Source Coding for Synthesizing Correlated Randomness

We consider a scenario wherein two parties Alice and Bob are provided $X_{1}^{n}$ and $X_{2}^{n}$ samples that are IID from a PMF $p_{X_1 X_2}$. Alice and Bob can communicate to Charles over (noiseless) communication links of rate $R_1$ and $R_2$ respectively. Their goal is to enable Charles generate samples $Y^{n}$ such that the triple $(X_{1}^{n},X_{2}^{n},Y^{n})$ has a PMF that is close, in total variation, to $\prod p_{X_1 X_2 Y}$. In addition, the three parties may posses shared common randomness at rate $C$. We address the problem of characterizing the set of rate triples $(R_1,R_2,C)$ for which the above goal can be accomplished. We provide a set of sufficient conditions, i.e., an achievable rate region for this three party setup. Our work also provides a complete characterization of a point-to-point setup wherein Bob is absent and Charles is provided with side-information.Comment: 30 page

Quantum soft-covering lemma with applications to rate-distortion coding, resolvability and identification via quantum channels

We propose a quantum soft-covering problem for a given general quantum channel and one of its output states, which consists in finding the minimum rank of an input state needed to approximate the given channel output. We then prove a one-shot quantum covering lemma in terms of smooth min-entropies by leveraging decoupling techniques from quantum Shannon theory. This covering result is shown to be equivalent to a coding theorem for rate distortion under a posterior (reverse) channel distortion criterion [Atif, Sohail, Pradhan, arXiv:2302.00625]. Both one-shot results directly yield corollaries about the i.i.d. asymptotics, in terms of the coherent information of the channel. The power of our quantum covering lemma is demonstrated by two additional applications: first, we formulate a quantum channel resolvability problem, and provide one-shot as well as asymptotic upper and lower bounds. Secondly, we provide new upper bounds on the unrestricted and simultaneous identification capacities of quantum channels, in particular separating for the first time the simultaneous identification capacity from the unrestricted one, proving a long-standing conjecture of the last author.Comment: 29 pages, 3 figures; v2 fixes an error in Definition 6.1 and various typos and minor issues throughou

Quantum Natural Gradient with Efficient Backtracking Line Search

We consider the Quantum Natural Gradient Descent (QNGD) scheme which was recently proposed to train variational quantum algorithms. QNGD is Steepest Gradient Descent (SGD) operating on the complex projective space equipped with the Fubini-Study metric. Here we present an adaptive implementation of QNGD based on Armijo's rule, which is an efficient backtracking line search that enjoys a proven convergence. The proposed algorithm is tested using noisy simulators on three different models with various initializations. Our results show that Adaptive QNGD dynamically adapts the step size and consistently outperforms the original QNGD, which requires knowledge of optimal step size to {perform competitively}. In addition, we show that the additional complexity involved in performing the line search in Adaptive QNGD is minimal, ensuring the gains provided by the proposed adaptive strategy dominates any increase in complexity. Additionally, our benchmarking demonstrates that a simple SGD algorithm (implemented in the Euclidean space) equipped with the adaptive scheme above, can yield performances similar to the QNGD scheme with optimal step size. Our results are yet another confirmation of the importance of differential geometry in variational quantum computations. As a matter of fact, we foresee advanced mathematics to play a prominent role in the NISQ era in guiding the design of faster and more efficient algorithms.Comment: 14 page

A Complete Solution to LTE-U and Wi-Fi Hidden Terminal Problem

With the exponential growth in mobile data traffic, mobile operators are facing the unfortunate limit on the availability of licensed spectrum which has however, led to the popularity of Long Term Evolution (LTE) in unlicensed spectrum (LTE-U). Undeniably, it is expected from LTE-U that it fairly shares the spectrum with Wi-Fi. Along with fair sharing, efficient utilization of the unlicensed spectrum is also equally important, which in some sense requires coordination between the two Radio Access Technologies (RATs) viz., LTE-U and Wi-Fi. In particular, the hidden terminal scenario between LTE-U and Wi-Fi, resulting mainly due to lack of coordination, threatens the spectrum utilization of unlicensed spectrum. Focusing on this hidden terminal problem between LTE-U and Wi-Fi, we highlight the deficiency of existing technologies from the Wi-Fi perspective, both at the user level and at the network level. We then propose a novel coexistence technique (similar to RTS-CTS mechanism in Wi-Fi) that solves the hidden terminal problem between LTE-U and Wi-Fi, and subsequently addresses the spectrum underutilization problem caused by hidden terminal collisions. The proposed mechanism achieves this by using a modified CTS frame of Wi-Fi. We have validated our proposed mechanism using a mathematical framework demonstrating its credibility

