Pulling in Wideband

June 1, 2009 By: Martin Unwin, Paul Blunt, Philip Jales, Ruediger M. Weiler, Stephen Hodgart GPS World

L1/E5 Receiver

New-generation GNSS signals can be combined to offer the use of dual-frequency techniques for the general public. The Galileo signal in the E5 band exemplifies such a wideband, widely available signal. Modulated by the alternate binary-offset carrier (AltBOC) (15, 10) with a bandwidth greater than 50 MHz, it makes receiver design challenging. The direct-conversion technique explained here enables a prototype dual-frequency receiver. We analyze its performance with live Galileo in-orbit validation element (GIOVE) signals on the L1 and E5 band.

Hardware set-up for the prototype direct-conversion based, dual-frequency GNSS receiver

Ongoing GNSS evolution introduces more wideband signals (greater than 10 MHz), intended to deliver better tracking accuracy and greater robustness against code multipath. These wideband signals are combined together with narrowband signals onto different carriers. FIGURES 1 and 2 overview the spectral use of modernized GPS and Galileo. The L1 and the L5 (E5a) frequency bands are common to both GPS and Galileo. Not considered here are GLONASS and Compass signals, but the principles and methods described apply equally.

FIGURE 1 Spectral use of GPS: open service in blue, restricted services in green and red

The binary offset carrier (BOC) is used for some signals, to ensure spectral separation. In the accepted notation BOC (f subcarrier , f code ), both frequencies are written as a multiple of the fundamental GNSS frequency 1.023 MHz. The subcarrier is always selected as equal or higher than the code frequency, as it specifies how far the main lobes of the signal spread out. For BOC (10, 5) modulation, each chip of the code is modulated by this pattern; FIGURE 3 shows the autocorrelation function.

FIGURE 2 Spectral use of Galileo: open services shown in blue, commercial services in green, and restricted in red

The subcarrier spreads the spectral energy of the signal away from the center of the band, leaving a null at the center frequency (FIGURE 4), an important feature for direct-conversion receivers. The autocorrelation function shows a multi-peak characteristic.

FIGURE 3 Chip pattern (left) and autocorrelation function of a sine BOC (10, 5) modulated signal

In addition to wideband signals using BOC modulation, other signals are modulated by phase-shift keying (PSK) with a higher bandwidth than the common C/A code signal from GPS. TABLE 1 summarizes the Galileo signals used in this article.

FIGURE 4 Power spectrum of a sine BOC (10, 5) modulated signal

The E5a/b signal has a bandwidth of more than 40 MHz, 20 times wider than GPS C/A code. The high bandwidth, while bringing significant advantages, also introduces technical hurdles for future use of these signals in mass-market applications. Antennas, amplifiers, and filters must cope with the high bandwidth. Analog-to-digital converters (ADCs) and the digital processing must run at high frequencies to fulfill the demands.