Federico Boccardi

Five disruptive technology directions for 5G

New research directions will lead to fundamental changes in the design of future fifth generation (5G) cellular networks. This article describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter wave, massive MIMO, smarter devices, and native support for machine-to-machine communications. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain.

Load & backhaul aware decoupled downlink/uplink access in 5G systems

Until the 4th Generation (4G) cellular 3GPP systems, a user equipments (UE) cell association has been based on the downlink received power from the strongest base station. Recent work has shown that - with an increasing degree of heterogeneity in emerging 5G systems - such an approach is dramatically suboptimal, advocating for an independent association of the downlink and uplink where the downlink is served by the macro cell and the uplink by the nearest small cell. In this paper, we advance prior art by explicitly considering the cell-load as well as the available backhaul capacity during the association process. We introduce a novel association algorithm and prove its superiority w.r.t. prior art by means of simulations that are based on Vodafones small cell trial network and employing a high resolution pathloss prediction and realistic user distributions. We also study the effect that different power control settings have on the performance of our algorithm.

Downlink and Uplink Decoupling: A disruptive architectural design for 5G networks

Cell association in cellular networks has traditionally been based on the downlink received signal power only, despite the fact that uplink and downlink transmission powers and interference levels differed significantly. This approach was adequate in homogeneous networks with macro base stations all having similar transmission power levels. However, with the growth of heterogeneous networks where there is a big disparity in the transmit power of the different base station types, this approach is highly inefficient. In this paper, we study the notion of Downlink and Uplink Decoupling (DUDe) where the downlink cell association is based on the downlink received power while the uplink is based on the pathloss. We present the motivation and assess the gains of this 5G design approach with simulations that are based on Vodafones LTE field trial network in a dense urban area, employing a high resolution ray-tracing pathloss prediction and realistic traffic maps based on live network measurements.

Why to decouple the uplink and downlink in cellular networks and how to do it

Ever since the inception of mobile telephony, the downlink and uplink of cellular networks have been coupled, that is, mobile terminals have been constrained to associate with the same base station in both the downlink and uplink directions. New trends in network densification and mobile data usage increase the drawbacks of this constraint, and suggest that it should be revisited. In this article we identify and explain five key arguments in favor of downlink/uplink decoupling based on a blend of theoretical, experimental, and architectural insights. We then overview the changes needed in current LTE-A mobile systems to enable this decoupling, and then look ahead to fifth generation cellular standards. We demonstrate that decoupling can lead to significant gains in network throughput, outage, and power consumption at a much lower cost compared to other solutions that provide comparable or lower gains.

Downlink and Uplink Cell Association With Traditional Macrocells and Millimeter Wave Small Cells

Millimeter wave (mmWave) links will offer high capacity but are poor at penetrating into or diffracting around solid objects. Thus, we consider a hybrid cellular network with traditional sub-6 GHz macrocells coexisting with denser mmWave small cells, where a mobile user can connect to either opportunistically. We develop a general analytical model to characterize and derive the uplink and downlink cell association in the view of the signal-to-interference-and-noise-ratio and rate coverage probabilities in such a mixed deployment. We offer extensive validation of these analytical results (which rely on several simplifying assumptions) with simulation results. Using the analytical results, different decoupled uplink and downlink cell association strategies are investigated and their superiority is shown compared with the traditional coupled approach. Finally, small cell biasing in mmWave is studied, and we show that unprecedented biasing values are desirable due to the wide bandwidth.

Spectrum Pooling in MmWave Networks: Opportunities, Challenges, and Enablers

Motivated by the specific characteristics of mmWave technologies, we discuss the possibility of an authorization regime that allows spectrum sharing between multiple operators, also referred to as spectrum pooling. In particular, considering user rate as the performance measure, we assess the benefit of coordination among networks of different operators, study the impact of beamforming at both base stations and user terminals, and analyze the pooling performance at different frequency carriers. We also discuss the enabling spectrum mechanisms, architectures, and protocols required to make spectrum pooling work in real networks. Our initial results show that, from a technical perspective, spectrum pooling at mmWave has the potential to use the resources more efficiently than traditional exclusive spectrum allocation to a single operator. However, further studies are needed in order to reach a thorough understanding of this matter, and we hope that this article will help stimulate further research in this area.

