Intelligent reflecting surfaces (IRS) have the potential to provide coverage extension and capacity improvement in many scenarios, particularly at millimeter wave (mmWave) and terahertz (THz) frequencies, by making the wireless channels controllable to mitigate blockages in propagation environments. In this paper, we present key propagation-related characteristics and optimal phase shift solutions of IRSs for different incident and reflection angles. Raytracing-based simulations of IRSs were performed at 28 GHz in both indoor and outdoor scenarios, and the results show that IRS can greatly extend coverage and improve capacity at low cost.

In this paper, we investigate the interference generated by the High Altitude Platform Station (HAPS) deployments to the Terrestrial Networks (TNs) and quantify interference impact on the TN performance for the co-channel and adjacent channel deployment scenarios. For each case, both the unsynchronized Time-Division Duplexing (TDD) and synchronized TDD scenarios are considered. For the co-channel case, we observe that the synchronized TDD scenario requires a smaller inter-system distance than the unsynchronized case with respect to either the 5th percentile loss in spectral efficiency (SE) or the 95th percentile of the interference-to-noise ratio (I/N). For the adjacent channel case, we conclude that the resulting I/N is always below the minimum threshold of -6dB for both unsynchronized and synchronized scenarios. In addition to considering the full buffer traffic model, we also present results for a bursty traffic model under varying traffic loads.

Technology advances have enabled unmanned aerial vehicles (UAV) to stay afloat in the stratosphere for several months as base station platforms with the communication payload powered by solar energy. These UAV-based High Altitude Platform Stations (HAPS) can provide i) connectivity for remote areas not served by terrestrial networks, ii) global coverage for IoT devices, and iii) services for public safety and transportation industries. At a typical altitude of 20 Km, an HAPS can cover a large service area with a higher throughput and a lower latency compared to satellite links.In this paper, we present a reference design of a New Radio (NR) HAPS communication system comprised of both service and feeder links that can serve multiple cells within a large coverage area. The use of an adaptive antenna array with beam steering enables the system to continuously serve users from the same cell while the aircraft conducts a repetitive flight pattern above the fixed service area. Achievable capacity and coverage of the service link design at a sub-6 GHz carrier frequency are first estimated in a link budget analysis and then evaluated through Monte Carlo simulations. We observe that good spectral efficiency and coverage can be expected for outdoor line-of-sight users in the downlink, while the uplink performance is limited by the transmit power and antenna gain of the user terminal. We also demonstrate that 5G NR in the mmWave band can be used for the feeder link and can satisfy the throughput and availability requirement.

Integrated Access and Backhaul (IAB) has been considered by the 3GPP as a New Radio (NR) Release 16 study item for 5G networks. The purpose of IAB is to realize low cost 5G deployments in millimeter wave bands using wireless relaying. The main benefit is to reduce the cost of backhaul by using wireless NR links between the infrastructure nodes. In this paper, we study the throughput performance of wireless relaying in 5G millimeter wave bands in the 3GPP Urban Micro scenario. We characterize the upper bound on the IAB performance using centralized scheduling and flexible time division duplex (TDD) switching, and also consider appropriate backhaul and uplink access power levels. Simulation results are presented for a scenario with multiple base stations deployed, each with multiple transmit-receive antenna array panels, and with a large number of users deployed in the system.

Multihop self-backhauling is a key enabling technology for millimeter wave cellular deployments. We consider the multihop link scheduling problem with the objective of minimizing the end-to-end delay experienced by a typical packet. This is a complex problem, and so we model the system as a network of queues and formulate it as a Markov decision process over a continuous action space. This allows us to leverage the deep deterministic policy gradient algorithm from reinforcement learning to learn the delay minimizing scheduling policy under two scenarios: 1) an ideal setup where a centralized scheduler performs all per slot scheduling decisions and has full instantaneous knowledge of network state and 2) a centralized scheduler, but where network state feedback and scheduling decisions are limited to once per frame, which is many slots. For the second scenario, we model the scheduler with a recurrent neural network to capture the evolution of the network state over the frame. Detailed system-level simulations show that for the more realistic second scenario, the delay experienced by the 5thpercentile packets under backpressure based scheduling and max-min scheduling can be up to 230% and 260%, respectively more than that under the proposed scheduler.

