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 MAC Control Elements

MAC Control Elements:

- Way of FAST Signaling Communication Exchange Between UE and eNodeB.

- Send as a part of MAC PDU.

- MAC control elements are always placed before any MAC SDU.

MAC Control Element Types:

• Buffer Status Report MAC Control Elements
• C-RNTI MAC Control Element
• DRX Command MAC Control Element
• UE Contention Resolution Identity MAC Control Element
• Timing Advance Command MAC Control Element
• Power Headroom MAC Control Element
• Extended Power Headroom MAC Control Element
• MCH Scheduling Information MAC Control Element
• Activation/Deactivation MAC Control Element

MAC CE Header:

- LCID field in MAC Subheader denotes MAC CE Type.

Values of LCID for DL-SCH:

Index	LCID values
00000	CCCH
00001-01010	Identity of the logical channel
01011-11010	Reserved
11011	Activation/Deactivation
11100	UE Contention Resolution Identity
11101	Timing Advance Command
11110	DRX Command
11111	Padding

Values of LCID for UL-SCH:

Index	LCID values
00000	CCCH
00001-01010	Identity of the logical channel
01011-11000	Reserved
11001	Extended Power Headroom Report
11010	Power Headroom Report
11011	C-RNTI
11100	Truncated BSR
11101	Short BSR
11110	Long BSR
11111	Padding

Buffer Status Report MAC Control Elements:

- Short BSR and Truncated BSR format :

- Long BSR format: 

- If extendedBSR-Sizes is not configured, the values taken by the Buffer Size field are in Table 6.1.3.1-1(3GPP TS 36.321). If extendedBSR-Sizes is configured, the values taken by the Buffer Size field are in Table 6.1.3.1-2(3GPP TS 36.321).

- Short BSR Header:

3D : MAC sub-header - Short BSR

R = 0

R = 0

E = 1

LCID = 11101 = Short BSR

1D : MAC sub-header - Short BSR

R = 0

R = 0

E = 0

LCID = 11101 = Short BSR

- Long BSR Header:

3E : MAC sub-header - Long BSR

R = 0

R = 0

E = 1

LCID = 11110 = Long BSR

1E : MAC sub-header - Long BSR

R = 0

R = 0

E = 0

LCID = 11110 = Long BSR

- Truncated BSR Header:

3C : MAC sub-header - Truncated BSR

R = 0

R = 0

E = 1

LCID = 11100= Truncated BSR

1C : MAC sub-header - Truncated BSR

R = 0

R = 0

E = 0

LCID = 11100= Truncated BSR

C-RNTI MAC Control Element Format : 

C-RNTI MAC control element

UE Contention Resolution Identity MAC Control Element :

UE Contention Resolution Identity MAC control element

- Has a fixed 48-bit size

- UE Contention Resolution Identity: This field contains the uplink CCCH SDU.

3C : MAC subheader - Contention Resolution

R = 0

R = 0

E = 1

LCID = 11100 = Contention Resolution

Timing Advance Command MAC Control Element :

Timing Advance Command MAC control element

- Timing Advance Command is of 6 bits in length. T_A (0, 1, 2… 63). 
Power Headroom MAC Control Element : 

Power Headroom MAC control element

Activation/Deactivation MAC Control Element : 

- The C_i field is set to "0" to indicate that the SCell with SCellIndex i shall be deactivated.

Activation/Deactivation MAC control element

Padding MAC Sub-Header:

1F : MAC subheader - Padding

R = 0

R = 0

E = 0

LCID = 11111 = Padding

All About MAC PDU

MAC PDU: 
- A MAC PDU consists of'

• MAC Header
• Zero or More MAC SDU, 
• Zero or More MAC Control Elements and
• Optionally Padding.

- MAC header and MAC SDU are of variable SIZE.
- The MAC header and subheaders are octet aligned.

- A MAC PDU sub-header consists of the six header fields R/R/E/LCID/F/L.

- For the last sub-header in the MAC PDU, for fixed sized MAC control elements and Padding consist of the four header fields R/R/E/LCID.