A dynamic clustering algorithm for downlink CoMP systems with multiple antenna UEs

Coordinated multi-point (CoMP) schemes have been widely studied in the recent years to tackle inter-cell interference. In practice, latency and throughput constraints on the backhaul allow the organization of only small clusters of base stations (BSs) where joint processing (JP) can be implemented. In this work, we focus on downlink CoMP-JP with multiple antenna user equipments (UEs) where the additional degrees of freedom are used to suppress the residual interference by using an interference rejection combiner (IRC) and allow a multi-stream transmission. The main contribution of this paper is the development of a novel dynamic BS clustering algorithm with corresponding UE scheduling. In particular, we first define a set of candidate BS clusters depending on long-term channel conditions. Then, in each time block, we develop a resource allocation scheme where: (a) for each candidate BS cluster, with corresponding scheduled UEs, a weighted sum rate is estimated and then (b) we select the set of non-overlapping BS clusters that maximizes the downlink system weighted sum rate. Numerical results show that much higher rates are achieved when UEs are equipped with multiple antennas and dynamic BS clustering is used.

Spectrum Sharing in mmWave Cellular Networks via Cell Association, Coordination, and Beamforming

Spectrum sharing is not typically used in current cellular networks, because it only provides a slight performance improvement while requiring a heavy coordination among different cellular operators. However, these problems can be potentially overcome in millimeter wave (mmWave) networks, thanks to beamforming both at base stations and at user equipments. This paper investigates up to which extent spectrum sharing in mmWave networks with multiple cellular operators is a viable alternative to the traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework to characterize the performance gain that the users can experience by spectrum sharing as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on a joint beamforming and base station association optimization with the objective of maximizing the long-term throughput of the users. Asymptotic and non-asymptotic performance analysis reveals that 1) spectrum sharing with light on-demand intra- and inter-operator coordination is feasible, especially at higher mmWave frequencies (e.g., 73 GHz); 2) directional communications at the user equipments substantially alleviate the disadvantages of spectrum sharing; 3) larger number of antenna elements can reduce the need for coordination and simplify the implementation of spectrum sharing; 4) while inter-operator coordination can be neglected in the large-antenna regime, intra-operator coordination can still bring gains by balancing the network load; and 5) critical control signals among base stations, operators, and user equipments should be protected from the adverse effects of spectrum sharing, e.g., by exclusive resource allocation. The results of this paper can provide fundamental insights for future standardization and spectrum policy.This paper investigates the extent to which spectrum sharing in millimeter-wave (mmWave) networks with multiple cellular operators is a viable alternative to traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework to characterize the performance gain that can be obtained when spectrum sharing is used, as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on joint beamforming and cell association optimization, with the objective of maximizing the long-term throughput of the users. Our asymptotic and non-asymptotic performance analyses reveal five key points: 1) spectrum sharing with light on-demand intra- and inter-operator coordination is feasible, especially at higher mmWave frequencies (for example, 73 GHz); 2) directional communications at the user equipment substantially alleviate the potential disadvantages of spectrum sharing (such as higher multiuser interference); 3) large numbers of antenna elements can reduce the need for coordination and simplify the implementation of spectrum sharing; 4) while inter-operator coordination can be neglected in the large-antenna regime, intra-operator coordination can still bring gains by balancing the network load; and 5) critical control signals among base stations, operators, and user equipment should be protected from the adverse effects of spectrum sharing, for example by means of exclusive resource allocation. The results of this paper, and their extensions obtained by relaxing some ideal assumptions, can provide important insights for future standardization and spectrum policy.

Hybrid Spectrum Sharing in mmWave Cellular Networks

While spectrum at millimeter wave (mmWave) frequencies is less scarce than at traditional frequencies below 6 GHz, still it is not unlimited, in particular if we consider the requirements from other services using the same band and the need to license mmWave bands to multiple mobile operators. Therefore, an efficient spectrum access scheme is critical to harvest the maximum benefit from emerging mmWave technologies. In this paper, we introduce a new hybrid spectrum access scheme for mmWave networks, where data packets are scheduled through two mmWave carriers with different characteristics. In particular, we consider the case of a hybrid spectrum scheme between a mmWave band with exclusive access and a mmWave band where spectrum is pooled between multiple operators. To the best of our knowledge, this is the first study proposing hybrid spectrum access for mmWave networks and providing a quantitative assessment of its benefits. Our results show that this approach provides advantages with respect to traditional fully licensed or fully pooled spectrum access schemes, though further work is needed to achieve a more complete understanding of both technical and nontechnical implications.

CoMPflex: CoMP for In-Band Wireless Full Duplex

In this letter, we consider emulation of a full duplex (FD) cellular base station (BS) by using two spatially separated and coordinated half duplex (HD) BSs. The proposed system is termed CoMP for In-Band Wireless Full Duplex (CoMPflex) and at a given instant it serves two HD mobile stations (MSs), one in the uplink and one in the downlink, respectively. We evaluate the performance of our scheme by using a geometric extension of the one-dimensional Wyner model, which takes into account the distances between the devices. The results show that CoMPflex leads to gains in terms of sum-rate and energy efficiency with respect to the ordinary FD, as well as with respect to a baseline scheme based on unidirectional traffic.