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

Picture 15. Throughput Formula

When there are 4 different data packet will be transmit in the same time

Picture 16. Illustrate 4 data packet transmit with 4 antenna port

ü  Increasing Performance Gain

When only have 1 data packet will be transmit, where is a same packet will be transmit.

Picture 17. Illustrate 1 data packet transmit with 4 antenna port

The Air Interface

In mobile or wireless communication, the air interface is the radio-based communication link between the UE and the active Base Station.

A.      Evolution of Cellular Networks

Cellular mobile networks have been evolving for money years, The initial networks are refer to as “First Generation” and now days 4G or “Fourth Generation” systems are being deployed. Pictures 1 below are describe Evolution of Cellular network:

 Picture 1. Evolution of Cellular Network

B.      3GPP Releases

From Evolution of Cellular Networks as a Pictures 1, one of them is 3GPP (Third Generation Partnership Program) evolution where is the most successful modulation techniques. Hardware vendors and software developer use these releases as part of their development road map.

Pictures 2 are describe how to 3GPP Releases:

Picture 2. 3GPP Evolution

C.      Radio Interface Techniques

In wireless cellular systems, mobiles have to share a common medium for transmission. There are various categories of assignment, the main four include as Pictures 3 below:

Picture 3. Radio Interface Techniques

D.      Transmission Modes

Pictures 4 and Pictures 5 are describe what is transmission Modes and the advantages

Picture 4. Transmission Modes

Picture 5. Duplex Technologies

E.       Spectrum Usage in LTE

LTE Radio Interface, namely the E-UTRA (Evolved – Universal Terrestrial Radio Access) will be operated in the difference signal as Table show on pictures 6. Frequency band Release as a picture 6 are used to identify center frequencies, Each Center frequencies from Bands are given EARFCN (E-UTRA Absolute Radio Frequency Channel Number), where in FDD requires 2 Center frequency (EARFCN) for UL & DL and for TDD only has 1 Center frequencies (EARFCN).

In the table below UL ARFCN are shows on NUL and for DL ARFCN are shows as NDL , for TDD EARFCN has same value on NUL and NDL that is why TDD only has 1 center frequencies. Telkomsel are used Band 8 for LTE 900 and Band 3 for LTE 1800.

Picture 6. LTE Release 10 Bands

Center frequencies (EARFCN) use for Network identification what band are used in the Network. Pictures 7 below are describing formula how to calculation carrier frequency EARFCN.

Picture 7. Carrier Frequency ERFCN Calculation

F.       Channel Coding in LTE

The term “channel coding” can be used to describe the overall coding for LTE channel. It can be used to describe one of the individual stages. LTE channel coding is typically focused on TB (Transport Block). This is a block of information which is provided by upper layer i.e MAC (Medium Access Control).

The figure 8 below summarizes the typical processes performed by the PHY (Physical Layer):

Picture 8. LTE Transport Channel Processing

Additional CRC on Transport Block are the method to detection error across air interface, which may occurred when data was being sent. In LTE CRC based on complex parity checking, after CRC Parity bits the Next stage in the processing of Transport Block is Code Block segmentation & CRC Attachment. This process to ensure the size each block is compatible with later stage. i.e

1. Max Transport block 6144 bits, if more divided some Code Block

2. Each Code Block has CRC include Turbo Coding

3. Add 16 bits of filler is required to ensure the segment meet a valid Turbo Coding Code block Size

Channel Coding in LTE Facilitated FEC (Forward Error Correction) across air interface. There are 4 main types Channel Codding:

1.    Repetition Coding è for coding the HI (HARQ Indicator) bit

HI sent “1”=ACK (Acknowledgement), “0”=NACK (Negative Ack)

 2. Block Coding è for CFI (Control Format Indicator)

     Used to convey vital information about size DL control

 3. Tail Biting Convolutional Coding

4. Turbo Coding

Rate Matching for turbo coded transport channel defined per code block & consist of interleaving the three information bit streams. Code Block Concatenation, in receive  side used to concatenation Code block to become transport block original which is  before transport block divide some code block in transmit side for sending in air interface

G.     Principles of OFDM

       

Picture 9. Principles of OFDM

In the LTE Air interface, there are 2 Multiples access techniques both based on OFDM (Orthogonal Frequency Division Multiplexing) shows on picture 9.

1. OFDMA è Used for Down Link

    (Orthogonal Frequency Division Multiplexing Access)

2. SC-FDMA è Used for Up Link

    (Single Carrier – Frequency Division Multiplexing)
  Why LTE system used SC-FDMA used SC-FDMA as Up link because Handset Specification need high processing capabilities and battery performance to used OFDMA method where is this systems have High Peak to Average Power Ratio.

OFDMA it was currently being used on various systems such as Wi-Fi & WiMAX since 1998, but  just adopted on LTE system because the fact nowadays Handset processing capabilities and battery performance both are improved.

On FDM Methods (Picture 10) to ensure each subcarrier which is transmit simultaneously does not interference with adjacent subcarrier, a Guard Band is Utilized, because additional Guard Band FDM become not spectrally efficient compare other systems

Picture10. Frequency Division Multiplexing

OFDM is based on FDM (Frequency Division Multiplexing) and this methods whereby Multiples Frequency are used to simultaneously transmit different information. On OFDM even though has the same concept with FDM but OFDM Drastically spectral efficiency by reducing the spacing between the subcarriers. OFDM systems still employ Guard Bands where is located at the Upper and lower parts of the channel and reduce adjacent channel interference.

The figure 11 illustrates how the subcarriers can overlap due to their orthogonality with the other subcarriers. When a subcarriers is at  its maximum the two adjacent subcarriers are passing through zero.

In addition, OFDM systems still employ guard bands. These are located at the upper and the lower parts of the channel and reduce interference.

Picture 11. OFDM Subcarriers

OFDM Subcarriers are generated and decoded using mathematical function called Fourier Transform

            1.        IFFT (Inverse Fast Fourier Transform)

         Is used in the transmitter to generate the wave form as describe on picture 12
       

Picture 12. IFFT (Inverse Fast Fourier Transform)

        2.       FFT (Fast Fourier Transform)

       At the receiver side, the signal is passed to FFT which analyses the complex/combined            waveform into the original stream as picture 13.
       

Picture 13. FFT (Fast Fourier Transform)