In this paper, we study the random-access procedure for Ambient-IoT (A-IoT). Traditional approaches based on a slotted-ALOHA-like access over TDM/FDM resources suffer from sizable fraction of idled and/or collided resources, leading to reduced system throughput. To address this, we consider CDM of transmissions from multiple A-IoT devices, which enables detecting multiple A-IoT devices simultaneously in a TDM/FDM resource. CDM of A-IoT transmissions faces a number of challenges, including timing misalignment and loss of orthogonality, due to significantly higher sampling frequency offset (SFO) of low-complexity and low-power A-IoT devices, as compared to regular devices. We address these issues by adding a prefix or postfix to A-IoT transmissions and proposing a new multi-SFO hypotheses and successive interference cancellation (SIC) based reader processing. We show through simulations that CDM using the proposed approach is able to achieve significant gains in the system throughput over slotted-ALOHA-based schemes for A-IoT devices with high SFOs.

Reconfigurable Intelligent surface (RIS) has emerged as a candidate technology for enhancing coverage in millimeter wave wireless networks at a lower energy and cost footprint. RIS has several antenna elements that can each be configured to reflect impinging electromagnetic waves after imparting a chosen phase shift with possibly some amplitude attenuation (a.k.a. chosen reflection coefficient). Over the canonical RIS-enabled communications scenario, an optimal choice of reflection coefficients (or optimal RIS pattern) can be efficiently determined for an ideal unit-amplitude unconstrained phase alphabet. However, for most practical RIS that entail finite alphabets with amplitude imbalance and per-group-of-elements control, efficiently determining optimal patterns remain open problems. In this paper we resolve two such open problems by designing optimal and efficient pattern determination algorithms for binary and quaternary alphabets. We show that our algorithms can noticeably improve over the state-of-art conventional heuristic, especially in the presence of high amplitude imbalance and more restrictive per-group control. The designed algorithms also yield companion sets, which we show offer very significant advantages in RIS pattern selection under interference limit (leakage suppression) constraints.

Deployments of millimeter wave systems suffer from increased propagation and blockage losses that lead to a focus on small cell coverage. With an increased densification of such systems, cooperative transmissions are possible at a user equipment (UE) from multiple transmission reception points (TRPs) and with repeater nodes (e.g., relay UEs, intelligent reflecting surfaces, reflectarrays) in the vicinity of a certain UE. In this context, this work proposes and studies the performance of a class of directional beam steering and co-phasing schemes that leverage the existing set of beam steering solutions for point-to-point transmissions. To benchmark the performance of the proposed schemes, an unrealizable upper bound to the SNR is first established. We then quantify the SNR gap between the proposed directional schemes and the unrealizable upper bound via theory and numerical studies and show that the proposed solutions are of low complexity and comparable performance to the unrealizable upper bound.

Hand and body blockage are known to result in significant performance degradation at millimeter wave carrier frequencies. A number of prior works have proposed statistical models to capture the spatial region lost due to blockage and the associated losses incurred in signal strength over this region. In particular, it is understood that while a hard hand grip can lead to substantial losses, a more relaxed/looser hand grip can lead to moderate losses. However, all these works consider only the signal strength degradation with blockage mitigation mechanisms consisting of densification of the network and the use of more antenna modules at the user equipment end. In practice, the number of antenna modules in a user equipment may be limited leading to the need for more novel blockage mitigation strategies. Towards this goal, in this work, we show that the presence of the hand on top/near an antenna module can distort the observed amplitude and phase of the array response. We also show that a non-directional analog beamforming codebook enhancement can de-randomize some of the phase distortions and recover some of the gains lost due to the hand with a directional beamforming codebook.

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.

The ongoing commercial deployment of millimeter wave systems brings into focus a number of practical issues in form factor user equipment (UE) design. With wavelengths becoming smaller, antenna gain patterns becoming directional, and link budgets critically dependent on beamforming, it becomes imperative to use a number of antenna modules at different locations of the UE for good performance. While more antennas/modules can enhance beamforming array gains, it comes with the trade-off of higher component cost, power consumption of the associated radio frequency circuitry, and a beam management overhead in learning the appropriate beam weights. Thus, the goal of a good UE design is to provide robust spherical coverage corresponding to good array gains over the entire sphere around the UE with a low beam management overhead, complexity, and cost. The scope of this paper is to study the implications of two popular commercial millimeter-wave UE designs (a face and an edge design) on spherical coverage. We show that the analog beam codebooks can result in good performance for both the designs, and the edge design provides a better trade-off in terms of robust performance (with hand blockage), beam management overhead, implementation complexity from an antenna placement standpoint, and cost.

