Millimeter Wave Cellular Networks Research Articles

RTTV-TCP: Adaptive congestion control algorithm based on RTT variations for mmWave networks

Internet applications such as video gaming virtual/ augmented reality necessitate efficient fifth-generation (5G) millimeter-wave (mmWave) cellular networks. The Transmission Control Protocol (TCP) is an essential protocol for network connectivity. However, TCP faces challenges in efficiently utilizing the available bandwidth of 5G mmWave cellular networks while maintaining low latency, mainly due to constraints like Non-Line of Sight (NLoS) conditions. This paper introduces Round-Trip-Time Variations-TCP (RTTV-TCP), enhancing TCP performance in 5G mmWave cellular networks. Simulation scenarios for a 5G mmWave cellular network have been conducted to evaluate RTTV-TCP’s performance, comparing it to legacy TCP variants such as NewReno, HighSpeed, CUBIC, Bottleneck Bandwidth and Round-trip propagation time (BBR), FB-TCP (Fuzzy Based-TCP). The results demonstrate that RTTV-TCP achieves higher average throughput than these TCP variants while maintaining the same level of delay in 5G mmWave cellular networks. RTTV-TCP outperforms NewReno and CUBIC by a very significant margin, demonstrating a 208% improvement compared to HighSpeed and a 6% increase compared to BBR protocol in the worst Packet Error Rate (PER) scenario and when the buffer size matches the Bandwidth Delay Product (BDP).

Joint Precoding for Active Intelligent Transmitting Surface Empowered Outdoor-to-Indoor Communication in mmWave Cellular Networks

Outdoor-to-indoor communications in millimeter-wave (mmWave) cellular networks have been one challenging research problem due to the severe attenuation and the high penetration loss caused by propagation characteristics of mmWave signals. We propose a viable solution to implement the outdoor-to-indoor mmWave communication with the aid of an active intelligent transmitting surface (active-ITS), where the active-ITS allows the incoming signal from an outdoor base station (BS) to pass through the surface and be received by indoor users (UEs) after shifting its phase and magnifying its amplitude. Then, the problem of joint precoding of the BS and active-ITS is investigated to maximize the weighted sum-rate (WSR) of the system. An efficient block coordinate descent (BCD) based algorithm is developed to solve it with the suboptimal solutions in nearly closed-forms. In addition, to reduce the size and hardware cost of active-ITSs, we provide a block-amplifying architecture to partially remove the circuit components for power-amplifying, where multiple transmissive-type elements (TEs) in each block share the same power amplifier. Simulations indicate that active-ITS has the potential of achieving a given performance with much fewer TEs compared to the passive-ITS under the same total system power consumption, which makes it suitable for application to the space-limited and aesthetic-needed scenario, and the performance degradation caused by the block-amplifying architecture is negligible.

Versatile Resource Management for Millimeter-Wave Cellular Network: Near Interference-Free Scheduling Methodology

In millimeter-wave (mmWave) cellular network, the highly directional beam generated by large-scale antenna array considerably depresses the inter-cell/intra-cell interferences. To fully exploit this feature in resource management, we introduce the concept of near interference-free (NIF) scheduling, representing a well-designed user scheduling scheme which avoids the possible strong interferences in mmWave network. In this paper, a generalized NIF scheduling methodology is proposed to jointly design user scheduling, beam scheduling and power allocation. The proposed methodology exhibits versatility to handle various optimization objectives, including the maximization of (weighted) sum rate, the maximization of minimum user rate, and the minimization of total transmit power. We take into account 3-dimensional user scheduling (over time, frequency and spatial domain) under the constraints induced by hybrid analog-digital beamforming. The directional beam feature is utilized to simplify the problem, and leads to linearity/convexity in the optimization of resource allocation. For each of the studied optimization objectives, simulation results validate that our proposed NIF scheduling methodology outperforms the state-of-the-art user scheduling schemes.

