Spread Spectrum: FHSS and DSSS Quiz Quiz

Explore key concepts of spread spectrum communication with this quiz on Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS), including real-world applications, advantages, and technical mechanisms. Enhance your understanding of how FHSS and DSSS improve wireless communication security and resistance to interference.

1. Principle of Operation

   Which statement best describes how Frequency Hopping Spread Spectrum (FHSS) transmits a signal over the radio spectrum?

   1. It amplifies the original signal power to override interference.
   2. It remains fixed on one frequency for the entire transmission duration.
   3. It combines multiple frequencies and transmits them simultaneously.
   4. It rapidly changes the carrier frequency among many channels in a specific pattern.

   Explanation: FHSS works by quickly switching the carrier frequency among various channels in a predetermined sequence, which helps reduce interference and makes eavesdropping more difficult. The option about combining frequencies describes frequency-division multiplexing, which is unrelated to FHSS. Staying fixed on a single frequency doesn't provide FHSS's anti-jamming benefits. Amplifying power to override interference is not specific to spread spectrum methods and doesn't relate to how FHSS works.

2. Direct Sequence Mechanism

   In Direct Sequence Spread Spectrum (DSSS), how is the original data signal altered before transmission?

   1. It is directly amplified to increase bandwidth.
   2. It is compressed to reduce its bandwidth requirements.
   3. It is multiplied by a pseudo-random noise code at a higher rate.
   4. It is modulated onto a single fixed carrier frequency without spreading.

   Explanation: DSSS modifies the original data by multiplying it with a fast pseudo-random noise code, spreading the signal across a wider bandwidth. Compressing the signal would do the opposite of spreading. Modulating onto a single frequency doesn't involve spreading, and simple amplification increases power, not bandwidth. Only the correct option describes the signature mechanism of DSSS.

3. Interference Resistance

   Suppose a wireless device operating in a crowded environment experiences frequent interference. Which feature of DSSS makes it particularly effective in minimizing the impact of narrowband interference?

   1. Spreading the signal across a wide frequency band.
   2. Encoding data as analog rather than digital signals.
   3. Using constant transmission power.
   4. Transmitting on randomly selected time slots.

   Explanation: DSSS distributes the signal over a broad frequency range so that interference affecting a small part of the spectrum has minimal effect on the overall transmission. Constant power doesn't help mitigate interference specifically. Random time slots refer more to time-division techniques, not DSSS. Encoding as analog or digital is unrelated to spread spectrum's interference immunity.

4. Synchronization Requirement

   Why is synchronization crucial between transmitter and receiver in both FHSS and DSSS systems?

   1. To ensure both sides follow the same spreading or hopping patterns.
   2. To compress the data before modulation.
   3. To allow simultaneous transmission on multiple channels.
   4. To avoid exceeding legal transmission power limits.

   Explanation: Both FHSS and DSSS rely on precisely matched patterns (hopping sequence or spreading code) for successful encoding and decoding of data. Exceeding power limits is about regulatory compliance, not synchronization. Data compression is not related to synchronization requirements, and simultaneous multi-channel transmission is not the defining feature of these techniques.

5. Application Scenario

   A company needs a wireless communication system resistant to intentional jamming for a secure facility. Given this scenario, why might FHSS be preferred over DSSS?

   1. Because FHSS compresses all data, minimizing required bandwidth.
   2. Because FHSS uses more transmission power, preventing signal loss.
   3. Because FHSS transmits data only once, reducing interception risk.
   4. Because FHSS changes frequencies rapidly, making it more difficult for jammers to disrupt communication.

   Explanation: FHSS's core strength is its ability to jump quickly among many frequencies, preventing a jammer from easily targeting the entire signal. More transmission power does not inherently increase resistance to jamming. FHSS does not transmit data only once; that's unrelated to anti-jamming. Compressing data is not a spread spectrum benefit and does not affect jamming resistance.