“Spatial Reuse” supports high-density deployment scenarios that had become a tough challenge for service providers.
The WiFi 6 “Spatial Reuse” focuses on two main directions:
- Increasing throughputs by enabling simultaneous data transmission within overlapping networks
- Reducing inter-BSS interference among overlapping networks in high-density environments
Before 11ac, WiFi devices followed the “CSMA/CA” mechanism, also known as “Listen-Before-Talk”, to complete and gain transmission opportunities. Before commencing transmission, WiFi devices always perform CCA (Clear Channel Assessment) to determine if the air is currently busy or accessible.
The IEEE 802.11 defines two thresholds: ED-CCA (-62 dBm@20 MHz) and CS-CCA (-82 dBm@20 MHz) for CCA to follow; in which “CS-CCA” was the main reason that restricted high-density deployments of WiFi BSSs. Since the evolution of services in high-density environments becomes a serious challenge for service providers, the 11ax standard introduces two new mechanisms to lift the “CS-CCA” restriction and make it a “dynamic” threshold (-62dBm ~ -82 dBm@20 MHz) instead of a “fixed” threshold (-82 dBm@20 MHz).
2.1. Concept of BSS Color
The concept of “BSS Color” was originally from IEEE 802.11ah, a sub-GHz band WiFi standard for IoT applications. The WiFi 6 defines 6 bits to identify the BSS with different colors, which means that there are a maximum of 63 BSS colors to identify.
With the identifier, the BSSs can operate on the same channel with different “colors” or “no color” to behave as “competitors”. They follow the most stringent rule, CS-CCA = -82 dBm@20 MHz, to compete for the air time.
On the other hand, BSSs operating on the same channel with the same “BSS Color” become “allies” and follow the loosened rule, CS-CCA = -62 dBm@20 MHz, to compete with each other.
The well-organized BSS operating channels with the effective geometry of installations and assignments of colors can significantly improve the deployment density of WiFi BSSs as well as network performance in the area.
2.2 Concept of OBSS-PD
“CS-CCA” is the standard-defined restriction for high-density deployment of BSSs. In addition to CS-CCA, the WiFi device’s transmission power is one of other factors. OBSS-PD utilizing TPC (Transmission Power Control) and DSC (Dynamic Sensitivity Control, CS-CCA in the range of -62 ~ -82 dBm@20 MHz) can further optimize the network performance in an area with high-density deployment.
Before the WiFi 6, WiFi STAs utilized “PS-Pool” or “U-APSD” as the power-saving mechanism, with which the “active” or “sleep” state is independently controlled by STAs. The advent of WiFi 6 introduces Target Wake Time (TWT) concept from IEEE 802.11ah as a centrally managed power-saving mechanism.