134 research outputs found

    Phase-locked loop, delay-locked loop, and linear decorrelating detector for asynchronous multirate DS-CDMA system

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    The performance of phase synchronization and code tracking of a digital phase-locked loop (PLL) and delay-locked loop (DLL), respectively, is investigated in wideband asynchronous multirate DS-CDMA system. Dynamic Partial Correlation (DPC) method is proposed to evaluate the autocorrelation and its power spectrum density (PSD) of the cross-correlated terms in the presence of multirate multiple access interference (MMAI) under additive white gaussian noise (AWGN) and fading channel environments. The steady-state probability density function (PDF) and variance of the phase estimator error and code tracking jitter is evaluated by solving the first-order Fokker-Planck equation. Among many linear multiuser detectors which decouple the multiple access interference from each of the interfering users, one-shot window linear decorrelating detector (LDD) based on a one bit period to reduce the complexity of the LDD has attracted wide attention as an implementation scheme. Therefore, we propose Hybrid Selection Diversity/ Maximal Ratio Combining (Hybrid SD/MRC) one-shot window linear decorrelating detector (LDD) for asynchronous DS-CDMA systems. The selection diversity scheme at the input of the Hybrid SD/MRC LDD is based on choosing the branch with the maximum signal-to-noise ratio (SNR) of all filter outputs. The MR Combining scheme at the output of the Hybrid SD/MRC LDD adopts to maximize the output SNR and thus compensates for the enhanced output noise. The Hybrid SD/MRC one-shot LDD with PLL is introduced to track its phase error and to improve the demodulation performance. The probability density functions of the maximum SNR of the SD combiner, the near-far resistance (NFR) of one-shot LDD by Gaussian approximation, and the maximum SNR of the MR combiner for Hybrid SD/MRC LDD are evaluated, and the bit error probability is obtained from these pdfs. The performance of Hybrid SD/MRC one-shot LDD is assessed in a Rayleigh fading channel

    Slow Frequency Hopping Assisted MC DS-CDMA using Large Area Synchronised Spreading Sequences

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    The family of Multi-Carrier Direct-Sequence CDMA (MC DS- CDMA) systems exhibits numerous attractive properties, which render them attractive candidates for next-generation wireless communications. We demonstrate that spreading codes exhibiting a so-called interference-free window (IFW) are capable of outperforming classic spreading codes, when the interfering multi-user and multipath components arrive within this IFW. The best possible quasi-synchronous timing of the spreading sequences has to be adjusted with the aid of accurate adaptive timing advance control, which has to be significantly more accurate than that used in the lower-bit-rate second-generation GSM system. Fortunately, the IFW duration may be extended with the advent of multi-carrier DS-CDMA proportionately to the number of subcarriers. Hence the resultant MC DS-CDMA system is capable of exhibiting a near-single-user performance without employing a multi-user detector. A deficiency of the resultant system is that the number of spreading codes exhibiting a certain IFW is limited and so is the IFW duration. This contribution sets out to mitigate the above-mentioned shortcomings so that when the users' delays are in the range of the IFW, we separate them with the aid of the unique, user-specific LAS spreading codes. By contrast, when the users roam at a high distance from the base-station and hence their received signal arrive outside the range of the IFW, we separate them using their unique frequency hopping patterns

    Equalization with oversampling in multiuser CDMA systems

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    Some of the major challenges in the design of new-generation wireless mobile systems are the suppression of multiuser interference (MUI) and inter-symbol interference (ISI) within a single user created by the multipath propagation. Both of these problems were addressed successfully in a recent design of A Mutually Orthogonal Usercode-Receiver (AMOUR) for asynchronous or quasisynchronous code division multiple access (CDMA) systems. AMOUR converts a multiuser CDMA system into parallel single-user systems regardless of the multipath and guarantees ISI mitigation, irrespective of the channel locations. However, the noise amplification at the receiver can be significant in some multipath channels. In this paper, we propose to oversample the received signal as a way of improving the performance of AMOUR systems. We design Fractionally Spaced AMOUR (FSAMOUR) receivers with integral and rational amounts of oversampling and compare their performance with the conventional method. An important point that is often overlooked in the design of zero-forcing channel equalizers is that sometimes, they are not unique. This becomes especially significant in multiuser applications where, as we will show, the nonuniqueness is practically guaranteed. We exploit this flexibility in the design of AMOUR and FSAMOUR receivers and achieve noticeable improvements in performance

    Multi-sensor frequency domain multiple access interference canceller for DS-CDMA systems

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    Direct sequence code division multiple access (DS-CDMA) signals exhibit cyclostationary properties which imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper a multiple access interference canceller (frequency shift canceller) that explores this property is presented. This linear frequency domain canceller operates on the spread signal in such way that the interference and noise at its output is minimized (minimum mean squared error criterion). The frequency shift canceller (FSC) performance was evaluated for a UMTS-TDD scenario and multi-sensor configurations, where the cases of diversity and beamforming were considered. All these configurations are evaluated concatenated with a parallel interference canceller (PIC-2D). The results are benchmarked against the performance of the conventional RAKE-2D detector, the conventional PIC-2D detector and single user scenario, and we observe considerable performance gains with the FSC especially for the diversity case and a performance close to the single user case when it was evaluated jointly with PIC-2D

    Design and performance of CDMA codes for multiuser communications

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    Walsh and Gold sequences are fixed power codes and are widely used in multiuser CDMA communications. Their popularity is due to the ease of implementation. Availability of these code sets is limited because of their generating kernels. Emerging radio applications like sensor networks or multiple service types in mobile and peer-to-peer communications networks might benefit from flexibilities in code lengths and possible allocation methodologies provided by large set of code libraries. Walsh codes are linear phase and zero mean with unique number of zero crossings for each sequence within the set. DC sequence is part of the Walsh code set. Although these features are quite beneficial for source coding applications, they are not essential for spread spectrum communications. By relaxing these unnecessary constraints, new sets of orthogonal binary user codes (Walsh-like) for different lengths are obtained with comparable BER performance to standard code sets in all channel conditions. Although fixed power codes are easier to implement, mathematically speaking, varying power codes offer lower inter- and intra-code correlations. With recent advances in RF power amplifier design, it might be possible to implement multiple level orthogonal spread spectrum codes for an efficient direct sequence CDMA system. A number of multiple level integer codes have been generated by brute force search method for different lengths to highlight possible BER performance improvement over binary codes. An analytical design method has been developed for multiple level (variable power) spread spectrum codes using Karhunen-Loeve Transform (KLT) technique. Eigen decomposition technique is used to generate spread spectrum basis functions that are jointly spread in time and frequency domains for a given covariance matrix or power spectral density function. Since this is a closed form solution for orthogonal code set design, many options are possible for different code lengths. Design examples and performance simulations showed that spread spectrum KLT codes outperform or closely match with the standard codes employed in present CDMA systems. Hybrid (Kronecker) codes are generated by taking Kronecker product of two spreading code families in a two-stage orthogonal transmultiplexer structure and are judiciously allocated to users such that their inter-code correlations are minimized. It is shown that, BER performance of hybrid codes with a code selection and allocation algorithm is better than the performance of standard Walsh or Gold code sets for asynchronous CDMA communications. A redundant spreading code technique is proposed utilizing multiple stage orthogonal transmultiplexer structure where each user has its own pre-multiplexer. Each data bit is redundantly spread in the pre-multiplexer stage of a user with odd number of redundancy, and at the receiver, majority logic decision is employed on the detected redundant bits to obtain overall performance improvement. Simulation results showed that redundant spreading method improves BER performance significantly at low SNR channel conditions

    Multiuser detection in CDMA using blind techniques

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    Thesis (Master)--Izmir Institute of Technology, Electronics and Communication Engineering, Izmir, 2004Includes bibliographical references (leaves: 63-65)Text in English; Abstract: Turkish and Englishxiv, 69 leavesIn code division multiple access (CDMA) systems, blind multiuser detection (MUD) techniques are of great importance, especially for downlinks, since in practice, it may be unrealistic for a mobile user to know the spreading codes of other active users in the channel. Furthermore, blind methods remove the need for training sequences which leads to a gain in the channel bandwidth. Subspace concept in blind MUD is an alternative process to classical and batch blind MUD techniques based on principle component analysis, or independent component analysis (ICA) and ICA-like algorithms, such as joint approximate diagonalization of eigen-matrices (JADE), blind source separation algorithm with reference system, etc. Briefly, the desired signal is searched in the signal subspace instead of the whole space, in this type of detectors. A variation of the subspace-based MUD is reduced-rank MUD in which a smaller subspace of the signal subspace is tracked where the desired signal is contained in. This latter method leads to a performance gain compared to a standard subspace method. In this thesis, blind signal subspace and reduced-rank MUD techniques are investigated, and applied to minimum mean square error (MMSE) detectors with two different iterative subspace tracking algorithms. The performances of these detectors are compared in different scenarios for additive white Gaussian noise and for multipath fading channels as well. With simulation results the superiority of the reduced-rank detector to the signal subspace detector is shown. Additionally, as a new remark for both detectors, it is shown that, using minimum description length criterion in subspace tracking algorithm results in an increase in rank-tracking ability and correspondingly in the final performance. Finally, the performances of these two detectors are compared with MMSE, adaptive MMSE and JADE detectors

    On the performance and capacity of space-time block coded multicarrier CDMA communication systems

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    Ph.DDOCTOR OF PHILOSOPH

    Frequency shift based multiple access interference canceller for multirate UMTS-TDD systems

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    Direct Sequence Spread Spectrum (DS-SS) signals exhibit cyclostationary properties which imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper we present a Multiple Access Interference Canceller that explores this property and applies to UMTS-TDD. This frequency domain canceller operates in the spreaded signal in such way that the interference and noise at its output is minimized (Minimum Mean Squared Error Criterium). The performance is evaluated in two con gurations: one including the Frequency Shift Canceller (FSC) and the other concatenated with Parallel Interference Canceller(PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector

    Wavelet-based multi-carrier code division multiple access systems

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