Faster frequency hopping for narrowband radio systems
Publish date: 2020-12-14
Report number: FOI-R--5071--SE
Pages: 39
Written in: Swedish
Keywords:
- frequency hopping
- follower jamming
- hop rate
- MFSK
- OFDM
- CPM
Abstract
Frequency hopping is the main jamming protection method in military tactical radio systems and follower jamming is an efficient jamming technique for frequency hopping radio systems. By measuring in real time which frequency is currently being used, the follower jammer can instantly concentrate the energy of the interference signal to the used frequency. A high hopping rate is required to provide good protection against follower jamming. The study investigates whether narrowband systems, with 50 kHz bandwidth, can achieve hop rates of 5 hop/ms with a data rate of at least 20 kbit/s. Various aspects of the effect of the hop rate are considered, such as safe distance to the jammer, propagation delay, the out-of-band properties of the communication signal and orthogonally hopping networks. Basic properties such as power spectrum, pulse shaping and bandwidth expansion are also addressed. In general, an increased hop rate modifies the spectrum of the frequency hopping signal in two ways: the bandwidth of the data-modulated signal increases and the out-of-band properties are affected. Some example systems based on MFSK, OFDM and CPM are analyzed with respect to information data rate, bandwidth and communication distance. MFSK can be used for fast frequency hopping, but not with a sufficiently high information data rate. The performance requirements are met for the examples based on OFDM and CPM at moderate ramp times and guard times for propagation delays.