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Lecture02 Spread Spectrum Communication
By
Engr. Muhammad Ashraf Bhutta
Spread-Spectrum Communication
•Since 1940 is in use for military purposes
• excellent immunity to interference and intentional jamming
•This technique, as the name implies, spreads the signal over a broader spectrum of frequencies than is usual
•By using a smaller portion of a greater bandwidth, less interference is produced between competing signals
Types of Spread-Spectrum Systems
•There are Three important types of spread-spectrum systems:
–Frequency-hopping
–Direct-sequence
–Time hoping
•Frequency-hopping systems are the simpler of the three systems available
•A frequency generator is used that generates a carrier that changes frequency many times a second according to a programmed sequence of channels known as pseudo-random (PN) noise sequence
•It is called this because if the sequence is not known, the frequencies appear to hop about unpredictably
•Two Types I) Fast FH ii) Slow FH
Frequency-Hopping Systems
Direct-Sequence Systems
•The data to be transmitted is combined with the PN
•Direct-sequence systems inject pseudo-random noise (PN) into the bit stream that has a much higher rate than the actual data to be communicated
–The PN bits are inverted when real data is represented by a one and leave the bit stream unchanged when a data zero is transmitted
–The extra bits transmitted this way are called chips
–Most direct-sequence systems use a chipping rate of at least ten times the bit rate
Direct-Sequence Spectrum
•The use of high-speed PN sequence results in an increase in the bandwidth of the signal, regardless of the modulation scheme used to encode the signal
Reception of Spread-Spectrum Signals
•For a frequency-hopped signal, a conventional narrowband receiver is needed that hops in the same way and is synchronized to the transmitter•The type of receiver used for spread-spectrum signals depends upon how the signal is generated•One way to synchronize the signals is to transmit a tone on a prearranged channel at the start of each transmission before it begins hopping•A more reliable method is to for the transmitter to visit several channels in a prearranged order before beginning a normal transmission
Reception of Direct-Sequence Spread-Spectrum
•Direct-sequence spread-spectrum transmissions require a wideband receiver with autocorrelation incorporated into it
•Autocorrelation involves multiplying the the received signal by a signal generated at the receiver from the PN code
•When the input signal corresponds to the PN code, the output will be large; at other times, the output will be small
What is a CDMA?
Code division Multiple Access is a digital technology pioneered by QUALCOMM that provides crystal clear voice quality in new generation wireless communication systems
CDMA provides better and most cost effective
Voice quality
Privacy
System capacity
Flexibility.
SMS
Internet
Main Issue in a Communication System
Capacity
Delay
Error detection/Correction
Transmission techniques
CDMA
TDMA
FDMA
Classification of wireless Communication systems
Simplex( one direction e.g paging)
Half-duplex(one way Comm.same frequency for TX and Rx , Push to talk)
Full Duplex
i)FDD( two separate Channel)
ii)TDD(Same channel on different TS)
Features and Advantages of CDMA
Universal Frequency Reuse
Fast and accurate Power Control
Rake Receiver
CDMA Hand off
FDMA/TDMA Frequency Reuse Frequency Reuse
CD CDMA MA Frequency Reuse
CDMA Frequency Reuse
CDMA Power Controlwithout
Power Control
Power Control
Power Control
Power Control
Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum
CDMA Rake Receiver
Multipaths
Multipaths
Multipaths
CDMA RAKE Receiver
CDMA RAKE Receiver
CDMA RAKE Receiver
Receive set
Correlator 1
Correlator 2
Correlator 3
Searcher correlator Calculate the time delay and signal strength
Combiner The combined signal
tt
s(t) s(t)
CDMA Soft Handoff
Soft Handoff
Soft Handoff
Soft Handoff Advantages
Softer Handoff