Satellite communication has the potential to provide coverage over large geographical areas. This ubiquity enables potential for providing connectivity for currently unserved remote areas as well as for moving platforms such as airplanes and ships. However, many satellite systems were developed on private networks, each having their individual solution for the satellite systems and for the end user equipment (UE). In order to extent the satellite-industry-wide standards to global telecommunication standards, there are currently activities in the 3rd generation partnership project (3GPP) to study the feasibility and needed standard adaptations for the 5th generation (5G) new radio (NR) 3GPP standard. In this paper, we provide an overview of the ongoing activity in 3GPP RAN working groups with focus on RAN2 and RAN3 aspects for enabling NR communication over geosynchronous orbit (GEO) and non-GEO satellite systems.

This paper presents long term evolution (LTE) uplink measurements taken with two drones operating in a public cellular network in rural environment. Three similar measurement scenarios with drone flight altitudes of 50 m and 100 m above ground level are studied with different measurement software and equipment. Four different key performance indicators (KPIs) are presented in the paper: Physical Resource Block (PRB) usage, Modulation and Coding Scheme (MCS) class, throughput and transmission power. These together help to analyse the overall interference behaviour, which is an essential part of the paper. The results show that aerial UEs add minor interference to the network, which decrease the MCS class of the ground level UEs and slightly increase their transmission power. The resulting data rates are roughly the same as them operating in the same cell because of increased PRB amount per user equipment (UE). This effect is similar to that of a ground level UE switching cell in the cell edge area to another cell. In addition, ground level cell edge area UE performs slightly poorer in comparison with a UE near the serving cell antenna when drones are utilized. Nevertheless, two drones operating in the air with smart phones connected to them do not have a critical effect on the performance of the normal ground level UEs from the throughput point of view, but slightly increase the resource utilization.

Beyond visual line-of-sight connectivity is key for use cases of unmanned aerial vehicles (UAVs) such as package delivery, infrastructure inspection, and rescue missions. Cellular networks stand ready to support flying UAVs by providing wide-area, quality, and secure connectivity for UAV operations. Ensuring reliable connections in the presence of UAV movements is important for safety control and operations of UAVs. With increasing height above the ground, the radio environment changes. Using terrestrial cellular networks to provide connectivity to the UAVs moving in the sky may face new challenges. In this article, we share some of our findings in mobility support for cellular connected UAVs. We first identify how the radio environment changes with altitude and analyze the corresponding implications on mobility performance. We then present evaluation results to shed light on the mobility performance of cellular connected UAVs. We also discuss potential enhancements for improving mobility performance in the sky.

In this paper we study multi-user interference alignment at system level. Interference alignment (IA) is a quite recently proposed transmission strategy with varying precoding solutions. In IA, precoders and receivers are designed cooperatively between transmission points such that interference is aligned in a smaller vector space at each receiver than it would be without IA. One known problem for all IA schemes is that they are prone to external interference, interference from transmission points that are outside of the cooperation set. In this paper, we evaluate the system level performance of a multi-user IA scheme with a 3GPP LTE-Advanced compliant system level simulator. We found that IA is very prone to external interference and that a receiver that is cancelling also external interference is outperforming IA.

In this paper, we consider sum rate maximizing linear zero interference precoder/receiver pair for single stream transmission per user. We parametrize the transmitter and receivers with a unit norm vector per user, leading to a transparent interpretation of the tradeoff between aligning with the eigenspace of the user and avoiding loss of power due to multiuser zero forcing. With this formulation the sum rate is maximized numerically for two users that have two receive antennas in spatially correlated and uncorrelated channels. The presented scheme outperforms zero interference transmissions based on maximum ratio combining receiver, especially in correlated channels.

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.

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.

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.

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 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.

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.

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.