Efficient Waveform Design for High-Bit-Rate W-band Satellite Transmissions

Abstract

In the EHF (Extremely High Frequency) domain, W-band (75-110 GHz) offers promising perspectives for future satellite communications, mainly in terms of large bandwidth availability for high-bit-rate transmission. In this work 1 , an innovative physical (PHY) layer design for broadband satellite connections operating in W-band is proposed, which is based on the Prolate Spheroidal Wave Functions (PSWFs). PSWF waveforms (originally proposed in short-range indoor Ultra Wideband (UWB) communications) are aimed at optimizing the tradeoff between the concentration of pulse energy in a finite time interval and in a limited bandwidth. In our paper, PSWF-based 4-ary PSM modulation, characterized by a nearoptimal compromise between spectral and envelope compactness, has been tested for the radio interface of a W-band geostationary (GEO) downlink connection. The effect of nonlinear distortions, introduced by power-efficient saturating amplifiers, can be drastically reduced without any power back-off and maintaining a very good spectral efficiency. Experimental results, obtained by means of realistic simulations, fully demonstrate the potential advantages taken by PSWF in terms of increased spectral efficiency, link availability and net payload rate with respect to state-of-the-art pulse-shaped modulations (raised-cosine filtered QAM and Gaussian Minimum Shift Keying), commonly employed in satellite communications.

Waveform design solutions for EHF broadband satellite communications

“…Unfiltered binary or quaternary modulations using rectangular pulse shaping (BPSK, QPSK), characterized by constant amplitude, are not acceptable for broadband transmission applications due to their high sidelobe power levels, not complying with the standard recommendation about spectrum usage. Frequency modulation schemes like Minimum Shift Keying (MSK) and Gaussian Minimum Shift Keying (GMSK) are resilient against nonlinear distortion and characterized by negligible sidelobe power level but, as shown in [6], are vulnerable to phase noise. We think that the effi- cient exploitation of EHF bandwidth portion will require a step-ahead in waveform design with respect to conventional state-of-the-art solutions.…”

“…These waveforms, based on the solution of specific differential equations, are characterized by optimal energy concentration in time and frequency domain. In particular, PSWF of order 1 and order 2 can provide RF modulated signals characterized by a near-optimal compromise between spectral compactness (sidelobe power level 31.5 dB below the main lobe [6]) and envelope compactness (their Peakto-Average-Power-Ratio (PAPR) is 1 dB [6]). Therefore, the effect of nonlinear distortions can be drastically reduced, while maintaining a spectral efficiency rather similar to bandwidth-limited raised cosine waveforms.…”

“…Nevertheless, it is worth mentioning that the generation of PSWFs is still unaffordable by state-ofthe-art signal processing architectures, at least at the high baud-rate considered in our simulations. Indeed, as shown in [6], 64 samples/pulse are required at minimum in order to maintain the optimal properties of these waveforms. This would result in a sampling rate of 320 GHz, clearly unreachable by state-of-the-art devices.…”

“…Nevertheless, for the applicability to a "dirty RF" environment, such as the EHF channel, the modification to Constant Envelope (CE) multicarrier waveforms, which are natively more robust to nonlin-ear distortions, should be considered. Other typologies of waveforms that have been proposed for satellite communication and, in particular, for EHF satellite communications are impulse-based ultra-wideband (UWB) [5,6].…”

Analysis of power spectral density of ultra wideband waveforms for pulse shape modulation

“…A M-ary PSM scheme based on the orthogonal properties of Hermite pulses is reported in Ghavami et al (2002), Wallace et al (2005) and de whereas such a scheme utilising PSWF is shown in Usuda et al (2004) to achieve high data rate in a severe multipath fading environment. A 4-ary PSM scheme based on PSWF acts as a physical layer for broadband satellite communications in W-band showing distinct advantages over raised cosine filtered quadrature amplitude modulation (QAM) (Sacchi et al, 2011). Performance comparison of UWB modulation schemes based upon the fifth derivative of Gaussian pulses is discussed in Abedi and Yagoub (2012).…”