DESIGN AND IMPLEMENTATION OF A FREQUENCY HOPPING SPREAD SPECTRUM SYSTEM

Abstract

In this thesis, the design, implementation, and testing of a complete fast frequency hopping spread spectrum (FFH-SS) system is demonstrated. By applying the suitable channel-coupling circuits, the system can be used in many applications including: power line carrier, remote meter reading, power management, load control and intrabuilding computer communications network. The proposed system uses binary frequency shift keying (BFSK) for data

modulation and demodulation. The BFSK signal is randomly hopped among 32 expected frequency slots, occupying a total bandwidth of320 kHz. The hopping rate is 2000 hops/sec while the data rate is 400 bits per sec (bps). The transmitter employs a direct digital frequency synthesizer (DDS) for both spreading and data modulation. The receiver utilizes a single-dwell stepped-serial-search (SSS) circuitry for initial code acquisition and a delay-locked loop (DLL) circuitry for code tracking. The two loops enable the receiver to dehop the received signal to the normal BFSK bandwidth. The data is then extracted using normal non-coherent BFSK demodulator. A simplified scheme for the receiver digital frequency synthesizer and a multiplier-less correlator circuits are introduced. Using this scheme, a single DDS is used for both acquisition, tracking and demodulation tasks. The operating characteristics of the acquisition channel, the root-mean square jitter and the S-curve performance of the tracking channel are obtained by means of computer simulation. The simulation process considers the discrepancies between ideal and implemented acquisition and tracking schemes. The bit error rate (BER) of the system is tested experimentally against different jamming signals for different signal-to-noise (SNR), and signal-to-jammer (SJR), ratios. A channel simulator circuit is designed and implemented to provide white noise, partial band, single tone, multiple tones, and swept-frequency jamming signals. Variable attenuation levels are provided to modify the SNR. The bit error rate (BER) is measured for each type of interference at different SNR's. The system achieved a BER of 6.25x10-6 at SNR of 19 dB in AWGN environment.