Sleep state
Figure 1: DRX in wireless network [6].
In the LTE DRX mechanism, the sleep/wake scheduling of each UE is determined by the following four parameters DRX Short Cycle, DRX Long Cycle, DRX Inactivity Timer, and DRX Short Cycle Timer. In LTE DRX, the sleep/wake-up mode consists of the three different states, namely, Inactivity period, Light Sleep period, and Deep Sleep period. The transition from the Inactivity period to the Light Sleep period is controlled [7].
Active state/On state
When a UE is actively monitoring the RF spectrum for data frames.
Sleep state/Off state
When a UE has powered down and not monitoring the RF spectrum.
UL retransmission timer
According to 3GPP specifications, UE shall monitor PDCCH when it expects UL grant for retransmission. This functionality is implemented in the System Simulator using UL retransmission Timer. This timer is triggered 4 sub-frames after the uplink transmission.
DL retransmission timer
One retransmission timer is configured for every HARQ process in the downlink which means eight retransmission timers.Main purpose of this timer is to keep UE awake when it expects retransmissions on DL-SCH.Retransmission is triggered ON 8ms after a downlink HARQ process is NACKED.The period of retransmission timer is kept long enough so that to avoid the case where during DL retransmission arrives but UE is sleeping [9].