Marco Luise

Carrier frequency acquisition and tracking for OFDM systems

We present and analyze a technique for fast acquisition and accurate tracking of the carrier frequency in orthogonal frequency division multiplexing (OFDM) receivers. The scheme is based on a data-aided frequency estimation algorithm. The presence of known symbol sequences periodically inserted in the OFDM frame allows the data demodulator to rapidly lock onto the carrier frequency during the acquisition phase, even in the presence of frequency offsets up to a few tenths of the overall signaling rate. Once acquisition is over, the circuit switches to a decision-directed mode to perform fine frequency tracking for reliable data demodulation. The algorithm performance is analyzed in terms of width of the lock-in frequency range and of lock-in probability in the acquisition mode, and of mean-square frequency estimation error in the tracking mode. Since OFDM is known to be extremely sensitive to carrier frequency errors, the impact of the carrier frequency synchronizer on the receiver error rate is also investigated.

Turbo synchronization: an EM algorithm interpretation

This paper is devoted to turbo synchronization, that is to say the use of soft information to estimate parameters like carrier phase, frequency offset or timing within a turbo receiver. It is shown how maximum-likelihood estimation of those synchronization parameters can be implemented by means of the iterative expectation-maximization (EM) algorithm [A.P. Dempster, et al., 1977]. Then we show that the EM algorithm iterations can be combined with those of a turbo receiver. This leads to a general theoretical framework for turbo synchronization. The soft decision-directed ad-hoc algorithm proposed in V. Lottici and M. Luise, [2002] for carrier phase recovery turns out to be a particular instance of this implementation. The proposed mathematical framework is illustrated by simulations reported for the particular case of carrier phase estimation combined with iterative demodulation and decoding [S. ten Brink, et al., 1998].

Low-complexity blind carrier frequency recovery for OFDM signals over frequency-selective radio channels

This paper introduces a blind (i.e., data independent) algorithm for carrier frequency offset recovery in an orthogonal frequency-division multiplexing (OFDM) receiver operating over frequency-selective fading channels. The main idea behind this algorithm is to exploit the time-frequency-domain exchange inherent to the modulation scheme. Due to this feature, a carrier frequency offset has a similar impact on OFDM as a clock timing offset has in a quadrature amplitude modulation (QAM) system. The scheme we propose is a variant of Oerder-Meyrs (1988) feedforward clock recovery. Its performance is assessed by simulation, and the results are compared to those obtained from Van de Beek-Sandell-Borjessons (1997) frequency synchronizer, which bears comparable complexity. The new scheme is shown to outperform the latter over frequency-selective fading channels, notably at medium to high signal-to-noise ratios. We also evaluated the efficiency of two different (time domain and frequency domain) offset correction strategies embedded in a particular OFDM receiver.

Mobile and Personal Communications in the 60 GHz Band: A Survey

This paper intends to present a summary of the technical issues arising in the exploitation of the 60 GHz mm-wave band for mobile and personal communications. The most significant applications proposed so far are surveyed, with particular emphasis placed on recent experimentation about millimeter-wave propagation for road/railway transportation as well as indoor scenarios. As a case study, the capacity of a (micro-)cellular Code Division Multiple Access (CDMA) network in the 60-GHz band is also evaluated in detail.

A digital chip timing recovery loop for band-limited direct-sequence spread-spectrum signals

Migration towards a full-digital implementation of modems is currently one of the main trends in transmission systems design. The authors describe a noncoherent all-digital delay lock loop (DDLL) suited for chip timing synchronization in band-limited direct sequence spread spectrum (DS/SS) systems, and they thoroughly analyze its performance. The key features of this novel scheme are represented by its low-complexity processing section together with its good tracking capability. Analytical expressions for the DDLL S-curve and steady-state timing jitter are derived and confirmed by a time-domain computer simulation. Furthermore, the Mean Time to Lose Lock (MTLL) of the loop is evaluated and some numerical results are reported. The proposed chip timing synchronization scheme reveals also an improved tracking performance when compared to the traditional analog DLL for rectangular chip DS/SS signals. >

Code-Aided Turbo Synchronization

The introduction of turbo and low-density parity-check (LDPC) codes with iterative decoding that almost attain Shannon capacity challenges the synchronization subsystems of a data modem. Fast and accurate signal synchronization has to be performed at a much lower value of signal-to-noise ratio (SNR) than in previous less efficiently coded systems. The solution to this issue is developing specific synchronization techniques that take advantage of the presence of the channel code and of the iterative nature of decoding: the so-called turbo-synchronization algorithms. The aim of this paper within this special issue devoted to the turbo principle is twofold: on the one hand, it shows how the many turbo-synchronization algorithms that have already appeared in the literature can be cast into a simple and rigorous theoretical framework. On the other hand, it shows the application of such techniques in a few simple cases, and evaluates improvement that can be obtained from them, especially in the low-SNR regime.

