A Novel Beamformed Control Channel Design for LTE with Full Dimension-MIMO

The Full Dimension-MIMO (FD-MIMO) technology is capable of achieving huge improvements in network throughput with simultaneous connectivity of a large number of mobile wireless devices, unmanned aerial vehicles, and the Internet of Things (IoT). In FD-MIMO, with a large number of antennae at the base station and the ability to perform beamforming, the capacity of the physical downlink shared channel (PDSCH) has increased a lot. However, the current specifications of the 3rd Generation Partnership Project (3GPP) does not allow the base station to perform beamforming techniques for the physical downlink control channel (PDCCH), and hence, PDCCH has neither the capacity nor the coverage of PDSCH. Therefore, PDCCH capacity will still limit the performance of a network as it dictates the number of users that can be scheduled at a given time instant. In Release 11, 3GPP introduced enhanced PDCCH (EPDCCH) to increase the PDCCH capacity at the cost of sacrificing the PDSCH resources. The problem of enhancing the PDCCH capacity within the available control channel resources has not been addressed yet in the literature. Hence, in this paper, we propose a novel beamformed PDCCH (BF-PDCCH) design which is aligned to the 3GPP specifications and requires simple software changes at the base station. We rely on the sounding reference signals transmitted in the uplink to decide the best beam for a user and ingeniously schedule the users in PDCCH. We perform system level simulations to evaluate the performance of the proposed design and show that the proposed BF-PDCCH achieves larger network throughput when compared with the current state of art algorithms, PDCCH and EPDCCH schemes

Scheduler Algorithms for MU-MIMO

In multi-user multiple input multiple output (MU-MIMO), the complexity of the base-station scheduler has increased further compared to single-user multiple input multiple output (SU-MIMO). The scheduler must understand if several users can be spatially multiplexed in the same time-frequency resource. One way to spatially separate users is through beamforming with sufficiently many antennas. In this thesis work, two downlink beamforming algorithms for MU-MIMO are studied: The first algorithm implements precoding without considering inter-cell interference (ICI). The second one considers it and attempts to mitigate or null transmissions in the direction of user equipments (UEs) in other cells. The two algorithms are evaluated in SU-MIMO and MU-MIMO setups operating in time division duplex (TDD) mode and serving with single and dual-antenna terminals. Full-Buffer (FB) and file transfer protocol (FTP) data traffic profiles are studied. Additionally, various UE mobility patterns, UE transmit antenna topologies, sounding reference signal (SRS) periodicity configurations, and uniform linear array (ULA) topologies are considered. Simulations have been performed using a system level simulation framework developed by Ericsson AB. Another important part of this thesis work is the functional verification of this simulation framework, which at the time of writing is still undergoing development. Our simulation results show that in SU-MIMO, the second algorithm, which considers ICI, outperforms the first one for FB traffic profile and all UE speeds, but not for FTP traffic profile and medium (30 km/h) or high (60 km/h) UE speeds. In this case, the first algorithm, which does not consider ICI, can be used with advantage. In MU-MIMO, cell downlink throughput gains are observed for the second algorithm over the first one for low and medium system loads (number of users). For both algorithms, the cell throughput is observed to decrease with increasing UE speed and sounding periodicity.Scheduling in modern wireless standards, e.g., 3G, 4G and future 5G, can be defined as the task of allocating time and frequency resources by the base station (BS) to each user equipment (UE) that wants to engage in communication. Resources are allocated every transmission time interval (TTI), which is typically one millisecond. There exist both uplink (from the UEs to the BS) and downlink (from the BS to the UEs) resource schedulers implemented in the e-Node B, i.e., the base station (BS) in Long Term Evolution (LTE). The aim of this thesis work is to study how various communication techniques proposed for 5G can increase the overall system throughput of the downlink (DL) when a realistic resource scheduler is used. In particular, we consider: (i) Beamforming, (ii) Multi-user multiple input multiple output (MU-MIMO), and (iii) Inter-cell interference (ICI) mitigation. Beamforming can be achieved by deploying a large number of antenna elements at the BS with the aim of increasing the signal to interference noise ratio (SINR) towards the UE. Contrary to single-user multiple input multiple output (SU-MIMO), in MU-MIMO more than one UE are scheduled for transmissions in the same time-frequency resource; this is possible by judiciously pairing various UEs which are spatially sufficiently separated (according to some metric that we will define later). ICI mitigation can be achieved by means of proper precoding at BS where the precoder attempts to mitigate the interfering signal from BS towards UEs belonging to neighboring cells. In this thesis work, we investigate the performance of two scheduler algorithms for MU-MIMO, using SU-MIMO as baseline. The first algorithm does not consider ICI while the second one does. Dual layer beamforming (that is, two independent data streams are transmitted to each UE) and time division duplex (TDD) are assumed. In TDD mode the BS acquires the channel information from sounding reference signals (SRS) transmitted in the uplink (UL) and, by virtue of channel reciprocity, reuses the so-obtained channel information in the downlink. The performance evaluation of the two algorithms is based on the following parameters: UE Traffic profile, UE speed, SRS UL antenna configuration, SRS parameters, and BS antenna topology. - UE speed includes 3,30, and 60 km/h. - UE traffic profile includes full-buffer (FB) and file transfer protocol (FTP). With FB traffic profile, UEs send/receive data to/from the BS all the time, while this is not the case in the FTP traffic profile case. Some examples of FTP traffic profiles may include chatty, video, VoIP, web, etc. - SRS UL antenna configuration includes: (i) Two SRS, in which each UE sends two SRS to the BS from two antennas, (ii) one SRS with antenna selection, in which each UE alternately sends one SRS to the BS from each of two antennas, and (iii) one SRS without antenna selection, in which each UE sends one SRS to the BS from only one antenna. For two SRS UE case (note that in the downlink two layers, and hence two UE antennas, are always used). - SRS parameters include SRS bandwidth and SRS periodicity. In this thesis work, full-bandwidth SRS (20 MHz) with various SRS periodicities such as 5 ms, 10 ms, 20 ms are considered. - BS antenna topology includes 8 and 64 antenna elements at the BS. The main result of this thesis work is that in both SU-MIMO and MU-MIMO with FB traffic profile, it is better to use the second algorithm which considers ICI rather than the first one which does not. However, with FTP traffic profile, this is not always the case

