Binary offset carrier modulation

Lore & Background

Binary offset carrier modulation was developed by John Betz to enable different satellite navigation systems to share the same frequency bands with reduced interference. The main idea behind BOC modulation is to reduce interference with BPSK-modulated signals, which have a sinc function shaped spectrum. While BPSK-modulated signals such as C/A GPS codes concentrate most of their spectral energy around the carrier frequency, BOC-modulated signals have low energy around the carrier frequency and two main spectral lobes further away from the carrier, hence the name split-spectrum.

BOC modulation has several variants, including sine BOC (sinBOC), cosine BOC (cosBOC), alternative BOC (altBOC), multiplexed BOC (MBOC), and double BOC (DBOC), some of which have been selected for Galileo GNSS signals. A BOC waveform is typically denoted as BOC(m, n), where the sub-carrier frequency and chip frequency are related to a reference chip frequency. A sine BOC(1, 1) modulation is similar to Manchester code: a '+1' is encoded as '+1 −1', and a '0' as '−1 +1'. For higher modulation orders, a '+1' is encoded as an alternating sequence of '+1 −1 +1 −1 +1 ...' with a specific number of elements, and a '0' (or '−1') as an alternating '−1 +1 ...' sequence.

BOC modulation is typically applied on CDMA signals, where each chip of the pseudorandom code is split into BOC sub-intervals. The power spectral density of a BOC-modulated signal depends on the modulation order. BOC-modulated signals create ambiguities in the correlation function, unlike BPSK signals, and can be processed either with a Full BOC receiver or via various unambiguous approaches.

Reader's Guide

Binary offset carrier modulation holds significance primarily for its role in enabling multiple satellite navigation systems to coexist with reduced mutual interference. By shifting spectral energy away from the carrier frequency into two distinct lobes, BOC modulation allows systems such as GPS, IRNSS, and Galileo to operate in overlapping frequency bands while maintaining compatibility. This split-spectrum characteristic also provides better theoretically achievable tracking capabilities when downmixed to complex baseband, due to the higher frequencies involved.

However, the proliferation of many BOC variants—sine BOC, cosine BOC, alternative BOC, multiplexed BOC, double BOC, and others—has made it difficult to obtain a complete picture of the modulation family. Early and sometimes recent publications on the topic often lack matched filters for pulse shaping and do not treat complex Gaussian noise correctly, leading to mathematically inconsistent baseband descriptions. When these standards are not followed, theoretical results become unreliable, regardless of the publication medium or peer review. The legacy of BOC modulation is thus one of both practical utility in global navigation systems and ongoing challenges in achieving rigorous theoretical modeling.

Did You Know?

• BOC modulation was developed by John Betz to allow interoperability of satellite navigation systems.
• BOC modulation is also known as split-spectrum modulation because its spectrum is divided into two parts.