Multiuser MIMO Wireless Communications: Optimal and Efficient Schemes For Rate Maximization and Power Minimization

Ph.DDOCTOR OF PHILOSOPH

D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies

This document provides the most recent updates on the technical contributions and research challenges focused in WP3. Each Technology Component (TeC) has been evaluated under possible uniform assessment framework of WP3 which is based on the simulation guidelines of WP6. The performance assessment is supported by the simulation results which are in their mature and stable state. An update on the Most Promising Technology Approaches (MPTAs) and their associated TeCs is the main focus of this document. Based on the input of all the TeCs in WP3, a consolidated view of WP3 on the role of multinode/multi-antenna transmission technologies in 5G systems has also been provided. This consolidated view is further supported in this document by the presentation of the impact of MPTAs on METIS scenarios and the addressed METIS goals.Aziz, D.; Baracca, P.; De Carvalho, E.; Fantini, R.; Rajatheva, N.; Popovski, P.; Sørensen, JH.... (2015). D 3. 3 Final performance results and consolidated view on the most promising multi -node/multi -antenna transmission technologies. http://hdl.handle.net/10251/7675

A survey on hybrid beamforming techniques in 5G : architecture and system model perspectives

The increasing wireless data traffic demands have driven the need to explore suitable spectrum regions for meeting the projected requirements. In the light of this, millimeter wave (mmWave) communication has received considerable attention from the research community. Typically, in fifth generation (5G) wireless networks, mmWave massive multiple-input multiple-output (MIMO) communications is realized by the hybrid transceivers which combine high dimensional analog phase shifters and power amplifiers with lower-dimensional digital signal processing units. This hybrid beamforming design reduces the cost and power consumption which is aligned with an energy-efficient design vision of 5G. In this paper, we track the progress in hybrid beamforming for massive MIMO communications in the context of system models of the hybrid transceivers' structures, the digital and analog beamforming matrices with the possible antenna configuration scenarios and the hybrid beamforming in heterogeneous wireless networks. We extend the scope of the discussion by including resource management issues in hybrid beamforming. We explore the suitability of hybrid beamforming methods, both, existing and proposed till first quarter of 2017, and identify the exciting future challenges in this domain

Enabling Efficient Communications Over Millimeter Wave Massive MIMO Channels Using Hybrid Beamforming

The use of massive multiple-input multiple-output (MIMO) over millimeter wave (mmWave) channels is the new frontier for fulfilling the exigent requirements of next-generation wireless systems and solving the wireless network impending crunch. Massive MIMO systems and mmWave channels offer larger numbers of antennas, higher carrier frequencies, and wider signaling bandwidths. Unleashing the full potentials of these tremendous degrees of freedom (dimensions) hinges on the practical deployment of those technologies. Hybrid analog and digital beamforming is considered as a stepping-stone to the practical deployment of mmWave massive MIMO systems since it significantly reduces their operating and implementation costs, energy consumption, and system design complexity. The prevalence of adopting mmWave and massive MIMO technologies in next-generation wireless systems necessitates developing agile and cost-efficient hybrid beamforming solutions that match the various use-cases of these systems. In this thesis, we propose hybrid precoding and combining solutions that are tailored to the needs of these specific cases and account for the main limitations of hybrid processing. The proposed solutions leverage the sparsity and spatial correlation of mmWave massive MIMO channels to reduce the feedback overhead and computational complexity of hybrid processing. Real-time use-cases of next-generation wireless communication, including connected cars, virtual-reality/augmented-reality, and high definition video transmission, require high-capacity and low-latency wireless transmission. On the physical layer level, this entails adopting near capacity-achieving transmission schemes with very low computational delay. Motivated by this, we propose low-complexity hybrid precoding and combining schemes for massive MIMO systems with partially and fully-connected antenna array structures. Leveraging the disparity in the dimensionality of the analog and the digital processing matrices, we develop a two-stage channel diagonalization design approach in order to reduce the computational complexity of the hybrid precoding and combining while maintaining high spectral efficiency. Particularly, the analog processing stage is designed to maximize the antenna array gain in order to avoid performing computationally intensive operations such as matrix inversion and singular value decomposition in high dimensions. On the other hand, the low-dimensional digital processing stage is designed to maximize the spectral efficiency of the systems. Computational complexity analysis shows that the proposed schemes offer significant savings compared to prior works where asymptotic computational complexity reductions ranging between $80\%$ and $98\%$. Simulation results validate that the spectral efficiency of the proposed schemes is near-optimal where in certain scenarios the signal-to-noise-ratio (SNR) gap to the optimal fully-digital spectral efficiency is less than $1$ dB. On the other hand, integrating mmWave and massive MIMO into the cellular use-cases requires adopting hybrid beamforming schemes that utilize limited channel state information at the transmitter (CSIT) in order to adapt the transmitted signals to the current channel. This is so mainly because obtaining perfect CSIT in frequency division duplexing (FDD) architecture, which dominates the cellular systems, poses serious concerns due to its large training and excessive feedback overhead. Motivated by this, we develop low-overhead hybrid precoding algorithms for selecting the baseband digital and radio frequency (RF) analog precoders from statistically skewed DFT-based codebooks. The proposed algorithms aim at maximizing the spectral efficiency based on minimizing the chordal distance between the optimal unconstrained precoder and the hybrid beamformer and maximizing the signal to interference noise ratio for the single-user and multi-user cases, respectively. Mathematical analysis shows that the proposed algorithms are asymptotically optimal as the number of transmit antennas goes to infinity and the mmWave channel has a limited number of paths. Moreover, it shows that the performance gap between the lower and upper bounds depends heavily on how many DFT columns are aligned to the largest eigenvectors of the transmit antenna array response of the mmWave channel or equivalently the transmit channel covariance matrix when only the statistical channel knowledge is available at the transmitter. Further, we verify the performance of the proposed algorithms numerically where the obtained results illustrate that the spectral efficiency of the proposed algorithms can approach that of the optimal precoder in certain scenarios. Furthermore, these results illustrate that the proposed hybrid precoding schemes have superior spectral efficiency performance while requiring lower (or at most comparable) channel feedback overhead in comparison with the prior art

