Paola Salmi

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.

Differential Post Detection Integration Techniques for Robust Code Acquisition

In this paper, the problem of code acquisition for spread-spectrum systems is investigated in the presence of frequency errors. The classic approach of noncoherent post detection integration (NCPDI) is contrasted with differential post detection integration (DPDI), presented in two different versions, namely DPDI-Real and DPDI-Abs, respectively, suited for fine and large frequency offsets. The differential schemes are characterized thoroughly introducing a novel analytical treatment that is validated by simulations. In order to fairly quantify the potential gains introduced by the differential solutions, specific detector parameter optimization is performed for each scheme. The interesting result is that the DPDI significantly outperforms the classic NCPDI approach for large frequency errors, and behaves close to it for small offsets. DPDI can, thus, be considered as a very robust PDI solution with moderate complexity.

A performance review of PSP for joint phase/frequency and data estimation in future broadband satellite networks

In unknown channels, nonadaptive maximum likelihood sequence detection (NA-MLSD) is far from optimum in the presence of channel-induced uncertainty. To counteract this uncertainty, conventional solutions augment MLSD with external channel estimators. However, for very low SNR values this estimation procedure becomes marginal and the conventional adaptive MLSD (CA-MLSD) approaches perform poorly. We consider joint data decoding and parameter estimation techniques, and in particular per survivor processing (PSP) techniques, as a potentially effective solutions to this problem, at the expense of increased complexity. The performance/complexity tradeoff is addressed for a convolutionally encoded Q-PSK return link of an SHF/EHF satellite system for multimedia services impaired by phase noise and frequency offsets. By contrasting NA-MLSD, CA-MLSD, and PSP, ranges are provided for channel conditions where the tradeoff favors a particular technique. Further, CA-MLSD and PSP are also contrasted in terms of scalar/ vector estimation performance.

Differential and non-coherent post detection integration techniques for the return link of satellite W-CDMA systems

In the paper, the preamble structure of IMT-2000 SW-CDMA air radio interface is considered in the evaluation of the performance of different post detection integration techniques for code acquisition in the return link of a satellite W-CDMA system. In particular, non-coherent post detection integration (NCPDI) and two different options of differential post detection integration (DPDI) techniques, namely DPDI-abs and DPDI-real, are investigated in terms of missed detection and false alarm probabilities. AWGN and Rician channels are considered in the presence of a frequency error. Both analytical and simulation results are provided.

Link performance of an ultra wide bandwidth wireless in-home network

We present an ultra wide bandwidth (UWB) wireless network for in-home cordless networking and wireless distribution of cable TV channels. The article describes the network services, the media access control, the physical link transmitter and receiver design and the link bit error rate (BER) performance. The physical link is based on an interference suppressing OFDM (IS-OFDM) in which the total number of frequency bins (subcarriers) of the UWB channel is divided into groups. Every frequency bin in a group carries all transmitted symbols for that group which are distinguished and separated from each other by orthogonal Hadamard sequences. The proposed design may then suppress narrow-band interference which is often present in UWB channels. In an indoor multipath channel, IS-OFDM has also a significant advantage over ordinary OFDM, which, as shown, does not offer acceptable performance for this type of channel. The IS-OFDM link then is expected to satisfy the high bit rate needs and meet the required quality of service with a minimum transmit power so that it does not cause interference beyond the perimeter of the home.

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.

Differential post detection integration technique in the return link of satellite CDMA systems

A performance evaluation is carried out of a differential post detection integration (DPDI) technique for code acquisition in the return link of a satellite CDMA system in the presence of Rician fading and non-ideal frequency recovery. In particular, the preamble structure of IMT-2000 SW-CDMA air radio interface is considered for evaluating the missed detection and false alarm probabilities. An analytical approach is used for a post detection integration length equal to one, whereas simulation results are provided both for validation purposes and for evaluation of more general cases. Finally, the achieved closed form expression for the false alarm probability allows the decision threshold to be computed analytically.

A novel up-link code-acquisition procedure based on a structured preamble sequence

A novel up-link one-shot code acquisition procedure is proposed based on a structured preamble sequence. The general strategy is applied to the context of third generation satellite networks. A multi-dwell approach is introduced to simplify the detection of a long preamble. The parallel detection of the two basic sequences forming the preamble is coordinated by a novel HLC (high level controller). Passive and active differential post detection integrations are selected to perform detection during the search and verification phases, respectively. Performance is evaluated and optimized with respect to the most significant system parameters.

Performance analysis of PSP based joint data decoding and phase/frequency estimation in satellite communications

The application of per-suvivor processing (PSP) to digital satellite communications at Ka-band and beyond is considered. The PSP approach is used in a convolutionally encoded M-PSK transmission scheme to effectively counteract the degradations caused by phase noise and frequency offsets. Second order tracking loops are considered for per-survivor parameter estimation. A comparison is carried out between conventional adaptive maximum likelihood sequence detection (CA-MLSD) and PSP schemes both in terms of detection and estimation performance. Emphasis is placed on the analysis of the estimation quality one of the most unexplored research areas in the field of PSP-based systems. To this aim, novel definitions for a decision-related tracker and virtual tracker are introduced and their performance analyzed. Monte-Carlo simulations are provided to show the improvement in both estimation and detection quality brought about by PSP with respect to CA-MLSD.

Imperfect orthogonal multiplexing in the forward link of CDMA networks

The Universal Mobile Telecommunications System (UMTS) will comprise both a terrestrial part (T-UMTS) and a satellite part (S-UMTS). The air interface will include code division multiple access as one of the operating modes, with direct sequence spreading based on scrambling and channelization codes. The latter are used for discriminating among different users/services, and play a fundamental role in the overall system performance. We consider Orthogonal Variable Spreading Factor codes for channelization, and we focus our attention on the forward link from the base station to the mobile terminal. Our aims are to describe analytically the transmitted multiplex signal passed through a high power amplifier (inevitable in S-UMTS), before and after the non-linear amplification, and to analyze the performance loss which derives from imperfect orthogonality induced by the non-linear device. To this end, we introduce a semi-analytic procedure based on the notion of channelization code power ratio, and we validate it through computer simulation. The major finding in our numerical results is that due to the orthogonality loss, it is impossible to fully exploit the forward link power control dynamics, with consequent capacity loss.