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 stani dard only supports Single User spatial multiplexing based MIMO (SUMIMO) 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, SUMIMO 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.