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Carrying out a Galileo/GPS system design

Posted: 17 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Bluetooth? Wi-Fi? Galileo/GPS system design? receivers?

GPS functionality is quickly becoming a major market driver for consumer electronics, emerging as a primary differentiator in next-generation systems. The ability to accurately determine one's position crosses automotive applications to personal navigation devices (PNDs) to cellphones.

Developers need to become familiar with various key technologies that make implementing GPS in CE devices possible, most specifically, the Galileo satellite network being developed and implemented under the auspices of the European Union. Through the availability of additional satellite signals, PNDs will be able to acquire and lock positions faster and more accurately than GPSonly devices, especially in urban environments.

GPS and Galileo use the same frequency band (and center frequency of 1.575GHz), so it is possible to use one radio that operates with both systems (i.e. a dual radio). Slight differences in signal acquisition, however, will need to be implemented in a configurable fashion. Specifically, Galileo signals use a 4MHz bandwidth (compared with 2MHz for GPS) and will implement a different coding scheme. From a baseband perspective, these modulation schemes can both be demodulated using correlators, so a single baseband processor can be used where a single flexible correlator block can be independently configured to demodulate Galileo and GPS signals simultaneously.

Cost savings
The key to a cost-effective enabling of Galileo/GPS functionality is to leverage the unused capacity of existing architectures and implement some portion of the baseband processing in software. Consumer Galileo/GPS becomes possible when baseband processing can be implemented in software on the applications processor. In this way, software-based Galileo/GPS is analogous to software-defined radio, since a single hardware radio supports multiple satellite systems. As wireless communication technologies continue to converge, it is foreseeable that consumer devices will use a multifunction radio supporting Bluetooth, Wi-Fi and Galileo/GPS using a configurable software baseband.

Developers have the option of continuing to implement GPS baseband processing in hardware and Galileo processing in software or of implementing both GPS and Galileo in software. While implementing both in software completely eliminates the need for a hardware baseband chip, both approaches reduce the cost of implementing location-based services in consumer applications. Further cost savings can be achieved by integrating fixed baseband processing such as correlation with the radio. Additionally, Galileo can be added to a software-based baseband device anytime without increasing overall hardware cost.

Key issue
Sensitivity is a key performance and accuracy issue for Galileo/GPS receivers. Signal acquisition requires a signal that is between -130dBm and -155dBm at the receiver (in assisted GPS systems), approximately 19-34dB below the noise floor seen by the RF front-end. The correlators despread a 2MHz-wide signal down to a 50Hz data signal, offering a 43dB correlation gain, which boosts desired signals above the noise floor so that they can be processed. However, any other communications signal that is close to the desired signal frequency or has harmonics in the desired band can act as a source of interference and further desensitize the radio.

The most common and destructive sources of interference come from within the PND itself. There are several methods for overcoming these internal transmission interference. Since the transmitted signal is known, it can be subtracted from the Galileo/GPS signal. Alternatively, a filter can be used to protect the incoming satellite signal by knocking down cellphone interference by more than 70dB.

A dual-radio architecture, however, has two optimal filters, given that GPS has a 2MHz-wide bandwidth and Galileo spreads across 4MHz. The modulation scheme of GPS is BPSK, and the modulation scheme for Galileo is binary offset carrier of 1,1. This allows both signals to occupy the same signal bandwidth, and the correlators themselves then become able to distinguish GPS signals from Galileo and vice versa.

Filters are also applied in the baseband processor. When the baseband is implemented in hardware, these filter values are fixed, limiting the applications for which the radio is optimized. When baseband filtering is implemented in software, these values can be changed to match particular signal conditions. Additionally, as advances in filtering algorithms are made, these can be applied to existing architectures. Given the extreme differences in handset architectures, such flexibility enables dual-radio architecture to serve optimally across multiple product lines.

Sensitivity can also be seriously degraded based on a noisy crystal or voltage-controlled crystal oscillator reference. In general, the more stable the clock source, the higher the cost, but also the faster the acquisition time.

Galileo can be added to a software-based baseband device.

GSM references are locked to the network, and frequency corrections are applied from time to time. Sometimes these corrections are implemented by the GSM baseband driving a DAC, which in turn drives a voltagecontrolled temperature-compensated crystal oscillator. Step changes in reference frequency will not allow a Galileo/GPS receiver to maintain signal lock with satellite signals, especially where weak signals are concerned. This will consequently cause a loss of position lock. Thus, it is safest to use a separate clock for the Galileo/GPS subsystem, although this has the consequence of increasing overall device cost.

- Malcolm Lomer
Product Marketing Manager, GNSS solutions
SiGe Semiconductor Inc.

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