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UNIT- 6Spread Spectrum Modulation Q1) Explain pseudo noise sequence in detail.A1) A pseudo-noise code (PN code) or pseudo-random-noise code (PRN code) is one that has a spectrum similar to a random sequence of bits but is deterministically generated. The most commonly used sequences in direct-sequence spread spectrum systems are maximal length sequences, Gold codes, Kasami codes, and Barker codes.A pseudorandom binary sequence (PRBS) is a binary sequence that, while generated with a deterministic algorithm, is difficult to predict and exhibits statistical behavior similar to a truly random sequence. PRBS generators are used in telecommunication, such as in analog-to-information conversion, but also in encryption, simulation, correlation technique.

A binary sequence (BS) is a sequence of N bits i.e.

ABS consist of ones and N-m zeros.

ABS is a pseudorandom binary sequence (PRBS) if its autocorrelation given by

Has only two values

is called the duty cycle of the PRBS, similar to the duty cycle of a continuous time signal. For a maximum length sequence, where

, the duty cycle is ½. Q2) What is Direct Sequence Spread Spectrum (DSSS) using BPSK modulation?A2) DSSS is a spread spectrum modulation technique used for digital signal transmission over airwaves. It was originally developed for military use, and employed difficult-to-detect wideband signals to resist jamming attempts. It is also being developed for commercial purposes in local and wireless networks.

The stream of information in DSSS is divided into small pieces, each associated with a frequency channel across spectrums. Data signals at transmission points are combined with a higher data rate bit sequence, which divides data based on a spreading ratio. The chipping code in a DSSS is a redundant bit pattern associated with each bit transmitted. This helps to increase the signal's resistance to interference. If any bits are damaged during transmission, the original data can be recovered due to the redundancy of transmission.

The entire process is performed by multiplying a radio frequency carrier and a pseudo-noise (PN) digital signal. The PN code is modulated onto an information signal using several modulation techniques such as quadrature phase-shift keying (QPSK), binary phase-shift keying (BPSK), etc. A doubly-balanced mixer then multiplies the PN modulated information signal and the RF carrier. Thus, the TF signal is replaced with a bandwidth signal that has a spectral equivalent of the noise signal. The demodulation process mixes or multiplies the PN modulated carrier wave with the incoming RF signal. The result produced is a signal with a maximum value when two signals are correlated. Such a signal is then sent to a BPSK demodulator. Although these signals appear to be noisy in the frequency domain, bandwidth provided by the PN code permits the signal power to drop below the noise threshold without any loss of information.

Q3) Derive SNR and processing gain of spread spectrum.A3) Assume coherent detection

SNR before spreading

SNR after spreading

Orthonormal basis vused at the receiver end

SNR before spreading (SNR)

SNR afterv spreading

is hence named the processing gain of spread spectrum technique.

Q4) Explain the concept of Jamming.A4) Spread spectrum signals are subject to the same jamming equations as any other signals, but the ability of the cooperative receiver to collapse the spectrum gives it a "processing gain" that reduces the effectiveness of the jamming. In general, the processing gain advantage is the same as the spreading ratio (i.e., the transmission bandwidth/the information bandwidth). It is also defined (in DSSS signals) as the code rate (used for spreading) divided by the data rate. Another applicable term is "jamming margin," which is defined by the following formula.M J = G p L SYS SNR OUTwhereM J= the jamming margin (in decibels);G P = the processing gain (in decibels);L SYS = the system losses (in decibels);SNR OUT= the required output SNR. Q5) Explain FHSS.A5) This is frequency hopping technique, where the users are made to change the frequencies of usage, from one to another in a specified time interval, hence called as frequency hopping. For example, a frequency was allotted to sender 1 for a particular period of time. Now, after a while, sender 1 hops to the other frequency and sender 2 uses the first frequency, which was previously used by sender 1. This is called as frequency reuse.The frequencies of the data are hopped from one to another in order to provide a secure transmission. The amount of time spent on each frequency hop is called as Dwell time.

Q6) Compare FHSS and DSSSA6)

FHSS	DSSS
Multiple frequencies are used	Single frequency is used
Hard to find the user’s frequency at any instant of time	User frequency, once allotted is always the same
Frequency reuse is allowed	Frequency reuse is not allowed
Sender need not wait	Sender has to wait if the spectrum is busy
Power strength of the signal is high	Power strength of the signal is low
Stronger and penetrates through the obstacles	It is weaker compared to FHSS
It is never affected by interference	It can be affected by interference
It is cheaper	It is expensive
This is the commonly used technique	This technique is not frequently used

Q7) What is spread spectrum and write some of its features.A7) The spread spectrum signals have the signal strength distributed as shown in the following frequency spectrum figure.

Spread Spectrum Signals

Following are some of its features −

Band of signals occupy a wide range of frequencies.

Power density is very low.

Energy is wide spread.

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