We investigate a K-user parallel packet-erasure broadcast channel. There is an ongoing effort to harness millimeter-wave bands, which are known to be unstable having high outage probabilities, by combining them with stable legacy bands. Motivated by this effort, we consider a heterogeneous scenario in which the parallel subchannels are categorized into two classes having different outage probabilities. For the two-user case, we characterize the sum capacity by developing an explicit achievable scheme and deriving a matching upper bound. In contrast to suboptimal schemes that apply coding on a per-subchannel basis only, our scheme applies coding across subchannels to exploit coding opportunities that arise from asymmetric outage probabilities more efficiently, thereby achieving optimality. By extending our scheme systematically to be applicable for the K-user case, we show that it can provide significant gains over existing schemes. Compared to the K-user scheme currently employed in practice, which allocates chunks of subchannels to users exclusively, we demonstrate the performance improvement attainable by our scheme to be substantial, as the multiplicative gain scales with K. Moreover, we find that our scheme outperforms a per-subchannel extension of state-of-the-art K-user schemes by large margins, further reducing the optimality gap. Our results suggest a potential coding scheme that can be employed in future wireless systems to meet ever-growing mobile data demands.

We investigate the benefits of incorporating relays in future multi-user wireless networks that seek to exploit unexplored bands of very high frequency spectrum, where transmitted signals are known to be highly susceptible to outages. To this end, we examine a two-user bursty MIMO Gaussian interference channel with an in-band relay, where Bernoulli random states conceptually capture signal outages. As our main result, we show that an in-band relay can provide a degrees of freedom (DoF) gain in this bursty channel. This beneficial role of in-band relays in the bursty channel is in direct contrast to their role in the non-bursty channel which is not as significant to provide a DoF gain. More importantly, we demonstrate that in certain antenna configurations, an in-band relay can help achieve interference-free performances with increased DoF. We find the benefits particularly substantial in high-outage circumstances, as the DoF gain can grow linearly with the number of antennas at the relay. In this paper, first we derive an outer bound from which we obtain a necessary condition for interference-free DoF performances. Then we develop a novel scheme that exploits information of the bursty channel states to achieve them.

Motivated by recent efforts to harness millimeter-wave (mmWave) bands, known to have high outage probabilities, we explore a K-user parallel packet-erasure broadcast channel that consists of orthogonal subchannels prone to packet-erasures. Our main result is two-fold. First, in the homogeneous channel where all subchannels have the same erasure probability, we show that the separation principle holds, i.e., coding across subchannels provides no gain. Second, in the heterogeneous channel where the subchannels have different erasure probabilities, we devise a scheme that employs coding across subchannels and show that the principle fails to hold, i.e., coding across subchannels provides a gain. Inspired by this finding, we demonstrate our scheme to be effective in harnessing the mmWave bands. Compared to the current approach in the 4G systems which allocates subchannels to users exclusively, we show that our scheme offers a huge gain. We find the gain to be significant in scenarios where the erasure probabilities are largely different, and importantly to increase with the growth of K. Our result calls for joint coding schemes in future wireless systems to meet growing mobile data demands.

We explore the top-K rank aggregation problem in which one aims to recover a consistent ordering that focuses on top-K ranked items based on partially revealed preference information. We examine an M-wise comparison model that builds on the Plackett-Luce (PL) model where for each sample, M items are ranked according to their perceived utilities modeled as noisy observations of their underlying true utilities. As our result, we characterize the minimax optimality on the sample size for top-K ranking. The optimal sample size turns out to be inversely proportional to M. We devise an algorithm that effectively converts M-wise samples into pairwise ones and employs a spectral method using the refined data. In demonstrating its optimality, we develop a novel technique for deriving tight $\ell_\infty$ estimation error bounds, which is key to accurately analyzing the performance of top-K ranking algorithms, but has been challenging. Recent work relied on an additional maximum-likelihood estimation (MLE) stage merged with a spectral method to attain good estimates in $\ell_\infty$ error to achieve the limit for the pairwise model. In contrast, although it is valid in slightly restricted regimes, our result demonstrates a spectral method alone to be sufficient for the general M-wise model. We run numerical experiments using synthetic data and confirm that the optimal sample size decreases at the rate of 1/M. Moreover, running our algorithm on real-world data, we find that its applicability extends to settings that may not fit the PL model.

We explore the role of a relay in multiuser networks where some physical perturbation shared around the users may generate data traffic for them simultaneously, hence cause their transmission patterns to be correlated. We investigate how the gain from the help of a relay varies with correlations across the users' transmission patterns in a bursty multiple access channel where the users send signals intermittently. As our main results, we show that in most cases a relay can provide a greater degrees-of-freedom (DoF) gain when the users' transmission patterns are more correlated. Furthermore, we demonstrate that the DoF gain can scale with the number of users.

We explore the benefits of relays1in multi-user wireless networks with bursty user traffic, where intermittent data traffic restricts the users to bursty transmissions. Specifically, we investigate a two-user bursty MIMO Gaussian interference channel (IC) with a relay, where two Bernoulli random states govern the bursty user traffic. We show that an in-band relay can provide a degrees of freedom (DoF) gain in this bursty channel. This beneficial role is in contrast to the role in the non-bursty channel which is not as significant to provide a DoF gain. More importantly, we demonstrate that for certain antenna configurations, an in-band relay can help achieve an interference-free performance with increased DoF. Particularly, we find the benefits of a relay substantial with low user traffic, as the DoF gain can scale linearly with the number of antennas at the relay. To this end, we derive an outer bound from which we obtain a necessary condition for interference-free DoF performances. Then, we develop a novel scheme that exploits information of the bursty traffic states to achieve the performances.