Improving Security and Reducing Interference with Frequency Hopping Spread Spectrum

Frequency Hopping Spread Spectrum (FHSS) transmission is repeated switching of radio frequencies to reduce interference. The procedure is also done to avoid interception. It is used to counter eavesdropping or to sidestep any jamming of telecommunications. FHSS also minimizes the unwanted effects of unintentional interference.

Definitions: Hop rate, hop interval, and hopset

FHSS is a periodic changing of the transmitted signal's carrier frequency. The transmission uses a set of M possible carrier frequencies {f1,f2,…..,f_M }  termed the hopset.

The Hop rate refers to the rate of movement of the frequency hopping spread spectrum between the transmission frequencies. The hop rate equals the reciprocal of dwell time at the FHSS center frequency.

Hopping takes place over a frequency band (the hopping band), which includes the M frequency channels. Each frequency channel can be described as a kind of spectral region that includes a single carrier frequency of the hopset as its center frequency, and has a sufficiently big bandwidth (B) to include maximum power in a signal pulse with a particular carrier frequency. The sequence of carrier frequencies is termed the frequency hopping pattern. Hop interval, as the name suggests, is the time interval between hops.

When it comes to transmission, the binary data is fed into a modulator using some kind of digital-to-analog encoding scheme. The latter can be Frequency Shift Keying (FSK) or Binary Phase Shift Keying (BPSK).

Frequency Shift Keying (FSK)

Frequency Shift Keying (FSK) describes the digital modulation technique where the carrier signal frequency varies as the digital signal changes. FSK is thus a frequency modulation scheme. The following figure shows the FSK signal waveform.

Figure 1: FSK signal waveform

In Figure 1, mc(t) represents the digital message modulated by two carrier frequencies fc1 and fc0. The fc1 is transmitted when the message signal is zero, and fc0 when the message signal is one. The data can be modified by more than two frequencies.  When this is done, it is termed Multiple Frequency Shift Keying (MFSK). MFSK describes a technique for digital data transmission using multiple tones, and one tone or multiple can be used simultaneously.

FHSS Detailed Description

The data gets modulated through the FSK scheme and gives the FSK signal, thus making it an MFSK signal.

Figure 2: FHSS block Diagram

The frequency synthesizers generate frequency hopping patterns determined by time-varying multilevel sequence as specified by output bits of code generators.

PN Generator

A pseudo-noise code or PN generator provides a table of frequencies and also indexes it. Each of the K bits makes particular mention of the 2K frequencies. For example, if K=3, then the total number of sequences will be eight, depending on the selected particular frequency code in the following table.

Table 1: PN Generator

The data modulated signal gets mixed with the synthesizer output pattern so that a frequency-hopping signal gets produced.

The receiver synthesizer at the demodulator end produces the pattern synchronized with the transmitter produced pattern. A fixed intermediate frequency offsets this pattern. It may be zero. The frequency hopping pattern is removed by the mixer from the received signal, and this is termed dehopping. The mixer output gets applied to the band pass filter, which removes all double frequency components and produces the data modulated dehopped signal.

The pattern must be a pseudo-random sequence of frequencies. If this is done, the signal will not be detected, and signal privacy will be maintained.

Figure 3: Frequency hopping pattern

The sequence must have a large period and be uniformly distributed over the frequency channels. It must be generated by a multilevel sequence having a wide linear span. The width detracts the opponent by inhibiting not only the capture but also storage of the period pattern. The sequence satisfies in its smallest degree any linear recursion like the linear span of any multilevel sequence. A wide linear span discourages pattern reconstruction from its short segment.

The code generator generated control bits set usually constitutes a symbol. The latter is drawn from a finite field having the required properties. The frequency hopping pattern is extracted by linking a distinct frequency with a particular symbol.

Dwell Interval

A dwell interval refers to a frequency hopping pulse with a fixed carrier frequency. This occurs during a portion of the hop interval.

Dwell Time

Dwell time is described as the duration of the dwell interval when the channel symbols get transmitted.  The hop duration T_h equals the sum of dwell time T_d and switching time T_sw. The switching time equals dead time. The latter refers to the interval duration when there are no signals in addition to pulse rise and fall times.

Even if the transmitted signal has no switching time, it will be present in the receiver’s dehopped signal due to the faulty synchronization between the received and the receiver-generated waveforms. The nonzero switching time that may include any intentional guard time decreases the transmitted symbol duration T_s. If T_so is the symbol duration in the absence of frequency hopping, then T_s=T_so (T_d/T_h ). The transmitted spectrum expands when the symbol duration reduces. This reduces the frequency channel numbers within the fixed hopping band. The receiver filtering will ensure that the rise and fall times of pulses have durations on the order of a symbol duration,T_sw> T_s.  For all practical systems, implementation of short switching time becomes an obstacle with the decrease of the hop rate.

FHSS modulation techniques

Digital methods like pulse-amplitude modulation, phase shift keying, and quadrature amplitude modulation are not preferred for FHSS, because they are ineffective in the light of coherent detection when compared to FSK.

Frequency hopping types include slow and fast frequency hopping.

Slow frequency hopping

When it comes to slow frequency hopping, multiple symbols get transmitted in a single frequency hop. One or multiple symbols get transmitted over the same carrier frequency. The value of slow frequency hopping is the reduction of radio signal fading effects. Unwanted effects from the interference of radio channels operating on the same frequency are also minimized. This signal can be detected by a jammer if the hop's carrier frequency is known.

Figure 4: Slow frequency hopping

Fast frequency hopping

Multiple hops in fast frequency hopping must transmit one symbol. A single symbol gets transmitted in different hops over multiple carriers. This signal escapes undetected by the jammer as one symbol gets transmitted over multiple carrier frequencies.

Figure 5: Fast Frequency Hopping

FHSS Applications

Bluetooth remains the most common FHSS application, with millions of users opting for this feature. The technology uses an FHSS variant and the latter is a platform for Bluetooth. Militaries all over the world use FHSS to communicate between members. This application finds favor as it is near impervious to jamming by enemies.