Download - Ch1 Fundamental of CDMA
Course Outline
• Introduction
• Ch1: Fundamental Of CDMA
• Ch2: UMTS Evolution & Air Interface.
• Ch3: The UMTS Network.
• Ch4: UMTS Transmission Chain.
• Ch5: UMTS Protocols
• HSPA Introduction
Transmission
Medium
As AMPS and NMT
As GSM and IS-136
FDMA TDMA
Advantages
•low cost hardware technology
•No need for network timing
•No restriction regarding the type of base
band (voice or data) or type of modulation
•Permits a flexible bit rate
•Easy for mobile or base stations to
initiate and execute hands off
Disadvantages
•The presence of guard bands
•Requires Ideal RF filtering to
minimize adjacent channel interference
•Requires network-wide timing
synchronization
•Requires signal processing for
advancing
Band Saturation.
Capacity (more than ½ The population of the globe use mobile
phones).
More BW efficiency.
New service.
Why do we need a new technology?
What about SDMA?
Switched Beam System Adaptive Array
Antenna
Array
Desired
User
Interfering
User Antenna
Array
Active
Beam
Signal Spreading
Fast Frequency Hopping
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.
Spread Spectrum
Anti-Jamming System
S(f)
f f0
Before spreading
Signal
S(f)
f f0
After spreading
Signal
S(f)
f f0
After despreading
signal
Interfering noise
f
S(f)
f0
Before despreading
Signal
Interfering noise
CDMA Spread Spectrum
As Channel BW get smaller More Capacity NO More necessary
Sharing the frequency by using Different codes
Signal Spreading Preview
Fast Frequency Hopping
This approach assumes that the two stations have:
A common knowledge of the frequency hopping plan.
Synchronized clocks
CDMA options
Direct sequence and freq hopping CDMA
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
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
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).
Bit Stream Multiplication Concepts
b(t) · c(t) = y(t)
b(t) · c(t) · c(t) = b(t)
Bit Stream Multiplication Concepts
Consider the difference between spreading and scrambling:
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
Input Data +1 -1 +1
+1 -1 +1 Divide by
Code Length
Receiver and Transmitter use identical code at same time offset
+1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 PN code used
in Transmitter
x x x
+8 -8 +8
Integrate
Result
Integrate Integrate Integrate
+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1 Transmitted
Sequence
= = =
+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 PN Code
Used in Receiver
x x x
Transmitter
Receiver
Bit Stream Multiplication Concepts
Coding Concept…
Transmitter
Input Data +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
Orthogonal code
in Transmitter
Transmitted
Sequence
Orthogonal
different Code
used in Receiver
0 0 0
Integrate
Result
0 Divide by
Code Length
x x x
Integrate Integrate
= = =
x x x
= = Receiver
Receiver and Transmitter use two uncorrelated codes at same time offset
+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
=
Integrate
0 0
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
Scrambling and Descrambling
Scrambling is often used for privacy. When scrambling is used
for encryption, then descrambling is used for decryption.
Repeated Spreading and Scrambling
CDMA systems use scrambling and spreading in several ways
for:
Encryption for privacy
Channel identification
Base station identification
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
Code Generation Circuitry
Long code = 242-1
Short code = 2 15-1
Code Generation Circuitry
D
Flip Flop D
Flip Flop D
Flip Flop
Quick Review
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
RF Transmission Delay and its Effect
Locking On to a Particular PN Sequence Cont.
Locking On to a Particular PN Sequence Cont.
Locking On to a Particular PN Sequence Cont.
Signal Correlation With Offsets Less Than 1 Time Chip
Signal Correlation With Offsets Less Than 1 Time Chip
Multi-User Downlink Overview
Multi-User Downlink Overview
Message detection
C3
Multi-User Uplink Overview
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.
Interference limited system
Multipath and Delay Spread
Non-RAKE CDMA Reception
RAKE Receiver Operation
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
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