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 LTE Radio Frame (FDD and TDD)

LTE Radio Frame: 
- Two Radio Frame Structures Supported.
- Type 1, applicable to FDD
- Type 2, applicable to TDD

Frame structure type 1:
- Applicable to both full duplex and half duplex FDD.
- Each radio frame is Tf = 307200 * Ts = 10ms long.
- Consists of 20 slots of lengthTslot = 15360*Ts = 0.5ms
- Numbered from 0 to 19
- A subframe is defined as two consecutive slots.
- Subframe i consists of slots 2i and 2i + 1.

Frame structure type 2:
- Each radio frame of length Tf  = 307200*Ts = 10ms long.
- Consists of two half-frames of length 153600*Ts = 5ms each
- Each half-frame consists of five subframes.

- The supported uplink-downlink configurations are listed below:

- Each subframe in a radio frame, “D” denotes the subframe is reserved for Downlink Transmissions.
- Each subframe in a radio frame, “U” denotes the subframe is reserved for Uplink Transmissions.
- Each subframe in a radio frame, “S” denotes the subframe is reserved for Special Subframe.
- Special Subframe consists of three fields DwPTS, GP and UpPTS.
- Both 5 ms and 10 ms downlink-to-uplink switch-point periodicity are supported.
- In case of 5 ms downlink-to-uplink switch-point periodicity, the special subframe exists in both half-frames.
- In case of 10 ms downlink-to-uplink switch-point periodicity, the special subframe exists in the first half-frame only.
- Subframes 0 and 5 and DwPTS are always reserved for downlink transmission.
- UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission.

All About Cell Specific Reference Signal

Cell Specific Reference Signal: (36.211)

- Transmitted in all downlink subframes in a cell supporting PDSCH transmission
- eNodeB transmit as Downlink Reference Signal.
- Equivalent to CPICH in UMTS network.
- Used by UE for Channel Estimation, Cell Selection, Cell Re-selection, and Handover.
- Allocated REs are distributed both in Time Domain and Frequency Domain.
- REs allocated to the Cell Specific Reference Signal are dependent update Physical Layer Cell Identity.
- RE allocation cycles once every 6 Physical Layer Cell Identities. E.g. Physical Layer Cell ID 6 has the same RE allocation as Physical Layer Cell ID 0.
- REs allocated to Cell Specific Reference Signal also depends on No Of Transmit Antenna.
- Once UE read the PSS and SSS, and consequently identified the Physical Layer Cell ID, then UE can find out the REs allocated to the Cell Specific Reference Signal and the Sequence used to generate the Cell Specific Reference Signal.
- If a RE is allocated to the Cell Specific Reference Signal on One Antenna Port, the corresponding RE on the other Antenna Ports are left Empty.

According to 36.211

All About Timing Advance (TA)

What is Timing Advance?

- UL Transmission in LTE is Not Synchronized.
- Used to control the Uplink Timing of Individual UE.
- Ensure that transmissions from All UE are Synchronized when received by the eNodeB.
- UE furthest from the eNodeB requires a larger Timing Advance to compensate for the Larger Propagation Delay.
- The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE is considered uplink
time aligned
- timeAlignmentTimerCommon(Common For All UEs In a Cell) included in SIB2.
- timeAlignmentTimerDedicated (UE specific value for Time Alignment Timer) is included in the RRC Connection Reconfiguration Message.

Timing Advance = 2 * Propagation Delay.

Timing Advance = N-TA * TS

Where,
0 < N-TA <=20152
TS = 1/30720 ms

So Maximum Timing Advance = 20512 * 1/30720 = 0.6677 ms.
Based on the speed of light this allows a maximum propagation distance of 100 km.

Timing Advance is initialized in RAR command using 11 bit TA command. 
Timing Advance in RAR takes a value from 0 - 1282

According to Spec 36.321:

N-TA = Signaled Value (TA Command) * 16
            (0 - 1282)

Once TA is initialized in the Random Access Response UE gets TA command from eNodeB using TA MAC Control Element.

