Spatial Throughput Research Articles

Performance Analysis and Optimization of UAV Integrated Terrestrial Cellular Network

Unmanned aerial vehicles (UAVs) have been extensively applied as aerial access points to assist the terrestrial wireless network. Despite the inherent potential, nevertheless, it still remains to explore whether the gain of UAV access points (UAPs) could be fully harvested, which is critically dependent on the factors including the flight altitude and deployment density of UAPs. In this light, we investigate the performance of a downlink UAV integrated terrestrial cellular network (UTCN) and analytically study the influence of varying UAP altitude and density on the spatial throughput (ST) of UTCN. In particular, we obtain the UAP altitude upper bound, below which more line-of-sight (LOS) connections could be provided to improve network ST. Otherwise, cross-layer interference over the LOS paths becomes dominant, which results in significant degradation of network ST. More importantly, we reveal the limitation of the application of UAPs by showing that there exists a critical UAP density, beyond which network ST would encounter a rapid decrease. To fully exploit the potential of UAPs, we further tailor a probabilistic interference avoidance scheme and study the optimization of the UAP activated probability. Remarkably, network ST could be substantially improved using the optimized activated probability, i.e., network ST could increase with the growing UAP density and converge to a positive constant instead of zero in the dense UAP regime. Therefore, the results of this paper could provide insight on the deployment and optimization of UTCN.

Delay Analysis of Spatially Coded MIMO-ZFBF With Retransmissions in Random Networks

For a low-mobile Poisson bipolar network and under line-of-sight/non-line-of-sight (LOS/NLOS) path-loss model, we study repetitive retransmissions (RR) and blocked incremental redundancy (B-IR). We consider spatially-coded multiple-input multiple-output (MIMO) zero-forcing beamforming (ZFBF) multiplexing system, whereby the packet success reception is determined based on the aggregate data rate across spatial dimensions of the MIMO system. Characterization of retransmission performance in this low-mobile configuration is practically important, but inherently complex due to a substantial rate correlation across retransmissions and intractability of evaluating the probability density function of aggregate data rate. Adopting tools of stochastic geometry, we first characterize the rate correlation coefficient (RCC) for both schemes. Our results show that, compared to the RR scheme, the B-IR scheme has higher RCC while its coverage probability is substantially larger. We demonstrate that the spotted contention between coverage probability and RCC causes the mean transmission delay (MTD) of B-IR to become either smaller or larger than the MTD of the RR scheme. Finally, we develop a numerical approximation of MTD, and evaluate the effective spatial throughput (EST), which is reciprocal to MTD, of the RR and B-IR schemes. Our numerical results highlight fundamental tradeoffs between densification, multiplexing gain, block length, and activity factor of nodes. We further observe that for dense networks: first, LOS component is considerably instrumental to enhance EST; second, EST of the B-IR scheme can be much higher than that of the RR scheme; third, when Doppler spread exists, it can improve MTD of B-IR, whereas it does not cast any meaningful effect on MTD of RR.

Spatial Throughput Analysis and Transmission Strategy Design in Energy Harvesting Cognitive Radio Networks

In this paper, we consider an energy harvesting cognitive radio network where each secondary transmitter (ST) harvests radio frequency energy from ambient primary transmitters (PTs), and communicates with its secondary receiver (SR) which suffers co-channel interference from PTs. A positive correlation is observed between the harvested energy at the ST and the aggregate interference at the SR, which illustrates that an ST will have a higher probability to harvest enough energy if there is strong interference at the SR. To exploit the positive correlation, we propose an interference threshold-based transmission strategy for STs, so as to protect secondary transmissions as well as increase the amount of harvested energy. We model the battery level of each ST as a finite state discrete-time Markov chain and derive the expression of the secondary spatial throughput as a function of the transmission probability and coverage probability. We investigate the impact of interference threshold and density of PTs on the secondary spatial throughput, and provide guidelines in the optimal design of these two factors to maximize the spatial throughput. To further improve the spatial throughput of a secondary network, we consider the successive interference cancellation strategy at SRs, and reveal its superiority in the high density regime of PTs.

