Bernd Bandemer

Linear MMSE Multi-User MIMO Downlink Precoding for Users with Multiple Antennas

In this paper, the multi-user MIMO downlink channel is considered, where a single transmitter (base station) sends data to several users within the same resource unit. Assuming perfect channel state information (CSI) at the transmitter, different linear precoding schemes are studied, aiming at mean square error minimizing transmission to all users. The exact MMSE solution can be obtained by exploiting uplink/downlink duality. A suboptimal solution based on necessary conditions for MMSE optimality may also be computed directly in the downlink domain. Our results indicate that the performance difference between the direct and duality-based method that is visible in uncoded BER curves vanishes when channel coding is introduced to the system. We also compare the MMSE-optimal approaches to other existing linear precoding schemes (BD, S-MMSE)

Experimental Characterization and Modeling of Outdoor-to-Indoor and Indoor-to-Indoor Distributed Channels

We propose and parameterize an empirical model of the outdoor-to-indoor and indoor-to-indoor distributed (cooperative) radio channel, using experimental data in the 2.4-GHz band. In addition to the well-known physical effects of path loss, shadowing, and fading, we include several new aspects in our model that are specific to multiuser distributed channels: 1) correlated shadowing between different point-to-point links, which has a strong impact on cooperative system performance; 2) different types of indoor node mobility with respect to the transmitter and/or receiver nodes, implying a distinction between static and dynamic shadowing motivated by the measurement data; and 3) a small-scale fading distribution that captures more severe fading than that given by the Rayleigh distribution.

On the capacity region for index coding

A new inner bound on the capacity region of the general index coding problem is established. Unlike most existing bounds that are based on graph theoretic or algebraic tools, the bound relies on a random coding scheme and optimal decoding, and has a simple polymatroidal single-letter expression. The utility of the inner bound is demonstrated by examples that include the capacity region for all index coding problems with up to five messages (there are 9846 nonisomorphic ones).

Spatial separation of multi-user MIMO channels

Since multi-antenna (MIMO) systems are becoming more popular thanks to their inherent potential for capacity improvement, interference from MIMO transceivers is an increasingly serious concern. Spatial multiplexing schemes are particularly vulnerable to multi-user interference. Fortunately, this interference can be mitigated, when the channel matrices show a sufficient spatial separation. In this paper, we quantify the separability of multi-user MIMO channels using actual measurements in a scenario where a single outdoor base station transmits to two indoor mobile receivers. To quantify the spatial distance between the two users, we compare the spatial correlation matrices using two simple measures: (i) matrix collinearity, and (ii) the condition number ratio. Both measures are directly linked to MIMO system performance. Our measurement-based evaluations demonstrate that the downlink channels of different users can have a significantly different spatial structure, even when the users are in the same room. This leads to the following conclusions: (i) new multi-user MIMO models are needed to describe the spatial characteristics of different users, and (ii) spatial interference can be well managed by appropriate scheduling and precoding algorithms.

Simultaneous nonunique decoding is rate-optimal

It is shown that simultaneous nonunique decoding is rate-optimal for the general K-sender, L-receiver discrete memoryless interference channel when encoding is restricted to randomly generated codebooks, superposition coding, and time sharing. This result implies that the Han-Kobayashi inner bound for the two-user-pair interference channel cannot be improved simply by using a better decoder such as the maximum likelihood decoder. It also generalizes and extends previous results by Baccelli, El Gamal, and Tse on Gaussian interference channels with point-to-point Gaussian random codebooks and shows that the Cover-van der Meulen inner bound with no common auxiliary random variable on the capacity region of the broadcast channel can be improved to include the superposition coding inner bound simply by using simultaneous nonunique decoding. The key to proving the main result is to show that after a maximal set of messages has been recovered, the remaining signal at each receiver is distributed essentially independently and identically.

Overhearing-Based Interference Cancellation for Relay Networks

We consider a wireless downlink system with one transmit base station (BS) and two receive subscriber stations (SS1 and SS2) in full frequency reuse. The primary subscriber (SS1) is served in two hops via a relay station (RS). The second subscriber (SS2) is located relatively close to both BS and RS. In a conventional paradigm, SS2 can not receive its message from BS when either BS or RS serves SS1, since the transmissions for SS1 cause strong interference at SS2. To improve system throughput, we propose an interference cancellation scheme at SS2 that allows BS to serve SS2 while RS relays. Exploiting the identity of the interfering messages in the two hops, SS2 overhears the interfering message at its initial transmission and cancels its interference latter. Theoretical analysis and realistic simulations show that the proposed scheme can significantly and practically boost system throughput.

On the sum capacity of a class of cyclically symmetric deterministic interference channels

Certain deterministic interference channels have been shown to accurately model Gaussian interference channels in the asymptotic low-noise regime. Motivated by this correspondence, we investigate a K user-pair, cyclically symmetric, deterministic interference channel in which each receiver experiences interference only from its neighboring transmitters (Wyner model). We establish the sum capacity for a large set of channel parameters, thus generalizing previous results for the 2-pair case.

On the noisy interference regime of the MISO Gaussian interference channel

In this work, we consider the two-user Gaussian MISO interference channel. We derive a criterion for sum-rate optimality of treating interference as noise at the receivers, which generalizes recent results for the SISO case [1]-[3]. In contrast to those results, however, the criterion is not in closed form. For the case where single-mode beamforming is applied at the transmitters, we propose a practical decision algorithm that will decide whether optimality holds for a given channel.

Optimal Achievable Rates for Interference Networks With Random Codes

The optimal rate region for interference networks is characterized when encoding is restricted to random code ensembles with superposition coding and time sharing. A simple simultaneous nonunique decoding rule, under which each receiver decodes for the intended message as well as the interfering messages, is shown to achieve this optimal rate region regardless of the relative strengths of signal, interference, and noise. This result implies that the Han-Kobayashi bound, the best known inner bound on the capacity region of the two-user pair interference channel, cannot be improved merely by using the optimal maximum likelihood decoder.

Capacity-Based Uplink Scheduling Using Long-Term Channel Knowledge

In the multi-user MIMO uplink channel, the presence of multiple receive antennas enables the base station to serve a number of users simultaneously, thus increasing overall system throughput. In practice, users have to be scheduled instead of being served all at once. In this work, a novel scheduling metric is proposed based on ergodic sum capacity. A low-SNR Taylor series of the capacity expression is combined with a receiver-correlation channel model to obtain a robust measure of spatial compatibility. Long-term channel knowledge at the base station is sufficient for metric evaluation, which significantly reduces the amount of required channel sounding, compared to previously proposed schemes.