A Review of MAC Scheduling Algorithms in LTE System

With the soaring demands for high speed data communication, as well as transmission of various types of services with different requirements over cellular networks, having a decent radio resource management is considered vital in Long Term Evolution (LTE) system. In particular, satisfying the quality of service (QoS) requirements of different applications is one of the key challenges of radio resource management that needs to be dealt by the LTE system. In this paper, we propose a cross-layer design scheme that jointly optimizes three different layers of wireless protocol stack, namely application, Medium Access Control, and physical layer. The cross-layer optimization framework provides efficient allocation of wireless resources across different types of applications (i.e., real-time and non real-time) run by different users to maximize network resource utilization and user-perceived QoS, or also known as Quality of Experience (QoE). Here, Mean Opinion Score is used as a unified QoE metric that indicates the user-perceived quality for real-time or multimedia services notably video applications. Along with multimedia services, the proposed framework also takes care of non-real-time traffic by ensuring certain level of fairness. Our simulation, applied to scenarios where users simultaneously run different types of applications, confirms that the proposed QoE-oriented cross-layer framework leads to significant improvement in terms of maximizing user-perceived quality as well as maintaining fairness among users.

n/a

With the onset of multimedia applications, Quality of Service (QoS) in 802.11-based wireless ad hoc networks has been an essential area to look into. Standard wireless protocols such as IEEE 802.11b and IEEE 802.11e have been deemed unsuitable for multimedia traffic transmission in an ad hoc (multihop) network environment due to the unique network characteristics. As an ad hoc network is infrastructureless and distributed in nature, it gives rise to several challenges in QoS provisioning. In this thesis, we focus on the research of three key areas on QoS in ad hoc networks namely, buffer management in the medium access control (MAC) layer, cross-layer design and flow admission control. Firstly, we study MAC layer buffer management in the IEEE 802.11e protocol. The IEEE 802.11e MAC protocol provides QoS for multimedia traffic by probabilistic priority queueing. Ad hoc networks tend to generate a large volume of network control packets for route maintenance and these control packets are typically queued in the highest priority queue. As a result, higher priority control traffic is always being served, consuming most of the bandwidth and results in poor network performance for data traffic. We examine this problem and propose a dynamic buffer management scheme to mitigate it. By setting different threshold levels in different priority queues, we allow lower priority data traffic to be served when the higher priority control traffic is deemed to be excessive. Simulation of our proposed scheme has shown that our approach yielded better network performance compared to the standard protocol in terms of packet delivery ratio and average delay. Secondly, we study the cross-layer interaction between the MAC and Network layers to identify its impact on QoS. We select the IEEE 802.11e MAC protocol and Ad-hoc On Demand Distance Vector (AODV) routing protocol in our study since they are standardized protocols for wireless networks. In IEEE 802.11e MAC protocol, lower priority traffic has a higher probability of being blocked. Hence, the MAC layer will inform the Network layer of a broken link through the link notification function. In this case, AODV will re-route the paths that traverse this link. As a result, higher priority traffic which shares the same link as the lower priority traffic will undergo the same re-routing procedures. We termed this phenomenon path correlation effect. Under high traffic load, the path correlation effect will be more intense and hence degrade the QoS of higher priority traffic. To break the path correlation effect, we propose a cross-layer design called the Bundled Virtual Circuit (BVC). Our proposed BVC scheme not only addresses the path correlation issues, but also provides priority treatment from the Network layer to the MAC layer. Through our simulations, our proposed cross-layer design approach is proven to provide better network performance than the 802.11e/AODV protocols in terms of packet delivery ratio and delay. Finally, we examine flow admission control at the routing layer in ad hoc networks. Standard routing protocols such as AODV performs routing on a shortest path basis. In the presence of multiple routes, hotspots are likely to form among the intermediate nodes and thus lead to network congestion. In addition, existing routes are not well-protected from the introduction of new routes. We propose a flow admission control algorithm to be used with AODV. Our algorithm allows routes to be created through available bandwidth and congestion estimation. During the route discovery phase, each node will be gauged on its estimated available bandwidth and its backlog of packets in the MAC buffer. If a node does not have sufficient bandwidth, or has too many backlogged packets compared to the requested flow rate, it will not participate in the route discovery process. From simulations, our new scheme is able to provide better network performance than the standard AODV protocol in terms of delay. Our work in this thesis focuses on QoS provisioning in ad hoc networks from the MAC and network layer perspectives. By studying the network characteristics and protocols’ behavior, we investigate the various problems of providing QoS in an ad hoc network environment and propose three different schemes to improve the QoS provisioning at the MAC and network layers. We believe that our work is able to provide some insights to the research area on QoS in ad hoc networks.

