Real-time GNSS software receivers Challenges, status and perspectives

The idea of a software receiver is to replace the data processing implemented in hardware with software and to sample the analog input signal as close to the antenna as possible. Thus, the hardware is reduced to the minimum (antenna and analog to digital converters) while all the signal processing is done in software. As current mobile devices (such as personal digital assistants and smartphones) include more and more computing power and system features it becomes possible to integrate a complete GNSS receiver with very few external components

Real-Time GNSS Software Receiver Optimized for General Purpose Microprocessors

A satellite navigation system (like GPS) allows an user to determine its own position everywhere and anytime on Earth. The process of calculating the position is relatively simple (use of trilateration). The main issue is to obtain and decode the transmitted information and to estimate accurately the Time Of Arrival (TOA) of the signals as they lie below the thermal noise floor. A technique called spread-spectrum has been applied for the transmission of these signals that distributes a narrow-band signal over a large bandwidth with the help of spreading codes. In the receiver, these known sequences (one for every satellite) are re-generated and correlated with the incoming signal. As the satellites are moving, the signals undergo additionally a Doppler frequency shift that also has to be compensated in the receiver. These correlation processes require a huge number of opera- tions which make them difficult to be executed in software. Current microprocessors and mobile devices (like smartphones and mobile computers) offer more and more processing power and system resources. Therefore, the interest in software receivers increased during the last years as they offer a great level of flexibility and allow a low-cost implementation with few additional components. The aim of this thesis is to develop and implement a real-time software receiver on a general purpose microprocessor. This includes an extensive study of the current state-of-the-art and the development and the imple- mentation of a new signal processing baseband architecture. The require- ments and the performance are finally evaluated with simulated and real signals

Real-Time GNSS Software Receiver: Challenges, Status, and Perspectives

Challenges and current status of algorithms of code and carrier generation and baseband processing, including single-instruction multiptle-data (SIMD) operations and bit-wise processing

Distributed Arithmetic for Efficient Base-Band Processing in Real-Time GNSS Software Receivers

The growing market of GNSS capable mobile devices is driving the interest of GNSS software solutions, as they can share many system resources (processor, memory), reducing both the size and the cost of their integration. Indeed, with the increasing performance of modern processors, it becomes now feasible to implement in software a multichannel GNSS receiver operating in real time. However, a major issue with this approach is the large computing resources required for the base-band processing, in particular for the correlation operations. Therefore, new algorithms need to be developed in order to reduce the overall complexity of the receiver architecture. Towards that aim, this paper first introduces the challenges of the software implementation of a GPS receiver, with a main focus given to the base-band processing and correlation operations. It then describes the already existing solutions and, from this, introduces a new algorithm based on distributed arithmetic

Real-time Carrier Generation for a GNSS Software Receiver

The growing market of GNSS capable mobile devices is driving the interest of software receiver solutions as they can share many resources with other system units, reducing both the size and the cost of their integration. However, a major issue with the software approach is the large computing resources required for the base-band processing and the carrier generation in particular. Several strategies have been proposed in the literature to overcome it, but they still suffer from their lack of flexibility and large memory requirements. In this context, IMT has developed a completely new receiver architecture for optimizing the processing resources of the base-band operations and the carrier generation especially. The general concept consists in batch processing the incoming samples for progressively reducing the data throughput, and consequently the global amount of arithmetic operations. This method not only allows the real-time generation of the carrier at any desired frequency but also greatly simplifies the further mixing operations with the incoming signal

Performances of a New Correlation Algorithm for a Platform-independent GPS Software Receiver

