LTE Terminology

eNB or eNodeB 

• It is similar to Base Station in GSM networks and NodeB in WCDMA.
• Prefix "e" stands for evolved.

UE 

• User equipment. Could be handphone, tablet, datacard. It is similar to mobile subscriber (MS) in GSM environment.

OFDMA

• Orthogonal Frequency Division Multiple Access.
• Is a technique for transmitting large amounts of digital data over a radio wave by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver.
• Used in physical layer of LTE Downlink. 
• It is digital modulation scheme & better in dealing with multi-path effect in radio transmission.

SC-FDMA

• Single Carrier Frequency Division Multiple Access.
• Used in physical layer of LTE Uplink due to low PAPR (Peak to Average Power) ratio.
• This increases the efficiency of power amplifier and hence increases the battery life

LTE Frame

• LTE frame are of 2 types FDD (Type 1) and TDD (Type 2). 
• In both the cases, frame is composed of 10 sub frames and each sub frame is made of 2 slots. Frame size is 10ms.

Resource Block (RB)

• The smallest block of resource that can be allocated to UE by eNB; 
• It is 12 sub-carriers for 7 symbols (Normal CP); 6 symbols (Extended CP)
• LTE Bandwidth (1.4KHz=6RB, 3MHz=15RB, 5MHz=25RB, 10MHz=50RB, 15MHz=75RB and 20MHz=100RB) 

Resource Element (RE)

• The smallest unit of radio resources, one sub-carrier for one symbol.

Slot

• 7 consecutive symbols for short Cyclic Prefix (Normal CP), 6 symbols for long cyclic prefix (Extended CP).

Sub-frame

• 2 consecutive timeslots.
• Each timeslot has 0.5ms. 

Reference Signal

• Similar to pilot carrier and is used for channel estimation at the receiver.

PCI

• Physical Cell Identity/ Identifier.
• Provide value for identifying eNodeBs.
• Created from two components; PSS & SSS
• PSS has the value 0, 1, or 2 (total 3)
• SSS can have a value between 0 to 167 (total 168)
• The PCI value is [(3(SSS)+PSS], resulting in a value between 0 - 503 (total is 504, coming from 168*3).
• PCI needs to plan very well to avoid high interruption of the Reference Signal (RS) which may result towards bad signal coverage. 
• Poor planning results in PCI conflicts or collisions which impact network performance.
  

 Synchronization signal

• Is used as preamble sequence in LTE for synchronization purpose.
• There are two synchronization signals, Primary (PSS) and Secondary SSS). 
• Primary Synchronization Signal: Determine Cell ID in a cell group 
• Secondary Synchronization Signal: Determine cell group ID
• Both are transmitted in slot 0 and slot 10 in all the frames. 
• It is same as preamble used in earlier systems and used for time, frequency synchronization purpose.
• 168 physical layer cell group & each group consist of 3 ID. Total is 504. That's make it the PCI (0-503). The PCI value is determined by [(3x SSS)+(PSS)].
• In 3G, PSC consists of 512 (0-511)  

P-SS

• Primary Synchronization Signal
• Is used for initial synchronization
• Determine Cell ID in a Cell Group

S-SS

• Secondary Synchronization Signal
• S-SS is used for frame boundary determination
• First signal transmitted upon access network
• Determine the Cell Group ID

S-GW

• Serving Gateway
• For user plane/ data plane
• Also the interface to MME during CSFB 

MME

• Mobility Management Entity
• For control plane 

X2 Interface

• Interface between eNodeB and eNodeB.

S1 Interface

• Interface between eNodeB and core network interface (MME/S-GW).

Uu Interface

• Interface between UE and eNodeB.

Control channel

• This channel carry the information to control, make, maintain and terminate the connection. 
• Used for the transfer of control plane information in LTE.

Data channel

• This channel carry traffic information. 
• Used for the transfer of user plane information.

