Telemetry system

Introduction

Telemetry is a key aspect in present automotive technology, both for racing and commercial applications (in new models development and prototype testing). The need for continuously increasing bit rates in order to allow a larger and larger number of sensors to be monitored, and the possibility to guarantee a proper radio coverage, also in difficult propagation conditions, require to develop sophisticated radio systems. However the requirements of reduced size and weight and low power consumption, particularly in racing applications, imposes stringent constraints during the development phase. In order to guarantee good performance in challenging radio propagation conditions a COFDM (Coded Orthogonal Frequency Division Modulation) approach has been selected. Even though the principles of COFDM have been studied for about thirty years, practical interest for this kind of modulation technique has only recently increased thanks to the significant improvements in Digital Signal Processors (DSPs) performances both in terms of CPU capacity and high speed internal RAM availability. The smart idea of COFDM systems is to split the base band serial data stream into many parallel streams with lower data rate, and to transmit each of them on a separate sub carrier. These carriers are forced to be orthogonal choosing an appropriate frequency spacing (among them). Due to the usual high number of transmitted carriers, larger symbol duration is obtained compared with single carrier transmission systems: up to 1 ms in practical applications. The resulting longer symbol duration allows inserting a guard interval or a cyclic prefix to each COFDM symbol. In this manner, it is possible to tolerate a partial symbols overlapping, since carrier orthogonality is guaranteed if the length of the circular prefix is chosen to be larger than the channel delay spread. This fact, combined with the spectrum partitioning provided by the multiplexing of data in the frequency domain, gives OFDM a high resistance to fast fading and frequency selective channels, and Doppler shift effects. Moreover in COFDM, the redundancy introduced by the convolutional codes allows to recover the symbols corrupted by noise due to channel selectivity. The designed architecture is completely based on a powerful general purpose DSP for the all modulation processing functions. The use of a Digital Signal Processor to perform data modulation, grants to have a fully software radio (SWR) system, very flexible, in order to reduce the hardware complexity and reconfiguration. In an initial phase, this architecture allows to implement different kinds of OFDM modulation schemes suited to the hardware platform, and later, the one that provides the best trade off in terms of BER performance, power consumption, bit rate, error protection, and other critical parameters in the challenging automotive environment, may be selected.