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 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