Wireless Cloaking System Based on Time-Reversal Multipath Propagation Effects

Robust Digital Retrodirective Beamforming Technique for Multipath Channel Environment

This paper presents channel precoding schemes to combat intersymbol interference (ISI) over a frequency-selective slow fading channel in wireless communication systems using /spl pi//4 differential quadrature phase shift keying (/spl pi//4-DQPSK) or minimum shift keying (MSK). Based on the dimension partitioning technique, the precoders predistort the phase of the transmitted signals in the forward link to combat ISI, keeping the transmitted signal amplitude constant. The precoding schemes can (i) ensure the stability of the precoders even in equalizing a non-minimum-phase channel, (ii) achieve ISI-free transmission without increasing the complexity of the portable unit receiver and (iii) reduce the envelope variations of transmitted signals such that a power efficient nonlinear amplifier can be used without causing undue distortion. Theoretical and simulation results are presented to demonstrate that the proposed channel precoders can outperform DFE when the ISI is severe and the precoding scheme for /spl pi//4-DQPSK can achieve a smaller envelope variation than the standard /spl pi//4-DQPSK.

To investigate the spike activities of cerebellar cortical cells in a computational network model constructed based on the anatomical structure of cerebellar cortex. The multicompartment model of neuron and NEURON software were used to study the external influences on cerebellar cortical cells. Various potential spike patterns in these cells were obtained. By analyzing the impacts of different incoming stimuli on the potential spike of Purkinje cell, temporal focusing caused by the granule cell-golgi cell feedback inhibitory loop to Purkinje cell and spatial focusing caused by the parallel fiber-basket/stellate cell local inhibitory loop to Purkinje cell were discussed. Finally, the motor learning process of rabbit eye blink conditioned reflex was demonstrated in this model. The simulation results showed that when the afferent from climbing fiber existed, rabbit adaptation to eye blinking gradually became stable under the Spike Timing-Dependent Plasticity (STDP) learning rule. The constructed cerebellar cortex network is a reliable and feasible model. The model simulation results confirmed the output signal stability of cerebellar cortex after STDP learning and the network can execute the function of spatial and temporal focusing.

Underwater sensor networks may be used in many underwater applications and some of these applications may transmit large amounts of data. Underwater acoustic communication is limited by several physical factors affecting communication. Time spread caused by multipath effects and frequency selective fading are major concerns. The choice of modulation technique is important. Due to the high data rate required in network applications coherent modulation techniques should be considered. In the choice of multiple access method code division multiple access is considered the most promising technique because of its robustness and resistance against frequency selective fading. Also by using a RAKE receiver multipath arrivals can coherently be combined. To get an idea of multipath effects for different propagation conditions the acoustic modeling program EasyPLR is used. Sound speed profiles from different seasons of 2005 are implemented to display the seasonal variations. The results indicate multipath effects and time spread for all seasons at short range. For ranges up to 3 km time spread is kept at tens of micro seconds.

Multiple-input/multiple-output (MIMO) frequency selective fading channels introduce both the intersymbol interference (ISI) and the inter-(sub)channel interference (ICI) that are the major performance limiting factors for MIMO wireless data communications. This paper focuses on joint suppression of such two interferences in receiver design under the criterion of the minimum mean-squared error (MMSE). The well-known Kalman filtering arises as a natural design tool to achieve the optimum joint suppression of the ISI and ICI in the sense of MMSE that results in a two-stage design method that suppresses the ISI and ICI in series. Specifically, a state-space solution is derived for the optimum decision-feedback equalizer (DFE) based on temporal Kalman filtering, and is shown to be equivalent to a conventional finite length DFE for ISI suppression. A novel spatial Kalman filtering scheme is proposed as the second stage to successively cancel the ICI. It is shown that the proposed two-stage method yields the realizable and optimum joint ISI and ICI suppression, while it admits lower computational complexity than the conventional finite length DFE for ISI suppression and the MMSE V-BLAST receiver for ICI suppression. The results are applicable to any linear channels that admit state-space realizations, including MIMO frequency selective time-varying channels, and are thus the most general. Simulation results show the effectiveness of the proposed two-stage method and its robustness to channel estimation uncertainties that may arise in practical systems.

The Fourth generation (4G) wireless networks are expected to afford high data rate transmission of multimedia services such as wireless video, wireless Internet access and mobile computing. Orthogonal frequency division multiplexing (OFDM) is an effective technique for the future 4G communications because of its great immunity to impulse noise and intersymbol interference. Ultra wide band (UWB) wireless technology is poised to replace high-speed data cables in homes and offices. UWB technology will be capable of transmitting hundreds of megabits per second over distances of several meters. Channel estimation in OFDM systems can conveniently be done by inserting a stream of pilot symbols at the transmitter and by using FIR interpolation filters at the receiver. One of the challenges in wireless system is the frequency selective fading caused due to multipath channel between the transmitter and receiver. To overcome such a multipath fading environment with low complexity and to increase the performance, this paper presents an importance of UWB OFDM for wireless fading channels.

This paper presents channel precoding schemes to combat intersymbol interference (ISI) over a frequency-selective slow fading channel in wireless communication systems using /spl pi//4 shifted quadrature phase-shift keying or minimum shift keying. Based on the dimension partitioning technique, the precoders predistort the phase of the transmitted symbol in the forward link to combat ISI, keeping the transmitted symbol amplitude constant. The proposed schemes can ensure the stability of the precoder even in equalizing a nonminimum-phase channel, combat ISI without increasing the complexity of portable unit receivers, and reduce the envelope variations of transmitted signals such that power-efficient nonlinear amplifiers can be used for the precoded signals without causing a significant undue distortion. Theoretical analysis and simulation results are presented to demonstrate the effectiveness of the proposed channel precoding schemes.

