Full-duplex asynchronous spread spectrum modem using a SAW convolver for 2.4-GHz wireless LAN

An asynchronous spread spectrum (SS) modem for 2.4-GHz wireless LAN has been implemented using an efficient ZnO-SiO₂-Si surface acoustic wave (SAW) convolver. The design of the highly efficient SAW convolver was developed at Tohoku University and commercially manufactured by Clarion Co., Japan. The modem can operate under full-duplex transmission in the same frequency range of the 2.4-GHz SS band. The SS modem is based on a direct-sequence/code-shift-keying (DS/CSK) method for the modulation. Pseudo-noise (PN) codes are chosen from a preferred pair of 127-chip m-sequences and the code rate is 14 MHz. The asynchronous demodulation is simply realized utilizing the coherent correlation characteristics of the SAW convolver. Under full-duplex transmission, the self-jamming caused by a transmitted signal in the modem itself is effectively reduced by an RF isolator and the SS processing gain. The implemented modem has been tested using a coaxial cable with attenuator. A bit error rate of 10^-6 under full-duplex transmission is observed at -78.3 dB of a desired to undesired signal ratio. The effective range is estimated on the basis of two-path propagation model. From self-jamming rejection of -78.3 dB, the effective range under real-time full-duplex is estimated to be about 200 m.