Performance of Small Cell Networks Under Multislope Bounded Pathloss Model: From Sparse to Ultradense Deployment

Network densification is proved to be the most efficient method to expand network capacity over the past few decades. Nevertheless, can network densification always boost network performance? To shed light on this, we explore the pros and cons of network densification in $k$ -dimension downlink small cell networks by analyzing transmission success probability (SP) and network spatial throughput (ST). Remarkably, different from long-distance propagation in sparse networks, short-distance propagation of distinctive features dominates as well under dense deployment of small cells. On this account, we apply a multislope bounded pathloss model (MBPM) to characterize signal attenuation within long and short propagation distance. Under MBPM, it is surprisingly found that network densification would enhance SP and ST only when small cell base station density $\lambda \left(\mathrm{SBS}/\mathrm{m}^{k}\right)$ is smaller than some critical densities. Beyond the critical densities, both SP and ST are proved to exponentially diminish with $\lambda$ and even approach zero when $\lambda$ is sufficiently large. In other words, ultradense deployment of small cells indeed deteriorates network performance. Besides, the numerically derived critical densities are helpful for the practical deployment of small cell networks.

Limitation of SDMA in Ultra-Dense Small Cell Networks

Benefitting from multi-user gain brought by multi-antenna techniques, space division multiple access (SDMA) is capable of significantly enhancing spatial throughput (ST) in wireless networks. Nevertheless, we show in this letter that, even when SDMA is applied, ST would diminish to be zero in ultra-dense networks (UDN), where small cell base stations are fully densified. More importantly, we compare the performance of SDMA, single-user beamforming (SU-BF) (one user is served in each cell) and full SDMA (the number of served users equals the number of equipped antennas). Surprisingly, it is shown that SU-BF achieves the highest ST and critical density, beyond which ST starts to degrade, in UDN. The results in this letter could shed light on the fundamental limitation of SDMA in UDN.

Mean Delay Analysis of MIMO-ZFBF Multiplexing in Random Networks Under LOS/NLOS Path-Loss Model

We analyze the performance of a multiple-input multiple-output multiplexing system in a Poisson bipolar network under line-of-sight/non-line-of-sight (LOS/NLOS) path-loss model and zero-forcing beamforming at receivers. The capacity and outage performance of such a configuration, commonly under the standard path-loss model, have been broadly analyzed; yet little is known about its local transmission delay with considering the traits of LOS/NLOS model. As the effective fading power gain on each data stream is Nakagami-type, and due to the interference correlation across data streams of a link as well as the retransmission attempts, the evaluation of the mean delay is more involved than the capacity/coverage evaluation. Our rigorous analysis provides a lower bound and an approximate upper bound on the mean delay as the functions of density, multiplexing gain, transmission activity, and LOS/NLOS model, which sheds some light on the effect of the LOS component in circumventing the possible divergence of the mean delay. Simulations show the lower bound is very accurate and demonstrate several aspects of multiplexing, path-loss traits, and interference correlation on the mean delay. Exploiting the analysis, we further explore the optimization of effective spatial throughput of the network.

Geometry Distance Estimated MAC Protocol for TV White Space

Regulatory bodies worldwide have quite recently approved the dynamic access of unlicensed networks to the TV white space (TVWS) spectrum. In TVWS, secondary users (SUs) are required to access a database periodically to acquire information on the spectrum usage of primary users (PUs). The spectrum database querying of SUs will solve the spectrum scarcity problem. A key technology in efficiently using the TVWS spectrum is designing an efficient medium access control (MAC) protocol. In this paper, we propose a geometry distance estimated medium access control (GMAC) protocol to mitigate the hidden and exposed terminal problems in TVWS. GMAC provides an efficient means of channel database querying to obtain the list of available channels to overcome interference with PUs. We develop a Markov chain model to characterize the saturation normalized throughput of our proposed GMAC protocol for the TVWS. In this study, transmitter power control is based on the geometrically estimated distance between SU communication pairs; this increases channel spatial reuse and throughput, and reduces PU outage probability. We also compare our proposed scheme with existing MAC protocols for TVWS. We show that the GMAC will improve the channel spatial reuse and normalized throughput, and reduce PU outage probability.

