DRX Operation in LTE

üDiscontinuous Reception (DRX) can be implemented in the UE to save battery power

–Discontinuous monitoring of PDCCH in DL

–UE can turn off its receiver during periods of inactivity

–Defined on a per UE basis, not per RB basis

üTwo modes of entering DRX

–Implicit transition based on timers

  –Explicit transition based on DRX command from the network via DRX Control Element

üDuring DRX, UE maintains a DRX cycle that is defined as a number of subframes.

  •UE monitors PDCCH for on-duration sub frames (1~200 subframes) and may turn off its receiver for rest of the DRX cycle

  •eNB does not schedule transmissions during off-period of the DRX cycle

  •Two DRX cycles: Short and Long

ü–UE starts with Short DRX cycle and transitions to long DRX cycle after the expiration of a timer

 DRX For Different Apps

üVoIP

–For voice over Internet Protocol (VoIP), since it is a bidirectional connection, the DRX is enabled only when the UE need not send or receive the packet.

–VoIP is characterized by the periodic arrival of fixed length packets for the duration of talk spurt. One way of enabling the DRX is to exploit this characteristic.

–Immediately after sending a packet, the eNB instructs the UE to go into DRX mode. The DRX cycle should be set such that the next packet is scheduled when the UE wakes up to read the PDCCH message on DL.

–Assuming that the VoIP packets are arriving at 20 ms and the power saving (neglecting the retransmissions, etc.) is approximately  60 to 70 percent.

üFTP:

–In FTP applications, a session includes a series of file transfers. It can be viewed that all the packets arrive at the same time and can be transmitted without interruption by thesleeping time of UE.

–Inactivity timer to 0 for FTP applications.

üVideo:

–Fixed Video Frame Rate-> Fixed No of Packets (Variable Size).

–Long DRX cycle of 100 ms, Short DRX cycle 12,25,50 ms.