dAna BayAnaKa: LTE Air Interface General Principles

A. Basic Part of LTE Air Interface

A.1. The Uu Interface and Protocols

The LTE air interface is identified as the E-UTRA (Evolved – Universal Terrestrial Radio Access) and can support Varying bandwidth options ranging from 1.4 MHz – 20 MHz. The interface is identified as “Uu” where is “U” indicating “User to Network” and “u” means “Universal”.

Picture 1. Uu Interface

The E-UTRA interface (Uu) provides connectivity between the User Equipment (UE) and the Evolved Node B (eNB). It can be logically split into 2 plane:

1. Control Plane

1.1. Provide by RRC (Radio Resource Control) and carries signaling between UE and eNB

1.2. Carries NAS (Non Access Stratum) signaling messages to the MME (Mobility Management Entity) which carried by RRC.

2. User Plane

User Plane focus on the delivery of IP datagrams to and from EPC (Evolved Packet Core) namely S-GW (Serving Gateway) and PDN-GW (Packet Data Network – Gateway).

Picture 2. LTE Control Plane and User Plane

Picture 3. E-UTRA Protocols

From picture 3 Above;

1. Control Plane & User Plane have the same protocol on Lower layer

2. Only Control Plane has RRC where NAS signaling uses this services

3. On the User plane IP datagrams are also mapped into PDCP

The following are describe E-UTRA protocols:

ü NAS Signaling; in terms of NAS Signaling, messages pass between the UE and the MME

ü RRC; messages to be transferred between the UE and the eNB it is uses the services of PHY, MAC, RLC & PDCP

ü PDCP (Packet Data Convergence Protocol) LTE implements PDCP in both Control & User Plane, unlike UMTS where PDCP only found in user plane. PDCP function are:

· In Control Plane è Encryption & Integrity Checking

· In User Plane è IP Header Compression, Encryption & sequencing and duplicated detection.

ü RLC (Radio Link Control), where is support three delivery services to higher to the lever:

· TM (Transparent Mode) è Provides a connectionless service for signaling

· UM (Unacknowledged Mode) è it has additional features of sequencing, segmentation & concatenation.

· AM (Acknowledged Mode) è offer Automatic Repeat Request such as retransmissions.

ü MAC (Medium Access Control) provides the following services:

· Channel Mapping è Maps the information received on LTE Logical channel to the transport channel.

· Channel Multiplexing è Multiplex different bearers (Radio/Multiple Radio) into the same TB (Transport Bock) thus increasing efficiency.

ü PHY (Physical Layer) provides the following services:

· Error Detection, FEC encoding/Decoding, Rate Matching, Mapping of Physical Channels

· Power Weighting, Modulation & Demodulation, Frequency & Time Synchronization

· Radio Measurements, MIMO Processing, Transmit Diversity, Beamforming, RF processing

A.2. LTE Channel Structure

In this subject we discuss about Channel structure where describe in picture 4.

Picture 4. Logical Channels

1. Logical Channels

The picture 4 shows Logical channels located between RLC & MAC layers. The various forms Control Logical Channels as shows picture 5 include:

· BCCH (Broadcast Control Channel)

This Downlink channel used to send SI (System Information) messages from eNB, define by RRC.

· PCCH (Paging Control Channel)

This Downlink channel used by eNB to send paging information

Picture 5. Control Logical Channels

Picture 6. CCCH & DCCH Signaling

· CCCH (Common Control Channel)

· Used to establish RRC (Radio Resouce Control) connection known as SRB (Signaling Radio Bearer)

· SRB Used for re-establishment procedures

· SRB 0 maps to CCCH

· DCCH (Dedicated Control Channel)

Provide bidirectional channel for signaling. Logically there are 2 DCCH activated :

· SRB 1 è Used for RRC messages carrying high priority NAS signaling

· SRB 2 è Used for carrying Low priority NAS signaling prior to SRB 1 establish

Release 8 LTE has one of Logical Channel carrying traffic namely DTCH (Dedicated Traffic Channel), this used to carry DRB (Dedicated Radio Bearer) information i.e. IP datagrams.

