Optimisation of wireless multimedia networks with MIMO antennas: a cross-layer approach

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The ultimate goal of future-generation wireless communications is to provide ubiquitous seamless connections between mobile terminals such as personal digital assistant (PDA), and computer servers so that users can enjoy high-quality multimedia services at anytime anywhere without wires. Fundamental physical challenges such as channel fading and interference, however, have put strains on the radio resources that make reliable wireless communications difficult to achieve. In response to the rapidly increasing demand of the data-rate requirements, of particular attention is the so-called MIMO antenna (or a configuration that has multiple antennas at both the transmitter and the receiver), which cleverly exploits the difference between spatial signatures among antennas to reproduce transmission bandwidth.The aim of this project is to devise a smart scheduling protocol that maximises the utilisation of a MIMO downlink channel where communication occurs from a multi-antenna base station to many multi-antenna mobile terminals, through adaptation to the channel variations. Specifically, our aim is to minimise the total power of the base station while satisfying the users' error-rate and delay constraints. The problem is important for providing high-quality multimedia services in wireless channels. The proposed research jointly deals with scheduling of the users that access the channel for each packet time (i.e., the main function in the medium access control (MAC) layer) and how they access the channel using MIMO transmission (i.e., the main function in the physical (PHY) layer).The merit of the proposed joint MAC-PHY optimisation can be explained as follows. Users generally situated at different locations and they fade independently. At a particular constant in time, a certain user may be in deep fade but some may not be, and this contributes the so-called multiuser diversity. Intelligent MAC scheduling can maximise the channel utilisation by adapting to the channel variations in time among users. On the other hand, from the PHY point of view, MIMO transmission can be optimised to take full advantage of the channel variations in space among the users.To maximise the channel utilisation, users who are experiencing a good channel should gain access to the channel. However, for a channel with MIMO antenna, a sense of goodness is ill defined as a user may not have unanimously better channels for all antennas than another user's. Besides, as we are promoting spectral reuse in downlink, inter-user interference will further exacerbate the definition of a good channel. In addition, as far as delay is concerned, even a user who has a bad channel should gain access to the channel before his/her data expire. These mutually conflicting objectives complicate the optimisation.To accomplish our objective, we need to overcome the above challenges and in the end be able to design the scheduling and transmission jointly that can observe and adapt to the channel variations. It is anticipated that through enhanced interaction between PHY and MAC, the system throughput can be improved far exceeding systems in use today.