- MAC Padding always placed at the end of the MAC PDU, with only exception when single-byte or two-byte padding is required.PDU subheaders have the same order as the corresponding MAC SDUs, MAC CE and Padding.
- A maximum of one MAC PDU can be transmitted per TB per UE.

MAC PDU consisting of MAC header, MAC control elements, MAC SDUs and padding

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 Carrier Aggregation (CA)

Carrier Aggregation (CA):

- LTE Release 10 Feature.

- Known as LTE Advance.

- DL Speeds upto 1 Gbps and UL Speeds upto 500Mbps.

- Backward Compatibility with Rel 8 and Rel 9.

- Can be used for both FDD and TDD.

- The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz.

- A maximum of 5 Component Carriers(CC) can be aggregated.

- The maximum aggregated bandwidth is 100 MHz.

- Each component carrier is treated as an R8 carrier.

- The spacing between center frequencies of contiguously aggregated component carriers will be a multiple of 300 kHz to be compatible with the 100 kHz frequency raster of Release 8/9 and at the same time preserve orthogonality of the subcarriers, which have 15 kHz spacing.

- For non-contiguous cases the CCs are separated by one, or more, frequency gap(s).

LTE-Advanced Support Three Component Carrier Aggregation Scenarios:

- Intra-Band Contiguous

- Intra-Band Non-contiguous

- Inter-Band Non-contiguous

PCell and SCell :
- The Cell UE selects during initial establishment (RRC Connection Request/RRC Connection Re-establishement Request) will become the PCell.
- eNodeB can add / delete Scell(s) using RRC Connection Reconfiguration message.

PCell Vs SCell:
- PCell always have both Uplink(UL) and Downlink(DL). Scell always have DL (While activated) but may or may not have UL.
- PCell is always activated whereas SCell has to be activated or deactivated using MAC-CE.
- UE does not required to acquire System Information and decode Paging from SCell.
- For Scell SI is passed to UE while adding the Scell. 
- When an Scell is added using RRC Connection Reconfiguration Message it remains in the deactivated state till it is activated using MAC-CE.
- If Scell activation/deactivation MAC-CE is received on Subframe n the Scell is activated/deactivated on Subframe n+24 or n+34.(TS 36.133 Section 7.7.2)
- When sCellDeactivationTimer expires then Scell is deactivated.
- Once Scell is deactivated 
         - PDCCH on Scell and PDCCH for Scell is not monitored. 
         - PUSCH is not transmitted and PDSCH is not received. 
         - The SRS is not transmitted.
         - The CQI/PMI/RI for Scell is not reported.


Activation/Deactivation MAC-CE:
- The MAC-CE can activate and deactivate Scell(s) which is already configured using RRC Connection Reconfiguration Meassage.
- Control Element is identified by a MAC PDU subheader with LCID.
.

Values of LCID for DL-SCH

Index	LCID values
11011	Activation/Deactivation

- fixed size and consists of a single octet containing seven C-fields and one R-field.

Activation/Deactivation MAC control element

- The Ci field is set to "1" to indicate that the SCell with SCellIndex i shall be activated.

- The Ci field is set to "0" to indicate thatthe SCell with SCellIndex i shall be deactivated.

- R: Reserved bit, set to “0”.
- 

Pcell and Scell Concepts:
- Pcell can be changed using RRC Connection Reconfiguration With MobilityControlInfo i.e. Handover.
- Scell can be changed using RRC Connection Reconfiguration message.
- During Radio Link Failure, the Scell is release first before initiating RRC Connection Re-establishment procedure.
- On receiving Handover Command i.e. RRC Connection Reconfiguration With MobilityControlInfo, UE deactivates the Scell, if configured.
- TTI Bundling is not supported when configured with one or more Scell with Configured Uplink.
- The RSRP and RSRQ measurement for Pcell shall follow time domain measurement resource restriction in accordance with measSubframePatternPCell, if configured.

