10. FDMA TDMA
•low cost hardware technology
•Permits a flexible bit rate
•No need for network timing
Advantages •Easy for mobile or base stations to
•No restriction regarding the type of base
initiate and execute hands off
band (voice or data) or type of modulation
•The presence of guard bands •Requires network-wide timing
•Requires Ideal RF filtering to synchronization
Disadvantages
minimize adjacent channel interference •Requires signal processing for
advancing
11. Why do we need a new technology?
Band Saturation.
Capacity (more than ½ The population of the globe use mobile
phones).
More BW efficiency.
New service.
15. Signal Spreading Preview
In DS Spreading, the incoming digital speech signal is
multiplied by a digital pseudo-noise (PN) code through a
process called bit stream multiplication.
The result of DS spreading is an encoded, spread spectrum, CDMA signal.
16. Spread Spectrum
Anti-Jamming System
S(f) S(f)
Signal
Signal
f0 f
f0 f
Before spreading After spreading
S(f)
S(f)
Interfering noise
Signal signal
Interfering noise
f0 f
Before despreading f0 f
After despreading
17. CDMA Spread Spectrum
NO More necessary
As Channel BW get smaller More Capacity
19. Signal Spreading Preview
Fast Frequency Hopping
This approach assumes that the two stations have:
A common knowledge of the frequency hopping plan.
Synchronized clocks
21. Signal Spreading Preview
Time Hopping
Time hopping assumes that the two stations in communication have:
A common knowledge of the time-hopping "plan."
Synchronized clocks
22. The Strengths of CDMA
Increased system capacity.
Cell sites can be larger.
Enhanced privacy due to digital coding of CDMA signals.
Enhanced call quality.
Lower transmit RF power levels, longer battery life, and increased
talk time for hand-held units.
Simplified System Planning.
Bandwidth on Demand
23. Bit Stream Multiplication Concepts
Bit stream multiplication is the process where an input bit
stream of information (usually a digital speech signal) called
b(t) is multiplied by a pseudo-noise (PN) code called c(t) to
produce a new composite output signal called y(t).
26. Spreading and Despreading
Remember that spreading occurs when a lower bit rate input
signal b(t) is multiplied by a higher chip rate spreading code
c(t) as shown.
Bit
Chip
27. Bit Stream Multiplication Concepts
Receiver and Transmitter use identical code at same time offset
Input Data +1 -1 +1
x x x
PN code used
in Transmitter +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1
Transmitter
= = =
Transmitted +1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1
Sequence
x x x
PN Code +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1
Used in Receiver
= = =
+1 +1 +1 +1 +1 +1 +1 +1 -1 –1 –1 –1 –1 –1 –1 -1 +1 +1 +1 +1 +1 +1 +1 +1 Receiver
Integrate Integrate Integrate
Integrate
Result +8 -8 +8
Divide by
Code Length
+1 -1 +1
28. Coding Concept…
Receiver and Transmitter use two uncorrelated codes at same time offset
Input Data +1 -1 +1
x x x
Orthogonal code
in Transmitter +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1
Transmitter
= = =
Transmitted
+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1
Sequence
x x x
Orthogonal +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1
different Code
used in Receiver = = =
+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1 Receiver
Integrate Integrate
Integrate
Integrate
Result 0
0 0
Divide by
Code Length 0 0 0
29. Spreading and Power Spectral Density
Spreading does not change total power. Spreading changes
how the power is distributed over frequency.
The following formulas hold true:
Fb =1/Tb (the bit rate of the input signal)
Fc =1/Tc (the chip rate of the spreading code)
G (processing gain) = Fc/Fb =Tb/Tc
G = Spreading factor
30. Scrambling and Descrambling
Scrambling is often used for privacy. When scrambling is used
for encryption, then descrambling is used for decryption.
31. Repeated Spreading and Scrambling
CDMA systems use scrambling and spreading in several ways
for:
Encryption for privacy
Channel identification
Base station identification
32. PN Codes
A binary sequence with fixed length and has noise-like
randomness
Nearly equal number of zeros and ones
Low correlation between shifted versions of the sequence
Low correlation with other user signals (interference) and noise
Good autocorrelation properties with own signal in
synchronization
36. RF Transmission Delay and its Effect
Radio signals typically travel at a rate of about five microseconds to
the mile. Assuming that a mobile is ten miles away from the base
station, a 50 microsecond (µs) delay would be the result.
Chip = .81 µsec
47. Interference limited system
Traditional Systems Performance is measured by signal-to-noise
ratio (S/N). The desired user's signal versus noise.
CDMA Systems Performance is measured by signal-to-
interference ratio (S/I). The desired user's signal versus
interference from other users signals.
52. Related Terms and Definitions
Term Definition
Narrow Band Signal Signal occupies a relatively small bandwidth
i.e. (GSM signal has 200KHz bandwidth)
Wide Band Signal Signal occupies relatively wide bandwidth
i.e. (WCDMA signal has 5 MHz bandwidth)
Pseudo Noise Signal Signal has a noise like behaviour
- actual noise never repeats -
Spreading Converting a signal with low bit rate into another
signal with much higher bit rate
Scrambling Converting a signal into another coded version of
it keeping the same bit rate
53. Related Terms and Definitions
Term Definition
Auto Correlation Measurement for how much a signal is
related to another version of itself
Cross Correlation Measurement for how a signal is related to
another different signal
Orthogonal Codes Codes has Auto Correlation = 1 and
Cross Correlation = 0
Pseudo Noise Codes Codes has Auto Correlation very close to 1
and
Cross Correlation very close to 0
