INTELLIGENT O&M OF WIRELESS NETWORKS
The Wi-Fi network system, similar to other wireless communications systems, is a self-interference system, where different APs can interfere with one another, affecting overall system capacity. For example, a high-power AP may interfere with adjacent APs if they work on overlapping channels. In this case, radio calibration is required to reduce inter-AP interference and improve overall system capacity and performance. In addition, Wi-Fi networks use spectrum resources on the free Industrial, Scientific and Medical (ISM) band, where short-range wireless communications devices (such as Bluetooth or ZigBee devices) and electromagnetic devices (such as microwave ovens) may also generate non-Wi-Fi interference. The radio calibration function can dynamically adjust the channels and power of APs managed by the same WAC and avoid non-Wi-Fi interference, ensuring that APs consistently work at the optimal performance.
Basic Radio Calibration
1. Basic radio calibration concepts
In wireless network deployment, if radio parameters are not adjusted and all wireless APs work on the same channel and use the maximum transmit power, the APs will interfere with each other, and STAs may be associated with remote APs with high transmit power. In this deployment policy, due to the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism and the hidden node problem of Wi-Fi networks, STA uplink and downlink communications become extremely unreliable. In addition, STAs may encounter frequent instances of extended latency and access issues, which severely affects user experience.
To address these problems, the following key radio parameters must be adjusted after Wi-Fi network deployment:
a. Transmit power
The transmit power of an AP determines the radio coverage and isolation between cells, with a higher transmit power indicating a higher downlink signal-to-noise ratio (SNR) and easier STA access. However, it should be noted that the transmit power of STAs is limited and is significantly lower than that of APs. If the transmit power of an AP is excessively high, STAs can receive data sent by the AP, but the AP cannot receive the data sent by the STAs.
b. Frequency band
The working frequency band of an AP determines the radio capacity and coverage. Currently, wireless local area networks (WLANs) use the 2.4 and 5 GHz frequency bands. As the 2.4 GHz frequency band has fewer channel resources and lower path loss than the 5 GHz frequency band, when APs are densely deployed, intrachannel interference on the 2.4 GHz frequency band is much stronger than that on the 5 GHz frequency band. In addition, a large amount of non-Wi-Fi interference exists on the 2.4 GHz frequency band from devices (such as cordless phones, Bluetooth devices, and microwave ovens). As such, the amount of information transmitted on the 2.4 GHz frequency band is significantly less than that on the 5 GHz frequency band.
The working channel of an AP indicates the frequency at which the AP works. If two adjacent APs work on the same channel, they will compete for channel resources, decreasing throughput and wasting the resources of other channels.
The bandwidth of an AP determines its maximum rate and the channel’s capacity. Obviously, the channel capacity for the 20 MHz bandwidth differs greatly from that for the 80 MHz bandwidth, and so the bandwidth must be properly allocated.
When planning radio parameters, we should be clear about the AP’s location, interference, and running services. Only by doing so can we configure the most appropriate transmit power, frequency band, channel, and bandwidth. This approach, however, is time- and labor-consuming, and is unable to respond immediately if severe interference occurs abruptly.
To address these problems, the automatic radio calibration function is utilized. When this function is enabled, a Wi-Fi network automatically detects neighbor relationships between APs, interference on each channel, and load information within a given period. Based on the detected information, the Wi-Fi network automatically calculates radio parameters and delivers them to APs.
2. Key technologies used in radio calibration
a. Obtaining network status information
i. Radio topology and interference identification
After radio calibration begins, all radios are scanned for a given duration. This involves each radio switching to other channels to send Probe Request frames, receive Probe Response frames, listen to Beacon frames and other frames, and obtain the transmit power by exchanging packets between APs over the air interface. The transmit power is carried in the vendor-defined field, and the difference between the obtained transmit power and receive power is the path loss between the APs. The physical distance between two radios can then be calculated using the path loss calculation formula provided by the IEEE TGac.
During channel scanning, an AP obtains a large number of packets by listening to 802.11 frames, including air interface packets from other APs managed by the same WAC, and packets from APs or wireless routers managed by other WACs. The AP considers the APs or wireless routers managed by other WACs as external Wi-Fi interference sources, and stores their MAC addresses, receive power, and frequencies.
Also during channel scanning, the AP enables the spectrum scanning function to identify non-Wi-Fi interferences through time domain signal collection, fast Fourier transformation, and spectrum profile matching, and stores the non-Wi-Fi interference information together with the receive power and frequency information.
As shown in Figure 7.39, each AP periodically uploads collected neighbor information as well as Wi-Fi and non-Wi-Fi interference information to the WAC, which then generates a topology matrix, a Wi-Fi interference matrix, and a non-Wi- Fi interference matrix after signal filtering.
ii. Radio load statistics
Load information is easy to collect. This is because APs record wireless and wired traffic statistics, and periodically
upload these statistics and user quantity information to the WAC. The WAC then can calculate radio load information.
By scanning spectrum resources and scanning and parsing 802.11 frames, the WAC can obtain topology information, channel interference information, and radio load statistics, enabling it to allocate channels and bandwidths to radios accordingly. The WAC preferentially allocates a channel with the minimum interference and the maximum bandwidth to a heavily loaded radio.
b. Automatic transmit power adjustment
In multi-AP networking scenarios, adjusting the transmit power of each radio can prevent coverage holes and overlapping, and can enable neighboring APs to fill any coverage holes upon an AP radio failure. As STAs cannot detect Wi-Fi signals in a coverage hole, avoiding coverage holes is extremely important.
