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.

Architecture of a Real-Time Platform Independant GPS L1 Software Receiver

Personal digital assistants or mobile phones applications are not anymore restricted to multimedia or wireless communications, but have been extended to handle Global Positioning System (GPS) functionalities. Consequently, the growing market of GPS capable mobile devices is driving the interest of software receiver solutions as they provide several advantages with respect to traditional hardware implementations. 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 a firmware update – for incorporating the latest developments, such as the exploitation of the future satellites 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 long integration process. With the increasing performance of modern processors, it becomes now feasible to implement in software a multi-channel GPS receiver operating in real-time. However, a major problem with the software architecture is the large computing resources required for the digital signal processing. Former studies have demonstrated that a straightforward transposition of traditional hardware based architectures into software would lead to an amount of integer operations which is not suitable for today's fastest computers. From this observation, several strategies have been proposed in the literature in order to reduce the complexity of the receiver operations. The first one relies on the utilization of advanced microprocessor instructions set which provides the capability of processing vectors of data by operating on multiple integer values at the same time. This results in significant gains in execution speed, but also severely limits the portability of the code, since the operations are tied to specific processors architectures. Another alternative consists in exploiting the native bitwise representation of the signal. The data bits are stored in separate vectors on which logical parallel operations can be performed. The objective is to take advantage of the universality, high parallelism, and speed of the bitwise operations for which a single integer operation translates into a few simple parallel logical relations. However, the inherent drawback of the bitwise processing is the lack of flexibility as the complexity becomes bit-depth dependent. This thesis has been carried out in the framework of a two-year industrial project (2007-2009) in collaboration with U-blox AG in Thalwil. It aimed to the realization of a multi-channel, platform-independent real-time GPS L1 software receiver. The main challenge of this project consisted in providing real-time performances while keeping the portability of the code to make the receiver suitable for any type of software implementation. In that sense, new techniques and algorithms have been developed for optimizing the processing chain in order to lower the processor load. The main idea consists in regrouping data which share the same characteristics, and process them in batches instead of sequentially. This way, it becomes possible to progressively reduce the data throughput and consequently the amount of operations to perform. A completely new receiver architecture has been proposed and validated through the realization of a functional prototype, thus demonstrating the feasibility of the concept.

EPFL2010

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

Marcel Baracchi Frei, Cyril Botteron, Pierre-André Farine

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.

Inst Navigation, 815 15Th Th St Nw, Ste 832, Washington, Dc 20005 Usa2009

Real-time Carrier Generation for a GNSS Software Receiver

Marcel Baracchi Frei, Cyril Botteron, Pierre-André Farine

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.

2009

Architecture of a Real-Time Platform Independant GPS L1 Software Receiver

EPFL2010