Stochastic Modeling of Beam Management in mmWave Vehicular Networks

Mobility management is a major challenge for the wide-spread deployment of millimeter-wave (mmWave) cellular networks. In particular, directional beamforming in mmWave devices renders high-speed mobility support very complex. This complexity, however, is not limited to system design but also the performance estimation and evaluation. Hence, some have turned their attention to stochastic modeling of mmWave vehicular communication to derive closed-form expressions characterizing the coverage and rate behavior of the network. In this article, we model and analyze the beam management for mmWave vehicular networks. To the best of our knowledge, this is the first work that goes beyond coverage and rate analysis. Specifically, we focus on a multi-lane divided highway scenario in which base stations and vehicles are present on both sides of the highway. In addition to providing analytical expressions for the average number of beam switching and handover events, we provide design insights for the network operators to fine-tune their network within the flexibility provided by the standard in the choice of system parameters, including the number of resources dedicated to channel feedback and beam alignment operations.

Coverage Enhancement in Millimeter-Wave Cellular Networks via Distributed IRSs

Intelligent reflecting surface (IRS) is a promising technology to provide line-of-sight (LOS) links for blocked paths, especially in millimeter wave (mmWave) cellular networks. However, in practice, it is difficult for IRSs to arbitrarily adjust the reflection angle to align served users. A promising solution is to deploy distributed IRSs to increase the probability that the users lie in the reflection directions. This paper develops a stochastic geometry-based approach for studying the coverage enhancement in mmWave cellular networks via distributed IRSs. Specifically, the locations of IRSs are modeled through a binomial point process centered at a base station, and the reflection beam of each IRS is pointed to a certain direction. Considering the difference between LOS and non-LOS mmWave transmissions, we propose a received signal strength indicator based association strategy to guarantee that the users receive the strongest average power. After characterizing the association probabilities and distance distributions, we derive the coverage probability for an arbitrary user and perform simplifications for enhancing the computation efficiency. The results are validated by simulations and reveal that distributed deployment of IRSs can achieve a better coverage probability than that of the centralized deployment, which validates the feasibility of enhancing system performance through distributed IRSs.

Uncoordinated Spectrum Sharing in Millimeter Wave Networks Using Carrier Sensing

We propose using Carrier Sensing (CS) for distributed interference management in millimeter-wave (mmWave) cellular networks where spectrum is shared by multiple operators that do not coordinate among themselves. In addition, even the base station sites can be shared by the operators. We describe important challenges in using traditional CS in this setting and propose enhanced CS protocols to address these challenges. Using stochastic geometry, we develop a general framework for downlink coverage probability analysis of our shared mmWave network in the presence of CS and derive the downlink coverage probability expressions for several CS protocols. Our work is the first to investigate and analyze (using stochastic geometry) CS for mmWave networks with spectrum and BS sites shared among non-coordinating operators. We evaluate the downlink coverage probability of our shared mmWave network using simulations as well as numerical examples based on our analysis. Our evaluations show that our proposed approach leads to an improvement in coverage probability, compared to the coverage probability with no CS, for higher values of signal-to-interference and noise ratio (SINR). Interestingly, our evaluations also reveal that for lower values of SINR, not using any CS is the best strategy in terms of the downlink coverage probability.

Performance Analysis of Inter-Cell Interference Coordination in mm-Wave Cellular Networks

In millimeter-wave (mm-wave) cellular networks, directional antenna arrays are typically adopted to mitigate the severe propagation loss. However, the interference caused by such highly directional beams may, in turn, result in a significant number of transmission failures, especially for dense networks. To tackle this problem, we propose two inter-cell interference coordination (ICIC) schemes in mm-wave bands: one is merely based on the path loss incorporating the blockage effect (PL-ICIC); the other considers both path loss and directivity gain (PG-ICIC). To fully investigate both schemes, we first derive an exact expression for the success probability (reliability) of the typical (served) user. We further provide an asymptotic analysis for the success probability and propose an effective approximation based on the asymptotic signal-to-interference ratio (SIR) gain relative to no ICIC. Secondly, to incorporate the cost of ICIC schemes, we derive the approximate normalized throughput taking into account that some users cannot be served due to limited resources. Numerical results show that the two proposed schemes provide significant reliability improvements in the low-SIR regime, and the higher the number of antennas, the wider the SIR range for which there is an improvement. In addition, compared with PL-ICIC, PG-ICIC balances the available resources among all users well.