There has been a strong interest in understanding hybrid precoding tradeoffs for millimeter wave (mmW) multi-input multi-output (MIMO) systems. A common assumption in most of these works is that the analog part of the hybrid precoder can only be designed with phase shifters. The consequent search for analog and digital precoding matrices is solved with different black box-type optimization algorithms. In contrast, this work motivates an analog precoding structure at the base-station end that can be realized with both phase shifters and gain controls. Such a structure is necessary for interference management in multi- user transmissions and is easily realized with low complexity and cost. We then propose a feedback framework of the top-P beams over a beam alignment phase from each user. This framework allows the base-station to reconstruct the channel matrix between it and each user, and to manage interference with a simple zeroforcing solution. Such a structured solution is implemented with the amplitude and phase control of the analog part of the hybrid precoder. We finally illustrate the performance improvement with the proposed solution over simpler constructions such as beam steering that can be implemented with phase shifters alone.

Autonomous vehicles (AVs) have the potential to transform road transportation by improving safety and increasing traffic/fuel efficiency. Currently, AVs rely primarily on line-of-sight sensing technologies to gather information about their surroundings. Some of the surrounding objects (e.g., vehicles, infrastructure, and pedestrians), however, can potentially communicate with the AV to help it create a better digital map of the world around it. In this work, we quantify the gains vehicle-to-vehicle (V2V) communication can provide for AVs. Specifically, we focus on the safety and maneuver planning of the AVs. Our preliminary findings indicate that V2V can reduce a significant fraction of AV collisions. Further, V2V can considerably reduce the maneuver completion time.

Given the cost and complexity associated with the deployment of a large number of radio frequency (RF) chains in millimeter wave (mmW) multi-input multi-output (MIMO) systems, this paper addresses the question of network efficiency considerations for hybrid precoding. We first establish the relevance of directional precoding structures as a low-complexity and robust solution to meet the data rate demands of single-user MIMO systems relative to the more complex and less robust eigen-mode-based precoding structures. Key to the relevance of the directional precoding structures is the sparsity of the mmW channel coupled with higher antenna dimensionality affordable due to smaller wavelengths. We then leverage the directional structure of the channel to propose a simple class of directional schedulers that offers a low-complexity and yet approximately fair separation plane in the user space. We then compare the performance of single-user precoding schemes with multi-user precoding schemes and show that from network efficiency considerations, it would be more worthwhile to expend the RF chain resource on multi-user transmissions.

The focus of this work is on cost and complexity- induced rank-constrained precoding for millimeter- wave (mmW) multi-input multi-output (MIMO) systems. The higher antenna dimensionality and sparse channel structure of mmW systems motivate directional precoding schemes over more complex and less robust eigen-mode-based precoding schemes. Building on a prior work that illustrated the superiority in design, robustness and performance of a directional rank-1 beamformer, this work considers higher-rank directional precoding structures for mmW systems. It is shown that directional precoders continue to enjoy similar properties in single-user MIMO (SU-MIMO) systems. But from a network efficiency consideration, it would be more worthwhile to expend the radio frequency (RF) chain resource on multi-user transmissions. Towards this goal, a directional scheduler and different directional beamforming structures are proposed here. These structures are shown to result in significant performance improvement over higher-rank SU-MIMO schemes.

In recent years, the public safety community has been aligning behind Long Term Evolution (LTE) technology as the basis for next generation public safety networks. Correspondingly, the 3rd Generation Partnership Project (3GPP) standards body has started multiple technical work initiatives on LTE enhancements to better support public safety use cases and requirements. Of particular importance to the public safety community is the addition of direct device-to-device (D2D) broadcast capability that can enable off-network push-to-talk (PTT) group communications in a manner equivalent to existing public safety communication systems. In this paper, we briefly explore the design objectives for, and challenges associated with, providing a D2D broadcast service. We then present and analyze a new distributed media access control (MAC) scheme for interference-aware coordination of both unicast and broadcast transmission. System-level simulation results are provided to validate the performance and the effectiveness of our approach, using a CSMA-based approach as a basis for comparison.

We study the problem of distributed time and frequency synchronization for device-to-device communication in LTE [1]. LTE is an OFDMA system that crucially uses tight time and frequency synchronization between UEs and the base station for intracell and intercell interference coordination. For consistency with the cyclic prefix length and tone spacing used in LTE, we target an accuracy of a few microseconds for time synchronization and 150 Hz for frequency synchronization. We examine the problem both from a link and a system-level perspective. Link-level challenges involve designing a synchronization signal for computationally and energy efficient receiver algorithms, and system-level challenges involve achieving consistent timing using a distributed algorithm while leveraging the multiuser aspect of the problem to improve performance.

