No. 3 (2001)
Published:
2001-09-30
ARTICLES FROM THIS ISSUE
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An introduction to envelope constrained filter design
Abstract
Envelope constrained filter design is concerned with the time domain synthesis of a filter whose response to a specified input signal stays within prescribed upper and lower bounds and in addition has minimal noise enhancement. In many practical applications, a ``soft`` approach, such as least mean square, is not the most suitable and it becomes necessary to use ``hard`` constraints such as the ones considered in the paper. We present an overview of key ideas related to robust continuous time envelope constrained filter design.
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Source enhanced linear prediction of speech incorporating simultaneously masked spectral weighting
Abstract
Linear prediction is the cornerstone of most modern speech compression algorithms. This paper proposes modifying the calculation of the linear predictor coefficients to incorporate a~weighting function based on the simultaneous masking property of the ear. The resultant prediction filter better models the perceptual characteristics of the source and results in the removal of more perceptually important information from the input speech signal than a standard LP filter. When employed in a low rate speech codec the net effect is an improvement in subjective quality, with no increase in transmission rate and only a modest increase in computational complexity.
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Walsh-chirp sequences for wireless applications
Abstract
This paper deals with a new method to design polyphase spreading sequences for DS CDMA wireless applications. The method is based on weighting symbols of the orthogonal Walsh sequences by the complex factors being symbols of baseband chirp sequences. The resulting sequences possess good aperiodic correlation properties, while maintaining the orthogonality. Because of the parametric design, the sequences can be optimized to achieve desired characteristics.
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Memory truncation and crosstalk cancellation for efficient Viterbi detection in FDMA systems
Abstract
In this paper, the design of optimal receive filter banks for frequency division multiple access (FDMA) over frequency selective channels is investigated. A new design strategy based on the principle of memory truncation, rather than equalization, is presented. Through the receive filters, each subchannel is truncated to a pre-defined length, and the final data recovery is carried out via low complexity Viterbi detectors. Both closed form designs and adaptive techniques are discussed. Design examples are presented for high speed transmission over copper wires. The examples show that memory truncation allows significant performance improvements over the often used minimum mean squared error (MMSE) equalization.
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From modelling of a CDMA transceiver in indoor environment to an ASIC circuit synthesis
Abstract
This paper presents the study, design and simulation of a multi-flow radio frequency transceiver based on a direct sequence spread-spectrum with a 2.4 GHz carrier. First, the functional model of differential QPSK modulation for digital transmission, and the different parts making up spread spectrum function (spreader, despreader, tracking and synchronising devices) have been studied, implemented, simulated and validated in noisy multi-users and multi-path environment by using a unified language. The results obtained by taking into account the home automation running constraints have allowed to determine some critical parameter values and so to integrate the digital functions in an ASIC circuit.
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Simulating capture behaviour in 802.11 radio modems
Abstract
Simulation plays an important role in the performance evaluation of MAC protocols. Building simulation models which are able to accurately model physical behaviour is fundamental to the outcome of such techniques. Through both qualitative and quantitative comparison of experimental trace data against simulation results obtained using delay, power, and hybrid capture models, this paper investigates the performance of various packet capture models in the simulation analysis of the 802.11 PHY and MAC layer protocols. We illustrate these models are unable to accurately describe the fairness properties of the experimental data. A new model is proposed, Message Retraining, to describe the operation of an 802.11 receiver. We illustrate that the Message Retraining reception model is able to model the fairness characteristics obtained with an IEEE 802.11 radio modem more accurately than the previous capture models.
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Error statistics for concatenated systems on non-renewal time-varying channels
Abstract
The statistics of the error process generated by a discrete super channel formed by the concatenation of a constrained encoder, a non-renewal finite state channel (FSC), and a constrained decoder is studied in this paper. First, recursions are developed for the error weight distribution. This statistics is relevant to the design of coding schemes and interleaving in concatenated systems. We also study the renewal nature of the residual error process as modified by the constrained decoder. Ferreira et al. conjectured that if the channel model is a renewal FSC, the super channel can be modeled as a similar renewal model. We use a statistics called the multigap distribution to analytically disprove this hypothesis. Furthermore, the effect of interleaving is investigated from a new perspective using the variance of the multigap distribution. -
Stop criteria for retransmission termination in soft-combining algorithms
Abstract
Soft-combining algorithms use retransmissions of the same codeword to improve the reliability of communication over very noisy channels. In this paper, soft-outputs from a maximum a posteriori (MAP) decoder are used as a priori information for decoding of retransmitted codewords. As all received words may not need the same number of retransmissions to achieve satisfactory reliability, a stop criterion to terminate retransmissions needs to be identified. As a first and very simple stop criterion, we propose an algorithm which uses the sign of the soft-output at the MAP decoder. The performance obtained with this stop criterion is compared with the one assuming a genius observer, which identifies otherwise undetectable errors. Since this technique needs always a particular number of initial retransmissions, we exploit cross-entropy between subsequent retransmissions as a more advanced but still simple stop criterion. Simulation results show that significant performance improvement can be gained with soft-combining techniques compared to simple hard or soft decision decoding. It also shows that the examined stop criteria perform very close to the optimistic case of a genius observer.
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Precise measurement of complex permittivity of materials for telecommunications devices
Abstract
In order to obtain precise complex permittivity of the dielectric materials obtained from the perturbation method a correction curve is made using the electromagnetic field simulator which applies transmission line modeling (TLM) method. In this experiment, generated microwave power with the frequency of 2.45 GHz is applied to heat dielectric material while measuring temperature dependence of complex permittivity of dielectric material. To obtain these objectives cavity resonator with cooling system is designed. It is found from the result that the accurate temperature dependence of complex permittivity of the materials can be obtained by the method presented here.
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Method of analytical regularization based on the static part inversion in wave scattering by imperfect thin screens
Abstract
The paper is focussed on the development of the method of analytical regularization (MAR) in electromagnetic wave scattering and absorption by imperfect scatterers shaped as thin screens.
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Optimum double-matched detection and its application to SSMA systems
Abstract
This paper is concerned with the issue of optimum detection of known signal in nonwhite noise and/or narrow-band interference. The detection is carried out in three steps. First, some function related to the power spectrum of interfering process I(z) is estimated via adaptation. Second, the signal interference is whitened due to I(z). Third, the replica of signal is filtered via I(z) to match it to the deformed signal in the previous step. The simulation of SS reception in presence of NB interference shows a high gain in comparison to classical single-matched detection.
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Photo-devices for optical controlling of microwave circuits
Abstract
The most important optical devices which can be used for controlling microwave circuits will be presented in the paper. The performance and the parameters of the devices such as semiconductor microwave optoelectronic switches, photodiodes and phototransistors were described. The influence of the optical illumination on their microwave parameters will be described in details, including the our own investigations and simulations results. Several applications of such devices and their potential possibilities will be presented.
