Daniel Tweed

Network-Aware Adaptive LBT (NALT) Coexistence Mechanism

In the absence of coordination between radio access technologies (RATs), and with the goal of deploying unlicensed LTE without requiring changes to the Wi-Fi MAC layer, it falls to the LTE base stations to ensure fair coexistence. The multiple access method used in Wi-Fi is designed for fair sharing of the channel with devices operating toward the same goal. Following this paradigm, if LAA-LTE is not carefully designed to ensure fairness, it can easily lead to Wi-Fi stations being barred from the channel. The greatest gains in fair coexistence are achieved when LAA-LTE behaves in as Wi-Fi-like a manner as possible; however, this may not allow LTE to make the best use of the channel. In this chapter, a Network-aware Adaptive LBT mechanism (NALT) is presented which passively monitors both channel conditions and usage activity to maximize transmission opportunities while respecting fair sharing of the channel, all in a way that is transparent to incumbent Wi-Fi devices. Simulation results are presented demonstrating the effectiveness of NALT in providing proportional fair sharing among LAA-LTE and Wi-Fi devices.

IEEE 802.11/Wi-Fi Medium Access Control: An Overview

In order to understand and analyze the interactions between U-LTE and Wi-Fi when sharing the 5 GHz frequency band, knowledge of the architectures and medium access control (MAC) mechanisms currently adopted by these two technologies is required. This chapter focuses on the Wi-Fi technology and begins with an overview of the evolution of IEEE 802.11/Wi-Fi. Five existing generations, as well as the next generation, of IEEE 802.11/Wi-Fi are presented. The majority of this chapter provides the underlying ideas and detailed mechanisms of the CSMA/CA MAC protocol. Important observations on how CSMA/CA senses and occupies the radio medium when the LTE network is operating in the vicinity are highlighted.

LTE-Advanced: An Overview

This chapter provides a high-level overview of LTE-Advanced (LTE-A) networks and associated technologies to form a basis for discussion of the coexistence issues that exist for unlicensed LTE and Wi-Fi. Understanding the underlying architecture and protocols employed in LTE-A networks will provide a comparative framework to grasp how, and at what levels, LTE and Wi-Fi networks may interact and interfere with each other, and form a greater understanding of the challenges to be address in designing coexistence mechanisms. Specifically, this chapter will overview the LTE-A network, as well as its capabilities and protocols, with specific emphasis on the physical layer and medium access sub-layers to illuminate specific sources of coexistence issues. Proposed changes which may be included in future LTE releases are discussed in the context of LTE/Wi-Fi coexistence.

Open Questions and Potential Research Directions

Since U-LTE is a nascent LTE technology, the coexistence of this technology and Wi-Fi remains one of the most active research topics and working areas. Based on observations obtained from the survey and our study presented in Chaps. 5 and 6, this chapter attempts to highlight a number of open research questions and issues. Potential solutions to those issues are also identified. Primarily, the cooperation of LTE and Wi-Fi so that they could have a better understanding of each other when operating in the same area using the same radio frequency band is suggested. Such understanding can be used to take more vigilant action and help to avoid aggressive channel access that could corrupt ongoing transmissions and to design relevant protocols for a fair spectrum sharing.

Requirements and Regulations in the 5 GHz Unlicensed Spectrum

Licenses and fees are not required for operators to use the 5 GHz unlicensed spectrum. However, in order to avoid interference and to ensure a fair use of this resource, numerous requirements and regulations are imposed by national and international organizations. When operating in this band, the emerging U-LTE technology needs to adhere to these regulations, as any other existing technologies, especially IEEE 802.11/Wi-Fi, would. This chapter provides an overview on the radio spectrum resources in these bands and the related management and allocation concerns. It then summarizes a number of the key requirements and regulations specified by the European Telecommunications Standards Institute (ETSI) and the Federal Communications Commission (FCC) on radio channels, operating channel selection, transmission power, and channel access rules. These technical details are the baselines to be followed when designing medium access control protocols for U-LTE and any other technologies operating in the 5 GHz unlicensed radio band.

U-LTE and Wi-Fi Coexistence: A Survey

Coexistence between U-LTE and Wi-Fi networks is a deciding factor on the acceptance of U-LTE. As a result, a large number of studies have been carried out to identify what could be the effects that U-LTE may cause to Wi-Fi and which mechanisms could be used to ensure that these two technologies share the 5 GHz unlicensed frequency band in an efficient and fair manner. This chapter presents a survey on related work to answer the following questions: (i) What issues arise from simultaneous operation of LTE and Wi-Fi in the same spectrum bands? (ii) Which technology is affected the most? and (iii) Which factors determine the impacts of U-LTE to Wi-Fi?. It also identifies the strengths and weaknesses of existing solutions and suggests potential strategies to improve performance of these two technologies.

Outage-Constrained Resource Allocation in Uplink NOMA for Critical Applications

In this paper, we consider the resource allocation problem for uplink non-orthogonal multiple access (NOMA) networks whose users represent power-restricted but high priority devices, such as those used in sensor networks supporting health and public safety applications. Such systems require high reliability and robust resource allocation techniques are needed to ensure performance. We examine the impact on system and user performance due to residual cancellation errors resulting from imperfect successive interference cancellation (SIC) and apply the chance-constrained robust optimization approach to tackle this type of error. In particular, we derive an expression for the user outage probability as a function of SIC error variance. This result is used to formulate a robust joint resource allocation problem that minimizes user transmit power subject to rate and outage constraints of critical applications. As the proposed optimization problem is inherently non-convex and NP-hard, we apply the techniques of variable relaxation and complementary geometric programming to develop a computationally tractable two-step iterative algorithm based on successive convex approximation. Simulation results demonstrate that, even for high levels of SIC error, the proposed robust algorithm for NOMA outperforms the traditional orthogonal multiple access case in terms of user transmit power and overall system density, i.e., serving more users over fewer sub-carriers. The chance-constrained approach necessitates a power-robustness tradeoff compared with non-robust NOMA but effectively enforces maximum user outage and can result in transmit power savings when users can accept a higher probability of outage.