Saturday, September 19, 2009

LTE Radio Interface

LTE is not complete without the radio interface. It has been my burning desire to understand the radio network of LTE. I did some research and this is the second post on radio side of the network.

LTE Radio Interface User Plane protocols


In downlink data from SAE will enter eNB. The data is an IP packet. The IP packet is several protocols and is passed to UE.

LTE Radio Interface Control Plane Protocols


The control has two more layers over PDCP. RRC layer is terminated at eNB, while NAS layer goes all the way to MME.

Lets take a look at each layer individually: -

NAS: Non-Access Stratum (3GPP TS 24.301)

NAS is responsible for EPS bearer management, authentication, paging and mobility handling in ECM IDLE state.

RRC: Radio Resource Control (3GPP TS 36.331)

This layer is responsible for Broadcast and paging. It also takes care of RRC connection management, radio bearer control, mobility functions and UE measurement reporting and control.

PDCP: Packet Data Control Protocol (3GPP TS 36.323)

This layer is responsible for IP header compression to avoid unnecessary overhead in the payload. This layer is also responsible for ciphering and integrity protection check.

RLC: Radio Link Control (3GPP TS 36.322)           

RLC is responsible for segmentation/concatenation, retransmission handling and in sequence delivery of messages to higher layers. RLC offers services to PDCP in form of radio bearer. These radio bearers are mapped to EPS bearers in EPC.

MAC: Media Access Control (3GPP TS 36.321)

Mac handles ARQ, uplink and downlink scheduling. The scheduling functionality is located in eNB. There is one MAC entity per cell for both uplink and downlink. The HARQ is present in both UE and eNB. MAC offers services to RLC inform of logical channels.

Physical Layer: (3GPP TS 36.201)

It handles coding/decoding, modulation/demodulation, multiple antennas etc. It offers services to MAC layer inform of transport channels.

LTE Channels Over view: (3GPP TS 36.300)


LTE Physical Channels: Downlink Channels

-       Physical Broadcast Channel: PBCH

-       Physical Control Format Indicator Format: PCFICH

o   This informs UE about number of OFDM symbols used for the PDCCH’s.

o   This is transmitted in downlink.

-       Physical Downlink Control Channel: PDCCH

o   Informs UE about resource allocation of PCH & DL-SCH and HARQ information related to DL-SCH

o   PCH: Paging channel. DL-SCH: Downlink Synchronization Channel.

-       Physical Hybrid ARQ Indicator Channel: PHICH

o   Carries Hybrid ARQ Ack/NAK’s in response to uplink transmission

-       Physical Downlink Shared Channel: PDSCH

o   Carries DL-SCH and PCH

-       Physical Multicast Channel: PMCH

o   Carries Multicast channel (MCH).

Uplink Channels

-       Physical Uplink Control Channel: PUCCH

o   Carries HARQ ACK/NAK in response to downlink transmission

o   Carries scheduling request.

-       Physical Uplink Share Channel: PUSCH

o   Carries UL-SCH

-       Physical Random Access channel: PRACH

o   Carries random access preamble.

LTE Transport Channels

The physical layer offers information transfer services to MAC and higher layers. The physical layer transport services are described by how and with characteristics data is transferred over the radio interface. (What kind of data is transferred is dealt in logical channels)

Downlink Transport Channels:

-       Broadcast channel: BCH

o   This channel is used to broadcast info in the entire cell.

o   It has fixed and pre defined Transport Format (not aware of TF’s yet)

-       Downlink Shared Channel: DL-SCH

o   This channel is used for transmitting downlink data.

o   It supports HARQ, dynamic link adaptation.

o   It I can be used to broadcast data in entire cell.

o   It supports UE discontinuous reception (DRX) to enable power saving in UE.

o   It also supports MBMS transmission.

-       Paging Channel: PCH

o   Used for transmitting paging information.

o   PCH supports DRX so that UE can sleep and wakeup to receive PCH in specific time intervals.

-       Multicast Channel: MCH

o   This channel is used to support MBMS.

Uplink Transport Channels:

-       Uplink Shared Channel: UL-SCH

o   Supports HARQ

o   Counter part of DL-SCH

-       Random Access Channel: RACH

Transport and Physical Channel Mapping

Downlink Channels:


Uplink Channels:


LTE Layer 2:

LTE layer 2 is split in MAC, RLC and PDCP.

Layer 2 Structure of downlink


Layer 2 Uplink Structure


The communication between two sub-layers is marked with circles. These are called Service Access Points (SAP). SAP between Physical layer and MAC sub-layer provides the transport channels. The SPA’s between MAC and RLC provide logical channels. Multiplexing several logical channels (i.e radio bearers) to same transport channel is preformed by MAC sub-layer.

Logical Channels:

MAC sub layer offers different kind of data services to RLC inform of logical channels. Logical channels define what type of data is transferred between UE and eNB. Logical Channels are classified into Control Channels (for control plane information transfer) and Traffic Channels (for transfer of user plane data)

Control Channels:

-       Broadcast Control Channel: BCCCH

o   This channel is used of broadcasting system control information.

o   This is downlink channel.

-       Paging Control Channel: PCCH

o   Downlink channel.

o   Transfers paging information and system information change notification.

o   This channel is used for paging when the network does not know the location cell of the UE.

-       Common Control Channel: CCCH

o   Channel of transmitting control information between UE and network.

o   This channel is used for UE’s having no RRC connection with the network.

-       Multicast Control Channel: MCCH

o   Point to Multi point downlink channel used for transmitting MBMS control information from the network to UE.

o   This channel is only used by UE’s that receive MBMS.

-       Dedicated Control Channel: DCCH

o   A point-to-point bi directional channel that transmits dedicated control information between a UE and the network.

o   Used by UE’s having an RRC connection.

Traffic Channels:

-       Dedicated Traffic Channel: DTCH

o   Uplink and downlink channel.

o   Point-to-point channel dedicated to one UE for transfer of user data.

-       Multicast Traffic Channel: MTCH

o   Point-to-Multipoint downlink channel for transmitting traffic data from network to UE.

Mapping logical and transport channels:





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