UAV-Aided Offloading for Cellular Hotspot

In conventional terrestrial cellular networks, mobile terminals (MTs) at the cell edge often pose a performance bottleneck due to their long distances from the serving ground base station (GBS), especially in hotspot period when the GBS is heavily loaded. This paper proposes a new hybrid network architecture by leveraging the use of unmanned aerial vehicle (UAV) as an aerial mobile base station, which flies cyclically along the cell edge to offload data traffic for cell-edge MTs. We aim to maximize the minimum throughput of all MTs by jointly optimizing the UAV's trajectory, bandwidth allocation and user partitioning. We first consider orthogonal spectrum sharing between the UAV and GBS, and then extend to spectrum reuse where the total bandwidth is shared by both the GBS and UAV with their mutual interference effectively avoided. Numerical results show that the proposed hybrid network with optimized spectrum sharing and cyclical multiple access design significantly improves the spatial throughput over the conventional GBS-only network; while the spectrum reuse scheme provides further throughput gains at the cost of slightly higher complexity for interference control. Moreover, compared to the conventional small-cell offloading scheme, the proposed UAV offloading scheme is shown to outperform in terms of throughput, besides saving the infrastructure cost.

Wireless Powered Asynchronous Backscatter Networks With Sporadic Short Packets: Performance Analysis and Optimization

In the fifth generation era, the pervasive applications of Internet of Things and massive machine-type communications have initiated increasing research interests on the backscatter wireless powered communication (B-WPC) technique due to its ultrahigh energy efficiency and low cost. The ubiquitous B-WPC network is characterized by nodes with dynamic spatial positions and sporadic short packets, of which the performance has not been fully investigated. In this paper, we give a comprehensive analysis of a multiantenna B-WPC network with sporadic short packets under a stochastic geometry framework. By exploiting a time-space Poisson point process model, the behavior of the network is well captured in a decentralized and asynchronous transmission way. We then analyze the energy and information outage performance in the energy harvest and backscatter modulation phases of the backscatter network, respectively. The optimal transmission slot length and division are obtained by maximizing the network-wide spatial throughput. Moreover, we find an interesting result that there exists the optimal tradeoff between the durations of the energy harvest and backscatter modulation phases for spatial throughput maximization. Numerical results are demonstrated to verify our analytical findings and show that this tradeoff region gets shrunk when the outage constraints become more stringent.

Cognitive Radio Networks With Primary Receiver Assisted Interference Avoidance Protocol

In this paper, a novel opportunistic spectrum access protocol, namely the primary receiver assisted interference avoidance (PRA-IA) protocol, is proposed and analyzed in cognitive radio (CR) networks to simultaneously exploit the underutilized positions of the primary network and avoid transmission collisions among secondary transmitters (STs). Particularly, the proposed PRA-IA protocol is comprised of two processing phases, i.e., the qualification phase and the contention phase. The qualification phase is designed to preselect the set of STs (denoted by eligible STs) which are in the “spatial holes” of the active primary receivers to guarantee the primary transmissions. The contention phase, on the other hand, aims to improve the performance of secondary transmissions by further resolving the potential collisions among the eligible STs based on the randomly generated backoff timer. With mathematical tools from stochastic geometry, the transmission probability of active STs, the coverage probability, and thereby the spatial throughput of the CR network under the PRA-IA protocol are characterized and analyzed. Furthermore, simulations are provided to verify the accuracy of the derived analytical results and demonstrate the impacts of key network parameters on network performance. From the numerical results, it is shown that the proposed PRA-IA protocol is superior to the PRA protocol on the spatial throughput tradeoff of the primary and secondary networks.

