JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III SPREAD SPECTRUM COMMUNICATIONS Previous Question Paper (February/March - 2022 - R16)

Frequency Hopping Spread Spectrum (FHSS) offers several distinct advantages over Direct Sequence Spread Spectrum (DSSS) systems:

• Near-Far Problem Mitigation: FHSS is inherently more resistant to the near-far problem because the signal is not continuously present in a single wide frequency band, reducing the impact of a strong nearby interferer on distant signals.
• Frequency Diversity: By rapidly changing carrier frequencies, FHSS provides robust protection against frequency-selective fading, as the signal effectively 'hops' out of deep nulls in the channel.
• Implementation Complexity: FHSS systems often require less stringent synchronization requirements compared to the precise chip-level synchronization needed for DSSS despreading.
• Interference Avoidance: FHSS can dynamically avoid narrowband interference by excluding specific frequencies from the hopping pattern, whereas DSSS spreads the interference across the entire bandwidth.
• Lower Processing Gain Requirements: FHSS can achieve high levels of security and anti-jamming performance without requiring the extremely high-speed spreading code generators necessary for high-gain DSSS systems.

A spread spectrum digital communication system is modeled by integrating a spreading process at the transmitter and a despreading process at the receiver to increase the signal bandwidth significantly beyond the minimum required for information transmission.

The model consists of the following key components:

1. Channel Encoder and Modulator: The input data is first processed by a standard digital modulation scheme (e.g., BPSK or QPSK) to produce a baseband signal with bandwidth B.
2. Spreading Code Generator: A pseudo-noise (PN) sequence generator produces a high-rate code signal that is independent of the data. This sequence is characterized by its chip rate, which is much higher than the data bit rate.
3. Spreader (Multiplier): The modulated signal is multiplied by the PN sequence. This operation spreads the signal spectrum to a much wider bandwidth W, where W >> B.
4. Channel: The spread signal is transmitted over a channel that may contain noise, interference, and jamming signals.
5. Despreader: At the receiver, the received signal is multiplied by a locally generated, synchronized replica of the original PN sequence. This process collapses the spread signal back to its original bandwidth B, while simultaneously spreading any narrowband interference or jamming signals across the wide bandwidth W.
6. Demodulator and Decoder: The despread signal is passed through a bandpass filter (which rejects the spread-out interference) and then demodulated to recover the original information bits.

The efficiency of this model is defined by the Processing Gain, given by the ratio of the spread bandwidth to the information bandwidth: