Binary offset carrier modulation (or BOCM) is a split-spectrum modulation scheme used by the Galileo navigation satellite system. BOCM is a square subcarrier modulation. A signal is multiplied by a rectangular subcarrier of a frequency that is equal to or higher than the CDMA rate. Following this subcarrier multiplication, the spectrum of the signal is divided into two parts.
The purpose for using BOC modulation in Galileo is to reduce interference with BPSK-modulated signals such as C/A GPS codes. These signals have sinc function shaped spectra, with most of their spectral energy concentrated around the carrier frequency. In comparison, the BOC-modulated signals have low energy around the carrier frequency and two main spectral lobes further away from the carrier.
As a result, the Galileo signals can easily coexist with GPS signals without danger of interference, despite sharing the same 20MHz band centred at 1575.42MHz. This greatly simplifies the circuitry of any combined GPS/Galileo receiver design.
With the first Galileo-capable GNSS receiver chipsets just starting to appear, manufacturers can immediately get to work perfecting their multi-GNSS receiver designs in advance of the launch of the Galileo system in 2014 using Spirent simulators. Spirent is the official supplier of RF constellation simulators for the Galileo programme.