which clearly hasn't recovered the original signal.
And discarded as noise due to huge frequency mismatch…

19. CDMA FORWARD LINK
The forward link uses the same frequency spectrum of 824-849 Mhz
(same as AMPS). Each carrier is 1.25MHz.
Four types of logical channel
Pilot,
Synchronization,
7 Paging, and
55 Traffic channels
Channels are separated using different spreading codes
QPSK is the modulation scheme
Orthogonal Walsh codes are used (64 total)
After orthogonal codes, they are further spread by short PN spreading
codes
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20. SANDIP BOSE ETCE FINAL YEAR CDMA 20
CDMA Forward Channel (IS 95)

21. CDMA MUX
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22. CDMA REVERSE CHANNEL
Fundamentally different from the forward channels
Uses OQPSK for power efficiency
QPSK demodulation is easy
869-894 MHz range.
No spreading of the data using orthogonal codes
Same orthogonal codes are used for WAVEFORM encoding
Two types of logical channels:
Access channels
Reverse traffic channels
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23. CDMA Reverse Channel (IS 95)
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24. CDMA DEMUX
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25. CDMA CHANNEL ARCHITECTURE
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CDMA Forward Link Channels
Pilot Channel
Sync Channel
Paging Channels
Traffic Channels

26. FREQUENCY REUSE
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A
F
E
D
B
G C
A
F
E
D
B
G C
A
F
E
D
B
G C
TDMA Systems:
frequency re-use factor 7 Frequency re-use
factor =1
CDMA Systems:
frequency re-use factor = 1
In CDMA, each of the cells are assigned to the same frequency band