Jung Hyun Bae

Advanced downlink MU-MIMO receiver for 3GPP LTE-A

Third Generation Partnership Project Long Term Evolution (3GPP LTE) provides potential of higher spectral efficiency by using multi-user multiple-input and multiple-output (MU-MIMO) system. Because of the coarse knowledge of channel state information at the transmitter (CSIT) under current standard, co-scheduled user equipments (UEs) may suffer large residual multi-user interference in MU-MIMO. In this paper, we investigate performances of various types of receivers in the cases where residual interference is not negligible. Interference ignoring receiver is expected to perform poorly in this scenario, and hence, we consider interference-aware receivers. Interference rejection combiner (IRC) and joint Max-Log-MAP receiver are considered as interference-aware receivers. In order to perform joint detection of the target and interference layers, UE needs to know interference modulation which current standard does not provide. Because of this, we consider modulation-estimation-based joint receiver as our advanced downlink MU-MIMO receiver. Performances of aforementioned receivers are investigated in several practically relevant cases. It is shown that modulation-estimation-based joint receiver can significantly outperform IRC or the joint receiver with assumption of fixed interference modulation.

The GDOF of 3-user MIMO Gaussian interference channel

The paper establishes the optimal generalized degrees of freedom (GDOF) of 3-user M × N multiple-input multiple-output (MIMO) Gaussian interference channel (GIC) in which each transmitter has M antennas and each receiver has N antennas. A constraint of 2M ≤ N is imposed so that random coding with message-splitting achieves the optimal GDOF. Unlike symmetric case, two cross channels to unintended receivers from each transmitter can have different strengths, and hence, well known Han-Kobayashi common-private message splitting would not achieve the optimal GDOF. Instead, splitting each users message into three parts is shown to achieve the optimal GDOF as well as O(1) capacity approximation.

Low-Complexity 2D LMMSE Channel Estimation for OFDM Systems

In this paper, we propose a novel method of reducing the complexity of a pilot-based two-dimensional linear minimum mean square error (2D LMMSE) channel estimation scheme for orthogonal frequency division multiplexing (OFDM) systems. We identify that the 2D LMMSE channel estimation method aided by pilots embedded in a two-dimensional OFDM resource grid can be decoupled into three steps: (a) pilot denoising, (b) interpolation in each of OFDM symbols including pilots, and (c) interpolation across OFDM symbols. In the denoising step, we process pilots only for noise reduction. In the following interpolation steps, we utilize two-dimensional channel correlation across subcarriers (frequency) and across OFDM symbols (time) in order to get channel estimates of interest. Under the assumption that the frequency and time correlations are disjoint and the channel correlation can be represented by the product of them, we show that the channel interpolation can be performed in two steps along the frequency domain first and then along the time domain. By taking advantage of this separation, we can reduce the complexity of 2D LMMSE considerably while still achieving the optimal performance of it.

Simple transmission strategies for interference channel

In this paper, we investigate performances of simple transmission strategies. We first consider two user SISO Gaussian symmetric interference channel (IC) for which Etkin, Tse and Wang proposed a scheme (ETW scheme) which achieves one bit gap to the capacity. We compare performance of point-to-point (p2p) codes with that of the ETW scheme in practical range of transmitter power. It turns out that p2p coding scheme performs better or as nearly good as the ETW scheme. Next, we consider K user SISO Gaussian symmetric IC. We define interference regimes for K user SISO Gaussian symmetric IC and provide closed-form characterization of the symmetric rate achieved by the p2p scheme and the ETW scheme. Using this characterization, we evaluate performances of simple strategies with K=3, and show the similar trend to two user case.

Outage-based ergodic link adaptation for fading channels with delayed CSIT

To deal with a time-varying nature of the wireless channel, the most modern wireless systems use link adaptation in which the transmission rate is adjusted according to the current fading status to ensure reliable communication. For example, the current long-term evolution (LTE) standards require the user equipment (UE) to report channel quality indicator (CQI) to the base station (BS) to determine the data rate of a transmission (code) block. Such link adaptation effectively eliminates fading-induced outage and provides ergodic throughput in the long run. When there is delay in CQI feedback, however, such knowledge of the channel status is inevitably outdated, and outage becomes problematic again. In this paper, we characterize a future channel in which the reported CQI is actually used as a conditional random variable given current channel observation. The dependence of the future channel to the current observation is captured by channel correlation. By considering an outage event as a failed transmission, we can compute probability of successful transmission for each value of transmission rate to determine the best transmission rate which maximizes the expected throughput. If ergodicity holds for a channel process, then such expected throughput becomes close to an empirical throughput in the long run.

On the achievable region with point-to-point codes for generalized interference networks

This paper discusses evaluation of the capacity region for interference networks with point-to-point (p2p) codes. Such capacity region has recently been characterized as union of several sub-regions each of which has distinctive operational characteristics. Detailed evaluation of this region, therefore, can be accomplished in a very simple manner by acknowledging such characteristics, which, in turn, provides an insight for a simple implementation scenario. Generalized message assignment is considered in this paper, and it is shown to provide strictly larger achievable rates than what traditional message assignment does when a receiver with joint decoding capability is used.