Optimal Bandwidth and Power Allocation for Sum Ergodic Capacity under Fading Channels in Cognitive Radio Networks

This paper studies optimal bandwidth and power allocation in a cognitive radio network where multiple secondary users (SUs) share the licensed spectrum of a primary user (PU) under fading channels using the frequency division multiple access scheme. The sum ergodic capacity of all the SUs is taken as the performance metric of the network. Besides all combinations of the peak/average transmit power constraints at the SUs and the peak/average interference power constraint imposed by the PU, total bandwidth constraint of the licensed spectrum is also taken into account. Optimal bandwidth allocation is derived in closed-form for any given power allocation. The structures of optimal power allocations are also derived under all possible combinations of the aforementioned power constraints. These structures indicate the possible numbers of users that transmit at nonzero power but below their corresponding peak powers, and show that other users do not transmit or transmit at their corresponding peak power. Based on these structures, efficient algorithms are developed for finding the optimal power allocations.Comment: 28 pages, 6 figures, submitted to the IEEE Trans. Signal Processing in June 201

Joint Bandwidth and Power Allocation with Admission Control in Wireless Multi-User Networks With and Without Relaying

Equal allocation of bandwidth and/or power may not be efficient for wireless multi-user networks with limited bandwidth and power resources. Joint bandwidth and power allocation strategies for wireless multi-user networks with and without relaying are proposed in this paper for (i) the maximization of the sum capacity of all users; (ii) the maximization of the worst user capacity; and (iii) the minimization of the total power consumption of all users. It is shown that the proposed allocation problems are convex and, therefore, can be solved efficiently. Moreover, the admission control based joint bandwidth and power allocation is considered. A suboptimal greedy search algorithm is developed to solve the admission control problem efficiently. The conditions under which the greedy search is optimal are derived and shown to be mild. The performance improvements offered by the proposed joint bandwidth and power allocation are demonstrated by simulations. The advantages of the suboptimal greedy search algorithm for admission control are also shown.Comment: 30 pages, 5 figures, submitted to IEEE Trans. Signal Processing in June 201

Why Chinese airlines haven’t become leading cargo carriers - analyzing air freight network and international trade drivers for mainland China

In contrast to the tremendous growth in the passenger sector, Chinese airlines and logistics operators still play relatively minor roles in the world’s air cargo market. This study investigates the air freight network within mainland China using complex network analysis, and identifies the key drivers for international trade delivered by air with an augmented gravity model. Our network analysis suggests that domestic air freights flow through a relatively small point-to-point network. Demands are concentrated in the catchments of metropolitan regions, where passenger hubs have not served as cargo gateways. International air cargo flow is more balanced than China’s overall merchandise trade. As a result, foreign carriers can cherry-pick the most lucrative markets and link them to their global networks. Gravity model estimation suggests that for China’s international trade by air, the composition of the economy is a more important driver than the simple size of the economy. Therefore, air freight demand in China was not as high as past GDP numbers suggested, but is likely to outpace overall economic growth in the years to come. This should help Chinese airlines to achieve their cargo ambitions in the long term

Diurnal Variation of Tropical Ice Cloud Microphysics: Evidence from Global Precipitation Measurement Microwave Imager Polarimetric Measurements

The diurnal variation of tropical ice clouds has been well observed and examined in terms of the occurring frequency and total mass but rarely from the viewpoint of ice microphysical parameters. It accounts for a large portion of uncertainties in evaluating ice cloud's role on global radiation and hydrological budgets. Owing to the advantage of precession orbit design and paired polarized observations at a high-frequency microwave band that is particularly sensitive to ice particle microphysical properties, three years of polarimetric difference (PD) measurements using the 166 GHz channel of Global Precipitation Measurement Microwave Imager (GPM-GMI) are compiled to reveal a strong diurnal cycle over tropical land (30degS-30deg N) with peak amplitude varying up to 38%. Since the PD signal is dominantly determined by ice crystal size, shape, and orientation, the diurnal cycle observed by GMI can be used to infer changes in ice crystal properties. Moreover, PD change is found to lead the diurnal changes of ice cloud occurring frequency and total ice mass by about 2 hours, which strongly implies that understanding ice microphysics is critical to predict, infer, and model ice cloud evolution and precipitation processes