Sandro Bellini

Compensation of tissue absorption in emission tomography

This paper presents a technique for overcoming the problem of tissue absorption in emission tomography. Given a set of equispaced projections in the interval (0, 2π), it is possible to derive an exact formula for recovering the spectrum of the image. The formula is obtained by solving a linear partial-differential equation that links the derivatives of the spectrum with respect to absorption and spatial frequencies. Examples of the application of the technique to synthetic and real data are given.

Digital frequency estimation in burst mode QPSK transmission

In burst digital transmission using PSK (phase shift keying) modulation with coherent detection, the recovery of the carrier reference phase and the symbol clock is a key aspect. If all users have a common clock synchronization, symbol timing needs not to be recovered in each burst. A digital processor for carrier recovery without preambles, in the presence of frequency offset, is considered. As an example, a 2 Mb/s QPSK transmission system is considered in which E/sub b//N/sub o/=10 dB, and the burst and estimation interval length L=15. Using the algorithm described and averaging eight successive estimated frequency offsets, in order to eliminate anomalous errors, the BER (bit error rate) degradation is equal to 0.14 dB when Delta f=20 kHz. >

A Low Complexity Turbo MMSE Receiver for W-LAN MIMO Systems

In this paper we consider an iterative detection and decoding scheme for Space-Frequency-Bit-Interleaved Coded Modulation (SF-BICM) MIMO-OFDM systems as a candidate receiver architecture for Next Generation Wireless LANs. This work is focused on the implementation complexity reduction of the overall turbo MIMO scheme through the simplification of the three main blocks: the Soft-Interference-Canceller, the MMSE-MIMO detector and the MIMO Soft-Symbol demapper which uses extrinsic soft information, produced by a Soft-Output-Viterbi-Algorithm (SOVA), to perform the LLRs calculation of the coded bits. A new receiver architecture is proposed, its computational complexity is estimated and compared with a more classical turbo MMSE receiver, both for 16-QAM and 64-QAM constellations.

Blind equalization and deconvolution

The identification (or inversion) of the impulse response or the transfer function of a linear channel, given only the channel output, has several applications: equalization of communication or data transmission channels; vocal tract identification; seismic data deconvolution for multiple reflection elimination; image deblurring; echographic data focusing, either in the acoustic case or in the electromagnetic one (synthetic aperture radar); etc. The aim of this paper is to clarify the connections between some of the techniques proposed so far to solve this problem.

Asymptotically efficient blind deconvolution

Abstract A solution is proposed for the discrete blind deconvolution problem, i.e., the estimation of the impulse response of a sampled system given its output and some statistical information on its input. The input sequence is supposed to be independent and identically distributed. The estimate uses the crosscorrelation between the output samples and a nonlinear function of the output samples. The technique is efficient when the residual unknown channel distortion is small, i.e., asymptotically. Therefore, it is recommended as a final step to clean up the noise left by any preferred blind deconvolution method. The variance of the estimation error and the optimal nonlinear function, which depends on the input probability density, are given. The variance is checked against the Cramer-Rao bound. The rms error of the suboptimal solution that adopts the nonlinear function ‘sign’ depends on simple moments of the input data sequence and on the probability density at the origin. Moderate losses ensue in the case of generalized Gaussians. When the distortion is not small, simulations show that this technique is still useful, but iterations are needed to remove the estimation bias. Polyspectral techniques, that could give unbiased solutions for any channel distortion, cannot be any better asymptotically. The paper discusses whey they could be much noisier. The extension of the technique described in this paper to complex data and impulse response and to parameter dependent distortions is straightforward and is briefly sketched.

On the Throughput of an ALOHA Channel with Variable Length Packets

In this paper we give a new simple expression for the probability of successful transmission on an infinite population ALOHA channel with variable length packets. Expressions for the throughput and the probability density of the packet length on the channel are derived in a straightforward way along with the best and the worst length densities.

Efficient OFDM Channel Estimation via an Information Criterion

In this paper, we consider joint estimation of the channel length and of the impulse response for OFDM systems, exploiting information criteria to find the best trade-off, in terms of Kullback-Leibler divergence, between noise rejection and channel description accuracy. So far, information criteria have not been used for practical channel length estimation methods, due to their prohibitive complexity. We show how to make them affordable, performing channel estimation in a recursive way that allows to establish the optimal channel length with a moderate incremental cost. With reference to IEEE 802.11 OFDM-based standards for WLAN, we investigate several cases, applying the joint channel length and impulse response estimation to many scenarios, ranging from the simplest pilot-aided channel estimation based on training sequences, to the most challenging data-aided channel tracking, driven either by detected or by decoded symbols. In all cases, the achieved performance and robustness are very good, with a very small increase in complexity w.r.t. estimation methods that assume fixed channel length.

On the selection of semi-orthogonal users for zero-forcing beamforming

We reconsider the role of user selection in multiuser MIMO broadcast channels (downlink), in the relevant regime where the number of users K is linear in the number of transmitter (base station) antennas M. User selection is known to achieve mutually quasi-orthogonal user channels and, at the same time, a multiuser diversity effect in terms of receiver SNR. These goals are achieved in the regime of fixed number of transmit antennas, and very large number of users. In contrast, we show that when K = O(M) these effects cannot be achieved, and the role of user selection is marginal. In terms of system design, our results suggest that only a small number K ≈ M of users should feedback their channel state information at each point in time. This greatly alleviates the burden of the channel state information feedback, while achieving essentially optimal performance.

Full Digital Adaptive Equalization in 64-QAM Radio Systems

In view of a digital implementation of most of the signal processing required in a QAM demodulation-equalization Structure, a performance analysis as a function of equalizer complexity, A/D conversion resolution, and precision of internal arithmetic is presented. The analysis is focused on a 140 Mbit/s 64-QAM modulation scheme (corresponding to CEPT-4 or 3 \times DS3 systems), but extension to other QAM constellations is straightforward. Considerations of currently available technology, overall computational complexity, and related optimal quantization formats are presented. For high-speed 64 QAM, it is shown that satisfactory performance may be expected from a full digital implementation of reasonable complexity, using current technology.

Turbo-LORD: A MAP-Approaching Soft-Input Soft-Output Detector for Iterative MIMO Receivers

In this paper a novel Soft-Input Soft-Output detector, namely Turbo-LORD, is proposed for iterative MIMO receivers. This is an improved version, capable of managing a priori information, of the Layered ORthogonal lattice Detector recently presented. The implementation is straight and efficient when there are only two transmitting antennas. However, problems arising with more than two antennas are also discussed, along with possible solutions. It is shown that notwithstanding the suboptimal low-complexity implementation, this iterative receiver misses the performance of the turbo MAP detector by only few tenths of dB in various configurations, with very high spectral efficiency.