| GPS Receiver RF Front-End Enables Use of a Laptop PC for Soft Baseband Processing |
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| Aug 31 2007 | |
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advertisement: After acquisition and tracking have established the initial synchronization, the system can decode the navigation bits. Data demodulation begins by despreading the 1.023-Mbps input signal to a 1,000-bps bit stream. Bit synchronization is then invoked to recover the 50-bps information from the 1,000-bps stream. For bit synchronization, we first need to identify the beginning of a bit in time. This is accomplished by finding the zero-crossing edge (at 0V), which indicates the beginning of a bit. When that edge is known, we can partition the 1,000-bps input stream at 20-ms intervals, knowing that the duration of a navigation data message (50 bits) is 20 ms. Finally, the bit samples in a 20-ms interval are summed and averaged to decode the navigation data.
The software that runs on the PC is the swGPS™ Spot Version 2 from NXP’s Software division. It can turn a notebook PC into a powerful location device, supporting navigation and a wealth of location- based services. The GPS front-end streams digitized IF data into the notebook via the industry-standard USB 2.0 interface. The Spot software uses the input to calculate a position fix and subsequently, to perform tracking. The code footprint on the host PC is about 750 kbytes and the software needs about 27 Mbytes of workspace when fed by a 12 Mbit/s GPS front end data stream. (For more information on the software, go to www.software.nxp.com or www.swgps.com.) A virtual COM port created by the software allows the software to link to a wealth of existing navigation and location applications. The software output interface conforms to NMEA 0183 and the application can run on Microsoft’s Windows XP and Vista operating systems. Additionally, the software can handle all available assistance data, whether supplied via industrystandard protocols or proprietary customer interfaces. When the software runs on a 1-GHz Pentium M system, the average processor load when tracking is 13%; when run on a Pentium Core Duo processor running at 2.18 GHz, the processor load is just 4%, both when performing updates every second. Software GPS techniques provide a high level of flexibility and simplicity for many potential applications. To support these possibilities, the MAX2741 RF front-end provides flexibility in frequency planning for software GPS receivers and traditional hardware implementations as well. Of course, every solution has its pros and cons — software GPS receivers require a high-performance processor and moderate amounts of memory. This article was written by David Weber, Strategic Applications Engineer, at Maxim Integrated Products Inc. in Sunnyvale, CA. Contact Mr. Weber at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it For more information, click here. (MATLAB is a registered trademark of The MathWorks; cdma2000 is a registered trademark of the Telecommunications Industry Association.) References [1] E. Kaplan. Understanding GPS: Principles and Applications. 2nd ed. Artech House Publishers, 1996. |
