Software-aided receiver techniques are often associated with military tactical radios. But such flexible technology also holds great promise for L1-ban civilian Global Positioning System (GPS) receivers. Of course, low-cost RF receiver circuitry is still needed with the software, and the model MAX2741 integrated circuit (IC) from MAXIM Integrated Products (Sunnyvale, CA) serves as a compact and inexpensive RF front end for the receiver.

Software techniques for GPS are paving the way for positioning capability in a wide range of communications and navigation systems.^1,2 Thanks to the development of very-large-scale-integration (VLSI) technology, GPS receivers using powerful central processing units (CPUs) and digital signal processing (DSP) can receive and decode GPS signals in real time with the help of software. These software-based GPS receivers offer considerable flexibility—in modifying settings to accommodate new applications without hardware redesign, in using the same board design for different frequency plans, and in implementing future upgrades. As an example, this article will focus on challenges of integrating GPS receiver capabilities with code-division-multiple-access (CDMA) communications systems.

A complete GPS system includes the constellation of satellites, the ground-control station, and user equipment (receivers). The constellation has 24 satellites, each identified by a unique pseudorandom-noise (PRN) code. For civilian GPS applications, these satellites communicate over the L1 band located at 1.57542 GHz.

A GPS receiver must acquire signals from at least four satellites to establish a reliable position. Acquisition and tracking of the signals is very complex, because each one varies with time as well as receiver location. The RF front end of a software-based GPS receiver (Fig. 1) first amplifies a weak incoming signal with a low-noise amplifier (LNA), and then downconverts the signal to a lower intermediate frequency (IF) of approximately 4 MHz. Frequency downconversion is accomplished via superheterodyne techniques, by mixing the input RF signal with the local-oscillator (LO) signal, using one or two mixer stages of conversion. The resulting analog IF signal is converted to a digital IF signal by an analog-to-digital converter (ADC).

DEVELOPMENT TIME
To significantly reduce the development time needed for GPS applications, the LNA, mixer, and ADC are available in an IC solution, model MAX2741.³ The two-stage receiver IC supports receiver sensitivity of –185 dBW for successful GPS operation indoors. The IC provides 80-dB cascaded gain with a cascaded noise figure of 4.7 dB. It has a 50dB IF automatic-gain-control (AGC) range and can tolerate CDMA out-of-band jammer levels as high as +13 dBm and –90-dBm in-band jammers at its input. It is supplied in a small 28-pin thin QFN package and runs from a 2.7-to- 3.0-V supply. It features an SPI control interface.

The MAX2741 L1-band GPS receiver IC amplifies an incident 1575.42MHz GPS signal that exceeds its sensitivity level, downconverts it to a first IF of 37.38 MHz, further amplifies it, and then downconverts to a second IF of 3.78 MHz. An internal 2- or 3-b ADC (selectable as 1-b sign, 1- or 2-b magnitude) samples the second IF and outputs a digitized signal to a baseband processor. The integrated frequency synthesizer enables flexible frequency planning, allowing the same GPS circuit board to implement any fixed reference frequency from 2 to 26 MHz, with a change of settings. The integrated reference oscillator enables operation with either a crystal or a temperature-compensated crystal oscillator (TCXO).

Traditional GPS receivers implement acquisition, tracking, and bit-synchronization operations in an application-specific integrated circuit (ASIC), but a software GPS receiver provides flexibility by implementing those blocks in software rather than hardware. By simplifying the hardware architecture, software makes the receiver smaller, cheaper, and more power-efficient. Software can be written in C/C++, MATLAB, and other languages, and ported into all operating systems (embedded OS, PC, Linux, and DSP platforms). Thus, software GPS receivers offer the greatest flexibility for mobile handsets, portable digital assistants (PDAs), and similar applications.

For civilian L1-band applications, the GPS system is actually a simple spread-spectrum communication system.⁴ Figure 2 shows the signal generation block for civilian applications. First, the 50-b/s navigation message is repeated 20 times to produce a 1000-b/s bit stream, then the repeated signal is spread by a unique Coarse/Acquisition (C/A) code with a length of 1023 chips (the rate at which the pseudorandom noise code is applied). The result is a baseband signal of 1.023 Mchips/s. As a result of this spread-spectrum approach, the total processing gain (G) of the GPS system can resolve a signal well below the thermal noise level.

Each satellite is assigned a unique C/A or Gold code.⁵ Because the Gold code exhibits excellent auto-correlation and cross-correlation properties, it is widely used in CDMA communication systems such as wideband CDMA (WCDMA), CDMA2000, and other variants. The baseband signal is processed with binary-phase-shift-keying (BPSK) modulation, then upconverted to the L1 band for transmission.

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 alignment.⁶

More explicitly, a GPS receiver must first determine whether it has line-of-sight visibility to certain satellites. Each satellite is distinguished by a unique C/A code (Gold 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 effect,⁷ which causes the frequency to deviate from its nominal value by 5 to 10 kHz, depending on the speed of the satellite with respect to the receiver.