With the development of wireless communications, the physical Wi-Fi transmission rate is soaring, which requires a good wireless environment. In actual scenarios, complex wireless environments always cause weak signals or no signal coverage. Smart antenna technology is introduced to Wi-Fi systems to improve wireless coverage performance and wireless user experience.
In the WLAN field, smart antennas can be implemented in the following methods:
l Chipset
Smart antenna implementation using chipsets is a part of 802.11 protocols, which is called beamforming. For details, see [From Beginner to Expert - WLAN Common Terms] Section 5 - Beamforming.
l Antenna array + Antenna selection algorithm
Antenna array implementation is a beam switching technology. An antenna array has multiple hardware antennas and intelligently selects multiple antenna elements based on the antenna selection algorithm to transmit and receive signals. Combining different antennas can form different transmit directions, providing STAs with optimal antenna combinations to improve receive signal quality and system throughput.
The following describes the smart antenna array and antenna selection algorithm for implementing smart antennas.
Smart Antenna Array
A smart antenna array consists of a series of antenna elements. The combination mode depends on the gain, polarization mode, and pattern of antenna elements. In most cases, there are specific requirements for the antenna element distance, relevance, and isolation.
Each antenna element can be a directional or omnidirectional antenna. The number of antenna elements determines the number of final beams.
STAs associated with an AP may enter the AP's coverage area from any position. Therefore, the AP needs to support omnidirectional signal receive to ensure STA access. Additionally, downlink broadcast packets are transmitted in all directions within the coverage. After a STA associates with the AP, the AP selects an optimal antenna combination based on the antenna selection algorithm to communicate with the STA.
Antenna Selection Algorithm
The antenna selection algorithm determines how to select an antenna from an antenna array.
The following describes antenna selection in three scenarios:
l Antenna selection when a STA associates with an AP for the first time
a. After a STA associates with an AP, the AP uses its default antenna to select a rate. Based on statistics about packets sent to the STA, the AP selects the rate at which the most packets are successfully sent as the initial probe rate.
b. At the initial probe rate, the AP detects antenna combinations to select an optimal one.
The AP traverses all antenna combinations and computes the packet error rate (PER) and received signal strength indicator (RSSI). The antenna combination with the lowest PER is preferred. If multiple combinations have the lowest PER, the one with the highest RSSI is preferred.
c. The AP adjusts the rate, and selects the optimal rate and antenna combination.
When PERs of multiple antenna combinations are within the specified threshold, the AP detects antenna combinations at a higher rate until it selects the optimal rate and antenna combination.
l Event-triggered antenna selection
When the wireless environment changes greatly, for example, interference increases or disappears, or STAs move, antenna detection is triggered to select a new antenna combination. The antenna selection method is the same as that when a STA associates with an AP for the first time. In this way, the AP re-selects the optimal rate and antenna combination.
l Periodic antenna selection
Better antenna combinations may always exist because of the wireless channel randomicity or wireless environment changes, even if the PER and RSSI do not change significantly. In this case, periodic antenna detection is recommended.
During the detection time, an AP sends probe packets using all antenna combinations to collect their working status and select the optimal one. Then the AP uses the selected antenna combination until the next detection. The PER and RSSI are used to evaluate antenna combinations. When the detection period elapses, the AP traverses all antenna combinations and compares PERs to determine the optimal combination.
