All About Connected Mode Discontinuous Reception (DRX)

Connected Mode DRX:

- DRX in connected mode is a power-saving method.

- DRX is a method by which the UE can switch off its receiver for a period of time.

- Applicable when UE is in RRC_CONNECTED state.

- When UE is in RRC Connected state UE may be configured with a UE specific DRX.

- if DRX is configured, the UE is allowed to monitor the PDCCH discontinuously in RRC Connected state.

- Controls the UE’s PDCCH monitoring activity for the UE’s C-RNTI, TPC-PUCCH-RNTI, TPC-PUSCH-RNTI and Semi-Persistent Scheduling C-RNTI (if configured).

- In the RRC_CONNECTED state DRX mode is enabled during the idle periods during the

packet arrival process,when there are no outstanding/new packets to be transmitted/received, eNB/UE may initiate the DRX mode.

- If the UE is configured with DRX, the UE may delay the measurement reporting for event triggered and periodical triggered measurements until the Active Time.

- cqi-Mask - Limits CQI/PMI/PTI/RI reports to the on-duration period of the DRX cycle.

DRX Cycle : 

-On Duration followed by a possible period of inactivity.

DRX Configuration: 

onDurationTimer : 

- The duration of 'ON time' within one DRX cycle.

- The number of consecutive PDCCH-subframe(s) UE monitors at the beginning of a DRX Cycle.

- If both Long DRX and Short DRX is configured for a particular UE, onDurationTimer i.e. on duration time during Short DRX cycle or Long DRX cycle should be same.

- When to onDurationTimer should be started depends on : 

-     If (Short DRX Cycle) && If ([(SFN * 10) + subframe number] modulo (shortDRX-Cycle) == (drxStartOffset) modulo (shortDRX-Cycle)) 

or

-     If (Long DRX Cycle) && If ([(SFN * 10) + subframe number] modulo (longDRX-Cycle) == drxStartOffset)

-     start onDurationTimer.

drx-InactivityTimer : 

- During Active time if UE receives PDCCH indicates a new transmission (DL or UL) drx-InactivityTimer started or restarted.

- Determines the number of consecutive PDCCH-subframe(s) UE monitors before going to sleep, after successfully decoding a PDCCH during active time. 

- Value in number of PDCCH sub-frames.

drx-RetransmissionTimer :

- PDCCH subframe(s) the UE should remain active as soon as a DL re-transmission is expected by the UE.

- Value in number of PDCCH sub-frames.

longDRX-Cycle :

- Once drxShortCycleTimer  expires Long DRX cycle starts.

- longDRX-Cycle and drxStartOffset . The value of longDRX-Cycle is in number of sub-frames.

- If shortDRX-Cycle is configured,the value of longDRX-Cycle shall be a multiple of the shortDRX-Cycle value.

- The value of drxStartOffset value is in number of sub-frames.

shortDRX-Cycle : 

- It is a optional one.

- If Short DRX is configured, if drx-InactivityTimer expires or a DRX Command MAC control element is received, drxShortCycleTimer  is started and short DRX cycle is used.

drxShortCycleTimer : 

- Specifies the number of time(s) the UE shall follow the Short DRX cycle.

- Value in multiples of shortDRX-Cycle. A value of 1 corresponds to shortDRX-Cycle, a

value of 2 corresponds to 2 * shortDRX-Cycle and so on.

Active Time:

• Time when UE continuously monitors PDCCH.
• Includes when onDurationTimer or drx-InactivityTimer or drx-RetransmissionTimer or mac-ContentionResolutionTimer is running.
• SR is sent on PUCCH and is pending.
• Uplink grant for a pending HARQ re-transmission can occur, and there is data in the corresponding HARQ buffer.
• A PDCCH indicating a new transmission addressed to the C-RNTI of the UE has not been received after successful reception of a RAR for the preamble not selected by the UE i.e. Dedicated RACH.

DRX Command MAC Control Element : 

- DRX Command MAC control element is identified by a MAC PDU subheader with LCID (11110).

- It has a fixed size of zero bits.

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)

All About RACH Contention Resolution Procedure

All About RACH Contention Resolution Procedure:

- RACH procedure can be Contention Based and Non-Contention Based.

Contention Based RACH Procedure:

-  In Contention based procedure UE selects a Random Access Resource i.e. UE selecting a PRACH resource, a Preamble Sequence and the next available Subframe for PRACH transmission.

- There are two types of contention based RACH procedure depending on MSG3 i.e. initial layer 3 message.

