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:

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.

All About Cell Search In LTE

Cell Search In LTE : 
- eNodeB broadcasts Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) to help UE with the Cell Search Process and Cell Id detection.
- There is total 504 Cell Ids (0 - 503) defined in LTE.
- These 504 Cell IDs are grouped in 168 Physical Layer Cell Identity Group.
- Each Physical Layer Cell Identity Group Consists of 3 Physical Layer Cell Identity.
- PSS and SSS is transmitted using central 62 sub carriers around the DC. The 5 REs above and below the Synchronization Signals are not used for transmission, i.e. they represents DTX periods.

Primary Synchronization Signal (PSS):
- PSS is allocated to Central 62 subcarriers.
- Belonging to the "Last Symbol" of slot 0 and slot 10 of every radio frame.
- So PSS is transmitted twice every 10 ms.
- Both Transmissions are Identical.
- PSS is used for :
          - Achieve SYMBOL, SLOT, and SUBFRAME synchronization.
          - Determine the Physical Layer Cell Identity (PCI) within the Physical Layer Cell Identity Group.
- There are 3 Physical Layer Cell Identity in each Group So PSS is generated using 1 of 3 different Sequences.

Secondary Synchronization Signal (SSS):
- SSS is allocated to Central 62 subcarriers.
- Belonging to the "Second Last Symbol" of slot 0 and slot 10 of every radio frame.
- So SSS is also transmitted twice every 10 ms.
- The 2 SSS transmissions within each radio frame use Different Sequences.
- This is to allow UE to differentiate between the 1st and 2nd transmission.
- This helps UE to determine the starting of each radio frame, i.e. to achive the Frame Synchronization.
- SSS is used for :
          - Achieve FRAME synchronization.
          - Determine the Physical Layer Cell Identity Group.
- There are 168 Physical Layer Cell Identity Group So SSS is generated using 1 of 168 different pairs of Sequences.

Cell Id Identification:
- Once UE read the PSS and SSS, UE will be able to get the Cell ID from the  Physical Layer Cell Identity and Physical Layer Cell Identity Group.
- Cell ID = 3 * Physical Layer Cell Identity Group + Physical Layer Cell Identity.

All About Resource Block (RB)

Resource Blocks (RB):
- Basic unit of resource for the LTE air-interface.
- eNodeB scheduler allocates RBs to UE to allow data transfer.
- Defined in both time and frequency domains.

In Time Domain:
- Occupies 0.5 ms slot in time domain.
- Consists of 7 OFDMA symbols when using Normal Cyclic Prefix.
- Consists of 6 OFDMA symbols when using Extended Cyclic Prefix.

In Frequency Domain: 
- Consists of 12 subcarriers.
- Each subcarrier is of 15 KHZ.
- Each RB occupy 12*15 = 180 KHZ in frequency domain.

- The GRID generated by One Sub-Carrier in the Frequency Domain and One Symbol in the Time Domain defines a RESOURCE ELEMENT (RE).
- RB consists of 84 (12*7) REs when using Normal Cyclic Prefix.
- RB consists of 72 (12*6) REs when using Extended Cyclic Prefix.
- A single RE can carry a Single Modulation Symbol (2 bits when using QPSK, 4 bits when using 16QAM, and 6 bits when using 64QAM).

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.