The mm-wave spectra at 71-76 GHz (70 GHz) and 81-86 GHz (80 GHz) have the potential to endow fifth-generation new radio (5G-NR) with mobile connectivity at gigabit rates. However, a pressing issue is the presence of incumbent systems in these bands, which are primarily point-to-point fixed stations (FSs). In this paper, we first identify the key properties of incumbents by parsing databases of existing stations in major cities to devise several modeling guidelines and characterize their deployment geometry and antenna specifications. Second, we develop a detailed uplink interference framework to compute the aggregate interference from outdoor 5G-NR users into FSs. We then present several case studies in densely populated areas, using actual incumbent databases and building layouts. Our simulation results demonstrate promising 5G coexistence at 70 and 80 GHz as the majority of FSs experience interference well below the noise floor, thanks to the propagation losses in these bands and the deployment geometry of the incumbent and 5G systems. For the few FSs that may incur higher interference, we propose several passive interference mitigation techniques such as angular-based exclusion zones and spatial power control. The simulation results show that the techniques can effectively protect FSs, without tangible degradation of the 5G coverage.

3GPP has completed the first phase of the standardization of 5G cellular systems covering bands up to 52.6 GHz. The 3GPP Rel-15 New Radio standard provides a comprehensive framework for supporting massive MIMO in a wide variety of use cases and deployment scenarios. The NR-MIMO framework supports large scale antenna arrays having arbitrary configurations and supports digital, hybrid, and analog array architectures for both FDD and TDD deployments. With the rollout of NR soon to commence, many questions have been raised on the benefits and performance of NR-MIMO especially compared to the FD-MIMO framework in Rel-13/14 of LTE. This paper provides detailed system level simulation results showing the performance characteristics of NR-MIMO as a function of a various system and deployment parameters such as the array configuration, deployment scenario, and transmission scheme. The purpose is to answer important questions for network planners looking to roll-out NR-MIMO in the near future.

A promising way of realizing the fifth generation (5G) wireless systems is to operate 5G deployments at higher frequency bands, specifically in the millimeter-wave (mmW) spectrum (30-300 GHz). Access to such spectrum bands will enable future 5G wireless systems to meet the 5G requirements of peak rate greater than 10 Gb/s, and cell edge rate of up to 1 Gb/s. However, the emerging 5G systems will need to coexist with a number of incumbent systems in these bands. This paper provides an extensive study of the co-channel coexistence of 5G in two critical mmW bands, 27.5-28.35 GHz (28 GHz) and 71-76 GHz (70 GHz) bands, where fixed satellite service (FSS) and fixed service (FS), such as wireless backhaul, are the predominant incumbent users. In the 28-GHz study, we show that interference from 5G into the FSS space stations can be kept below the FSS interference protection criterion. We also characterize the minimum separation distance between the FSS earth stations (ESs) and 5G in order to protect the 5G system from interference due to the ESs transmissions. In the 70-GHz study, we show that the 5G-to-FS interference could be a potential issue in certain scenarios, but we introduce techniques to significantly suppress this interference, while maintaining acceptable performance of the 5G systems. For each study, we suggest appropriate deployment strategies for a 5G system based on our results.

In conventional applications of time division duplex (TDD) in cellular systems, the time resource split between uplink (UL) and downlink (DL) is fixed across all base stations (BSs) in the network. This leads to under utilization of BS resources when there is a mismatch between the expected and experienced UL/DL traffic in a given cell. A dynamic split that varies in each cell is desirable, but is challenging due to the high interference experienced by UL receivers in one cell from DL transmissions in adjacent cells. This paper analyzes the performance of UL users in dynamic TDD enabled next generation cellular networks using a stochastic geometry framework. The analysis highlights the trade-off between spectral efficiency and resource utilization for dynamic TDD. With appropriate interference mitigation, dynamic TDD offers a significant gain in data rates as compared to static TDD, which is higher when the BSs are lightly loaded and/or the fraction of UL users is low.

This paper investigates the impact of radar interference on LTE by directly injecting narrow-band radar waveforms into a state of the art LTE down-link simulator. We show detailed results demonstrating the impact of interference on various parts of LTE signaling including reference symbols. Then we show how the presence of radar interference causes errors in the noise and interference estimates, impacts decoding in the LTE soft decoder, and ultimately leads to erroneously received Sub-frames. We propose a mechanism to combat such interference and improve LTE throughput in the presence of narrow-band radar using selective erasures. We simulate the proposed scheme and show that it is extremely effective in mitigating narrow-band interference and realizing the throughput expected from LTE.