A Non-Cooperative Game-Based Distributed Beam Scheduling Framework for 5G Millimeter-Wave Cellular Networks

This paper studies the problem of distributed beam scheduling for 5G millimeter-Wave (mm-Wave) cellular networks where base stations (BSs) belonging to different operators share the same spectrum without centralized coordination among them. Our goal is to design efficient distributed scheduling algorithms to maximize the network utility, which is a function of the achieved throughput by the user equipment (UEs), subject to the average and instantaneous power consumption constraints of the BSs. We propose a Media Access Control (MAC) and a power allocation/adaptation mechanism utilizing the Lyapunov stochastic optimization framework and non-cooperative games. In particular, we first decompose the original utility maximization problem into two sub-optimization problems for each time frame, which are a convex optimization problem and a non-convex optimization problem, respectively. By formulating the distributed scheduling problem as a non-cooperative game where each BS is a player attempting to optimize its own utility, we provide a distributed solution to the non-convex sub-optimization problem via finding the Nash Equilibrium (NE) of the game whose weights are determined optimally by the Lyapunov optimization framework. Finally, we conduct simulation under various network settings to show the effectiveness of the proposed game-based beam scheduling algorithm in comparison to that of several reference schemes.

Doppler-tolerant sequence design for positioning high-speed vehicles in millimeter-wave cellular systems

Future fifth-generation (5G) New Radio (NR) services consider positioning an integral part of a system that provides high precision using millimeter-wave (mmWave) cellular networks. While the 5G standard for positioning is not fully defined yet, the positioning reference signal (PRS) used in long-term evolution (LTE) system is being considered. Since the conventional LTE PRS is designed for LTE systems below 6 GHz, it is more sensitive and inefficient for high-speed vehicle (HSV) experiencing a high Doppler effect in mmWave cellular systems. In this paper, a new sequence, called an oscillatory pulse sequence (OPS), is proposed for the positioning of HSVs in mmWave cellular systems. The ambiguity function (AF) and cross-ambiguity function of OPS are analyzed to examine its autocorrelation and cross-correlation properties in the existence of Doppler shift. The performance of the proposed OPS is compared with the performance of LTE PRS, Zadoff-Chu, and linear-frequency modulated waveforms with respect to correlation, time ambiguity, and Doppler tolerance. The analytic and simulation results show that the OPS provides an accurate timing (distance) estimation in the existence of a high Doppler shift, which is the ideal characteristic of the AF for positioning. The OPS is especially suitable for orthogonal frequency division multiplexing (OFDM) systems because it can be easily multiplexed with other signals in the frequency domain.

D2D Communication Underlaying Microwave and Millimeter-Wave Cellular Networks Using CIPC

D2D communication is one of the promising technologies to improve network performance through direct wireless communication between users, without, or with limited participation of the base station (BS). In this paper, we investigate the performance of D2D communication underlaying microwave ( μ-wave) and millimeter-wave (mm-wave) cellular networks using stochastic geometry. Rayleigh fading is considered to characterize the μ-wave channels in both links, while the Rician fading is used to characterize the mm-wave channel in the D2D and cellular desired link. The D2D communication is established when the losses between the D2D transmitter and the D2D receiver are lower compared to the losses that exist between the D2D transmitter and the BS receiver. On the other hand, to minimize the interference caused by the coexistence of cellular and D2D communication and increase network performance, the Channel Inversion-Based Power Control (CIPC) is studied. In this way, the Complementary Cumulative Distribution Function (CCDF) for the successful transmission is obtained. Based on the analytical and numerical results, we show an important increase in the probability of successful transmission using the CIPC and considering the channel gains condition to establish the D2D communication compared to the traditional mode in which power control is not used and no conditions are considered to establish the D2D communication.

