Radio-frequency energy harvesting from ambient cellular energy and drone-based receivers close to the sensors can be effective tools to prolong the lifespan of wireless sensor networks (WSNs). Moreover, the energy management policy of a sensor plays a critical role in increasing the reliability of data transmission under severe energy constraints. Thus, in this paper, we develop an optimal transmission policy to reduce the outage such that a sensor decides on when to transmit and how much power to use given the uncertainties of the harvested energy. To this end, we model sensors with a finite-level battery and a buffer with discrete states, whereas the cellular base stations from which the energy is harvested are modeled according to a Poisson point process. The drone is assumed to be hovering at a fixed height above the field of sensors, and assume path loss and small scale fading with varying intensities depending on the line-of-sight nature of the link. An outage is assumed to occur due to incorrect reception of transmissions and buffer overflow. We formulate the problem as a Markov decision process, and utilize a Q-learning based algorithm to generate the optimal transmission policy. Our numerical results show that the proposed policy significantly outperforms the previously proposed policies under all conditions.

This paper investigates the throughput improvement of cell-free massive multiple-input multiple-output (MIMO) systems by non-orthogonal multiple access (NOMA) for future cellular networks under stochastic access point and user locations. In this context, the node locations are modeled with Poisson point processes. The time division duplexing mode is employed, and uplink channels are estimated locally using uplink pilots. Furthermore, unique pilot sequences are used between NOMA clusters, while pilot reuse occurs within each cluster to strike a balance between the training overhead and the number of clusters. Matched-filter-based precoding is utilized for downlink transmission. The aggregate received signal is analytically characterized by deriving the moment generating function and approximations via moment matching. Then, the asymptotic achievable rates of the NOMA users are derived, thereby quantifying the adverse impact of error propagation owing to imperfect successive interference cancellation. Special scenarios with prior downlink channel state information and log-distance power control are also considered. We show that NOMA greatly increases the achievable average rate, especially under low path loss exponents and dense networks, while user fairness may be boosted by the adoption of a log-distance transmit power control scheme with proper parameter selection (i.e. lower values for the power control parameter).

The next generations of cellular wireless systems promise an improved user experience with respect to the throughput, reliability, latency, and connectivity. To this end, multiple-input multiple-output (MIMO) and massive MIMO systems hold enormous potential. Moreover, due to heterogeneity and densification, user and base station locations are increasingly random. Thus, in this paper, we characterize the performance of a typical downlink user within a massive MIMO Voronoi cell when the base stations employ matched filter based precoding in the downlink. We consider a typical Voronoi cell where the base stations follow a Poisson point process (PPP), and also consider the users to be randomly distributed according to a PPP independent from the base stations. Furthermore, a wireless channel with log-distance path loss and Rayleigh fading is assumed. Using a novel framework to model the signal to noise ratio, the outage probability of a user is derived in closed-form while taking into account full and partial loading for the cell's base station due to the randomness of user numbers. Numerical results show that the outage probability is heavily dependent on the base station density, and that the performance decreases when the maximum number of users served by a base station increases.

Simultaneous wireless information and power transfer (SWIPT) is increasingly seen as a viable technique to power energy constrained user devices while transmitting data. SWIPT can be readily implemented within the recently proposed cell-free massive MIMO networks, where a large number of interconnected access points (APs) simultaneously serve users. This paper characterizes the coexistence and interplay between these two technologies by using tools from stochastic geometry. To this end, we consider a spatially random network, where the APs are modeled stochastically using a Poisson point process, and a time-switching protocol is used for the SWIPT operation at the users. A time-division-duplexing protocol is considered in which uplink pilots are used to obtain channel state information at the APs, while conjugate beamforming is performed in the downlink. Moreover, we consider blockages due to obstacles in the channel and the resulting line-of-sight and non-line-of-sight conditions affecting the fading and path loss. We derive the mean and variance of the harvested energy along with the average achievable rate in the downlink for an energy user. The tradeoff between the downlink data throughput and harvested energy is quantified, and thereby, we show that spatially-distributed APs in a cell-free arrangement can boost the energy-rate trade-off of SWIPT.

