Helka-Liina Määttänen

System-level performance of LTE-Advanced with joint transmission and dynamic point selection schemes

In this article, we present a practical coordinated multipoint (CoMP) system for LTE-Advanced. In this CoMP system, cooperation is enabled for cell-edge users via dynamic switching between the normal single-cell operation and CoMP. We first formulate a general CoMP system model of several CoMP schemes. We then investigate a practical finite-rate feedback design that simultaneously supports interference coordination, joint transmission (JT), and dynamic point selection (DPS) with a varying number of cooperating transmission points while operating a single-cell transmission as a fallback mode. We provide both link-level and system-level results for the evaluation of different feedback options for general CoMP operation. The results show that there are substantial performance gains in cell-edge throughputs for both JT and DPS CoMP over the baseline Release 10 LTE-Advanced with practical feedback options. We also show that CoMP can enable improved mobility management in real networks.

Beamforming Codebooks for Two Transmit Antenna Systems Based on Optimum Grassmannian Packings

Precoding codebook design for limited feedback MIMO systems is known to reduce to a discretization problem on a Grassmann manifold. The case of two-antenna beamforming is special in that it is equivalent to quantizing the real sphere. The isometry between the Grassmannian G2,1ℂ and the real sphere S2 shows that discretization problems in the Grassmannian G2,1ℂ are directly solved by corresponding spherical codes. Notably, the Grassmannian line packing problem in ℂ2, namely maximizing the minimum distance, is equivalent to the Tammes problem on the real sphere, so that optimum spherical packings give optimum Grassmannian packings. Moreover, a simple isomorphism between G2,1ℂ and S2 enables to analytically derive simple codebooks in closed-form having low implementation complexity. Using the simple geometry of some of these codebooks, we derive closed-form expressions of the probability density function of the relative SNR loss due to limited feedback. We also investigate codebooks based on other spherical arrangements, such as solutions maximizing the harmonic mean of the mutual distances among the codewords, which is known as the Thomson problem. We find that in some special cases, Grassmannian codebooks based on these other spherical arrangements outperform codebooks from Grassmannian packing.

Design and Evaluation of LTE-Advanced Double Codebook

In this paper we address codebook design for LTE-Advanced downlink. In order to meet the peak spectral efficiency requirements of up to 30 bit/s/Hz, LTE-Advanced supports up to eight transmit antennas in downlink, enabling spatial multiplexing transmissions with up to eight parallel streams. In addition to high-order single-user (SU-) MIMO transmissions, LTE-Advanced provides state-of-the-art multi-user (MU-) MIMO support with seamless operation between SU-MIMO and MU-MIMO. Codebook design is the basis of such precoded closed-loop multi-antenna schemes. We investigate multiple options for LTE-Advanced codebook design, providing a solution flexible enough to support both single-user and multi-user operation as well as different base station antenna setups while taking into account practical system design constraints. We provide thorough codebook performance evaluation in realistic LTE-Advanced system scenarios.

System-Level Performance of Interference Suppression Receivers in LTE System

In wireless networks with smaller cell sizes and dense network deployments the spectral efficiency is limited by inter-cell interference. To improve signal-to-noise-plus-interference-ratio, interference can be mitigated using advanced receivers like an interference rejection combining (IRC) receiver. The performance of a practical IRC-receiver is dependent on the quality of the channel and interference covariance estimation. Interference covariance can be estimated for example by using reference signals. As the interference structure is dependent on several components, such as scheduler decisions made and precoders used by other cells, receiver evaluations are performed on system level. Because of full network system level simulations are very complex and highly computationally intensive, simplifications in modeling are needed. Typically time-frequency resolution can be reduced and only the fast-fading coefficients of the interfering links are generated. The lack of symbol samples requires a model to estimate the losses of realistic IRC receivers. In this paper, we show system level results for practical IRC algorithms.

Precoder partitioning in closed-loop MIMO systems

We study unitary precoding for multistream MIMO systems with partial channel state information at the transmitter. We introduce a quantization scheme in which the full space of non-equivalent precoding matrices is partitioned into Grassmannian and orthogonalization parts. The Grassmannian part is used for maximizing the power after precoding and the orthogonalization part is used for removing cross talk between the data streams. We show that orthogonalization improves the attainable capacity when the receiver is linear. We give a parametrization for the non-equivalent orhogonalization matrices and a metric which measures the orthogonality of the transmission. Optimal orthogonalization codebooks for two-stream transmission are presented. When feedback is limited, the optimal partitioning of feedback bits between Grassmannian and orthogonalization parts becomes an issue. In correlated scenarios, the number of feedback bits may be significantly reduced by investing bits into the orthogonalization part.

Performance Evaluations for Multiuser CQI Enhancements for LTE-Advanced

In LTE-Advanced, multiuser MIMO has been identified as a key technique for increasing system spectral efficiency in closed-loop MIMO. Transmission parameters like precoding and transmission rate are adapted based on finite rate feedback from the users. Switching between single user and multiuser transmission modes is possible without higher layer signaling, which means that the feedback should be designed to sustain efficiently both single user and multiuser transmissions. In this paper, performance of LTE-Advanced is evaluated with system level simulations. The focus is on studying the viability of an additional multiuser specific channel quality feedback. The baseline in the comparison is single stream single user feedback consisting of a preferred precoding vector and a channel quality feedback. This baseline is compared to feedback schemes that aim at improving the multiuser performance by an additional multiuser-specific CQI. Also, the gain from a user specific scaling for the channel quality feedback performed by the base station is evaluated.

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.

CSI Feedback for Dynamic Switching Between Single User and Multiuser MIMO

We consider channel state information (CSI) feedback in 3GPP Long Term Evolution (LTE)-Advanced context. In LTE-Advanced, switching between single user and multiuser transmission schemes is possible without higher layer signaling, which means that the feedback should support both single user and multiuser transmissions. Typically, the CSI feedback consists of a precoding matrix index (PMI) and channel quality indication(s) (CQI). For PMI feedback, we consider different PMI selection schemes and study whether there is a tradeoff between single user and multiuser specific codeword selection metrics. For multiuser CQI, we consider different CQI estimation strategies for two paired users, which is the primary case in LTE-Advanced. The schemes include single user single stream and two stream CQIs and several multiuser specific CQI estimation options. We find that estimating the multiuser CQI as an average over unitary pairs or as the minimum of the signal-to-interference and noise ratios of the unitary pairs offers a stable, well-performing options for different signal-to-noise ratio (SNR) ranges and antenna correlation values.

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.

Orthogonalizing Transmission in MIMO with Linear Receiver and Finite MCS Set

Using channel dependent precoding, a MIMO channel can be orthogonalized, which provides throughput gains especially if a linear receiver is used. When there is a limited set of modulation and coding schemes (MCS), however, we show that orthogonalization is not always the throughput maximizing strategy. Close to the upper and lower ends of operation points of the MCS set, it may be optimal to anti-orthogonalize the channel instead.