FadeNet: Deep Learning-Based mm-Wave Large-Scale Channel Fading Prediction and its Applications

Accurate prediction of the large-scale channel fading is fundamental to planning and optimization in 5G millimeter-wave cellular networks. The current prediction methods, which are either too computationally expensive or inaccurate, are unsuitable for city-scale cell planning and optimization. This paper presents FadeNet, a convolutional neural-network enabled alternative for predicting large-scale fading with high computation speed and accuracy. By using carefully designed input features and neural-network architecture to capture topographical information, FadeNet accurately predicts the large-scale fading from a base station to each location in its coverage area. Evaluations on realistic data, derived from millimeter-wave cells across multiple cities, suggest that FadeNet can achieve a prediction accuracy of 5.6 decibels in root mean square error. In addition, by leveraging the parallel processing capabilities of a graphics processing unit, FadeNet can reduce the prediction time by $40X-1000X$ in comparison to industry prevalent methods like ray-tracing. Generalizations of FadeNet, that can handle variable topographies and base station heights, and its use for optimal cell site selection are also explored.

Optimal and Approximation Algorithms for Joint Routing and Scheduling in Millimeter-Wave Cellular Networks

Millimeter-wave (mmWave) communication is a promising technology to cope with the exponential increase in 5G data traffic. Such networks typically require a very dense deployment of base stations. A subset of those, so-called macro base stations, feature high-bandwidth connection to the core network, while relay base stations are connected wirelessly. To reduce cost and increase flexibility, wireless backhauling is needed to connect both macro to relay as well as relay to relay base stations. The characteristics of mmWave communication mandates new paradigms for routing and scheduling. The paper investigates scheduling algorithms under different interference models. To showcase the scheduling methods, we study the maximum throughput fair scheduling problem. Yet the proposed algorithms can be easily extended to other problems. For a full-duplex network under the no interference model, we propose an efficient polynomial-time scheduling method, the {\em schedule-oriented optimization}. Further, we prove that the problem is NP-hard if we assume pairwise link interference model or half-duplex radios. Fractional weighted coloring based approximation algorithms are proposed for these NP-hard cases. Moreover, the approximation algorithm parallel data stream scheduling is proposed for the case of half-duplex network under the no interference model. It has better approximation ratio than the fractional weighted coloring based algorithms and even attains the optimal solution for the special case of uniform orthogonal backhaul networks.

A Magneto-Electric Dipole Antenna Array for Millimeter Wave Applications

In this paper, a low-profile broadband antenna is proposed for future 5G millimeter-wave cellular wireless networks. The proposed antenna is a modified Magneto-Electric (ME) dipole, which consists of four metallic plates, grounded vias, an aperture fed, a ground plane, and a microstrip line feed. The antennas are built on RT/Duroid 5880 substrates and have been realized by the printed circuit board technique. A single-element with an overall of 10×10×1.04mm3 (~1.26λo×1.26λo×0.13λo at 38GHz) exhibits an impedance matching of 27.9% (32.2-42.8GHz) for |S11|<–10dB and a realized gain up to 7.5dBi over the frequency band. The usefulness of these antennas as beamforming radiators is demonstrated by a 1×4 element linear array. Also, a wide-band excitation is applied for the linear ME dipole array to realize a broadband array. The simulated results proved the proposed array can operate in a frequency band spreading from 31.4GHz to 42.1GHz with a gain of 12.5dBi and a side-lobe of -13dB

The Meta Distributions of the SIR/SNR and Data Rate in Coexisting Sub-6GHz and Millimeter-Wave Cellular Networks

Using stochastic geometry tools, we develop a systematic framework to characterize the meta distributions of the downlink SIR/SNR and data rate of the typical device in a cellular network with coexisting sub-6GHz and millimeter wave (mm-wave) spectrums. Macro base-stations (MBSs) transmit on sub-6GHz channels (which we term “microwave” channels), whereas small base-stations (SBSs) communicate with devices on mm-wave channels. The SBSs are connected to MBSs via a microwave (μwave) wireless backhaul. The μwave channels are interference limited and mm-wave channels are noise limited; therefore, we have the meta distribution of SIR and SNR in μwave and mm-wave channels, respectively. To model the line-of-sight (LOS) nature of mm-wave channels, we use Nakagami-m fading model. To derive the meta distribution of SIR/SNR, we characterize the conditional success probability (CSP) (or equivalently reliability) and its b th moment for the typical device (a) when it associates to a μwave MBS for direct transmission, and (b) when it associates to a mm-wave SBS for dual-hop transmission (backhaul and access transmission). Performance metrics such as the mean and variance of the local delay (network jitter), mean of the CSP (coverage probability), and variance of the CSP are derived. Closedform expressions are presented for special scenarios. The extensions of the developed framework to the μwave-only network or mm-wave only networks where SBSs have mm-wave backhauls are discussed. Numerical results validate the analytical results. Insights are extracted related to the reliability, coverage probability, and latency of the considered network.