Optimization of Massive Full-Dimensional MIMO for Positioning and Communication

Massive Full-Dimensional multiple-input multiple-output (FD-MIMO) base stations (BSs) have the potential to bring multiplexing and coverage gains by means of three-dimensional (3D) beamforming. Key technical challenges for their deployment include the presence of limited-resolution front ends and the acquisition of channel state information (CSI) at the BSs. This paper investigates the use of FD-MIMO BSs to provide simultaneously high-rate data communication and mobile 3D positioning in the downlink. The analysis concentrates on the problem of beamforming design by accounting for imperfect CSI acquisition via Time Division Duplex (TDD)-based training and for the finite resolution of analog-to-digital converter (ADC) and digital-to-analog converter (DAC) at the BSs. Both \textit{unstructured beamforming} and a low-complexity \textit{Kronecker beamforming} solution are considered, where for the latter the beamforming vectors are decomposed into separate azimuth and elevation components. The proposed algorithmic solutions are based on Bussgang theorem, rank-relaxation and successive convex approximation (SCA) methods. Comprehensive numerical results demonstrate that the proposed schemes can effectively cater to both data communication and positioning services, providing only minor performance degradations as compared to the more conventional cases in which either function is implemented. Moreover, the proposed low-complexity Kronecker beamforming solutions are seen to guarantee a limited performance loss in the presence of a large number of BS antennas.Comment: 30 pages, 6 figure

Reduced Switching Connectivity for Large Scale Antenna Selection

In this paper, we explore reduced-connectivity radio frequency (RF) switching networks for reducing the analog hardware complexity and switching power losses in antenna selection (AS) systems. In particular, we analyze different hardware architectures for implementing the RF switching matrices required in AS designs with a reduced number of RF chains. We explicitly show that fully-flexible switching matrices, which facilitate the selection of any possible subset of antennas and attain the maximum theoretical sum rates of AS, present numerous drawbacks such as the introduction of significant insertion losses, particularly pronounced in massive multiple-input multiple-output (MIMO) systems. Since these disadvantages make fully-flexible switching suboptimal in the energy efficiency sense, we further consider partially-connected switching networks as an alternative switching architecture with reduced hardware complexity, which we characterize in this work. In this context, we also analyze the impact of reduced switching connectivity on the analog hardware and digital signal processing of AS schemes that rely on channel power information. Overall, the analytical and simulation results shown in this paper demonstrate that partially-connected switching maximizes the energy efficiency of massive MIMO systems for a reduced number of RF chains, while fully-flexible switching offers sub-optimal energy efficiency benefits due to its significant switching power losses.Comment: 14 pages, 11 figure

Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems

We study optimal delivery strategies of one common and $K$ independent messages from a source to multiple users in wireless environments. In particular, two-layered superposition of broadcast/multicast and unicast signals is considered in a downlink multiuser OFDMA system. In the literature and industry, the two-layer superposition is often considered as a pragmatic approach to make a compromise between the simple but suboptimal orthogonal multiplexing (OM) and the optimal but complex fully-layered non-orthogonal multiplexing. In this work, we show that only two-layers are necessary to achieve the maximum sum-rate when the common message has higher priority than the $K$ individual unicast messages, and OM cannot be sum-rate optimal in general. We develop an algorithm that finds the optimal power allocation over the two-layers and across the OFDMA radio resources in static channels and a class of fading channels. Two main use-cases are considered: i) Multicast and unicast multiplexing when $K$ users with uplink capabilities request both common and independent messages, and ii) broadcast and unicast multiplexing when the common message targets receive-only devices and $K$ users with uplink capabilities additionally request independent messages. Finally, we develop a transceiver design for broadcast/multicast and unicast superposition transmission based on LTE-A-Pro physical layer and show with numerical evaluations in mobile environments with multipath propagation that the capacity improvements can be translated into significant practical performance gains compared to the orthogonal schemes in the 3GPP specifications. We also analyze the impact of real channel estimation and show that significant gains in terms of spectral efficiency or coverage area are still available even with estimation errors and imperfect interference cancellation for the two-layered superposition system