Rajarajan Balraj

MU-MIMO in LTE Systems

A relatively recent idea of extending the benefits of MIMO systems to multiuser scenarios seems promising in the context of achieving high data rates envisioned for future cellular standards after 3G (3rd Generation). Although substantial research has been done on the theoretical front, recent focus is on making Multiuser Multiple-Input Multiple-Output (MUMIMO) practically realizable. This paper presents an overview of the different MU-MIMO schemes included/being studied in 3GPP standardization from LTE (long-term evolution) to LTE Advanced. MU-MIMO system concepts and implementation aspects have been studied here. Various low-complexity receiver architectures are investigated, and their performance assessed through link-level simulations. Appealing performance offered by low-complexity interference aware (IA) receivers is notably emphasized. Furthermore, system level simulations for LTE Release 8 are provided. Interestingly, it is shown that MU-MIMO only offers marginal performance gains with respect to single-user MIMO. This arises from the limited MU-MIMO features included in Release 8 and calls for improved schemes for the upcoming releases.

Multi-user MIMO and Carrier Aggregation in 4G systems: The SAMURAI approach

The market success of broadband multimedia-enabled devices such as smart phones, tablets, and laptops is increasing the demand for wireless data capacity in mobile cellular systems. In order to meet such requirements, the introduction of advanced techniques for increasing the efficiency in spectrum usage was required. Multi User -Multiple Input Multiple Output (MU-MIMO) and Carrier Aggregation (CA) are two important techniques addressed by 3GPP for LTE and LTE-Advanced. The aim of the EU FP7 project on “Spectrum Aggregation and Multi-user-MIMO: real-World Impact” (SAMURAI) is to investigate innovative techniques in the area of MU-MIMO and CA with particular focus on the practical, real-life, implementation and system deployment aspects. In the present paper, we provided an overview of the up-to-date SAMURAI contributions together with a description of the SAMURAI demonstrators developed as core part of the project.

Impact of feedback and user pairing schemes on receiver performance in MU-MIMO LTE systems

This paper addresses the problem of feedback design for MU-MIMO (multi-user multiple input multiple output) transmission in 3GPP Long Term Evolution (LTE) systems. The CSI (channel state information)-based feedback introduced in the latest release of LTE, Rel-10 is adopted in MU-MIMO transmission and potential performance gains are demonstrated over Rel-8 CQI/PMI (channel quality indicator/precoding matrix indicator)-based feedback mechanism at the link level. The LTE downlink performance has been evaluated for various MU-MIMO feedback schemes and for low complexity receivers. The results have shown that the CSI-based feedback can make a significant tradeoff between receiver performance and complexity in MU-MIMO therewith reducing the design requirements for advanced receiver concepts.

Interference estimation for multi-layer MU-MIMO transmission in LTE-Advanced systems

The aim of this paper is to get an insight of the interference estimation for multi-layer multi-user multiple-input and multiple-output (ML-MU-MIMO) transmission for LTE-Advanced (long term evolution) systems. Different interference-aware receivers have been investigated in ML-MU-MIMO with the presence of co-layer, intra-cell and inter-cell interferences. User-specific reference signal (UE-RS) based channel estimation and various interference covariance estimation schemes in LTE-Advanced systems are investigated together analytically and numerically. Our investigation has shown the significant influence of the interference estimation schemes on the system performance at the link level. Furthermore, the simulation results have shown the advanced MMSE receiver together with UE-RS based interference estimation as the most robust receiver structure for the signal detection in ML-MU-MIMO transmission.

On the performance of CoMP transmission in unsynchronized networks with timing offset

Coordinated Multipoint (CoMP) transmission is a promising technology in 4G Long Term Evolution (LTE) systems to enable coverage and high data rates for cell edge User Equipments (UEs). However, the presence of Timing Offset (TO) between the received signals from multiple transmitters at the UE could prove prohibitive for CoMP transmission. Such TO affects the estimation of the transmit covariance matrix for Pre-coding Matrix Index (PMI) selection significantly. Especially, the promised advantages of closed-loop CoMP transmission vanishes and it performs even worse than the open-loop in presence of a large TO due to the improper PMI selection. With analytical and numerical results we show that keeping the TO small, the promised advantages of closed-loop CoMP transmission can be guaranteed and as a result near optimum performance is achieved for CoMP transmission in unsynchronized networks with TO present.

Reliable implicit feedback generation in unsynchronized CoMP transmission

Coordinated multipoint (CoMP) transmission is a key technique in 4G long term evolution (LTE) systems to enable broadband communication and coverage for user equipments (UEs) at the cell edge. However, as the transmit signals will reach the UE at several and varying delays, unsynchronized CoMP transmission might experience large timing offsets (TOs), imposing a significant performance loss. In this paper we focus on the impact of implicit feedback generation in closed-loop CoMP transmission operating in joint processing mode. To enable the expected advantages, reliable channel state information is crucial for practical realization of closed-loop CoMP transmission. We show that a spacing in terms of subcarriers between the reference signals in the orthogonal frequency division multiplexing symbols imposes a phase offset on the transmit covariance matrix in presence of TO, affecting the feedback generation and limiting the expected performance. We provide a solution to overcome this limitation by estimating and compensating this phase offset. We show analytically and numerically that keeping the phase offset close to zero, reliable feedback generation can be guaranteed and, as a result, near optimum performance is achieved in closed-loop CoMP transmission in unsynchronized networks with TO present.