Massimo Neri

On the adaptive DVB-S2 physical layer: design and performance

The successful DVB standard has now evolved into the DVB-S2 standard, which promises to bring very significant capacity gains. The main DVB-S2 feature is its adaptive air interface, where coding and modulation techniques are varied flexibly to maximize performance and coverage. This article addresses the design of the entire DVB-S2 communication chain, considering practical algorithms for coding, modulation, predistortion, carrier and SNR estimation, frame synchronization, and data recovery. The design complexity is exacerbated by the fact that DVB-S2 foresees 28 different coding/modulation pairs, demanding specific optimization and variable frame length. The performance achieved considering all possible impairments is compared to the ideal performance achievable in the Gaussian channel in terms of integral degradation, which ranges from 0.4 to 2,5 dB in going from QPSK to 32-APSK.

LTE adaptation for mobile broadband satellite networks

One of the key factors for the successful deployment of mobile satellite systems in 4G networks is the maximization of the technology commonalities with the terrestrial systems. An effective way of achieving this objective consists in considering the terrestrial radio interface as the baseline for the satellite radio interface. Since the 3GPP Long Term Evolution (LTE) standard will be one of the main players in the 4G scenario, along with other emerging technologies, such as mobile WiMAX; this paper analyzes the possible applicability of the 3GPP LTE interface to satellite transmission, presenting several enabling techniques for this adaptation. In particular, we propose the introduction of an inter-TTI interleaving technique that exploits the existing H-ARQ facilities provided by the LTE physical layer, the use of PAPR reduction techniques to increase the resilience of the OFDM waveform to non linear distortion, and the design of the sequences for Random Access, taking into account the requirements deriving from the large round trip times. The outcomes of this analysis show that, with the required proposed enablers, it is possible to reuse the existing terrestrial air interface to transmit over the satellite link.

DVB-S2 LDPC Decoding Using Robust Check Node Update Approximations

Broadband satellite services to fixed terminals are currently offered in the forward link by the 2nd generation (2G) digital video broadcasting satellite (DVB-S2) standard. For this standard the use of powerful low-density parity-check (LDPC) error correcting codes has been adopted performing within approximately 1 dB from the Shannon capacity limit. This paper studies and compares for the first time in a systematic manner different approximation methods used in check node update computation of DVB-S2 LDPC decoding with the aim of reducing computational complexity. Various performance evaluation results are presented for a wide range of DVB-S2 parameters, such as LDPC codeword size, coding rate, modulation format and including several decoding algorithms. It is shown that the proposed check node update approximations have a robust behavior, i.e. the resulting performance is quite independent of the DVB-S2 modulation and coding parameters. It is further shown that these approximations perform very close to the optimal sum-product algorithm (SPA) in degradation, which is less than 0.2 dB. Despite this small degradation, the reduction in computational complexity compared to the optimal SPA is significant and can be as high as 40% in computational time savings.

On the use of OFDM radio interface for satellite digital multimedia broadcasting systems

The paper considers the application of Orthogonal Frequency Division Multiplexing (OFDM) techniques in the physical layer of a satellite system devoted to the provision of digital multimedia broadcasting (DMB) services, which are presently the focus of extensive research and development endeavours. In particular, the paper demonstrates that the inherently high spectrum efficiency of OFDM can be retained even in unfavourable propagation channels affected by heavy non-linear distortion introduced by the on-board high power amplifier. This encouraging performance is achieved through the proper design and application of coding and pre-distortion techniques, as described in the paper. Based on these results, the paper considers also the comparison of OFDM and High Speed Downlink Packet Access (HSDPA) techniques. HSDPA is the standardized radio interface for 3G terrestrial mobile networks.

