High-throughput collision-free client polling in multi-AP WLANs

In wireless local area networks (WLANs), collision-free channel access is desirable for real-time services that require guaranteed bandwidth and bounded delay. In WLANs with a single access point (AP), collision-free access can be achieved by applying the point coordination function (PCF), where the AP polls all associated clients for data transmissions. However, in larger WLANs with multiple APs, if there is no inter-AP coordination mechanism, concurrent transmissions from nearby basic service sets (BSSs) may collide and thus degrade service performance even if all APs operate on the PCF mode. So far only few schemes have been proposed to resolve this problem, and the client throughput in these schemes is quite limited due to the inaccurate modeling of polling conflicts. In this paper, we study client polling in multi-AP WLANs, with the objective of providing high-throughput, collision-free channel access for each client, and maximizing network capacity. We first give a WLAN framework in which the PCF of all APs is coordinated and clients are polled in a time slotted manner.We then formulate client polling into a time slot allocation problem and propose a collision-free polling scheme consisting of three procedures: (1) a basic polling procedure that determines the minimum number of time slots required to poll every client once to obtain the polling frequencies of all clients; (2) a complementary polling procedure that makes extra polls for APs that have idle time slots without causing collisions, to improve spatial reuse of the network; (3) a backup poll selecting procedure that finds backup clients to poll in case the current polled client has no data to transmit, to utilize the otherwise wasted bandwidth. We have conducted extensive simulations and compared it with two existing schemes. The simulation results show that the proposed scheme can provide high throughput and uniform channel access time for all clients, while boosting spatial reuse 2 to 3 times compared to other schemes.