Young-Han Nam

Full-dimension MIMO (FD-MIMO) for next generation cellular technology

This article considers a practical implementation of massive MIMO systems [1]. Although the best performance can be achieved when a large number of active antennas are placed only in the horizontal domain, BS form factor limitation often makes horizontal array placement infeasible. To cope with this limitation, this article introduces full-dimension MIMO (FD-MIMO) cellular wireless communication system, where active antennas are placed in a 2D grid at BSs. For analysis of the FD-MIMO systems, a 3D spatial channel model is introduced, on which system-level simulations are conducted. The simulation results show that the proposed FD-MIMO system with 32 antenna ports achieves 2-3.6 times cell average throughput gain and 1.5-5 times cell edge throughput gain compared to the 4G LTE system of two antenna ports at the BS.

Evolution of reference signals for LTE-advanced systems

3GPP LTE Release 10 standards (also known as LTE-Advanced) adopted some of the state-of-the-art radio access technologies that include carrier aggregation, eight-layer downlink spatial multiplexing, and four-layer uplink spatial multiplexing. For facilitating these enhancements, reference signals have significantly evolved in LTE-Advanced. This article examines underlying design principles of the LTE-Advanced reference signals. Specifically, newly introduced dedicated demodulation reference signals and channel state information reference signals for downlink and improvements of demodulation reference signals and sounding reference signals in uplink are discussed.

Fulfilling the promise of massive MIMO with 2D active antenna array

In this paper, we study practical 2-Dimensional (2D) active antenna array configurations for Massive MIMO systems and evaluate the system-level performance with the form factor at the base stations operating in cellular frequency band. To this end, we develop a 3-Dimensional Spatial Channel Model (3D SCM) to facilitate accurate performance evaluation and to build insights for system design. System-level simulation is conducted for various 2D antenna array configurations (8Hx8V, 8Hx4V) by using the 3D SCM developed. Simulation results show that 2-5 times cell average throughput gain and 1.5-9 times cell edge throughput gain can be achieved compared to the 4G LTE system.

A non-asymptotic throughput for massive MIMO cellular uplink with pilot reuse

This paper considers an uplink massive MIMO scheme that employs a pilot reuse scheme. For the massive MIMO scheme, a non-asymptotic cell throughput lower bound that is valid for any M is derived and the bound is shown to be tight by simulations. The lower bound is dependent upon three interference terms: intra-cell interference, inter-cell interference, and interference due to pilot contamination. A set of conditions on which each interference term dominates is examined by analysis and simulations, where the conditions are defined in terms of network topology, pilot reuse number and M. For example, for a dense network with M = 100, the largest cell throughput is achieved with pilot reuse number 2, in which case the traditional inter-cell interference dominates rather than the pilot contamination.

Cooperative communication technologies for LTE-advanced

The LTE-Advanced (LTE-A) system is currently under development to allow for significantly higher spectral efficiency and data throughput than LTE systems. In a wireless system based on orthogonal frequency division multiplexing (OFDM) with frequency reuse factor one such as LTE, the achievable cell spectral efficiency is often limited by the inter-cell interference or coverage shortage of base stations. Hence in LTE-A, coordinated multi-point (CoMP) transmission/reception (a.k.a. multi-cell MIMO or base station cooperation) and relaying technologies are being introduced to clear these major performance hurdles. In this paper, overall picture of cooperative communication technologies being discussed in LTE-A systems including CoMP and relaying is presented, together with considerations on system design.

Proportional fair scheduling for multi-cell multi-user MIMO systems

In this paper, a multi-cell multi-user MIMO scheme with advanced user pairing and scheduling algorithm is proposed to effectively explore multi-user diversity and to combat both intra-cell and inter-cell interference in the downlink of wireless communication systems. The optimality of the proposed user paring and scheduling algorithm is shown under generalized proportional fair metrics. System-level simulations are conducted to verify the performance of the proposed scheme with the agreed LTE-Advanced (LTE-A) system parameters. The results suggest the proposed scheme is a promising candidate for improving both the cell-edge user throughput and average cell throughput, and furthermore for fulfilling the IMT-Advanced (IMT-A) requirements.

Overview of Full-Dimension MIMO in LTE-Advanced Pro

Multiple-input multiple-output (MIMO) systems with a large number of base station antennas, often called massive MIMO, have received much attention in academia and industry as a means to improve the spectral efficiency, energy efficiency, and processing complexity of next generation cellular systems. The mobile communication industry has initiated a feasibility study of massive MIMO systems to meet the increasing demand of future wireless systems. Field trials of the proof-of-concept systems have demonstrated the potential gain of the Full-Dimension MIMO (FD-MIMO), an official name for the MIMO enhancement in the 3rd generation partnership project (3GPP). 3GPP initiated standardization activity for the seamless integration of this technology into current 4G LTE systems. In this article, we provide an overview of FD-MIMO systems, with emphasis on the discussion and debate conducted on the standardization process of Release 13. We present key features for FD-MIMO systems, a summary of the major issues for the standardization and practical system design, and performance evaluations for typical FD-MIMO scenarios.

Cooperative communications for LTE-advanced—relay and CoMP†

SUMMARY The Long Term Evolution-Advanced (LTE-A) system is currently under development to allow for significantly higher spectral efficiency and data throughput than the LTE systems. In a wireless system based on orthogonal frequency division multiplexing with frequency reuse factor one, the achievable cell spectral efficiency is often limited by the inter-cell interference or coverage shortage of base stations. In LTE-A, coordinated multi-point transmission/reception (a.k.a. multi-cell MIMO or base station cooperation) and relaying technologies are being introduced to clear these major performance hurdles. In this paper, cooperative communication technologies being discussed in LTE-A systems are presented, together with considerations on system design. Copyright

On the Optimality of Lattice Coding and Decoding in Multiple Access Channels

In this paper, we consider a class of multiple access lattice space-time coding and decoding schemes. We prove an extended version of the Minkowski-Hlawka theorem, utilizing independent Loeliger ensembles. Applying this extension, we prove that lattice coding and decoding achieves the optimal diversity-multiplexing tradeoff of multiple access channels.

Cooperation through ARQ

We consider the cooperative multiple access channel with single antenna sources and a multiple antenna destination. In order to fully exploit the degrees of freedom in this channel, multiple sources must transmit independent streams of information simultaneously. Under the standard coherent model, we argue that no half-duplex cooperation protocol can simultaneously achieve the maximum diversity and multiplexing gains of this channel. We avoid this limitation by developing a framework for cooperation through an automatic retransmission request (ARQ) mechanism. In the proposed cooperation protocol, users are only allowed to cooperate after receiving the first round of ACK/NACK signals. Our characterization of the achievable diversity-multiplexing tradeoff with the proposed protocol reveals that it attains both full-diversity and full-rate. Furthermore, our analysis reveals the asymptotic optimality of the proposed protocol, as the number of ARQ rounds grows. To establish the gain offered by the proposed protocol, we compare its tradeoff curves with that of the multiple access ARQ channel.