# Implementation of a Fast Link Rate Adaptation Algorithm for WLAN Systems
## 1 Introduction  
The wireless channel features variations over time in the temporal domain and across frequency in the spectral domain are characterized by delay spread and Doppler spread, respectively, giving rise to frequency selectivity and time selectivity, respectively. Both the physical layer (PHY) and the data link layer or the medium access control (MAC) sublayer of the air interface (or access mode) should be designed in consideration of this wireless channel. In other words, signal transmission techniques should be effectively adjusted according to the varying channel quality or channel status. For instance, the access point (AP) in wireless local area networks (WLANs) may send a known signal to the station and subsequently the station can send to the AP a feedback signal with channel state information that recommends which signal transmission techniques are adequate for the channel. Taking this feedback into account, the AP may choose effective transmission techniques with which to send data to the station. This is known as closed-loop link adaptation. As another way, the station sends a known signal to the AP (via the uplink) and from the quality of this signal, the AP predicts the status of the downlink channel, assuming channel reciprocity. This is known as open-loop link adaptation. Closed-loop link adaptation assumes perfect channel knowledge available at the transmitter from either receiver feedback or channel sounding, incurring high computational complexity and communication overhead. A link adaptation algorithm where the receiver feedback is considered through the acknowledgment (ACK) of the transmitted data is a closed-loop link adaptation algorithm [1]. On the other hand, open-loop link adaptation utilizes only the transmitter's statistics and can maintain seamless interoperability and coexistence with legacy WLAN devices [2; 3]. In a target system that is based on the time division duplex (TDD), the wireless channel holds channel reciprocity for a relatively short time and accordingly open-loop link adaptation is an amenable choice in IEEE 802.11 with multi-user multi-input multi-output (MIMO). When the AP senses that the wireless channel condition is good or the signal-to-noise ratio (SNR) level is high, it will switch to another modulation and coding scheme (MCS) available in the WLAN which offers a higher data rate or enhanced throughput [4]. One of the most widely used basic link rate adaptation algorithm is automatic rate fallback (ARF) [5] which is based on statistical count of frames. After first ACK miss, the retransmission is still performed at the same rate but after second ACK miss, the second retry and subsequent transmission attempts are performed at the fallback rate. When the No. of successively received good ACKs reaches 10, the rate is upgraded. However, if the first transmission fails right after the rate upgrade, the rate is immediately decreased. A drawback of ARF is that if the channel changes quickly, the rate in ARF cannot be adapted effectively. Another statistical-count-based link adaptation algorithm called dynamic link adaptation [6] employs a success counter and a failure counter such that it increases the rate when the success counter reaches the threshold value S and decreases the rate when the failure counter reaches the threshold value F. If the channel changes very slowly, the number of retransmission attempts in both ARF and dynamic link adaptation increases. To address these two problems with ARF, adaptive ARF [7] was proposed such that if the first transmission fails right after the rate upgrade, not only the rate is immediately switched back to the previous rate but also the success threshold is doubled. On the other hand, if the rate is decreased owing to two consecutive fails, the success threshold is set to the initial value, 10. More variant algorithms exist as the statistical-count-based link adaptation using frame-based measurement.