Fog-Aided Cognitive Radio Networks With Guard Zone and Interference Cancellation Based Opportunistic Spectrum Access

We investigate the spatial spectrum sharing mechanism design in fog-aided cognitive radio (CR) networks with the methods of guard zone and interference cancellation. Particularly, it is assumed that the studied CR network is comprised of a library of multimedia files $\mathcal {F}$ with size $F$ and two types of users, namely the primary and secondary users, with fog computing capability. The primary users are authorized devices and have privilege to preferentially utilize the bandwidth resources. The secondary users, on the other hand, are unauthorized devices and can access the spectrum only if the primary transmissions are unaffected. To constrain the unintended secondary interference at the active primary receivers (PRs) while enhance the spectrum accessibility of the secondary users, the guard zone and interference cancellation-based opportunistic spectrum access (GZ-IC OSA) protocol is proposed and analyzed. Under the GZ-IC OSA protocol, by exploiting the cached content as side information, the $f$ -th file sent by a secondary transmitter (ST) can be effectively cancelled at the unintended active PRs which cache the same file. As such, under the GZ-IC OSA protocol, a ST is allowed to transmit the $f$ -th file in $\mathcal {F}$ as long as it is outside the union of the guard zones of all active PRs which cache other files except the $f$ -th file. Assuming decentralized probabilistic caching, under the GZ-IC OSA protocol, we derive the first-order moment measure of the access probabilities of primary transmitters (PTs) and STs, respectively. Further, we capture the conditional spatial distributions of active PTs and STs as piecewise functions with respect to the radius $R_{d}$ of the guard zone. Finally, we evaluate the performance of the proposed GZ-IC OSA protocol in terms of coverage performance and spatial throughput based on the derived conditional spatial distributions.

Closed-Form Approximations for Coverage Probability of Multistream MIMO-ZFBF Receivers in HetNets

The evaluation of the coverage probability of multistream multiple-input multiple-output (MIMO) communications in HetNets subject to noise, fading, and intercell interference is undoubtedly intricate. Unfortunately, the current literature misses its comprehensive evaluation, as the effects of noise and cross-stream correlation are often overlooked. Furthermore, computationally friendly expressions of the coverage probability allowing engineering insights and adaptive system design are lacking. For multistream MIMO zero-forcing beamforming, in this paper we tackle these issues by considering scenarios where a receiver is in the coverage if all of its data-streams are successfully decoded. Assuming the max signal-to-interference-plus-noise ratio (SINR) cell association (CA), we adopt the stochastic geometry tools to provide an upper bound and an easy-to-compute closed-form lower bound on the coverage probability, while their accuracies are confirmed against extensive simulations. Our contributions are as follows. We prove that full correlation of data streams of a given link slightly reduces the coverage performance. We show that from a coverage probability perspective, the single stream communication is preferable. We exploit our analysis to explore several pertinent design issues, which have not fully discussed in the literature. Our results demonstrate tradeoffs between densification and multiplexing gains. We, further, see that by appropriately designating feedback channel with modest capacity 8 bits per frame per user, the spatial throughput grows by nearly 180 $\%$ over the conventional 1-bit feedback scenario. Finally, We present important extensions of our analysis to underlay spectrum sharing, practical aspects of the max-SINR CA, and a nonhomogeneous path loss environment.