LAW: A Novel Mechanism for Addressing Hidden Terminal Problem in LTE-U and Wi-Fi Networks

Recently, the use of LTE in unlicensed spectrum (LTE-U) has gained a lot of attention. One of the daunting tasks before any such employment was to ensure the fair sharing of unlicensed spectrum between LTE-U and Wi-Fi radio access technologies (RATs), which now seems to be well addressed in the literature. However, along with fair sharing, the efficient utilization of unlicensed spectrum is also of profound significance, which pushes the need for coordination between LTE-U and Wi-Fi. Hence, this letter proposes a novel LTE-U and Wi-Fi (LAW) inter-RAT coordination mechanism for a more efficient utilization of the unlicensed spectrum. The aim is to address the inter-RAT hidden terminal problem between LTE-U and Wi-Fi and, thereby, offer better spectral efficiency. We modify the regular clear-to-send (CTS)-to-self frame and suggest the transmissions of modified CTS-to-self from LTE-U nodes to solve this hidden terminal issue. Further gains are extracted by allowing the simultaneous transmissions of LTE-U and Wi-Fi whenever possible

On the Impact of Duty Cycled LTE-U on Wi-Fi Users: An Experimental Study

The deployment of LTE in unlicensed spectrum is a plausible solution to meet explosive traffic demand from mobile users. However, fair coexistence with the existing unlicensed technologies, mainly Wi-Fi, needs to be ensured before any such deployment. Duty cycled LTE (LTE-U) is a simple and an easily adaptable scheme which helps in fair coexistence with the Wi-Fi. Nonetheless, the immense deployment of Wi-Fi necessitates a user-oriented study to find the effects of LTE-U operation, primarily in scenarios where the LTE-U eNB remains hidden from Wi-Fi Access Point. To delineate these effects, we perform a user-level Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) throughputs study of Wi-Fi in the presence of LTE-U using a testbed. Since, TCP is a more complicated protocol, we analyzed the Congestion Window and Round Trip Time data to comprehend the throughput results. This further explains the unfairness in throughput distribution among Wi-Fi users. Furthermore, we also notice inability among the disadvantaged users to receive the periodic Wi-Fi beacon frames successfully. The reasons and the subsequent consequences of throughput unfairness and beacon losses, are carefully elaborated. Also, to validate the beacon loss results, we present a beacon loss analysis which provides a mathematical expression to find the beacon loss percentage. Finally, we examine the results and highlight a need for incorporating additional functionalities in either LTE-U or Wi-Fi to overcome the present challenges

LTE-U and Wi-Fi hidden terminal problem: How serious is it for deployment consideration?

The deployment of LTE in unlicensed spectrum is a plausible solution to meet explosive traffic demand from mobile users. However, fair coexistence with the existing unlicensed technologies, mainly Wi-Fi, needs to be ensured before any such deployment. Duty cycled LTE (LTE-U) is a simple and an easily adaptable scheme which helps in fair coexistence with the Wi-Fi. Nonetheless, the immense deployment of Wi-Fi necessitates a user-oriented study to find the effects of LTE-U operation, primarily in scenarios where the LTE-U eNB remains hidden from Wi-Fi Access Point. To comprehend these effects, we perform a user-level throughput study of Wi-Fi in the presence of LTE-U using a testbed and observe a clear unfairness in throughput distribution among Wi-Fi users. Furthermore, we also notice inability among the disadvantaged users to receive the periodic Wi-Fi beacon frames successfully. The reasons and the subsequent consequences, of throughput unfairness and beacon losses, are carefully elaborated. Also, to validate the beacon loss results, we present a beacon loss analysis which provides a mathematical expression to find the beacon loss percentage. Finally, we examine the results and highlight a need for incorporating additional functionalities in either LTE-U or Wi-Fi to overcome the present challenges