Low-Complexity Multicarrier Waveform Processing Schemes fo Future Wireless Communications

Wireless communication systems deliver enormous variety of services and applications. Nowa- days, wireless communications play a key-role in many fields, such as industry, social life, education, and home automation. The growing demand for wireless services and applications has motivated the development of the next generation cellular radio access technology called fifth-generation new radio (5G-NR). The future networks are required to magnify the delivered user data rates to gigabits per second, reduce the communication latency below 1 ms, and en- able communications for massive number of simple devices. Those main features of the future networks come with new demands for the wireless communication systems, such as enhancing the efficiency of the radio spectrum use at below 6 GHz frequency bands, while supporting various services with quite different requirements for the waveform related key parameters. The current wireless systems lack the capabilities to handle those requirements. For exam- ple, the long-term evolution (LTE) employs the cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) waveform, which has critical drawbacks in the 5G-NR context. The basic drawback of CP-OFDM waveform is the lack of spectral localization. Therefore, spectrally enhanced variants of CP-OFDM or other multicarrier waveforms with well localized spectrum should be considered. This thesis investigates spectrally enhanced CP-OFDM (E-OFDM) schemes to suppress the out-of-band (OOB) emissions, which are normally produced by CP-OFDM. Commonly, the weighted overlap-and-add (WOLA) scheme applies smooth time-domain window on the CP- OFDM waveform, providing spectrally enhanced subcarriers and reducing the OOB emissions with very low additional computational complexity. Nevertheless, the suppression perfor- mance of WOLA-OFDM is not sufficient near the active subband. Another technique is based on filtering the CP-OFDM waveform, which is referred to as F-OFDM. F-OFDM is able to provide well-localized spectrum, however, with significant increase in the computational com- plexity in the basic scheme with time-domain filters. Also filter-bank multicarrier (FBMC) waveforms are included in this study. FBMC has been widely studied as a potential post- OFDM scheme with nearly ideal subcarrier spectrum localization. However, this scheme has quite high computational complexity while being limited to uniformly distributed sub- bands. Anyway, filter-bank based waveform processing is one of the main topics of this work. Instead of traditional polyphase network (PPN) based uniform filter banks, the focus is on fast-convolution filter banks (FC-FBs), which utilize fast Fourier transform (FFT) domain processing to realize effectively filter-banks with high flexibility in terms of subcarrier bandwidths and center frequencies. FC-FBs are applied for both FBMC and F-OFDM waveform genera- tion and processing with greatly increased flexibility and significantly reduced computational complexity. This study proposes novel structures for FC-FB processing based on decomposition of the FC-FB structure consisting of forward and inverse discrete Fourier transforms (DFT and IDFT). The decomposition of multirate FC provides means of reducing the computational complexity in some important specific scenarios. A generic FC decomposition model is proposed and analyzed. This scheme is mathematically equivalent to the corresponding direct FC imple- mentation, with exactly the same performance. The benefits of the optimized decomposition structure appear mainly in communication scenarios with relatively narrow active transmis- sion band, resulting in significantly reduced computational complexity compared to the direct FC structure. The narrowband scenarios find their places in the recent 3GPP specification of cellular low- power wide-area (LPWA) access technology called narrowband internet-of-things (NB-IoT). NB-IoT aims at introducing the IoT to LTE and GSM frequency bands in coexistence with those technologies. NB-IoT uses CP-OFDM based waveforms with parameters compatible with the LTE. However, additional means are needed also for NB-IoT transmitters to improve the spec- trum localization. For NB-IoT user devices, it is important to consider ultra-low complexity solutions, and a look-up table (LUT) based approach is proposed to implement NB-IoT uplink transmitters with filtered waveforms. This approach provides completely multiplication-free digital baseband implementations and the addition rates are similar or smaller than in the basic NB-IoT waveform generation without the needed elements for spectrum enhancement. The basic idea includes storing full or partial waveforms for all possible data symbol combinations. Then the transmitted waveform is composed through summation of needed stored partial waveforms and trivial phase rotations. The LUT based scheme is developed with different vari- ants tackling practical implementations issues of NB-IoT device transmitters, considering also the effects of nonlinear power amplifier. Moreover, a completely multiplication and addition- free LUT variant is proposed and found to be feasible for very narrowband transmission, with up to 3 subcarriers. The finite-wordlength performance of LUT variants is evaluated through simulations

