Showing papers by "Wan Choi published in 2017"

Content Placement for Wireless Cooperative Caching Helpers: A Tradeoff Between Cooperative Gain and Content Diversity Gain

Abstract: Depending on what and how caching helpers cache content in their finite storage, the caching helpers can offer either a content diversity gain by serving diverse content or a cooperative gain by jointly transmitting the same content. This paper identifies a tradeoff between the content diversity gain and the cooperative gain according to content placements and proposes a probabilistic content placement to optimally balance the tradeoff. Using stochastic geometry, we quantify this tradeoff by deriving the cache hit rate and the rate coverage probability. To efficiently control the tradeoff, we determine the near-optimal caching probabilities that maximize the average content delivery success probability with the cooperative caching helpers. Our analysis and numerical results reveal that our proposed content placement outperforms the conventional caching schemes, such as caching with uniform probabilities, caching the most popular contents, and caching the content maximizing the cache hit, in terms of the average content delivery success probability.

Optimal Content Placement for Wireless Femto-Caching Network

Abstract: This paper investigates optimal content placement for wireless femto-caching network. The average bit error rate (BER) is formulated as a function of content placement under wireless fading. To minimize the average BER, we propose a greedy algorithm finding optimal content placement with low-computational complexity. Exploiting the property of the optimal content placement which we derive, the proposed algorithm can be performed over considerably reduced search space. Contrary to the optimal content placement without consideration of wireless fading aspects, we reveal that optimal content placement can be reached by balancing a tradeoff between two different gains: file diversity gain and channel diversity gain. Moreover, we also identify the conditions that the optimal placement can be found without running the proposed greedy algorithm and derive the corresponding optimal content placement in closed form.

Beamforming for Full-Duplex Multiuser MIMO Systems

Abstract: We solve a sum rate maximization problem of full-duplex (FD) multiuser multiple-input–multiple-output (MU-MIMO) systems. Since additional self-interference (SI) in the uplink channel and cochannel interference (CCI) in the downlink channel are coupled in FD communication, the downlink and uplink multiuser beamforming vectors are required to be jointly designed. However, the joint optimization problem is nonconvex and difficult to solve due to the coupled effect. To properly address the coupled design issue, we reformulate the problem into an equivalent uplink channel problem by using the uplink and downlink channel duality known as multiple-access channel–broadcast channel duality (MAC–BC duality). Then, using a minorization–maximization (MM) algorithm based on an affine approximation, we obtain a solution for the reformulated problem. In addition, without any approximation and thus performance degradation, we develop an alternative algorithm based on iterative water filling (IWF) to solve the nonconvex problem. The proposed algorithms warrant fast convergence and low computational complexity.

Optimal file storing with cache memory in amorphous femto helper aided networks

Abstract: In this paper, we consider a cellular network, where a macro BS serves a single user with the aid of helper nodes, and find the optimal way to save a file in the helper nodes. Storing a file into distributed storage affects both hitting probability and successful recovery of the data at a user. With MDS coding, cache equipped helper nodes can store small fragments to improve the hitting probability. When a file is relatively important, we show that the optimal way to save a file is to save either the whole file or the smallest MDS fraction constrained on the amount of fragments. For less important file, we show that the conventional replica storing is optimal in terms of hitting probability.

The Degrees of Freedom of the Interference Channel With a Cognitive Relay Under Delayed Feedback

Abstract: This paper studies the interference channel with a cognitive relay under delayed feedback. Three types of delayed feedback are studied: delayed channel state information at the transmitter, delayed output feedback, and delayed Shannon feedback. Outer bounds are derived for the degrees of freedom (DoF) region of the two-user multiple-input multiple-output interference channel with a cognitive relay with delayed feedback as well as without feedback. For the single-input single-output scenario, optimal schemes are proposed based on retrospective interference alignment. It is shown that while a cognitive relay without feedback cannot improve the sum-DoF in the two-user single-input single-output interference channel, delayed feedback in the same scenario can increase the sum-DoF to $4/3$ . For the multiple-input multiple-output case, achievable schemes are obtained via extensions of retrospective interference alignment, leading to the DoF regions that meet the respective upper bounds.

