1,373 research outputs found
Felicità e malinconia. Gozzano e i Crepuscolari, a cura di R. Carnero, Milano, Baldini Castoldi Dalai, 2006
No full textRecensione al volume Felicità e malinconia. Gozzano e i Crepuscolari, a cura di R. Carnero, Milano, Baldini Castoldi Dalai, 200
Efficient trellis search techniques for adaptive MLSE on fast Rayleigh fading channels
No full textWe analyze trellis search algorithms for joint sequence estimation and channel tracking, assuming rapidly varying frequency-selective Rayleigh fading. The Generalized Viterbi Algorithm (GVA) and the M-Algorithm (MA) are considered for approximately searching the maximum likelihood path in the trellis diagram and compared with the Viterbi Algorithm (VA). All algorithms perform channel tracking utilizing Per-Survivor Processing (PSP) techniques by associating a channel estimate
to each hypothetical trellis path, according to the Least Mean Square (LMS) or Recursive Least Square (RLS) algorithm.
To reproduce a typical mobile digital communication system,
a Time-Division Multiple Access (TDMA) data frame
is assumed, where each user transmits a block of information
symbols with known preamble and tail. For Doppler bands up to one hundredth the symbol frequency and a three path delay profile, the MA is shown to exhibit a slightly better performance than the GVA for an equal number of survivors, and outperform the VA
A Recursive Formulation for Quadratic Detection on Rayleigh Fading Channels
No full textWe address the problem of optimal detection of a random signal transmitted over a time-varying frequency-selective correlated Rayleigh fading channel. We present a general recursive solution which may be operated at full complexity to provide optimal detection or at reduced complexity, using per-survivor processing (PSP) techniques, to yield a suboptimal receiver
On the Use of Oversampling in Adaptive Receivers for Frequency-Selective Rayleigh Fading Channels
No full textIn this work, we investigate Maximum Likelihood Sequence Estimation (MLSE) of data sequences transmitted over time-varying frequency-selective Rayleigh fading channels and consider adaptive receivers which realize joint data estimation and channel tracking utilizing Per-Survivor Processing (PSP) techniques. In order to obtain a sufficient statistics for data detection, the received signal is oversampled with respect to the symbol rate. Accordingly, a time-discrete channel model with fractional spacing, with respect to the symbol interval, is assumed and an algorithm which performs fractionally-spaced
PSP-based channel tracking is analyzed. We show that receivers which employ this algorithm outperform traditional adaptive receivers based on one sample per symbol interval. Two samples per symbol interval are shown to be sufficient to attain the optimal performance for these receivers
On the Performance of the Quadratic Receiver for Rayleigh Fading Channels
No full textWe investigate maximum likelihood sequence estimation (MLSE) of data sequences transmitted over time-varying frequency-selective Rayleigh fading channels. We present the optimal receiver designed to exploit perfect channel state information (CSI) at the beginning of the data sequence and the optimal receiver which does not use this information. For any given data sequence length, a performance range for the optimal detection can be identified: this range is lower bounded by the optimal receiver with perfect CSI and upper bounded by that without CSI. Furthermore, this range narrows for increasing data sequence lengths. The performance of these optimal receivers is compared to that of some suboptimal receivers based on per-survivor processing (PSP) which perform joint sequence detection and channel tracking using the least mean square (LMS) algorithm. In terms of performance, these suboptimal receivers are shown to be intermediate between the optimal receivers with and without initial CSI. In addition, we show that the performance of both optimal receivers degrades significantly if they are provided with imperfect information about the signal-to-noise ratio or channel delay profile-information which is not necessary to the suboptimal PSP-based receivers
Optimal versus PSP-based sequence estimation for Rayleigh fading channels
No full textWe investigate maximum likelihood sequence estimation (MLSE) of data sequences transmitted over time-varying frequency-selective Rayleigh fading channels. We present the optimal receiver designed to exploit perfect channel state information (CSI) at the beginning of the data sequence and the optimal receiver which does not use this information. For any given data sequence length a performance range for optimal detection can be identified: this range is lower bounded by the optimal receiver with perfect CSI and upper bounded by that without CSI. For increasing data sequence lengths, this range becomes narrower, since the performance of the two optimal receivers tends to converge to a common level. As a consequence, the performance of the optimal detector can be inferred and compared to that of practical receivers based on per-survivor processing (PSP)
An efficient time-discrete symbol-spaced simulation model for mobile radio channels
No full textWe propose a fully parametric, time-discrete, symbol-spaced simulation model of frequency-selective mobile radio channels. The model encapsulates transmission and receiver filters and a physical channel accounting for both Rayleigh and Rice fading. The simulator allows one to accurately trade between complexity and quality of representation of the propagation channel, making use of a digital filter of variable order to implement the Doppler spectrum of the fading processes. Numerical results assessing the performance of the model in terms of the simulation time and the power spectrum of the fading processes are provided. A communication system with an adaptive receiver is employed to test the reliability of the propagation model. Increasing the degree of accuracy in the approximation of the Doppler spectrum, the performance of the considered receiver, expressed in terms of symbol error rate, rapidly converges to a specific function of the signal to noise ratio
Recursive Optimal Detection of Linear Modulations in the presence of Random Fading
No full textIn this paper, the problem of optimal detection of a linearly modulated digital signal transmitted over a fading channel is addressed. First, the issue of preliminary front-end processing in order to extract a sufficient statistics for information sequence detection from the continuous-time received signal is considered. Then, recursive algorithms are derived, which approximate optimal detection under the assumption of a linear and noisy channel modeled by a stochastic time-varying impulse response with arbitrary known statistics. Specific solutions are proposed for the Rayleigh fading channel typical of mobile radio communications. The information symbols are assumed to be grouped into blocks which are preceded and followed by known preamble and postamble, respectively This model encompasses both Time Division Multiple Access (TDMA) schemes and pilot symbol assisted transmission. Two detection schemes are considered and relevant recursive formulations proposed. More specifically, optimal detection in complete absence of Channel State Information (CSI) (blind detection) or in presence of perfect CSI at the beginning of the data block (trained detection) are investigated. It is shown that optimal detection performance may be practically attained by sampling the received signal with few samples per symbol interval and searching a trellis diagram of adequate size by means of a Viterbi algorithm. State-complexity reduction techniques based on Per-Survivor Processing (PSP) for both types of detection schemes are also considered
New Recursive Formulations of Optimal Detectors for Rayleigh Fading Channels
No full textWe investigate maximum likelihood sequence estimation (MLSE) of data sequences transmitted over time-varying frequency-selective Rayleigh fading channels. We present new recursive formulations for optimal detection in presence of a perfect channel state information (CSI) at the beginning of the data sequence or in complete absence of CSI. We then derive a reduced state-complexity solution based on per-survivor processing (PSP) techniques for both optimal detection schemes. Performance evaluations are also provided. In particular, we show that the optimal detector performance may be practically attained with PSP-based reduced-complexity detectors. Furthermore the considered receivers are shown to approach in several cases the lower bound of an exhaustive sequence search
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