dAna BayAnaKa: 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.