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The most commonly used setting in the coded caching literature consists of the following four elements: (i) homogeneous file sizes, (ii) homogeneous cache sizes, (iii) user-independent homogeneous file popularity (i.e., all users share the same file preference), and (iv) worst-case rate analysis. While recent results have relaxed some of these assumptions, deeper understanding of the full heterogeneity setting is still much needed since traditional caching schemes place little assumptions on file/cache sizes and almost always allow each user to have his/her own file preference through individualized file request prediction. Taking a microscopic approach, this paper characterizes the exact capacity of the smallest 2-user/2-file (N = K = 2) problem but under the most general setting that simultaneously allows for (i) heterogeneous files sizes, (ii) heterogeneous cache sizes, (iii) user-dependent file popularity, and (iv) average-rate analysis. Solving completely the case of N = K = 2 could shed further insights on the performance and complexity of optimal coded caching with full heterogeneity for arbitrary N and K. 

This paper presents a circuit for simultaneous reception of optical power and data using a solar cell. The circuit employs a switched-inductor boost DC-DC converter for energy harvesting and a low-power thresholding receiver for data reception. The thresholding data receiver comprises a current-sense resistor that monitors the current output of the solar cell, an instrumentation amplifier, a band-pass filter and a comparator. A system-level analysis of an optical communication system employing the proposed circuit is presented along with a circuit-level analysis and implementation. As a proof-of-concept, the proposed circuit for simultaneous power and data reception is implemented using off-the-shelf components and tested using a custom-built test setup. Measurement results, including harvested power, electronic noise and bit error rate (BER), are reported for a GaAs solar cell and a red LED light source. Results show that 223 μW of power are harvested by the DC-DC converter at a distance of 32.5 cm and a radiated power of 9.3 mW. At a modulation depth of 50% and a transmission speed of 2.5 kbps, a BER of 1.008×10^-3 is achieved. Measurement results reveal that the proposed solution exhibits a trade-off between harvested power, transmission speed and BER. 

Coded caching is a technique for reducing congestion in communication networks by prefetching content during idle periods and exploiting multicasting opportunities during periods of heavy traffic. Most of the existing research in this area has focused on minimizing the worst case (i.e., peak) rate in a broadcast link with multiple identically distributed user requests. However, modern content delivery networks are investing very heavily in profiling their users and predicting their preferences. The minimal achievable rate of a coded caching scheme with heterogeneous user profiles is still unknown in general. This paper presents the first steps towards solving that problem by analyzing the case of two users with distinct but overlapping demand sets. Specifically, it provides a complete characterization of the uniform-average-rate capacity when the sets overlap in just one file and shows that such capacity can be achieved with selfish and uncoded prefetching. Then, it characterizes the same capacity under selfish and uncoded prefetching when the demand sets overlap in two or more files. The paper also provides explicit prefetching schemes that achieve those capacities. All our results allow for arbitrary (and not necessarily identical) users’ cache sizes and number of files in each demand set. 

An optical wireless communication approach that exploits the photo-luminescent radiation of LEDs is presented. In this approach the photo-luminescence of an array of LEDs is modulated by varying the impedance connected to the LEDs. The LEDs are also employed to harvest radiant energy making possible fully passive optical communications tags. Possible applications of this approach include short-range underwater communications. Initial experimental results suggest that communication speeds of few kilobits per second can be achieved.