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 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.