Design and Performance of Predistortion Techniques in Ka-band Satellite Networks ¤

In a broadband satellite network supporting multicast/broadcast services along with point-to-point applications one of the most critical issues is related to the presence of the on-board high power amplifier (HPA) which causes non-linear distortion, with very severe impact when using high order modulations. The goal of this paper is to investigate advanced predistortion techniques and to assess the robustness of distortion resilient modulation schemes. In particular, we present a novel predistorter based on the analytical inversion of an extended Saleh model (ESM) for the HPA characteristics, fitted by using the Minimum Mean Square Error (MMSE) criterion, and the performance evaluation of a Look-UpTable (LUT) based predistortion scheme for uncoded transmission. The ESM parameters can be tuned to improve the match between the analytic HPA characteristics and the measured curves in a selected input/output power range, so as to boost the performance under specific operating conditions. These parameters are used both to derive the inverted HPA characteristics of the analytic predistorter, and to obtain the values to be stored in the LUT. Numerical results show that the ESM-based predistorter outperforms the classic Saleh model (SM) predistorter. Further, the optimization of LUT size, in terms of complexity and performance, and indexing strategy is reported in the paper.

OFDM vs. HSDPA comparison for satellite digital multimedia broadcasting systems

The paper considers the comparison of orthogonal frequency division multiplexing (OFDM) and high speed downlink packet access (HSDPA) techniques in the physical layer of a satellite system devoted to the provision of digital multimedia broadcasting (DMB) services. In particular, the paper demonstrates that the inherently high spectrum efficiency of OFDM can be retained even in unfavorable propagation channels affected by heavy non-linear and linear distortion, respectively introduced by the on-board high power amplifier and by frequency selective propagation. This encouraging performance, achieved through the proper design and application of coding and pre-distortion techniques, is compared with the standardized HSDPA radio interface for 3G terrestrial mobile networks, and is shown to be in line on non-selective channels. Advantages for OFDM are demonstrated in frequency selective channels, using low complexity equalization in the frequency domain

Joint Precoding and Predistortion Techniques for Satellite Telecommunication Systems

We analyze the application of Tomlinson-Harashima precoding to satellite transmission. Precoding counteracts channel dispersion at the price of increasing the dynamic range of the signal to be transmitted. This conflicts with the requirements of typical satellite transmission, where amplifiers with nonlinear characteristics are used, and their incoming signal is desired to present a reduced dynamic range. We propose fractional predistortion as a valid countermeasure against the effects of the nonlinear amplifier. Simulations allow us to obtain the performance of the proposed scheme when applied to two typical satellite propagation environments and compare it to an equivalent scheme using standard decision-feedback equalization. We conclude that a system using predistortion is more sensitive to the effects of satellite transmission-related nonlinearities than the equivalent DFE system

Performance of turbo coding for satellite UMTS multimedia broadcast multicast services

In this paper the forward link performance of a satellite based UMTS system designed for the delivery of multimedia broadcast multicast services is investigated for different physical layer parameter combinations in AWGN and Rice radio channels. Guidelines for the choice of these parameters are given as a trade-off between bit error rate and processing delay. Also, the effects on the performance of the internal turbo interleaver and of the inter- and intra-frame interleavers are analyzed and described.

Unequal error protection: a turbo multi level coding approach

In this paper we address the study of systems implementing unequal error protection (UEP) by means of the multi level coding (MLC) technique, taking into account the impact of asymmetric constellations and the effect of puncturing. UEP can lead to significant improvements when applied to multimedia broadcast and multicast services (MBMSs), which should represent an important share of the consumer market in the next years. Theoretical considerations and performance assessment of an MLC system implemented by means of turbo codes are undertaken, considering several modulation options.

Joint Symbol Timing and Carrier Frequency Recovery for DVB-SH System

In this paper, we present a novel joint symbol timing and carrier frequency synchronization algorithm for orthogonal frequency division multiplexing (OFDM) system with very low hardware complexity. Although we developed the proposed algorithm with reference to the DVB-SH standard, it is easily adaptable to other OFDM based systems. A well-known problem in OFDM modulation is its vulnerability to synchronization errors. In particular, accurate recovery of carrier frequency and symbol timing is crucial for the proper demodulation of the received packets. The proposed algorithm has low hardware complexity, and at the same time achieves very good performance in both AWGN and multipath channels. Finally, the paper includes also a detailed comparison of the proposed technique with several time and frequency synchronization algorithms already presented in the scientific literature and compliant with the DVB-SH physical layer.