Vehicular safety applications have much significance in preventing road accidents and fatalities. Among others, cellular networks have been under investigation for the procurement of these applications subject to stringent requirements for latency, transmission parameters, and successful delivery of messages. Earlier contributions have studied utilization of Long‐Term Evolution (LTE) under single cell, Friis radio, or simplified higher layer. In this paper, we study the utilization of LTE under multicell and multipath fading environment and introduce the use of adaptive awareness range. Then, we propose an algorithm that uses the concept of quality of service (QoS) class identifiers (QCIs) along with dynamic adaptive awareness range. Furthermore, we investigate the impact of background traffic on the proposed algorithm. Finally, we utilize medium access control (MAC) layer elements in order to fulfill vehicular application requirements through extensive system‐level simulations. The results show that, by using an awareness range of up to 250 m, the LTE system is capable of fulfilling the safety application requirements for up to 10 beacons/s with 150 vehicles in an area of 2 × 2 km2. The urban vehicular radio environment has a significant impact and decreases the probability for end‐to‐end delay to be ≤100 ms from 93%–97% to 76%–78% compared to the Friis radio environment. The proposed algorithm reduces the amount of vehicular application traffic from 21 Mbps to 13 Mbps, while improving the probability of end‐to‐end delay being ≤100 ms by 20%. Lastly, use of MAC layer control elements brings the processing of messages towards the edge of network increasing capacity of the system by about 50%.

When a mobile operator migrates its network from the third generation (3G) system to Long Term Evolution (LTE), both 3G and LTE will coexist for a period of time. Since the 3G circuit-switched (CS) voice mechanism is more mature and available than that for the LTE Voice over Internet Protocol (VoIP), the operator may consider CS fallback (CSFB) as a solution for providing reliable voice calls. According to the 3G Partnership Project (3GPP) CSFB procedure, when a mobile user in the LTE network has an incoming or an outgoing call, the user equipment (UE) falls back from LTE to Universal Mobile Telecommunications System (UMTS). When the call is complete and released, the UE immediately returns to LTE. If the next activity for the UE is another voice call, immediately switching from UMTS to LTE may not be efficient. In this case, the UE has to perform another CSFB. To resolve this issue, we suggest delaying the returns to avoid unnecessary CSFBs, which is called delayed return (DR). Based on the measurements from the real UMTS and LTE networks, we develop an analytic model for investigating the performance of the CSFB with DR. This paper indicates that the DR scheme can effectively reduce the CSFB costs by up to 60%.

One of the critical components of the Smart Grid (SG) is the Smart Meter (SM), which reports power consumption to a centralized control center at regular intervals. The Long Term Evolution (LTE) network has received much attention as a promising platform for SG communications due to its high bandwidth and low latency over a large coverage area. Nevertheless, when a huge number of SMs access the LTE network, the preamble collision becomes an issue. The negative effects of preamble collision are packet loss, delay, reestablishment attempts, and physical channel utilization, which degrade the performance of the LTE network. Hence, a Random Access (RA) mechanism is required for preamble signature to occupy Resource Blocks (RBs) for SMs. In this paper, we introduce a hybrid procedure that combines contention and non-contention based random access methods. The significance of this method is that, although SMs have fixed periodic communications, preamble signatures do not need to be reserved. The simulation results are compared with LTE standard. Packet loss is reduced by approximately 43%. SM latency is decreased by 11 ms at 90% of probability delay. Re-establishment attempts are lower than standard by proximately 76%. Finally, physical channel utilizations are reduced by around 3%.