Multi-UAV Assisted Multi-Tier Millimeter-Wave Cellular Networks for Hotspots With 2-Tier and 4-Tier Network Association

The evolution of the fifth-generation (5G)/ beyond 5G (B5G) mobile communication networks greatly depends on the seamless integration of terrestrial and sky networks over advanced multi-tier heterogeneous network (HetNet) infrastructure. By using the huge potential of unmanned aerial vehicles (UAVs) and mm-Wave, we develop a novel and practical multi-UAV assisted multi-tier HetNet model which consists of clustered aerial UAV base stations (U-BSs) and Poisson point process (PPP) based ground BSs (G-BSs) for emergency and crowded hotspot area. In a typical cluster, both ground user equipments (GUEs) and UAVs are modeled as Poisson cluster process (PCP) and scattered around the common cluster center. Specially, the analysis is conducted by adding two additional tiers which consist of multiple intra-cluster U-BSs and single G-BS in representative cluster so that the point sets of U-BSs and G-BSs are partitioned into two sub-sets, respectively, and a 4-tier network model is formed. To find the contribution of the inter-cluster associations on the performance of networks, we construct 2-tier and 4-tier association schemes. The 2-tier association allows a GUE associated only with intra-cluster U-BSs and G-BS, while the 4-tier association allows all inter/intra-cluster U-BSs and G-BSs can be associated. Then, according to stochastic geometry methods, statistical description of the downlink interferences experienced at a typical GUE is derived. This derivation yields the downlink coverage probabilities and total network throughput are obtained. It is achieved that association probabilities greatly depend on the distribution of daughter point process. UAVs' height and the distributions of U-BSs and GUEs impose non-monotonous impact on coverage probabilities. Due to severe inter-cluster interference, the achievable average area throughput is not always increasing with SINR so that there is an optimal SINR threshold. In addition, our analysis finds the proposed 4-tier association scheme outperforms the simple 2-tier one in terms of the average area throughput.

Energy and Spectral Efficiency Analysis for a Device-to-Device-Enabled Millimeter-Wave OFDMA Cellular Network

The spectral efficiency (SE) and energy efficiency (EE) performance of a millimeter-wave (mmWave) cellular network is studied where a user device can associate with a base station (BS) or another user for device-to-device (D2D) communication based on an interference-aware D2D distance threshold. Using the tools of stochastic geometry, the mean interference, coverage probability, area SE, and network EE are derived under the proposed association scheme. Performance of the proposed scheme is compared with that of the minimum path loss (Min PL)-based and maximum biased-received-power (Max BRP)-based association schemes. The proposed scheme is shown to give the best coverage probability performance in noise-limited networks, while the three schemes converge in performance in interference-limited networks in the high coverage threshold regime (>20 dB). Further, the proposed scheme achieves up to 60% increase in the area SE and EE, compared to the Min PL-based scheme that gives the next best performance. Lastly, the paper proposed a goal attainment algorithm that achieves up to a seven-fold decrease in the mean deviation from a preset SE objective and 50% savings in EE, compared to the achievable performance under a constant transmit power and bandwidth allocation scheme.

