Wireless Communications

Driven by the visions of Internet of Things and 5G communications, recent years have seen a paradigm shift in mobile computing, from the centralized mobile cloud computing toward mobile edge computing (MEC). The main feature of MEC is to push mobile computing, network control and storage to the network edges (e.g., base stations and access points) so as to enable computation-intensive and latency-critical applications at the resource-limited mobile devices. MEC promises dramatic reduction in latency and mobile energy consumption, tackling the key challenges for materializing 5G vision. The promised gains of MEC have motivated extensive efforts in both academia and industry on developing the technology. A main thrust of MEC research is to seamlessly merge the two disciplines of wireless communications and mobile computing, resulting in a wide-range of new designs ranging from techniques for computation offloading to network architectures. This paper provides a comprehensive survey of the state-of-the-art MEC research with a focus on joint radio-and-computational resource management. We also discuss a set of issues, challenges, and future research directions for MEC research, including MEC system deployment, cache-enabled MEC, mobility management for MEC, green MEC, as well as privacy-aware MEC. Advancements in these directions will facilitate the transformation of MEC from theory to practice. Finally, we introduce recent standardization efforts on MEC as well as some typical MEC application scenarios.

A Survey on Mobile Edge Computing: The Communication Perspective

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

The vision of next generation 5G wireless communications lies in providing very high data rates (typically of Gbps order), extremely low latency, manifold increase in base station capacity, and significant improvement in users’ perceived quality of service (QoS), compared to current 4G LTE networks. Ever increasing proliferation of smart devices, introduction of new emerging multimedia applications, together with an exponential rise in wireless data (multimedia) demand and usage is already creating a significant burden on existing cellular networks. 5G wireless systems, with improved data rates, capacity, latency, and QoS are expected to be the panacea of most of the current cellular networks’ problems. In this survey, we make an exhaustive review of wireless evolution toward 5G networks. We first discuss the new architectural changes associated with the radio access network (RAN) design, including air interfaces, smart antennas, cloud and heterogeneous RAN. Subsequently, we make an in-depth survey of underlying novel mm-wave physical layer technologies, encompassing new channel model estimation, directional antenna design, beamforming algorithms, and massive MIMO technologies. Next, the details of MAC layer protocols and multiplexing schemes needed to efficiently support this new physical layer are discussed. We also look into the killer applications, considered as the major driving force behind 5G. In order to understand the improved user experience, we provide highlights of new QoS, QoE, and SON features associated with the 5G evolution. For alleviating the increased network energy consumption and operating expenditure, we make a detail review on energy awareness and cost efficiency. As understanding the current status of 5G implementation is important for its eventual commercialization, we also discuss relevant field trials, drive tests, and simulation experiments. Finally, we point out major existing research issues and identify possible future research directions.

Next Generation 5G Wireless Networks: A Comprehensive Survey

The increasing demand of mobile Internet and the Internet of Things poses challenging requirements for 5G wireless communications, such as high spectral efficiency and massive connectivity. In this article, a promising technology, non-orthogonal multiple access (NOMA), is discussed, which can address some of these challenges for 5G. Different from conventional orthogonal multiple access technologies, NOMA can accommodate much more users via nonorthogonal resource allocation. We divide existing dominant NOMA schemes into two categories: power-domain multiplexing and code-domain multiplexing, and the corresponding schemes include power-domain NOMA, multiple access with low-density spreading, sparse code multiple access, multi-user shared access, pattern division multiple access, and so on. We discuss their principles, key features, and pros/cons, and then provide a comprehensive comparison of these solutions from the perspective of spectral efficiency, system performance, receiver complexity, and so on. In addition, challenges, opportunities, and future research trends for NOMA design are highlighted to provide some insight on the potential future work for researchers in this field. Finally, to leverage different multiple access schemes including both conventional OMA and new NOMA, we propose the concept of software defined multiple access (SoDeMA), which enables adaptive configuration of available multiple access schemes to support diverse services and applications in future 5G networks.

Non-orthogonal multiple access for 5G: solutions, challenges, opportunities, and future research trends

Wireless distributed machine learning is envisaged to facilitate advanced learning services and applications in wireless networks consisting of devices with limited computing capability. Distributed machine learning algorithms are more vulnerable in wireless systems since information exchange in learning is limited by wireless resources and channel conditions. Moreover, their performance can be significantly degraded by attacks of the Byzantine devices, and information distorted by channel fading can be treated as Byzantine attacks. Consequently, protection of wireless distributed machine learning from Byzantine devices is paramount. Leveraging over-the-air computation, we put forth a novel wireless distributed stochastic gradient descent system which is resilient to Byzantine attacks. The proposed learning system is underpinned by two novel and distinct features which enable more accurate and faster distributed machine learning resilient to Byzantine attacks: (1) collecting training data in the PS to obtain its own training results and (2) grouping the distributed devices. We derive upper bounds of the mean square error of the global parameter when the proposed algorithms are used in the cases with and without Byzantine devices, and prove the convergence of the proposed algorithms with the derived bounds. The effectiveness of the algorithms is validated by showing the accuracy and convergence speed.