Interference caused by other users in a multi-user environment degrades the system performance significantly. Conventionally, user separation in a multi-user environment is achieved by signal separation in time, frequency or code. The use of multiple elements antenna (MEA) systems can apply separation of the users in space by targeting the transmitted power to the user of interest. Inevitably in that case, the separation is not ideal and there is interference on the other users in the systems. Moreover in a wideband transmission, frequency selective fading causing inter symbol interference (ISI) is a big challenge for point to point wireless communications. In this paper, we study the feasibility of applying the time reversal technique (TR) in multiple input single output (MISO) systems to alleviate the effect of ISI. The ability of TR to focus the signal on the receiver of interest in a multi-user environment is also considered. The studies are supported by measured complex channel impulse responses with 10 MHz bandwidth centered at 2.14 GHz. For an 8/spl times/1 MISO system, using TR the root mean square delay spread is reduced by a factor of 2. To evaluate the capability of the TR in reducing the ISI, simulation of the bit error rate (BER) was made. The irreducible BER of the TR-MISO system was shown to be lower than that of the SISO system by at least an order of magnitude. By using TR and assuming that two users are communicating simultaneously with the same BS the two signals have 17 dB of isolation.

This paper presents an optimized turbo equalizer based on Laot's turbo equalization scheme by means of soft intersymbol interference (ISI) cancellation. Due to Berrou's turbo-principle, common turbo equalization schemes use the extrinsic information as a-priori input for the equalizer. In contrast to this, we propose the use of the full soft output channel decoder's information for the turbo interference canceller. The performance of this improved turbo equalizer by means of soft interference cancellation is evaluated in the environment of two channels, which introduce severe ISI. Simulation results show that the proposed turbo equalizer achieves a performance gain of up to 4 dB compared to Laot's original approach. The performance gain is achieved without any increase of computational complexity. Moreover, we apply a modified extrinsic information transfer (EXIT) chart to the proposed turbo equalizer and illustrate the convergence advantages in comparison to Laot's approach.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text M. Durst, G. Zhu, and C. Xu, "Effects of Refractive-Index Mismatch and Scattering on Simultaneous Spatial and Temporal Focusing," in Frontiers in Optics 2007/Laser Science XXIII/Organic Materials and Devices for Displays and Energy Conversion, OSA Technical Digest (CD) (Optica Publishing Group, 2007), paper FTuU2. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

We propose a two-dimensional simultaneous spatial and temporal focusing (SSTF) microcopy employing a VIPA and a grating, which can improve both axial resolution and background noise compared to 1-D SSTF microscopy.

In this paper, an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system is investigated. For downlink, users perform Information Decoding (ID) and Energy Harvesting (EH) simultaneously. For uplink, users transmit information to Base Station (BS), while BS performs the Non-Orthogonal Multiple Access (NOMA) to decode information from users. In order to maximize the total uplink ID rate in the condition that the total downlink ID rate is ensured, a joint uplink and downlink optimization method based on power and subcarrier allocation is proposed. As shown in simulation results, compared with the existing method, the proposed method can implement the maximum harvested energy for users in the downlink and achieve higher total uplink ID rate.

During the last decade we have experienced an increasing demand for wireless communication systems, that require the implementation of different functions, including equalization, to establish a reliable connection between nodes of the network. The correlation and statistical properties of the training sequences have been exploited in wireless systems in the past, however there are few studies that compare the impact of these sequences on the efficacy of the equalization process where they are implemented. Our experimental design considers five well known training sequences of the same length, including Barker, Willard, Neuman-Huffman, a Random binary sequence, and the Zadoff-Chu (ZC) code, and a new proposed sequence. The simulation results show that Random code is the best performing training sequence, especially when it is used in environments with strong levels of inter-symbol interference (ISI), ZC performs better for conditions with lower ISI and our proposed sequence named EoP performs in second place for any ISI level.

In this paper an efficient implementation of decision feed back equalizer (DFE) is carried out using novel memory less distributed arithmetic (NMLDA) filter. In wireless transmission systems, DFEs are used to mitigate the inter-symbol interference (ISI). The ISI is occurred due to multi-path propagation of the transmitted signal. High data rate systems demand higher order filters in DFE architectures which increase complexity in hardware design. In our proposed NMLDA design, we have used multiplexers and enhanced compressor adders in place of memory unit and conventional adders. The proposed design occupies lower area and gives higher throughput, when compared to MAC based filter and all other memory based DA filter architectures. By using proposed NMLDA based DFE, the ISI errors in transmission signal, will be minimized and the performance of the transmission system will be enhanced. We have synthesized the NMLDA of 32-tap, 16-tap, 8-tap and 4-tap filters and implemented them on FPGA device. The proposed design has nearly 70% less number of logical elements than OBC DA and 50% less than MDA and offers better throughput than the existing designs when implemented on Altera Cyclone III EP3C55F484C6.

A wavelet packet function based multiple access (WPMA) system is developed in this paper to maximize capacity and improve receiver performance over frequency selective multipath fading channels. To design an efficient receiver that mitigates residual multiple access interference (MAI) and intersymbol interference, while improving receive signal-to-interference and noise ratio (SINR) simultaneously on the uplink; a multichannel decision feedback equalizer (DFE) following a wavelet packet function based RAKE receiver is proposed. Simulation results show that, over GSM TU channels the developed receiver performs quite well if the power of each user is perfectly controlled or the space diversity combining (SDC) technique is applied.