Application of chirp to spread-ALOHA system

This paper proposes a spread-ALOHA system using chirp. The system is composed of the modulator containing the chirp filter and the demodulator containing the chirp matched filter, the quasi-coherent detector and the block demodulator. In the proposed system, the data transmission is realized by using the chirp signal with phase information corresponding to binary data and then, using the chirp effect, the frequency component of the transmitted signal is spread continuously. In addition, since the respective bits composing a packet are compressed in time by the correlation processing using the chirp matched filter, the multiplexed packets are separated in individual packets based on the timing difference among the received packets. The carrier synchronization, which is required in the demodulation process, is achieved by quasi-coherent detection and the block demodulation. The frequency and the phase offset occurring in the quasi-coherent detection are corrected by a method of highly accurate frequency detection based on the fast-Fourier transform in the block demodulator. Furthermore, assuming various conditions, quantitative evaluations are made on the bit-error rate performance by means of computer simulation. It is verified that a high interference immunity is realized by the frequency spread using chirp, and frequency offset generated in the quasi-coherent detection is well estimated and corrected.