Ultra wide band communication systems using orthogonal frequency coded SAW correlators

This paper examines ultra-wideband (UWB) communication systems utilizing orthogonal frequency coded surface acoustic wave (SAW) correlators. Orthogonal frequency coding (OFC) and pseudo-noise (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; allowing for improved correlation gain. The transceiver approach is still very similar to that of the CDMA approach but provides greater code diversity. Use of 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. The OFC SAW correlator consists of a dispersive OFC transducer and an apodized output transducer. The dispersive filter was designed using seven contiguous chip frequencies within the transducer. Each chip is weighted in the transducer to account for the varying conductance of the chips and to compensate for the output transducer apodization. Since each chip in the OFC transducer has a different local carrier frequency, inter-electrode reflections are greatly reduced compared to a single carrier frequency. In general, this allows quarter-wavelength electrodes to be used in narrow bandwidth correlator designs; however, for bandwidths above 25%, as in the ultra-wideband case, sixth-wavelength electrodes are used to eliminate bulk mode conversion effects within the transducer. Experimental correlator results of an OFC SAW correlation filter are presented. The dispersive filter was designed using seven contiguous chip frequencies within the transducer. SAW correlators with fractional bandwidth of approximately 29% were fabricated on lithium niobate (LiNbO₃) having a center frequency of 250 MHz and the filter has a processing gain of 49. A coupling of modes (COM) model is used to predict the experimental SAW filter response. Good correlation between the predicted COM responses and t- e measured device data is obtained and presented. Discussion of the filter design, analysis and measurements are presented. Results are shown for operation in a matched filter correlator for use in an UWB communication system and compared to predictions, showing good results. The results demonstrate that OFC SAW devices can be used for UWB communication transceivers.