中文摘要：

IEEE802．11n协议的MAC层增强为长距离单向链路提供了高吞吐量，使其具备了用于长距离宽带接入的基本条件。针对长距离下侦听失效和ACK超时的问题，调整不同距离下的帧间距和ACKTimeout。在确保侦听有效和ACK不超时的前提下，建立基于退避时间和退避等级的二维Markov模型，计算了带有DCF功能的长距离802.11n双向链路的吞吐量和时延。同时，优化最小竞争窗口和最大重传次数，提升了链路性能。结果表明，优化参数后802．11n协议在长距离双向链路中的性能较好，具有一定实用性。

英文摘要：

The IEEE802.11n protocol with MAC enhance providing high throughput to long-range unidirectional link, has the basic condition for long-distance broadband access. The frame spacing and ACKTimeout at different distances was adjusted for the problems of long-range listening failure and ACK timeout. The two-dimensional Markov model based on backoff time and backoff level was established under the premise of ensuring that the listening was valid and the ACK was not timed. The throughput and delay of the long-distance 802.11n bidirectional link with DCF function were calculated. The minimum number of competing windows and the maximum number of retransmissions was optimized to improve link performance. The results show that the 802.11n protocol has better performance in long distance bidirectional link and certain practicability.