The millimeter-wave (mm-Wave) frequency band ~30-300 GHz has received significant attention lately as a prospective band for 5G systems. Millimeter-wave frequencies have traditionally been used for backhaul, satellite and other fixed services. While these bands offer substantial amount of bandwidth and opportunity for spatial multiplexing, the propagation characteristics for terrestrial mobile usage need to be fully understood prior to system design. Towards this end, this paper presents preliminary indoor measurement results obtained using a channel sounder equipped with omni- and directional antennas at 2.9 GHz and 29 GHz as a comparative study of the two bands. The measurements are made within a Qualcomm building in Bridgewater, NJ, USA, for two separate floors, each representing a different yet representative type of office plan. We present measurements and estimated parameters for path loss, excess delay, RMS delay and analyze the power profile of received paths. In addition, we present several spherical scans of particular links to illustrate the 3-D angular spread of the received paths. This work represents initial results of an ongoing effort for comprehensive indoor and outdoor channel measurements. The measurements presented here, along with cited references, offer interesting insights into propagation conditions (e.g. loss, delay/angular spread etc.), coverage and robustness for mobile use of millimeter-wave bands. We believe additional extensive measurement campaigns in diverse settings by academia and industry would help facilitate the generation of usable channel models.

Dedicated Short-Range Communications (DSRC) has been designed to support vehicular communications. In the U.S., DSRC operates in the 5.9 GHz licensed spectrum band. Its physical (PHY) and medium access control (MAC) layers, defined in the IEEE 802.11p standard, are based on the IEEE 802.11 family of Wi-Fi standards. Vehicular communication environments differ significantly from the sparse and low-velocity nomadic use cases of a typical Wi-Fi deployment. Thus, there are many challenges to adapt Wi-Fi technologies to support the unique requirements of vehicular communications such as achieving high and reliable performance in highly mobile, often densely populated, and frequently non-line-of-sight environments. The automotive and the communications industries, academia, and governments around the world have been devoting tremendous efforts to address these challenges, and significant achievements have been made. Remaining challenges can be addressed by the future versions of DSRC. In this paper, we investigate the current technologies used by DSRC to support vehicle safety communications, analyze existing and possible DSRC performance enhancements that can be realized in the near term, and provide a few initial thoughts on the DSRC evolution path.

This paper proposes FlashLinQ-a synchronous peer-to-peer wireless PHY/MAC network architecture. FlashLinQ leverages the fine-grained parallel channel access offered by OFDM and incorporates an analog energy-level-based signaling scheme that enables signal-to-interference ratio (SIR)-based distributed scheduling. This new signaling mechanism, and the concomitant scheduling algorithm, enables efficient channel-aware spatial resource allocation, leading to significant gains over a CSMA/CA system using RTS/CTS. FlashLinQ is a complete system architecture including: 1) timing and frequency synchronization derived from cellular spectrum; 2) peer discovery; 3) link management; and 4) channel-aware distributed power, data rate, and link scheduling. FlashLinQ has been implemented for operation over licensed spectrum on a digital signal processor/field-programmable gate array (DSP/FPGA) platform. In this paper, we present FlashLinQ performance results derived from both measurements and simulations.

Ad hoc wireless networks without central authority have attracted much attention in both academic society and industry. A key aspect of communications in such networks is the large dynamic range of interference. OFDMA (multiplexing simultaneous transmissions in different frequency tones) is an effective way of dealing with interference. On the other hand, there are many known distortions in transmit and receive chains of the communication devices, e.g. non linearity of the power amplifier, which can compromise the OFDMA orthogonality among the resources assigned to different transmissions. These distortions, together with the large dynamic range one would expect in ad hoc communications, can pose a threat to the effectiveness of communications in OFDMA ad hoc networks. In this paper, we propose and evaluate different signaling choices for communications in ad hoc networks, via both analysis and simulations. In particular, we observe that OFDMA based single tone signaling, i.e. each user transmits only on a single frequency tone, outperforms other signaling choices in many aspects.

Recent deployments of data-rich smart phones has provided a fresh impetus for designing, deploying and understanding the performance of wide area ad-hoc networks. The most popular medium access mechanism for such ad hoc networks is CSMA/CA with RTS/CTS. In this paper, using tools from stochastic geometry, we study and optimize the throughput performance of such networks. We show that in ad-hoc networks enabled with SIR based scheduling, a simple modification to the transmit power level - setting it to be inversely proportional to the square root of the link gain - leads to large improvements in network throughput. This simple power-level selection is optimal over the class of all ”local” transmit power selection strategies when channels are stationary, and further is at most a factor of two away from optimality in the fading case. Using stochastic geometric techniques, we also provide analytical expressions for the medium access probability in different scenarios.

