Extended Time-division AltBOC Modulation Technique for GNSS

Zhihui Zhou, J. Wei, Z. Tang, T. Yan

Abstract:	In the Galileo system, an AltBOC(15,10) modulation method is adopted in its E5a and E5b band, AltBOC is a new method proposed by Galileo to achieve constant-envelope modulation for dual-frequency signals. And it also becomes a modulation baseline for B2 band of BeiDou system. However, AltBOC modulation has lower efficiency and great satellite implementation and matched receiving complexity. To reduce the complexity, the time division technique was introduced to AltBOC by Tang Zuping and NAGARAJ C, they are TD-AltBOC (Time Division AltBOC, Tang Zuping) and TMOC-QPSK (time-multiplexed off-set carrier quadrature phase shift keying, NAGARAJ C). And TD-AltBOC technique is a feasible modulation for Beidou B2 signal to replace AltBOC. But these two authors only focus on the situation that the four components have the same power. For GNSS signal design, how to split power between data component and pilot component is usually a trade-off result between navigation data bit rate and signal power split, so it is necessary for a variable power split among the four components. In this paper, an extended time-division alternate binary offset carrier (ETD-AltBOC) modulation with variable power assignment is proposed. Different from existing time-multiplexed schemes, it is more flexible to adapt variable power ratio between signal components. In TD-AltBOC, Data component at upper sideband and data component at lower sideband are transmitted in odd slot. Pilot component at upper sideband and pilot component at lower sideband are transmitted in even time slot, any time slot is a 2-codes AltBOC modulation. Easy to know, the two components in 2-codes AltBOC modulation can have arbitrary power ratio. in other words, any time slot in ETD-AltBOC can have arbitrary power ratio but the total power at every time slot must be the same. Which two components will be transmitted is not limit. It is easy to achieve constant envelope by ETD-AltBOC modulation when the sum of any two components is equal to the sum of other components. in order to escape the hopping of frequency, we assume that one component from upper sideband and the other component from lower sideband are transmitted in any time slot in this paper. In the rest of paper, power ratio a:b:c:d represents the power of data component at upper sideband is a, the power of pilot component at upper sideband is b, the power of data component at lower sideband is c and the power of pilot component at lower sideband is d respectively. As an example, the power ratio like a:a:b:b or a:b:a:b which is common used in engineering can achieve by ETD-AltBOC. For power ratio a:a:b:b, data component at upper sideband and data component at lower sideband are transmitted in odd slot, the power ratio is a:b. Pilot component at upper sideband and pilot component at lower sideband are transmitted in even time slot, the power ratio is a:b. For power ratio a:b:a:b, data component at upper sideband and pilot component at lower sideband are transmitted in odd slot, the power ratio is a:b. Pilot component at upper sideband and data component at lower sideband are transmitted in even time slot, the power ratio is b:a. For more general case, when there is not exit the situation that the sum of any two components is equal to the sum of other components. We process four adjacent time slots as a group. Data component at upper sideband and data component at lower sideband are transmitted in the first time slot. Pilot component at upper sideband and pilot component at lower sideband are transmitted in the second time slot. Pilot component at upper sideband and data component at lower sideband are transmitted in the third time slot. Data component at upper sideband and pilot component at lower sideband are transmitted in the second time slot. Take power ratio 4:5:7:4 as an example, the power of the first time slot is 1:4, the power of the second time slot is 3:2, the power of the third time slot is 3:2, and the power of the four time slot is 2:3. Analysis results show that ETD-AltBOC technique can achieve similar power spectrum density and function with Galileo AltBOC modulation. Power ratio of four signal components is adjustable. This method is available for most power splitting schemes. Moreover it can reduce satellite implementation and matched receiving complexity. And adjust the power allocation of transmitted signal has no effect on receivers. So ETD-AltBOC is a feasible modulation for Beidou B2 signal.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1859 - 1879
Cite this article:	Zhou, Zhihui, Wei, J., Tang, Z., Yan, T., "Extended Time-division AltBOC Modulation Technique for GNSS," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1859-1879.
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