Search

This paper proposes a narrowband interference (NBI) suppression algorithm for Direct Sequence-Code Division Multiple Access systems. The NBI is considered from heterogeneous networks, and predicted based on its cyclostationary characteristic using a nonlinear feed-forward neural network predictor which eliminates the nonlinearity of the spread spectrum (SS) signal in the NBI prediction. To further improve the suppression performance, this paper exploits the structure of the spreading code, and proposes an iterative code-aided algorithm to jointly estimate the NBI and the SS signal. Simulation results reveal that the proposed algorithm largely outperforms the conventional linear prediction filtering and linear-conjugate linear polyperiodically time-varying filtering methods in both the signal to interference plus noise ratio improvement and the bit error rates, when it operates in NBI-contaminated environments. [ABSTRACT FROM AUTHOR]

It is well known that power control error (PCE) is a critical issue in CDMA cellular systems. In this paper, the bit error rate (BER) of a direct sequence-code division multiple access (DS-CDMA) receiver with imperfect power control, adaptive beamforming, and voice activity is derived in frequency-selective Nakagami fading channels. We discuss the effects of PCE, Nakagami- m fading parameter, and channel's multipath intensity profile as average signal strength and rate of average power decay and their effects on the BER performance of DS-CDMA cellular systems. In this paper, the RAKE receiver consists of three stages. In the first stage, with conjugate gradient adaptive beamforming algorithm, the desired users' signal in an arbitrary path is passed and the inter-path interference is canceled in other paths in each RAKE finger. Also in this stage, the multiple access interference (MAI) from other users is reduced. Thus, the matched filter (MF) can be used for the MAI reduction in each RAKE finger in the second stage. In the third stage, the output signals from the MFs are combined according to the conventional maximal ratio combining principle and then are fed into the decision circuit for the desired user. How the Nakagami fading parameters, power control imperfections, or the number of resolvable paths affect the reverse link capacity of the system is discussed in detail. Analytical and simulation results are also given for systems with different processing gains and number of BSs in the cell-selection process with various Nakagami fading parameters. [ABSTRACT FROM AUTHOR]

Present day services and its users need new technology so as to integrate multimedia services along with existing ones within the same Bandwidth. Future communication networks need wide Bandwidth & high-speed access. Ever since the World War 2, spread spectrum techniques have been widely used in a variety of military and civilian applications. A method of Spread Spectrum Multiple Access technique called Frequency Hopping/Multiple Carrier Direct Sequence-Code Division Multiple Access [1] (FH/MC DSCDMA), is an emerging system for future communication system. In this work an FH/MC DS-CDMA scheme is synthesized in VLSI chip of Xilinx's XC4000 architecture. The adoptability of the proposed system for future generation mobile as well as existing generation is always higher. Multiple carriers can be allotted as per user requirements with high secrecy and multiple encryptions. The performance analysis for the above scheme is shown supportive in terms of BER, power requirements, complexity, etc., compared to existing and evolving techniques. Here as a part of the research works a method for quick tracking of frequency-hopped signals is performed in on-chip. [ABSTRACT FROM AUTHOR]

In this contribution, a novel soft-output ant colony optimization (SO-ACO)-based multiuser detector (MUD)--namely the MUlti-input-Approximation (MUA) assisted SO-ACO-based MUD--is proposed for the synchronous direct-sequence code-division-multiple-access (DS-CDMA) uplink (UL). The previously proposed conventional ACO based MUDs were unable to provide soft log-likelihood ratio (LLR) values for the channel decoder. Hence, to solve this open problem, we commence by proposing the MAximum-Approximation (MAA) assisted SO-ACO algorithm, leading to a novel MUA assisted SO-ACO algorithm, which subsumes the MAA algorithm as a particular case and outperforms the MAA algorithm. More explicitly, at a signal-to-noise ratio (SNR) of 13 dB, the BER performance of the conventional coding (CC) aided CDMA UL employing the MAA SO-ACO is improved from 5.2 ⋅ 10-6 to 2.7 ⋅ 10-6 by employing the MUA SO-ACO. Our numerical results also demonstrate that the MUA assisted SO ACO-MUD is capable of approaching the optimum performance of the Bayesian detector, when K : 32 UL users are supported with the aid of 31-chip Gold codes, while the complexity of the former is a fraction of 10-8 lower than that of the latter. [ABSTRACT FROM AUTHOR]

