Surface acoustic wave convolvers for spread-spectrum mobile and wireless communications

This paper reviews three surface acoustic wave (SAW) convolver types for use in real-time spread-spectrum code-division multiple access (CDMA) mobile and wireless communications in noisy electromagnetic environments. Operational tradeoffs include convolution efficiency η _c, operational frequency, and code-length capability. The first involves Rayleigh-wave propagation on single-crystal SAW piezoelectrics for IF signal processing with η_c~-50 to -70 dBm^-1, while the second is based on Sezawa wave processing in thin-film layered SAW structures, with η_c~-40 dBm^-1. Both can employ internal SAW-based synchronous/asynchronous correlation of lengthy pseudo-noise (PN) chip codes with direct sequence (DS) and frequency hopping (FH), suitable for low-power indoor and outdoor communications. One potential disadvantage of these two is in the frequency doubling of the convolved output. The third SAW-based design, that avoids frequency doubling, employs external mixing of signal and reference codes. While it has currently been applied to short-code modulation/demodulation (e.g. Barker codes), it offers higher conversion efficiency-in competition with both digital and analog signal processing using non-SAW techniques