PBCH

• Physical Broadcast Channel.
• It carries only MIB (Master Information Block)
• It uses QPSK modulation.

PDSCH

• Physical Downlink Shared Channel. It's RB is shared among all other active connections.
• Used to transmit paging message, RRC signalling messages (Signalling radio bearer will use this channel) and transfer application data.
• It also broadcast information that are not transmitted on the PBCH which include System Information Blocks (SIB). 
• This channel uses QPSK, 16QAM, 64QAM modulation types (based on adaptation algorithm by eNodeB - depending on radio condition & buffer capacity).
• For SIB and Paging, it uses QPSK (the most robust scheme).

PDCCH

• Physical Downlink Control Channel.
• Used to carry DCI (Downlink Control Information) - information about number of OFDMA symbols used after decoding PCFICH.
• Normally, a maximum of three or four OFDM symbols can be used for PDCCH.
• These symbols are always at the start of each sub-frame.
• This channel also used to inform the UE about the resource allocation of PCH (paging channel) & DL-SCH (downlink shared channel).
• Among other function, indicating the modulation, coding and hybrid-ARQ (HARQ) information related to DL-SCH.
• It uses QPSK modulation .

PCFICH

• Physical Control Format Indicator Channel
• Physical Channel that carries CFI (Control Format Indicator)
• CFI is a indicator telling how many OFDM symbols are used for carrying control channel (e.g, PDCCH and PHICH) at each sub-frame.
• If CFI is set to be 1 for a sub-frame, it means one symbol (the first symbol) at the sub-frame is used for PDCCH allocation. If CFI is 2, it means two symbols (the first and the second symbol) are used for PDCCH and so on.
•  

PHICH

• Physical Hybrid ARQ Indication Channel.
• A control channel on LTE downlink.
• Used to indicate the reception of PUSCH transmission from a mobile/UE. 
• The PHICH indicates either "ACK" or "NACK", which the UE will decode to find out if
  Carries Hybrid ARQ ACK/NAKs in response to uplink transmissions (whether re-transmission is required or not). 
• Uplink transmission -> from UE to network.

PCH

• Paging channel
• It carries RRC paging message (RRC procedure) using RLC (radio link control) and PCCH (physical control channel)
• The RRC procedure is a procedure to waking up the UE which is Idle (RRC Idle state) using paging cycle (part of SIB2).
• If there is any Paging for it then it will react to it by triggering a RRC connection request message. If no Paging, then it will cont to sleep (saving resources & UE battery).
• Mean, it support the UE discontinuous reception (DRX) to enable UE power saving (DRX cycle is indicated by the network to the UE).

RS

• Reference Signal, used both in uplink and downlink
• Only exist at PHY layer.
• Used as pilot subcarrier in LTE for channel estimation and tracking.
• Is not for delivering any specific information.
• The main purpose is to deliver the reference point for the downlink power. 
• RS are of two types viz. Demodulation RS (DRS) and Sounding RS (SRS)

SRS

• Sounding Reference Signal, used in uplink only.
• SRS is used for channel quality estimation purpose
• Used to estimate UL channel quality and provide input to UL scheduler, as well as help timing advance.

DMRS

• Demodulation Reference Signal, used both in uplink and downlink.
• DRS is used for sync and channel estimation purpose
• This is a reference signal for PUSCH implying that eNodeB would not be able to decode PUSCH if this PUSCH DMRS is bad.
• PUSCH DMRS always takes up the center symbol of a slot (meaning symbol 3 and symbol 10 of a UL sub-frame). 

PRACH

• Physical Random Access Channel used in uplink
• Carries the random access preamble.

PUSCH

• Physical Uplink Shared Channel
• Carries the UL-SCH.

PUCCH

• Physical Uplink Control Channel
• Carries Hybrid ARQ ACK/NAKs in response to downlink transmission (network to UE);
  - Carries Scheduling Request (SR);
  - Carries CQI reports.
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 to be cont..