MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107

• DEPARTMENT- E&TC ENGG.
• SEMISTAR- 5^TH
• SUBJECT-A & D Communication
• TOPIC – 6 – DIGITAL MODULATION TECHNIQUE
• NAME OF TOPIC – SPREAD SPECTRUM
• PREPARED BY - U S Panda (Sr. Lect. E & TC Engineering)
• AY – 2021-2022, WINTER-2021

Spread Spectrum

Analog or digital data

Analog signal

Spread data over wide bandwidth

Makes jamming and interception harder

Frequency hoping

Signal broadcast over seemingly random series of frequencies

Direct Sequence

Each bit is represented by multiple bits in transmitted signal

Chipping code

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Spread Spectrum Concept

Input fed into channel encoder

Produces narrow bandwidth analog signal around central frequency

Signal modulated using sequence of digits

Spreading code/sequence

Typically generated by pseudonoise/pseudorandom number generator

Increases bandwidth significantly

Spreads spectrum

Receiver uses same sequence to demodulate signal

Demodulated signal fed into channel decoder

General Model of Spread Spectrum System

Gains

Immunity from various noise and multipath distortion

Including jamming

Can hide/encrypt signals

Only receiver who knows spreading code can retrieve signal

Several users can share same higher bandwidth with little interference

Cellular telephones

Code division multiplexing (CDM)

Code division multiple access (CDMA)

Pseudorandom Numbers

Generated by algorithm using initial seed

Deterministic algorithm

Not actually random

If algorithm good, results pass reasonable tests of randomness

Need to know algorithm and seed to predict sequence

Frequency Hopping Spread Spectrum (FHSS)

Signal broadcast over seemingly random series of frequencies

Receiver hops between frequencies in sync with transmitter

Eavesdroppers hear unintelligible blips

Jamming on one frequency affects only a few bits

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Basic Operation

Typically 2^k carriers frequencies forming 2^k channels

Channel spacing corresponds with bandwidth of input

Each channel used for fixed interval

300 ms in IEEE 802.11

Some number of bits transmitted using some encoding scheme

May be fractions of bit (see later)

Sequence dictated by spreading code

Frequency Hopping Example

Frequency Hopping Spread Spectrum System (Transmitter)

Frequency Hopping Spread Spectrum System (Receiver)

Slow and Fast FHSS

Frequency shifted every T_c seconds

Duration of signal element is T_s seconds

Slow FHSS has T_c ≥ T_s

Fast FHSS has T_c < T_s

Generally fast FHSS gives improved performance in noise (or jamming)

Slow Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

Fast Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

FHSS Performance Considerations

Typically large number of frequencies used

Improved resistance to jamming

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Direct Sequence Spread Spectrum (DSSS)

Each bit represented by multiple bits using spreading code

Spreading code spreads signal across wider frequency band

In proportion to number of bits used

10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code

One method:

Combine input with spreading code using XOR

Input bit 1 inverts spreading code bit

Input zero bit doesn’t alter spreading code bit

Data rate equal to original spreading code

Performance similar to FHSS

Direct Sequence Spread Spectrum Example

Direct Sequence Spread Spectrum Transmitter

Direct Sequence Spread Spectrum Transmitter

Direct Sequence Spread Spectrum Using BPSK Example

Approximate�Spectrum of �DSSS Signal

Code Division Multiple Access (CDMA)

Multiplexing Technique used with spread spectrum

Start with data signal rate D

Called bit data rate

Break each bit into k chips according to fixed pattern specific to each user

User’s code

New channel has chip data rate kD chips per second

E.g. k=6, three users (A,B,C) communicating with base receiver R

Code for A = <1,-1,-1,1,-1,1>

Code for B = <1,1,-1,-1,1,1>

Code for C = <1,1,-1,1,1,-1>

CDMA Example

CDMA Explanation

Consider A communicating with base

Base knows A’s code

Assume communication already synchronized

A wants to send a 1

Send chip pattern <1,-1,-1,1,-1,1>

A’s code

A wants to send 0

Send chip[ pattern <-1,1,1,-1,1,-1>

Complement of A’s code

Decoder ignores other sources when using A’s code to decode

Orthogonal codes

CDMA for DSSS

n users each using different orthogonal PN sequence

Modulate each users data stream

Using BPSK

Multiply by spreading code of user

CDMA in a DSSS Environment

Seven Channel CDMA Encoding and Decoding

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