LTE Physical Layer

LTE Physical Layer:                                                         Reference: 3GPP TS 36.206

• The Figure below shows the physical layer interfaces the Medium Access Control (MAC) sub-layer of Layer 2 and the Radio Resource Control (RRC) Layer of Layer 3.
• The physical layer offers a transport channel to MAC.
• MAC offers different logical channels to the Radio Link Control (RLC)

Service provided to higher layers:

•    Error detection on the transport channel and indication to higher layers
•    FEC encoding/decoding of the transport channel
•    Hybrid ARQ soft-combining
•    Rate matching of the coded transport channel to physical channels
•    Mapping of the coded transport channel onto physical channels
•    Power weighting of physical channels
•    Modulation and demodulation of physical channels
•    Frequency and time synchronization
•    Radio characteristics measurements and indication to higher layers
•    Multiple Input Multiple Output (MIMO) antenna processing
•    Transmit Diversity (TX diversity)
•    Beam-forming
•    RF processing.

LTE physical signals

Downlink physical signals	Purpose
Primary synchronization signal	Used for cell search and identification by the UE. Carries part of the cell ID (one of three orthogonal sequences)
Secondary synchronization signal	Used for cell search and identification by the UE. Carries the remainder of the cell ID (one of168 binary sequences)
Reference signal	Used for downlink channel estimation. Exact sequence derived from cell ID (one of 3 x 168 = 504 pseudo random sequences)
Uplink physical signals	Purpose
Reference signals(demodulation and sounding)	Used for synchronization to the UE and for UL channel estimation

LTE physical channels

DL channels	Full name	Purpose
PBCH	Physical broadcast channel	Carries cell-specific information
PMCH	Physical multicast channel	Carries the MCH transport channel
PDCCH	Physical downlink control channel	Scheduling, ACK/NACK
PDSCH	Physical downlink shared channel	Payload
PCFICH	Physical control format indicator channel	Defines number of PDCCH OFDMA symbols per subframe (1, 2, 3, or 4)
PHICH	Physical hybrid ARQ indicator channel	Carries HARQ ACK/NACK
UL channels	Full name	Purpose
PRACH	Physical random access channel	Random Access Procedure for Call Setup
PUCCH	Physical uplink control channel	Scheduling, ACK/NACK
PUSCH	Physical uplink shared channel	Payload

  

Physical layer procedures

-        Cell search

-        Power control

-        Uplink synchronisation and Uplink timing control

-        Random access related procedures

-        HARQ related procedures