253 research outputs found
Age-Optimal Information Updates in Multihop Networks
The problem of reducing the age-of-information has been extensively studied
in the single-hop networks. In this paper, we minimize the age-of-information
in general multihop networks. If the packet transmission times over the network
links are exponentially distributed, we prove that a preemptive Last Generated
First Served (LGFS) policy results in smaller age processes at all nodes of the
network (in a stochastic ordering sense) than any other causal policy. In
addition, for arbitrary general distributions of packet transmission times, the
non-preemptive LGFS policy is shown to minimize the age processes at all nodes
of the network among all non-preemptive work-conserving policies (again in a
stochastic ordering sense). It is surprising that such simple policies can
achieve optimality of the joint distribution of the age processes at all nodes
even under arbitrary network topologies, as well as arbitrary packet generation
and arrival times. These optimality results not only hold for the age
processes, but also for any non-decreasing functional of the age processes.Comment: arXiv admin note: text overlap with arXiv:1603.0618
Capacity of Compound MIMO Gaussian Channels with Additive Uncertainty
This paper considers reliable communications over a multiple-input
multiple-output (MIMO) Gaussian channel, where the channel matrix is within a
bounded channel uncertainty region around a nominal channel matrix, i.e., an
instance of the compound MIMO Gaussian channel. We study the optimal transmit
covariance matrix design to achieve the capacity of compound MIMO Gaussian
channels, where the channel uncertainty region is characterized by the spectral
norm. This design problem is a challenging non-convex optimization problem.
However, in this paper, we reveal that this problem has a hidden convexity
property, which can be exploited to map the problem into a convex optimization
problem. We first prove that the optimal transmit design is to diagonalize the
nominal channel, and then show that the duality gap between the capacity of the
compound MIMO Gaussian channel and the min-max channel capacity is zero, which
proves the conjecture of Loyka and Charalambous (IEEE Trans. Inf. Theory, vol.
58, no. 4, pp. 2048-2063, 2012). The key tools for showing these results are a
new matrix determinant inequality and some unitarily invariant properties.Comment: 8 pages, submitted to IEEE Transactions on Information Theor
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