All About Idle Mode Discontinuous Reception (DRX)

DRX: 

- In LTE, DRX mode can be enabled in both RRC_IDLE and RRC_CONNECTED states.

- DRX is used to reduce power consumption.

Idle Mode DRX: 

- Applicable when UE is in RRC_IDLE state.

- The UE is registered with the evolved packet system mobility management (EMM_REGISTERED) but does not have an active session (ECM_IDLE).

- In this state the UE can be paged.

- UE monitors paging messages using idle mode DRX configuration.

- Idle mode DRX configuration is broadcast within System Information Block 2(SIB2).

- Idle mode DRX configuration is used to calculate Paging Frame(PF) and Paging Occation(PO).

- One Paging Occasion (PO) is a subframe where there may be P-RNTI transmitted on PDCCH addressing the paging message.

- One Paging Frame (PF) is one Radio Frame, which may contain one or multiple Paging Occasion(s).

-When DRX is used the UE needs only to monitor one PO per DRX cycle.

PF is given by following equation: 

     SFN mod T= (T div N)*(UE_ID mod N) 

T : DRX cycle of the UE. T = min(The UE specific DRX , Default DRX value).
The UE specific DRX value allocated by upper layers, and default DRX value broadcast in system information. 
If UE specific DRX is not configured by upper layers, the default value is applied. 

N: min(T,nB)

nB: Broadcast within System Information Block 2(SIB2) and can take values 4T, 2T, T, T/2, T/4, T/8, T/16, T/32.

N can have values of T, T/2, T/4, T/8, T/16, T/32.

UE_ID: IMSI mod 1024.

IMSI is given as sequence of digits of type Integer

Index i_s pointing to PO from subframe pattern defined below will be derived from following calculation: 

     i_s = floor(UE_ID/N) mod Ns 

Ns: max (1,nB/T)