All About Automatic Neighbor Relation (ANR)

Automatic Neighbor Relation (ANR):
- Automatic Neighbor Relation (ANR) function is to relieve the operator from the burden of
manually managing Neighbor Relations (NRs).
- The ANR function resides in the eNB and manages the conceptual Neighbor Relation Table (NRT).
- The Neighbor Detection Function finds new neighbors and adds them to the NRT.
- ANR also contains the Neighbor Removal Function which removes outdated NRs.
- An existing Neighbour Relation from a source cell to a target cell means that eNB controlling the source cell:

• Knows the ECGI/CGI and PCI of the target cell.
• Has an entry in the Neighbour Relation Table for the source cell identifying the target cell.
• Has the attributes in this Neighbour Relation Table entry defined, either by O&M or set to default values.

- NRT contains : 

• Target Cell Identifier (TCI)
• Identifies the target cell. 
• For E-UTRAN, the TCI corresponds to the E-UTAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell. 

Each NR has three attributes: 

• NoRemove : eNB shall not remove the Neighbor cell Relation from the NRT.
• NoHO : Neighbor cell Relation shall not be used by the eNB for handover reasons.
• NoX2 attribute : Neighbor Relation shall not use an X2 interface in order to initiate procedures towards the eNB parenting the target cell.

NR Table

- Part of Self-Configuring Networks in SON.
- Manual Addition of cells in the neighbor list is a tough task. It is becomes very difficult when there are exiting networks like 2G,3G.
- For LTE there can be Intra LTE, Inter LTE and Inter RAT neighbors.
- There can be:

• Intra-Freq Automatic Neighbor Relations. 
• Inter-Freq/Inter System Automatic Neighbor Relations. 

Intra-Freq Automatic Neighbor Relations:

- Intra-Frequency neighbors can be added automatically as part of Intra-Freq handover procedure.

-  As a process for Intra Frequency HO procedure the serving eNodeB instructs each UE  to perform Intra-Frequency Measurement on neighboring cell by sending RRC Connection Reconfiguration message with measurement control information.

Automatic Neighbour Relation Function

- Consider as in the above picture, Serving Cell is CellA(UE is in RRC Connected State) and Cell B is the Target Cell: 
1.

• The UE sends a measurement report regarding cell B. 
• This report contains Cell B’s PCI, but not its ECGI. 
• eNodeB checks if the reported PCI is already included in the Neighbor Database, then the HO proceeds in the normal way.
• If reported PCI is not included in the Neighbor Database then eNB proceeds to add the PCI to its NRT.

2.

• Once eNB receives a UE measurement report containing the PCI, the eNB instructs the UE with another RRC Connection Reconfiguration Message, using the newly discovered PCI as parameter.
• Instruct UE to read the ECGI, the TAC and all available PLMN ID(s) of the related neighbor cell. 
• To do so, the eNB may need to schedule appropriate idle periods to allow the UE to read the ECGI from the SIB1 of the detected neighbor cell. 
• The UE reads the requested information from SIB1 on PDSCH. UE needs to read MIB on PBCH, then DCI with SI-RNTI on PDCCH to read SIB1 on PDSCH.

3. 

• When the UE has found out the new cell’s ECGI, the UE reports the detected ECGI to the serving cell eNB. 
• In addition the UE reports the tracking area code and all PLMN IDs that have been detected. 
• If the detected cell is a CSG or hybrid cell, the UE also reports the CSG ID to the serving cell eNB.

4. 
The eNB decides to add this neighbor relation, and can use PCI and ECGI to:

• Look up a transport layer address to the new eNB.
• Update the Neighbor Relation List.
• If needed, setup a new X2 interface towards this eNB. 

Inter-Freq/System Automatic Neighbor Relations:
- Can be done as a part of the normal inter-frequency and inter-system handover procedure.
- Inter-frequency and Inter-RAT measurement requires compressed mode to be configured.
- The eNodeB instructs UE to start inter-frequency and inter-RAT measurement using RRC Connection Reconfiguration message.
- The UE searches for the neighbor cells, identifies and reports them to eNodeB.
- The format of PCI depends upon the RAT of the Cell being measured.

Measurement Info Reported By UE

- Neighbor cell addition procedure:

Automatic Neighbour Relation Function in case of Inter Frequency/System Neighbor

- Once UE receives the Measurement Report for the Inter Frequency/RAT cell, 

• eNodeB checks if the reported PCI is already included in the Neighbor Database, then the HO proceeds in the normal way.
• If reported PCI is not included in the Neighbor Database then eNB proceeds to add the PCI to its NRT

- eNodeB instruct UE using another RRC Connection Reconfiguration message to decode the Global Cell Identity (CGI) from the system information.
- The UE may be required to report additional information depending on the system being measured.
- The eNB updates its inter-RAT/inter-frequency Neighbour Relation Table.
- In the inter-frequency case and if needed, the eNB can use the PCI and ECGI for a new X2 interface setup towards the new eNB.

