Magnitude modulation for VSAT's low back-off transmission

Abstract

This paper addresses the problem of controlling the envelope's power peak of single carrier modulated signals, band limited by root-raised cosine (RRC) pulse shaping filters, in order to reduce power amplifier back-off for very small aperture terminals ground stations. Magnitude modulation (MM) is presented as a very efficient solution to the peak-to-average power ratio problem. This paper gives a detailed description of the MM concept and its recent evolutions. It starts by extending the look-up-table (LUT) based approach of the MM concept to M -ary constellations with M ≤ 16. The constellation and RRC symmetries are explored, allowing considerable reduction on LUT computation complexity and storage requirements. An effective multistage polyphase (MPMM) approach for the MM concept is then proposed. As opposed to traditional LUT-MM solutions, MM coefficients are computed in real-time by a low complexity multirate filter system. The back-off from high-power amplifier saturation is almost eliminated (reduction is greater than 95%) with just a 2-stage MPMM system even for very demanding roll-off cases (e.g., α = 0.1). Also, the MPMM is independent of modulation in use, allowing its easy application to constellations with M >16.Index Terms: Back-off, magnitude modulation (MM), multirate filtering, peak-to-average power ratio (PAPR), very small aperture terminals (VSAT).

Ring-type Magnitude Modulation for LINC: a pragmatic approach to the efficiency challenge

“…Magnitude modulation techniques [17], [18], [27], can provide a suitable solution. MM was originally developed to limit the maximum excursion of bandlimited SC signals of generic M-ary constellations (thus requiring for linear amplification) in order to reduce input back-off requirements of linear HPAs to improve SC transmitter's energy efficiency; moreover, since MM operates before pulse shaping it does not affect signal bandwidth.…”

“…Unlike other MM methods (e.g. multistage polyphase magnitude modulation (MPMM) [18]), the task of developing a real-time method is not straightforward due to the number of RMM parameters (two amplitude boundaries and two coefficients) aiming at obtaining a magnitude modulated signal having an envelope with very low dynamic range and without zero crossings, which is why the RMM coefficients are computed a priori and stored in LUTs, as in classical MM methods [17], [18], [28]. Therefore, in order to adjust a given symbol using the RMM technique, it is only necessary to search each LUT for the current shift register's state, and fetch the corresponding RMM coefficients; the base architecture of an OQPSK transmitter employing MM is shown in Fig.…”

“…Accordingly, by evaluating independently the in-phase and quadrature symbol components, the coefficient calculation routine that is performed by the end of each iteration is: Equation (20) shows that the RMM coefficients depend on the defined amplitude boundaries, here depicted by [k]. Hence, the solutions obtained using the proposed RMM method are substantially different from the ones derived in [17], [18]. Additionally, (20) holds when RMM has no effect, since [k]=0, ∀k, yields the RMM coefficients m I =m Q =1.…”

“…Despite the MM coefficients' dependency on the pulse shaping filter (as in [18], [28]), the reasoning and the algorithms described in this paper are equally valid for any parameter set, which makes it possible to obtain a similar performance from transmitter schemes with a different parameter configuration (e.g., a different roll-off or oversampling factor).…”

“…Nonetheless, OQPSK bandlimited signals close to the Nyquist limit still present substantial envelope fluctuations. Regarding this, magnitude modulation (MM) techniques [17], [18] are known to be effective methods of controlling the signal's envelope excursions without spreading the transmitted signal's spectrum or noticeably affecting transmission's bit error rate (BER) performance. However, when applied to bandlimited OQPSK signals, conventional magnitude modulation (MM) techniques only control the maximum upper envelope excursion, with the resulting signals still having a high dynamic range (although with lower PAPR), with frequent zero crossings, not allowing an efficient LINC transmission.…”