Byzantine Fault Tolerant Distributed Stochastic Gradient Descent Based on Over-the-Air Computation

Disclosed is a repeater system having an oscillation preventing function and an automatic reverse output disabling function, and a control method thereof, in which the reverse output of the repeater system is disabled upon detection of a non-subscriber interval in the service area or a reverse oscillation so as to prevent a drop in the capacity of a base station transceiver system and protect a reception path. The repeater system includes: a first duplexer coupled to an external base station transceiver system; a transmitter for amplifying a signal received from the first duplexer into a service frequency; a second duplexer for transmitting the output signal of the transmitter to a mobile station located in a corresponding service area in a wireless manner, and receiving a signal from the mobile station; a receiver for amplifying the signal received from the second duplexer; and a controller, coupled to the output signal of the receiver, for detecting the power level of the receiver, the controller preventing the output of the receiver being sent to the first duplexer when the detected power level is above a first reference value during a first time period or below a second reference value during a second time period.

Control Method for a Repeater System Having Oscillation Preventing Function and Automatic Reverse Output Disabling Function for Non-Subscriber

From the perspective of a single user, occupying all available radio access technologies (RAT) is always beneficial for increasing data rate. However, due to the interactions among the users, occupying multiple RATs at each user might not be optimal in a network view point. This paper studies the optimal association strategy to maximize the system throughput in multi-RAT cellular networks. To this end, with stochastic geometry, we derive the network sum rates by considering two association modes; single RAT association (altruistic association) and multiple RATs association (selfish association). By comparing the network sum rates for the two association modes, we show that optimally balancing the number of users between two association modes is necessary to improve the performances. With some numerical examples, we investigate how various system parameters, such as the number of carrier components, user density, and density of access points, etc, affect on the optimal portion of the users between two association modes.

Performance enhancement via RAT association control in multi-RAT cellular networks

Coordinated beamforming (CBF) provides improved sum rate with less overhead and computational complexity, and its theoretical performance analysis is mostly finished in a spatially uncorrelated environment. In this paper, we investigate the effect of spatial correlation on the sum rate of a two-cell multiuser downlink channel. We analyze the scaling laws of the sum rate with a large number of users. To improve the performance, different roles are assigned to the two cells: One is a master cell, and the other is a slave cell. The base station (BS) of the master cell requests the interference cancelation to the slave cell, and the BS of the slave cell performs precoding to cancel the interference influencing over the master cell, although the achievable rate of the slave cell is decreased. The decision of the master and slave cells can be affected by the number of users and the degree of spatial correlations. From computer-based simulations, we show that our proposed algorithm can achieve a higher sum rate than other previous approaches with lower overhead.

Asymptotic Sum Rate in Spatially Correlated Two-Cell Channel With User Scheduling

In Compressed Sensing based Random Access CHannel (CS-RACH) protocol, a base station leverages compressed sensing technique to detect the active users in the cell coverage and estimate the channel gain between the users and the base station. In a real communication scenario, activity of a specific user usually varies time to time and thus can be seen as a random variable following ON/OFF distribution. Meanwhile, the performance of compressed sensing technique is dependent on the sparsity of the estimating vector, which is closely related to the user activity in CS-RACH scenario. In this perspective, we analyze the condition of the user activity for the stable operation of the protocol. Particularly, we use the least absolute shrinkage and selection operator (LASSO) approach, which gives a closed form expressions of the sparsity condition for the successful active user detection in an asymptotic manner. As a result, we obtain the condition of the user activity for stable operation of CS-RACH and verify the result with numerical simulations.

Wan Choi

Papers

Byzantine Fault Tolerant Distributed Stochastic Gradient Descent Based on Over-the-Air Computation

Control Method for a Repeater System Having Oscillation Preventing Function and Automatic Reverse Output Disabling Function for Non-Subscriber

Performance enhancement via RAT association control in multi-RAT cellular networks

Asymptotic Sum Rate in Spatially Correlated Two-Cell Channel With User Scheduling

Analysis on User Activity in Compressed Sensing based Random Access