pn. ds, fh & activities

7/31/2019 PN. DS, FH & Activities

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Nirika Bakliwal

Nivedita Saha

Prateek Gurjar

Praveen Jain

Rohit Singh

Sourabh Boyat


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Today, spread spectrum devices usingmicro-chips make pagers, cellularphones, and, yes, communication onthe internet possible. Many units canoperate at once using the samefrequencies.

Most important, spread spectrum is thekey element in anti-jamming devicesused in the government's 25 billionMilstar system. Milstar controls all theintercontinental missiles in U.S.weapons arsenal.

Fifty-five years later, Lamarr wasrecently given the EFF (ElectronicFrontier Foundation) Award for theirinvention. The co-inventor, Antheil wasalso honored; he died in the sixties.


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Occupies larger BW

Uses kind of coding

Code word being independent of info carried

Pseudorandom in nature

Appears like random noise

Only specially designed receiver can demodulate it.


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The generator generate a PN binary sequence which isimpressed on the transmitted signal

The modulated signal along with PN sequence travelsover the channel and spreads over a wide frequencybandThus the output of modulated signal is spread

spectrum signal.

At transmitter:-


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PSEUDO NOISE (PN) SEQUENCE

Basically a shift register

D i/p to a flip flop is connected to the Q o/p of theprevious flip flop.


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Maximum length of sequence is 2m

-1.this isbecause the state 000..0 must be excluded

MAXIMUM LENGTH SEQUENCE:-

COMMONLY USED PN SEQUENCE HAVINGLONG PERIODSREQUIRES A LINEAR FEEDBACK SHIFTREGISTER

The state of each flip flop changes & get shifted tonext flip flop corresponding to each pulse of theclock.

The character generated depends on no. of flipflop & the selection of which flip flop outputs areconnected to the inputs of ex-or gate.


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PROPERTIES

BALANCE PROPERTY

NO. OF 1S IS ALWAYS 1 MORE THAN THE NO. OFZEROS

RUN PROPERTYo AMONG THE RUNS OF 1S & 0S IN EACH PERIOD

,ONE HALF THE RUN OF EACH KIND ARE OFLENGTH ONE,1/4THARE OF LENGTH TWO,1/8THAREOF LENGTH THREE & SO ON.

o HERE RUN MEANS A SUBSEQUENCE OF

IDENTICAL SYMBOLS(1s & 0s)

CORRELATION PROPERTY

AUTO-CORRELATION OF A MAXIMUM LENGTH

SEQUENCE IS PERIODIC & BINARY


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Direct Sequence (DS) - A carrier is modulated by adigital code sequence in which bit rate is muchhigher than the information signal bandwidth.

The received spread spectrum signal for a singleuser can be represented as

Sss(t)= (2.Es /Ts )1/2 m(t).p(t).cos(2 fc t+)

m(t) is data sequencep(t) is PN spreading sequencefc is carrier frequency is carrier phase angle at t=0.


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m(t)

The data waveform is atime sequence non-overlapping rectangularpulses

An amplitude equal to+1 or -1. (Bipolar)

Time duration is Ts

p(t)

Represents a chip

The data waveform is a

time sequence non-overlapping rectangularpulses

The data waveform is atime sequence non-

overlapping rectangularpulses

Time duration is Tcbut Tc


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The United States GPS, European Galileo andRussian GLONASS satellite navigation systems

DS-CDMA (Direct-Sequence Code Division MultipleAccess) is a multiple access scheme based on DSSS, byspreading the signals from/to different users with differentcodes. It is the most widely used type of CDMA.

Cordless phones operating in the 900 MHz, 2.4 GHz and5.8 GHz bands

IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead)

Automatic meter reading

IEEE 802.15.4 (used, e.g., as PHY and MAC layerfor ZigBee, or, as the physical layer for Wireless HART)

Radio-controlled model vehicles


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Frequency Hopping (FH) Frequency-hoppingspread spectrum(FHSS) is a method of transmittingradio signals by rapidly switching a carrier amongmany frequency channel, using a pseudorandom

sequence known to both transmitter and receiver.

A set of possible carrier frequencies is called ahopset.

Hopping occurs at a frequency band that includes anumber of channels

Bandwidth of a channel used in the hopset is calledinstantaneous bandwidth Wss.


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Bandwidth of the spectrum over which thehopping is occurring is called total hoppingbandwidth .

Time duration between a hop is called thehop duration or hopping period Th.

Processing gain= Wss/Th


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Frequency shifted every Tcseconds

Duration of signal element is Tsseconds

Slow FHSShas Tc T

s

Fast FHSShas Tc< Ts Generally fast FHSS gives improved

performance in noise (or jamming)


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Spread-spectrum signals are highly resistantto deliberate jamming, unless the adversaryhas knowledge of the spreadingcharacteristics.

Military radios use cryptographic techniquesto generate the channel sequence under thecontrol of a secret Transmission Security

Key (TRANSEC) that the sender and receivershare in advance.

FHSS is mainly used in 802.11 wireless LANsand provides 1 or 2 Mbps transmission in the

2.4 GHz band


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COLUMN A COLUMN B

A. Sss(t)= (2.Es /Ts )1/2

m(t).p(t).cos(2 fc t+)

B. Wss/ Th

C. Frequency Shifting

D. 0T n(t).p1(t). cos(2fst)dt

E. Ksinc(f)

I. Processing Gain

II. x(t)ejt X(w-)

III. Transfer function for an

equalizer

IV.

Transmitted DS-SSwave

V. Gaussian random

variable representing

noise


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COLUMN A COLUMN B

A. Sss(t)= (2.Es /Ts )1/2

m(t).p(t).cos(2 fc t+)

B. Wss/ Th

C. Frequency Shiftingtheorem

D. 0T n(t).p1(t). cos(2fst)dt

E. Ksinc(f)

IV. Transmitted DS-SS

wave

I. Processing Gain

II. x(t)ejt X(w-)

V. Gaussian random

variable representing

noise

III. Transfer function for ane ualizer


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A. Reduced receiver sensitivity to interference is an

advantage in DS modulation.

B. Joint probability of mutually exclusive event is 1.

C. The Area under PDF curve is always 1.

D. CDF is not a monotonously non-decreasing function.

E. Companding is used to overcome quantizing noise in

PCM.

F. PCM systems use non-uniform quantization in order to

cut down the required bandwidth of transmission.

G. Granular noise is associated with QAM

H. ASK is most affected with noise


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A. TrueB. False

C. True

D. True

E. True

F. False

G. False

H. True


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Block Diagram of a Communication System

Input TRANSDUCER


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Block Diagram of Regenerative repeaterAmplitude Equalizer

FILL IN THE BLOCK


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FILL IN THE BLOCK

PPM Modulator

Reference Pulse Generator


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A Companding ModelUniform Quantizer


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RECAPITULATION


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ARRANGE ALL THE PICTURES IN THE

EXACT SEQUENCE AS SHOWN ABOVE


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B N C Cord Digital to BCD Convertor Analog DV Convertor

DAC Function Generator DSO

RC Filter

Digital Multimeter TDM Transmission


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We would like to finally thank PoojaMaam for making the session interactivefor us and Thank all of you for being

patient listeners.

pn. ds, fh & activities

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