AN APPLICATION OF CHIRP TO SPREAD-ALOHA SYSTEM

This paper proposes a spread-ALOHA system using chirp. The system is c omposed of the modulator containing the chirp filter and the demodulat or containing the chirp matched filter, the quasi-coherent detector an d the block demodulator. In the proposed system, the data transmission is realized by using the chirp signal with phase information correspo nding to binary data and then, using the chirp effect, the frequency c omponent 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 mult iplexed packets are separated in individual packets based on the timin g difference among the received packets. The carrier synchronization, which is required in the demodulation process, is achieved by quasi-co herent detection and the block demodulation. The frequency and the pha se offset occurring in the quasi-coherent detection are corrected by a method of highly accurate frequency detection based on the fast-Fouri er transform in the block demodulator. Furthermore, assuming various c onditions, quantitative evaluations are made on the bit-error rate per formance by means of computer simulation. It is verified that a high i nterference immunity is realized by the frequency spread using chirp, and frequency offset generated in the quasi-coherent detection is well estimated and corrected.