# A Multi-Service Adaptive Semi-Persistent LTE Uplink Scheduler for Low Power M2M Devices
## 4 Implementation of DRX Power Savings with Adaptive SPS  
### Standard DRX Mechanism  
The 3 GPP has standardized the DRX power-saving mechanism [12] to allow the UEs to enter a sleep mode when there is no data to be received. The UEs are configured with two parameters, i.e., the sleep period and the inactivity timer which determines how often and for how long the UE can remain in sleep by turning off its transceiver. A DRX cycle consists of a sleep period and an ON-period. In the sleep period, the UE is in power-saving state and does not monitor the PDCCH and any downlink packets arriving at the eNodeB destined for this UE needs to wait. At the end of the sleep period, the UE enters ON-period and monitors the PDCCH for possible scheduled transmissions. Two types of DRX cycles, short and long DRX are supported. The short DRX cycle has a short sleep period and it will be repeated until the expiry of a short DRX cycle timer, expressed in number of short DRX cycles, with no traffic. Upon expiry of the short DRX cycle timer, the long DRX cycle is started and is continued until traffic is indicated in the ON-period of the UE. If there is any PDCCH grant for this UE during the ON-period, the UE becomes active to process data and starts an inactivity timer. Whenever there is a new transmission or reception during the active state of the UE, the inactivity timer is reset and only upon expiry of it without any activity, the UE can start DRX cycle.  
Figure 5 shows the DRX mechanism for uplink traffic in LTE. For initial packet arrival at the UE in its active state, a request in sent to the eNodeB using the Physical Uplink Control Channel (PUCCH) and the UE actively listens to the PDCCH for scheduling grant.  
Upon reception of the grant in PDCCH, the UE starts inactivity timer and transmits in the PRBs allocated to it in the Physical Uplink Shared Channel (PUSCH). If the BSR sent along with the data indicated the requirement of further allocations, new PDCCH grants are sent to the UE which makes it reset the inactivity timer. If the BSR was zero then the UE waits until the inactivity timer is expired. If there is new packet arrival before the timer expiry, again the SR process is executed. Otherwise, upon timer expiry, the UE can enter sleep state. If there is any packet arrival in sleep state, the UE wakes up immediately and sends SR. If the sleep timer is expired without any packet arrival then the UE enters ON-state. The UE actively monitors PDCCH in this state and in case of packet arrival starts the SR process. Only upon the expiry of ON-period without any event it can enter sleep state.  
The DRX mechanism can save significant amount of UE power in the presence of occasional downlink traffic for the UE. However, for uplink-heavy M2M traffic with small and/or random inter-arrival gap, the UE has to wake up every time an uplink packet is  
Figure 5: Standard Discontinuous Reception (DRX) mechanism from User Equipment (UE) perspective (uplink only traffic).  
generated. Moreover, for such arrival pattern, in most cases, there is data remaining in the UE buffer after each uplink transmission; thus keeping the UE awake and monitoring. All these factors prevent the UE to benefit from the DRX mechanism because the probability of the UE entering the sleep state and completing the sleep period becomes very low.