Relay-Aided Outdoor-to-Indoor Communication in Millimeter-Wave Cellular Networks

In this article, we examine the feasibility of serving an indoor mobile terminal (MT) by an outdoor base station (BS) in urban millimeter-wave (mmWave) cellular networks. Toward this, we consider two scenarios: 1) the signal reaches the reference MT directly by penetrating the building where it is located and 2) the communication is assisted by a relay placed on the rooftop of the building that conveys through a waveguide the desired signal to an indoor transmit antenna located at the floor where the reference MT is located. By using a stochastic-geometry-based framework, we derive for both scenarios, analytical expressions for the coverage and average rate, which exhibit a good match with respect to the simulations. In addition, from these expressions, we gain important insights regarding the performance comparison of the two scenarios. In particular, it is analytically proved and validated by the simulations that relaying notably enhances the performance of the low-rate MTs even under substantial waveguide losses. However, for such losses, the average rate in the relaying case can become worse than the case without relays.

Uplink Performance Analysis in D2D-Enabled Millimeter-Wave Cellular Networks With Clustered Users

In this paper, an analytical framework is provided to analyze the uplink performance of device-to-device (D2D)-enabled millimeter-wave (mm-wave) cellular networks with clustered D2D user equipments (UEs). The locations of cellular UEs are modeled as a Poisson point process, while the locations of potential D2D UEs are modeled as a Poisson cluster process. Signal-to-interference-plus-noise ratio outage probabilities are derived for both cellular and D2D links using tools from stochastic geometry. The distinguishing features of mm-wave communications such as directional beamforming and having different path loss laws for the line-of-sight and non-line-of-sight links are incorporated into the outage analysis by employing a flexible mode selection scheme and Nakagami fading. Also, the effect of beamforming alignment errors on the outage probability is investigated to get insight into the performance in practical scenarios. Moreover, area spectral efficiency of the cellular and D2D networks is determined for both underlay and overlay types of sharing. Optimal spectrum partition factor is determined for overlay sharing by considering the optimal weighted proportional fair spectrum partition.

On the Throughput Performance of TCP Cubic in Millimeter-Wave Cellular Networks

In this paper, we study a cross-layer analysis framework for the performance evaluation of TCP over millimeter-Wave (mmWave) fading channels in the fifth-generation (5G) cellular networks, when the truncated incremental redundancy hybrid automatic repeat request (IR-HARQ) scheme and adaptive modulation and coding (AMC) are employed. Specifically, the throughput performance of TCP Cubic, which is one of the most widely deployed TCP variants in the Internet, is investigated. For this purpose, the mmWave fading channel, approximated by Nakagami-m distribution, is captured by a finite-state Markov chain (FSMC) to develop a transmission loss model. A loss-based TCP model, which is analyzed based on the transmission loss model, is then used to analytically derive the TCP throughput performance. The numerical results quantitatively show the effect of different parameters/settings of AMC, IR-HARQ, blockages and mmWave fading channels on the TCP performance and support the optimal selection of parameters to maximize the system throughput. Monte Carlo simulations are also performed to validate the analytical results.

Effect of Users Height Distribution on the Coverage of mmWave Cellular Networks With 3D Beamforming

In this paper, we study the effect of users' height distribution on the coverage probability of millimeter-wave (mmWave) cellular networks that utilize three-dimensional beamforming (3DBF). The users and base stations (BSs) are equipped with multiple antennas and both line-of-sight (LOS) and non-LOS links exist in the channel which are, respectively, modeled by the Nakagami-m and Rayleigh distribution. In this setup, we investigate the tilt angle optimization of the BS antenna arrays for maximizing the coverage probability under two regimes of noise limited and interference limitedd. In both cases, by adopting a stochastic geometry approach, we analytically derive the coverage probability, and then, find the optimal tilt angle that maximizes this probability. In addition, in the noise limited regime, we show that the optimal tilt angle depends on the average distance between each user and its serving BS and also their effective height. In the interference-limited regime, we further consider different rules for associating users to the BSs. Meanwhile, since in this regime, the tilt angle optimization is very complex, we propose a low complexity approach to find the optimal tilt angle that has a performance close to the optimal solution. We further study the asymptotic behavior when the density of the BSs or signal-To-interference ratio tends to infinity or zero. Finally, through the numerical simulations, we show that using the 3DBF and also incorporating the users' height distribution in the tilt angle optimization lead to a substantial improvement in the coverage probability of the mmWave cellular networks.