- For RRC Connection Establishment and RRC Connection Reestablishment the MSG3 is transferred on CCCH logical channel i.e. initial layer 3 message is a CCCH SDU. 
- Contention Resolution is based upon the reception of Contention Resolution Identity MAC CE. In this case a new CRNTI is allocated to the UE.
-  If ( CCCH SDU was included in Msg3 ) && If ( PDCCH transmission is addressed to its Temporary C-RNTI ) && If ( The MAC PDU is successfully decoded ) && If ( MAC PDU contains a UE Contention Resolution Identity MAC control element ) 
{
      If ( UE Contention Resolution Identity included in the MAC control element == CCCH SDU transmitted in Msg3) 
                    Then Consider Contention Resolution successful.
}

- For  Intra-System Hand Over and Uplink/Downlink data arrives while UE is in Non-synchronized RRC Connected State the MSG3 is transferred using the DCCH logical channel and contention resolution is based upon the reception of PDCCH on already allocated C-RNTI.

RACH Sequence

Non Contention Based RACH Procedure:
- ra-PreambleIndex (Random Access Preamble) and ra-PRACH-MaskIndex (PRACH Mask Index) explicitly signalled and and ra-PreambleIndex not equal to 000000.
- During Intra System HO(RRC Connection Reconfiguration with Mobility Control Info) or PDCCH order will provide the information about  ra-PreambleIndex and ra-PRACH-MaskIndex.


  

All About Random Access Procedure In LTE

When UE Initiate Random Access Procedure In LTE:
- Transition from RRC Idle to RRC Connected mode.
- Completing an Intra-System Hand Over.
- Uplink data arrives while UE is in Non-synchronized RRC Connected State(Time alignment Timer Expiry).
- Downlink data arrives while UE is in Non-synchronized RRC Connected State(Time alignment Timer Expiry).
- RRC Connection Re-establishment.

PRACH : Physical Random Access Channel:
- Used to transfer RANDOM ACCESS PREAMBLES to initiate RANDOM ACCESS PROCEDURE.
- Do not transfer RRC Signaling Messages or Application Data.

RACH - Config Common Information: 

RACH-ConfigCommon IE

RACH-ConfigDedicated IE

RACH Procedure:
- RACH Procedure can be
             - Contention Based.
             - Non Contention Based.
- Contention Based procedure involves the UE selecting a Random Access Resource i.e. UE selecting a PRACH resource, a Preamble Sequence and the next available Subframe for PRACH transmission.
- Non-Contention Based procedure involves the eNodeB allocating the Random Access Resource i.e. eNodeB allocating ra-PreambleIndex and ra-PRACH-MaskIndex.
- Contention Based RACH Procedure can be applicable for all RACH reasons but Non Contention Based RACH Procedure can be applicable for :
             - Completing an Intra-System Hand Over.
             - Downlink data arrives while UE is in Non-synchronized RRC Connected State.

Random Access Resource Selection:

Random Access Group Selection:
If (ra-PreambleIndex == Allocated By eNB && ra-PRACH-MaskIndex == Allocated By eNB)
      {
      If (ra-PreambleIndex != 000000)
           {
            Random Access Preamble && PRACH Mask Index = Allocated By eNB.
            }
       }
Elseif
      {
      If (MSG3 == Not Transmitted Yet)
           {
            If (Random Access Preambles group B == present)
                 {
                 If ( sizeof(MSG3) > messageSizeGroupA && pathloss < (P-CMAX,c– preambleInitialReceivedTargetPower – deltaPreambleMsg3 – messagePowerOffsetGroupB))
                      {
                       select the Random Access Preamble from  Random Access Preambles group B
                       }
                  Else
                      {
                       select the Random Access Preamble from  Random Access Preambles group A
                       }
                  }
              Else
                  {
                   select the Random Access Preamble from  Random Access Preambles group A
                   }
             }
       Elseif (MSG3 == Re-transmitted)
              {
               select the Same Group of Random Access Preambles = used for the preamble transmission attempt corresponding to the first transmission of Msg3
               }
       }

Random Access Preamble Selection Within the Group:
- Randomly select a Random Access Preamble within the selected group.
- The random function shall be such that each of the allowed preamble will have equal probability.
- set PRACH Mask Index to 0.

PRACH Resource Selection:
- Subframe for PRACH transmission selected using restrictions given by the prach-ConfigIndex and the PRACH Mask Index.
- physical layer timing requirements i.e. UE may take into account the possible occurrence of measurement gaps when determining the next available PRACH subframe.