Transitioning a Terrestrial Trunked Radio (TETRA) network to a Long-Term Evolution (LTE) network in public protection and disaster relief (PPDR) systems is a path to providing future services requiring high radio interface throughput and allowing broadband PPDR (BB-PPDR) radio communications. Users of TETRA networks are currently considering how to deploy a BB-PPDR network in the coming years. This study offers several radio planning considerations in TETRA to LTE migration for such networks. The conclusions are obtained from a case study in which both measurements and radioelectric coverage simulations were carried out for the real scenario of the Murcia Region, Spain, for both TETRA and LTE systems. The proposed considerations can help PPDR agencies efficiently estimate the cost of converting a TETRA network to an LTE network. Uniquely in this study, the total area is divided into geographical areas of interest that are defined as administrative divisions (region, municipal areas, etc.). The analysis was carried out using a radio planning tool based on a geographic information system and the measurements have been used to tune the propagation models. According to the real scenario considered, the number of sites needed in the LTE network—for a specific quality of service (90% for the whole region and 85% for municipal areas)—is a factor of 2.4 higher than for TETRA network.

The purpose of the paper is to examine if public long term evolution (LTE) network is suitable for smart grid (SG) automatic metering usage, in a suburban scenario, without causing significant hindrance to typical LTE traffic. In addition, the same generalized suburban scenario is evaluated with a hybrid sensor - LTE network where wireless sensor network (WSN) cluster heads are also equipped with LTE remote terminal units (RTUs). The simulation topology is formed as a suburban area that consists of single RTU per apartment/house for SG traffic and an average 3.7 people per habitat, each with their user equipment, using one of the available carriers. Normal SG traffic is generated in both directions, uplink and downlink. The worst-case usage factor is considered as a power outage, where all SG meters in the area of influence simultaneously detect and attempt to report such a failure. Thus, emergency SG traffic is generated simultaneous by all RTUs. To alleviate the problem of the high momentary SG emergency traffic load, two distinct solutions are proposed and simulated. As the first solution, an artificial, up to one-second random delay is introduced in the transmission scheduling of emergency messages. The second solution applies low-power WSNs at the cluster level and cluster heads that contain both 802.15.4 and LTE communication interfaces, i.e., a hybrid sensor-LTE network. The simulation results show that both of the solutions that try to alleviate the high instantaneous traffic loads can reach the quality of service criteria for SG traffic without compromising other public LTE traffic. The deciding factor to use either pure LTE approach or a hybrid sensor-LTE network comes from non-functional requirements of the system.

Long Term Evolution (LTE) was developed by 3GPP to cope with the increasing demand for better Quality of service (QoS) and the emergence of bandwidth-consuming multimedia applications. Today's data transmission networks face extreme challenges in providing high data rate and low latency. Scheduling paradigms such as Round Robin, Best Channel Quality Condition and Proportional Fair are commonly adopted in current LTE downlink scheduling algorithms, but they are far from optimal for satisfying latency requirements. In this paper, we first survey the state of the art downlink scheduling algorithms in LTE and identify their main challenges. We then formulate the LTE downlink scheduling problem as an optimization problem in order to meet the flow deadlines, then incorporate the formulation within the surveyed scheduling algorithms, to produce better performance. We consider strict deadlines for different types of packets with the goal of maximizing resource distribution. Additionally, in our formulation the buffer state for each user is taken into consideration in order to minimize the packet loss. We evaluate the proposed formulation using LTE-Sim and study its positive impact on the existing LTE downlink scheduling algorithms; the performance in terms of QoS, packet loss and fairness is improved throughout all evaluations.

