Elektrotehniˇski vestnik 69(1): 34–39, 2002 Electrotechnical Review, Ljubljana, Slovenija
A 2MSK receiver based on the regeneration of the larger MSK signal component Tomaˇz Javornik, Gorazd Kandus Dept. of Digital Communications and Networks, Institut Joˇzef Stefan, Jamova 39, Ljubljana, 1000, Slovenia E-mail:
[email protected],
[email protected] Abstract. A differential 2MSK receiver, based on regeneration of the MSK signal component with larger amplitude in the receiver, was designed and analysed. The proposed receiver structure is relatively simple and has only 1dB lower power efficiency than the optimal maximum likelihood sequence estimator. Sensitivity of the receiver to non-optimum amplification of the received signal, to error in the symbol timing recovery and to the phase error of the regenerated MSK signal was investigated. No appreciable degradation of the system performance is found for a symbol timing recovery error less than 1/32 of the symbol duration and for a phase difference between regenerated MSK and received 2MSK signal less than π/32 radians. A 3dB loss in receiver power efficiency is observed for an amplification error of ±1.5dB of the received signal. Key words: 2MSK, non-coherent and differential receiver
Sprejemnik signala 2MSK s pomoˇcjo remodulacije komponente MSK z veˇcjo amplitudo Povzetek. V delu smo opisali delovanje in prouˇcili lastnosti diferencialnega sprejemnika 2MSK, ki za detekcijo podatkov, preneˇsenih v komponenti MSK z veˇcjo amplitudo, uporablja diferencialno fazno detekcijo signala 2MSK. Detektirane podatke nato uporabi za regeneracijo komponente MSK z veˇcjo amplitudo, le-to pa odˇsteje od sprejetega signala 2MSK. Razlika signalov je signal MSK iz katerega z diferencialno detekcijo doloˇci preostale podatke. Opisani sprejemnik je preprost, pri tem da so njegove lastnosti v Gaussovem kanalu slabˇse le za ˇ je napaka pri 1dB v primerjavi z optimalnim sprejemnikom. Ce regeneraciji takta signala manjˇsa kot 1/32 simbolnega intervala ali pa je fazna razlika med sprejetim signalom 2MSK in regeneriranim signalom MSK manjˇsa kot π/32 radijanov, poslabˇsanje ˇ je napaka v ojaˇcenju signala lastnosti sprejemnika ni opazno. Ce 1.5dB, se moˇcnostna uˇcinkovitost signala zmanjˇsa za 3dB. Kljuˇcne besede: 2MSK, nekoherentni in diferencialni sprejemnik
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Introduction
The increasing demand for radio frequency spectrum has led to investigation of modulation schemes with higher bandwidth efficiency. One possibility of increasing bandwidth efficiency of the modulated signal is to combine phase and amplitude modulation schemes. Multiamplitude modulation schemes such as quadrature amplitude shift keying (QASK) and multi-amplitude phase shift keying (MAPSK) are in general not suitable for mobile Received 19 Jun 2001 Accepted 10 December 2001
and satellite communication systems, due to sensitivity to distortion caused by nonlinear amplifiers. However, the 2MSK signals are rather insensitive to nonlinear amplification [5]. The 2MSK modulation scheme can be represented as a shaped QASK modulation scheme [2, 3]. Coherent quadrature 2MSK receivers are usually used for shaped QASK signal detection, which requires carrier recovery, symbol timing recovery and automatic gain control [2]. Alternatively, the 2MSK signal can be obtained by superposition of two minimum shift keying (MSK) signals with different amplitudes [6]. The maximum likelihood sequence estimator (MLSE) is the optimal detection procedure in this case for estimating the transmitted data from signals corrupted by white Gaussian noise. However, MLSE detection is complex and difficult to implement in digital communication systems. Various simplified detection schemes for MSK signals are known [1, 4], such as differential detection and discriminator detection. A loss of about 1dB in system power efficiency of the MSK signal was reported [1] when differential detection was used instead of MLSE detection. Therefore, we chose differential detection for 2MSK receiver implementation. Firstly, a differential phase detection is used to estimate the information digit carried by the MSK component with the larger amplitude. The estimated digit value is used in the 2MSK receiver to regenerate the MSK signal with the larger amplitude. This regenerated MSK component is then subtracted from the received 2MSK signal.
A 2MSK receiver based on the regeneration of the larger MSK signal component At this point, the MSK component with the smaller amplitude is obtained. After that, differential detection of the smaller MSK component is used to estimate the second information digit transmitted by the 2MSK signal.
