Abdallah K. Farraj

Queue performance measures for cognitive radios in spectrum sharing systems

This paper investigates the queuing system of a cognitive user who is trying to communicate, using spectrum sharing methods, over a channel reserved to a licensed user. For the cognitive user to use the channel, its transmission power should satisfy the outage probability requirement of the primary user. In this environment, the cognitive user is found to have an M/G/1 queue model, where the queue service rate is equal to the cognitive channel capacity. Some queue performance measures are investigated. These include the first two moments of the service time process, mean waiting and transit times, mean number of waiting and transit packets, mean duration of server busy period, and mean number of packets served during a busy period. We also investigate the effects of changing some of the communication environment parameters on the queue behavior.

Impact of Quality of Service Constraints on the Performance of Spectrum Sharing Cognitive Users

This article investigates the impact of choosing a specific quality of service constraint on the performance of the cognitive users in a spectrum sharing cognitive environment. To communicate over a wireless channel reserved to a primary user, the cognitive user has to satisfy the primary user’s quality of service constraint. Cognitive systems under interference temperature and outage probability constraints are investigated in this work. The outage probability of the primary user in an interference temperature-constraint environment is analyzed, and the performance measures of the cognitive users are developed. A comparative study of the cognitive user’s performance under equivalent outage probability and interference temperature constraints is conducted as well. Numerical results are presented to verify the theoretical analysis and compare the performance measures under these constraints. Results of this work illustrate the effects of the communication environment parameters on the cognitive users and detail the performance differences between the equivalent outage probability-constraint and interference temperature-constraint cognitive systems.

Performance of Primary Users in Spectrum Sharing Cognitive Radio Environment

This paper investigates the performance of the primary user in a multiuser cognitive radio environment. Using spectrum sharing method, multiple cognitive users compete to share a channel dedicated to the primary user in order to transmit their data to a common receiver unit. Only one cognitive user is scheduled to share the channel, and to do so, its transmit power should not increase the primary user’s outage probability above a certain limit. The average bit error rate and the average channel capacity of the primary user are derived in this setup for different cognitive scheduling schemes. The performance measures of the primary user are found to depend on the cognitive user’s scheduling criterion and on the parameters of the communication environment.

Scheduling in a Spectrum-Sharing Cognitive Environment Under Outage Probability Constraint

This article investigates a multiuser cognitive environment where secondary users compete to communicate over a channel licensed to a primary user using spectrum sharing. In this environment, both the primary and secondary users transmit to the same receiver unit, and the transmission power of the scheduled secondary user should satisfy the outage probability requirement of the primary user. Secondary users are ranked according to their channel strength, and performance measures are derived as a function of a generic channel rank. Bit error rate, channel capacity, and generated interference are investigated as performance measures. In addition, the performance of the proportional fair, random selection, and round-robin scheduling algorithms are investigated. Numerical results are presented to verify the theoretical analysis and investigate the effects the parameters of the communication environment have on the performance measures and the transmission power of the cognitive users.

Channel Characterization for Acoustic Downhole Communication Systems

The ability to communicate between downhole and surface instruments became a critical need as well operators monitor flow rate, temperature, and pressure data to facilitate well performance optimization and maintenance. The use of wire lines for communication between downhole and surface is common, but these installations present cost, maintenance, and reliability issues. Wireless communications technology using acoustic waves is an interesting alternative to these wired systems. While the acoustic technology offers great benefits, a clear understanding of its propagation aspects inside the wells is lacking. A testbed was built to investigate the propagation of acoustic signals over production pipes. The testbed comprises an acoustic tool that transmits data to the well surface without cables, an internally-developed receiver unit, and five segments of 7 inch production tubing that form a pipe string. Acoustic waves propagate in this setup by vibrating the pipes body, without interfering with the surrounding medium. Moreover, to study the effect of cemented pipes on wave propagation, the third pipe segment was encased in concrete. An impulse signal was fed to the acoustic tool, and channel impulse response readings were taken along the pipe string. The measurements were analyzed to understand the propagation aspects of acoustic waves. This work shows that acoustic waves experience dispersion and frequency-dependent attenuation over the pipe string; the pipe string appears as a frequency selective channel. The concrete segment filters out a considerable amount of energy in the higher frequency band and introduces further attenuation and dispersion. Signal processing algorithms are proposed to reduce the distortion and dispersion introduced by the pipe string channel on the acoustic waves. Technical contributions include: finding the impulse response of the channel along the pipe string, investigating the power delay profile, power spectral density, and signal-to-noise ratio measures, and studying the channel dispersion parameters.

Performance of Cognitive Radios in Dynamic Fading Channels Under Primary Outage Constraint

This article investigates the performance of a cognitive user in a dynamic fading environment. The cognitive user communicates, using a spectrum-sharing technology, over a channel reserved to a primary user (PU). The transmit power of the cognitive user satisfies the outage probability constraint of the PU. The channels of the primary and cognitive users experience independent and non-identically distributed fading models. Nakagami-

Efficient Decoding for Generalized Quasi-Orthogonal Space–Time Block Codes

m

Minimum Cross Correlation Spreading Codes

m and hyper-Nakagami-