What Is CoSR?

Why Is CoSR Needed?

In high-density scenarios, co-channel interference usually occurs because APs are densely deployed. To detect co-channel interference and avoid conflicts common in high-density scenarios, Wi-Fi introduces the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. The downside of this mechanism, however, is that APs in close proximity operating on the same channel cannot communicate with their respective STAs at the same time. Communication between AP1 and STA1 cannot occur at the same time as communication between AP2 and STA2, although the two communication sessions are not related. As such, the conventional clear channel assessment (CCA) used by CSMA/CA for channel detection is no longer able to meet the communication requirements.

In Wi-Fi 6, the spatial reuse (SR) mechanism is implemented by setting two thresholds for APs: signal detection (SD) thresholds for "my" BSS (MYBSS) frames and for overlapping BSS (OBSS) frames. For MYBSS frames, the threshold is set as low as possible to increase the coverage and thereby ensure as few MYBSS frames as possible are lost. For OBSS frames, the threshold is set as high as possible to reduce the coverage. A STA considers a channel to be interference-free and open for communication, as long as the signal received signal strength indicator (RSSI) is within the specified OBSS threshold. That is, when AP1 is communicating with STA1, AP2 can still communicate with STA2, thereby achieving the SR effect.

According to the existing SR mechanism of Wi-Fi 6, AP2 determines its own transmit power. The trouble is, information obtained by AP2 is limited. As a result, the transmit power that it determines is inaccurate, which in turn affects the data transmission of AP1.

In response, Huawei proposed an enhanced SR mode — CoSR.

How Does CoSR Work?

In CoSR, APs coordinate with one another to achieve the SR effect. Specifically, the sharing AP precisely controls the transmit power of shared APs. This eliminates interference from the shared APs to the sharing AP while increasing the gain of the shared APs.

The key to CoSR is coordinated scheduling. A prerequisite of coordinated scheduling is coordinated scheduling measurement, which enables APs to obtain information about other co-channel APs in a timely manner.

Coordinated scheduling measurement

An AP periodically measures the signal strength of uplink ACK frames from STAs (both associated and non-associated) on the same channel. Then the AP exchanges with co-channel neighboring APs measurement information about STAs associated with the neighboring APs. AP1 measures the signal strength of STA1 (associated STA) and STA2 (non-associated STA) on the same channel at the same time, and synchronizes the signal strength of STA2 to AP2. The same process applies to other co-channel neighboring APs (AP3, in this example).

In this phase, there is no sharing or shared AP. Coordinated scheduling measurement is performed among all co-channel APs. Therefore, AP2 also synchronizes the measured signal strength of STA1 to AP1. This information synchronization process is important to the shared AP selection during subsequent coordinated scheduling.

Determining the APs and STAs that participate in CoSR

The sharing AP needs to be determined first. The AP that preempts the transmit opportunity (TXOP) on the air interface is defined as the sharing AP (AP1 in the following figure, for example). The sharing AP first determines the STA to communicate with in the local cell, which is the sharing STA. Then, the sharing AP selects the shared AP. The selected shared AP should be as far away as possible from the STAs associated with the sharing AP to maximize reuse efficiency of the air interface.

This distance is determined based on the previous coordinated scheduling measurement information.

After the sharing and shared APs are determined, the sharing AP first sends a coordination frame in the current transmit window to instruct the shared AP to perform coordinated transmission. Additionally, the sharing AP specifies the maximum transmit power of the shared AP to ensure that the sharing and shared APs do not interfere with each other during data transmission. Then, the sharing AP sends its own data frame. At this moment, the shared AP selects a STA in its own cell as the shared STA according to the coordination frame and sends a data frame to it. Specifically, the shared STA is the STA that is closest to the shared AP and has the strongest signal. At this point, the CoSR process is complete.

Instead of the fixed implementation mechanism, CoSR adjusts the AP roles and packet rate continuously to ensure the optimal coordinated transmission effect. If the transmission failure rate of the sharing AP is high, the system does not allow it to function as the sharing AP. If the shared AP fails to send packets, the system reduces the packet transmission rate slightly so that the next coordinated transmission can succeed. If the next transmission succeeds, the system increases the packet transmission rate slightly to achieve an optimal transmission effect.

What Are the Application Scenarios of CoSR?

CoSR is mainly used in high-density office scenarios where APs are densely deployed and co-channel interference exists. In this scenario, according to CSMA/CA, APs in close proximity operating on the same channel cannot communicate with their respective STAs at the same time. With CoSR enabled, the APs can communicate with their STAs at the same time, thereby improving the capacity of the entire network.