Anti-jam Communications Using Frequency-Hopped OFDM and LDPC with Erasure Decoding ("Minotaur")

We consider frequency-hopped OFDM (FH-OFDM) as the basis for anti-jam communications achieving very high processing gain. A conventional OFDM signal is hopped across a number of frequency sub-bands. Our design hops a 1 MHz bandwidth OFDM waveform to one of 400 frequencies (26 dB processing gain), occupying a large total bandwidth of 400 MHz. On each hop we assume an independent channel realization and jammer state. Employing 16-QAM and rate-1/2 coding, we achieve a data rate of 1.22 Mbps after accounting for all overheads, such as cyclic prefix, pilots, and peak-to-average control. We examine the impact of coding across hops in fading channels to achieve frequency diversity, and design custom LDPC codes for the resulting short block length (1248 information bits). We find that under jammed conditions erasure decoding improves BER by an order of magnitude, and develop a simple yet effective "in-band" approach to jammer state estimation. A non-real-time prototype was tested in the lab, sending the 400 MHz signal over a wired channel. Jamming levels high enough to completely preclude non-hopped communication degrade our system by only 2-5 dB, depending on channel conditions.