High Efficiency Narrowband

The Intelligent Transportation System (ITS) relies heavily on data communications systems to link eld equipment such as traac sensors, changeable message signs and incident response vehicles with traac operations centers. This paper has two main purposes. The rst is to present a review of data communications concepts essential in the design of wireless modems for the ITS 220 MHz spectral allocations. The second is to propose an architecture for such modems which ooers a spectral eeciency much higher than that available in current products. The new modem architecture is implemented using the combination of a digital signal processor, discrete electronics, and high level software. All sophisticated signal processing is implemented in software to provide both high performance and exibility. The system forms an ideal testbed for the design of future enhancements and modiications to t speciic ITS applications. A prototype has been constructed and subjected to full bench testing. Preliminary eld testing has been performed and more is in progress. INTRODUCTION The Federal Highway Administration (FHWA) has identiied a set of seven features that form a core infrastructure for the deployment of Intelligent Transportation System (ITS) traac management and traveler information services in metropolitan areas. Data communication systems linking eld equipment with transportation infrastructure are a fundamental requirement of all of the core infrastructure features. In 1992, ve frequency pairs in the 220-222 MHz Land Mobile radio band were allocated to the FHWA for a period of fteen 1 years. These frequencies will be used by the FHWA in ITS applications which have national implications and beneets, including selected research experiments and operational tests. The ITS 220 MHz channels are narrowband channels, each with a usable bandwidth of about 4 kHz. Due to the limitedbandwidth, achieving signiicant data transmission rates over these channels requires a very eecient modem design. We deene the spectral (or bandwidth) eeciency of a communication link at the physical layer to be the data rate available to higher level functions divided by the bandwidth of the link. Standard twisted-pair telephone circuits also have an available bandwidth of about 4 kHz, yet modem technology has now evolved to the point where maximum data transmission rates are at 28,800 bits per second (28.8 kbps). This amounts to a spectral eeciency of almost 9 bps/Hz. In certain situations the spectral eeciency of narrowband wireless data transmission can be made to approach the eeciency of wireline data transmission, but it …