Development of a CSMA mac protocol and throughput analysis for mu-mimo WLAN

Abstract

With the proliferation of wireless devices as well as high bandwidth consuming ap- plications, wireless local area networks (WLANs) has not only turned into the most popular form of wireless communication but also increased the emphasis on through- put in a signi cant manner. Multi-user multiple-input and multiple-output (MU- MIMO), an advance form of MIMO, has caught the eyes of the researchers due to its enormous capacity in enhancing the performance of wireless networks. IEEE 802.11 provides the physical layer (PHY) and medium access control (MAC) speci cations for WLANs. Carrier sense multiple access with collision avoidance (CSMA/CA) is mainly utilized as the access mechanism in IEEE 802.11 based WLANs. However, the traditional CSMA/CA MAC protocol cannot use the full bene ts from MU- MIMO technique in WLANs. The random access nature of the users to the shared medium is responsible for this. Therefore, MU-MIMO WLANs under the tradi- tional CSMA/CA MAC protocol cannot employ all the resources simultaneously which results in the ine ective utilization of the resources. In this thesis, we propose a carrier sensing based MAC protocol for MU-MIMO based WLANs system which can extract the full bene ts of MU-MIMO technique. In the proposed protocol, multi-contention based users selection in uplink and con- current multi-packet transmission in downlink are incorporated by introducing an additional control packet and using the existing control packets with some modi ca- tions into the frame structure. By utilizing these control packets, our protocol not only ensures the proper utilization of all the available resources but also e ciently encounters the joint uplink-downlink access based problems raised in MU-MIMO based WLAN environment. We consider a single rate and single channel WLAN. The transmission power of each antenna is assumed to be equal. We model the WLAN system under the proposed MAC protocol as a discrete time Markov chain (DTMC). Based on the Markov chain, an analytical model is developed to compute the total throughput of a WLAN under saturation. The wireless characteristics, i.e., path loss, Rayleigh fading and log-normal shadowing under the physical interference model are taken into account while modeling the throughput. These considerations make our analysis more practical. The analytical model developed for throughput computation under the proposed MAC protocol is validated via simulation. We develop a MATLAB based simulator to nd out the simulation results. For nding out di erent insights of the proposed protocol, the performances of this protocol are compared with CSMA/CA MAC and Uni-MUMAC [38] protocols. The simulators for CSMA/CA MAC and Uni-MUMAC protocols are also developed in MATLAB to nd out their throughput performances. Using the simulation and analytical results, we demonstrate that the proposed MAC protocol signi cantly improves the throughput performance compared to the tradi- tional CSMA/CA MAC protocol in WLANs. We also observe that our proposed MAC protocol provides better throughput than the Uni-MUMAC protocol. Throughput performance of the proposed MAC protocol is analyzed by varying medium access rate of the users, data transmission rate, variance in shadowing and the number of users. The uplink throughput decreases with increasing the medium access rate of the AP. The throughput of downlink transmissions shows opposite nature compared to the uplink throughput under this condition. However, we obtain the opposite results when the AP accesses the channel with a xed rate and the medium access rate of the users is increased. The throughput gains for both uplink and downlink transmissions decrease with increasing of data transmission rate. Interestingly we nd that the impact of shadowing is insigni cant on both uplink and downlink throughputs. Furthermore, with increasing the number of users, the uplink throughput increases and the downlink throughput decreases.