| 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: Each satellite is assigned a unique C/A code, also called a gold code5. Because the gold code exhibits excellent auto- and cross-correlation properties, it is widely used in CDMA communication systems such as WCDMA, cdma2000®, and others. The baseband signal is modulated with binary phase-shift keying (BPSK), and upconverted to the L1 band for transmission.
Acquiring the Signal Because GPS is a CDMA communications system, the receiver must synchronize the pseudorandom noise (PRN) code as a prerequisite to demodulating the data. Code synchronization is usually achieved in two steps: code acquisition for the coarse-code alignment, and code-phase tracking for the fine alignment6. More explicitly, a GPS receiver must first determine whether it has lineof- sight visibility to certain satellites. As we know, each satellite is distinguished by a unique C/A code. When the satellite is visible, acquisition determines the signal’s frequency and code phase, which in turn establishes the corresponding demodulation parameters. The received-signal frequency varies due to the Doppler effect7, which causes the frequency to deviate from its nominal value by 5 kHz to 10 kHz, depending on the speed of the satellite with respect to the receiver. In the receiver, the GPS signal is first downconverted to in-phase and quadrature (I and Q) components. A pair of IQ correlators then correlates the I and Q baseband signals with the locally generated PRN sequence. After integrating over the duration of one bit, the I-Q correlator outputs are summed to provide an output-decision variable. Whenever the decision variable exceeds a certain threshold value, the system assumes the corresponding acquisition was successful and proceeds to the tracking mode. Otherwise, the relative phase of the locally generated PRN sequence and the oscillator frequency are adjusted to update the decision variable, and the above process is repeated. The simple logic structure of the serialsearch method makes it feasible for implementation in an ASIC, but for software implementations it is not practical because the search space is huge. Assuming the system tolerates a 500- Hz carrier-frequency offset and the Doppler frequency is 10 kHz, the search space for a software implementation is roughly 2 × (10,000/500) × 1023 = 40,920. Obviously, a serial-search acquisition would be difficult in software. Another acquisition method, called frequency-domain parallel code-phase acquisition, is less complex to implement with software. In this method, the Doppler-frequency and code-phase searches are combined into one search, which, after an FFT transform of the PRN code, reflects all code-phase information into the frequency domain. We then need only to search the space over the Doppler-frequency offset, thereby implementing a fast and effective software search. |
