DESIGN CONSIDERATIONS FOR DIRECT RF SAMPLING RECEIVER IN GNSS ENVIRONMENT Ville Syrjälä, Mikko Valkama, Markku Renfors Tampere University of Technology Institute of Communications Engineering P.O Box 553, FIN-33101 Tampere, Finland ABSTRACT In this paper, an advanced direct RF sampling receiver architecture is studied for the GNSS environment. The architecture is based on sampling the signal directly at RF, which in the GNSS case are in the 1.5 GHz range. The high-frequencies in the signals to be sampled pose then very high demands for the accuracy and quality of the sampling process, and thus quantization and especially the timing jitter must be considered in detail. The study shows that the quantization and jitter requirements are, however, feasible when the pre- sampling filtering is done properly. 1. INTRODUCTION Not only the wide scale changes in technology of Global Navigation Satellite Systems (GNSS), but also the demand for low-cost and flexible GNSS receivers has been greatly increasing in the last few years in many application areas. Car navigation systems are becoming almost an essential part of better-class cars, and the demand for GNSS integrated cellular phones is also increasing [5]. The car navigation systems need to be as cheap as possible and cellular phone integrated navigation systems must also be small and have low power consumption. In addition, as the cellular phones of course need receivers for capturing the cellular signals, there is big interest to receive Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS) and GPS/Galileo signals, all with one flexible receiver. Direct RF Sampling (DRFS) receiver architecture is one promising approach for building flexible radio receivers. The idea of DRFS is to sample the signal at as early stage of the receiver as possible. The down- conversion of the high-frequency signals can be done within the sampling process itself, using aliasing in a controlled manner. So there is no need for analog mixers and thus the analog component count decreases. Analog components are usually bulky, expensive and use quite much power so the basic concept of the DRFS gets very interesting. In addition, the flexibility of the DRFS receiver is at a very high level because the signal is sampled directly from the radio frequencies (RF). Naturally, the signal must be filtered before sampling but the filtering can be done so that the signals of more than one communications systems can be processed and received. The topic of DRFS has not yet been thoroughly investigated and there are only a few papers that consider the practical implementation of the DRFS receiver principle. Thus, the idea is very promising but more research needs to be done on the topic. The direct RF sampling receiver architecture has been considered in [1], [2] and [12] with focus on the GNSS case, and more generally in [4]. One of the main problems in a DRFS receiver is the high demands for the quality of the sampling process due to the high frequency of the sampled signals [6], [15]. This paper concentrates on the requirements the DRFS receiver has for the sampling process in GNSS application. First, the sampling jitter and the quantization effects on the signal-to-noise ratios (SNR) are considered. Then, the sampling jitter and quantization level requirements for the sampling of a GNSS signal centered at 1575.42 MHz (E2-L1-E1) band are considered. After the requirements have been derived by using system calculations, the results are verified using simulations. 2. BASIC NON-IDEALITIES IN SAMPLING The main sources of noise in a sampling system are the limited number of bits in the quantization and unintentional deviations called jitter in the moments the samples are taken. Limits in these non-idealities are of