We study an approach for sharing channels to improve network utilization in packet-switched cellular networks. This scheme exploits unused resources in neighboring cells without the need for global coordination. We formulate a minimax approach to optimizing the allocation of channels in this sharing scheme. We develop a distributed algorithm to achieve this objective and study its convergence. We illustrate, via simulation results, that the distributed channel sharing scheme performs better than the fixed channel scheme over a wide variety of traffic conditions.

We present a novel static power control scheme to improve system capacity in wireless cellular networks. Our basic idea is to reduce intercellular interference and improve the capture probability by coordinating transmission powers of users in different cells. This coordination is determined beforehand and no real-time coordination is required. Power control is static and fixed. We formulate and solve a generic optimal scheduling problem with our coordination scheme. We find that the optimal scheduling policy is in a simple form of bang-bang control, which is illustrated for a specific case with the uniform fairness constraint. We evaluate, via numerical analysis and simulation, both throughput and delay, and compare them with other schemes. We find that the coordination scheme can achieve significant performance improvement, in terms of both maximum throughput and throughput-delay tradeoff, over a wide range of capture ratio values.

Enhancing system capacity while maintaining quality of service is an important issue in wireless cellular networks. In this paper, we present a localized channel sharing scheme to address this problem. Our basic idea is to allow channels to be shared between adjacent cells at the expense of a smaller initial allocation of channels per cell. We show that this tradeoff results in a better utilization of network resources. An important feature of our sharing scheme is that channel management is localized between adjacent cells, and no global coordination or optimization is required, thus making it suitable for implementation. The sharing scheme can also facilitate handoff processing. We provide numerical results comparing our scheme with the channel reservation technique, and find a significant performance improvement over a wide range of traffic parameters and a variety of quality of service requirements.

We present a new scheme that addresses the call handoff problem in mobile cellular networks. Efficiently solving the handoff problem is important for guaranteeing quality of service to already admitted calls in the network. Our scheme is based on a new approach called channel carrying: when a mobile user moves from one cell to another, render certain mobility conditions, the user is allowed to carry its current channel into the new cell. We propose a new channel assignment scheme to ensure that this movement of channels will not lead to any extra co-channel interference or channel locking. In our scheme, the mobility of channels relies entirely on localized information, and no global coordination is required. Therefore, the scheme is simple and easy to implement. We further develop a hybrid channel carrying scheme that allows us to maximize performance under various constraints.

We present a novel reduced-power channel reuse scheme to improve the spectrum efficiency in wireless packet cellular networks. The basic idea is to reduce intercellular interference and improve the capture probability by an a priori assignment of power levels of channels used in different cells. We formulate and solve an optimal channel-selection problem for our scheme. We find that the optimal policy is in a form of bang-bang control. We illustrate our channel-selection solution by a case study with uniform fairness constraint. We evaluate, via numerical analysis and simulation, both throughput and delay of the new scheme, and compare them with other schemes. We find that our scheme can achieve significant performance improvements, in terms of both the maximum throughput and throughput-delay tradeoff, over a wide range of capture ratio values.

Enhancing system capacity is an important issue in wireless cellular networks. In this paper, we present a new channel sharing scheme to address this problem. Our basic idea is to allow channels to be shared between adjacent cells. We propose a fixed channel assignment scheme to maximize channel reuse efficiency while allowing channel sharing. An important feature of our sharing scheme is that channel management is localized between adjacent cells, and no global coordination or optimization is required thus simplifying implementation. We provide simulation results comparing our scheme with the fixed channel assignment scheme.

We present a new scheme that addresses the call handoff problem in mobile cellular networks. Efficiently solving the handoff problem is important for guaranteeing quality of service (QoS) to already admitted calls in the network. Our scheme is based on a new concept called channel carrying: when a mobile user moves from one cell to another, under certain mobility conditions, the user is allowed to carry its current channel. We propose a new channel assignment scheme to ensure that this movement of channels will not lead to any extra co-channel interference or channel locking. In our scheme, the mobility of the channels relies entirely on localized information, and no global coordination is required. Therefore, the scheme is simple and easy to implement. We further develop a hybrid channel carrying scheme that allows us to maximize the performance under various constraints. We provide numerical results comparing our scheme with the traditional channel reservation techniques. We find that our scheme outperforms the reservation scheme over a broad range of traffic parameters.