The fifth generation (5G) new radio (NR) of mobile communications is designed to support two major class of services with vastly heterogeneous requirements: ultra-reliable low-latency communication (URLLC) and enhanced mobile broadband (eMBB). To enable the coexistence of URLLC and eMBB in the same radio spectrum in a cost-effective manner while guaranteeing the latency and reliability performance of URLLC, 5G NR has developed an innovative preemption and superposition framework which allows a dynamic multiplexing of the two services in an interleaved manner. In this tutorial paper, we review the key technological innovations in this framework in NR Release 15 and 16, namely, preemption indication and enhanced power control, and explain the design principles underlying these technologies. System-level and link-level simulations are provided to illustrate the performance benefits of these designs.

Tactile internet is defined as a network that offers a response to a physical process or an object in perceived real time. The development of new radio (NR) and long-term evolution (LTE) technologies with tailored high-reliability and low-latency design is expected to reliably transmit data in milliseconds, leading to the promising realization of tactile internet with applications in areas such as factory automation, education, gaming, and healthcare. In this paper, from a physical layer and medium-access control (PHY/MAC) perspective, we discuss the systems design of ultrareliable and low-latency communications (URLLC) in NR and LTE technologies, both belonging to the fifth-generation (5G) wireless technologies. Motivated by the safety and privacy requirements of tactile internet, we also outline the 5G security landscape, major categories of attackers, and potential countermeasures. Finally, we provide a case study of factory automation as an example of the proposed 5G-based tactile internet.

A base-station (BS) equipped with multiple antennas can use its spatial dimensions in three different ways: 1) to serve multiple users, thereby achieving a multiplexing gain; 2) to provide spatial diversity in order to improve user rates; and 3) to null interference in neighboring cells. This paper answers the following question: What is the optimal balance between these three competing benefits? We answer this question in the context of the downlink of a cellular network, where multi-antenna BSs serve multiple single-antenna users using zero-forcing beamforming with equal power assignment, while nulling interference at a subset of out-of-cell users. Any remaining spatial dimensions provide transmit diversity for the scheduled users. Utilizing tools from stochastic geometry, we show that, surprisingly, to maximize the per-BS ergodic sum rate, with an optimal allocation of spatial resources, interference nulling does not provide a tangible benefit. The strategy of avoiding inter-cell interference nulling, reserving some fraction of spatial resources for multiplexing, and using the rest to provide diversity, is already close-to-optimal in terms of the sum-rate. However, interference nulling does bring significant benefit to cell-edge users, particularly when adopting a range-adaptive nulling strategy where the size of the cooperating BS cluster is increased for cell-edge users.

This paper studies the physical-layer benefits of low latency operation in long-term evolution (LTE) networks. Latency reduction can be achieved by reducing the transmission time interval (TTI) from 1ms to the duration of only a few orthogonal frequency-division multiplexing symbols. The TTI shortening potentially enables faster link adaptation, thereby enhancing system performance. However, enabling low latency operation in a backward compatible manner requires a careful design and performance characterization. This paper conducts a link-level performance analysis of a low latency LTE network in both downlink and uplink with different transmission schemes and under various operating regimes. Our results reveal that a low latency LTE network can provide reasonable link-level performance improvements as compared to a legacy LTE network.

We propose a time division duplex (TDD) based network architecture where a macrocell tier with a “massive” multiple-input multiple-output (MIMO) base station (BS) is overlaid with a dense tier of small cells (SCs). In this context, the TDD protocol and the resulting channel reciprocity have two compelling advantages. First, a large number of BS antennas can be deployed without incurring a prohibitive overhead for channel training. Second, the BS can estimate the interference covariance matrix from the SC tier which can be leveraged for downlink precoding. In particular, the BS designs its precoding vectors to transmit independent data streams to its users while being orthogonal to the subspace spanned by the strongest interference directions; thereby minimizing the sum interference imposed on the SCs. In other words, the BS “sacrifices” some of its antennas for interference cancellation while the TDD protocol allows for an implicit coordination across both tiers. Simulation results suggest that, given a sufficiently large number of BS antennas, the proposed scheme can significantly improve the sum-rate of the SC tier at the price of a small macro performance loss.

Employing centralized resource management schemes is generally infeasible in large-scale networks. The deployment of heterogeneous Femtocell Access Points (FAPs) over the cellular licensed spectrum is therefore challenging. In particular, the resulting inter-node interference inhibits the network performance. In this paper, we design a hierarchical, distributed, interference management scheme that exploits the benefits of clustering. First, in order to reduce the cross-tier interference, each FAP independently identifies vacant subbands for potential transmission. Then, by exchanging some simple messages with its immediate neighbors in an iterative fashion, coalition clusters are formed. Given the small population of each group, centralized resource management is subsequently performed to avoid intra-cluster interference. Different clusters, however, may still share a fraction of common idle channels, which degrades system performance. Therefore, this paper further considers inter-cluster interference management to determine the set of privileged FAPs that can share a subband via solving a binary power control optimization problem. While the optimal solution requires prohibitive complexity, this paper provides tight bounds on the sum rate of the binary power control problem. The simulation results show that, in a high interference regime, inter-cluster coordination provides a significant performance improvement compared to the case of no coordination.

This paper considers a multi-source orthogonal frequency division multiplexing (OFDM)-based network of access points wherein dedicated relays use the decode-and-forward relaying. We investigate the joint problem of transmission strategy selection (relaying v/s direct), relay assignment, and power allocation to maximize the minimum rate across sources in two different cooperative scenarios: subcarrier-based and block-based relaying. In the subcarrier-based scheme, each subcarrier is treated as an independent transmission; however, in addition to the synchronization problems caused, it is likely impractical for a relay to decode a subset of subcarriers. Thus, we study relay selection for the entire OFDM block. The key difference from previous work is that we consider resource allocation across source-relay, relay-destination, and source-destination channels. Second, a simple, distributed, block-based scheme is proposed. Simulation results using the COST-231 channel model reveals that the performance of this heuristic scheme tracks that of the optimum block-based scheme while it significantly decreases the required computational complexity.

We consider a multi-source mesh network of static access points wherein sources use decode-and-forward to cooperate with each other. All transmissions use orthogonal frequency division multiplexing (OFDM). Our objective is to maximize the minimum achievable rate across all flows. We find a tight upper bound on the performance of the subcarrier-based cooperation and show that selecting a single relay for each subcarrier is optimal for almost all subcarriers. The solution to the related optimization problem simultaneously solves the relay, power, and subcarrier assignment problems. Second, unlike previous works, we also consider relay selection for the entire OFDM block. This addresses the fact that, in addition to the synchronization problems caused, it is likely impractical for a relay to only decode a subset of subcarriers. We propose three selection-based cooperation schemes to relay the entire OFDM block with varying complexity. Simulation results show that under the COST-231 channel model, the performance of the simplest scheme almost exactly tracks that of an exhaustive search.