All About Self Organizing Network (SON)

Self Organizing Network : 
- Aims to reduce the network operational cost and improve user experience.
- Addition of new network element should be in Plug and Play mode.
- SON introduced in Rel 8 version of the 3GPP specification.
- Fully implemented in Rel 9 and Rel 10.
- 3 main concepts :

• Self - Configuring Network
• Self - Optimizing Network
• Self - Healing Network

Self Configuring Network :

- Aimed at automating the deployment of new eNodeB.
- eNodeB should be added to the Network as Plug and Play.
- Introducing a new eNodeB to the system should be automatic and eNodeB will be able to allocate appropriate Physical Layer Cell Identity.
- ANR is part of Self Configuring Network.
- Should be easy to deploy and should be less sensitive to error.
- Most used during initial network deployment.

Self - Optimizing Network :

- Aimed at automating network performance improvement.

- Includes 

• Coverage and capacity optimization
• Handover optimization
• RACH optimization
• Reduce inter-cell interference between eNodeB.

- Aimed at reducing the requirement of drive test.

- Useful throughout the life of the network

Self - Healing Network :

- Aimed at automating the fault handling.

- Faults should be automatically detected and corrected.

- Reduces outage time.

- Improves network performance

- Improves end-user experience.

- If one the eNodeB goes bad then self - healing allows neighboring eNodeBs to compensate by using their unused power headroom.

- It also uses redundancy within the network. 

- Useful throughout the life of the network

All about Buffer Status Reporting (BSR)

Buffer Status Reporting (BSR) :

- The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE.

Type Of BSR:

- UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR".

- UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR".

- retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"

- periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR".

When UE will Report Which Type:

- periodicBSR-Timer expires, "Periodic BSR".

For Regular and Periodic BSR:

 If (More than one LCG has data available for transmission in the TTI where the BSR is transmitted)

      Report Long BSR.

 else,

      Report Short BSR.

For Padding BSR:

If (Number of padding bits => size of the Short BSR plus its subheader) && If (Number of padding bits < size of the Long BSR plus its subheader)

Then Short BSR or Truncated BSR Can be transmitted instead of Padding.

If (More than one LCG has data available for transmission in the TTI where the BSR is transmitted): 
Then : 

     Report Truncated BSR of the LCG with the highest priority logical channel with data available for transmission;
Else

     Report Short BSR.

Elseif (Number of padding bits => Size of the Long BSR plus its subheader)         

     Report Long BSR.

All About HARQ

HARQ :

- It is a re-transmission technique used by LTE for re-transmitting of UL & DL Data.
- HARQ (Hybrid ARQ) = ARQ (Automatic Repeat Request) +  FEC (Forward Error Correction).

- The HARQ makes use of ARQ along with an Error Correction technique called 'Soft Combining', which no longer discards the received corrupted data.

- Using 'Soft Combining' data packets that are not decoded are not discarded anymore. The received signal is stored in a 'buffer', and combined with next re transmission.

- Hybrid ARQ (HARQ) leads to higher efficiency in transmission and error correction.
- There is one HARQ entity per UE with 8/16 stop-and-wait processes for each HARQ entity.
- It means Sender will not send new data or re-transmitted data until he will not get ACK or NACK from receiver respectively.
- As Sender is waiting for ACK/NACK from receiver, hence it decreases the through put. To overcome this issue, LTE uses multiple parallel HARQ Process with different process ID.
- FDD-LTE uses 8 HARQ Parallel Process having unique process ID 0, 1, 2...7 (3 bits reserve for HARQ Process ID in DCI Messages).
- In TD-LTE, it uses 16 HARQ Parallel Process ID having unique process ID 0, 1…15(4 bits reserve for HARQ Process ID in DCI Messages).
- Both Incremental redundancy(IR) and Chase combining(CC) are supported.
- The number of HARQ re transmissions targeted by the HARQ protocol depends on the network provided configuration.

FEC (Forward Error Correction) :

FEC or Channel Coding is a technique used for controlling and correcting error in LTE Data transmission.

Channel coding supported for LTE Data Transmission

HARQ with Soft Combining :

In practice, incorrectly received coded data blocks are often stored at the receiver rather than discarded, and when the re-transmitted block is received, the two blocks are combined. This is called Hybrid ARQ with soft combining

Soft Combining Techniques

- IR requires larger receiver buffer than CC but can achieve better performance than CC.
- CC is simple HARQ and requires small receiver buffer.

Chase Combining :

- Every re-transmission = The same information (data and parity bits).
- Receiver uses maximum-ratio combining to combine the received bits with the same bits 
from previous transmissions.
- All transmissions are identical So Chase combining seen as additional repetition coding.
- This scheme achieves gain with small buffer size in a receiver. 
- The buffer size becomes the number of coded symbols of one coded packet

Incremental Redundancy :

- To transmit additional redundant information in each re-transmission and receiver decode on each re-transmission. 
- Every retransmission contains different information than the previous one.
- IR requires larger size of buffer in a receiver than Chase Combining. The buffer size becomes the number of coded bits of total transmitted coded packets.

Redundancy Versions (RV) :

- Different combinations of systematic data bits + FEC bits.
- LTE HARQ has 4 RVs typically of a packet (0,1,2,3).

Difference between LTE HARQ used in UL and DL:

• UL: A synchronous HARQ mode is used.
• DL: An adaptive, asynchronous HARQ.

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 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.