This paper tackles the downlink user scheduling and transmit beamforming problems in MIMO-OFDMA by extending a recent algorithm that maximizes the weighted sum rate (WSR) to all users in MIMO flat fading channels. The proposed method has a complexity that is proportional to the number of OFDMA subcarriers, which makes it practically attractive. Having assigned users to each subcarrier and designed beamformers for each user in each subcarrier, it remains to find the best, in terms of rate maximization, adaptive modulation and coding (AMC) mode to use for each data stream. The latter problem is solved in the second half of the paper through viewing the channel from the base station to the k-th receiver as a multiple access channel (MAC) with Nk“users”, where Nkis the number of antennas at receiver k. The proposed method maps the available AMC modes to the space of allowed theoretical rates, using the signal-to-noise ratio (SNR) gap to capacity concept, and selects the operating point with the largest sum-rate.

In this paper, the variational expectation-maximization (VEM) algorithm is used to provide channel estimates in a space-time decoder. The proposed estimator works for many types of space-time codes (STC), including full-rate full-diversity (FRFD) codes, Bell Laboratories layered space-time architecture (BLAST) and orthogonal STC's. The principle idea is to treat the channel coefficients as the unknown parameters, and the transmitted symbols as the unobserved variables in the EM algorithm. The posterior distribution of the symbols is approximated by a factorized distribution, whose Kullback-Liebler divergence with the true distribution is minimized, thereby explaining the ``variational'' aspect of the technique. The channel estimates may then be used for coherent decoding -- here we use the K-best detector for a 2×2 system. Simulation results show that the new channel estimator can reduce system complexity greatly compared with traditional schemes, while achieving near-ideal performance.

This work proposes a per-subband multiple input multiple output (MIMO) precoder selection technique for point-to-point MIMO orthogonal frequency division multiplexing (OFDM) based bit interleave coded modulation (BICM) systems with the soft-output minimum mean square error (MMSE) receiver. Given a pre-designed precoder codebook, the codeword/precoder that maximizes the mean of the mutual information per coded bit (MMIB) on all subcarriers within a subband is selected. The main advantages of this technique are the following: i) the precoder selection metric is explicitly related to BICM performance, thus it outperforms the previously proposed precoding techniques; ii) with commonly used unitary precoding codebooks, this technique works for an arbitrary number of transmit streams unlike the minimum singular value based method which does not work when the number of input streams is the same as the number of transmit antennas; iii) when multiple packets are transmitted and one precoder is used for these transmitted packets, an algorithm that combines the MMIB of each packet is proposed using an upper bound on the average packet error rate.

In this paper, a new opportunistic feedback and user selection protocol is proposed for the multiple antenna downlink. The proposed scheme uses the structure of zero-forcing dirty paper coding (DPC) at the transmitter and selects users using a contention-based feedback channel. The user selection procedure is divided into stages and within each stage, a user is selected for one spatial channel (defined by the zero-forcing dirty paper coding strategy). The user feedback is controlled by a feedback probability and a feedback threshold. This method of feedback significantly reduces the resources used for feedback compared to dedicating feedback resources for each user. We optimize an approximation to the sum rate expression with respect to these parameters and illustrate the sum rate performance by simulations. The simulation shows that we can achieve good sum rate performance despite the reduction in feedback resources.

For a multiuser bit interleaved coded OFDM (BIC-OFDM) system, we study the methods of time domain equalization and per-subcarrier maximum likelihood (ML) frequency domain equalization when there is timing asynchronism among the users. For the time domain equalizer structure, a coded pairwise error probability (PEP) approximation is derived, which leads to a training-based space-time design method that takes the error rate performance into consideration. We compare the time domain equalization and per-subcarrier ML frequency domain equalization methods, and conclude that at high SNR's, the time domain equalization approach is preferable.

A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station

In this paper, we investigate a multiple input multiple output (MIMO) multiuser relay channel, where a source with multiple antennas sends data to multiple users via a relay with multiple antennas. The relay applies linear processing to the received signal and forwards the processed signal to multiple users. In our system model, the direct links from the source to the users are neglected. We propose algorithms to compute achievable sum rates of this system based on dirty paper coding. An achievable sum rate defines a sum rate that can be achieved in the MIMO multiuser relay channel with zero error probability for any user, hence it is also a lower bound of the capacity of this channel. These algorithms also produce coefficients of the precoder at the source node and the coefficients of the linear processing unit at the relay. Simulations show that the proposed system architecture and algorithms achieve sum rate performance that is close to the derived performance upper bound

We propose a multichannel feedback protocol to enable local scheduling with channel state information for wireless networks with orthogonal frequency division multiplexing (OFDM). In our proposed protocol, the frequency subcarrier domain is shared by multiple control channels, on which request-to-send (RTS) and clear-to-send (CTS) are exchanged. These control channels are created using random spreading signatures. Channel state information of the simultaneous transmissions, which defines the gains of these channels, is exchanged on these control channels and channel state information tables that contain the channel information of both the desired link and the interfering links are created at the transmit nodes, then scheduling decisions are made based on the channel information tables. We show that the proposed protocol improves the network throughput compared to IEEE 802.11 style protocols for a wireless local area network topology