Orthogonal Frequency Coding for Use in Ultra Wide Band Communications and Correlators

This paper presents the development of surface acoustic wave (SAW) correlators using orthogonal frequency coding (OFC) for use in ultra-wideband (UWB) spread spectrum communication systems. The use of ultra-short pulses, producing very wide bandwidths and low spectral power density, are the widely accepted approach for UWB communication systems. This approach is simple and can be implemented with current DSP technologies. However, SAW devices have the capability of producing wide bandwidths and relatively high frequency operation. As an example, Brocato, et al., has recently shown the numerous advantages of utilizing SAW correlators in UWB communication transceivers at center frequencies up to 5.6 GHz and fractional bandwidths up to 50% (Brocato, 2004). Their approach was to use a PN coded SAW transducer which produces a PN code sequence on an RF carrier. Information is encoded by pulse-phase-modulation used to excite the SAW which can be amplified and then transmitted. Reception is achieved by correlation of the matched filter received response and base-band envelope detection to extract the pulse phase. This approach, using SAW correlators, eliminates many of the costly components that are needed in the IF block in the transmitter and receiver, and reduces much of the signal processing requirements. This paper introduces OFC SAW transducers which can be used in UWB communication transceivers. OFC and PN coding provides a means for UWB spreading of data. The use of OFC spectrally spreads a PN sequence beyond that of CDMA because of the increased bandwidth. The transceiver approach is still very similar to that of the CDMA approach but provides greater code diversity. Experimental results of a SAW filter designed with OFC transducers are presented. The SAW correlation filter was designed using seven contiguous chip frequencies within the transducer. SAW correlators with 29% fractional bandwidth were fabricated on lithium niobate (LiNbO₃) having a center f- requency of 250 MHz. A coupling of modes (COM) model is used to predict the SAW filter response experimentally and is compared to the measured data. Good correlation between the predicted COM responses and the measured device data is obtained. Discussion of the design, analysis and measurements are presented. The results show that OFC SAW devices can be used for UWB communication transceivers