Performance Analysis and Enhancement of MIMO aware MAC Protocols in WLANs

Abstract

Multiple Input Multiple Output (MIMO) is a radio communication technology that uses multiple antenna elements at both transmitting and receiving ends. Multiple antenna elements in MIMO systems could be exploited to boost up channel capacity or to attain transmission reliability without additional bandwidth and power. Wireless networks deployed with MIMO systems can utilize these features by employing spatial multiplexing and/or spatial diversity. Recently, MIMO systems have gained increased interest. Most of the existing wireless networks are looking forward to meet their ever increasing capacity demand by exploiting MIMO offered spectral efficiency. With the IEEE 802.11n amendment, MIMO systems have already been introduced to Wireless Local Area Networks (WLANs). IEEE 802.11n supports both spatial multiplexing and spatial diversity. Regarding spatial multiplexing, however, the latest WLAN standard only supports Single User spatial multiplexing based MIMO (SU-MIMO) transmission. SU-MIMO is point-to-point MIMO communication, where all concurrently transmitted data streams are destined for a single receiver. But due to various network characteristics, SU-MIMO is not always applicable. For instance, unless all the queues of corresponding antenna elements have enough packets to send, it's not worth applying SU-MIMO. In addition, SU-MIMO is worthy only when the antenna elements are evenly distributed in a transmitter-receiver pair. Apart from that, SU-MIMO is advantageous mostly when the transmitting node also deals with data packets of only one node at a time. Otherwise, if a transmitting node deals with data packets of various nodes, like Access Point (AP), it requires to go through multiple transmission attempts, which ultimately increases delay. To deal with inexpedient issues of SU-MIMO, Multi-User spatial multiplexing based MIMO (MU-MIMO) transmission is being widely studied. MU-MIMO refers to point-to-multipoint MIMO communication or vice versa. In downlink, it is point-to-multipoint MIMO communication, whereas, in uplink, it is multipoint-to-point MIMO communication. Realizing MU-MIMO in existing WLANs requires significant changes in the MAC protocol. Either dominant MAC protocol Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) needs to be replaced by novel MU-MIMO aware MAC protocol or it should be upgraded into MU-MIMO aware CSMA/CA. Nevertheless, the simplest approach would be upgrading the widely deployed CSMA/CA. Therefore, in this dissertation, we investigate different modification approaches that best represent the possible modifications in CSMA/CA to upgrade it into MU-MIMO aware CSMA/CA. We consider both uplink and downlink MU-MIMO communications. We also provide detailed performance analysis to understand their performance benefits and trade-off. As both theoretical and practical understanding have already shown the superiority of MU-MIMO in many cases, MU-MIMO can be expected to become one of the basic essentials of future wireless networks. Even so, MU-MIMO still has some limitations. For example, when the data transmission duration varies between the links due to difference in link quality or data length, MU-MIMO may fail to utilize the available MIMO capacity to its full potential. Therefore, in this dissertation, we also analyze a MAC layer solution to address such issues. We present a MAC layer protocol design that can switch between MU-MIMO and multiple SU-MIMO transmissions. After taking into account the achievable performance, presented protocol selects a transmission mode in such a way that the available MIMO capacity is maximally utilized.