Mobile Air Interface

- Overview

An air interface or access mode is a communication link between two stations in mobile or wireless communication. The technology used for the radio transmission between mobile devices and the base station in a cellular network (see RAN). The air interface involves the physical and data link layers (layers 1 and 2) of the OSI model for connectivity. The air interface (also known as the radio interface) defines the frequency, channel bandwidth, and modulation scheme.

- The Air Interface Physical Layer

The physical connection of the air interface is usually radio based. This is usually a point-to-point link between the active base station and the mobile station. Techniques such as Opportunity Driven Multiple Access (ODMA) may allow flexibility in which devices play which roles. Some types of wireless connections are capable of broadcasting or multicasting.

Multiple links can be created in a limited spectrum via FDMA, TDMA or SDMA. Some advanced forms of transmission multiplexing combine frequency and time division methods, such as OFDM or CDMA. 

In cellular telephone communications, the air interface is the radio frequency portion of the circuit between the cellular telephone or wireless modem (usually portable or mobile) and the active base station. The active base station changes periodically as a user moves from one cell to another in the system. Each transition is called a handover.

In radio and electronics, an antenna is an electrical device that converts electrical energy into radio waves and vice versa. It is usually used with a radio transmitter or radio receiver. During transmission, a radio transmitter supplies current oscillating at radio frequency to the antenna terminals, and the antenna radiates the energy in the current as electromagnetic waves (radio waves). Antennas focus radio waves in a certain direction. Often, this is called the cardinal direction. Because of this, less energy is emitted in other directions. 

The gain of an antenna in a given direction is usually referenced to a (assumed) isotropic antenna, which emits strong radiation uniformly in all directions. Antenna gain is the power in the strongest direction divided by the power emitted by an isotropic antenna emitting the same total power. In this case, the antenna gain (Gi) is usually specified in dBi, or decibels isotropic. 

- The Air Interface Data Link Layer

The Data Link layer in the air interface is usually further divided than the simple Media Access Control (MAC) and Logical Link Control (LLC) sublayers in other OSI terminology. While the MAC sublayer is generally unmodified, the LLC sublayer is subdivided into two or more additional sublayers according to the standard. Common sublayers include: 

• radio link control
• Packet Data Convergence Protocol
• radio resource control

Especially in mobile telecommunications and Internet broadband (...) Maximum combined input ratio for SNR estimation 

• The signals from each channel are summed together
• The gain of each channel is directly proportional to the RMS signal level and inversely proportional to the mean squared noise level in that channel.
• Each channel uses a different scaling constant.

Smart matrix arrays for combining input signal gains separate them from filters and different types of output multiplexing schemes are used to approach multiple users such as CDMA, FDMA, WCDMA, TDMA and ODMA. This way calls and web services are methods and authenticate only subscribers.

- The LTE Air Interface Physical Layer

The air interface waveform of LTE and NR, like many other modern digital communication standards, is based on orthogonal frequency division multiplexing (OFDM). OFDM is highly spectrally efficient and allows high data rate transmission with low receiver complexity even in a dispersive radio channel.

The LTE air interface physical layer offers data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service:

• 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 synchronisation
• Radio characteristics measurements and indication to higher layers
• Multiple Input Multiple Output (MIMO) antenna processing
• Transmit Diversity (TX diversity)
• Beamforming
• RF processing

- The 5G NR (New Radio) Air Interface

The 5G New Radio (5G NR) is a new air interface being developed for 5G. 5G NR was defined in 5G R15. 5G NR is being developed from the ground up in order to support the great variety of services, devices & deployments which 5G will encompass, including diverse spectrum requirements, building on established LTE technologies to ensure backwards and forwards compatibility.

Like LTE, 5G NR uses orthogonal frequency division multiplexing but makes it highly flexible. For example, variable subcarrier spacing, flexible radio frame structure including a self-contained slot, and carrier bandwidth parts are introduced. Both sub 7 GHz spectrum (called frequency range 1 or FR1) and millimeterwave spectrum (called frequency range 2 or FR2) are supported. The new high performance channel coding techniques of low density parity check coding and polar coding are defined. Spatial multiplexing techniques used in LTE, SU-MIMO and MU-MIMO3) are enhanced in 5G. NR is a beamformed air interface with fewer beams at low frequency bands and more beams at high frequency bands. 5G supports hybrid beamforming where both digital beamforming (available in LTE) and analog beamforming are combined. Massive MIMO (mMIMO) in 5G enables enhanced combining of beamforming methods with spatial multiplexing.

While 5G NR provides a flexible air interface, it is advantageous in transitioning from 4G to 5G to use dynamic spectrum sharing (DSS) to dynamically allocate 4G and 5G subcarriers in the same channel. With DSS, mobile operators can simultaneously support 4G LTE, 5G NSA and 5G SA devices. DSS was introduced in R15, further refined in 5G R16 and 5G R17 and will probably continue to be refined in future releases,
especially to improve the scheduling of resources between and within 4G and 5G subcarriers and across multiple cells. While the transition from one wireless generation to another in a specific band has been a painful experience in the past, it will be much easier with 5G thanks to DSS.

[More to come ...]