Stanislaus Iwelski

On the Equivalence of MMSE and IRC Receiver in MU-MIMO Systems

In order to combat the residual interference in multiuser multiple-input and multiple-output systems (MU-MIMO), a number of low-complexity interference aware type of receivers have been considered. In this letter, single layer Minimum Mean Square Error (MMSE) and Interference Rejection Combiner (IRC) are applied in MU-MIMO transmission. Based on some very simple but extremely useful matrix manipulations, their equivalence is proven analytically and demonstrated via Monte Carlo simulations.

Feedback Generation for CoMP Transmission in Unsynchronized Networks with Timing Offset

Coordinated multipoint (CoMP) transmission is a promising technique in long term evolution (LTE) systems to provide coverage and broadband communication for cell edge user equipments (UEs). However, as signals from multiple transmitters may reach the UE at different times, CoMP networks might experience high time offsets, leading to significant performance loss in closed-loop transmission. In this paper we show that spacing between reference signals (RSs) imposes a phase offset on the transmit covariance matrix in presence of timing offset (TO), affecting the feedback generation. The promised advantages of closed-loop CoMP transmission vanish in presence of TO due to improper precoding matrix index (PMI) selection. Keeping the phase shift close to zero, reliable PMI selection can be guaranteed and as a result near optimum performance is achieved for closed-loop CoMP transmission in unsynchronized networks with TO present.

Analysis of CQI Prediction for MU-MIMO in LTE Systems

The aim of this paper is to get an insight of multiuser multiple-input and multiple-output (MU-MIMO) channel quality indicator (CQI) prediction for long term evolution (LTE) systems. MU-MIMO CQI predictions have been investigated for LTE Release 8 (rank one, Transmission Mode 5) and for LTE Release 9 (adaptive rank, Transmission Mode 8) at the link level. The relationship between CQI reporting and channel conditions is analytically analysed and demonstrated. The tradeoff between the performance gain and the feedback overhead w.r.t. MUMIMO CQI prediction is investigated as well. The results have shown how MU-MIMO CQI prediction is influenced by channel correlation and that significant improvements can be achieved through rank adaption and receiver choice. The results have also shown the potentially most efficient system configuration (feedback design and CQI prediction scheme) to facilitate MUMIMO transmission for practical LTE systems.

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.

Evaluation of Implicit Feedback in Coordinated Multipoint Transmission beyond LTE-Advanced

Coordinated Multipoint (CoMP) was adapted as a key technique to improve the coverage of high data rates and the cell-edge user experience in beyond LTE- Advanced systems. CoMP simultaneously supports multiple Transmission Points (TPs) to serve mobile devices in the high interference areas, which occur between cells. In theory it can provide significant gains in the order of high double-digit percentages for cell-edge user throughput while increasing overall network efficiency. The aim of this paper is to get an insight of the system performance and configuration flexibility of CoMP transmission in beyond LTE-Advanced systems. Different channel state information feedback concepts for CoMP networks have been analyzed theoretically and verified through Monte Carlo simulations. Complexity and implementation issues have been addressed as well. The results have shown that downlink CoMP schemes are very effective in improving the overall system performance. Furthermore, the investigation has shown that different feedback approaches bring different performance gains but also different complexity and implementation challenges. Finally, the most effective feedback approach and system configuration for CoMP scenarios investigated in this study is suggested.

Cooperative interference mitigation in heterogeneous LTE networks

Small cell transmission is a promised technique to enable the tremendous data growth in next generation long term evolution (LTE) networks. However, emerging inter-cell interference is the dominant limiting factor achieving higher network capacities. In this paper we focus on cooperative interference mitigation techniques in interfering heterogeneous network (HetNet) deployments to assist the cell edge user equipment (UE) suffering from dominant co-channel interference (CCI). As a handover might be refused and conventional precoding matrix index (PMI) selection schemes limit reliable downlink (DL) transmission, the co-scheduled UEs being in good channel condition delegate the precoding vector selection to that victim UE. We provide a practical implementation for the cooperative interference mitigation algorithm keeping computational complexity and feedback overhead low. With numerical results at the link level we show that CCI is almost suppressed and as a result near optimum DL performance is achieved at the cell edge UE in strong interfering HetNet deployments.

Robust FFT Window Replacement in Non-Ideal CoMP Networks with Timing Offset

Coordinated multipoint (CoMP) transmission is one of key technologies to provide broadband communication and coverage for cell edge user equipments (UEs) in long term evolution (LTE) systems. The basic concept behind CoMP transmission are geographically distributed transmitters working jointly in coordinated manner. As the transmit signals reach the UE at different times, CoMP networks might experience high timing offsets (TOs) leading to a fast Fourier transform (FFT) window mismatch. Positive TOs with synchronization signals reaching the UE prior to data signals impose an acceptable performance loss, whereas the presence of negative TOs disables CoMP transmission saturating in error floor at unacceptable bit error rate. In this paper a novel approach to overcome negative TO is proposed that guaranties robust performance while keeping system complexity low. Shifting the FFT window towards the data signal by the estimated TO, the negative TO turns positive and as a result near optimum performance is achieved.

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.

Advanced receiver design for interfering small cell deployments in LTE networks

Small cells are low power nodes, which have lower transmit power than the macro eNodeB. They are regarded as one of the important solutions in long term evolution advanced (LTE-A) to deal with the dramatically increased mobile data traffic. Small cells deployments in LTE-A release 12 with 4 small cells in a cluster suffer from stronger inter-cell interference than heterogeneous network (HetNet) in the previous release of LTE-A, where one small cell exists in a cluster. We investigate the joint least squares channel estimation (JLSCE) over demodulation reference signals (DMRS) spreading in time and frequency resources in LTE-A systems. Furthermore, different receiver structures using least square channel estimation (LSCE) and JLSCE are investigated in small cell deployments with the presence of strong inter-cell interference. Our results show the linear minimum mean square error (LMMSE)-interference rejection combining (IRC) with LSCE is the most efficient receiver structure compared to the enhanced (e)LMMSE-IRC with JLSCE, interference cancellation (IC) with eLMMSE-IRC and maximum ratio combining (MRC) receiver structures for signal detection in small cell deployments.

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.