Discontinuous Reception (DRX)
One of the important activities performed by UE during RRC Idle mode is listening to paging message.
The UE should listen to the paging message to know about incoming calls. In LTE, the UE needs to monitor PDCCH continuously (every 1ms) for P-RNTI to listen to paging message and this drains UE battery power heavily.
Instead, if the UE is allowed to monitor P-RNTI in PDCCH, only at predetermined period say every 60ms or 100ms, it will reduce UE’s power consumption.
The process of monitoring PDCCH discontinuously for P-RNTI to listen to paging message during RRC idle state is known as Discontinuous Reception (DRX) in RRC_Idle.
This provides method for the UE to micro-sleep even in the active state to reduce power consumption while providing high QOS and connectivity. Meaning that UE is not monitoring the PDCCH in the given sub frame and it is allowed to go UE in a power saving mode.
Discontinuous Reception for Paging:
This provides method for the UE to micro-sleep even in the active state to reduce power consumption while providing high QOS and connectivity. Meaning that UE is not monitoring the PDCCH in the given sub frame and it is allowed to go UE in a power saving mode.
Discontinuous Reception for Paging:
Paging is a procedure to transmit paging message to the UE in RRC Idle mode or EMM registered state. Paging message may be triggered either by MME in core network or eNB in Radio Access Network.
Paging information for the UE is being carried on the PDSCH in the resource blocks indicated by the PDCCH.
Paging indication on PDCCH is single fixed indicator (FFFE) called the paging RNTI (P-RNTI)
Different group of UE's monitor different sub frames for their paging messages.
UE's in idle mode monitor the PDCCH channel for P-RNTI used for paging to receive paging message from eNB.
Paging information for the UE is being carried on the PDSCH in the resource blocks indicated by the PDCCH.
Paging indication on PDCCH is single fixed indicator (FFFE) called the paging RNTI (P-RNTI)
Different group of UE's monitor different sub frames for their paging messages.
UE's in idle mode monitor the PDCCH channel for P-RNTI used for paging to receive paging message from eNB.
The UE needs to monitor the PDCCH only at certain UE specific paging occasion, i.e. only at specific subframe in specific radio frame. At other times the UE may go for sleep mode to save battery power.
Paging Frame (PF ) : One paging frame (PF) is one Radio
Frame in which the UE is looking for
paging message. One Paging Frame may
contain one or multiple Paging
Occasion(s).
Paging Occassion (PO): It is a sub-frame where there may be
P-RNTI transmitted on PDCCH
addressing the paging message for the
UE.There is always only one paging
occasion for each UE in a DRX cycle.
Frame in which the UE is looking for
paging message. One Paging Frame may
contain one or multiple Paging
Occasion(s).
Paging Occassion (PO): It is a sub-frame where there may be
P-RNTI transmitted on PDCCH
addressing the paging message for the
UE.There is always only one paging
occasion for each UE in a DRX cycle.
To find out PF (Paging Frame) and PO (Paging Occasion ), the UE uses DRX parameters broadcasted in the system information SIB2.
What are the DRX Parameters ?
1] DRX cycle: Indicates the number of radio frames in the paging
cycle. Larger the value of this parameter, lesser
the UE battery power consumption.
Whereas smaller the value of DRX cycle, increase in
the UE battery power consumption
cycle. Larger the value of this parameter, lesser
the UE battery power consumption.
Whereas smaller the value of DRX cycle, increase in
the UE battery power consumption
2] nB : This cell specific parameter indicates number of
paging occasions in a cell specific DRX cycle.
Configuration of nB value depends on paging
capacity required in a cell. The larger the value
of nB is configured to, the larger the paging
capacity. Likewise, the smaller the paging capacity
when the value of nB is smaller.
paging occasions in a cell specific DRX cycle.
Configuration of nB value depends on paging
capacity required in a cell. The larger the value
of nB is configured to, the larger the paging
capacity. Likewise, the smaller the paging capacity
when the value of nB is smaller.
With this we can infer that a maximum of 1 radio frame is available per DRX cycle of the UE for paging. Also, the minimum
of 1 sub frame per radio frame (nB ≤ 1T) and maximum
of 4 sub frames (for nB=4T) are available for paging message for the UE.
Calculation of Paging Frame Number:
Both the eNB and the UE calculate the Paging Frame number by the following relation:
Paging Frame Number = SFN mod T = (T/N) * (UE ID MOD N)
The eNB will receive UE_ID from the MME in PAGING S1AP message as "UE Identity Index Value" for MME initiated paging message.
The UE will find out the UE_ID by using the relation IMSI mod 1024
The IMSI is given as sequence of digits of type Integer (0..9). The IMSI shall, in the formulae above, be interpreted as a decimal integer number, where the first digit given in the sequence represents the highest order digit.
For example: IMSI = 12 (digit1=1, digit2=2). In the calculations, this shall be interpreted as the decimal integer "12", not "1x16+2 = 18".
Calculation of Paging Occasion (Paging Sub frame Number in a Paging Frame)
Paging Occasion is a function of i_s, Ns and predefined sub frame pattern i_s => Index pointing to predefined table corresponding subframe.
= Floor (UE_ID/N)MOD Ns
Paging Occasion for the UE is obtained from the predefined subframe pattern (36.304 section 7.2, shown below for quick reference) based on the value of Ns, i_s. For example, if Ns=2 and i_s = 1, UE interprets that sub frames 4 and 9 are configured for paging in the cell and subframe 9 is paging occasion for it.
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