A Survey on QoE-oriented Wireless Resources Scheduling <s> A. QoE-unaware Schedulers <s> We propose an efficient way to support quality of service of multiple real-time data users sharing a wireless channel. We show how scheduling algorithms exploiting asynchronous variations of channel quality can be used to maximize the channel capacity (i.e., maximize the number of users that can be supported with the desired QoS). <s> BIB001 </s> A Survey on QoE-oriented Wireless Resources Scheduling <s> A. QoE-unaware Schedulers <s> An efficient channel scheduler called the adaptive EXP/PF algorithm is proposed to provide real-time service with different QoS as well as a non-real time data services demanded by mobile users in an adaptive modulation and coding in time division multiplexing (AMC/TDM) systems. The adaptive EXP/PF algorithm guarantees transmission time delay bounds for real-time services as well as maximizes the system throughput. Simulation results show that the proposed adaptive EXP/PF algorithm controls the channel resource among real time and non-real time service users automatically to guarantee delay bound for real time service users. <s> BIB002 </s> A Survey on QoE-oriented Wireless Resources Scheduling <s> A. QoE-unaware Schedulers <s> Wireless technology advancements made opportunistic scheduling a popular topic in recent times. However, opportunistic schedulers for wireless systems have been studied since nearly twenty years, but not implemented in real systems due to their high complexity and hardly achievable requirements. In contrast, today's popularity of opportunistic schedulers extends to implementation proposals for next generation cellular technologies. Motivated by such a novel interest towards opportunistic scheduling, we provide a taxonomy for opportunistic schedulers, which is based on scheduling design's objectives; accordingly, we provide an extensive review of opportunistic scheduling proposals which have appeared in the literature during nearly two decades. The huge number of papers available in the literature propose different techniques to perform opportunistic scheduling, ranging from simple heuristic algorithms to complex mathematical models. Some proposals are only designed to increase the total network capacity, while others enhance QoS objectives such as throughput and fairness. Interestingly, our survey helps to unveil two major issues: (i) the research in opportunistic is mature enough to jump from pure theory to implementation, and (ii) there are still under-explored and interesting research areas in opportunistic scheduling, e.g., opportunistic offloading of cellular traffic to 802.11-like networks, or cooperative/distributed opportunistic scheduling. <s> BIB003
As previously mentioned, any scheduling strategy comprises many trade-offs among complexity, efficiency and fairness. In the case of schedulers that do not take QoE into account, these trade-offs also result from the significance that the different scheduling algorithms give to the communication channel characteristics and to QoS parameters. 1) Channel-unaware Strategies: The schedulers that implement these approaches assume that the transmission channel is error-free and time-invariant, which are unrealistic assumptions when dealing with wireless channels. Nevertheless, these strategies form the basis for more complex algorithms. First In First Out (FIFO) can be regarded as the simplest scheduling scheme, in which users are served according to the order of their resource request. Even though this approach is very simple, it is inefficient and unfair. The Round-Robin (RR) strategy tries to add some fairness to the FIFO approach, namely by allocating an equal share of resources to each user in a round-robin manner. Thus, this scheduling algorithm is fair regarding the amount of time in which the channel is occupied by each user. However, since channels conditions are not taken into account, this type of schedulers are not fair in terms of system throughput, nor they are efficient because the different requirements of the applications (e.g., bitrate) are also not considered. 2) Channel-aware/QoS-unaware Strategies: A wireless resource scheduler can take into account the channel state information that is usually fed back to the base stations, so as to enhance the efficiency of its scheduling algorithm. Maximum Throughput (MT) is an example of a policy that, in each scheduling period, prioritizes the resources to the user experiencing the best channel conditions. In this manner, these schedulers provide the highest system throughput, so that the best possible spectral efficiency is attained. Nevertheless, MT scheduling algorithms are very unfair to users with poor channel conditions (e.g., users further away from the base station) and can even make them suffer of starvation. The concept adopted by Proportional Fair (PF) schedulers  provides a trade-off between fairness and spectral efficiency. Within this approach, past average throughput acts as a weighting factor in an MT-like strategy, i.e., if two users can achieve the same throughput (based on the experienced channel conditions), then the user that has been served with lower average throughput in the past is prioritized. This means that users with poor conditions will be surely served within a certain amount of time. 3) Channel-aware/QoS-aware Strategies: In order to attain a certain performance level, different applications have different requirements, which are typically mapped into QoS parameters. Accordingly, scheduling algorithms should also take into account these QoS parameters. For instance, some scheduling strategies try to guarantee a minimum data rate for the users, whereas others deal with delay constrains. This last approach is more common among QoS-aware schedulers, since many applications, such as real-time flows, video streaming or Voice over IP (VoIP) calls, require that their packets are delivered within a certain deadline. The Modified Largest Weighted Delay First (M-LWDF) BIB001 algorithm and the Exponential/PF (EXP/PF) BIB002 scheme are two of the most popular QoS-aware scheduling strategies, as they provide a good trade-off among spectral efficiency, fairness and QoS provisioning. Besides taking service delay requirements into account, both algorithms support different services and treat differently real-time flows. All the above mentioned scheduling strategies, either channel-unaware or channel-aware (with or without taking into account QoS requirements), are just an illustrative sample of what can be found in the literature. The reader is suggested to refer to  - BIB003 for more examples and details on scheduling algorithms that are not QoE-oriented.