An ultra-wideband RF transmitter with local oscillator (LO) leakage reduction method for direct-sequence code division multiple access (DS-CDMA) is presented. To achieve ultra-wideband gain flatness, the method of combining two shunt-peaking loads with different resonant frequencies is used. A double balanced mixer with a linear transconductance stage is used to increase the linearity of the mixer. To reduce the LO leakage at the output port, an additional DC feedback loop is applied to the linear transconductor. Measurement results show 9.5 dB gain with ±0.8 dB variation over 3.1–5 GHz range. The output P1dB is approximately -3 dBm and the output IP3 is more than 9.8 dBm for a frequency greater than 3–5 GHz. By the LO leakage reduction method, the LO leakage output power is 5 dB lower than without it. The transmitter consumes 54 mW with a 1.8 V supply and is realized with a 0.18 μm RF CMOS technology. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 3140–3145, 2007; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.22960 [ABSTRACT FROM AUTHOR]

This work presents the development and implementation of a Local Positioning System (LPS) based on the transmission of ultrasonic signals, proposed for indoor positioning of mobile robots. The LPS consists of several ultrasonic beacons located in the environment, all of them emitting periodically and simultaneously. A portable receiver collects all the transmissions and autonomously computes its position. Neither infrared nor radiofrequency are required for synchronization in the localization process, compared to other related works. Direct Sequence Code Division Multiple Access (DS-CDMA) techniques are used to allow simultaneous emission from several beacons; each one transmitting a different 255-bit Kasami code. The portable receiver can detect the time of arrival for the different signals coming from the beacons, by carrying out all the correlations with the emitted codes. Absolute position of the receiver is determined by hyperbolic trilateration, using the Differences in Times-of-Arrival (DTOA) between a reference beacon and the other ones. The main advantage of this proposal is to permit the coexistence of an unlimited number of portable receivers, what is useful in applications like multirobot cooperation. [Copyright &y& Elsevier]

In this paper, we proposed a new multicarrier, direct sequence code division multiple access, the so-called premulticoded MC-DS-CDMA system. In this scheme, the basic MC-DS-CDMA system is augmented with a precoder and a multicode encoder at the transmitter. The multicode encoder is an inverse Walsh-Hadamard transform (IWHT) unit, while the precoder is a recently suggested "constant amplitude" encoder that guarantees that the IWHT output stream has binary level. While IWHT, multicarrier (MC) transmission, and direct sequence spreading provide us with frequency diversity, the constant amplitude precoder ensures that the transmitted waveform is spread BPSK signal, minimizing peak to average-power ratio (PAPR) problem. The multicode and (MC) combination also addresses the need for flexible high data rate services in present day wireless communications. In the receiver, after matched filtering or correlation demodulation for each subcarrier and WHT operation, a detection strategy based on minimum-mean-square-error (MMSE) criterion is used. We have derived an upper bound for the bit error rate (BER) performance in Nakagami fading channel. Numerical results are compared with those obtained from a single carrier DS-CDMA system (SC-DS-CDMA) and a MC-DS-CDMA scheme without the precoding and IWHT units. We show that our proposed MC.DS-CDMA scheme has a much better BER performance compared with both of these schemes. [ABSTRACT FROM AUTHOR]

Power control is an important factor for direct-sequence code division multiple access (DS-CDMA) cellular radio systems to achieve higher communication link quality and better system capacity. In order to track the desired signal-to-interference-plus-noise ratio (SINR) under round-trip delay, multiple access interference (MAI), channel fading, and noise, a time delay-based state-space model is developed for representing the tracking error dynamics and a state feedback controller is introduced for SINR tracking control. Then the power tracking problem can be regarded as a control problem. In this paper, a robust H∞ power tracking control design is proposed to achieve a robust optimal SINR tracking from the minimization of the worst-case effect point of view. This robust optimal power tracking design problem can be transformed to solving the eigen-value problem (EVP) under some linear matrix inequality (LMI) constraints. The LMI Matlab toolbox can be used to efficiently solve the EVP via convex optimization to achieve a robust optimal SINR tracking design. Under the proposed distributed framework, the information of channel gain is not needed. [ABSTRACT FROM AUTHOR]