Optimization of Load Balancing in Hybrid LiFi/RF Networks

Light fidelity (LiFi) uses light emitting diodes (LEDs) for high-speed wireless communications. Since an LED lamp covers a small area, a LiFi system with multiple access points (APs) can offer a significantly high spatial throughput. However, the spatial distribution of data rates achieved by LiFi fluctuates because users experience inter-cell interference from neighboring LiFi APs. In order to guarantee a quality of service (QoS) for all users in the network, an RF network is considered as an additional wireless networking layer. This hybrid LiFi/RF network enables users with low levels of optical signals to achieve the desired QoS by migrating to the RF network. With regard to moving users, the hybrid LiFi/RF system dynamically allocates either a LiFi AP or an RF AP to users based on their channel state information. In this paper, a dynamic load balancing scheme is proposed, which considers the handover overhead in order to improve the overall system throughput. Joint optimization algorithm (JOA) and separate optimization algorithm (SOA), which jointly and separately optimize the AP assignment and resource allocation, respectively, are proposed. Simulation results show that SOA can offer a better performance/complexity tradeoff than JOA for system load balancing.

Improvement of an octave bandwidth bowtie antenna design based on the analysis of a mimo efficiency metric in Random-Los

A multiple-input multiple-output (MIMO) efficiency metric based on the probability of detection (PoD) of data bitstreams is used to improve the design of a self-grounded dual-polarized wideband bowtie antenna over an octave bandwidth. The analysis is based on the ideal digital threshold receiver model for the Zero-Forcing algorithm. The random line-of-sight (Random-LOS) channel model with two orthogonal linearly polarized waves coming from the same angle-of-arrival is considered in the evaluation. The MIMO efficiency performance is presented along with the reflection coefficient, the directivity and the radiation patterns of the antennas. Corresponding spatial throughput coverage patterns based on 2-bitstream MIMO efficiency evaluated at the 95% PoD level are also provided. The loss of orthogonality as well as the power imbalance of the 2-bitstreams has been identified as the main reasons to MIMO efficiency performance degradation in Random-LOS.

An Analysis Framework for Interuser Interference in IEEE 802.15.6 Body Sensor Networks: A Stochastic Geometry Approach

Interuser interference occurs when multiple body sensor networks (BSNs) are transmitting simultaneously in close proximity to each other. Interference analysis in BSNs is challenging due to the hybrid medium access control (MAC) and the specific channel characteristics of BSNs. This paper presents a stochastic geometry analysis framework for interuser interference in IEEE 802.15.6 BSNs. An extended Matern point process is proposed to model the complex spatial distribution of the interfering BSNs caused by the hybrid MAC defined in IEEE 802.15.6. We employ a stochastic geometry approach to evaluate the performance of BSNs, considering the specific channel characteristics of BSNs near the human body. Performance metrics are derived in terms of outage probability and spatial throughput in the presence of interuser interference. We conduct performance evaluation through extensive simulations and show that the simulation results fit well with the analytic results. Insights are provided on the determination of the interference detection range, the BSN density, and the design of MAC for BSNs.

Cost-Efficient Optimization of Base Station Densities for Multitier Heterogeneous Cellular Networks

Heterogeneous cellular networks have emerged as the primary solution for overwhelming traffic demands. This study addresses the architecture optimization of multitier open-access downlink heterogeneous cellular networks. A distinguishing feature of this work is that the transmission requirements of mobile users and the profit requirements of operators are all considered. Spatial throughput is maximized under practical constrains on the network deployment cost and traffic loads of individual tiers. By using stochastic geometry, the problems of optimizing the densities of base stations (BSs) in different tiers are shown to be linear-fractional programming that can be further transformed into linear programming by employing Charnes–Cooper transformation. The linear-fractional programming can then be solved efficiently. For a two-tier network, the optimal BS densities are derived in closed form by using the graphical method. Our results indicate that enlarging the densities of BSs may not always improve network throughput and that the optimal densities of each tier exist to satisfy the communication demands of mobile users and the profit requirements of operators. Numerical results demonstrate the significant throughput gain from the optimal deployment of multitier heterogeneous cellular networks.

