This paper presents an interference cancellation and multipath mitigation algorithm for use in Global Positioning System (GPS) with an array antenna. It is shown that interference signals and multipath signals are effectively suppressed using a serial subspace projection method without any knowledge of the incoming directional information. After the subspace projections, a beamformer is used to maximize the SNR of the received signal. The enhancement in the performance is presented in terms of the cross correlation value and beam patterns.

This paper extends previous analytical approaches for the study of CDMA systems to the relevant case of multipath environments where users can operate at different bit rates. This scenario is of interest for the Wideband CDMA strategy employed in UMTS, and the model permits the performance comparison of classic and more innovative spreading signals. The method is based on the characteristic function approach, that allows to model accurately the various kinds of interferences. Some numerical examples are given with reference to the ITU-R M.1225 Recommendations, but the analysis could be extended to different channel descriptions.

In this paper, sampling rate selection diversity (SRSD) scheme for Direct-Sequence/Spread-Spectrum (DS/SS) is proposed. In DS/SS communication systems, oversampling may be employed to increase the signal-to-noise ratio (SNR). However, oversampling enlarges the power consumption because signal processing of the receiver has to be carried out at a higher clock rate. Higher sampling rate does not always maximize the SNR. In the proposed SRSD scheme, the power consumption can be reduced by selecting the optimum sampling rate depending on the characteristics of the channel. The proposed SRSD scheme can also reduce the BER more than the conventional oversampling scheme under certain channel conditions.

We propose a novel asynchronous direct-sequence code-division multiple access (DS-CDMA) using feedback-controlled spreading sequences (FCSSs) (FCSS/DS-CDMA). At the receiver of FCSS/DS-CDMA, the code-orthogonalizing filter (COF) produces a spreading sequence, and the receiver returns the spreading sequence to the transmitter. Then the transmitter uses the spreading sequence as its updated version. The performance of FCSS/DS-CDMA is evaluated over time-dispersive channels. The results indicate that FCSS/DS-CDMA greatly suppresses both the intersymbol interference (ISI) and multiple access interference (MAI) over time-invariant channels. FCSS/DS-CDMA is applicable to the decentralized multiple access.

In this paper, we propose a multipath en-route filtering method to deal with the problems caused by black hole attacks and selective forwarding attacks. Our result shows that the method is more resilient to these problems up to a certain number of compromised nodes than the statistical en-route filtering scheme.

The effect of wall and indoor scatterers on the indoor multiple input multiple output (MIMO) communication system is investigated by using the hybrid technique of finite difference time domain (FDTD) method and method of moments (MoM). MIMO channel capacity with the wall reflection is investigated with consideration of the eigenvalue of channel covariance matrix, the received power and the effective multipaths of MIMO system. It is found that the stronger side wall reflection can lead to the higher MIMO channel capacity. MIMO system with indoor scatterers is also analyzed and compared with the line of sight (LOS) indoor MIMO system. It is found that the scatterer material has different effect on the received power and the effective multipaths of MIMO system.

Direction of arrival estimation of coherent multipath waves by using superresolution technique often requires decorrelation preprocessings. Spatial smoothing preprocessings are the most popular schemes as the techniques. In mobile environment, position change of the target/transmitter often brings us decorrelation effect. In addition, multiple signals transmitted by an antenna array, such as a MIMO transmitter, can also cause the same effect. These effects can be categorized as the spatial smoothing preprocessing at the transmitter. In this paper, we analyze the spatial smoothing effect at the transmitter in the presence of multipath coherent waves. Theoretical and simulation results show that the spatial smoothing at the transmitter has a good feature in comparison with the conventional SSP at the receiving array. We also show that better decorrelation performance can be obtained when the SSPs at the transmitter and receiving array are applied simultaneously.

In this paper, a sparse multipath channel estimation algorithm is proposed for multiple-input multiple-output (MIMO) single-carrier systems with frequency-domain decision feedback equalizer (FD-DFE). This algorithm exploits the orthogonality of an optimal MIMO preamble based on repeated, phase-rotated, Chu (RPC) sequences, leading to a dramatic reduction in computation. Furthermore, the proposed algorithm employs an improved non-iterative procedure utilizing the Generalized AIC criterion (GAIC), resulting in further computational saving and performance improvement. The proposed scheme is simulated for 802.16d SCa-PHY and SUI-5 sparse channel model under a 22 spatial multiplexing scenario, with the MIMO FD-DFE as the receiver. The result shows that the channel estimation accuracy is significantly improved, and the performance loss compared to the known channel case is only 0.7 dB at the BER of 10-3.

