time-hopping

Time-hopping is a signal-spreading technique where a transmitter divides data into short pulses and scatters them across time slots according to a pseudorandom sequence known only to the intended receiver. Instead of occupying a continuous channel, the signal hops in time much as frequency-hopping spreads energy across the frequency spectrum. The receiver synchronizes with the same pseudorandom code and reconstructs the message by collecting pulses from their scattered time slots. In practice, pulse widths typically measure in nanoseconds to microseconds, with the hopping pattern repeating at intervals of milliseconds or longer.

The technique excels in hostile radio environments. An unauthorized listener without knowledge of the hopping sequence perceives only brief, scattered energy bursts that appear as noise or momentary interference. A jammer attempting to disrupt the signal must either jam across all possible time slots, spreading its power thinly, or correctly predict the next hop. Time-hopping resists both targeted jamming and unintentional interference better than fixed-schedule transmission. This property makes it valuable in military communications, cellular systems, and industrial wireless networks operating in congested spectrum.

Time-hopping pairs naturally with spread-spectrum modulation. Ultra-wideband (UWB) systems typically employ time-hopping as their fundamental mechanism, generating very short pulses spread across wide bandwidths and pseudorandomly delayed. A time-hopping receiver must maintain strict timing synchronization, usually within a fraction of the pulse width. Synchronization drift causes pulses to fall outside their assigned collection windows, raising the error rate. Receiver complexity grows with the required timing precision and the length of the pseudorandom sequence.

Distinction from frequency-hopping

Frequency-hopping and time-hopping are complementary techniques. Frequency-hopping shifts the carrier frequency; time-hopping shifts the transmission moment. Time-hopping consumes less instantaneous power since energy is concentrated in brief pulses rather than spread continuously. However, time-hopping demands tight synchronization and works best with short-pulse waveforms. Frequency-hopping tolerates loose synchronization and suits narrowband systems. Some advanced systems combine both, achieving deeper protection against jam.

The pseudorandom sequence itself must possess strong cryptographic properties: long period, low autocorrelation, and resistance to prediction. Weak sequences allow an eavesdropper to detect the pattern and decode the signal. Most practical systems derive the hopping code from a shared secret key between transmitter and receiver, similar to the keying in frequency-hopping systems.