Non-orthogonal multiple access (NOMA) and millimeter wave communications are key technologies for the fifth-generation of cellular networks and beyond, and the coexistence of these two techniques is critical. This paper characterizes the system performance through the outage probability and achievable downlink rate for large-scale millimeter wave NOMA networks by using stochastic geometry. In order to reflect spatial randomness, we consider homogeneous Poisson point processes to model the base stations and user equipments. Moreover, blockages which affect the channel characteristics, power allocation based on the combined channel gain, and imperfections in the successive interference cancellation are considered. The aggregate co-channel interference at a user is characterized based on the moment generating function. Finally, the outage probability and downlink rate are derived for a two-user NOMA scenario under two user-base station association schemes: 1) closest base station association and 2) closest line-of-sight base station association. It is seen that using NOMA under millimeter wave channels increases the achievable downlink rate while keeping the performance impact on individual users low, and that the closest line-of-sight base station association scheme is comparatively advantageous. Moreover, a dense base station deployment generally improves the performance further.

Device-to-device (D2D) where users communicate directly with each other with limited base station involvement can significantly improve spectral efficiency, energy efficiency, and throughput in future cellular networks. Moreover, RF (radio frequency) energy harvesting (EH) promises to prolong the battery life and improve energy efficiency of D2D communication. Mobility diversity refers to the gains accrued via user mobility. Is it possible to exploit the mobility of D2D terminals via user movements to enhance their ability to harvest RF energy? To this end, we analyze the performance of a mobile D2D device powered by EH from the transmissions of underlying cellular base stations (BSs), whose locations are modeled as a homogeneous Poisson point process. We model the movements of D2D nodes via a modified random waypoint model. Log-distance path loss is considered, and it is assumed that EH takes place solely within harvesting zones surrounding each BS and that each D2D user requires a fixed number of charging time slots before being able to transmit. We derive the probability of a D2D device being within an EH region surrounding BSs after multiple transitions, and the probability of being within the the fully charged state using a Markov-chain approach taking into account temporal effects. It is shown that the number of transitions required to be within a harvesting region increases significantly when the harvesting threshold power increases.

Cell-free massive multiple-input multiple-output (MIMO) is a form of distributed massive MIMO aiming to provide massive access and improve spectral efficiency by inheriting favorable properties of traditional massive MIMO, while mitigating detrimental effects such as shadowing and spatially correlated fading. This paper investigates how the throughput of cell-free massive MIMO is affected by non-orthogonal multiple access (NOMA) for the next-generation cellular networks under stochastic access point (AP) and user locations. Thus, we consider homogeneous Poisson point processes (PPPs) to model node locations, while considering a Rayleigh channel with log-distance path loss. The time division duplexing (TDD) mode is employed and uplink channels are estimated autonomously/locally at each AP via uplink pilots sent by users. Moreover, while unique pilots are used between NOMA clusters, pilots are reused within each cluster in order to strike a balance between the training overhead and number of clusters. Matched filter based precoding is performed within the downlink based on the estimated channels. The aggregate signal received from all access points is characterized based on the moment generating function and approximated via moment matching. Thereby, the achievable rates for the users are derived, under the consideration of error propagation due to imperfect successive interference cancellation (SIC). It is shown that NOMA increases the overall rate under environments with low path loss exponents and networks with high access point densities, while careful power allocation can significantly improve user fairness.

Massive multiple-input multiple-output (MIMO) systems operating within the millimeter wave frequency range offer exciting opportunities for the future fifth-generation (5G) wireless networks. While the increased bandwidth and spectral efficiency are attractive, transitioning to millimeter massive MIMO presents significant challenges with respect to blockages, high attenuation, and channel estimation errors. To address these challenges, this paper evaluates the outage performance of a millimeter wave cellular network using massive MIMO under a stochastic set-up subject to pilot contamination and matched-filter precoding. We model the cellular users and base stations with Poisson point processes. Furthermore, we consider blockages from random objects, and employ different path loss and fading models for the line-of-sight (LOS) and non-line-of-sight scenarios. Moreover, both fixed power transmissions and path loss inversion-based power control are considered along with the sectored antenna patterns. Using stochastic geometry, we derive the moment generating function of the interference experienced by a typical cellular user and its outage probability. It is observed that the environments with different path loss exponents have varying behaviour for similar blockage sizes and densities. In addition, the ratio of the number of cellular users to that of base stations, and the antenna beamwidth are critical parameters affecting the outage performance.

