Ultra Wide Band Communication Systems using Orthogonal Frequency Coded SAW Correlators D. R. Gallagher, N.Y. Kozlovski and D. C. Malocha School of Electrical Engineering and Computer Science University of Central Florida, Orlando, Florida 32816 danielgallagher@ieee.org Abstract— This paper examines ultra-wideband (UWB) com- munication 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 band- width; 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 the 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. I. I NTRODUCTION Ultra-wideband (UWB) communications has recently re- ceived great attention as an emerging new technology due to its numerous communications advantages. The ability to share the FCC allocated frequency spectrum, large channel capacity and data rate, simple transceiver architecture and high performance in noisy environments has paved the way for emerging wireless technologies. The use of ultra-short pulses has become the widely accepted method for achieving the LNA LO PA OFC SAW OFC SAW Correlator LNA Receiver Transmitter Fig. 1. Conceptual block diagram of UWB OFC transmitter and receiver. very wide bandwidths and low power spectral density needed for UWB communications. The “impulse radio” method of UWB communications is effective and simple; however the implementation of more complex signals are not feasible with even the fastest silicon technologies. Surface acoustic wave (SAW) devices, however, allow for simple generation and detection of complex UWB communication. The numerous advantages of SAW devices for UWB communications were recently demonstrated using a pseudo-noise (PN) coded SAW transducer to implement a CDMA coded signal on a single frequency RF carrier [2]. The use of a SAW correlator elim- inates the need for costly high speed silicon CMOS devices as well as many of the costly components needed in the IF section. The use of OFC in UWB SAW correlators provides several advantages over CDMA including an increased range due to enhanced processing gain and greater multiple access opera- tion due to greater code diversity [3]. The UWB OFC SAW device is capable of being used as both code generator and matched filter in an UWB transceiver as shown in Fig. 1. II. ORTHOGONAL FREQUENCY CODING BACKGROUND OFC is a spread spectrum coding technique that has been successfully implemented in SAW tags and sensors [4], [5]. The technique uses multiple orthogonal chips, each τ chip long. In the frequency domain, the local chip frequencies are separated by 1/τ chip . The final criteria requires that f chip ·τ chip must equal an integer number of half carrier cycles. A well known example signal is a linear stepped chirp, which contains a series of local chips with contiguous or- thogonal frequencies and linear group delay. For OFC, a