Mohammad Saquib

Novel Automatic Modulation Classification Using Cumulant Features for Communications via Multipath Channels

Nowadays, automatic modulation classification (AMC) plays an important role in both cooperative and non-cooperative communication applications. Very often, multipath fading channels result in the severe AMC performance degradation or induce large classification errors. The negative impacts of multipath fading channels on AMC have been discussed in the existing literature but no solution has ever been proposed so far to the best of our knowledge. In this paper, we propose a new robust AMC algorithm, which applies higher-order statistics (HOS) in a generic framework for blind channel estimation and pattern recognition. We also derive the Cramer-Rao lower bound for the fourth-order cumulant estimator when the AMC candidates are BPSK and QPSK over the additive white Gaussian noise channel, and it is a nearly minimum-variance estimator leading to robust AMC features in a wide variety of signal-to-noise ratios. The advantage of our new algorithm is that, by carefully designing the essential features needed for AMC, we do not really have to acquire the complete channel information and therefore it can be feasible without any a priori information in practice. The Monte Carlo simulation results show that our new AMC algorithm can achieve the much better classification accuracy than the existing AMC techniques.

Decorrelating detectors for a dual rate synchronous DS/CDMA system

This paper addresses the use of decorrelating detectors for a dual rate DS/CDMA system that serves both low bit rate and high bit rate users. All users have the same BPSK modulation technique and the same chip rate. The differences in bit transmission rates result in different processing gains for each class of user. We assume that in an interval of duration T/sub 0/, a low rate user transmits one bit while a high rate user transmits M bits. Applying a standard decorrelator to the interval of duration T/sub 0/ yields an M bit processing delay for high rate users and a computational complexity that grows with M. We propose a decorrelator that generates bit decisions for each high rate user in every subinterval of duration T/sub 0//M. To decode a low rate user, a soft decoding rule is considered. The soft decoding rule will apply maximal ratio combining on M separate decorrelated outputs of each low rate user. The soft decoding dual rate decorrelator eliminates the bit processing delay for high rate users and also reduces the computational complexity of a standard decorrelator. The proposed soft decoding decorrelator is found to perform nearly as well as the standard decorrelator while retaining the near-far resistance property.

Decorrelating Detectors for a Dual Rate Synchronous DS/CDMA System

This paper addresses the use of decorrelating detectors for a dual rate DS/CDMA system that serves both low bit rate and high bit rate users. We assume that in an interval of duration T0, a low rate user transmits one bit while a high rate user transmits M bits. Applying a standard decorrelator to the interval of duration T0 yields an M bit processing delay for high rate users and a computational complexity that grows with M. In this paper, we propose a decorrelator that generates bit decisions for each high rate user in every subinterval of duration T0/M. To decode a low rate user, a soft decoding rule applies maximal ratio combining on M separate decorrelated outputs of each low rate user. The soft decoding dual rate decorrelator eliminates the bit processing delay for high rate users and also reduces the computational complexity of a standard decorrelator. Further, it is shown that the asymptotic efficiency of the standard decorrelator is greater than or equal to the proposed decorrelator. By evaluation, it is observed that when the signature sequences have good correlation properties, the proposed soft decoding decorrelator is found to perform nearly as well as the standard decorrelator. However, when the signature sequences have high cross-correlation, then the proposed decorrelator suffers in terms of bit error rate while retaining the near-far resistance property.

Performance Analysis of a Cognitive Network with Dynamic Spectrum Assignment to Secondary Users

In a cognitive wireless network, sudden arrival of a primary user could force one or more secondary users to terminate their ongoing communication. This situation leads us to propose a cognitive wireless network which dynamically assigns service rates to the secondary users depending on the available network spectral resource. Our analysis is based on a two-dimensional Markov chain with three state variables. Performance metrics for the secondary users such as dropping probability, blocking probability and completion probability are derived. Our numerical result demonstrates that, depending on the network condition, the proposed dynamic spectrum assignment scheme is capable of reducing the dropping probability substantially and yielding higher throughput via increasing the maximum allowable service rate. Analysis of blocking probability implies that higher value of the maximum allowable service rate may or may not make the network more selective in terms of admitting new secondary users.

