29 research outputs found

    Interference Suppression in WCDMA with Adaptive Thresholding based Decision Feedback Equaliser

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    WCDMA is considered as one of the 3G wireless standards by 3GPP. Capacity calculation shows that WCDMA systems have more capacity compared to any other multiple access technique such as time division multiple access (TDMA) or frequency division multiple access (FDMA). So it is widely used. Rake receivers are used for the detection of transmitted data in case of WCDMA communication systems due to its resistance to multipath fading. But rake receiver treat multiuser interference (MUI) as AWGN and have limitation in overcoming the effect of multiple access interference (MAI) when the SNR is high. A de-correlating matched filter has been used in this thesis, which eliminates and improves system performance. But the given receiver works well only in the noise free environment. A DFE, compared to linear equaliser, gives better performance at severe ISI condition. The only problem in this equalisation technique is to select the number of symbols that are to be fed back. This thesis gives an idea on multiple symbol selection, based on sparity where an adaptive thresholding algorithm is used that computes the number of symbols to feedback. Simulated results show a significant performance improvement for Regularised Rake receiver along with thresholding in terms of BER compared to a rake receiver, de-correlating rake receiver and regularised rake receiver. The performance of the receiver in different channels is also analysed

    On receiver design for an unknown, rapidly time-varying, Rayleigh fading channel

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    Interference Mitigation in Wireless Communications

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    The primary objective of this thesis is to design advanced interference resilient schemes for asynchronous slow frequency hopping wireless personal area networks (FH-WPAN) and time division multiple access (TDMA) cellular systems in interference dominant environments. We also propose an interference-resilient power allocation method for multiple-input-multiple-output (MIMO) systems. For asynchronous FH-WPANs in the presence of frequent packet collisions, we propose a single antenna interference canceling dual decision feedback (IC-DDF) receiver based on joint maximum likelihood (ML) detection and recursive least squares (RLS) channel estimation. For the system level performance evaluation, we propose a novel geometric method that combines bit error rate (BER) and the spatial distribution of the traffic load of CCI for the computation of packet error rate (PER). We also derived the probabilities of packet collision in multiple asynchronous FH-WPANs with uniform and nonuniform traffic patterns. For the design of TDMA receivers resilient to CCI in frequency selective channels, we propose a soft output joint detection interference rejection combining delayed decision feedback sequence estimation (JD IRC-DDFSE) scheme. In the proposed scheme, IRC suppresses the CCI, while DDFSE equalizes ISI with reduced complexity. Also, the soft outputs are generated from IRC-DDFSE decision metric to improve the performance of iterative or non-iterative type soft-input outer code decoders. For the design of interference resilient power allocation scheme in MIMO systems, we investigate an adaptive power allocation method using subset antenna transmission (SAT) techniques. Motivated by the observation of capacity imbalance among the multiple parallel sub-channels, the SAT method achieves high spectral efficiency by allocating power on a selected transmit antenna subset. For 4 x 4 V-BLAST MIMO systems, the proposed scheme with SAT showed analogous results. Adaptive modulation schemes combined with the proposed method increase the capacity gains. From a feasibility viewpoint, the proposed method is a practical solution to CCI-limited MIMO systems since it does not require the channel state information (CSI) of CCI.Ph.D.Committee Chair: Professor Gordon L. StBe

    Iterative multiuser detection with integrated channel estimation for turbo coded DS-CDMA.

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    In present days the demand of high bandwidth and data rate in wireless communications is increasing rapidly to accommodate multimedia applications, including services such as wireless video and high-speed Internet access. In this thesis, we propose a receiver algorithm for mobile communications systems which apply CDMA (Code division multiple access) as multiple access technique. Multiuser Detection and turbo coding are the two most powerful techniques for enhancing the performance of future wireless services. The standardization of direct sequence CDMA (DS-CDMA) systems in the third generation of mobile communication system has raised the interest in exploiting the capabilities and capacity of this type of Technology. However the conventional DS-CDMA system has the major drawback of multiple Access Interference (MAI). The MAI is unavoidable because receivers deal with the information which is transmitted not by a single information source but by several uncoordinated and geographically separated sources. To overcome this problem MUD is a promising approach to increase capacity. (Abstract shortened by UMI.)Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2005 .C465. Source: Masters Abstracts International, Volume: 45-01, page: 0404. Thesis (M.Sc.)--University of Windsor (Canada), 2005