Throughput and Fairness Analysis of Wi-Fi and LTE-U in Unlicensed Band

Shortage of available licensed spectrum is a major barrier to the development of 5G networks. The deployment of long term evolution (LTE) in unlicensed spectrum (LTE-U) is a promising solution to overcome such a barrier. However, the interaction between LTE-U and Wi-Fi in unlicensed spectrum has not been well understood. In this paper, we use stochastic geometry to develop a framework for a multi-radio access technologies (multi-RAT) heterogeneous network, which consists of an LTE-U tier and a Wi-Fi tier. To reduce the intra- and inter-RAT interference, LTE-U employs an ALOHA-like random access scheme and Wi-Fi performs carrier sensing and energy detection before transmission. We derive the coverage probability and spatial throughput of Wi-Fi and LTE-U networks, and perform the asymptotic analysis when the density of Wi-Fi and LTE-U nodes approach infinity. Based on our analysis, we investigate the effect of network parameters on the coverage probability and spatial throughput of these networks. Furthermore, in order to achieve weighted max-min fairness, we optimize the retention probability of LTE-U nodes to maximize the minimum weighted spatial throughput of Wi-Fi and LTE-U networks.

Study on Formation of Dislocation Contrast in 4H-SiC Wafer in Mirror Projection Electron Microscopy Image

A mirror projection electron microscopy (MPJ), non destructive, high spatial resolution and high throughput method, is useful for defect inspection in silicon carbide (SiC) wafer. Previously, it was demonstrate that three type of typical dislocations in 4H-SiC, threading screw dislocation (TSD), threading edge dislocation (TED) and basal plane dislocations (BPD) can be identified in MPJ image. Origin of the contrast of dislocations in MPJ image was revealed by observation of the same wafer at as-grown and after CMP processing. Streak of TSD spot is due to surface morphology, and the contrast of BPD isn’t due to surface morphology but due to charging on dislocation line.

Throughput analysis of cognitive wireless networks with Poisson distributed nodes based on location information

This paper provides a statistical characterization of the individual achievable rates in bits/s/Hz and the spatial throughput of bipolar Poisson wireless networks in bits/s/Hz/m2. We assume that all cognitive transmitters know the distance to their receiver’s closest interferers and use this side-information to autonomously tune their coding rates to avoid outage events for each spatial realization. Considering that the closest interferer approximates the aggregate interference of all transmitters treated as noise, we derive closed-form expressions for the probability density function of the achievable rates under two decoding rules: treating interference as noise, and jointly detecting the strongest interfering signals treating the others as noise. Based on these rules and the bipolar model, we approximate the expected maximum spatial throughput, showing the best performance of the latter decoding rule. These results are also compared to the reference scenario where the transmitters do not have cognitive ability, coding their messages at predetermined rates that are chosen to optimize the expected spatial throughput – regardless of particular realizations – which yields outages. We prove that, when the same decoding rule and network density are considered, the cognitive spatial throughput always outperforms the other option.

Spatial Throughput Maximization of Wireless Powered Communication Networks

Wireless charging is a promising way to power wireless nodes' transmissions. This paper considers new dual-function access points (APs) which are able to support the energy/information transmission to/from wireless nodes. We focus on a large-scale wireless powered communication network (WPCN), and use stochastic geometry to analyze the wireless nodes' performance tradeoff between energy harvesting and information transmission. We study two cases with battery-free and battery-deployed wireless nodes. For both cases, we consider a harvest-then-transmit protocol by partitioning each time frame into a downlink (DL) phase for energy transfer, and an uplink (UL) phase for information transfer. By jointly optimizing frame partition between the two phases and the wireless nodes' transmit power, we maximize the wireless nodes' spatial throughput subject to a successful information transmission probability constraint. For the battery-free case, we show that the wireless nodes prefer to choose small transmit power to obtain large transmission opportunity. For the battery-deployed case, we first study an ideal infinite-capacity battery scenario for wireless nodes, and show that the optimal charging design is not unique, due to the sufficient energy stored in the battery. We then extend to the practical finite-capacity battery scenario. Although the exact performance is difficult to be obtained analytically, it is shown to be upper and lower bounded by those in the infinite-capacity battery scenario and the battery-free case, respectively. Finally, we provide numerical results to corroborate our study.