The Research On The Degrees Of Freedom Region Of Multi-user MIMO Broadcast Channel

The channel state information(CSI) is very important to wireless networks. Such as the multi-user(MU) multi-input multi-output(MIMO) broadcast channel(BC). When the base station does not know CSI at transmitter(CSIT), according to traditional methods, no matter whether the clients know their CSI at receiver(CSIR), time division multiple access(TDMA) is degrees of freedom(DoF) optimal. But for the two-user BC, if the transmitter does not know CSIT and only one receiver knows its CSIR, Grassmannian-Euclidean superposition can get a bigger DoF region than TDMA. Based on the existing research on the BC, we consider the DoF region of BC with different CSI.We study the DoF region of multi-user BC with mixed CSI, where only part of the users have CSI at both side. The study of BC with unequal CSI is motivated by the downlink scenarios where users have different mobility, only two kinds within all the receivers: dynamic receivers and static receivers. For dynamic receivers with high-mobility, we assume that its movement is fast enough so there is no CSI at both side(transmitter and receiver). For static receivers with low-mobility, obtaining CSI is very easy, we assume that there is CSI at both side. Focus on this model, we propose a scheme which is a combination of zero-forcing pre-coding(or block diagonalization) and Grassmannian-Euclidean superposition. Compared to the traditional TDMA, this scheme can get a significant DoF improving. We first consider the case with only one dynamic receiver at receiving side and then discuss the general situation.Beside this, we also study the DoF region of the two-user non-coherent BC. As for the point-to-point non-coherent MIMO channel, we have already know the asymptotic capacity of this model in terms of transmitter antenna number M, receiver antenna number N and coherent interval T. We are also familiar with the geometric interpretation behind it. For the two-user MIMO broadcast channel, when one of the users is in coherent communication and the other is in non-coherent communication, we can also use Grassmannian-Euclidean superposition to get a bigger DoF region than TDMA. In this paper, we research the two-user non-coherent MIMO BC, and more particularly, all of the three points have no CSI and the coherent interval of the two users is uncertain. We start from two special cases and present a new solution which can attain a bigger DoF region than TDMA. We put emphasis on the choice of antenna numbers and the comparison with TDMA. This paper also explores the DoF region in a more general situation where the coherent interval of the two users has no relationship and we present a possible boundary of the DoF region.