Device-to-device (D2D) networks underlaying a millimeter-wave cellular network have great potential for capacity growth. Thus, it is important to characterize the outage of such a D2D link incorporating millimeter-wave propagation effects, user association rules, power control, and spatial randomness. To this end, we model the locations of cellular transmitters and receivers as homogeneous Poisson point processes and those of the D2D nodes as a Matérn cluster process, and incorporate blockages due to random objects, sectored antenna patterns, log-distance path loss, and Nakagami-m fading. Furthermore, we consider antenna gain inversion-based power control, and peak power constraints for D2D devices along with distinct path loss exponents and distinct fading severities for line-of-sight (LOS) and non-LOS scenarios. With the aid of stochastic geometry tools, we derive closed-form expressions of the moment generating function of the aggregate interference on a D2D receiver node and its outage probability for two transmitter-receiver association schemes- nearest association and LOS association. We finally show that the feasibility of millimeter-wave D2D communication relies heavily on the D2D cluster radii, peak power thresholds, and node densities. Furthermore, these parameters affect the performance of the desired link more than the interference and noise.

Massive multiple-input multiple-output (MIMO) enabled base stations employing non-orthogonal multiple access (NOMA) hold immense potential in increasing the spectral efficiency of future cellular networks. In this paper, we evaluate the achievable rate of N-user NOMA under a Poisson process of massive MIMO enabled base stations. We adopt a timedivision duplexing (TDD) mode, where the uplink pilots are reused among the different base stations, and within each NOMA cluster of each base station, while matched-filter based precoding is employed in the downlink. The achievable rate by a typical NOMA user is characterized by taking into account imperfect successive interference cancellation (SIC) and error propagation. To this end, the moment generating function of the interference from other base stations due to pilot contamination is derived along with the signal detection probability. It is shown that NOMA can significantly improve the rate performance under most system parameters, and that the rate performance can be increased further through denser networks. Moreover, we show that the individual user rates and fairness amongst users within a NOMA cluster are significantly impacted by the specific power allocation algorithm.

Non-orthogonal multiple access (NOMA) and millimeter wave communications are key technologies for the fifth generation (5G) of cellular networks in order to increase the spectral efficiency, ensure massive connectivity, and to provide large bandwidths to mobile internet services. However, the coexistence of these two techniques is critical, especially under increasingly random dense base station assignments. To this end, this paper characterizes the system coverage for users utilizing NOMA in a stochastic multi cell set-up under millimeter wave frequencies. We employ the Poisson point process (PPP) to model the base stations and user devices. Moreover, our system model considers the power allocation, imperfections in the successive interference cancellation (SIC), and blockages that affect the channel path loss and fading characteristics. The aggregate co-channel interference experienced at a user is characterized based on the moment generating function, and the outage probability is derived for a two user NOMA scenario. Analytical and numerical results show that NOMA is a feasible candidate technique to increase the spectral efficiency under millimeter wave channels if careful power allocation is implemented, with limited performance penalties for each user. It is also shown that the performance is further improved by utilizing a dense base station deployment.

Millimeter wave communication is a promising concept for the fifth generation (5G) of cellular wireless networks due to the large available bandwidth while device to device (D2D) communication among nearby devices which saves network resources is also gaining attention. As such, D2D networks underlaying millimeter wave cellular systems hold massive potential. However, the performance of such a D2D network incorporating spatial randomness and power control has not yet been characterized. To fill this knowledge gap, we develop a comprehensive analysis of the performance of a D2D receiver. To this end, we model cellular transmitters and receivers as homogeneous Poisson point processes and the D2D network as a Matern cluster process, and incorporate blockages due to random objects, sectored antenna patterns, log-distance path loss, and Nakagami-m fading. Furthermore, we consider path loss and antenna gain inversion based power control, and peak power constraints for D2D devices along with distinct path loss exponents and fading severities for line-of-sight and non-line-of-sight scenarios. With the aid of stochastic geometry tools, we derive closed-form expressions of the moment generating function of the aggregate interference experienced by a D2D receiver and its outage probability. We finally show that the feasibility of millimeter wave D2D communication relies heavily on the D2D cluster radii, peak power thresholds, and node densities.

Energy harvesting underlay device-to-device (D2D) networks are a promising solution to increase spectral and energy efficiency of wireless systems. However, to what extent is the performance of such networks affected by spatial randomness, temporal correlations, power control procedures, and channel uncertainties? To answer this question, we consider an environment with a multi channel primary user network whose nodes and D2D transmitters are spatially distributed as a homogeneous Poisson point process and the wireless signals are subject to log-distance path loss, Rayleigh fading, and path loss inversion based power control. We derive expressions for the ambient radio frequency power available for harvesting at a D2D transmitter, and approximate it using a Gamma distribution. Furthermore, we use a Markov chain model to derive the probability of a successful energy harvest for single slot and multi slot harvesting schemes, and derive the coverage performance of a D2D receiver when a D2D transmitter gets assigned to a sub-band randomly. It is concluded that a D2D receiver sensitivity between -120 dBm and -100 dBm is optimum for both single and multi-slot harvests, and that a higher primary transmitter density is detrimental to multi slot harvesting when the D2D transmitter-receiver distance increases.