The use of millimeter wave (mmWave) frequencies for access links promises to provide an incredible user experience given the large bandwidths available. Using a recently-developed mmWave indoor channel model for the lower E-band (71-76 GHz), we investigate the system-level performance of a mmWave system in a small office-type environment. Particular attention is paid to the performance with different antenna array types. The results show that peak data rates in excess of 14 Gbps are possible and "everywhere" data rates of more than 100 Mbps are seen for all indoor locations. Also the results show that it is best to position the antennas in the azimuth dimension at the access point as opposed to elevation, and that there is little performance difference between circular and linear arrays with the same number of antenna elements. Finally the results demonstrate that the mobile can benefit from having arrays of directional patch antennas as long as there are sufficient patch arrays around the mobile to provide omni-like coverage.

Massive MIMO is considered an important technique for improving cellular system capacity through the use of high gain adaptive beamforming and high-order spatial multiplexing. For downlink Frequency-Division Duplex (FDD) cellular systems, limited feedback strategies can provide the information needed for computing the transmit weights for Single User or Multi-User MIMO. However, for MU-MIMO transmission accurate channel knowledge is critical for obtaining the best performance. In this paper, we explore the performance characteristics of several limited feedback schemes with 2D cross-polarized arrays for evolutions beyond the current 3GPP LTE Rel-12 standard. We characterize the performance of product codebook-based feedback along with several scalar quantization schemes, all schemes having different levels of feedback overhead requirements. This is compared against a high performing Rel-12 compatible vertical sectorization scheme at the low end and also against the performance with ideal knowledge of the downlink covariance matrix at the high-end. We consider a 64 element transmit array with 16 transceiver units as a case study on the effects of various restrictions on the feedback scheme, such as a DFT beam structure, block diagonal structure, and Kronecker azimuth/elevation structure.

National Telecommunications and Information Administration (NTIA) has proposed vast exclusions zones between radar and Worldwide Interoperability for Microwave Access (WiMAX) systems which are also being considered as geographic separations between radars and 3.5 GHz Long Term Evolution (LTE) systems without investigating the changes induced by the distinct nature of the LTE systems as opposed to WiMAX. This paper performs a detailed system-level analysis of the interference effects from shipborne radar systems into LTE macro cells and outdoor small cells. Even though the results reveal impacts of radar interference on LTE systems performance, they provide clear indications of conspicuously narrower exclusion zones for LTE vis-à-vis those proposed for WiMAX and pave the way toward deploying 3.5 GHz LTE within the exclusion zones.

Elevation spread of departure angles (ESD) is the key parameter characterizing a 3D fast-fading channel model. 3D channel modeling is currently being studied in 3GPP to enable the development of MIMO techniques exploiting both azimuth and elevation dimensions of the channel. In this paper we use ray-tracing techniques to estimate the ESD behavior in Manhattan and Copenhagen city environments and compare it with the 3GPP ESD model. We also investigate the ESD spread within a cluster of rays and show that the ESD within a cluster depends significantly on the distance of the mobile from the base-station. This characterization addresses a gap in the existing literature. Simulation results show that the performance of MIMO techniques such as vertical sectorization depends significantly on the ESD of the underlying environment. This observation also underscores the importance of an accurate ESD model.

Massive MIMO involves the use of large scale antenna arrays for high-gain adaptive beamforming and high-order spatial multiplexing. An important design challenge in Massive MIMO systems is the acquisition of channel state information at the transmit array, where accurate channel knowledge is critical for obtaining the best performance with Multi-User MIMO transmission on the downlink. In this paper, we explore the use of a product codebook feedback methodology for two-dimensional antenna arrays where the codebook feedback strategy is decomposed into two separate feedback processes, one for azimuth and one for elevation. We specifically address the case where the transmit array consists of cross-polarized antennas and show how two separate codebook feedback processes can reduce reference signal overhead and simplify the mobile complexity while providing significant gains in performance over existing LTE configurations.

