Ridha Hamila

Highly efficient techniques for mitigating the effects of multipath propagation in DS-CDMA delay estimation

Delay estimation in direct-sequence code-division multiple-access (DS-CDMA) systems is necessary for accurate code synchronization and for applications such as mobile phone positioning. Multipath propagation is among the main sources of error in the DS-CDMA delay estimation process, together with multiple access interference and non-line-of-sight (NLOS) propagation. This paper provides a review of main delay estimation techniques, existing in the literature so far, which are able to cope with multipath propagation, together with our novel delay estimation techniques proposed in the context of DS-CDMA systems. The performance of all these techniques is compared through analysis and simulations, considering also their relative computational complexity and required prior information. Starting from the traditional delay locked loops (DLL) and their improved variants, we discuss several recently introduced delay estimation techniques able to cope with multipath propagation. The characterization of these methods is given in a unified framework, suited for both rectangular and root raised cosine pulse shapes. The main focus in the performance comparison of the algorithms is on the closely-spaced multipath scenario, since this situation is the most challenging for achieving diversity gain with low delay spreads and for estimating LOS component with high accuracy in positioning applications.

Teager energy and the ambiguity function

The connection between the Teager energy operator and the ambiguity function is established in this correspondence after defining the Teager operator over complex-valued signals. This relation allows the use of the Teager energy in estimating the second moment angular bandwidth and the moments of a signal duration (spread) and that of its spectrum.

Extended Kalman filter channel estimation for line-of-sight detection in WCDMA mobile positioning

In mobile positioning, it is very important to estimate correctly the delay between the transmitter and the receiver. When the receiver is in line-of-sight (LOS) condition with the transmitter, the computation of the mobile position in two dimensions becomes straightforward. In this paper, the problem of LOS detection in WCDMA for mobile positioning is considered, together with joint estimation of the delays and channel coefficients. These are very challenging topics in multipath fading channels because LOS component is not always present, and when it is present, it might be severely affected by interfering paths spaced at less than one chip distance (closely spaced paths). The extended Kalman filter (EKF) is used to estimate jointly the delays and complex channel coefficients. The decision whether the LOS component is present or not is based on statistical tests to determine the distribution of the channel coefficient corresponding to the first path. The statistical test-based techniques are practical, simple, and of low computation complexity, which is suitable for WCDMA receivers. These techniques can provide an accurate decision whether LOS component is present or not.

Subchip multipath delay estimation for downlink WCDMA system based on Teager-Kaiser operator

Accurate detection and estimation of overlapping fading multipath components is vital for many communication systems, particularly for positioning technologies. Traditional approaches used for channel estimation generally fail in estimating closely-spaced multipath components in code-division multiple access (CDMA) systems. Here, we present a highly efficient technique for asynchronous downlink WCDMA multipath delay estimation with subchip resolution capability based on nonlinear Teager-Kaiser operator concept. The behavior of this technique is influenced considerably by the pulse shape waveform. Both rectangular and root raised cosine pulse shaping filters are considered.

Polynomial-based maximum-likelihood technique for synchronization in digital receivers

Maximum-likelihood estimation theory provides a general framework for developing near-optimum (with respect to the Cramer-Rao bound) synchronization schemes for digital communication systems. A new technique for jointly estimating the symbol timing and carrier phase in digital receivers for linear digital modulations with nonsynchronized sampling is established for both data-aided and nondata-aided systems using a block-based feedforward architecture. This technique is a practical, fully digitally implemented synchronization concept based on a low-order polynomial approximation of the likelihood functions using the Farrow-based interpolator. It is shown that low oversampling, like two samples per symbol, can be used in timing recovery without compromising the system performance. This results in efficient overall receiver implementation. It is also demonstrated that the quality of the interpolator design has a big effect on the performance of the synchronization scheme. Frequency-domain optimized polynomial interpolator designs provide significantly better performance than the well-known Lagrange interpolators.

Performance analysis of relay selection schemes in underlay cognitive networks with Decode and Forward relaying

In underlay cognitive networks with regenerative relaying, secondary users operating with primary user are adhering to stringent interference constraint which limits their transmission power and coverage area. In order not to violate this interference limit, underlay network will make the use of relays to transmit signal over the secondary network. The retransmitting relay will be selected among the available secondary users; hence, relay selection becomes more challenging due to strict interference limits. Relay selection is based not only on interference limit, but also on threshold constraint which ensure the satisfactory reception of the signal by the relays and the destination and which is an important criterion since the relaying protocol used is Decode and Forward (DF). This paper proposes three new relay selection schemes based on relay to destination link and relay to primary link qualities. We derive closed form expressions of the probability density function (PDF) of the SNR at the secondary destination, the outage probability and the average bit error probability. Analytical results are validated by simulations and they provide comparative analysis of the different relay selection scheme proposed herein.

I/Q Imbalance in Multiple Beamforming {OFDM} Transceivers: SINR Analysis and Digital Baseband Compensation

In this paper, we start by investigating the impact of joint transmit-receive I/Q imbalance on the performance of direct-conversion beamforming OFDM transceivers. We derive a new analytical expression for the average subcarrier SINR of the I/Q imbalance-ignorant beamformer in terms of the I/Q imbalance level at both the transmit and receive sides, the size of the beamforming array, and the input SNR level. This expression motivates the need for I/Q imbalance compensation and provides valuable design insights when examining several special and limiting cases. Next, we derive the throughput-maximizing multiple beamforming transmit/receive coefficients under a general system model with an asymmetric frequency-dependent (FD) joint transmit-receive I/Q imbalance. In addition, we propose a low-complexity pilot-aided scheme for the estimation of the channel and I/Q imbalance parameters. Our simulation results demonstrate that the proposed generalized multiple beamforming scheme is highly effective in mitigating I/Q imbalance effects at practical complexity levels.

Blind Estimation of Large Carrier Frequency Offset in Wireless OFDM Systems

In this paper, a highly efficient blind band-edge filter-based technique is introduced for the estimation of carrier frequency offsets that are much larger than the subcarrier spacing in orthogonal-frequency-division-multiplexing (OFDM) systems. The proposed scheme is shown to have a high accuracy and an acquisition range that is much larger than the subcarrier spacing limitation inherent to other synchronization techniques. The performance of the proposed system as well as other well-referred techniques is assessed over additive white Gaussian noise and frequency-selective fading channels using different pulse shaping filters

Cloud-Ready Biometric System for Mobile Security Access

In this contribution, we introduce an application that allows a mobile phone to be used as a biometric-capture device for secure access to the cloud. In this application, the biometric capture and recognition are performed during a standard web session, using JQuery, a new mobile web framework which provides the technology needed to build web pages that act more like mobile applications rather than traditional web pages. The mobile user obtains service catalog through an interface developed for the Android system. We have used Hadoop, an open source cloud computing environment, to establish the connection between mobile user and server in the cloud via Ethernet, WiFi or 3G.

A survey of wireless data center networks

Data centers are becoming more and more popular for a wide variety of applications. However, the efficiency of data centers are restricted by many issues like cabling and maintenance problems in addition to performance problems like oversubscription. Wireless technology was proposed as it has the capability and the flexibility to offer feasible approaches to solve some of these problems. In this paper, we conduct a deep investigation about wireless data center networks, describe some works that enhance the performance of the network, then give some remarks and critiques.