Random Access Preamble Power : 
- PREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower (RACH-ConfigCommon) + deltaPreambleMsg3 (UplinkPowerControlCommon) + (PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep (RACH-ConfigCommon)
- Physical layer transmit a preamble using the selected PRACH, corresponding RA-RNTI, preamble index and PREAMBLE_RECEIVED_TARGET_POWER

Random Access Response (RAR) Reception:
- Once RACH is transmitted UE shall monitor PDCCH regardless of the possible occurrence of a measurement gap.
- RAR is identified by RA-RNTI.
- RA Response window starts at subframe that contains the end of the preamble transmission + three subframes.
- RA Response window has a length ra-ResponseWindowSize(SIB2) subframes
- The RA-RNTI is calculates as :
          RA-RNTI= 1 + t_id+10*f_id
          where t_id = Subframe within which start of preamble was transmitted(0<= t_id<=9).
                    f_id = Frequency domain index of the PRACH within the subframe.(0<= f_id<=5).
- For FDD there can be maximum one set of RBs allocated to PRACH within a subframe so f_id is always 0.
- So for FDD RA-RNTI= 1 + t_id.

All About PLMN Selection in LTE

What Is PLMN?
- Public Land Mobile Network
- PLMN ID - Unique identification of PLMN
- PLMN ID (not more than 6 digits) = MCC(Mobile Country Code) + MNC(Mobile Network Code)
- Each operator providing mobile services has its own PLMN.

Types Of PLMN:

- RPLMN : Registered PLMN
               - Derived from PLMN ID in Tracking Area Identity (TAI = PLMN + TAC).
               - Stored in EFEPSLI in USIM/NVMEM

- EPLMN : Equivalent PLMN
               - Received in Attach accept / Tracking Area Accept.
               - Regarded by the UE as equivalent to each other for PLMN selection and cell selection/re-selection.
               - UE shall update or delete this list at the end of each attach or combined attach or tracking area updating or combined tracking area updating procedure.
                - When the UE is switched off, it shall keep the stored list so that it can be used for PLMN selection after switch on.

- HPLMN : Home PLMN
               - Derived from IMSI
               - IMSI (not more than 15 digits) = PLMN ID + MSIN = MCC + MNC + MSIN 

- EHPLMN : Equivalent Home PLMN
               - Stored in USIM in EFEHPLMN

- UPLMN : User Controlled PLMN
               - Stored in USIM in EFPLMNwACT

- OPLMN : Operator Controlled PLMN
               - Stored in USIM in EFOPLMNwACT

- VPLMN : Visitor PLMN
               - Given by Physical Layer Search.

- FPLMN : Forbidden PLMN
               - Stored in USIM in EFFPLMN

PLMN Selection Modes: 
- Automatic PLMN Selection 
- Manual PLMN Selection

Automatic PLMN Selection Mode:   
- RPLMN and EPLMNs
- Either the HPLMN(if the EHPLMN (in USIM) list is not present or is empty) or the highest priority EHPLMN that is available(if the EHPLMN list is present).
- Each PLMN/Access Technology combination in the "User Controlled PLMN Selector with Access Technology"(UPLMNwACT) data file in the USIM(in Priority Order).
- Each PLMN/Access Technology combination in the "Operator Controlled PLMN Selector with Access Technology" (OPLMNwACT) data file in the USIM(in Priority Order).
- Other PLMN/Access Technology combinations with received high quality signal received in the random order (Phy Detected).
- Other PLMN/Access Technology combinations in order of decreasing signal quality (Phy Detected).

EXCEPTION:
- If the UE is in Automatic Network selection mode and if finds coverage of an EHPLMN, the UE may register to the highest priority EHPLMN available and not return to the register PLMN or equivalent PLMN.
- If the EHPLMN list is not present or is empty, and the HPLMN is available, the UE may register on the HPLMN and not return to the registered PLMN or equivalent PLMN.
- The Operator shall be able to control by USIM configuration whether an UE that supports this option is permitted to perform this alternate behaviour.

- EF - LRPLMNSI (Last RPLMN Selection Indication) plays an important role to decide this.
- EF - LRPLMNSI = '00' The UE shall attempt registration on the last RPLMN at switch on or recovery from out of coverage.
- EF - LRPLMNSI = '01' The UE shall attempt registration either on the EHPLMN or the last RPLMN at switch on or recovery from out of coverage.


Manual PLMN Selection Mode:   
- UE displays PLMNs to the USER.
- Includes FPLMNs as well.