Binary offset carrier modulation

Binary Offset Carrier (BOC) modulation currently used in Galileo 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 (CDMA) 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

References

• Betz J. The offset carrier modulation for GPS modernization. In Proceedings of ION Technical meeting, (Cambridge, Massachusetts) June 1999; 639–648. (ION-AM99)
• J. Betz, “Design and performance of code tracking for the GPS M code signal,” MITRE, Mclean, Va, USA, September 2000, http://www.mitre.org/work/tech_papers/tech_papers_00/ betz_codetracking/ (MITRE00)
• Galileo Open Service Signal In Space Interface Control Document http://www.gsa.europa.eu/go/galileo/os-sis-icd/galileo-open-service-signal-in-space-interface-control-document (SIS-ICD08)
• Hein G, Irsigler M, Rodriguez JA, Pany T. Performance of Galileo L1 signal candidates. In CDROM Proceedings of European Navigation Conference GNSS, May 2004. (ENC-GNSS04)
• 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)
• GJU. Galileo standardisation document for 3GPP. Galileo Joint Undertaking (GJU) webpages, http://www.galileoju.com/page.cfm?voce=s2&idvoce=64&plugIn=1 (GJU)
• www.septentrio.com/papers/GallileoAltBOC_paperFinal.pdf
• 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 (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, Dec. 11-13, 2006 (ESA06)

External links

• Binary Offset Carrier (BOC) signal generator in Matlab, http://www.mathworks.com/matlabcentral/fileexchange/12829

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