Mobile Communication Technologies : Evolution of CDMA

2. CDMA?
What does it stand for?
➲ Code
➲ Division
➲ Multiple
➲ Access
1
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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:
Spreading can also be achieved by hopping the narrowband
information signal over a set of frequencies. The type of spreading can
be classified as fast or slow depending upon the rate of hopping to the
rate of information.
2. Direct Sequence Spread Spectrum:
The information signal is inherently narrowband, on the order of less
than 10KHz. The energy from this narrowband signal is spread over a
much larger bandwidth by multiplying the information signal by a
wideband spreading code. DSS technique is used in the IS-95 CDMA
cellular system.
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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 spreaded by the modulating spreading
signal
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13. DIRECT SEQUENCE SPREAD SPECTRUM TECHNOLOGY
A number of spread spectrum RF signals thus generated share the
same frequency spectrum and thus the entire bandwidth available
in the band will be used by each of the users using same frequency
at the same time.
On the receiver side only the signal energy with the selected
binary sequence code is accepted and information content is
recovered. The other user signals, whose codes do not match
contribute only the noise and are not “de-spread” back in
bandwidth.
This transmission and reception of signals differentiated by
“codes” using the same frequency simultaneously by a number of
users is known as Code Division Multiple Access (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 (242 )
b) Short codes (215 )
 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 +1 -1
W2 :
W4 :
W8 :

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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 same Spreading 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 wrong Spreading 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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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