# Energy Efficient Resource Allocation for M2M Devices in LTE/LTE-A
## 2 Overview and Related Works
### Related Works  
Many studies have addressed radio resource allocation for M2M devices in LTE/LTE-A technology [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. In [10], a thorough survey on LTE uplink schedulers for M2M devices is presented. In this survey, authors indicate that existing schedulers could be classified into three main categories as follows:  
Figure 1: Frame in long term evolution (LTE). Figure 2: UE-RB metric matrix M.  
(i) power saving schedulers which aim to reduce energy consumption in M2M devices [11; 12]; (ii) QoS based schedulers which aim to provide QoS handling for each type of M2M applications [13; 14]; (iii) multi-hop schedulers. Multi hop communication is proposed to reduce the number of base stations and improve system performance by coverage extension [15; 16]. [PERSON] et al. [17] proposed the use of group-based radio resource allocation, where M2M devices are clustered based on their transmission protocols and their QoS requirements. The algorithm aims to maximise sum-throughput while satisfying SC-FDMA constraints and QoS requirements of M2M devices. [PERSON] et al. [18] proposed an uplink LTE scheduler that considers the smart metring and real-time traffic coexistence. In this propositional scheduler, a set of relays is considered to provide the link among the base station and the smart meters. Resource blocks are allocated to users considering their QoS class identifier (QCI) and their channel quality. The authors argue that the mechanism outperforms traditional schedulers by serving more users while satisfying their delay constraint. In [19], authors proposed a combination between round robin (RR) and first maximum expansion (FME) [30]. Depending on the type of traffic flow, either RR or FME schedulers is applied. RR is used for real time M2M applications, whereas FME is used for non-real time applications. This mechanism allows one to maximise the throughput and to ensure the fairness among users with higher priority for real time applications, but it does not consider power consumption constraints for M2M devices. In [20], [PERSON] et al. proposed a statistical priority scheduler which allocates RBs to an M2M device having the lowest statistical priority metric. This metric is a term that indicates the uniqueness of the information carried by certain data packets sent by machine type communications devices (MTCDs). References [21; 22; 23; 24] considered both channel condition and maximum delay tolerance when allocating resource blocks to M2M devices. [PERSON] et al. in [24] proposed two uplink scheduling schemes for the LTE based cellular systems with different objectives. The first one allocates RBs according to the device's channel quality. However, the second algorithm prioritises devices with less delay tolerance and tries to allocate RBs with the best channel quality to that device. Both proposed schedulers do not ensure fairness between devices. In [25; 26], delay-aware uplink schedulers were proposed. Both studies classify the M2M applications into classes and give the highest priority to the critical M2M class. [PERSON] et al., in [27], proposed M2M uplink scheduling which aims at achieving a balance between satisfying system deadline and throughput maximisation. Depending on the network operating conditions and priorities, scheduling metric is adjusted; it can be channel quality based, delay based or a combination of them. The authors of [28] proposed a predictive packet scheduling scheme for event-driven M2M applications. In a group of M2M devices, if one device sends a scheduling request (SR) to the base station, then there is a high probability that the nearby devices will also send SRs later. Therefore, in order to reduce the delay, the scheme schedules radio resources to these nearby devices before they send their SRs. The scheduler considers only event-driven applications and does not take into account other types of M2M applications. Besides, these studies [25; 26; 27; 28] do not consider the coexistence of M2M and H2H devices.