    Near maximum likelihood multiuser receivers for direct sequence code division multiple access

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    Wideband wireless access based on direct-sequence code-division multiple access (DS-CDMA) has been adopted for third-generation mobile communications systems. Hence, DS-CDMA downlink communications systems form the platform for the work in this thesis. The principles of the spread spectrum concept and DS-CDMA technology are first outlined, including a description of the system model and the conventional receiver. The two classes of codes used in this system, namely spreading codes and forward error correction codes (including Turbo codes), are discussed. Due to the fact that practical communications channels are non-ideal, the performance of an individual user is interference limited. As a result, the capacity of the system is greatly restricted. Fortunately, multiuser detection is a scheme that can effectively counteract this multiple access interference. However, the optimum multiuser detection scheme is far too computationally intensive for practical use. Hence, the fundamental interest here is to retain the advantages of multiuser detection and simplify its implementation. The objective of the thesis is to investigate the optimum multiuser receiver, regarded on a chip level sampling basis. The aim is to reduce the complexity of the optimum receiver to a practical and implementable level while retaining its good performance. The thesis first reviews various existing multiuser receivers. The chip-based maximum likelihood sequence estimation (CBMLSE) detector is formulated and implemented. However, the number of states in the state-transition trellis is still exponential in the number of users. Complexity cannot be reduced substantially without changing the structure of the trellis. A new detector is proposed which folds up the original state-transition trellis such that the number of states involved is greatly reduced. The performance is close to that of the CBMLSE. The folded trellis detector (FTD) can also be used as a preselection stage for the CBMLSE. The FTD selects with high accuracy the few symbol vectors that are more likely to be transmitted. The CBMLSE is then used to determine the most likely symbol vector out of the small subset of vectors. The performance of this scheme is as good as the CBMLSE. The FTD is also applied in an iterative multiuser receiver that exploits the powerful iterative algorithm of Turbo codes

    Channel modeling, estimation and equalization in wireless communication

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    The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Title from title screen of research.pdf file viewed on (May 25, 2006)Includes bibliographical references.Vita.Thesis (Ph. D.) University of Missouri-Columbia 2005.Dissertations, Academic -- University of Missouri--Columbia -- Electrical engineering.Channel modeling, estimation and equalization are discussed throughout this dissertation. Relevant research topics are first studied at the beginning of each chapter and the new methods are proposed to improve the system performance. MLSE is an optimum equalizer for all the case. However, due to its computational complexity, it is impractical for today technologies in third generation wireless communication. Thus, a suboptimum equalizer so-called perturbation equalizer is proposed, which outperforms the RSSE equalizer in the sense of bit error rate or computational complexity. In order to improve the system performance dramatically, the iterative equalization algorithm is implemented. It has been shown that the turbo equalization using the trellis based Maximum A Posteriori equalizer is a powerful receiver that yielding the optimum system performance. Unfortunately, due to its exhausted computational complexity, a suboptimal equalizer is required. An improved DFE algorithm, which only requires low computational complexity, is proposed for turbo equalization. The promising simulation results indicate that the proposed equalizer provides significant improvement in bit error rate while compared to the conventional DFE algorithm. Prior to channel equalization, channel estimation enable us to extract the necessary channel information from the pilot symbols for equalizers. Least-squares algorithm is a promising estimation algorithm providing the channel is time-invariant in a given period. Based on the derivations, we show that the channel is no longer constant and a new least-squares based algorithm is proposed to estimate the channel accurately. Simulation results convince us that the new algorithm provides the equalizer more reliable information. Besides, antenna diversity is another promising technique implemented practically to improve the system performance provided that the channels of antennas are not correlated. A new three dimensional multiple-input multiple-output abstract model is proposed for the investigation and understanding of the correlation of fading channel. The new model allows us to consider the channel correlation of which the mobile stations receive the incoming waves from any directions and angle spreads. Based on this abstract model, the closed form and mathematical tractable formula is derived for space-time correlation function. The new function can be further simplified other known special cases