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence-code division multiple access (DS-CDMA) signals to improve the probability of error (Pe) performance with minimal signal bandwidth variations. First, we derive the Pe expression over a quasi-synchronous additive white Gaussian noise channel for DS-CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS-CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half-sine and rectangular waveforms. Especially, the Pe of the optimum wavelet-based scheme becomes significantly better than those of the conventional chip waveforms-based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet-based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half-sine and rectangular-based systems, respectively, for a Pe value of 10-3. Copyright © 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In this paper, we present a systematic study of the subspace-based blind channel estimation method. We first formulate a general signal model of multiple simultaneous signals transmitted through vector channels, which can be applied to a multitude of modern digital communication systems. Based on this model, we then propose a generalized subspace-based channel estimator by minimizing a novel cost function, which incorporates the set of kernel matrices of the signals sharing the target channel via a weighted sum of projection errors. We investigate the asymptotic performance of the proposed estimator, i.e., bias, covariance, mean square error (MSE), and Cramér-Rao bound, for large numbers of independent observations. We show that the performance of the estimator can be optimized by increasing the number of kernel matrices and by using a special set of weights in the cost function. Finally, we consider the application of the proposed estimator to a downlink code division multiple access (CDMA) system operating in a frequency-selective fading channel with negligible intersymbol interference (ISI). The results of the computer simulations fully support our analytical developments. [ABSTRACT FROM AUTHOR]

The major limitation on the performance and capacity of direct sequence code division multiple access (DS-CDMA) communication systems is the multiple access interference (MAI) due to simultaneous transmission of several users. The linear minimum mean square error (MMSE) detector is a well-known method to suppress MAI adaptively and blindly, however, it is sub-optimal because of the inherent nonlinearity of the system. Therefore, in this paper, two nonlinear blind adaptive interference cancellation algorithms (the exact Newton (EXN) and the approximate Newton (APN)) were proposed and developed based on the 2nd order Volterra expansion. A complete performance analysis of the conventional matched filter (MF) detector, linear adaptive detector (which employs the standard Newton algorithm) and the proposed two nonlinear adaptive (EXN and APN) detectors was carried out in various DS-CDMA systems. Numerical results show that the three Newton type adaptive blind multiuser detectors yield significant bit error ratio (BER) improvement over the conventional MF detector in the presence of strong MAI. Further, the two nonlinear adaptive algorithms always outperformed the linear algorithm. Most attractively, the APN algorithm offers lower computation complexity, higher numerical stability and almost identical BER performance in comparison with the EXN algorithm. [Copyright &y& Elsevier]

Detection structures for a direct sequence (US) code division multiple access (CDMA) multiuser uplink using codes with a period of several symbols, called long codes, are investigated. In a first part, we propose a multirate description of the uplink. Based on this formalism, we revisit the linear and decision feedback minimum mean square error (MMSE) detectors. In order to identify the best attainable performance, we focus on the infinite size versions. In a second part, reduced complexity solutions are put forward and discussed. Finally, the performance of the proposed detectors is investigated and compared with that of the classical rake receiver and the single-bit horizon detectors. The relevance of the different solutions is discussed in the light of the channel length. [ABSTRACT FROM AUTHOR]

In cellular mobile communications, the mobile station must detect the received level so as to determine the position of the mobile station. In a broadband DS-CDMA system, RAKE reception is usual since the paths can be separated. Thus, the detection accuracy should be improvable by the path diversity effect. This paper considers a DS-CDMA system, and the accuracy of the received level detection is analyzed theoretically and by computer simulation for the case in which both path diversity and antenna space diversity are applied. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(2): 51–60, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1153 [ABSTRACT FROM AUTHOR]

