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

E-mail

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