The stadium environment represents one of the tougher environments for cellular systems given the extreme density of users and the potential for a large amount of uplink traffic. This paper presents the design and evaluation of 2D antenna arrays employed with azimuth and elevation sectorization in the stadium environment to improve the 3GPP LTE mobile network performance. A typical stadium model is introduced and different deployment options for a 2D antenna array with multiple sector beams are considered. It is found that only 2 to 4 antenna sites are sufficient to provide effective stadium coverage with up to 44 parallel spatially multiplexed sectors by using the antenna arrays consisting of 10×16 (elevation × azimuth) cross-polarized antenna element pairs. For more efficient performance the beams of each antenna array should be oriented towards the nearest stadium stands under the antenna suspension point. Using the results of system-level simulations it is estimated that the spectral efficiency of the stadium deployment can be up to 1.5-1.8 bit/s/Hz/sec resulting in the overall throughput of up to 1.5 Gbit/s which should be sufficient to provide the connectivity for tens of thousands people attending the stadium events.

Recently, there has been interest in extending MIMO processing techniques to exploit the elevation dimension of the multipath channel in addition to the azimuth dimension. In this paper, we explore the use of beamspace methods for creating virtual antenna ports in the vertical direction to enable a base station antenna array to exploit the elevation dimension of the multipath channel. We consider two ways of configuring the vertical antenna array: vertical sectorization, which creates additional sectors in the vertical domain, and vertical beamforming, which simply leverages the additional vertical antenna ports with existing MIMO processing techniques. For Rel-8/10 LTE, we show that both vertical sectorization and vertical beamforming can provide significant gains in the average and cell-edge throughputs, both in full buffer traffic as well as bursty traffic. We also highlight the sensitivity of the results to the specific assumptions of the underlying multipath channel characteristics and other system parameters.

Recently there have been proposals to extend MIMO processing to the elevation dimension in addition to the azimuth direction. To accurately assess the promised gains of these "3D-MIMO" techniques, a channel model is needed that accurately accounts for the elevation angles of the rays. In addition it would be desirable for the 3D channel model to be a simple extension of an already defined 2D channel model to allow for ease of implementation and to assist the 3GPP standardization effort in the 3D MIMO area. In this paper we propose an extension of the ITU 2D channel model to 3D by adding a distance dependent elevation spread based on observations from ray tracing. Through system-level simulations we observe that the behavior of 3D MIMO is greatly impacted by the modeling of the 3D channel.

In this paper, LTE Rel-10 eICIC, an advanced interference mitigation technique using semi-static coordination and DL CoMP techniques relying on dynamic scheduling are jointly applied in a heterogeneous network comprising of macro and low power nodes. It is shown that these algorithms, when applied jointly and if supported with appropriately designed feedback schemes, can provide complementary benefits and can significantly improve both system capacity and user experience in the network. Using covariance matrix feedback, we observe that the joint application of CoMP and eICIC can provide more than 20% system throughput improvement and more than 40% cell-edge throughput improvement that exceed the gains of either CoMP or eICIC applied individually.

It has been recognized that a primary method of further enhancing the downlink spectral efficiency of LTE Release-10 is to introduce support for coordinated multi-point (CoMP) transmission. This paper investigates the performance improvements due to downlink CoMP within the framework of LTE (specifically LTE Release-11). A realistic estimate of achievable CoMP throughput performance is provided with respect to variations in deployment scenario, amount of feedback information and at different traffic loads. It is observed that the performance gains due to CoMP are limited to cell edge UE (user equipment) throughput improvements of up to 30%. Further, we show that dynamic cell selection can realize most of the CoMP gains with less feedback than joint transmission and it is more robust under practical conditions.

A multihop relay extension for IEEE 802.16e systems is the subject of ongoing standardization activities within the IEEE 802.16j Task Group. The emerging IEEE 802.16J standard enhances the 802.16e PHY and MAC to enable support of multihop routes between a mobile station and a base station through intermediate relay stations. Since it is believed that the capacity of a single-hop 802.16e system is uplink-limited, this paper evaluates potential capacity gains attained with the relay enhancement of the 802.16e uplink. The capacity here denotes either cumulative throughput for data traffic or total number of users for voice traffic supported, under certain system-specific constrains detailed below. We first develop a simplified one-dimensional model of a relay-enhanced 802.16e system and estimate the capacity gains via analysis and numerical optimization. Motivated by the capacity gains predicted by this first-order analysis, simulation results obtained from a full two-dimensional simulator modeling a realistic deployment of a relay-enhanced system are then presented. Based on the simulation results, a parametric analysis of relay deployment cost vs. the capacity gain is also presented.

