Title: Features and Applications of the Adaptable Flexiband USB3.0 Front-end
Author(s): Alexander Rügamer, Frank Förster, Manuel Stahl, Günter Rohmer
Published in: Proceedings of the 27th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2014)
September 8 - 12, 2014
Tampa Convention Center
Tampa, Florida
Pages: 330 - 362
Cite this article: Rügamer, Alexander, Förster, Frank, Stahl, Manuel, Rohmer, Günter, "Features and Applications of the Adaptable Flexiband USB3.0 Front-end," Proceedings of the 27th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 330-362.
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Abstract: In 2012 we introduced the Flexiband multi-system, multi-band USB front-end [1]. In the meantime, this front-end has been used and validated in numerous scientific and industrial projects. In this paper we introduce new features and possibilities, which were mainly developed for customized applications but are now also available for anyone interested. The Flexiband USB front-end is a flexible, portable, and affordable front-end recording solution that can easily be adapted to the reception of all GNSS bands available supporting up to 80 MHz of RF bandwidth. Thanks to its modular concept, the Flexiband not only supports a set of pre-selected configurations but can also be set up for multi-antenna inputs, user selectable bandwidth, intermediate frequencies, and customized ADC sampling rates and resolutions. A regular Flexiband unit consists of up to three analog reception boards, a carrier board with ADCs and FPGA, and a USB3.0 interface board. A common antenna input port is supported as well as separate front-end input signals for up to three antenna inputs. Three dual-channel analog to digital converters (ADC) sample the incoming signal with 80 MSPS and 8 bits I/Q. The raw data stream is received by an FPGA in which different digital operations like filtering, mixing, data rate and bit width reduction are applied. A digital automatic gain control (AGC) is implemented in the FPGA and can be switched on or off by the user. Finally, a single multiplexed data stream is formed together with a checksum. This multiplexed stream is send via an USB3.0 interface to the PC. Date rates of up to a 1280 MBit/s or 160 MByte/s raw data stream are supported. The Flexiband graphical user interface (GUI) software receives the raw multiplexed stream, checks its integrity and demultiplexes it. The data streamed can be either written to hard disk or send to a customer application (e.g. a software receiver). The raw samples can be stored as a multiplexed data stream, in an 8 bit/sample format, or directly as a .mat file for MATLAB. The user-friendly Flexiband GUI can do much more than just receiving, checking for packet loss, demultiplexing the received raw data and having nice visualization options like spectrum and histogram view. The GUI also features the possibility to upgrade the firmware of the receiver modules as well as to reprogram the FPGA. Since finally the digital signal conditioning inside the Flexiband FPGA determines the signal to be received with its parameters, different configuration files can be selected. E.g. using three wideband analog receiver modules with 80 MHz bandwidth for GPS/Galileo L5/E5, 40 MHz GPS L2 and 40 MHz GPS/Galileo L1/E1/G1, the user can select with the help of the FPGA configuration what is best for his current project: With just one mouse click is it possible to configure this example from e.g. a single frequency, narrow-band GPS L1 with 2 bit @ 4 MHz sampling rate to wideband Galileo E5 with 8 bit resolution and 80 MHz sampling rate I/Q or a triple band GPS/Galileo L5/E5a, L2C, L1/E1 receiver. This degree of freedom is very useful if the user wants to use the Flexiband for different applications. Those applications scenarios could be e.g. 1) GPS L1 real-time software receiver, 2) Multi-band, multi-system GPS/Galileo receiver 3) Interference detection station. All these applications with completely different front-end requirements are feasible with the same Flexiband hardware unit, just by selecting the specific FPGA configuration. The Flexiband design is open in a way that an FPGA development kit is available. This gives the user the possibility to implement their own digital filters, mixers, and downsampling configurations. Having synthesized and implemented the VHDL code, the new customized firmware can be directly uploaded to the Flexiband device, making it ready for a new specific application. E.g. the customized filters can be used to implement an equalizer for signal S-curve bias equalization needed in timing receivers. Or the user can implement a pulse blanker running on the FPGA in real time before the data gets post processed on the PC. In a similar way also a software development kit is available. This kit is needed to adapt the Flexiband interface to the customer’s software defined radio (SDR) solution so that live data received by the Flexiband can directly be processed in real time. The Flexiband can also be used to embed digital sensor data in the received raw GNSS data stream. 67 versatile general purpose input/output (GPIO) provide many possibilities to connect digital sensors with the Flexiband unit’s carrier board FPGA. Such sensors could be inertial measurement systems but also any other sensor that should be precisely synchronized, on sample level accuracy, with the GNSS raw data stream. Different Flexiband front-ends can be synchronized among each other. The synchronization of two Flexibands featuring a 6 input signal system was successfully evaluated for a small, portable and lightweight postprocessing beamforming recording solution. Moreover, either the internal reference oscillator or an external high quality frequency reference can be used, e.g. a reference clock from an IMU or from a reference trajectory system. Thanks to an energy saving design the Flexiband front-end can be powered by the USB3.0 interface only, making it perfectly suited for mobile recording campaigns with e.g. a notebook as a data recording device. [1] Rügamer, A., Förster, F., Stahl, M., Rohmer, G., "A Flexible and Portable Multiband GNSS front-end System," Proceedings of the 25th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2378-2389.