How Frequency Hopping Works
In FHSS, the carrier frequency jumps between many narrow channels according to a pseudo-random (PN) code. The transmitter and receiver use the same code and stay synchronized, so the receiver always knows which channel to listen on next. To an outsider the signal looks like short bursts scattered over a wide band, which makes it hard to jam or intercept.
The total bandwidth occupied is much wider than the data bandwidth. The ratio of the two (roughly the number of hop channels) is called the processing gain.
Slow Hopping vs Fast Hopping
Slow frequency hopping: several data symbols are sent on each hop. The hop rate is lower than the symbol rate. Fast frequency hopping: each data symbol is spread over several hops. The hop rate is higher than the symbol rate.Fast vs Slow Frequency Hopping: Comparison Table
| Feature | Slow hopping | Fast hopping |
|---|---|---|
| Hop rate vs symbol rate | Hop rate < symbol rate | Hop rate > symbol rate |
| Symbols per hop | Several symbols per hop | One symbol spread over several hops |
| Chip rate | Equal to the symbol rate | Equal to the hop rate |
| Effect of a jammed channel | Several symbols are lost, so error-correction coding is essential | Only part of each symbol is lost, so the symbol can still be recovered |
| Resistance to jamming and fading | Lower | Higher (frequency diversity) |
| Frequency synthesizer | Slower and cheaper | Must switch very fast, so it is costly |
| Complexity and cost | Simpler, cheaper | More complex, more expensive |
| Typical use | Bluetooth, GSM frequency hopping | Military and anti-jam communication |
Advantages and Disadvantages of FHSS
- Advantage: Strong resistance to narrowband jamming and interference.
- Advantage: Hard to intercept without knowing the hopping code.
- Advantage: Many users can share the same band using different hopping codes.
- Advantage: Reduces the effect of multipath fading.
- Disadvantage: Needs a wide bandwidth and accurate synchronization.
- Disadvantage: Frequency synthesizers for fast hopping are complex and costly.
Real-World Example: Bluetooth
Classic Bluetooth uses slow frequency hopping in the 2.4 GHz ISM band. It hops among 79 channels, each 1 MHz wide, at 1600 hops per second. This lets many Bluetooth devices work near each other and near Wi-Fi without serious interference.
Practice MCQs for Job and University Exams
1. In fast frequency hopping:
(a) Several symbols are sent per hop
(b) The hop rate is higher than the symbol rate
(c) The hop rate equals zero
(d) No PN code is used
Answer: (b)
2. In slow frequency hopping, the chip rate is equal to:
(a) The hop rate
(b) The symbol rate
(c) The carrier frequency
(d) The PN code length
Answer: (b)
3. Which type of hopping gives better protection against jamming?
(a) Slow hopping
(b) Fast hopping
(c) Both are equal
(d) Neither
Answer: (b)
4. The hopping pattern in FHSS is controlled by a:
(a) Pseudo-noise (PN) code
(b) Huffman code
(c) Line code
(d) Parity bit
Answer: (a)
5. Classic Bluetooth hops among how many 1 MHz channels?
(a) 23
(b) 40
(c) 79
(d) 124
Answer: (c)
6. The processing gain of an FHSS system is approximately equal to:
(a) The number of hop channels
(b) The symbol rate
(c) The carrier frequency
(d) One
Answer: (a)
Frequently Asked Questions (FAQs)
Q: What is the main difference between fast and slow hopping?
A: In slow hopping, several symbols are sent on each frequency. In fast hopping, each symbol is spread over several frequencies, so the carrier changes faster than the data.
Q: What is the difference between FHSS and DSSS?
A: FHSS spreads the signal by jumping the carrier between channels over time. DSSS (Direct Sequence Spread Spectrum) spreads it by multiplying the data with a fast PN code, so the signal occupies the whole wide band at once. DSSS is used in CDMA and GPS.
Q: Who invented frequency hopping?
A: Actress Hedy Lamarr and composer George Antheil patented a frequency hopping system in 1942 to protect radio-guided torpedoes from jamming.
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