In this paper, a non-data aided minimum mean square error (MMSE) receiver with enhanced multiple access interference (MAI) suppression is proposed for direct-sequence code-division multiple-access (DS-CDMA) systems over a multipath fading channel. The design of the proposed receiver is via the following procedure: First, an adaptive correlator is constructed based on the linearly constrained minimum variance (LCMV) criterion to collect each multipath signal and suppress MAI blindly. A maximum ratio combiner is then utilized to coherently combine the correlator outputs. With a set of judicious chosen weight vectors, effective diversity combining can successfully suppress MAI and the desired signals can be effectively retained. Finally, further performance improvement against the finite data sample effect is achieved using a decision-aided scheme in which the channel response is obtained by the decision data and incorporated with the MMSE method to compute the refined weight vector. Performance analysis based on the output signal-to-interference-plus-noise ratio (SINR) is done to examine the efficacy of the proposed non-data aided MMSE receiver, which can offer the similar results as those of the MMSE receiver with the channel estimation correctly obtained beforehand. Computer simulation results then confirm correctness of the analysis results and demonstrate that the proposed blind receiver can successfully resist MAI as well as the finite data sample effect, and significantly outperform than the conventional blind receivers.

We developed a comprehensive simulation system for evaluating satellite-based navigation services in highly built-up areas; the system can accommodate Global Positioning System (GPS) multipath effects, as well as line-of-sight (LOS) and dilution of position (DOP) issues. For a more realistic simulation covering multipath and diffracted signal propagations, a 3D-ray tracing method was combined with a satellite orbit model and three-dimensional (3D) geographic information system (GIS) model. An accuracy estimation model based on a 3D position determination algorithm with a theoretical delay-locked loop (DLL) correlation computation could measure the extent to which multipath mitigation improved positioning accuracy in highly built-up areas. This system could even capture the multipath effect from an invisible satellite, one of the greatest factors in accuracy deterioration in highly built-up areas. Further, the simulation results of satellite visibility, DOP, and multipath occurrence were mapped to show the spatial distribution of GPS availability. By using object-oriented programming, our simulation system can be extended to other global navigation satellite systems (GNSSs) simply by adding the orbital information of the corresponding GNSS satellites. We demonstrated the applicability of our simulation system in an experimental simulation for Shinjuku, an area of Tokyo filled with skyscrapers.

The performance of M-ary code shift keying (CSK) in multipath fading channels is analyzed in this letter. In conventional spread spectrum (SS) systems, which employ a single spreading code, the interference due to the delay signal depends on the auto-correlation property of the spreading code. In CSK, since one code is selected from a code set that contains multiple codes, the effect of the delay signal differs from that for conventional SS systems. Analytical results reveal that, in the multipath fading channel, the error rate of CSK depends on the cross-correlation property with the other codes in addition to the auto-correlation of transmitted code.

Multipath routing employs multiple parallel paths between the source and destination for a connection request to improve resource utilization of a network. In this paper, we present an integrated design of multipath routing with delay constraints and failure survivability in MPLS networks. By combining the failure survivability schemes into the multipath routing algorithms, path protection or restoration policies will enable the network to accommodate link failures and at the same time achieve significant improvement on network resource utilization. We propose a number of multipath routing algorithms, working-backup path selection and bandwidth allocation schemes. We evaluate the performance of the proposed schemes in terms of call blocking probability, network resource utilization and load balancing factor. Extensive simulation results validate the effectiveness of the proposed schemes. In particular, we compare these multipath schemes to the existing failure recovery schemes that mostly focus on single path routing. The results demonstrate that the proposed integrated design framework can provide effective network failure survivability, and also achieve better load balancing and/or higher network resource utilization.

This paper surveys and introduces propagation studies and models that are expected to contribute to the development of broadband wireless systems. The survey focused on theory-based propagation models, experimental measurement data useful for modeling, and transmission characteristic evaluations using propagation models. The survey did not attempt to cover all papers in the research fields, but rather took key papers for various relevant subjects and described them in some detail. The basic characteristics of multipath propagation are summarized from the viewpoints of narrow-band (NB), wide-band (WB), and ultra wide-band (UWB). Recent studies on spatio-temporal propagation models and the relationship between models and systems are introduced. To clarify the relationship between OFDM, which is a representative of wideband data transmission schemes, and wave propagation factors, problems due to large delay spread and large Doppler spread are highlighted. Finally, studies on UWB propagation measurement and propagation models are introduced.