Cluster-based collaborative transmission method in wireless communication system and apparatus therefor

Abstract: The present application discloses a method for receiving, by a terminal, data in a wireless communication system. Specifically, the method for receiving data comprises the steps of: receiving a first division code from a first helper node in a first time unit; overhearing a second division code transmitted from a second helper node to another terminal in a second time unit; and acquiring the data by using the first division code and the second division code.

The K-User Linear Deterministic Broadcast Channel with Receiver Memory

Abstract: This paper considers a cache-aided linear deterministic broadcast channel where receivers have memories. A novel multi-rate coded multicasting is proposed, which utilizes different signal strength levels at the receivers contrary to the conventional coded multicasting. We derive the required transmission time for given memory size, and analyze the memory- transmission time tradeoff which characterizes the relationship between memory size and required transmission time satisfying all user requests. A lower bound on the memory-transmission time tradeoff is also derived using cut-set bound and optimality of the proposed scheme is evaluated by comparing the achievable tradeoff of the proposed scheme with the lower bound. It is shown that the proposed scheme achieves much less required transmission time than the conventional coded multicasting when the signal strength levels at the users are different.

Method and apparatus for index-coded multicast transmission in a wireless communication system

Abstract: A method for transmitting downlink data in a multicast scheme by a base station in a wireless communication system is disclosed. The method includes receiving a first request signal requesting first data from a first user equipment (UE), receiving a second request signal requesting second data from a second UE, generating a transmission message by applying an exclusive OR (XOR) operation to the first data and the second data, and transmitting the transmission message to the first UE and the second UE in the multicast scheme.

Optimization of Index Code and Transmission Time for minimum Outage in Broadcast Channels

Abstract: In this paper, we find the optimal index coding scheme with transmission time allocation that minimizes overall outage probability in a single-input single-output broadcast channel. In our system model, a transmitter has a set of files (i.e., a file library), and each user requests a single file in the library. Meanwhile, each user has a memory so that may already know some files. When the transmitter can only use limited transmission time to serve the users, the overall outage probability varies according to the index code and transmission time allocation. To find the optimal index code and time allocation, we first derive the optimal time allocation for a given index code. Then, we propose the optimal index code searching algorithm. We also propose a pruning algorithm, which reduces the searching complexity while maintaining the optimality. Our simulation results show that our proposed scheme well minimizes the overall outage probability, and our proposed pruning algorithm significantly reduces the complexity of our proposed scheme.

Gains and limits of diversity techniques in cognitive radio systems

Abstract: Cognitive radio (CR) is one of promising techniques to alleviate the spectrum scarcity resulting from the increasing demand for wireless services and the exclusive spectrum allocation policy. CR allows unlicensed users (secondary users) to access the under-utilized spectrum assigned to licensed users (primary users) if quality-of-service (QoS) requirements of primary users can be satisfied. However, the performance of secondary users’ communications is severely degraded by the QoS constraints imposed on the secondary users for the primary users. Diversity techniques for the secondary users help overcome the performance degradation by mitigating or exploiting the fluctuation of fading channels. We overview recent results on diversity techniques in CR and discuss fundamental limits and gains of diversity techniques inCR systems.

Cognitive Relay in Interference Channel with Delayed Feedback: Degree of Freedom Region

Abstract: This paper studies a two-user single-input single- output (SISO) interference channel with a cognitive relay (ICCR) under delayed feedback. We consider three types of feedback: delayed channel state information at transmitter (CSIT), delayed output feedback, and no feedback. For each feedback information, we derive the optimal degrees of freedom (DoF) region of the two-user ICCR where delayed feedback information is available at all transmitters. We show that while a cognitive relay with no feedback cannot improve the sum DoF beyond 1 in the two-user interference channel, delayed feedback can increase the sum DoF to 4/3.