Frekvenshopping

Frekvenshopping, also known as frequency-hopping spread spectrum (FHSS), is a method of transmitting radio signals by rapidly switching a carrier frequency among many discrete channels within a band according to a pseudorandom sequence. The transmitter and receiver must stay synchronized to hop together, so the same data is received as long as the hopping sequence is shared and kept secret or controlled.

How it works: The signal is spread over a wide portion of the available spectrum by hopping between frequencies in short intervals, or dwells. This creates a pseudo-spread spectrum that reduces the impact of narrowband interference and makes it harder to eavesdrop on the communication without knowledge of the hopping pattern. Data is conveyed over these hops, with the rate limited by the dwell time and the bandwidth of each channel.

History and usage: FHSS has military origins and later found civilian applications. It was widely used in early cordless phones and some wireless devices. Bluetooth originally employed FHSS in its classic form, with a fast hop rate across multiple 1 MHz channels; newer Bluetooth versions incorporate adaptive frequency hopping to avoid crowded frequencies.

Advantages and limitations: FHSS offers resilience to interference, multipath fading, and some degree of security through obscurity, as the hopping pattern is not readily known to outsiders. Limitations include the need for precise synchronization between transmitter and receiver, potentially lower data throughput for a given spectral bandwidth, and regulatory constraints on hopping behavior and dwell times.

Regulatory and practical notes: In many regions FHSS is permitted in ISM bands under specific rules, with requirements on dwell time, channel spacing, and total bandwidth to ensure coexistence with other users.