A class of band-limited chip waveforms for DS-CDMA systems

In Direct Sequence Code Division Multiple Access （DS-CDMA） systems, the chip waveform affects the implementation, system bandwidth, envelope uniformity, eye pattern and Multiple user Access Interference （MAI）. In this paper, based on an elementary density function of a second order polynomial, a class of second order continuity pulses is proposed. From this class of pulses, we can find some members having faster decaying rate, bigger eye opening, more uniform envelope and stronger anti-MAI capability than the Nyquist waveform. The normalized-bandwidth-pulseshape-factor product, the decaying rate of the tail of the time waveform, the opening of the eye diagram, and the envelope uniformity of the second order continuity pulses are addressed in the paper that provide the basic information for the selection of the chip pulse for CDMA systems.     

Time-limited leases in radio systems [Topics in Radio Communications]

A time-limited lease is a set of rights that expires after a specified duration. We analyze ways to use leases to facilitate innovation in radio devices and wireless communication. In our vision, manufacturers include in their devices a simple, secure subsystem that contains a clock and disables specific transmit capabilities if no extension message is received by the end of the lease period. When devices provide this support, regulators may use certification leases rather than permanent grants to accelerate deployment of innovative radios. Spectrum rights holders may use leases to reduce risk in secondary spectrum market transactions. Firms collaborating in innovative wireless service business models can better retain control of their respective rights. We examine both the technical and policy issues associated with leases.     

On chip-matched filtering and discrete sufficient statistics forasynchronous band-limited CDMA systems

The problem of generating discrete sufficient statistics for signal processing in code-division multiple-access (CDMA) systems is considered in the context of underlying channel bandwidth restrictions. Discretization schemes are identified for (approximately) bandlimited CDMA systems, and a notion of approximate sufficiency is introduced. The role of chip-matched filtering in generating accurate discrete statistics is explored. The impact of approximate sufficiency on performance is studied in three cases: conventional matched filter (MF) detection, minimum mean-squared-error detection, and delay acquisition. It is shown that for waveforms limited to a chip interval, sampling the chip-MF output at the chip rate can lead to a significant degradation in performance. Then, with equal bandwidth and equal rate constraints, the performance with different chip waveforms is compared. In all three cases above, it is demonstrated that multichip waveforms that approximate Nyquist sine pulses achieve the best performance, with the commonly used rectangular chip pulse being severely inferior. However, the results also indicate that it is possible to approach the best performance with well-designed chip waveforms limited to a chip interval, as long as the chip-MF output is sampled above the Nyquist rate     

A deterministic multiuser code-timing estimator for long-code band-limited CDMA systems

In this letter, we present a deterministic multiuser code-timing estimator for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems with aperiodic long spreading codes and band-limited chip waveforms. A key feature of the proposed estimator is that it captures and capitalizes a deterministic structure of the overall interference, namely multi-access interference (MAI) and intersymbol interference (ISI), in the frequency domain. This allows complete interference elimination in a deterministic manner, which is in general more effective and data-efficient than stochastic approaches. Numerical results show that the proposed estimator can achieve fast acquisition; it is also near-far resistant, providing accurate code acquisition for even overloaded systems (i.e., systems with more users than the processing gain) in multipath fading environments.     

An improved design of chip waveforms for band-limited DS-CDMA systems

This paper introduces an efficient and improved design of chip waveforms to minimize the multiple-access interference in band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. For ease of implementation, the DS-CDMA system employs a time-limited chip waveform, whereas its band limitation is ensured by the low-pass filters at both the transmitter and receiver ends. The design uses sinusoids to synthesize the time-limited chip waveform so that the portion of its spectrum across the specified bandwidth is as flat as possible. It is shown that by using a simple series expansion (with only a few terms) the synthesized chip waveforms significantly outperform the spreading/despreading waveforms previously proposed, particularly for large values of the chip duration-bandwidth product.     

Second order polynomial class of chip waveforms for band-limited DS-CDMA systems

The conventional frequency domain square-root raised cosine (Nyquist) chip waveform has much poorer anti-multiple-access-interference (anti-MAI) capability than the optimal band- limited waveform in direct sequence code division multiple access (DS-CDMA) systems. However, the digital implementation of the optimal chip pulse is very costly due to the slow decaying rate of the time waveform. In addition, its eye diagram and envelope uniformity are worse than the Nyquist pulse for a wide range of roll-off factor, which will incur performance degradation due to timing jitters and post non-linear processing. In this paper, based on an elementary density function of a second-order polynomial, a class of second-order continuity pulses is proposed. From this class of pulses, we can find some members having faster decaying rate, bigger eye opening, more uniform envelope and stronger anti-MAI capability than the Nyquist waveform. The normalized-band-width-pulse-shape-factor product, the decaying rate of the tail of the time waveform, the opening of the eye diagram, and the envelope uniformity of the second-order continuity pulses are addressed in the paper that provide the basic information for the selection of the chip pulse for CDMA systems.     

