Downlink transmission into frequency-selective links experiencing different coherence bandwidths and coherence times has obvious practical relevance, but is often addressed via assuming for all users the coherence conditions corresponding to the fastest, most dispersive user. This paper demonstrates an alternative approach that directly exploits the differences between coherence bandwidth and coherence time of users under frequency-selective conditions, showing that it can lead to significant new efficiencies and gains. More specifically, we study a two-user downlink frequency-selective channel under three broad conditions of disparity between the link qualities: when the disparity is in coherence time, in coherence bandwidth, and in both coherence time and coherence bandwidth. Each of the coherence scenarios calls for a distinct treatment; for each, we provide an analysis demonstrating the gains in achievable rates due to exploiting coherence disparity. Numerical simulations demonstrate the advantages of the proposed schemes.

The broadcast channel under delayed, mixed, hybrid, or no CSIT Is a subject of much Interest but has been studied only under i.i.d. fading and perfect CSIR for all users. Models that go beyond i.i.d. fading and perfect CSIR are of practical Importance since users may experience unequal fading block-length (coherence time) and unequal CSIR availability due to different mobility and scattering environment. This paper studies a two-user MISO broadcast channel with hybrid CSIR, where one static user (with slower fading) has CSIR and one dynamic user (with faster fading) does not have free CSIR. Under this hybrid CSIR condition, the paper studies the degrees of freedom under various CSIT scenarios: no, delayed, and hybrid CSIT. For no CSIT, we provide an outer bound that meets the achievable degrees of freedom region when the coherence times of the users are the same. For both delayed and hybrid CSIT, the achievable regions partially meet their corresponding outer bounds, and furthermore the corresponding gaps decrease with the dynamic user coherence time.

Antenna correlation is prevalent in higher frequencies as well as in massive MIMO, thus the study of correlated MIMO broadcast channels is becoming a subject of increasing interest. This paper explores the fundamental limits of such systems, focusing on cases where correlation eigenspaces are neither independent nor identical, so that known beam-space division techniques do not directly apply. We begin by introducing a simple but novel tight outer bound on the degrees of freedom of noncoherent point-to-point MIMO channels under transmit antenna correlation. We then analyze the performance of a two-user MIMO broadcast channel when one correlation eigenspace is a subspace of the other. We extend the result to K-user MIMO broadcast channel. Our results show that it is possible to exploit the differences between the correlation structure of transmit antennas towards different receivers to extract degrees of freedom gains out of the system. The extent of these gains are highlighted via several examples.

In practice, wireless nodes may have different mobility and/or different scattering environment leading to disparity in coherence time and/or coherence bandwidth. Coherence diversity, as a novel source of gain, exploits fading condition disparity of different nodes to provide gain. Downlink transmission for two wireless nodes is considered in three scenarios of fading conditions disparity: in coherence time, in coherence bandwidth, and in both coherence time and coherence bandwidth. Coherence diversity gain is calculated in the three scenarios using product superposition transmission, and furthermore, numerical simulations are presented to demonstrate the gains.

In practical multiuser wireless networks, different links often experience unequal coherence lengths due to differences in mobility as well as scattering environment, a common scenario that has largely been neglected in the fundamental studies of the wireless channel. A key feature of unequal coherence lengths is that the per-transmission cost of acquiring CSI and its effect on achievable rates may vary significantly among the nodes, thus pre-existing receive CSI on a typical node should not be assumed as it will hide this key feature of the problem. In this paper, the method of product superposition is employed to find the achievable degrees of freedom region of multiuser broadcast channel where the users coherence lengths have arbitrary integer ratios. The achievable degrees of freedom region meets the outer bound when the transmitter has fewer antennas than the receivers, or when all receivers have the same number of antennas, hence for this class of antenna configurations the optimal degrees of freedom is now known.

Individual links in a wireless network may experience unequal fading coherence times due to differences in mobility or scattering environment. This paper studies broadcast and multiple access channels whose nodes experience unequal fading block lengths. Channel state information (CSI) is not available at the transmitters, and the cost of acquiring CSI at the receivers is fully accounted for in the degrees of freedom. In the broadcast channel, the method of product superposition is employed to find the achievable degrees of freedom. When the number of symbols in any fading block is at least twice the number of antennas at any active node and the fading block lengths have integer ratios, achievable degrees of freedom meet the upper bound in four cases: when the transmitter has fewer antennas than the receivers, when all receivers have the same number of antennas, when the coherence time of one receiver is much shorter than all others, or when all receivers have identical block fading length. The degrees of freedom region of the broadcast under identical coherence times was also previously unknown and is settled by the results of this paper. The disparity of coherence times leads to gains that are distinct from those arising from other techniques, such as spatial multiplexing or multiuser diversity. This new class of gains is denoted coherence diversity. The inner bounds in the broadcast channel are further extended to fading block lengths of arbitrary ratio or alignment. In addition, in the multiple access channel with unequal coherence times, achievable and outer bounds on the degrees of freedom are obtained.

The degrees of freedom (DoF) of the MIMO broadcast channel under various conditions remains an important open problem. This paper makes contributions towards completing the picture of the DoF of broadcast channels in the absence of CSIT (Channel State Information at Transmitter). First, the DoF of the quasi-static coherent broadcast channel (with CSIR) is established. Second, the DoF of the two-user asymmetric MIMO broadcast channel without CSIR is established, and is extended to multiple users with arbitrary antenna configurations. Third, an achievable DoF for mixed-CSIR multiuser broadcast channel, which was only known for certain antenna configurations, is extended to other antenna configurations.

We consider a wireless communication scenario with K source-destination pairs communicating through several half-duplex amplify-and-forward relays. We design the relay beamforming matrices by minimizing the total power transmitted from all the relays subject to quality of service constraints on the received signal to interference-plus-noise ratio at each destination node. We propose a novel method for solving the resulting nonconvex optimization problem in which the problem is decomposed into a group of second-order cone programs (SOCPs) parameterized by K real parameters. Grid search or nested bisection can be used to search for the optimal values of these parameters. We provide numerical simulations showing the superior performance of the proposed algorithms compared to earlier suboptimal approximations and their ability to approach the globally optimal solution of the non-convex problem.