/ˌɛf eɪtʃ ɛs ɛs/
n. "Pseudo-random carrier frequency switching spreading narrowband signal across wide spectrum for Bluetooth anti-jamming."
FHSS, short for Frequency-Hopping Spread Spectrum, transmits data bursts across rapidly changing carrier frequencies (1600 hops/sec in Bluetooth) following a PN sequence known to TX/RX, minimizing narrowband interference impact since each hop duration (~625μs) sees only fraction of total energy. Contrasts WiFi DSSS by occupying full channel briefly vs spreading chip sequence continuously, enabling multiple piconets sharing 2.4GHz ISM band with low mutual interference.
Key characteristics of FHSS include: Pseudo-Random Hop Sequence generated via LFSR across 79 x 1MHz channels (Bluetooth Classic); Fast Hop Rate 1600 hops/sec (400ms period/79 channels); Adaptive FHSS blacklists interfered channels; Short Dwell Time 625μs/classic slot limits jammer effectiveness; CDMA Capability multiple transmitters share band via unique hop patterns.
Conceptual example of FHSS usage:
/* Bluetooth Classic FHSS hop sequence generator */
#define NUM_CHANNELS 79
#define HOP_PERIOD 625 // microseconds
uint8_t hop_sequence[NUM_CHANNELS];
uint16_t clock; // 12.5ms native + 625us slots
uint8_t current_channel;
void generate_bluetooth_hop_seq(uint16_t clk_28) {
// Native clock → 5-bit channel select via permutation
uint8_t page = (clk_28 >> 7) & 0x1F; // Page hop bits
uint8_t inquiry = (clk_28 >> 3) & 0x1F; // Inquiry bits
current_channel = permute(page ^ inquiry ^ clk_28);
set_rf_channel(current_channel); // Synth tune 2402 + ch MHz
}
void transmit_slot() {
// 625us dwell: transmit GFSK packet
transmit_gfsk_packet(data_buffer, 366); // 366us max
delay_us(625); // Slot time
generate_bluetooth_hop_seq(++clock); // Next hop
}Conceptually, FHSS evades interference by transmitting brief packets across wide spectrum—Bluetooth piconets hop pseudorandomly while AFH detects/avoid busy channels, ensuring reliable PAN links amid WiFi 2.4GHz congestion. Military origins (Hedy Lamarr torpedo guidance) evolved to civilian WPANs where hop synchrony via master clock maintains TDD timing; contrasts SerDes fixed carriers requiring CTLE equalization, with spectrum analyzers revealing FHSS as impulsive wideband noise versus OFDM comb lines.