TA command in MAC Control Element is of 6 bit length. Takes a value of 0 - 63.

According to Spec 36.321:

Timing Advance calculated from TA value received from TA MAC Control Element :

N-TA-New = N-TA-OLD + (TA -31) * 16

Subtracting 31 from the TA command received in MAC Control Element Allows eNodeB to move Timing Advance in Both in Positive and Negative direction.

Timing Advance Command Received in the Nth Subframe Applied to (N+6)th subframe.

The UE shall not perform any uplink transmission except the Random Access Preamble transmission when TATimer is not running.

When due to timing advance (X+1) subframe overlaps with subframe X, UE should transmit all subframes till X subframe and do not transmit overlapping part of subframe (X+1)

All About SRS (Sounding Reference Signal)

What is SRS?

- A Uplink Reference Signal.
- Not Associated with transmission of PUSCH or PUCCH.
- Use to measure Uplink Channel Quality over a Section of the Channel Bandwidth.
- Can be used by eNodeB to do Frequency Selective Scheduling  and Link Adaptation Decisions.
- eNodeB instructs UE to transmit SRS over a specific Section Of  The Channel Bandwidth
- eNodeB instructs UE to transmit SRS using a combination of  Common Information in SIB2 and UE  specific Dedicated  Information  in an RRC Connection Reconfiguration Message.
- SRS is always transmitted using the Last Symbol Of The Subframe.
- UE never instructed to send SRS over Entire Channel Bandwidth,  as it is NOT necessary to transmit  SRS within the RBs reserved  for PUCCH. PUCCH RBs are located at the Two Edges of the Channel  Bandwidth.
- SRS is used for Frequency Selective Scheduling of PUSCH, Not  PUCCH.

SRS Information In SIB2:

SRS Information In RRC Connection Reconfiguration Message: 

SRS-BandwidthConfig:(C-SRS)
- Broadcast on SIB2.
- Value from 0-7.
- Common to all UE within the Cell

SRS-Bandwidth:(B-SRS)
- Can be included in RRC Connection Reconfiguration Message.
- Can take values 0-3
- Can be UE specific.

According to 36.211: 

M-SRS : No of Resource Block over which the Sounding Reference Signal is Transmitted.

N0 - N3: One of parameter to decide the Starting position of the SRS in the Frequency Domain.

FreqDomainPosition: Received in RRC Connection Reconfiguration Message also has an impact on the Starting Position In The Frequency Domain.

SRS-SubframeConfig & SRS-ConfigIndex: The set of Subframes within which the SRS is transmitted is determined by Cell Specific SRS-SubframeConfig in SIB2 and UE specific SRS-ConfigIndex within in RRC Connection Reconfiguration Message.

SRS-SubframeConfig: 
- Takes a value between 0-14.
- Common within the Cell.
- Talks about in which subframe(s) SRS can be transmitted.

According to 36.211: 

SRS-ConfigIndex: (I-SRS)

According to 36.213: 

The SRS can transmitted in Subframes which satisfy:

(10 * nf +k-SRS -T-offset ) mod T-SRS =0

where,
  nf = SFN No (0-1023).
  k-SRS= SF No (0-9).

Duration: 
- Received in RRC Connection Reconfiguration Message.
- Takes a value TRUE or FALSE.
- TRUE - UE should Continue Transmitting SRS until instructed       otherwise.
- FALSE - UE should complete only a Single Transmission. 

TransmissionComb:
- Received in RRC Connection Reconfiguration Message.
- Allows 2 UE to Frequency Multiplex their SRS with in the Same     Resource Block

CyclicShift:(n-CS-SRS)
- Received in RRC Connection Reconfiguration Message.

SRS-HoppingBandwidth:(B-hop)

- Received in RRC Connection Reconfiguration Message.
- Allow SRS to move in the Frequency Domain Between Transmission.