What Is Air Interface Slicing?

High-Reliability Slicing Queue

The air interface slicing feature redefines the priorities of air interface queues, and places the High Reliability (HR) queue at the position with the highest priority. In the new queue design, the HR queue with the highest priority is reserved for high-reliability services such as programmable logic controller (PLC) to ensure that these services are scheduled first.

Time Domain Slicing

Co-channel interference between APs affects data transmission reliability. Time domain slicing can be used to enable these APs to transmit in different time slices to avoid conflicts. For example, AP1, AP2, and AP3 are three co-channel APs. Co-channel interference exists between AP2 and AP1 and between AP2 and AP3, whereas no co-channel interference exists between AP1 and AP3.

Co-channel interference between APs

In this case, the entire network-level coordinated resource allocation can be used to divide time-domain resources by period and reserve some time-domain resources for specified APs. For example, group 1 {AP1, AP3} and group 2 {AP2} are allocated to two different time slices. When group 1 sends packets, group 2 waits silently; and vice versa. The rest of the time, APs can freely contend for resources. This prevents conflicts between co-channel APs and improves the reliability of packet transmission. In addition, when resources are periodically reserved, APs can provide a deterministic transmission latency.

Coordinated resource allocation

Time Synchronization

Time synchronization is critical for ensuring that co-channel APs do not conflict with each other, due to time asynchronization during time domain slicing. Time synchronization is implemented by calculating the time deviation by measuring Beacon frames between APs. An AP sends Beacon frames at the interval of the target Beacon frame transmission time and fills the high-precision sending timestamp in Beacon frames. After receiving the timestamp, the neighboring co-channel AP aligns it with the locally measured timestamp by SN and compensates for the channel delay to achieve time synchronization. The following uses two APs as an example to describe the time synchronization process.

AP1 periodically sends Beacon frames. AP1 sends a Beacon frame at . AP2 receives the Beacon frame at and records the timestamp locally. The time deviation between APs can be calculated using the following formula:

A Beacon frame sent at does not carry the timestamp , and AP1 sends another Beacon frame at , carrying the timestamp of the previous frame. AP2 receives the frame at and parses the packet to obtain the timestamp .

Time synchronization mechanism between APs

On a large-scale network, a master AP is specified as the reference clock of the entire network according to the network topology or AP positions. The master AP sends a Beacon frame to broadcast its high-precision timestamp and layer. All co-channel APs within the coverage area of the master AP synchronize time and update layers based on the information carried in the Beacon frame. The AP at the second layer calculates the timestamp based on the time difference with the master AP, saves the SN and timestamp, and fills in the next Beacon frame. The next-level AP calculates the time difference with the master AP based on the timestamp of the reception link. Other APs complete time synchronization layer by layer.

Clock synchronization on a large-scale network

Robust Scheduling

Wi-Fi packets are transmitted at a high rate over the air interface to obtain a high throughput. However, a higher packet transmission rate requires a higher signal-to-noise ratio (SNR). In other words, a higher packet transmission rate indicates inefficient anti-interference capability. The high rate increases the possibility of transmission failures, and the retransmission increases the delay. Considering the small volume of service traffic on industrial control devices, robust scheduling reduces time delays through the following improvements:

• Low-speed Modulation and Coding Scheme (MCS) preference: Packets are transmitted at more stable rates to improve transmission reliability.
• Increased transmit power: The transmit power of packets is increased so that the peer device has sufficient SNR to correctly demodulate packets.
• Hardware retransmission: Hardware retransmission technology is used to reduce the delay caused by packet retransmission.
• Limited packet sending duration: When control applications and quality inspection applications coexist, Huawei's air interface slicing dynamically determines the single-packet sending duration of different application traffic, thereby reducing the transmission delay for sending control application traffic.

Interference Visualization

Two key technologies in the WLAN channel access mechanism are Clear Channel Assessment (CCA) and Distributed Coordination Function (DCF). The CCA technology is used to evaluate the usage status of an air interface, and the DCF technology is used to implement a random backoff mechanism of air interface contention. When there are a relatively small quantity of network nodes, this decentralized access mechanism reduces costs and improves efficiency. However, as the network scale expands, the mechanisms cause various problems, such as hidden nodes, conflicts, and long waiting times for air interface access, and finally bring uncertain network delays. The preceding problems are called WLAN interference. Interference visualization can identify network interference and optimize the network based on the following principles:

• Identifying network interference: On the basis of high-precision time synchronization between APs, the central node collects the characteristics of received and sent packets (including the packet sending direction, timestamp, type, rate, delay, and number of retransmission times) on the air interface of co-channel APs. The graphical user interface (GUI) tool is used to draw an air interface fingerprint of multiple APs to clearly show how multiple co-channel APs contend for air interface resources, whether there is interference, and the interference severity.
• Optimizing network: Based on interference identification, the system adjusts related air interface parameters and performs closed-loop control. If the loop cannot be closed, an alarm is generated to give a warning and air interface interference information is retained, helping O&M personnel locate the root cause and optimize the network.

Healthcare Interconnection Scenario

Nowadays, a hospital typically builds a dedicated IoT network to carry healthcare services, dealing with the challenges in healthcare data collection. Wireless healthcare IoT terminals require Wi-Fi to provide different Service Level Agreements (SLAs) and reserve bandwidth for key terminals or applications. For example, nurses need to use monitors and ventilators to monitor patients' vital signs to learn patients' conditions in a timely manner. Another example is that doctors need to invoke the image materials from the Picture Archiving and Communication System (PACS) at any time during mobile ward rounds. These healthcare terminals need to be frequently moved to different wards. Air interface slicing technology can provide high-reliability and low-latency wireless networks for these terminals.

Typical networking in the healthcare interconnection scenario

Industrial Interconnection Scenario

In an industrial interconnection scenario, devices, such as robot arms and palletizers, implement automatic production under the management of the programmable logic controller (PLC) system. When servers deliver job tasks and collect information, ultra-high reliability and low latency are required to ensure production services. Air interface slicing technology reserves bandwidth resources for key services, meeting high-reliability and low-latency requirements.

Typical networking in the industrial interconnection scenario