Performance evaluation for band-limited DS-CDMA systems based on simplified improved Gaussian approximation

The standard Gaussian approximation (SGA) for error analysis of direct-sequence code-division multiple-access (DS-CDMA) systems is very optimistic in many cases. Improved Gaussian approximation (IGA) is a technique that produces accurate error probabilities, but is still computationally intensive. Simplified IGA (SIGA) has complexity similar to that of SGA and, at the same time, provides sufficient accuracy. In this paper, we consider SIGA for DS-CDMA systems employing random sequences in a band-limited scenario. The validity of IGA for band-limited systems is established in a rigorous mathematical sense. Then a key parameter in SIGA is derived via a frequency-domain approach. Applications to a number of typical chip waveforms, including the popular sinc and raised-cosine pulses, are investigated. Performance comparison with IGA-based lower and upper bounds shows that SIGA yields very accurate probability of error.     

Inversion of band-limited reflection seismograms: Theory and practice

This paper examines the problem of recovering the acoustic impedance from band-limited normal incidence reflection seismograms. Recognizing the inherent nonuniqueness in the inversion, we proceed by constructing an impedance model which satisfies the processed seismogram, has a minimum of structural variation, honors any point impedance constraints that are provided, and incorporates information from stacking velocities. The constrained inversion is carried out in a single operation using linear programming methods. The constructed impedance is consistent with available geological and geophysical information and therefore constitutes a well-constrained estimate of the true earth impedance. A basic assumption in our inversion is that each seismic trace is a band-limited representation of the true reflectivity function. When seismic data do not conform with this assumption, pre-inversion processing of the data is required; this involves a series of data checks and possible corrections. A complete processing sequence incorporating all steps of the practical inversion is presented and illustrated with field data examples.     

A simple and accurate method of probability of bit error analysisfor asynchronous band-limited DS-CDMA systems

This paper considers probability of bit error (P_e) analysis in asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. It presents a simple and accurate method of P_e analysis. The proposed method can serve as an attractive alternative to the only two techniques currently available for band-limited systems: the standard Gaussian approximation (SGA) and the characteristic function method. The former is prone to inaccuracy while the latter, large computational complexity. The method generalizes the simplified improved Gaussian approximation (SIGA) derived previously for rectangular pulses. This paper also outlines a generalization of another method referred to as the improved Gaussian approximation (IGA). Numerical examples demonstrate the far greater accuracy of the generalized SIGA with respect to the SGA. The examples consider the IS-95 and square-root raised cosine (Sqrt-RC) pulses as well as uniform and nonuniform received power conditions     

Performance of the deliberate clipping with adaptive symbolselection for strictly band-limited OFDM systems

The performance of the strictly band-limited OFDM systems with deliberate clipping is examined in terms of the peak-to-average power ratio (PAPR) and the resultant bit error performance. The clipping is performed on the OFDM signals sampled at the Nyquist rate, followed by the ideal low-pass filter, Since the low-pass filter considerably enlarges the PAPR, there is a severe limitation in PAPR reduction capability. Thus, in order to achieve further reduction of the PAPR, the application of the adaptive symbol selection scheme is also considered. It is shown that the significant PAPR reduction with moderate complexity can be achieved by the combination of the clipping and the adaptive symbol selection. The price to be paid for PAPR reduction by this scheme is its performance degradation. The paper theoretically analyzes the bit error rate performance of the OFDM system with the Nyquist-rate clipping combined with the adaptive symbol selection, and considers the use of the forward error correction for compensation of the degradation. It is shown that even though the clipping scheme causes severe loss in required signal-to-noise ratio, the use of a powerful channel coding scheme such as turbo codes significantly alleviates the bit error rate performance degradation     

“信号与系统”混合式双语课程教学改革

在我国电力与能源学科逐渐走向国际化的新形势下，《信号与系统》是电子信息与电气类专业重要的一门必修课程，全面提高其双语课程的教学效果，可有效地帮助学生与国际接轨，培养具有扎实专业知识和高水平外语能力的、面向国际化的复合型新工科人才。本文从“新工科”大背景出发，对信号与系统课程的教学内容、教学技术、教学手段和方法以及考核评价的改革进行了探索。     

Accurate computation of bit error probabilities of band-limited asynchronous DS-CDMA systems using higher order moments

An existing method called the simplified improved Gaussian approximation (SIGA) was previously applied to compute the bit error probabilities (BEPs) of band-limited asynchronous direct sequence code division multiple access (DS-CDMA) systems employing general pulse shaping (IEEE Trans. Commun., vol. 50, p. 656, 2002). The SIGA method uses moments up to the second order and is more accurate than the standard Gaussian approximation (SGA) (IEEE Trans. Commun., vol. 50, p. 656, 2002). In this paper, a new method that uses moments up to the fourth order is proposed for computing the BEP. The method is derived from a five-point Chebyshev interpolation formula and is inherently more accurate than the SIGA. Like the SIGA, the new method requires the evaluation of only closed-form expressions and the error function. The new method achieves higher accuracy with a modest increase in computational complexity.     

