Air Interface Technology

Definition

Air Interface refers to the wireless transmission interface between the base station and the terminal, and it is the most critical physical layer interface in mobile communication systems. In LTE, it is called the Uu interface, and in 5G NR, it corresponds to the wireless interface between NG-RAN and UE. Air interface technology determines spectrum efficiency, coverage capability, latency, and reliability, and it is the core of the performance differences among various generations of mobile communications.

Key Technology Components

Evolution of Air Interfaces in Various Generations

Standard	Multiple Access	Bandwidth	Peak Rate
2G GSM	TDMA	200kHz	~100kbps
3G WCDMA	CDMA	5MHz	~14Mbps
4G LTE	OFDMA	1.4–20MHz	~150Mbps
5G NR	OFDMA	100MHz+	~Gbps

IoT Air Interface Characteristics

IoT standards such as NB-IoT, LTE-M have simplified and enhanced the air interface: narrowband design, repeated transmission, low-order modulation, power-saving modes, etc., which ensure coverage and power consumption while meeting the small data volume and low-speed requirements of IoT.

How resource blocks and slots cap the bytes in one frame

The air interface cuts spectrum into resource blocks and schedules them to a device by subframe or slot. An IoT packet is often too small to fill a resource block, but the scheduler still assigns whole blocks, so “the service is only a few dozen bytes” does not mean the air-interface overhead is only a few dozen bytes. Control signaling, reference signals, and hybrid automatic repeat feedback all take resources. If the uplink must first do timing advance and a power ramp, the air time of the first packet is longer than the send time the application sees.

When the subcarrier spacing widens, the same bandwidth holds shorter symbols, delay falls, and the link becomes more sensitive to frequency error. Narrowband IoT makes the bandwidth very narrow. In exchange, each transport block is smaller and the repetition count is higher. Do not select from the peak-rate line alone. Read the modulation, coding, and repetition the scheduler actually grants under the current coverage. That is how many bytes this frame can hold.

How the modulation order steps down when channel quality falls back

Adaptive modulation switches between high and low order from the channel quality the device reports. When quality is good, one symbol carries more bits. When quality is poor, it falls back to a lower order that tolerates interference, sometimes only far enough to get signaling out. The condition is several reports in a row below the threshold, not one brief fade. If the threshold is too aggressive, the rate chatters between two steps and the upper layer sees a sawtooth throughput. If it is too conservative, a link that could use a high order stays on a low order, and a firmware upgrade stretches out.

If a module locks the modulation order, adaptation on the network side does nothing. Locked high, the cell edge fails to decode in volume. Locked low, a nearby device wastes resource blocks. The air-interface symptom here is a rising block error rate, not a lost IP address. Read the block errors and the channel quality before deciding whether to move the installation.

Retransmission in a weak field pulls power consumption up with it

After the first transmission fails, hybrid automatic repeat sends only extra redundancy. The device must stay awake for the feedback and then send the next redundancy version. The worse the coverage, the more repetitions, the longer the radio stays on, and the higher the average current. Looking only at “it arrived in the end” makes the link look fine while the battery budget is already over. Read the retransmission count, the received level, and the radio-on time of one report together. All three getting worse is a weak field, not an occasional server timeout.

A SimtoLink IoT SIM card and communication module both have to cross this air interface before they can attach. The card sets the subscriber identity and the subscribed rate. The module sets radio capability and power-saving behavior. Air-interface quality is set by the base station and the installation. If the online rate on the platform falls and the reference-signal received power stays low, fix coverage and the antenna before restarting the card again. A metal cabinet indoors can disguise an air-interface problem as a card fault. Opening the cabinet and measuring again is a necessary step.

The first uplink packet never leaves when timing advance will not lock

Uplink on the air interface has to align with the base-station time base first. The farther the device is from the base station, the later the signal arrives, and the earlier the device must transmit. If the advance exceeds what the module can do, or multipath makes the estimate jump, an uplink grant can already have been received and the physical layer still discards the data as misaligned. The application sees a successful send and an empty server log. Downlink broadcast is still normal, because downlink does not depend on the device advance. “I can hear the base station and cannot report” should start with whether timing advance is stable, not with a new server address.

The advance keeps changing while the device moves. If a vehicle module locks the advance at the value from installation, uplink fails for a whole stretch after it leaves the original cell. A fixed device is the opposite: the advance should be stable. If it jumps by a large amount, a reflected path is probably changing, and the antenna position is more worth checking than the card status. A SimtoLink module has to support the advance range of the radio standard in use. The card itself does not compute this value. Next to a large metal structure, record whether the advance and the block errors jump together, then decide whether to move the antenna or change the report period.

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