162 research outputs found
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems
Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER
Parameters of Communication Systems Based on OFDM-CDMA
CĂl disertaÄnĂ prĂĄce leĆŸĂ v oblasti modelovĂĄnĂ a vyhodnocenĂ bezdrĂĄtovĂœch komunikaÄnĂch systĂ©mĆŻ s dvojrozmÄrnĂœm rozprostĂrĂĄnĂm signĂĄlu a jejich klĂÄovĂœch parametrĆŻ v zĂĄvislosti na vybranĂœch vlastnostech modernĂho bezdrĂĄtovĂ©ho komunikaÄnĂho ĆetÄzce. VĂœzkumnĂ© metody pouĆŸitĂ© v tĂ©to prĂĄci spoÄĂvajĂ pĆedevĆĄĂm ve vĂœvoji softwarovĂ©ho simulĂĄtoru pro prostĆedĂ Matlab, s jehoĆŸ pomocĂ, a s vyuĆŸitĂm statistickĂ©ho pĆĂstupu, jsou navrĆŸenĂ© algoritmy ovÄĆeny. DĂĄle je pouĆŸit simulĂĄtor fyzickĂ© vrstvy dle 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), vyvinutĂœ na TechnickĂ© univerzitÄ ve VĂdni. Tento pĆedstavuje ideĂĄlnĂ platformu pro implementaci metody dvojrozmÄrnĂ©ho (2D) rozprostĂrĂĄnĂ a jejĂ vyhodnocenĂ s pĆihlĂ©dnutĂm k souÄasnĂœm bezdrĂĄtovĂœm komunikaÄnĂm systĂ©mĆŻm. ZjiĆĄtÄnĂ prezentovanĂĄ v tĂ©to prĂĄci pĆedstavujĂ pĆedevĆĄĂm ovÄĆenĂ ĂșÄinnosti systĂ©mu nazvanĂ©ho jako Variable Spreading Factor - Orthogonal Code Frequency Division Multiplex (VSF-OFCDM), kterĂœ vyuĆŸĂvĂĄ principu 2D rozprostĂrĂĄnĂ signĂĄlu a zjiĆĄtÄnĂ, ĆŸe VSF-OFCDM systĂ©m pĆekonĂĄvĂĄ systĂ©my vyuĆŸĂvajĂcĂ Orthogonal Division Frequency Multiplex (OFDM), nebo Code Division Multiple Access (CDMA). DĂĄle byla navrĆŸena metoda 2D rozprostĂrĂĄnĂ signĂĄlu v systĂ©mu LTE, kde se tĂ©ĆŸ potvrdila jejĂ ĂșÄinnost. DĂky ĂșÄinnÄjĆĄĂmu potlaÄenĂ vlivu rychlĂ© variace pĆenosovĂ©ho kanĂĄlu v zĂĄvislosti na frekvenci a Äase, dosahuje systĂ©m VSF-OFCDM znatelnÄ vyĆĄĆĄĂ datovĂ© prostupnosti.The focus of this research is in the area of modeling and evaluating of the wireless systems with two dimensional signal spreading, itâs key parameters and dependencies on other features in modern wireless communication chain. The research method adopted in this dissertation includes a development of Matlab based simulators which exploits a statistical approach to show a contribution of proposed algorithms. Furthermore, a model of physical layer of the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), developed by the Vienna University of Technology, was utilized as a simulation environment suitable for implementation of a two dimensional (2D) signal spreading method and its evaluation as well as comparison of achieved results with the state-of-the-art systems. The findings from this research provide evidence that the Variable Spreading Factor - Orthogonal Code Frequency Division Multiplex (hereafter VSF-OFCDM) employing a 2D spreading is a promising wireless access scheme superior to Orthogonal Division Frequency Multiplex (OFDM) or Code Division Multiple Access (CDMA) and is capable to significantly increase the data rates in wireless transmission due to the capability of such system to effectively cope with fast time and frequency fluctuations in the wireless transmission channel.
Multi-carrier CDMA using convolutional coding and interference cancellation
SIGLEAvailable from British Library Document Supply Centre-DSC:DXN016251 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Performance studies and receiver design of a MB-OFDM UWB system
Master'sMASTER OF ENGINEERIN
Capacity, coding and interference cancellation in multiuser multicarrier wireless communications systems
Multicarrier modulation and multiuser systems have generated a great deal of research during the last decade. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation generated with the inverse Discrete Fourier Transform, which has been adopted for standards in wireless and wire-line communications. Multiuser wireless systems using multicarrier modulation suffer from the effects of dispersive fading channels, which create multi-access, inter-symbol, and inter-carrier interference (MAI, ISI, ICI). Nevertheless, channel dispersion also provides diversity, which can be exploited and has the potential to increase robustness against fading. Multiuser multi-carrier systems can be implemented using Orthogonal Frequency Division Multiple Access (OFDMA), a flexible orthogonal multiplexing scheme that can implement time and frequency division multiplexing, and using multicarrier code division multiple access (MC-CDMA). Coding, interference cancellation, and resource sharing schemes to improve the performance of multiuser multicarrier systems on wireless channels were addressed in this dissertation.