Multidimensional modulation and coding for band-limited digitalchannels

A class of multidimensional signals, based on generalized group alphabets, is introduced and its basic properties are derived. The combination of generalized group alphabets and coding is also examined: two coding schemes are considered-G. Ungerboeck's scheme (1982) for combination with convolutional codes, and V.V. Ginzburg's (1984) scheme for combination with block codes. It is concluded that the performance of these schemes makes them attractive for transmission over band-limited digital channels     

Model-driven systems development and integration environment

Telecommunications operators are undergoing massive transformations in order to metamorphose themselves into the ICT world and compete with agile, lean IT organisations. The main challenges facing telecommunications operators, such as BT, are to reduce costs and increase agility in deploying software systems for provisioning ICT services. Despite using reusable capabilities and COTS packages, the major source of increased cost lies in the heavy integration tax we incur for integrating diverse systems implemented on diverse platforms and middleware, with heterogeneous data and process models. This paper looks at cost implications of lengthy and often manual migration to-and-from systems and platforms, and shows the clear business benefits of model-driven development (MDD) as defined by the Object Management Group (OMG). It is clearly demonstrated that model-driven development has matured into a practical, industrialised, scalable and evolvable technology, culminating from decades of R&D on specification and design languages, executable formalisms and domainspecific languages and language transformations.     

One-dimensional normal-incidence inversion: A solution procedure for band-limited and noisy data

In this paper we present a one-dimensional normal-incidence inversion procedure for reflection seismic data. A lossless layered system is considered which is characterized by reflection coefficients and traveltimes. A priori knowledge for the unknown parameters, in the form of statistics, is incorporated into a nonuniform layered system, and a maximum a posteriori estimation procedure is used for the estimation of the system's unknown parameters (i.e., we assume a random reflector model) from noisy and band-limited data. Our solution to the inverse problem includes a downward continuation procedure for estimation of the states of the system. The state sequences are composed of overlapping wavelets. We show that estimation of the unknown parameters of a layer is equivalent to estimation of the amplitude and detection of the time delay of the first wavelet in the upgoing state sequence of the layer. A suboptimal maximum-likelihood deconvolution procedure is employed to perform estimation and detection. The most desirable features of the proposed algorithm are its layer-recursive structure and its ability to process noisy and band-limited data.     

Reflection coefficients of band-limited spectra and band-limitedspectrum modeling

We study the properties of the reflection coefficients of band-limited spectra. We show that they must belong to simple subsets of the unit disk. Then, the problem of band-limited spectrum modeling is addressed in a more appropriate way than with approaches based on correlation coefficients. In particular, we derive a new band-limited spectrum model for band-limited signals with first correlation coefficients matching the signal ones. The practical behavior of the model is illustrated with an example     

Orbital and site diversity systems in rain environment: Radar-derived results

During the summers of 1981 and 1982, an experiment was carried out in the Po River Valley with the scope of evaluating the statistical performance of diversity systems for earth to satellite links experiencing rain attenuation. The data were collected by anS-band meteorological radar scanning a32deginclined plane having the SIRIO path as its highest radial. Attenuation values directly measured through the satellite beacon at 11.6 GHz were used to "calibrate" the radar on event basis, in order to properly convert reflectivity into specific attenuation. In a previous paper the performance of site diversity (SD) systems was presented for earth terminals spaced from 1 to 20 km. In this paper, the same set of data is processed to simulate orbital diversity (OD) systems, which employ two satellites angularly spaced and one earth terminal. Aperture anglesthetaup to95deghave been considered. The results show that this diversity scheme already has a significant gain withtheta = 30degwhere at 10 dB single link attentuation the gain (as defined by Hodge) is 2.5 dB and it increases steadily asthetaincreases. The comparison between orbital diversity and site diversity performances shows that the site separation is linearly related to the orbital diversity aperture angle for any single link attenuation of the site diversity configuration.     

PV power systems and the environment: results of an expert workshop

Photovoltaic (PV) power systems can lead to significant reduction of emissions to the environment. Contrary to conventional fossil fuel-based electricity production, the environmental aspects of PV power systems are mostly related to indirect processes such as cell and module manufacturing and ‘end of life’ waste management. Careful assessment of such environmental aspects throughout all life-cycle stages is required to reveal the contribution that PV power systems can make to environmental sustainability within the energy sector. An expert workshop was held in Utrecht, The Netherlands, on 25–27 June 1997 that addressed issues and approaches regarding the environmental aspects of PV power systems, including energy payback times, CO₂ mitigation potential, environmental life-cycle assessment and health and safety assessment and control. Various issues of environmental importance were identified during the workshop and recommendations were made for further work to ensure that PV power systems will indeed fulfil the promise of environmental sustainability. © 1998 John Wiley & Sons, Ltd.