The DTCH is a bidirectional channel that can operate in either RLC AM or UM mode, this configured by RRC and based on the QoS (Quality of Service) of E-RAB (EPS Radio Access Bearer).

2. Transport Channels

There are 5 Transport channels LTE Release 8

· BCH (Broadcast Channel)

This is fix format channel which occurs once per frame and carries the MIB (Master Information Block).

· PCH (Paging Channel)

Used to carry the PCCH i.e. paging messages, it also utilizes DRX (Discontinuous Reception) to improve UE battery life.

· DL-SCH (Downlink – Share Channel)

This is main downlink channel for data and signaling, facilities the sending of System Information messages. Dynamic scheduling (eNB controlled) and link adaptation. In addition support HARQ (Hybrid Automatic Repeat Request) operation

· RACH (Random Access Channel)

This channel carries limited information and is used in conjunction with Physical channel and preambles to provide contention resolution procedures.

· UL-SCH (Uplink Share Channel)

Similar to the DL-SCH

3. Physical Channels

· Downlink Physical Channels

· PBCH (Physical Broadcast Channel) è Carries the BCCH

· PCFICH (Physical Control Format Indicator Channel) è Indicated the number of OFDM symbols used for PDCCH

· PDCCH (Physical Downlink Control Channel) è Resource allocation

· PHICH (Physical Hybrid ARQ Indicator Channel) è Part of HARQ process

· PDSCH (Physical Downlink Shared Channel) è carries the DL-SCH

· Uplink Physical Channels

· PRACH (Physical Random Access Channel)

· PUSCCH (Physical Uplink Control Channel)

· PUSCH (Physical Uplink Share Channel)

4. Radio Channel

The term “Radio Channel” is typically used to describe the overall channel, i.e. the downlink and uplink carrier for FDD or the single carrier for TDD.

Picture 7. Downlink Channel/Uplink Channel Mapping

A.3. LTE Frame Structure

There are 2 Type of LTE structure, the following figure describing the type of LTE structure:

· LTE Frame Structure Type (Type 1)

Picture 8. LTE Frame Structure (Type 1 FDD)

Picture 9. Normal And Extended Cyclic Prefix

Legend:

Ø Type 1; radio frame structure 10ms in duration, consist of 20 sub frame & 0,5ms slot

Ø Ts consist of a guard period i.e the cyclic prefix, Tb data duration 2048 LTE times unit 15 kHz for both Normal & Extended CP

Ø Normal Cyclic Prefix used for reange frequency below 14 KM and Extended Cyclic Prefix used when the range of the cell needs to be extended

Picture 10. Downlink CP Parameters

· Type 2 TDD Radio Frame

Picture 11. Type 2 TDD Radio Frame

A.4. OFDM Signal Generation

Picture 12. OFDM Signal Generation

There are various Physical Layer stages involved in the generation of DL & UL. The picture stages for PDSCH. The initial stage of physical layer processing is “scrambling”. Scrambling effectively randomizes interfering signals using a pseudo-random scrambling process, where the information with scrambling code based on the physical cell ID and RNTI. This stages is applied to the signal in order to provide interference rejection properties thus which scrambling less interference.

Modulation Mapper

Modulation Mapper converts the scrambled bits to complex-valued modulation symbols

Picture 13. Modulation Mapper

Picture 14.Codeword, Layer and Antenna Port

The use of layer and multiple antenna ports is related to diversity and MIMO (Multiple Input Multiple Output). The term “rank” is typically applied to the number of layer.

Codeword is packet name where is define processing after Rate Matching process in Physical layer and before scrambling process. In Rate Matching process and other process before in Physical layer called as Code Block. 2 Codeword Maximum can be used and these are mapped onto layers. In more than 2 Rank codeword the codeword separate and transfer simultaneously. It is important to note that the number of modulation symbols on each layer needs to be the same.

At least 2 advantages Codeword and maximum Rank used (related to diversity and MIMO) use Rank 4 (4 Antenna Port):

ü Increasing Throughput