Personal Digital Assistants (PDA’s) or mobile phones applications are not anymore restricted to wireless communications or multimedia, but have been extended to handle Global Navigation Satellite System (GNSS) functionalities. Consequently, the growing market of GNSS capable mobile devices is driving the interest of software solutions as they provide several advantages with respect to the conventional hardware architectures currently implemented in the mass market receivers. First, they share the same system resources such as the processor, embedded memory and power with other system units, reducing both the size and the costs of their integration. Second, they can be easily reprogrammed - via firmware updates - for incorporating the latest developments, such as the exploitation of the future GNSS signals or some improved multipath mitigation techniques. Finally, they offer a more flexible solution for rapid research and development as compared to conventional hardware receivers where the chip design is fixed and obtained after a very long integration process. This paper will first introduce the challenges of the software implementation of a GNSS receiver, with a main focus given to the base-band processing and the correlation issues. It will then describe the already existing solutions with their respective advantages and drawbacks, and provide an estimation of their complexity in term of computational load. From this, a new developed algorithm will be introduced, and its performances in term of integer operations needed to perform the correlation analyzed and compared to the other solutions

Batch processing for efficient base-band operations in real-time GNSS software receivers

Personal Digital Assistants (PDAs) or mobile phones applications are not anymore restricted to multimedia or wireless communications, but have been extended to handle Global Navigation Satellite System (GNSS) functionalities. The new generation of portable devices is becoming suitable for use as navigators thanks to their large displays and their improved storage capacities. But the use of the GNSS technology in the cellular handsets has some drawbacks. The functionalities are usually implemented using a standalone hardware module, introducing a relatively expensive item to integrate into such cost sensitive devices. However, with the increasing performance of modern embedded processors, it becomes now feasible to implement a GNSS receiver in software, where all the basic operations are performed on a general purpose microprocessor. Consequently, the growing market of GNSS capable mobile devices is driving the interest of software solutions, since they provide several advantages with respect to the conventional hardware architectures currently implemented in the mass market receivers. First, the receiver and the device can share the same system resources, reducing both the size and the costs of their integration. Second, they can be easily reprogrammed - via web firmware updates - for incorporating the latest developments. Software receivers thus constitute a very flexible solution for rapid research and development. However, implementing a GNSS receiver in software is not straightforward. The large computing resources required for performing the different operations on a microprocessor constitute a major issue, as compared to a hardware design where the chip is specifically designed for this task and can thus handle a much higher data throughput. This paper introduces a new receiver architecture based on batch processing of the incoming samples. Data sharing the same characteristics are regrouped into batches and processed collectively instead of sequentially. This way, it becomes possible to progressively reduce the data throughput and, consequently, the computational load of the base-band operations

A Galileo E1b,c RF front-end for Search-and-Rescue applications

The current Search-and-Rescue (SAR) service, which is based on the Cospas-Sarsat system, suffers from major limitations such as poor position accuracy, long alert times and high false alarm rate. Two types of distress signals are used, the 121.5MHz/(up to 100mW) signal and the 406.8MHz/5W signal, the latter being able to carry digitally encoded data, such as the beacon’s position. As a consequence, most distress beacons available today contain a GNSS receiver in order to be able to include the position into the distress message. Galileo will importantly contribute to the improvement of the SAR system. Indeed, the Galileo satellites will include a transponder in order to re-broadcast the 406.8MHz message, which will allow a better coverage of the earth (27 Galileo satellites plus the current seven Cospas-Sarsat satellites) and also a shorter alert time. They will also include a return link message (RLM) in the Galileo E1b open service signal, which will reduce the number of false alarms. Galileo is therefore a great opportunity for the development of a new generation of beacons which will include a Galileo receiver and therefore be able to take advantage of the better coverage provided by the Galileo constellation to provide shorter alert times and of the RLM to reduce the number of false alarms. One of the major issue when designing a Galileo receiver to be operated in a distress beacon is to design a front-end that is sensitive enough to pick the very weak E1b signal in the presence of the strong distress signals. Indeed, in order to receive the RLM message, the Galileo receiver will have to track E1b while the distress signals are emitted. This paper presents a Galileo radio frequency front-end designed in order to operate in the presence of such signals