Long term evolution (LTE) carrier aggregation with 5 GHz unlicensed national informational infrastructure band has been pointed out by the industry as a good solution to handle the rapidly increasing amounts of data traffic. To provide fair coexistence of LTE-licensed assisted access (LTE-LAA) and Wi-Fi on 5 GHz, several coexistence mechanisms have already been proposed. This paper provides a comprehensive survey of the coexistence of LTE-LAA and Wi-Fi on 5 GHz with corresponding deployment scenarios. We first analyze coexistence-related features of those two technologies, including motivation, LTE carrier aggregation with unlicensed band, LTE and Wi-Fi medium access control protocols comparison, coexistence challenges and enablers, performance difference between LTE-LAA and Wi-Fi, as well as co-channel interference. Second, we further extensively discuss current considerations about the coexistence of LTE-LAA and Wi-Fi. Third, influential factors for the classification of small cell scenarios, as well as four representative scenarios are investigated in detail. Then we explore a relatively smooth technical route for solving coexistence-related problems, which practically takes features of a specific scenario as the base for designing deployment mode of LTE-LAA and/or Wi-Fi. A scenario-oriented decision making procedure for the coexistence issue and the analysis on an example deployment scenario, including design and performance evaluation metrics focusing on the concept of the scenario-oriented coexistence are presented. We finally forecast further research trends on the basis of our conclusion.

Coexistence of LTE-LAA and Wi-Fi on 5 GHz With Corresponding Deployment Scenarios: A Survey

Future 5th generation networks are expected to enable three key services—enhanced mobile broadband, massive machine type communications and ultra-reliable and low latency communications (URLLC). As per the 3rd generation partnership project URLLC requirements, it is expected that the reliability of one transmission of a 32 byte packet will be at least 99.999% and the latency will be at most 1 ms. This unprecedented level of reliability and latency will yield various new applications, such as smart grids, industrial automation and intelligent transport systems. In this survey we present potential future URLLC applications, and summarize the corresponding reliability and latency requirements. We provide a comprehensive discussion on physical (PHY) and medium access control (MAC) layer techniques that enable URLLC, addressing both licensed and unlicensed bands. This paper evaluates the relevant PHY and MAC techniques for their ability to improve the reliability and reduce the latency. We identify that enabling long-term evolution to coexist in the unlicensed spectrum is also a potential enabler of URLLC in the unlicensed band, and provide numerical evaluations. Lastly, this paper discusses the potential future research directions and challenges in achieving the URLLC requirements.

/pdf/enabling-technologies-for-ultra-reliable-and-low-latency-44un8h2u5m.pdf

Enabling Technologies for Ultra-Reliable and Low Latency Communications: From PHY and MAC Layer Perspectives

Ever since the inception of mobile telephony, the downlink and uplink of cellular networks have been coupled, that is, mobile terminals have been constrained to associate with the same base station in both the downlink and uplink directions. New trends in network densification and mobile data usage increase the drawbacks of this constraint, and suggest that it should be revisited. In this article we identify and explain five key arguments in favor of downlink/uplink decoupling based on a blend of theoretical, experimental, and architectural insights. We then overview the changes needed in current LTE-A mobile systems to enable this decoupling, and then look ahead to fifth generation cellular standards. We demonstrate that decoupling can lead to significant gains in network throughput, outage, and power consumption at a much lower cost compared to other solutions that provide comparable or lower gains.

Why to decouple the uplink and downlink in cellular networks and how to do it

Remarkable research activities and major advances have been occurred over the past decade in multiuser multiple-input multiple-output (MU-MIMO) systems. Several transmission technologies and precoding techniques have been developed in order to exploit the spatial dimension so that simultaneous transmission of independent data streams reuse the same radio resources. The achievable performance of such techniques heavily depends on the channel characteristics of the selected users, the amount of channel knowledge, and how efficiently interference is mitigated. In systems where the total number of receivers is larger than the number of total transmit antennas, user selection becomes a key approach to benefit from multiuser diversity and achieve full multiplexing gain. The overall performance of MU-MIMO systems is a complex joint multi-objective optimization problem since many variables and parameters have to be optimized, including the number of users, the number of antennas, spatial signaling, rate and power allocation, and transmission technique. The objective of this literature survey is to provide a comprehensive overview of the various methodologies used to approach the aforementioned joint optimization task in the downlink of MU-MIMO communication systems.

