Frequency-hopping spread spectrum is designed for robust operation in noisy environments by transmitting short packets at different frequencies across wide portions of channel bandwidth. The receiver correlates these “near” signals against each other and selects the best signal for demodulation, which typically gives better performance compared with non-frequency hopping receivers operating under similar conditions.
In the spread spectrum, the information is transmitted in short breaks of data at different carrier frequencies. In the frequency-hopping spread spectrum (FHSS), each component is transmitted at a different carrier frequency. Conversely, multi-carrier systems (such as OFDM) transmit multiple signals on a single carrier frequency. The spread spectrum can be used to send independent digital data streams across a noisy channel by assigning different ‘slots’ to each signal.
In FHSS systems, the transmitted power is concentrated on one or a few carriers at a time. The carrier frequencies are chosen in accordance with a pseudo-random sequence or hopping sequence that changes periodically, so as to prevent long-term predictability of the carrier frequencies used. The receiver correlates received signals against the sequence of the received signals to determine which doesn’t interfere from noise and interference.
The exact form and implementation of the frequency-hopping sequence are different for each radio system. The most widely used sequence is the binary offset code, which is used in Bluetooth and IEEE 802.15.4. Several sequence types have been proposed, all of which implement the same concept: a pseudo-random sequence that changes over time, making it difficult to predict the next frequency. Some systems use multiple parallel sequences so that even if one sequence is compromised, others can be used to communicate.
The output of the pseudo-random number generator (PRNG) is fed into a counter that sequentially counts up to the total number of frequencies in one hop set (N). The PRNG and counter are synchronized with an exactly matching clock at both sender and receiver sides.
Advantages of FHSS:
Some of the major advantages are as follows:
- The processing gain PG is higher than that of DSSS system.
- The synchronization is not greatly dependent on the distance.
- The serial search system with FHSS needs shorter time for acquisition.
Issues Related to Frequency-Hopping Spread Spectrum in Wireless Networks:
- Frequency-hopping spread spectrum uses complex mathematical formulas that change over time and simultaneously change at both sender and receiver sides. The complexity of calculating the formula using a processor or microprocessor is hard to understand.
- The accuracy of the frequency hopping spread spectrum is too high (complexity) for some real-world scenarios such as military communication because the number of different sequences available is limited by the memory and computational power at both sender and receiver sides, which restricts its flexibility in assigning certain slots to certain users.
- Frequency-hopping spread spectrum is too complex for some real-world scenarios, because it may cause interference between some channels, causing a clash and lack of mutual understanding among users.
- The performance of the frequency hopping spread spectrum can be influenced by temperature (temperature affects the speed of frequency changing); it may also be affected by interference between channels.
Conclusion:
In many wireless networks, we use the frequency hopping spread spectrum for the purpose of improving communication quality and reliability. By using FHSS, it is possible to make communication more resistant to interference-causing noise. The most common way of implementing FHSS is through a pseudo-random frequency hopping sequence that changes over time.