OFDMA Based High Throughput Channel Access Strategies For The Next Generation WLAN

The IEEE 802.11 ax standard,which will be released in 2020,introduces the Orthogonal Frequency Division Multiple Access(OFDMA)mechanism for the first time,in order to deal with channel competition in dense network environments and improve spectrum efficiency and network throughput.The 802.11ax-based wireless local area network(WLAN)can enhance the capacity of multi-user parallel transmissions effectively,improve the data transmission efficiency of the physical layer,support an increasing number of mobile users to access the Internet,and cope with complex and diverse network environments.However,the 802.11 ax WLAN faces some serious problems,such as a large number of users transmitting data simultaneously in a dense network environment,shortage of channel resource,and a high probability of channel conflicts,etc.These problems lead to low overall channel utilization,long packet latency,and low network throughput,which are appealing for being solved.This paper focuses on the channel access protocols and channel resource allocation schemes in the IEEE 802.11 ax standard,and aims to improve the WLAN throughput and satisfy Quality of Service(Qo S)requirements.The main work and innovation of this dissertation are as follows:The Retransmission Number Aware Channel Access(RNACA)scheme is proposed,applying OFDMA mechanism.The uplink OFDMA-based random access(UORA)mechanism,provided by the IEEE 802.11 ax standard,supports multiple users to simultaneously transmit uplink data in different resource units(RU).With the UORA protocol,an access point(AP)uses a trigger frame to dynamically announce randomaccess resource units(RA-RUs)for which the STAs associated with the AP contend.These STAs use the back-off mechanism in channel competition and transmit data simultaneously over the obtained RUs.The disadvantage of the UORA mechanism is that,when the number of RUs is enough for the STAs,a failed transmission causes the sender's OFDMA contention window to be doubled,which brings in unnecessary packet delay and reduces channel utilization efficiency and network throughput.The RNACA presented in this paper can overcome the above shortcoming existing in the UORA mechanism.In the RNACA,it is defined the probability that takes into account the number of retransmissions,the number of RUs,and the number of STAs.When an STA is required to retransmit due to channel conflict,the STA uses the defined probability to decides on doubling its contention window size or not,targeting at improving network throughput.Probability theory is applied in the mathematical model for the RNACA,which leads to the successful probability of one transmission conducted by the sender,the packet delay,and the network throughput.From this,the throughput maximization problem is formulated,in which the maximum number of transmission trials is chosen as the optimization variable.The solution to the optimization problem yields the optimal maximum number of retransmissions,which is applied in the RNACA scheme.Simulation results show that the proposed RNACA scheme can achieve a higher throughput and a lower packet delay,compared with the UORA introduced in the IEEE 802.11 ax standard.The uplink channel reservation resource allocation(UCR)scheme,based on OFDMA mechanism,is proposed.With the development of Internet technology,higher real-time services have higher Qo S requirements in data delivery.In order to meet the Qo S requirements of users in dense network environments and improve user experience,the proposed UCR scheme makes efforts on satisfying the Qo S requirements of lowlatency,high-throughput,and low packet loss ratio for the video STAs to deliver uplink data.In the UCR scheme,STAs are classified as the video STAs with a higher priority in data delivery and the non-video STAs with a lower priority according to the needs of data delivery.In addition,the AP retains channel resources the video STAs so that they can continuously conduct long-time data delivery,improving the success ratio of channel access and the throughputs at the video STAs and reducing packet delay.The traditional RTS(Request to Send)/CTS(Clear to Send)scheme with four times of hands-shaking is extended in the UCR,and the RTS,CTS,and acknowledgement(ACK)frames in the traditional RTS/CTS mechanism are extended to Qo S-RTS,Qo S-CTS,and Qo S-ACK frames.The frame structures of them are defined to support the UCR.The simulation results show that the UCR scheme can improve the throughputs and reduce packet delay at the video STAs.The outcomes achieved in this dissertation are applicable in the newest generation WLAN,which is built on the IEEE 802.11 ax standard,so that throughput is enhanced.