Coexistence research for LTE systems is indispensable to ensure that the time division duplex (TDD) Long Term evolution (LTE) system can coexist with the frequency division duplex (FDD) LTE system operating at the adjacent frequency band within the same geographical area. Previous studies on this topic only focus on the throughput loss of the victim system from the data channel viewpoint. In this paper, the performance of control channels (CCHs) in the typical coexistence scenario is investigated, since the performance of communication systems is largely dependent on the quality of CCHs. The CCHs' interferences are analyzed in detail, including the coexistence issue for CCHs, Co-Channel Interference (CCI) analysis, and Adjacent Channel Interference (ACI) on CCHs. The simulation results indicate that the ACI generated by the Base Station (BS) of collocated LTE system degrades the performance of the uplink (UL) CCHs of the victim LTE system significantly.

The fifth-generation (5G) mobile network is facing severe challenges of the traffic demand surge and spectrum scarcity. To improve the network capacity and data rate, the deployment of Long Term Evolution (LTE) cellular system on unlicensed band is considered as one of the promising solutions. But the interference problem among LTE and WiFi system on the same unlicensed band hinders the deployment process, which is a critical issue unsolved for network operators. Therefore, a novel channel access scheme integrating the energy detection scheme and back-off mechanism in LTE system has been proposed in this paper, to minimize the interference to WiFi system. The throughput of both LTE and WiFi systems and the optimal sensing time and back-off time are theoretically analyzed with proofs. Furthermore, an efficient adaptive reconfiguration strategy of access parameters is proposed based on the channel occupancy cognition results. Simulation results verify that the proposed channel access scheme with optimal back-off time can double the throughput of WiFi system when coexisting with LTE system on unlicensed band.

Cyber-physical systems (CPSs) represent a new generation of control systems where distributed local control systems are connected not only physically, but also computationally by means of communication networks. CPSs target introducing intelligence beside traditional monitoring and control functionalities in a way that optimize the performance of the overall system. However, the realization of many CPS applications requires reliable communication systems that provide quality of service (QoS) control. In this domain, Long Term Evolution (LTE) standard offers a comprehensive QoS frame work. Nevertheless, commercial implementations of the standard provide only best effort type of service. In this paper, we demonstrated the benefits of using LTE networks with QoS support for CPSs by comparing the performance of a CPS application over LTE network with and without QoS support. The results clearly indicate the benefit to enable the QoS features in commercial implementations of LTE in order to realize reliable CPS applications.

In order to coexist with the WiFi systems in the unlicensed spectrum, Long Term Evolution (LTE) networks can utilize periodically configured transmission gaps. In this paper, considering a time division duplex (TDD)-LTE system, we propose a Q-Learning based dynamic duty cycle selection technique for configuring LTE transmission gaps, so that a satisfactory throughput is maintained both for LTE and WiFi systems. By explicitly taking the impact of IEEE 802.11n beacon transmission mechanism into account, we evaluate the coexistence performance of WiFi and LTE using the proposed technique. Simulation results show that the proposed approach can enhance the overall capacity performance by 19% and WiFi capacity performance by 77%, hence enabling effective coexistence of LTE and WiFi systems in the unlicensed band.

The 3GPP Long Term Evolution (LTE) Release-8 specifications are designed to deliver higher peak data rates, higher throughput and lower air-interface latency compared to 2G and 3G systems. This higher performance will make it possible to support more demanding applications beyond web-browsing and voice, requiring higher data rates and stricter QoS constraints. Video services are becoming increasingly popular over the Internet indicating that the demand for such high data-rate video applications over cellular wireless will continue to grow. However, in order to make these services commercially viable in a LTE system it is necessary for the air-interface to deliver high quality services to a significant number of users simultaneously. In this paper we investigate the video capacity of a LTE air-interface using realistic video traffic models. An LTE air-interface can support multiple-antenna transmit arrays and several multiple antenna transmission modes to increase system capacity. We investigate the benefits of using 4 transmit antennas compared to 2 transmit antennas on the video capacity of an LTE system. The results from our investigation indicate that the capacity benefits with 4 transmit antennas are much higher with video services than those observed with other traffic models such as the full-buffer traffic model. The results also show that a 10MHz TDD LTE system can service upto 48 users per sector with 256Kbps video streams in the downlink indicating that such services can be commercially viable.

QoS-priority based load balancing algorithm for LTE systems with mixed users