JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III Year II Semester SPREAD SPECTRUM COMMUNICATIONS Previous Question Paper (July - 2023 - R16)

A Spread Spectrum System is a communication technique in which the transmitted signal occupies a bandwidth much larger than the minimum bandwidth required to transmit the information signal. This spreading of the signal is achieved by using a spreading code that is independent of the actual data.

Key characteristics of these systems include:

• The signal bandwidth is spread over a wide frequency range, often much greater than the information bandwidth.
• The spreading is performed by a function (typically a pseudo-noise sequence) that is known to both the transmitter and the receiver.
• The signal power spectral density is very low, often below the noise floor, which provides inherent Low Probability of Intercept (LPI).
• These systems exhibit high resistance to interference, jamming, and multipath fading.

The relationship between the transmitted signal bandwidth (W) and the information bandwidth (R) is defined by the Processing Gain (PG):

Common types include Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS).

A Hybrid Direct Sequence Frequency Hop Spread Spectrum (DS/FHSS) system is a communication technique that combines the advantages of both Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) to provide enhanced security, robustness against jamming, and increased processing gain.

In this hybrid configuration, the transmitted signal is spread using both methods simultaneously or in a cascaded manner:

• Direct Sequence Component: The information signal is multiplied by a high-rate Pseudo-Noise (PN) code sequence, which spreads the baseband signal over a wide bandwidth. This provides the system with inherent processing gain and multi-path rejection capabilities.
• Frequency Hopping Component: The carrier frequency of the already spread DS signal is changed (hopped) periodically according to a specific hopping pattern generated by a second PN code. This forces the signal to occupy different frequency slots over time, making it extremely difficult for an interceptor to track or jam the signal.

The total processing gain of the hybrid system is the product of the processing gains of the individual DS and FH components, expressed as:

By utilizing both techniques, the system achieves a higher level of Low Probability of Intercept (LPI) and Anti-Jam (AJ) performance compared to using either method alone, as the adversary must simultaneously contend with the wideband nature of the DS signal and the time-varying center frequency of the FH signal.