MHOMS: high-speed ACM modem for satellite applications

This article presents the novel FPGA-based 1 Gb/s near-Shannon-limit ACM modem developed within the MHOMS program with particular focus on the advanced modem algorithm solutions devised. A number of powerful FEC schemes have been analyzed as possible candidates for the MHOMS modem, and the final selection is justified in terms of the best tradeoff between complexity and performance. State-of-the-art modulation and demodulation algorithms are also presented, including nonlinearity dynamic precompensation techniques and innovative synchronization strategies required by the selected powerful modulation and coding schemes. Overall modem performances are also shown for a variety of spectral efficiencies.

Advances in satellite CDMA transmission for mobile and personal communications

An ubiquitous network for multimedia personal communications (Personal Communications Network (PCN)) with small, individual low-cost terminals is one of the most ambitious worldwide projects for the 21st century that are being pursued nowadays. In the development of such PCN, Geostationary, Medium, and Low Earth Orbiting (GEO, MEO, LEO) satellite constellations will play a fundamental role to provide worldwide coverage for most services required by the end-user. The characteristics of efficiency and flexibility inherently required by that scenario suggest, amidst other possibilities, to take into special consideration a radio interface based on code division multiple access (CDMA) to ensure, in addition to the features mentioned above, a sufficient grade of power and spectral efficiency of the relevant satellite radio link. The aim of this paper is a review of the current status of those issues in the field of satellite CDMA transmission systems design that, in our opinion, appear fundamental to the successful operation of an efficient PCN. In particular, we survey the techniques for multiplexing, coding and transmission of direct-sequence spread spectrum (DS/SS) signals, and we touch upon the techniques for the minimization of the self-noise effect, and the related topics of power-control and multiuser detection. We also shortly address in this respect some technological aspects related to an efficient modem design via digital signal processing techniques. The final part of the paper deals more specifically with some typical issues of satellite transmission, namely the minimization of the detrimental effects of the nonlinear satellite transponder and of multipath propagation; the applicability of diversity reception to a multisatellite network is also addressed as possible means of performance boost.

Optimization of symbol timing recovery for QAM data demodulators

This paper describes an effective technique for the optimization of the clock recovery circuit in an all-digital modem for linearly modulated signals. Starting from the concept of prefiltering of the data signal (already pursued by these authors in the context of analog data receivers), it is shown how to design an optimum digital prefilter for the minimization of jitter due to both Gaussian and pattern noise in the closed-loop clock recovery scheme by Gardner (1986). The numerical results of the theoretical analysis, obtained after iterative resolution of a constrained-minimum problem via the Lagrange multiplier method, are checked by simulation and can be nicely justified by the consideration of the frequency response of the optimum prefilter. The key outcome of such an approach is the demonstration of a substantial performance improvement in terms of steady-state clock jitter, even with remarkably simple FIR prefilters with a small number of taps.

Decision-directed coherent delay-lock tracking loop for DS-spread-spectrum signals

The authors present a nonconventional joint data demodulation-pseudo-noise (PN) code tracking scheme for direct sequence (DS) spread-spectrum (SS) signals which solves problems of component imbalance and sensitivity with hardware simplicity and no performance degradation. An integrate-and-dump Costas loop is used for carrier recovery and data demodulation of the SS signal. Both data and carrier are then used to derive the baseband error signal of the code tracking loop. Moreover, a single passband correlator is used to perform the early-late correlation, leading to a hardware complexity equivalent to that of the tau-dither scheme, but without its loss in performance. Results of a thorough theoretical analysis of the system in an additive Gaussian noise (AWGN) environment are reported. They provide performance curves in terms of steady-state jitter and mean time to first lock loss. A superior jitter performance for low values of E/sub b//N/sub 0/ with respect to a traditional noncoherent delay lock loop (DLL) is shown, along with the potential gain of Manchester coding upon the more usual NRZ format. >