А.G. Khantimirov – Assistant,

Siberian Federal University (Krasnoyarsk)

E-mail: akhantimirov@sfu-kras.ru

I.N. Ryzhenko – Leading Engineer,

LLC «Phoenix Engineering Bureau» (Krasnoyarsk)

E-mail: rodgi@kras.ru

А.Е. Lutsenko – Expert,

LLC «Phoenix Engineering Bureau» (Krasnoyarsk)

E-mail: LUAE@fenix24.org

Е.S. Byvshev – Engineer-programmer,

LLC «Phoenix Engineering Bureau» (Krasnoyarsk)

E-mail: egorsergsss@gmail.com

Modern communication systems are using a large number of different modulations. The DVB-S2 standard describes such modulations as QPSK, 8PSK, 16APSK, 32APSK. A limitation for the choice of modulations is the characteristic of the ratio of signal energy per symbol to channel noise. By modifying the modulation and decrease the lower bound of the ratio, we can increase the spectral efficiency. In this paper we describe modified phase-shift keying modulation for eight points. One of the points is moved to the center of the signal structure, the rest are distributed around the circle. This modulation named 1+7APSK. This modification leads to an increase in the Euclidean distance between points. This allows us to increase the noise immunity of the modulation.

For the practical implementation of this modulation, it is necessary to change the following modules of the demodulator. First module, it is phase locked loop (PLL). The determination of phase error is performed without known data (NDA), using a blind method of synchronization. To determine and correct a phase error, the received signal is transferred from the in-phase-quadrature basis (I, Q) to the amplitude-phase basis using the CORDIC algorithm. The center point must be cut off in amplitude so that the PLL module is not disturbed. 

Second module is descrambler. Descrambling for modified constellation differ from standard descrambling of 8PSK. Instead of reversing I and Q components of signal constellation, this descrambler reversing only Q component. Thus, points with codes 110 and 111, 010 and 101, 100 and 011 are scrambled, respectively. While points with codes 000 and 001 are not scrambled, that is, sequences of such points can affect the signal spectrum. 

Third module is module of computing Log-Likelihood Ratio (LLR). This module calculates the Euclidean distance to the nearest zero and one of each of the three bits of the character code. Depending on the distance, the point is assigned an LLR metric for each bit. We have to change coordinates of points of signal constellation for this module to proper work.

As a result, we achieved ~ 1 dB difference between 8PSK and 1+7APSK modulations on high code speed without significant changes in hardware implementation.

1. Morello A., Mignone V. DVB-S2: The second generation standard for satellite broad-band services. Proceedings of the IEEE. 2006. № 1. P. 210–227.
2. Dvornikov S.V., Pshenichnikov A.V., Ekonom V.P. Metod otsenki pomekhoustoychivosti signal'nykh konstruktsiy kvadraturnoy modulyatsii s transformirovannymi konstellyatsionnymi diagrammami. Radiopromyshlennost'. 2017. №. 1. S. 51–56. [in Russian]
3. Xiang X., Valenti M.C. Improving DVB-S2 performance through constellation shaping and iterative demapping. 2011-MILCOM 2011 Military Communications Conference. IEEE, 2011. P. 549–554.
4. Nour C.A., Douillard C. Rotated QAM constellations to improve BICM performance for DVB-T2. 2008 IEEE 10th International Symposium on Spread Spectrum Techniques and Applications. IEEE, 2008. P. 354–359.
5. Xiang X. et al. Constellation labeling optimization for bit-interleaved coded APSK. Sensors and Systems for Space Applications IX.  International Society for Optics and Photonics. 2016. V. 9838.
6. Valenti M. A guided tour of CML, the coded modulation library. Morganown, W. Va.: Iterative Solutions. 2008.
7. Ip E., Kahn J.M. Feedforward carrier recovery for coherent optical communications. Journal of Lightwave Technology. 2007. № 9. P. 2675–2692. 
8. Datta D., Mitra P., Dutta H.S. Implementation of Universal Modulator Using CORDIC Architecture in FPGA. International Conference on Computational Intelligence, Communications, and Business Analytics. Springer, Singapore. 2018. P. 434–441.
9. Linn Y. Efficient phase detectors for M-PSK. 2013 IEEE Pacific Rim Conference on Communications, Computers and Signal Processing (PACRIM). IEEE, 2013. P. 26–31.
10. Lee I.K. et al. Multi-level modulation LDPC decoding algorithm for new generation DVB-S2 system. 24th AIAA International Communications Satellite Systems Conference. 2006. P. 1–8.