Procedure	Pcell	Scell
Radio Link Monitoring	Y	N
System Information Acquisition	Y	N
Random Access Procedure	Y	N
Time Alignment	Y	N
SPS	Y	N
Connected Mode DRX	Y	Y
Paging	Y	N
NAS Mobility Information	Y	N
Data Transmission	Y	Y
S-measure Criteria	Y	N
Power Control	Y	Y
Link Adaptation	Y	Y
Pathloss Meaurement	Y	Y= when pathloss reference=Scell N= when pathloss reference=Pcell

Channel	Pcell	Scell
PDCCH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PDCCH when CCS but eNodeB might be transmitting PDCCH.)
PDSCH	Y	Y
PUCCH	Y	N
PUSCH	Y	Y
PRACH	Y	N
SRS	Y	Y
PBCH	Y	N - UE does not look at PBCH But eNodeB might be transmitting it.
PSS/SSS	Y	N - UE does not look at PSS/SSS but to get timing information eNodeB might be transmitting PSS/SSS.
PCFICH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PCFICH when CCS but eNodeB might be transmitting PCFICH.)
PHICH	Y	Y - No Cross Carrier Scheduling N - Cross Carrier Scheduling (UE does not look at PHICH when CCS but eNodeB might be transmitting PHICH.)

Cross Carrier Scheduling(CCS):
- Downlink Scheduling or Uplink Grant information of One Component Carrier(CC) can be carried by the PDCCH of another Component Carrier(CC).
- 3 bit CIF field indicates target CC.
- Pcell shall always be scheduled by Pcell only.
- Scell can be cross scheduled by Pcell or by other Scell.
- UE indicates whether it supports CCS or not.
- Cross Carrier Scheduling is not applicable for PDCCH order. It is transmitted on Pcell.
- CCS is applicable for aperiodic SRS transmission.

- The cif-Presence-r10 in physicalConfigDedicated indicates whether CIF will be present in PDCCH of Pcell.

- The RadioResourceConfigDedicatedSCell-r10.PhysicalConfigDedicatedSCell-r10.CrossCarrierSchedulingConfig-r10 indicates CCS status of Scell.

- cif-Presence indicates whether carrier indicator field is present (value TRUE) or not (value FALSE) in PDCCH DCI formats.

- pdsch-Start indicates the starting OFDM symbol of PDSCH for the concerned SCell. Values 1, 2, 3 are applicable when dl-Bandwidth for the concerned SCell is greater than 10 resource blocks, values 2, 3, 4 are applicable when dl-Bandwidth for the concerned SCell is less than or equal to 10 resource blocks.

- schedulingCellId Indicates which cell signals the downlink allocations and uplink grants, if applicable, for the concerned SCell.(When Scell cross scheduled  other Scell.)

- The other-r10.schedulingCellId-r10 and cif-Presence-r10 of that cell should be consistent.

Carrier Aggregation and Measurement Events:

- Definition of Serving Cell Measurement is Modified.

- For Event A1 and Event A2 The Carrier Frequency in Measurement Object indicates whether this event is for Pcell or any Scell. 

- The eNodeB shall configure separate A1/A2 events for each serving cell.

- Event A3 - Neighbor becomes offset better than Pcell.

- Event A5 - Pcell becomes worse than theshold1 and neighbour becomes better than threshold2.

        - For Event A3 and Event A5 the frequency mentioned in the associated measObjectEUTRA indicates neighbours.

        - For Event A3 and Event A5 the Scell become neighbouring cell.

- Event B2 - Pcell becomes worse than theshold1 and inter RAT neighbour becomes better than threshold2.

- Event A6 - Intra Frequency Neighbour becomes offset better than Scell.

- No change in the definition of Event A4 and Event B1.

Carrier Aggregation and Periodic Measurement:

- If (Purpose == reportStrongestCells && reportAmount > 1)

      UE initiates a first MR immediately after the quantity to be reported becomes available for the Pcell.

- If (Purpose == reportStrongestCells && reportAmount == 1)

      UE initiates a first MR immediately after the quantity to be reported becomes available for the Pcell and for the strongest cell among the applicable cells.

- If (Purpose == reportStrongestCellsForSON)

      UE initiates a first MR when it has determined the strongest cells on the associated frequency.

Carrier Aggregation and Measurement Gap:

- UE shall be able to carry out Measurement on any serving frequency without measurement gap i.e. intra-frequency measurement for any serving frequency.

- UE may required measurement gap to perform inter-frequency or inter-RAT measurement.

Typical CA Call Flow: