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

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

Interference characterization and suppression for multiuser direct-sequence spread-spectrum system

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 175-184).In this thesis we investigate efficient modulation and detection techniques for the uplink (i.e. transmission from mobile to base station) of a DS-CDMA network. Specifically, the thesis contains three parts. In the first part, we focus on the mobile transmitter. In particular, we evaluate and compare the spectral efficiency of two promising variable rate DS-CDMA transmission techniques, multicode (MCD) and variable-spreading-gain (VSG), under the presence of multiple-access (user-to-user) interferences (MAI) and multipath interferences. The uniqueness of our study is that in bit-error-rate evaluation, instead of approximating the interference as Gaussian noise (which has been done in most of the previous studies), we incorporate both power and distribution of interferences into consideration. We show where the Gaussian assumption may give misleading answers and how our results in these cases are different from those obtained in the past. In part two and three of the thesis, we focus on the base station receiver. Specifically, we present effective joint detection techniques that have good performance-complexity tradeoff. Part two of the thesis introduces a class of novel multistage parallel interference cancellation algorithms based on stage-by-stage minimum mean-squared error (MMSE) optimization. We show that this scheme is capable of achieving significantly better performance than other algorithms with similar complexity. Part three of the thesis presents a low-complexity dual-mode multiuser detector that dynamically switches its detection mode between the matched-filter receiver and the decorrelator. We show that this detector is capable of achieving the performance of a decorrelator but with significant savings in processing power and complexity.by Mingxi Fan.Ph.D

Energy-Efficient Resource Allocation in Wireless Networks: An Overview of Game-Theoretic Approaches

An overview of game-theoretic approaches to energy-efficient resource allocation in wireless networks is presented. Focusing on multiple-access networks, it is demonstrated that game theory can be used as an effective tool to study resource allocation in wireless networks with quality-of-service (QoS) constraints. A family of non-cooperative (distributed) games is presented in which each user seeks to choose a strategy that maximizes its own utility while satisfying its QoS requirements. The utility function considered here measures the number of reliable bits that are transmitted per joule of energy consumed and, hence, is particulary suitable for energy-constrained networks. The actions available to each user in trying to maximize its own utility are at least the choice of the transmit power and, depending on the situation, the user may also be able to choose its transmission rate, modulation, packet size, multiuser receiver, multi-antenna processing algorithm, or carrier allocation strategy. The best-response strategy and Nash equilibrium for each game is presented. Using this game-theoretic framework, the effects of power control, rate control, modulation, temporal and spatial signal processing, carrier allocation strategy and delay QoS constraints on energy efficiency and network capacity are quantified.Comment: To appear in the IEEE Signal Processing Magazine: Special Issue on Resource-Constrained Signal Processing, Communications and Networking, May 200

Design and performance of CDMA codes for multiuser communications

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

Near far resistant detection for CDMA personal communication systems.

The growth of Personal Communications, the keyword of the 90s, has already the signs of a technological revolution. The foundations of this revolution are currently set through the standardization of the Universal Mobile Telecommunication System (UMTS), a communication system with synergistic terrestrial and satellite segments. The main characteristic of the UMTS radio interface, is the provision of ISDN services. Services with higher than voice data rates require more spectrum, thus techniques that utilize spectrum as efficiently as possible are currently at the forefront of the research community interests. Two of the most spectrally efficient multiple access technologies, namely. Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) concentrate the efforts of the European telecommunity.This thesis addresses problems and. proposes solutions for CDMA systems that must comply with the UMTS requirements. Prompted by Viterbi's call for further extending the potential of CDMA through signal processing at the receiving end, we propose new Minimum Mean Square Error receiver architectures. MMSE detection schemes offer significant advantages compared to the conventional correlation based receivers as they are NEar FAr Resistant (NEFAR) over a wide range of interfering power levels. The NEFAR characteristic of these detectors reduces considerably the requirements of the power control loops currently found in commercial CDMA systems. MMSE detectors are also found, to have significant performance gains over other well established interference cancellation techniques like the decorrelating detector, especially in heavily loaded system conditions. The implementation architecture of MMSE receivers can be either Multiple-Input Multiple Output (MIMO) or Single-Input Single-Output. The later offers not only complexity that is comparable to the conventional detector, but also has the inherent advantage of employing adaptive algorithms which can be used to provide both the dispreading and the interference cancellation function, without the knowledge of the codes of interfering users. Furthermore, in multipath fading channels, adaptive MMSE detectors can exploit the multipath diversity acting as RAKE combiners. The later ability is distinctive to MMSE based receivers, and it is achieved in an autonomous fashion, without the knowledge of the multipath intensity profile. The communicator achieves its performance objectives by the synergy of the signal processor and the channel decoder. According to the propositions of this thesis, the form of the signal processor needs to be changed, in order to exploit the horizons of spread spectrum signaling. However, maximum likelihood channel decoding algorithms need not change. It is the way that these algorithms are utilized that needs to be revis ed. In this respect, we identify three major utilization scenarios and an attempt is made to quantify which of the three best matches the requirements of a UMTS oriented CDMA radio interface. Based on our findings, channel coding can be used as a mapping technique from the information bit to a more ''intelligent" chip, matching the ''intelligence" of the signal processor

Wavelet-based multi-carrier code division multiple access systems

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Advancements of MultiRate Signal processing for Wireless Communication Networks: Current State Of the Art

With the hasty growth of internet contact and voice and information centric communications, many contact technologies have been urbanized to meet the stringent insist of high speed information transmission and viaduct the wide bandwidth gap among ever-increasing high-data-rate core system and bandwidth-hungry end-user complex. To make efficient consumption of the limited bandwidth of obtainable access routes and cope with the difficult channel environment, several standards have been projected for a variety of broadband access scheme over different access situation (twisted pairs, coaxial cables, optical fibers, and unchanging or mobile wireless admittance). These access situations may create dissimilar channel impairments and utter unique sets of signal dispensation algorithms and techniques to combat precise impairments. In the intended and implementation sphere of those systems, many research issues arise. In this paper we present advancements of multi-rate indication processing methodologies that are aggravated by this design trend. The thesis covers the contemporary confirmation of the current literature on intrusion suppression using multi-rate indication in wireless communiquE9; networks

Analisis Performansi Multiuser Detection Minimum Mean Square Error Successive Interference Cancellation (MMSE-SIC) pada Sistem MC-CDMA

ABSTRAKSI: Perkembangan teknologi wireless cellular menuntut kebutuhan akan efisiensi spektrum, performansi yang tinggi, penambahan laju data, serta layanan baru. Salah satunya adalah muncul metode multicarrier spread spectrum (MC-SS) yang merupakan perpaduan antara metode spektral tersebar (CDMA) dengan transmisi multicarrier (OFDM). Akan tetapi, pada teknik akses jamak berbasiskan CDMA dimana user menduduki timeslot dan spektrum frekuensi yang sama berpotensi menimbulkan MAI. Oleh karena itu, dikembangkan beberapa metode untuk menanggulangi permasalahan MAI. Salah satu metode tersebut ialah metode deteksi multiuser pada receiver atau yang dikenal dengan Multi User Detection (MUD). Terdapat beberapa metode MUD yang bergantung pada kompleksitas dan performansi sistem yang diharapkan. MMSE dan SIC detector merupakan kategori suboptimum MUD dimana MMSE bekerja dengan menekan interferensi serta SIC yang sensitif dengan distribusi daya sinyal terima. Pada tugas akhir ini telah disimulasikan dan dianalisa performansi sistem MCCDMA dengan menggunakan penggabungan MMSE dan SIC pada kondisi multiuser. Hasil simulasi menunjukkan adanya perbaikan performansi yang diberikan MMSE-SIC pada sistem MC-CDMA. Hal ini ditunjukkan dengan adanya perbaikan SNR sebesar 5 dB pada kondisi 5 user dengan kecepatan 3 km/jam.Kata Kunci : MC-CDMA, MMSE, SIC, MMSE-SIC.ABSTRACT: The requirements of high spectrum efficiency, high performance, high data rate and new services have motivated the evolution in wireless cellular technology. One of them is multi-carrier spread spectrum (MC-SS) method which is the combination between spread spectrum and multi-carrier transmission (OFDM). On the other hand, in the CDMA based multiple accesses where several users occupy the same time slot and frequency, there are high potential MAI occurs. Therefore, some methods are developed to minimize MAI problems. One of those is the multi user detection (MUD) method. There are various kind of MUD method which depends on the desired system complexity and performance. MMSE dan SIC detectors are of the suboptimum MUD which MMSE is employed to suppress interference and SIC is sensitive to ther receive signal power distribution. In this final assignment, the performance of MMSE-SIC for MC-CDMA system has been simulated and analyzed. The simulation result indicate that the MMSE-SIC provide the improvement of MC-CDMA system performance. This is indicated by the 5 dB improvement of SNR assuming 5 active users with 3 km/h user’s velocity.Keyword: MC-CDMA, MMSE, SIC, MMSE-SI