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.
- Active Time: The time data packets are received by a UE.
- DRX Inactivity Timer: A timer used to initiate the sleep states.
Sleep state/Off state
When a UE has powered down and not monitoring the RF spectrum.
- Listen state: A very short period of time when the UE powers up during
DRX short cycle to monitor the RF spectrum for incoming Physical
Downlink Control Channel (PDCCH)transmissions.DRX Short Cycle: A period
of time when the UE is in sleep state but periodically transitions to
the listen state to monitor the PDCCH for incoming data frames.
- DRX Short Cycle Timer: During the DRX short cycle the DRX Short Cycle
Timer sets off the listen state.
- DRX Long Cycle: A time span longer than the DRX Short Cycle where an
UE stays in the sleep state and wakes up only at the end of the cycle to
monitor the PDCCH.
- DRX Long Cycle Timer: During the DRX long cycle the DRX long Cycle
Timer set off the listen state [1].
- DL-related scheduling messages These messages are needed for a scheduled UE to be able to properly
receive, decode and demodulate DLSCH. Moreover, information about
DL-SCH resource allocation, transport format, and information about
hybrid ARQ are also included [8].
- UL-related scheduling messages These messages contain scheduling grant information. These are
intended to inform a scheduled UE what uplink resources and transport
format to use for UL-SCH. The PDCCH is located in the first (up to
three) OFDM symbols in the first slot of a subframe [5].
- SR-pending timer
According to the specifications, a scheduling request shall be
considered pending until UL grant is received and UE shall monitor the
PDCCH whenever the UL grant is expected. This functionality is implemented
using SR-pending Timer in the System Simulator. SR-pending Timer is
triggered ON whenever a scheduling request is made by UE and it keeps
the UE awake as long as it has received the UL grant from the eNB.The
duration of this timer is flexible in the sense that it could be as
short as one TTI during low traffic load scenario where UE actually
can receive a grant in 1ms. Whereas, in cases of high traffic load, it
may take as many as several TTIs for UE to receive the uplink grant
which results in longer awake time. Since SR-pending Timer is
independent of On-duration Timer and any UE can send a scheduling
request independent of its DRX status, it may trigger ON anytime
during the DRX cycle [6].
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].