The phenomenal growth of mobile data demand has brought about increasing scarcity in available radio spectrum. Meanwhile, mobile customers pay more attention to their own experience, especially in communication reliability and service continuity on the move. To address these issues, LTE-Unlicensed, or LTEU, is considered one of the latest groundbreaking innovations to provide high performance and seamless user experience under a unified radio technology by extending LTE to the readily available unlicensed spectrum. In this article, we offer a comprehensive overview of the LTEU technology from both operator and user perspectives, and examine its impact on the incumbent unlicensed systems. Specifically, we first introduce the implementation regulations, principles, and typical deployment scenarios of LTE-U. Potential benefits for both operators and users are then discussed. We further identify three key challenges in bringing LTE-U into reality together with related research directions. In particular, the most critical issue of LTE-U is coexistence with other unlicensed systems, such as widely deployed WiFi. The LTE/WiFi coexistence mechanisms are elaborated in time, frequency, and power aspects, respectively. Simulation results demonstrate that LTE-U can provide better user experience to LTE users while well protecting the incumbent WiFi users’ performance compared to two existing advanced technologies: cellular/WiFi interworking and licensed-only heterogeneous networks (Het-Nets).

LTE-unlicensed: the future of spectrum aggregation for cellular networks

Millimeter-wave (mmWave) communications is a promising enabling technology for high rate (Giga-bit) multimedia applications. However, because of the high propagation loss at 60 GHz band, mmWave signal power degrades significantly over distance. Therefore, a traffic flow being transmitted over multiple short hops can attain higher throughput than that over a single long hop. In this paper, we first design a hop selection metric for the piconet controller (PNC) to select appropriate relay hops for a traffic flow, aiming to improve the flow throughput and balance the traffic loads across the network. We then propose a multi-hop concurrent transmission (MHCT) scheme to exploit the spatial capacity of mmWave WPANs by allowing nodes to transmit concurrently in communication links without causing harmful interference. The analysis of concurrent transmission probability and time division multiplexing demonstrates that the MHCT scheme is capable of improving the time slot utilization. Extensive simulations are conducted to validate the analytical results and demonstrate that the proposed MHCT scheme can improve the average traffic flow throughput and network throughput.

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Enabling Multi-Hop Concurrent Transmissions in 60 GHz Wireless Personal Area Networks

The pervasive adoption of IEEE 802.11 radios in the past decade has made possible for the easy Internet access from a vehicle, notably drive-thru Internet. Originally designed for the static indoor applications, the throughput performance of IEEE 802.11 in the outdoor vehicular environment is, however, still unclear especially when a large number of fast-moving users transmitting simultaneously. In this paper, we investigate the performance of IEEE 802.11 DCF in the highly mobile vehicular networks. We first propose a simple yet accurate analytical model to evaluate the throughput of DCF in the large scale drive-thru Internet scenario. Our model incorporates the high-node mobility with the modeling of DCF and unveils the impacts of mobility (characterized by node velocity and moving directions) on the resultant throughput. Based on the model, we show that the throughput of DCF will be reduced with increasing node velocity due to the mismatch between the MAC and the transient high-throughput connectivity of vehicles. We then propose several enhancement schemes to adaptively adjust the MAC in tune with the node mobility. Extensive simulations are carried out to validate the accuracy of the developed analytical model and the effectiveness of the proposed enhancement schemes.

MAC in Motion: Impact of Mobility on the MAC of Drive-Thru Internet

Mobile cloud computing is a promising technique that shifts the data and computing service modules from individual devices to a geographically distributed cloud service architecture. A general mobile cloud computing system is comprised of multiple cloud domains, and each domain manages a portion of the cloud system resources, such as the Central Processing Unit, memory and storage, etc. How to efficiently manage the cloud resources across multiple cloud domains is critical for providing continuous mobile cloud services. In this paper, we propose a service decision making system for interdomain service transfer to balance the computation loads among multiple cloud domains. Our system focuses on maximizing the rewards for both the cloud system and the users by minimizing the number of service rejections that degrade the user satisfaction level significantly. To this end, we formulate the service request decision making process as a semi-Markov decision process. The optimal service transfer decisions are obtained by jointly considering the system incomes and expenses. Extensive simulation results show that the proposed decision making system can significantly improve the system rewards and decrease service disruptions compared with the greedy approach.

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An SMDP-Based Service Model for Interdomain Resource Allocation in Mobile Cloud Networks

The IEEE 802.11-based WiFi wireless technology is one of the most promising technologies to provide ubiquitous networking access. The IEEE 802.11 working group has always strived to improve this wireless technology through creating new amendments to the base 802.11 standard. Recently, IEEE 802.11n amendment was created to enhance 802.11 for higher throughput operation. Not only new Physical Layer enhancements are standardized, but new Medium Access Control Layer mechanism are also defined. In this paper, we examine the network performance enhancement by the proposed 802.11n MAC layer features: aggregation, block acknowledgement, and reverse direction mechanism. We implemented a new 802.11n module in the NS-2 simulation platform. The simulation results demonstrated the effectiveness of 802.11n MAC layer enhancement. VoIP performance is effectively improved with 802.11n MAC enhancement.

