# A Multi-Service Adaptive Semi-Persistent LTE Uplink Scheduler for Low Power M2M Devices
###### Abstract
Moreover, during this whole process, the M2M devices need to actively monitor the PDCCH for possible grants which drains their battery. Given the actual amount of M2M user data transmitted in the uplink is often very small (in order of few Kbytes), the time and resources consumed in the dynamic scheduling process of the LTE standard is extravagant and imprudent. For an efficient and long-term solution of the M2M communications, the signaling overhead of the scheduling process as well as power consumption of the devices need to be reduced [10]. In the 3 GPP standards, Semi-Persistent Scheduling (SPS) [11] was proposed to resolve control channel congestion for voice traffic with a deterministic flow and predictable burst size, but it fails to meet the requirements of diverse M2M applications with stochastic flow and different burst sizes. The standards [12] also propose the Discontinuous Reception (DRX) mechanism as an effective power saving mechanism that allows the LTE User Equipment (UE) to turn their transceivers off when there is no incoming traffic. The DRX mechanism seems very attractive for low power M2M communications because of a small data per terminal in irregular intervals. The sleep cycles of many delay tolerant M2M applications can be scheduled so that they are allowed to transmit only in network-permitted time intervals, thus alleviating the network load in peak times and also saving device power. However, again, many M2M applications have a random traffic arrival pattern and variable burst sizes depending on specific events and triggers, and variable delay tolerances depending on the event type. The random arrival of uplink packets non-synchronized with the DRX sleep cycle may result in either unacceptably long packet delays or the UE seldom entering sleep state. These problems need to be addressed to achieve the utmost benefits of the SPS and DRX mechanisms for low control-overhead and low power requirements of M2M communications. In view of these unique challenges, in this paper, we propose a multi-service adaptive semi-persistent scheduler which allocates the radio resources to the M2M devices/gateways in a semi-persistent way to reduce downlink control signaling yet adaptive to the actual M2M data burst sizes and also sensitive to the different QoS (delay budget) requirements of various M2M services. Our proposed scheduler can also alter the SPS allocations in real-time to make room for high priority M2M transmissions with small and strict delay requirements by pre-empting the already allocated but less delay-sensitive traffic classes. Afterwards, to support low-power devices, we design a mechanism to deploy the DRX mechanism in harmony with the adaptive SPS transmission intervals to maximize device sleep cycles without compromising the class-specific delay requirements. The proposed algorithm is simulated using the Riverbed modeler and the performance is analyzed and compared with the dynamic scheduler in terms of QoS, power consumption and channel utilization. The idea of our adaptive SPS algorithm was proposed in [13; 14; 15] and we also derived a statistical model for the device queue size and packet delay for static and adaptive persistent allocations for any arrival process in [16]. In this paper, we present an extended work by introducing a system model for multi-service adaptive SPS algorithm with priority-based pre-emptive feature to distinguish between traffic classes and maximize overall QoS satisfaction and also enhance the algorithm by combining with DRX for supporting power-constrained M2M devices. The novel contributions of this paper are listed below:  
* Design of a system model which offers different capacities to multi-service M2M traffic classes with variable burst sizes and packet delay budgets. * Introduction of a control mechanism to vary the semi-persistent period according to the required QoS for different adaptive allocation policies.