Binary offset carrier modulation

Binary offset carrier modulation (BOC modulation) was developed by John Betz, PhD, in order to allow interoperability of satellite navigation systems. It is currently used in the US GPS system, Indian IRNSS system and in Galileo and is a square sub-carrier modulation, where a signal is multiplied by a rectangular sub-carrier of frequency $f_{sc}$ equal or higher to the chip rate. Following this sub-carrier multiplication, the spectrum of the signal is divided into two parts, therefore BOC modulation is also known as a split-spectrum modulation.

The main idea behind BOC modulation is to reduce the interference with BPSK-modulated signal, which has a sinc function shaped spectrum. Therefore, BPSK-modulated signals such as C/A GPS codes have most of their spectral energy concentrated around the carrier frequency, while BOC-modulated signals (used in Galileo system) have low energy around the carrier frequency and two main spectral lobes further away from the carrier (thus, the name of split-spectrum).

BOC modulation has several variants: sine BOC (SinBOC), cosine BOC (CosBOC), Alternative BOC (AltBOC), multiplexed BOC (MBOC), Double BOC (DBOC) etc. and some of them have been currently selected for Galileo GNSS signals.

A BOC waveform is typically denoted via BOC(m,n) or BOC$(f_{sc}\;,f_{c})$, where $f_{sc}$ is the sub-carrier frequency, $f_{c}$ is the chip frequency, $m=f_{sc}/f_{ref}$, $n=f_{c}/f_{ref}$, and $f_{ref}=1.023$ Mcps is the reference chip frequency of C/A GPS signal.

A sine BOC(1,1) modulation is similar to Manchester code, that is, in digital domain, a '+1' is encoded as a '+1 −1' sequence, and a '0' is encoded as a '−1 +1' sequence. For an arbitrary $N_{BOC}=2m/n$ modulation order, in sine BOC(m,n) case, a '+1' is encoded as an alternating sequence of '+1 −1 +1 −1 +1 ...', having $N_{BOC}$ elements, and a '0' (or '−1') is encoded as an alternating '−1 +1 ...' sequence, also having $N_{BOC}$ elements.

BOC modulation is typically applied on CDMA signals, where each chip of the pseudorandom code is split into BOC sub-intervals, as explained above (i.e., there are $N_{BOC}$ BOC intervals per chip).

The power spectral density of a BOC-modulated signal depends on the BOC modulation order $N_{BOC}=2{\frac {f_{sc}}{f_{c}}}=2{\frac {m}{n}}$ and its derivation can be found, for example, in.

BOC-modulated signals, by difference with BPSK signals, create the so-called ambiguities in the correlation function. The BOC-modulated signals in GNSS can be processed either with a Full BOC receiver or via various unambiguous approaches.

Further reading

• Ries L, Lestarquit L, Armengou-Miret E, et al. A software simulation tool for GNSS2 BOC signals analysis. In Proceedings of ION GPS, (Portland, OR) September 2002; 2225-2239 (ION-GPS02)
• E. S. Lohan, A. Lakhzouri, and M. Renfors, “Binary-offset-carrier modulation techniques with applications in satellite navigation systems,” Wiley Wireless Communications and Mobile Computing, vol. 7, no. 6, pp. 767–779, 2006, http://www3.interscience.wiley.com/cgi-bin/fulltext/112693999/PDFSTART (Wiley06)
• Raghavan SH, Holmes JK. Modeling and simulation of mixed modulation formats for improved CDMA bandwidth efficiency. In Proceedings of Vehicular Technology Conference 2004; 6: 4290-4295 (VTC04).
• D. Margaria, F. Dovis, P. Mulassano, An Innovative Data Demodulation Technique for Galileo AltBOC Receivers, Journal of Global Positioning Systems, Journal of Global Positioning Systems, Vol.6, No.1, pp. 89–96, ISSN 1446-3156, 2007, http://www.gmat.unsw.edu.au/wang/jgps/v6n1/v6n1p10.pdf (GPSJournal07)
• D. Margaria, F. Dovis, P. Mulassano, Galileo AltBOC Signal Multiresolution Acquisition Strategy, IEEE Aerospace and Electronic Systems Magazine, Vol.23, No.11, pp. 4–10, ISSN 0885-8985, November 2008. (Margaria08)
• E. S. Lohan, A. Lakhzouri, M. Renfors, ``Complex Double-Binary-Offset-Carrier modulation for a unitary characterization of Galileo and GPS signals, IEE Proceedings on Radar, Sonar, and Navigation, vol. 153(5), pp. 403-408, Oct 2006.[IEE06]
• Avila-Rodriguez, J.A., Hein, G.W., Wallner, S., Issler, J.L., Ries, L., Lestarquit, L., De Latour, A., Godet, J., Bastide, F., Pratt, T., Owen, J. The MBOC Modulation- A Final Touch for the Galileo Frequency and Signal Plan, http://www.insidegnss.com/node/174 Inside GNSS (InsideGNSS07)
• Avila-Rodriguez, J.A., Wallner, S., Hein, G.W., Eissfeller, B., Irsigler, M., Issler, J.L.: A vision on new frequencies, signals and concepts for future GNSS systems, Proceedings of ION GNSS 2007, Fort Worth, Texas, USA, 25–28 September 2007 (ION-GNSS07)
• Avila-Rodriguez, J.A., Hein, G.W., Wallner, S., Issler, J.L., Ries, L., Lestarquit, L., De Latour, A., Godet, J., Bastide, F., Pratt, T., Owen, J.: The MBOC Modulation: The Final Touch to the Galileo Frequency and Signal Plan, Proceedings of ION GNSS 2007, Fort Worth, Texas, USA, 25–28 September 2007 (ION-GNSS07bis)
• E.S. Lohan and M. Renfors, ``On the performance of Multiplexed-BOC (MBOC) modulation for future GNSS signals, in Proc. of European Wireless Conference, Apr 2007, Paris, France.(EW07)
• Avila-Rodriguez J.A., Wallner S., Hein G.W.: MBOC: The New Optimized Spreading Modulation Recommended for Galileo E1 OS and GPS L1C, ESA Navitec 2006, Noordwijk, The Netherlands, 11-13 Dec. 2006 (ESA06)