Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays

Massive MIMO (multiple-input multiple-output) is no longer a "wild" or "promising" concept for future cellular networks - in 2018 it became a reality. Base stations (BSs) with 64 fully digital transceiver chains were commercially deployed in several countries, the key ingredients of Massive MIMO have made it into the 5G standard, the signal processing methods required to achieve unprecedented spectral efficiency have been developed, and the limitation due to pilot contamination has been resolved. Even the development of fully digital Massive MIMO arrays for mmWave frequencies - once viewed prohibitively complicated and costly - is well underway. In a few years, Massive MIMO with fully digital transceivers will be a mainstream feature at both sub-6 GHz and mmWave frequencies. In this paper, we explain how the first chapter of the Massive MIMO research saga has come to an end, while the story has just begun. The coming wide-scale deployment of BSs with massive antenna arrays opens the door to a brand new world where spatial processing capabilities are omnipresent. In addition to mobile broadband services, the antennas can be used for other communication applications, such as low-power machine-type or ultra-reliable communications, as well as non-communication applications such as radar, sensing and positioning. We outline five new Massive MIMO related research directions: Extremely large aperture arrays, Holographic Massive MIMO, Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive MIMO.Comment: 20 pages, 9 figures, submitted to Digital Signal Processin

Evolution Toward 5G Mobile Networks - A Survey on Enabling Technologies

In this paper, an extensive review has been carried out on the trends of existing as well as proposed potential enabling technologies that are expected to shape the fifth generation (5G) mobile wireless networks. Based on the classification of the trends, we develop a 5G network architectural evolution framework that comprises three evolutionary directions, namely, (1) radio access network node and performance enabler, (2) network control programming platform, and (3) backhaul network platform and synchronization. In (1), we discuss node classification including low power nodes in emerging machine-type communications, and network capacity enablers, e.g., millimeter wave communications and massive multiple-input multiple-output. In (2), both logically distributed cell/device-centric platforms, and logically centralized conventional/wireless software defined networking control programming approaches are discussed. In (3), backhaul networks and network synchronization are discussed. A comparative analysis for each direction as well as future evolutionary directions and challenges toward 5G networks are discussed. This survey will be helpful for further research exploitations and network operators for a smooth evolution of their existing networks toward 5G networks

Enhanced Mobile Networking using Multi-connectivity and Packet Duplication in Next-Generation Cellular Networks