Performance Limits of Microwave and Dual Microwave/Millimeter Wave Band Networks

Traditionally, wireless networks communicate over the conventional microwave band (sub-6 GHz) as it supports reliable communication over a large geographic area. The ever increasing demand for bandwidth to support the rising number of consumers and services, however, is fast depleting the available microwave spectrum. As such, complementing the microwave spectrum with additional bandwidth from the millimeter-wave (mm-wave) band has been envisioned as a promising solution to this problem. Since transmissions in the mm-wave band are typically achieved with highly directional steerable antenna arrays to counter the severe path-loss in mm-wave frequencies, the resulting mm-wave links are typically rendered highly directional, which can often be modeled as directional point-to-point links. However, mm-wave transmissions are inherently unreliable compared to those in the microwave band. Hence, communicating simultaneously over both bands in an integrated mm-wave/microwave dual-band setup is emerging as a promising new technology. In this dual-band setting, high-rate data traffic can be carried by relatively unreliable high-bandwidth mm-wave links, while control signals and moderate-bandwidth traffic can be communicated over the relatively reliable microwave band. In this thesis, we first study two dual-band multi-user networks that model two important aspects of wireless communication: inter-user interference and relay-cooperation. The broad goal of this study is to characterize information-theoretical performance limits of such networks, which can then be used to obtain insights on the optimal encoding/decoding strategy, effective resource allocation schemes, etc. In the first part of this thesis, we study a two-transmitter two-receiver dual-band Gaussian interference channel (IC) operating over an integrated mm-wave/microwave dual-band. This channel models a setting where a pair of single-transmitter single-receiver links communicate simultaneously, and thus mutually interfere. Here, transmissions in the underlying microwave band are modeled as a two-user conventional Gaussian IC (GIC). In contrast, a transmitter in the mm-wave band is assumed to be capable of communicating to either the desired destination or the interfered destination via a point-to-point direct-link or a cross-link, respectively. The dual-band IC is first classified into 3 classes according to the interference level in the underlying microwave GIC, and then sufficient channel conditions are obtained under which the capacity region of the 3 classes are characterized. For cases in which the sufficient conditions do not necessarily hold, approximate capacity results are obtained that characterizes the capacity region to within 1/2 bit per channel use per user. The performance of the dual-band IC is likely to be impacted significantly by the point-to-point nature and large bandwidth of the mm-wave links, and specifically by whether the mm-wave spectrum is used as direct-links or cross-links. Transmitting in either the direct-links only or the cross-links only is not optimal for all channel conditions, and there exists a non-trivial trade-off between the two modes. To understand the impact of this trade-off on the performance of the dual-band IC, we study the power allocation scheme over the mm-wave direct and cross-links that maximizes the sum-rate of the channel. The resulting power allocation strategy is characterized in closed form, which possesses rich properties and reveals useful insights into the trade-offs in such networks. In the second part of this thesis, we study a fading Gaussian multiple-access relay channel (MARC) over an integrated mm-wave/microwave dual-band, where two sources communicate to a destination with the help of a relay. In the dual-band MARC, transmission in the underlying microwave band is modeled as a conventional Gaussian MARC. However, similar to that in the dual-band IC, a mm-wave transmitter in this channel is modeled as being able to communicate to either the destination or the relay by creating a direct-link or a relay-link, respectively. For dual-band MARC, we characterize an achievable region and a set of rate upper bounds, and then obtain sufficient channel conditions under which its capacity region is characterized. Similar to the dual-band IC, the performance of the dual-band MARC will likely be significantly affected by whether the mm-wave band is used as direct-links or relay-links, and a non-trivial trade-off between the two modes exists in this case as well. To understand this trade-off, we study the transmission power allocation scheme over the mm-wave direct and relay-links that maximizes the sum-rate of the dual-band MARC. The resulting power allocation scheme, characterized in closed form, is observed to have rich structural properties, which reveal insights into the trade-offs in relay cooperation in dual-band networks. While dual-band communication is a promising technology, currently the bulk of the connectivity is still supported by the microwave band. However, the problem of interference mitigation for conventional microwave bands is still open even for the basic case of a two-user IC. Motivated by this, in the third part of the thesis, we study the performance limits of the multiple-access interference channel (MAIC) which models the interference during cellular uplink over the conventional single band. Focusing on the weak interference case, which provides a more realistic model of the inter-cell interference, we characterize an achievable strategy and 3 novel upper bounds on the sum-rate in the partially symmetric case, thereby providing improved sum-rate upper and lower bounds in these cases