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The 2MSK signal can be presented as a superposition of two MSK signals with different amplitudes 2E s2M SK (t, α, β) = (sM SK (t, α) + 2sM SK (t, β)) 5T (1) where sM SK (t, α) is the MSK component with the smaller amplitude, sM SK (t, β) is the MSK component with the larger amplitude, E is the average signal energy per symbol interval and T is the duration of one symbol interval. Sequences α and β represent input data with values ±1. The MSK signal is an angle modulated signal, so that the information is carried in the phase of the MSK signal. The MSK signal envelope is constant. The amplitude of the MSK component with the smaller amplitude is set to 1 and that of the MSK component with the larger amplitude is set to 2. s (t, α) = Re{ejΦM SK (t,α) ej2πfc t }, (2) M SK
where fc is the signal carrier frequency and ΦM SK (t, α) is the information carrying parts of the MSK signal phases, which is =
ΦM SK (t, an , an−1 , αn−1 ) = n = π aj q(t − jT ) = (3) j=−∞
t − nT π an + an−1 + αn−1 , 2T 2 nT ≤ t < (n + 1)T,
= π
where phase response q(t) is t 1, the SNR of the subtracted 2MSK signal, which is further on detected as an MSK component with smaller amplitude, is decreased drastically.
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Figure 5. Degradation of the receiver performance due to symbol timing recovery error %(5 $:*1 UHPRGXODWRU SKDVH
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Figure 6. Degradation of the receiver performance due to phase adjustment error
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4 Figure 4. Degradation of the receiver performance due to signal amplification error
The receiver sensitivity to the timing symbol recovery errors is analysed next, and the results are plotted in Fig. 5. Almost no system performance degradation is observed when the sampling instance is delayed or advanced by 1/32 of the symbol interval duration. 2.5dB greater received signal power is required for the same BER, when the sampling instance is misplaced for less than 1/16 of the symbol interval. Larger differences from optimal sampling instances cause serious degradation of the system performance. The remodulator phase error is analysed next. The results are plotted in Fig. 6. A phase error up to ±1/32 radians does not cause serious degradation of the receiver performance. 3dB degradation is observed when the locally generated MSK signal is phase shifted by ±1/16 radians from the received 2MSK signal phase. Much higher degradation is observed for higher phase differences between the received 2MSK signal and the locally generated MSK signal.
Conclusion
A differential 2MSK receiver based on regeneration of the MSK component with the larger amplitude is presented and analysed. The estimated data carried by the MSK component of the 2MSK signal with the larger amplitude are used to regenerate the MSK component in the receiver. The regenerated MSK component is subtracted from the received 2MSK signal to obtain the MSK component with the smaller amplitude, which is then differentially detected. The proposed differential 2MSK receiver needs an additional 1dB of the received signal power for the same BER as the MLSE receiver. The receiver performance is not decreased when the symbol timing error is less than 1/32 of the symbol interval and the phase difference between the receiver generated MSK signal and the received 2MSK signal is less than ±1/32 radians. A 2MSK signal amplification error higher than 1.5dB can cause significant system performance degradation.
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References
[1] J. B. Anderson, T. Aulin, C.-E. Sundberg, Digital phase modulation, Plenum Press, New York, 1986.
[2] W. J. Weber, P. H. Stanton, J. T. Sumida, A Bandwidth Compressive Modulation System Using Multi-Amplitude Minimum Shift Keying (MAMSK), IEEE Trans. Communications, Vol. COM-26, No. 5, pages 543-551, May 1978. [3] W. J. Weber, Differential Encoding for Miltiple Amplitude and Phase Shift Keying Systems, IEEE Trans. Communications, Vol. COM-26, No. 3, pages 385-391, March 1978. [4] N. A. B. Svensson, C-E. W. Sundberg, Performance Evaluation of Differential and Discriminator Detection of Continuous Phase Modulation, IEEE Trans. Veh. Technol., Vol. VT-35, No. 3, pages 106-117, August 1986. [5] J-S. Seo, K. Feher, Bandwidth Compressive 16-State SQAM Modems Through Saturated Amplifiers, IEEE Trans. Communication, Vol. COM-35, No. 3. pages 339345, March 1987. ˇ [6] G. Kandus, D. Lazi´c, V. Senk, Some Characteristics of Multi-Amplitude CPM, Proceedings on 8th European Conference on Electrotechnics, Area Communication, EUROCON 88, Stockholm, Sweden, pages 60-63, 1988.
Tomaˇz Javornik received his B.Sc., M.Sc. and Ph.D. degrees in electrical engineering from the University of Ljubljana in 1987, 1990 and 1993, respectively. Since 1987 he has been with the Joˇzef Stefan Institute, Ljubljana, where he is currently a researcher at the Department of Digital Communications and Networks. His current research interests include modulation and coding techniques and optimum detection in communication systems. Gorazd Kandus received his B.Sc., M.Sc. and Ph.D. degrees in electrical engineering from the University of Ljubljana, Slovenia, in 1971, 1974 and 1991, respectively. He is currently the head of the Department of Digital Communications and Networks at the Jozef Stefan Institute and Associate Professor at the Faculty of Electrical Engineering, University of Maribor. His main research interests include design and simulation of mobile and wireless communication systems and introduction of new telecommunication services.
