Timo Roman

Reference Signal Design for Flexible MIMO Operation in LTE-Advanced Downlink

In this paper we address the issue of demodulation reference signal design for LTE-Advanced downlink. Practical systems such as LTE-Advanced should operate in highly time-frequency selective environment. Hence, they impose a very tight trade-off between the radio resources available for reference signals used for channel estimation and spectral efficiency targets to be met. As an additional constraint, LTE-Advanced brings to real deployments state-of-the-art multi-antenna technologies such as multi-user MIMO and cooperative MIMO. The difficult radio environment and the multitude of different multi-antenna techniques set very high challenges to the reference signal design. In this paper we discuss selected issues of realistic demodulation reference signal design for LTE-Advanced. We highlight the trade-offs that are needed for enabling advanced MIMO technologies, while at the same time allowing low complexity channel estimation at the receiver side without increasing related signaling overhead. Based on the analysis we propose a certain reference signal design for LTE-Advanced that fulfills all the required design criteria.

CQI-report optimization for multi-mode MIMO with unitary codebook based precoding

In this paper we consider the CQI reporting problem in MIMO downlink with adaptive switching between single user and multiuser mode and finite rate feedback. The feedback consist of channel directional information, which is the precoding weight, and channel quality information, which is a quantized value of estimated post processing SINR. The quantization of the channel directional information causes multiuser interference so that the post processing SINR after a linear receiver in multiuser mode is considerably different from the SINR in single user mode. The accuracy of the channel quality information plays a relevant role for adaptive modulation and coding. In order to achieve good performance without increasing feedback load by reporting accurate channel quality for both modes, we consider differential reporting for the multiuser mode. We optimize the channel quality reporting by considering an optimum differential quantization between single user and multiuser SINR.

Carrier frequency synchronization for mobile television receivers

In this paper, we introduce a novel subspace-based approach for carrier frequency offset (CFO) estimation in OFDM. The proposed estimator exploits both scattered and continual pilots commonly used in mobile television systems such as DVB-H. Rapid channel estimation and CFO compensation is necessary in order to deal with high mobility, time-slicing and potential handovers during the receiver switch-off time. The proposed method achieves frequency synchronization within a single OFDM block. No extensive time averaging is needed, which makes the approach very attractive for time and frequency selective channels where the CFO may be time varying. It is particularly suitable to burst type of transmission used, for example in DVB-H. Simulation examples are provided within the framework of mobile DVB-H systems.

Differential CQI optimization for MIMO with codebook based precoding

In this paper we reduce the feedback overhead in two-stream MIMO with per stream link adaptation and codebook based precoding. The amount of feedback may be reduced by indicating the CQI of one stream in a differential manner. The performance of the system depends on how the differential feedback is designed. We optimize the differential feedback for the weaker data stream by considering an optimum differential quantization between the streams. The off-set CQI values for the weaker stream are found by maximizing the throughput on the weaker data stream using the distribution of the difference between the post processing SINR values of the streams. The SINR difference distribution is found in semi-analytic form for 2 × 2 MIMO with codebook based precoding and a linear receiver. The offset values are calculated for both i.i.d. and correlated flat Rayleigh fading channels for a given number of relative feedback bits.

Layer arrangement for single-user coordinated multi-point transmission

Coordinated multi point transmission is an attractive technology to minimize interference in single frequency networks and such to improve data rates. In this paper, we study closed-loop single user joint transmission (JT-MIMO) system. We propose to feedback a layer arrangement matrix together with per transmission point channel state information (CSI). In case of two layers, we provide a simple method to find an optimal layer arrangement. Furthermore, we propose a low complexity algorithm when an exhaustive search over the different layer arrangements becomes unbearable. System level simulation results show that the flexible layer arrangement outperforms traditional rank adaptation with only small amount of additional feedback.