An overview of the 5G New Radio (NR) physical downlink control channel (PDCCH) design of 3GPP Release 15 is presented and performance enhancements are proposed. A novel PDCCH hash function is shown to outperform the legacy hash function by about one order of magnitude in terms of inter-user blocking probability while maintaining the frequency-selective scheduling capability for downlink control information. Enhanced PDCCH candidate overbooking and mapping/dropping algorithms are investigated. Link level performance results of NR PDCCH with precoder cycling transmit diversity are presented for various aggregation levels and physical resource configuration options.

In this paper we investigate and enhance the user-pairing probability in a system employing multi-user superposition transmission (MUST). In order to improve the multiuser pairing probability, we propose an additional feedback of the second best channel state information (CSI) consisting of channel quality indicator (CQI) and precoding matrix indicator (PMI). We analyze the pairing-probability and show that the additional feedback increases significantly the pairing possibilities at the scheduler. By system level simulations we confirm that the proposed enhanced feedback is improving significantly MUST performance in the context of MUST operation on the same-beam with Gray-mapped super-constellation. In addition, we suggest a simple link-to-system mapping for maximum likelihood (ML) MUST receiver, which can re-use legacy mutual-information-to-block-error-rate tables.

In this paper, we investigate a codeword refinement technique called double codeword coding in the context of MIMO feedback. In the proposed method, the second codeword is fed back in addition to the best codeword as their interpolation at the transmitter improves the channel knowledge. The derivation of the optimal interpolation parameter is presented together with the derivation of an upper-bound to the interpolation parameter based on channel quality feedback available at the transmitter. We show that double codeword feedback refinement has slightly lower complexity than alternative techniques and still achieves similar performance. Furthermore, double codeword coding can be generalized to higher transmission ranks. Using extended multiuser MIMO link simulations, we verify that the double codeword feedback is able to provide significant gains in 3GPP LTE context.

In this paper we address the design of MIMO double codebook structure for cross-polarized antenna arrays. Double codebooks consist of two codebook components, a wide-band long-term component and a short-term sub-band component. Herein we study particularly the two layer short-term sub-band component and provide the complete set of structure types leading to orthogonal constant modulus codewords and cross-polarization phase of M-PSK alphabet. By 3GPP LTE system simulations we show that a novel, two layer structure type, provides significant improvements in channels with large angular spreads.

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 G2,1ℂand the real sphereS2shows that discretization problems in the Grassmannian G2,1ℂare 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 G2,1ℂandS2enables 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.

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.

This paper presents a new low complexity method of providing improved channel state information at the transmitter within closed-loop MIMO systems. We propose to feedback the second best codeword in addition to the best codeword. At the transmitter, these two precoding codewords are interpolated using geodesic. The optimal interpolation parameter is derived in closed-form for single-stream transmission. The performance of the proposed method with b-bit codewords is shown to be close to a codebook with 2b-bit codewords, while our method has significantly lower computational complexity.

MIMO-OFDM systems with channel reciprocity allow spatial multiplexing using eigenbeamforming. However, eigenbeamforming requires singular value decomposition, which is phase ambiguous and causes the precoded channel to become phase discontinuous in frequency domain. This changes statistical properties of channel components, which degrades the performance of channel estimation. In this paper we propose phase unifying method to improve channel statistics and consequently decrease the error in channel estimation. In addition, we investigate the 2D-Wiener channel estimation with respect to different channel frequency correlation functions. The performance is evaluated using LTE link simulator.

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.