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IEEE 802.11n MAC Enhancement and Performance Evaluation

An analytical model to study the performance of wireless local area networks (WLANs) supporting asymmetric nonpersistent traffic using the IEEE 802.11 distributed coordination function mode for medium access control (MAC) is developed. Given the parameters of the MAC protocol and voice codecs, the voice capacity of an infrastructure-based WLAN, in terms of the maximum number of voice connections that can be supported with satisfactory user-perceived quality, is obtained. In addition, voice capacity analysis reveals how the overheads from different layers, codec rate, and voice packetization interval affect voice traffic performance in WLANs, which provides an important guideline for network planning and management. The analytical results can be used for effective call admission control to guarantee the quality of voice connections. Extensive simulations have been performed to validate the analytical results

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Voice capacity analysis of WLAN with unbalanced traffic

In this paper, an analytical model is developed for the performance study of an IEEE 802.11n wireless local area network (WLAN) supporting voice and video services, considering the new features of the medium access control (MAC) protocol proposed in IEEE 802.11n, i.e., frame aggregation and bidirectional transmission. We show that these enhanced MAC mechanisms can effectively improve the network capacity by not only reducing the protocol overheads, but also smoothing the AP-bottleneck effect in an infrastructure-based WLAN. Voice and video capacity under various MAC mechanisms are compared as well.

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Supporting voice and video applications over IEEE 802.11n WLANs

There is an increasing interest in 60 GHz millimeter-wave (mmWave) communication technologies for multi-Gigabit wireless personal area networks (WPAN), aiming to support broadband multimedia applications. Internet Protocol TV (IPTV) is an emerging killer application which requires high data rate and stringent quality of services (QoS) in terms of delay and packet loss. In this paper, we propose a method to efficiently support high definition video flows in a mmWave-based WPAN with QoS guarantee, considering the characteristics of both the IPTV traffic and the mmWave communication technology. We first quantify the effective bandwidth of IPTV video sources using a simple, two-level Markov traffic model. Considering the overheads of the protocol stack in mmWave WPANs, we then quantify the minimum channel time needed for each IPTV flow. Since mmWave-based WPANs will deploy directional antennas to not only extend the transmission range, but also reduce the interference level to neighboring flows, we further propose an admission control scheme and scheduling algorithm to improve the network resource utilization by taking advantage of concurrent transmissions. Extensive simulations with NS-2 using real video traces have validated our analysis and demonstrated the efficiency and effectiveness of the proposed schemes, which will be an enabling technology for future mmWave-based WPANs supporting IPTV services.

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Resource Management and QoS Provisioning for IPTV over mmWave-based WPANs with Directional Antenna

We analyze the capacity of UWB networks supporting multimedia services by calculating the number of multimedia connections that can be supported in a UWB network based on IEEE 802.15.3 Medium Access Control (MAC) protocol, taking into consideration the overheads from different layers. We then propose how to increase the capacity by improving the MAC protocol design. To fully explore the potential of UWB technologies which favor concurrent transmissions if the interference is appropriately controlled, we study the capacity of cellular-like UWB networks. Our findings, which should provide important guidelines for UWB network planning, are a) the inter-cell interference of UWB networks is closely related to the Riemann Zeta function, and to guarantee the bounded inter-cell interference of UWB networks, the path loss exponent α must be larger than 2; b) the total throughput in an area is a concave function of the cell size; c) the best distance between adjacent cells is a function of path loss exponent, background noise level, and cross-correlation of the target signal and the interfering signal; and d) with the optimal cell size, a single flow's throughput is reduced by 2/α due to inter-cell interference. Simulation results are given to demonstrate the accuracy of the analysis.

Capacity of UWB networks supporting multimedia services

Although asymptotic bounds of wireless network capacity have been heavily pursued, the answers to the following questions are still critical for network planning, protocol and architecture design: Given a three-dimensional (3D) network space with the number of active users randomly located in the space and using the wireless communication technology, what are the expected per-flow throughput, network capacity, and network transport capacity? In addition, how can the protocol parameters be tuned to enhance network performance? In this paper, we focus on the ultra wideband (UWB) based wireless personal area networks (WPANs) and provide answers to these questions, considering the salient features of UWB communications, i.e., low transmission/interference power level, accurate ranging capability, etc. Specifically, we demonstrate how to explore the spatial multiplexing gain of UWB networks by allowing appropriate concurrent transmissions. Given 3D space and the number of active users, we derive the expected number of concurrent transmissions, network capacity and transport capacity of the UWB network. The results reveal the main factors affecting network (transport) capacity, and how to determine the best protocol parameters, e.g., exclusive region size, in order to maximize the capacity. Extensive simulation results are given to validate the analytical results.

Lin Cai

Papers

Voice capacity analysis of WLAN with unbalanced traffic

Supporting voice and video applications over IEEE 802.11n WLANs

Resource Management and QoS Provisioning for IPTV over mmWave-based WPANs with Directional Antenna

Capacity of UWB networks supporting multimedia services

Capacity analysis of UWB networks in three-dimensional space