In a broadband DS-CDMA system, control of transmitter power is indispensable in order to solve the distance problem of mobile stations, in which the transmitter power of mobile stations is adaptively controlled so that the received power at the base station remains constant. In a DS-CDMA system, the paths can be separated and RAKE reception can be applied, which helps to reduce the transmitted power as well as the error in transmitter power control. This paper considers the case in which transmitter power control by feedback is applied to a DS-CDMA system, and analyzes the effects of control parameters such as the control time interval, the maximum amount of control, and the control step width, on the error of the transmitter power control, considering path and antenna diversity. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(12): 100–111, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1145 [ABSTRACT FROM AUTHOR]

Wideband DS-CDMA must have path diversity reception (RAKE reception) because the received waves are separated into multiple paths as the spread bandwidth widens. Analysis of the reception characteristics which takes into account the actual path characteristics is essential to produce an optimal design for RAKE reception. In this paper, we illustrate the received level characteristics when the number of correlators is limited and the received levels are combined by a RAKE receiver based on a specific path selection scheme in a practical antenna diversity reception model having received level correlation between the antennas and different antenna gains. © 2001 Scripta Technica, Electron Comm Jpn Pt 1, 84(10): 36–54, 2001 [ABSTRACT FROM AUTHOR]

Multistage interference cancelers are deemed to be one of the most realizable schemes for asynchronous DS-CDMA. While limited to SD (Soft Decision)-based parallel multistage cancellation, this paper covers (1) theoretical relations introduced between suppressing factors and interference canceling gains, and (2) computer simulation results confirming the validity of the theory. There are two ways of setting suppressing factors. One is to set common values for all stages, and another is to set an individual value for each stage. This paper also discusses the differences between the two types of setting suppressing factors. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(12): 1–11, 2000 [ABSTRACT FROM AUTHOR]

A microcellular DS-CDMA radio link design methodology and an evaluation method are proposed in this paper. The normalized link capacity (link capacity/process gain) and maximum average transmitting power of a mobile station are analyzed for the reverse link in a wideband propagation environment. The bit error rate (BER) is derived for an exponential delay profile with the parameter of normalized transmission bandwidth (transmission bandwidth B × delay spread τs). The average interference noise power of the other users from their own cells and that from other cells are derived based on power control error and the standard deviation of short-term fading. From these results, we derive the normalized link capacity and the maximum average mobile station transmitting power. The transmitting power is reduced by more than 40% for Bτs≤ 2 by combining space and path diversity. The link capacity is proportional to the number of antenna branches. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(9): 51–60, 1999 [ABSTRACT FROM AUTHOR]

This paper discusses the problem of coexistence of DS-CDMA (direct sequence code division multiplex access) and TDMA (time division multiple access) systems where their signals are transmitted in the same frequency band. The DS-CDMA signal is robust against narrowband interference, but there is a limit to its immunity against interference. This paper proposes, from such a viewpoint, a method using a complex multirate filter bank, as a way to reject TDMA signals, which act as narrowband interference to the DS-CDMA signal, in the coexistence system. By the proposed method, a notch filter can be composed with an arbitrary frequency resolution at an arbitrary frequency, by constructing the complex version of the conventional multirate filter bank. It is shown that, by eliminating the TDMA signal by the proposed method, the bit error rate of the DS-CDMA signal is improved, indicating that the proposed system is useful in the coexistence problem. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(5): 34–41, 1999 [ABSTRACT FROM AUTHOR]

This paper considers the DS-CDMA with an imperfect transmit power control and analyzes the relationship between the reverse-link capacity and the average transmit power, considering the space/path (RAKE) diversity. A simple formula for calculation is derived. Using the space diversity, an instantaneous large transmit power can be avoided when the transmit power is controlled. It is shown that, at that time, the interference from other cells can be reduced and the reverse-link capacity can be increased. Especially, when the spreading bandwidth is narrow and the single-path reception is used, the space diversity is shown to be necessary. It is known in CDMA that the link capacity can be increased by sectorizing the cell. In the practical antenna, however, where the directivity of the antenna overlaps with other sectors, there can be interference from adjacent sectors. A calculation formula is derived considering the antenna directivity. Using that formula, the reverse-link capacity and the average transmit power are examined precisely for the case where the pilot symbol interpolation QPSK coherent detection is used in the data modulation/demodulation. Finally, the effect of the shadowing correlation is investigated. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80 (4): 72–82, 1996 [ABSTRACT FROM AUTHOR]