Performance of multiple access schemes applied to a downlink multiuser wireless system was studied from an information theory perspective and from a more practical perspective. For time, frequency, and code division, implemented using OFDMA and MC-CDMA, the system outage capacity region was calculated for a correlated fading channel. It was found that receiver complexity determines which scheme offers larger capacity regions, and that OFDMA results in a better compromise between complexity and performance than MC-CDMA. From the more practical perspective of bit error rate, the effects of channel coding and interleaving were investigated. Results in terms of coding bounds as well as simulation were obtained, showing that OFDMAbased orthogonal multiple access schemes are more sensitive to the effectiveness of the code to provide diversity than non-orthogonal, MC-CDMA-based schemes.
While cellular multiuser schemes suffer mainly from MAI, OFDM-based broadcasting systems suffer from ICI, in particular when operating as a single frequency network (SFN). It was found that for SFN the performance of a conventional OFDM receiver rapidly degrades when transmitters have frequency synchronization errors. Several methods based on linear and decision-feedback ICI cancellation were proposed and evaluated, showing improved robustness against ICI.
System function characterization of time-variant dispersive channels is important for understanding their effects on single carrier and multicarrier modulation. Using time-frequency duality it was shown that MC-CDMA and DS-CDMA are strictly dual on dispersive channels. This property was used to derive optimal matched filter structures, and to determine a criterion for the selection of spreading sequences for both DS and MC CDMA. The analysis of multiple antenna systems provided a unified framework for the study of DS-CDMA and MC-CDMA on time and frequency dispersive channels, which can also be used to compare their performance
Initial synchronisation of wideband and UWB direct sequence systems: single- and multiple-antenna aided solutions
This survey guides the reader through the open literature on the principle of initial synchronisation in single-antenna-assisted single- and multi-carrier Code Division Multiple Access (CDMA) as well as Direct Sequence-Ultra WideBand (DS-UWB) systems, with special emphasis on the DownLink (DL). There is a paucity of up-to-date surveys and review articles on initial synchronization solutions for MIMO-aided and cooperative systems - even though there is a plethora of papers on both MIMOs and on cooperative systems, which assume perfect synchronization. Hence this paper aims to ?ll the related gap in the literature
Channel Estimation in Multicarrier Communication Systems
The data rate and spectrum efficiency of wireless mobile communications have been significantly improved over the last decade or so. Recently, the advanced systems such as 3GPP LTE and terrestrial digital TV broadcasting have been
sophisticatedly developed using OFDM and CDMA technology. In general, most mobile communication systems transmit bits of information in the radio space to the receiver. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need of strong equalizer which requires knowledge on the channel impulse response (CIR).This is primarily provided by a separate channel estimator. Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples.
In this thesis we investigate and compare various efficient channel estimation schemes for OFDM systems which can also be extended to MC DS-CDMA systems.The channel estimation can be performed by either inserting pilot tones into all subcarriers of OFDM symbols with a specific period or inserting pilot tones into each OFDM symbol. Two major types of pilot arrangement such as block type and comb type
pilot have been focused employing Least Square Error (LSE) and Minimum Mean Square Error (MMSE) channel estimators. Block type pilot sub-carriers is especially suitable for slow-fading radio channels whereas comb type pilots provide
better resistance to fast fading channels. Also comb type pilot arrangement is sensitive to frequency selectivity when comparing to block type arrangement. However, there is
another supervised technique called Implicit Training (IT) based channel estimation which exploits the first order statistics in the received data, induced by superimposing
periodic training sequences with good correlation properties, along with the information symbols. Hence, the need for additional time slots for training the equalizer is avoided. The performance of the estimators is presented in terms of the mean square estimation error (MSEE) and bit error rate (BER)
Improved decoder metrics for DS-CDMA in practical 3G systems
While 4G mobile networks have been deployed since 2008. In several of the more developed markets, 3G mobile networks are still growing with 3G having the largest market -in terms of number of users- by 2019. 3G networks are based on Direct- Sequence Code-Division Multiple-Access (DS-CDMA). DS-CDMA suffers mainly from the Multiple Access Interference (MAI) and fading. Multi-User Detectors (MUDs) and Error Correcting Codes (ECCs) are the primary means to combat MAI and fading. MUDs, however, suffer from high complexity, including most of sub-optimal algorithms. Hence, most commercial implementations still use conventional single-user matched filter detectors. This thesis proposes improved channel decoder metrics for enhancing uplink performance in 3G systems. The basic idea is to model the MAI as conditionally Gaussian, instead of Gaussian, conditioned on the usersâ cross-correlations and/or the channel fading coefficients. The conditioning implies a time-dependent variance that provides enhanced reliability estimates at the decoder inputs. We derive improved log-likelihood ratios (ILLRs) for bit- and chip- asynchronous multipath fading channels. We show that while utilizing knowledge of all usersâ code sequences for the ILLR metric is very complicated in chip-asynchronous reception, a simplified expression relying on truncated group delay results in negligible performance loss. We also derive an expression for the error probability using the standard Gaussian approximation for asynchronous channels for the widely used raised cosine pule shaping. Our study framework considers practical 3G systems, with finite interleaving, correlated multipath fading channel models, practical pulse shaping, and system parameters obtained from CDMA2000 standard. Our results show that for the fully practical cellular uplink channel, the performance advantage due to ILLRs is significant and approaches 3 dB
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