An asynchronous multirate decorrelator

This paper examines truncated window decorrelators for an asynchronous direct-sequence code-division multiple-access system supporting users transmitting at different bit rates. We decode a user by extending the observation window over a sufficient number of its bits. To characterize practical window sizes, simple upper and lower bounds for the asymptotic efficiency of both the truncated window and infinite window decorrelators are developed. Empirical results show that as the length of the observation window increases, the bounds converge rapidly to the asymptotic efficiency of the infinite window decorrelator. The complexity of the receiver depends strongly on the ratio of the maximum to minimum bit rates.

Optimal call admission to a mobile cellular network

To fulfil the future demand for and increase the reliability of the mobile cellular service, the authors have explored the use of a call admission policy that constitutes a dynamic channel allocation scheme for a cellular system. The call admission policy optimizes a weighted blocking criteria for a queueing network model of a mobile cellular system. In this context, the call admission problem is formulated as a Markov decision process. The value iteration method has been applied to a uniformized chain of a cellular highway system. The effectiveness of a call admission policy for maximizing the call completion rate and for reducing the handoff dropping is studied. Channel reservation policies for handoffs are also examined for different system parameters. In addition, several symmetry properties of the optimal policies which effectively truncate the size of policy space, are mathematically established.

A Fault-Tolerant Interconnect Mechanism for NMR Nanoarchitectures

Redundancy techniques, such as N -tuple modular redundancy (NMR), has been widely used to correct faulty behavior of components and achieve high reliability. Almost all redundancy-based strategies rely on a majority voting. The voter, therefore, becomes a critical unit for the correct operation of any NMR system. In this paper, we propose a voterless fault-tolerant strategy to implement a robust NMR system design. We show that using a novel fault-tolerant communication mechanism, namely logic code division multiple access, we can transfer data with extremely low error rates among N modules and completely eliminate the need for a centralized voter unit. Such a highly reliable strategy is vital for future nanosystems in which high defect rate is expected. Experimental results are also reported to verify the concept, clarify the design procedure, and measure the systems reliability.

Pilot-aided chip-interleaved DS-CDMA transmission over time-varying channels

Time-varying multipath fading associated with the wireless link limits the capacity of a wireless system. In order to adapt to this adverse radio environment efficiently, we investigate the use of a pilot-aided fade-resistant transmission scheme for the uplink of a chip-interleaved code-division multiple-access (CDMA) system. We analyze the tradeoff between the number of diversity branches and the channel estimation error. We derive the optimum ratio of pilot signal energy to information signal energy. Our numerical study indicates that the proposed system is capable of outperforming the conventional CDMA system depending on the transmitter energy and channel condition.

Analysis of the Channel Energy Capture in Ultra-Wideband Transmitted Reference Systems

In this letter, we analyze the expected value of the channel energy capture as a function of the integration interval of the correlator in a transmitted reference system. A modified Saleh-Valenzuela channel model is employed. We observe that for the system specifications (such as channel model, data rate, and signal-to-noise ratio) considered here, an integration interval that captures approximately 84%-89% of the expected value of the channel energy provides a bit-error probability close to the minimum one.

Performance Analysis Framework and Vertical Handover Triggering Algorithms for Voice over WLAN/Cellular Network

The development of handheld user equipments (UE) capable of operating over both cellular networks and wireless local area networks (WLAN) is an important step towards the evolution of next-generation integrated networks. For real-time applications, maintaining a seamless connectivity and an acceptable level of quality is quite important. Efficient vertical handoff algorithms are required to ensure the best exploitation of the available resources under certain quality constraints. This paper proposes a realistic performance analysis framework upon addressing relevant factors that affect the performance of a voice application within a heterogeneous network environment. While the existing works in this field relies on received signal strength (RSS) samples to estimate the WLAN usability, this paper also proposes an algorithm that takes into account the real-time packet error pattern along with RSS.