Modern cellular communication systems need to handle an enormous number of users and large amounts of data, including both users as well as system-oriented data. 5G is the fifth-generation mobile network and a new global wireless standard that follows 4G/LTE networks. The uptake of 5G is expected to be faster than any previous cellular generation, with high expectations of its future impact on the global economy. The next-generation 5G networks are designed to be flexible enough to adapt to modern use cases and be highly modular such that operators would have the flexibility to provide selective features based on user demand that could be implemented without investment in additional infrastructure. Thus, the underlying cellular network that is capable of delivering these expectations must be able to handle high data rates with low latency and ultra-reliability to fulfill these growing needs. Communication in the sub-6 GHz range cannot provide high throughputs due to the scarcity of spectrum in these bands. Using frequencies in FR2 or millimeter wave (mmWave) range for communication can provide large data rates and cover densely populated areas, but only over short distances as they are susceptible to blockages. This is why dense deployments of mmWave base stations are being considered to achieve very high data rates. But, such architectures lack the reliability needed to support many V2X applications, especially under mobility scenarios. As we have discussed earlier, 5G and beyond 5G networks must also account for UE\u27s mobility as they are expected to maintain their level of performance under different mobility scenarios and perform better than traditional networks. Although 5G technology has developed significantly in recent years, there still exists a critical gap in understanding how all these technologies would perform under mobility. There is a need to analyze and identify issues that arise with mobility and come up with solutions to overcome these hurdles without compromising the performance of these networks. Multi-connectivity (MC) refers to simultaneous connectivity with multiple radio access technologies or bands and potentially represents an important solution for the ongoing 5G deployments towards improving their performance. To address the network issues that come with mobility and fill that gap, this dissertation investigates the impact of multi-connectivity on next-generation networks from three distinct perspectives, 1) mobility enhancement using multi-connectivity in 5G networks, 2) improving reliability in mobility scenarios using multi-Connectivity with packet duplication, and 3) single grant multiple uplink scheme for performance improvement in mobility scenarios. The traditional macro-cell architecture of cellular networks that cover large geographical areas will struggle to deliver the dense coverage, low latency, and high bandwidth required by some 5G applications. Thus, 5G networks must utilize ultra-dense deployment of access points operating at higher mmWave frequency bands. But, for such dense networks, user mobility could be particularly challenging as it would reduce network efficiency and user-perceived service quality due to frequent handoffs. Multi-connectivity is seen as a key enabler in improving the performance of these next-generation networks. It enhances the system performance by providing multiple simultaneous links between the user equipment (UE) and the base stations (BS) for data transfer. Also, it eliminates the time needed to deal with frequent handoffs, link establishment, etc. Balancing the trade-offs among handoff rate, service delay, and achievable coverage/data rate in heterogeneous, dense, and diverse 5G cellular networks is, therefore, an open challenge. Hence, in this dissertation, we analyze how mobility impacts the performance of current Ultra-dense mmWave network (UDN) architecture in a city environment and discuss improvements for reducing the impact of mobility to meet 5G specifications using multi-connectivity. Current handover protocols, by design, suffer from interruption even if they are successful and, at the same time, carry the risk of failures during execution. The next-generation wireless networks, like 5G New Radio, introduce even stricter requirements that cannot be fulfilled with the traditional hard handover concept. Another expectation from these services is extreme reliability that will not tolerate any mobility-related failures. Thus, in this dissertation, we explore a novel technique using packet duplication and evaluate its performance under various mobility scenarios. We study how packet duplication can be used to meet the stringent reliability and latency requirements of modern cellular networks as data packets are duplicated and transmitted concurrently over two independent links. The idea is to generate multiple instances (duplicates) of a packet and transmit them simultaneously over different uncorrelated channels with the aim of reducing the packet failure probability. We also propose enhancements to the packet duplication feature to improve radio resource utilization. The wide variety of use cases in the 5G greatly differs from the use cases considered during the design of third-generation (3G) and fourth-generation (4G) long-term evolution (LTE) networks. Applications like autonomous driving, IoT applications, live video, etc., are much more uplink intensive as compared to traditional applications. However, the uplink performance is often, by design, lower than the downlink; hence, 5G must improve uplink performance. Hence, to meet the expected performance levels, there is a need to explore flexible network architectures for 5G networks. In this work, we propose a novel uplink scheme where the UE performs only a single transmission on a common channel, and every base station that can receive this signal would accept and process it. In our proposed architecture, a UE is connected to multiple mmWave capable distributed units (DUs), which are connected to a single gNB-central unit. In an ultra-dense deployment with multiple mmWave base stations around the UE, this removes the need to perform frequent handovers and allows high mobility with reduced latency. We develop and evaluate the performance of such a system for high throughput and reliable low latency communication under various mobility scenarios. To study the impact of mobility on next-generation networks, this work develops and systematically analyzes the performance of the 5G networks under mobility. We also look into the effect of increasing the number of users being served on the network. As a result, these studies are intended to understand better the network requirements for handling mobility and network load with multi-connectivity. This dissertation aims to achieve clarity and also proposes solutions for resolving these real-world network mobility issues

X-TCP: A Cross Layer Approach for TCP Uplink Flows in mmWave Networks

Millimeter wave frequencies will likely be part of the fifth generation of mobile networks and of the 3GPP New Radio (NR) standard. MmWave communication indeed provides a very large bandwidth, thus an increased cell throughput, but how to exploit these resources at the higher layers is still an open research question. A very relevant issue is the high variability of the channel, caused by the blockage from obstacles and the human body. This affects the design of congestion control mechanisms at the transport layer, and state-of-the-art TCP schemes such as TCP CUBIC present suboptimal performance. In this paper, we present a cross layer approach for uplink flows that adjusts the congestion window of TCP at the mobile equipment side using an estimation of the available data rate at the mmWave physical layer, based on the actual resource allocation and on the Signal to Interference plus Noise Ratio. We show that this approach reduces the latency, avoiding to fill the buffers in the cellular stack, and has a quicker recovery time after RTO events than several other TCP congestion control algorithms.Comment: 6 pages, 5 figures, accepted for presentation at the 2017 16th Annual Mediterranean Ad Hoc Networking Workshop (MED-HOC-NET