Abstract-We explore the available degrees of freedom (DoF) for the two user MIMO interference channel, and find a general inner bound and a genie aided outer bound that give us the exact # of DoF in many cases. We also study a share-and-transmit scheme and show how the gains of transmitter cooperation are entirely offset by the cost of enabling that cooperation so that the available DoF are not increased.

Abstract-We explore the available degrees of freedom (DoF) for the two user MIMO interference channel, and find a general inner bound and a genie aided outer bound that give us the exact # of DoF in many cases. We also study a share-and-transmit scheme and show how the gains of transmitter cooperation are entirely offset by the cost of enabling that cooperation so that the available DoF are not increased.

Abstract-It has been shown that random beamforming using partial channel state information (CSI) achieves the same throughput scaling as obtained from dirty paper coding for a broadcast (downlink) channel with transmit antennas and users where is large [1]. In this paper, we apply this scheme to wideband MIMO broadcast channels. By using OFDM, an -tap wideband channel can be decomposed to parallel narrowband channels (subcarriers), where > . Neighboring subcarriers are highly correlated. Therefore, we consider neighboring subcarriers as a cluster and find the closed form solution for the joint characteristic function of SINR values at two subcarriers in a cluster. We show numerically how the knowledge of the quality of the center subcarrier sheds light about the quality of other subcarriers in the same cluster, and address the issue of cluster size. In addition, through complex and asymptotic analysis, we show that for cluster size of order √ log (for large ), users need only feedback the best SINR at the center subcarrier of each cluster in order for the transmitter to perform opportunistic beamforming and maintain the same throughput scaling as when full CSI is available. Using simulation results, we verify our analytical result and show that even fewer feedback can be tolerated, and larger clusters ( 2 ) can be implemented for a small throughput hit.Index Terms-Channel state information, OFDM, broadcast channel, random beamforming, multi-user diversity, wireless communications.

It has been recently shown that opportunistic transmit beamforming using partial channel state information (CSI) achieves the same throughput scaling obtained from dirty paper coding for a broadcast channel with fixed number of transmit antennas and many receivers [ 11. In this paper, we study the generalization of this scheme to wideband broadcast channels. By using orthogonal frequency division multiplexing, an L-tap wideband channel can be decomposed to N parallel narrowband channels, where N is larger than L. Neighboring subchannels are therefore highly correlated, and it is intuitive to say that each group of neighboring subchannels (forming a duster) can be characterized by one channel quality. We show in this paper that users need only feedback the best signal-to-noise-plus-interference ratio at the center of each cluster. Our results indicate that for cluster size of order &, where K is the number of users, this feedback scheme maintains the same throughput scaling as when full CSI is known. Simulation results show that larger cluster sizes (&) can also be implemented for a small throughput hit.