An Overview on Resource Allocation Techniques for Multi-User MIMO Systems

Ever since the inception of mobile telephony, the downlink and uplink of cellular networks have been coupled, i.e. mobile terminals have been constrained to associate with the same base station (BS) in both the downlink and uplink directions. New trends in network densification and mobile data usage increase the drawbacks of this constraint, and suggest that it should be revisited. In this paper we identify and explain five key arguments in favor of Downlink/Uplink Decoupling (DUDe) based on a blend of theoretical, experimental, and logical arguments. We then overview the changes needed in current (LTE-A) mobile systems to enable this decoupling, and then look ahead to fifth generation (5G) cellular standards. We believe the introduced paradigm will lead to significant gains in network throughput, outage and power consumption at a much lower cost compared to other solutions providing comparable or lower gains.

Why to Decouple the Uplink and Downlink in Cellular Networks and How To Do It

The choice of OFDM-based multi-carrier access techniques for LTE marked a fundamental and farsighted parting from preceding 3GPP networks. With OFDMA in the downlink and SC-FDMA in the uplink, LTE possesses a robust and adaptive multiple access scheme that facilitates many physical layer enhancements. Despite this flexibility, scheduling in LTE is a challenging functionality to design, especially in the uplink. Resource allocation in LTE is made complex, especially when considering its target packet-based services and mobility profiles, both current and emerging, in addition to the use of several physical layer enhancements. In this paper, we offer a tutorial on scheduling in LTE and its successor LTE-Advanced. We also survey representative schemes in the literature that have addressed the scheduling problem, and offer an evaluation methodology to be used as a basis for comparison between scheduling proposals in the literature.

Uplink Scheduling in LTE and LTE-Advanced: Tutorial, Survey and Evaluation Framework

3GPP's Long Term Evolution is defined by the standardization body's Release 8 and 9, and provides more than a substrate for 3GPP's IMT-Advanced candidate, namely LTE-Advanced, which is due to be defined in Release 10. Both LTE and LTE-Advanced have SC-FDMA in their uplink, a multi-carrier access technique requiring contiguous subcarriers allocations for each UE. No scheduling algorithm, however, is dictated by the standard and several proposals have hence been presented to implemented by vendors. A definite scheduling requirement is the support of QoS attributes of different types of uplink traffic. Our intent in this study is evaluate the connection-level performance of representative scheduling proposals, with focus on QoS aspects. Specifically, we utilize a mixed type of traffic flows and evaluate the schedulers in terms of per-user throughput, delay, packet loss and fairness.

/pdf/evaluating-uplink-schedulers-in-lte-in-mixed-traffic-1c5dx9oepa.pdf

Evaluating Uplink Schedulers in LTE in Mixed Traffic Environments

The choice of SC-FDMA for uplink access in Long Term Evolution (LTE) facilitates great flexibility in allocating medium resources to users while adapting to medium condition. A multicarrier multiple access technique, SC-FDMA gains an advantage over OFDMA in that it reduces the energy requirements in user equipment. 3GPP Releases 8 and 9, however, do not detail a specific scheduler and, accordingly, proposals have been made in the literature in designing an efficient and capable uplink scheduler for LTE. This paper presents a preliminary performance evaluation for representative proposals, and offers medium of comparison in order to highlight the individual characteristics of each proposals.

/pdf/comparing-uplink-schedulers-for-lte-1o1022znfz.pdf

Mohamed Salah

Papers

Uplink Scheduling in LTE and LTE-Advanced: Tutorial, Survey and Evaluation Framework

Evaluating Uplink Schedulers in LTE in Mixed Traffic Environments

Comparing uplink schedulers for LTE