Frequency hopping spread spectrum is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter and receiver. The data signal is modulated with a narrow-band carrier signal that "hops" in a random but predictable sequence form frequency to frequency as a function of time over a wide band of frequencies. FHSS is used to avoid interference, and to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications.
It was invented and patented in 1942 by the actress Hedy Lamarr and paintist George Antheill.
Principle of frequency hopping spread spectrum
FHSS has a very simple and interesting principle : a wide frequency band is divided into multiple channels and communications are hopping sequentially from one channel to another in a sequence and a rate agreed to advance between the transmitter and receiver.
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| Figure 1 : showing channel assignment and channel use |
Basic approach of FHSS
- Some number of channels are allocated for a frequency hopping(FH) signal.
- It has 2^k carrier frequencies forming 2^k channels.
- Spacing between carrier frequencies(i.e, the width of each channel) corresponds to the bandwidth of the input signal.
- Transmitter operates in one channel at a time for a fixed interval.
- During that interval,some number of bits is transmitted using some encoding scheme.
- Spreading code dictates the sequence of channels used.
- Both transmitter and receiver use the same code to tune into a sequence of channels in synchronization.
FHSS (Transmitter)
- Binary data are fed into a modulator using some digital to analog encoding scheme, such as FSK or BPSK resulting signal Sd(t) which is centered on some base frequency.
- Pseudonoise (PN) source serves as an index into a table of frequencies.
- Each k bits of the PN source (i.e., spreading code) specifies one of the 2^k carrier frequencies.
- At each successive interval, a new spreading code ( k bits) is generated, a new carrier frequency is selected.
- Frequency synthesizer generates a constant frequency tone whose frequency hops among a set of 2^k frequencies, with the hopping pattern determined by k bits from the PN sequence. It is known spreading or chipping signal c(t).
- c(t) is then modulated by the signal produced from the initial modulator to produce a new signal with the same shape but now centered on the selected carrier frequency.
- Bandpass filter is used to block the difference frequency and pass the sum frequency, yielding the final FHSS signal s(t).
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| Figure 2 : Transmitter block |
FHSS (Receiver)
- Signal s(t) is multiplied by a replica of the spreading signal c(t) to yield a product signal Sd(t).
- Bandpass filter is used to block the sum frequency and pass the difference frequency.
- Output signal of bandpass filter is then demodulated to recover the binary data.
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| Figure 3: Receiver block |
Block diagram of frequency hopping spread spectrum technique
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| Figure 4: Block diagram representing FHSS technique
The frequency of the carrier c(t), i.e. the sequence of channels, depends on the spreading code,which is generated by a pseudonoise (PN) source. Every Tc seconds, the PN source produces a new k-bit value. This value is then used to look up a channel in the channel table, and that determines the frequency of c(t) for that time interval. For example, consider a system where k=4, and for the first time interval, the PN source generates teh value 1101 . For that time interval, the signal will be transmitted on channel 13. (The channel table will have 2^k-1=2 =15 entries, indexed from 1 to 15. The PN sequence will repear itself with a period of 2^k -1).
One popular way to generate a PN sequence is with a linear feedback shift register (LFSR). Consider the LFSR described by the sequence 10011,which can also be described in the below diagram :
Figure 5: The LFSR corresponding to generator 1011
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In the FHSS implementation used in this experiment, the spreading code is determined from the state of the registers as shown in Figure 5. After each time instance, the values in each register "shift" as follows (for the LFSR generated by "10011"):
- r [n+1] = r [n]
- r [n+1] = r [n]
- r [n+1] = r [n]
- r [n+1] = r [n] + r [n]
r3 r2 r1 r0
e.g. if the values are r3=1, r2 =1, r1 =0, r0 =1, then the channel at f13 will be
used (1101 =13).
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| Figure 6: showing hopping of frequency |
The first four values for the LFSR corresponding to the generator 10011, with initial register values 1111, would be: 15,7,14,5
For the example above, the complete frequency hopping sequence would be:
f15,f7,f14,f5,f10,f13,f3,f6,f12,f1,f2,f4,f8,f9,f11 after which it will repeat.
The result obtained is shown in the figure below :
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| Figure 7: Frequency hopping shown in figure |
Types of FHSS
There are two types of frequency hopping :
- Slow frequency hopping spectrum
- Fast frequency hopping spectrum
1.Slow frequency hopping spectrum
Tc >Tb
where
Tc – Chip period
Tb- Bit period
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| Figure 8: Slow frequency hopping spectrum |
2.Fast frequency hopping spectrum
2.Fast frequency hopping spectrum
Tc <Tb
where
Tc – Chip period
Tb- Bit period
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| Figure 9: Fast frequency hopping spectrum |
Advantages of FHSS :
- FHSS signals are highly resistant to narrowband interference because the signal hops to a different frequency band.
- Signals are difficult to intercept if the frequency-hopping pattern is not known.
- FHSS transmissions can share a frequency band with many types of conventional transmissions with minimal mutual interference. FHSS signals add minimal interference to narrowband communications, and vice versa.
- It can be programmed to avoid some portions of spectrum.
- FHSS needs shorter time for acquisition.
- Very large bandwidth.
Disadvantages of FHSS :
There are some disadvantages of frequency hopping spread spectrum (FHSS) which are given below,
- It is need complex frequency synthesizers.
- Not useful for range and range rate measurements.
- It requires error correction.
Applications of FHSS :
- Military use
- Bluetooth
- Walkie-Talkies
- Other radios
Military use
- Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics.
- Military radios use cryptographic techniques to generate the channel sequence under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.
- By itself, frequency hopping provides only limited protection against eavesdropping and jamming.
- Most modern military frequency hopping radios also employ separate encryption devices such as the KY-100.
- Military radios that use frequency hopping include the JTIDS/MIDS family, the HAVE QUICK Aeronautical Mobile communications system, and the SINCGARS Combat Net Radio, Link-16.
Bluetooth
Adaptive Frequency hopping spread spectrum (AFH) (as used in Bluetooth) improves resistance to radio frequency interference by avoiding crowded frequencies in the hopping sequence. This sort of
adaptive transmission is easier to implement with FHSS.
Walkie-talkies
- Some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios are marketed under the name eXtreme Radio Service (eXRS).
- Motorola has deployed a business-banded, license free digital radio that uses FHSS technology: the DTR series, models 410, 550 and 650.
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