    近距離MIMOシステムの通信容量に関する研究

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    Recently, the near field communication, abbreviated NFC, which is a form of contactless communication between devices like smartphones or tablets, is emerging quickly. Contactless communication allows a user to wave the smartphone over a NFC compatible device to send information without needing to touch the devices together or go through multiple steps setting up a connection. Fast and convenient, NFC technology is popular in parts of Europe and Asia, and is quickly spreading throughout the whole world. Over the past decade, we have witnessed the rapid evolution of Multiple-Input Multiple-Output (MIMO) systems which promise to break the frontiers of conventional architectures and deliver high throughput by employing more than one element at the transmitter (Tx) and receiver (Rx) in order to exploit the spatial domain. This is achieved by transmitting simultaneous data streams from different elements which impinge on the Rx with ideally unique spatial signatures as a result of the propagation paths‘ interactions with the surrounding environment. For exchanging massive information, for instance the videos or photos, between two devices, the future NFC systems will require higher channel capacity than current systems. Therefore, the MIMO system, which has a wider bandwidth, multi-value modulation system, and spatial multiplexing scheme, is the appropriate candidate to be employed in the high-speed NFC systems. Contrary to conventional MIMO systems, near-field MIMO communication systems transfer data in a very short range, the transmission lines are formed in parallel without multipath, and the LOS (line-of-sight) paths are the major components. The conventional MIMO works in a multipath-rich propagation environment, and is expected to achieve a high channel capacity by utilizing multipath components. The near-field MIMO, however, transfers data directly from the transmitter to the receiver, without any fading caused by multipath components. In the near-field MIMO system, a higher channel capacity results from a higher SNR and lower spatial correlation characteristics. Considering the short distance, the LOS components from each of the Tx elements arrive at the Rx array with a spherical wavefront. Therefore the beamwidth of the antenna element radiation pattern affects not only the receiving gain but also the spatial correlation characteristics. Usually, the conventional dipole antennas are used to investigate the MIMO channel capacity. However, the conventional dipoles are omni-directional in the horizontal plane. In this paper, a bi-directional element named dual-dipole element is utilized to improve the channel capacity. In the dual-dipole array, two half wave-length dipole antennas are settled parallel as only one element. By changing the internal distance between the two dipoles in one Tx element, the HPBW (half power beam width) of the element can be adjusted. Therefore, the shape of the radiation pattern can be determined by the internal distance between the two dipoles in one element. The effect of the HPBW on the channel capacity is investigated in detail. The narrower beam width of the Tx element can result for a higher SNR in the facing Rx element, however, at the meantime the power in the other sub channels will decrease. Hence, it is expected that there would be an optimum HPBW when the system could obtain the maximum channel capacity. And we find out the optimum HPBW for the near-field MIMO system with dual-dipole arrays. In addition, the improvement in the channel capacity from the conventional dipole array is considerable. Basically two factors determine the capacity of a MIMO system—the path loss and the multipath richness. The dual-dipole arrays lead to much lower path loss than the conventional dipole arrays, hence, the channel capacity improves significantly. However, the multipath richness rarely exists in the near-field MIMO. So far, all the researches on the near-field MIMO are in the free space without any obstacle. However, due to the short transfer distance of the near-field MIMO, a tiny variation of the channel will lead to a significant difference on the channel capacity. Therefore, we employ metal wire in the near-field MIMO system to increase the multipath richness and clarify the effect of obstacles in the system. The characteristics of the single metal wire are detailed investigated. And the most significant aspect is the location of the metal wire placed in the system. Generally, an object placed between two transmission antennas will decrease the channel capacity of the system. Here, we try to determine the optimal location of the object between the opposing antennas. We expect that the optimal location will alleviate the deterioration in the capacity caused by the object. However, the simulation results indicate that if the metal wire is placed in an appropriate location, a higher channel capacity can be obtained. In addition, we can set multiple metal wires in the optimum locations to achieve higher channel capacity. The different types of objects in the different types of arrays are also researched. Finally, this paper clarifies the frequency dependency of channel capacity in near-field MIMO system with metal wire. As the frequency increases, the absolute value of the channel capacity decreases. The improvement on channel capacity of using a metal wire also changes with frequency. In addition, when the frequency is very large, the effect of the metal wire is negligible. The proper location for the metal wire is found related with the corresponding wavelength of each specific frequency. Confidently, the research of the effect of the element HPBW and the objects between Tx and Rx introduced in this study can be beneficially applied in actual network preparation of future near-field MIMO wireless communications in which the improvement in the channel capacity are required
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