Nowadays, MIMO systems are playing an important role in wireless communications. In this paper, we propose a spatial-temporal adaptive MIMO beamforming scheme for single carrier transmission in frequency-selective fading channels with the assumption of perfect channel state information (CSI) at both the transmitter and receiver. The transmit and receive weight vectors for detecting the preceding signal and the receive weight vectors for detecting the delayed signals of the preceding signal are designed by an iterative update algorithm. Based on minimum mean square error (MMSE) method, the delayed versions of the preceding signal are exploited to maximize the output signal to interference and noise ratio (SINR) instead of suppressing them at the receiver. The improvement of output SINR is useful for MIMO systems to enhance the high-quality communication in broadband wireless systems.

The Multiple Access Interference (MAI) and the Multipath Fading (MPF) restrict the performance of Code-Division Multiple-Access (CDMA) systems. The Multiuser Detection (MUD) based on Particle Swarm Optimization algorithm (PSO) with Rake processing is proposed in this paper to overcome these obstacles, followed by full details of how to apply the Binary PSO MUD (BPSO-MUD) on a CDMA system. Simulations show that the BPSO-MUD has significantly better performance than the Conventional Detection (CD).

A method of testing distributed amplifiers is presented; multipath interference (MPI) is detected as a beat spectrum between a multipath signal and a direct signal using a frequency-modulated test signal. A test signal with an approximately 450 MHz frequency deviation at an 80 kHz modulation frequency is emitted from a directly modulated DFB-LD by a pulse stream passing through an equalizer. A receiver consisting of a photodiode and an electrical spectrum analyzer (ESA) detects a baseband power spectrum peak appearing at the frequency of the test signal frequency deviation. MPI is converted from the spectrum peak power using a calibration chart. The test method has decreased the minimum detectable MPI as low as -70 dB, compared with that of -50 dB of conventional test methods. The detailed design and performance of the proposed method are discussed, including the calibration procedure, computer simulations for evaluating systematic errors caused by the repetition rate of the frequency modulated test signal and the fiber length under test, and experiments on single-mode fibers and distributed Raman amplifiers.

Software defined radio, which uses reconfigurable signal processing devices, requires the determination of multiple unknown parameters to realize the potential capabilities of adaptive communication. Evolutional algorithms are optimal multi dimensional search techniques, and are well known to be effective for parameter determination. This letter proposes an evolutional algorithm for learning the mobile time-varying channel parameters without any specific assumption of scattering distribution. The proposed method is very simple to realize, but can provide precise channel estimation results. Simulations of an OFDM system show that for an example of OFDM communication under the time-varying fading channel, the proposed learning method can achieve the better BER performance.

This paper investigates the impact of chip duty factor (DF) in DS-UWB system with Rake receiver over AWGN and UWB indoor multipath environment corresponding to system parameters such as spreading bandwidth and chip length. Manipulating DF in DS-UWB system offers several advantages over multipath channel and thus, capable of improving system performance for better quality of communication. Although employing lower DF generally improves performance, in some exceptional cases on the other hand, degradation can be observed despite decreasing DF. Therefore, the objective of this paper is to clarify the relationship between DF and DS-UWB system performance. We discovered that with constant processing gain and spreading bandwidth, performance improvement can be observed at DF lower than 0.17. Additionally, with spreading bandwidth as tradeoff parameter, significant performance improvement can only be observed below DF of 0.85.

Multipath search based instantaneous root-mean-squared (RMS) delay spread (RDS) estimators mainly depend on path detection or multipath search. This paper proposes a novel method for multipath search through Minimum Descriptive Length (MDL) criterion, and hence a novel instantaneous RDS estimation method for wireless OFDM systems. compared with the conventional multipath search based instantaneous RDS estimators, the proposed estimator doesn't need any a priori information about the noise variance and the channel power delay profile (PDP) while the performance is improved. Simulation results demonstrate that the proposed estimator is also insensitive to the variance of SNR and robust against the frequency selectivity, as well as the vehicle speed.

Many applications of OFDM systems require Doppler spread estimation. This is quite difficult in multi-path fading channels with no strong direct path. This letter proposes a novel Doppler spread estimation method, which uses the mean square (MS) value of channel impulse response's time derivative. The proposed method is very simple compared with the previously proposed methods. Simulation results show that it allows easy and precise Doppler spread calculation for OFDM by using the channel estimation based on either pilot tones or pilot symbols.