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The emerging IEEE 802.11ac standard introduces spatial division multiplexing access, in its downlink version, to the wireless local access world. Through this multi-user multiple input multiple output (MU-MIMO) technique the access point (AP) can simultaneously transmit multiple independent groups of streams to different stations. The consequent precoding requires precise channel knowledge and thus regular channel sounding and feedback. The interval to choose for this sounding procedure is an important factor. A compromise between precise channel knowledge at the AP and reduction of the associated overhead is needed. The first is a physical (PHY) layer parameter and the second is evaluated at the medium access control (MAC) layer. In this paper we present a PHY+MAC analysis of the impact of the channel sounding interval on MU-MIMO transmissions for different scenarios. [ABSTRACT FROM PUBLISHER]

Most of studies dealing with single user multiple input multiple output (SU-MIMO) and multiple user MIMO (MU-MIMO) rely on physical (PHY) layer only or medium access control (MAC) layer only simulations. In the first case, the apparent considerable performance gains enabled by space division multiple access (SDMA) in downlink transmissions are often obtained without taking into account sounding and channel access induced overhead. In the second case, crosstalk interference (CTI) between stations served by the SDMA transmission is not always accounted for. Consequently, in order to take into account all the parameters and characteristics of these two transmission schemes, precise PHY+MAC modeling is needed. A realistic and fair comparison can thus be performed. The new approach presented in this paper is based on a multi-user simulation platform containing an all inclusive PHY layer combined with an elaborated MAC layer working in a symbiotic manner. In order to discriminate the characteristics of each transmission scheme, the SU/MU comparison is done for single antenna stations, channel inversion being used as the precoding scheme. In addition, the influence of CTI in SDMA transmissions is also studied so as to compare the real gains of MU-MIMO with the generally accepted gains. [ABSTRACT FROM PUBLISHER]