IEEE 802.11 Wireless Local Area Network (WLAN) Physical Layer (PHY) offers multiple data rates. In multi-rate WLANs, 802.11 Medium Access Control (MAC) protocol, Distributed Coordination Function (DCF), essentially provides equal transmission opportunities to each sender host, causing the degradation of system throughput due to some hosts using low rates. In this paper we propose a cross-layer link adaptation scheme which improves throughput by adjusting MAC parameters according to PHY data rates to achieve time-based fairness instead of throughput-based fairness. Most importantly, in comparison with other approaches using a deterministic manner for temporal fairness (e.g. OAR), our random-access-based approach in statistic can be more effective to restrict the maximum packet delay within a bound when the number of stations grows. We conduct the transmission scenario of VoIP applications to evaluate the performance of our proposed scheme and the results demonstrate its effectiveness.