Overlapping coverage generates interference and affects users’ roaming experience. If the overlapping coverage area of two intrafrequency APs is large, co-channel interference increases and the APs’ throughput is affected. For example, if a STA associated with АР 1 is also located in the core coverage area of AP 2, the STA does not proactively roam when the signal strength of AP 1 is strong. However, the user experience is poor because AP 1 cannot receive uplink packets from the STA due to unbalanced uplink and downlink power.
Utilizing the automatic transmit power adjustment algorithm, an AP can determine the coverage boundary by a certain SNR and automatically adjust its own transmit power to ensure both the coverage area and signal quality.
An AP’s transmit power decreases as the number of neighbor APs increases, as shown in Figure 7.40, where each circle represents the coverage area of the corresponding AP. After AP 4 is added, the transmit power of the other APs decreases due to automatic power adjustment.
However, if an AP goes offline or fails, the transmit power of its neighbor APs increases, as shown in Figure 7.41.
FIGURE 7.40 An AP’s transmit power decreases when the number of neighbor APs increases.
c. Automatic channel and frequency bandwidth adjustment
After obtaining the neighbor relationship topology, external interference information, and long-term load statistics, a WAC can use the Dynamic Channel Assignment (DCA) algorithm to allocate channels and bandwidths to APs. Due to the limited number of channels on the 2.4 GHz frequency band, the bandwidth of each 2.4 GHz channel is typically fixed at 20 MHz. However, as there are more channels available on the 5 GHz frequency band and IEEE 802.11 standards support wider bandwidth, 5 GHz channel resources should be fully utilized to maximize system bandwidth, improve system throughput, and
FIGURE 7.41 When an AP goes offline or fails, the transmit power of neighbor APs increases.
satisfy customer demands. Utilizing the DCA algorithm, a WAC can dynamically allocate 5 GHz channels and bandwidths to APs based on the topology, interference, and load information.
On a Wi-Fi network, adjacent APs must work on nonoverlapping channels to avoid radio interference. Figure 7.42
FIGURE 7.42 Before and after channel adjustment.
shows an example of channel distribution before and after channel adjustment. Before channel adjustment, both AP 2 and AP 4 use channel 6. After channel adjustment, each AP is allocated an optimal channel to minimize or avoid adjacent- channel and co-channel interferences, ensuring reliable data transmission on the network. As such, AP 4 now uses channel 11 so that it does not interfere with AP 2.
Figure 7.43 shows a distribution example of 2.4 and 5GFIz channels for seven APs that provide continuous signal coverage
after the DCA algorithm is implemented. According to the figure, the co-channel and adjacent-channel interferences are alleviated on the 2.4 GHz frequency band and eliminated on the 5 GHz frequency band after automatic channel adjustment is implemented using the DCA algorithm.
In practice, however, it has been determined that the radio traffic on 2.4 GHz channel 44 reaches the upper limit in a period of time, and the traffic in the core area is significantly higher than that in edge areas. In this case, the bandwidth needs to be dynamically adjusted. After the Dynamic Bandwidth Selection (DBS) algorithm is enabled, frequency bandwidths are dynamically allocated to APs.
After the frequency bandwidth of the radio on 5 GHz channel 44 is adjusted to 80 MHz, as shown in Figure 7.44, service capability is greatly improved and the radio traffic no longer approaches the limit. In addition, the user access capacity in the core area can be improved when APs are not densely deployed. The frequency bandwidths of other radios in the core area can also be increased from 20 to 40 MHz based on service demands, enabling all the 13 channels on the 5 GHz frequency band to be fully utilized.
d. Automatic frequency band adjustment
In high-density scenarios, dual-band APs capable of supporting both 2.4 and 5 GHz frequency bands are usually deployed to meet the higher capacity requirements of customers. In most cases, these APs are deployed within a closer proximity to one another. As only four nonoverlapping channel combinations are available on the 2.4 GHz frequency band, a large number of AP radios utilize these channels, restricting system capacity gains and leading to severe co-channel interference between APs. Such radios are known as redundant radios.
Huawei’s dual-band APs support 2.4 GHz-to-5GHz radio switchover, which prevents overlapping of 2.4GHz radios and improves overall system capacity. For AP models that do not support 2.4 GHz-to-5 GHz radio switchover, redundant 2.4GHz radios can be disabled or switched to monitoring mode to reduce co-channel interference through Dynamic Frequency Selection (DFS) or Dynamic Frequency Assignment (DFA).
3. Radio calibration framework
Figure 7.45 shows the basic radio calibration framework.
The configurations that must be performed for basic radio calibration on the customer interface include enabling radio calibration and delivering related parameter settings. When radio calibration is triggered on the device side — such as global calibration triggered by commands or at a scheduled time, partial calibration triggered when APs experience severe interference, and coverage hole compensation upon AP faults or recovery — the APs enable the air scan function, send the detected air interface data to the WAC for processing, and obtain the network topology and radio load statistics. The WAC then delivers the obtained information to the core calibration algorithm module for processing, obtains the radio calibration result, and delivers the result to the APs as a configuration item. By this point, the entire radio calibration process is complete.
4. Customer benefits of radio calibration
Radio calibration is more and more widely applied on Wi-Fi networks, and offers the following customer benefits:
FIGURE 7.45 Basic radio calibration framework.
a. Retains optimal network performance. Radio calibration enables real-time intelligent radio resource management, enabling Wi-Fi networks to quickly adapt to environment changes and maintain optimal network performance.
b. Reduces network deployment and O&M costs. Radio calibration automatically manages radio resources, lowering the skill requirements for O&M personnel and reducing labor costs.
c. Improves Wi-Fi network reliability. Radio calibration can quickly eliminate the impact of network performance deterioration, improving system reliability and user experience through automatic radio monitoring, analysis, and adjustment.