Incremental redundancy, or Hybrid type-II ARQ (HARQ), algorithms use a combination of forward error correction and retransmissions to guarantee reliable packet data communications. In this work, we propose a HARQ algorithm that exploits received packet reliability to improve system performance. Specifically, the receiver uses the average magnitude of the log-likelihood ratios of the information bits as the packet reliability metric, which is then used to determine the sizes of subsequent retransmissions. The proposed retransmission strategy attempts to maximize user throughput while satisfying a maximum packet delay constraint. The performance of our reliability-based HARQ algorithm is evaluated in static and time-varying channels through simulations. Furthermore, analytical results on the relationship between the reliability metric, the code rate and the block error rate are presented.

Incremental redundancy, or hybrid type-II ARQ (HARQ), algorithms use a combination of forward error correction and retransmissions to guarantee reliable packet data communication. In our previous work (V. Tripathi et al. 2003), we have proposed an adaptive HARQ algorithm that exploits received codeword reliability for optimizing the size of the subsequent retransmissions. The optimization is based on two mappings: the codeword reliability to block error rate (BLER) mapping and the coding rate to codeword reliability mapping. We provide analytical approximations for these mappings for any convolutional code. Through simulations, the approximations are shown to be accurate in the range of interest.

Hybrid ARQ (HARQ) schemes use a combination of forward error correction and retransmissions to guarantee reliable packet data communications. In this work, we propose a HARQ scheme that exploits channel state information and received packet quality to improve system performance. Specifically, the receiver uses the average magnitude of the log-likelihood ratios corresponding to the received information bits, in order to determine the sizes of subsequent retransmissions. The proposed retransmission strategy attempts to maximize user throughput while satisfying a maximum packet delay constraint. The performance of our reliability-based type II HARQ (RBHARQ) scheme is evaluated in static and time-varying channels through simulations.

This paper considers the problem of multiuser detection for a system in which each user employs nonlinear modulation, with an emphasis on noncoherent detection techniques which do not require knowledge of the users' channel parameters at the receiver. Our goals are to gain fundamental insight into the capabilities of multiuser detection in such a setting, and to provide practical algorithms that perform better than conventional matched-filter reception. We begin by providing fundamental performance benchmarks by considering coherent maximum-likelihood (ML) detection, which requires knowledge of the users' channel parameters, as well as noncoherent detection, formulated in a non-Bayesian generalized likelihood ratio test (GLRT) framework. The asymptotic performance of each detector, as the noise level vanishes, is characterized, yielding simple geometric criteria for near-far resistance. In general, both the ML and GLRT detectors have complexity which is exponential in the number of users. We, therefore, propose the more practical sequential decision projection (SDP) detector which has complexity which is quadratic in the number of users. It is shown that the SDP detector has nonzero asymptotic efficiency if the users' powers are suitably disparate.

The use of channel feedback from receiver to transmitter is standard in wireline communications. While knowledge of the channel at the transmitter would produce similar benefits for wireless communications as well, the generation of reliable channel feedback is complicated by the rapid time variations of the channel for mobile applications. The purpose of this paper is to provide an information-theoretic perspective on optimum transmitter strategies, and the gains obtained by employing them, for systems with transmit antenna arrays and imperfect channel feedback. The spatial channel, given the feedback, is modeled as a complex Gaussian random vector. Two extreme cases are considered: mean feedback, in which the channel side information resides in the mean of the distribution, with the covariance modeled as white, and covariance feedback, in which the channel is assumed to be varying too rapidly to track its mean, so that the mean is set to zero, and the information regarding the relative geometry of the propagation paths is captured by a nonwhite covariance matrix. In both cases, the optimum transmission strategies, maximizing the information transfer rate, are determined as a solution to simple numerical optimization problems. For both feedback models, our numerical results indicate that, when there is a moderate disparity between the strengths of different paths from the transmitter to the receiver, it is nearly optimal to employ the simple beamforming strategy of transmitting all available power in the direction which the feedback indicates is the strongest.