Spectrum and energy constraints are fundamental barriers to the future growth of wireless communication networks, and to break this gridlock is the promise of energy harvesting cognitive radio (CR) networks. To this end, this paper investigates the feasibility of energy harvesting underlay CR networks with the primary system employing power control. We consider primary and underlay nodes distributed randomly in R2as homogeneous Poisson point processes (PPP). Underlay transmitters scavenge power from primary transmitters, and are able to transmit as long as they are outside a guard region surrounding a primary receiver. The primary and underlay systems are assumed to perform power control based on path loss inversion, and that a underlay transmitter requires N charging slots to fully charge its batteries after depletion. We consider two cases of power depletion after a underlay transmission: 1) full power depletion, and 2) partial power depletion based on distances to the intended receivers. We derive the probability of a successful charge by mapping the PPP of primary transmitters to an equivalent PPP incorporating random transmit powers, and use a Markov chain to derive the probability of a successful transmission while incorporating temporal effects for the two aforementioned power depletion scenarios. We show that the probability of successful transmission is not greatly affected by the guard distance, and that it drops by approximately 10 fold for each 15 dB increase in the threshold received power level required for an energy harvest. We further show that energy harvesting is most feasible when the threshold power required for a harvest is lower than the receiver sensitivity of a primary receiver.

While massive MIMO based underlay cognitive radio (CR) networks are a promising concept in the next generation of wireless networks to increase spectral efficiency, reusing the same pilot sequences in both networks cause pilot contamination leading to residual interference. Thus, this paper investigates the affects of pilot contamination on a random CR network underlaid upon a random primary network where both networks employ path loss inversion based power control. A Matern cluster process is considered for the underlay system, while homogeneous Poisson point processes are considered for the primary transmitters and receivers. We derive the moment generating function of the normalized aggregate interference at an underlay receiver, its first two moments, and the outage probability. Finally, it is shown that the underlay cluster radius, ensured received power levels through power control and the different node densities have a significant effect on the outage of an underlay receiver.

This paper investigates the effects of different cooperative sensing strategies on erroneous spectrum sensing for an interweave cognitive radio network. The setup is as follows. Primary receiver nodes and secondary nodes are randomly distributed in R2. We model them as two independent homogeneous Poisson point processes. Beacon (out-of-band) signals, periodically transmitted by primary receivers, indicate to the secondary nodes that spectrum is occupied. Whenever beacon detection fails, the transmissions of secondary nodes generate harmful interference. Thus, to alleviate this issue, the misdetection probability of secondary nodes must be reduced. To this end, we propose two strategies: 1) a secondary node cooperates with its closest neighbour, and 2) a secondary node cooperates with M random neighbours within a given radius. Furthermore, along with these co-operation strategies, we investigate three primary beacon detection methods for secondary nodes: 1) separately decoding each primary beacon, 2) detecting only the closest primary receiver?s beacon, and 3) detecting the aggregate beacon signal from all primary receivers. For the exponential path loss and Rayleigh fading considered, we derive the total misdetection probability for each scheme along with the resulting outage probability of a primary receiver. We finally show through numerical results that M co-operation works better for lower reception thresholds, and that for a reception threshold of -120 dBm, a 104fold decrease in the misdetection probability is achievable.

In an underlay cognitive radio network, the secondary (underlay) transmitters generate interference to a primary receiver, while an underlay receiver is subject to interference from both the primary transmitters and other underlay transmitters not associated with it. Although guard regions, maximum allowable underlay transmit powers, and contention distances help guarantee a minimum performance to the primary network, no such safeguard exists for the underlay network. To this end, this paper characterizes the aggregate interference on an underlay receiver while considering power control and receiver association schemes for both networks. Transmitters and receivers of both networks are assumed to be distributed as independent Poisson fields in the 2-D plane, and all links undergo exponential path loss and Rayleigh fading. We derive the moment generating function of the aggregate interference on an underlay receiver and its outage probability. We show that the interference from the primary network does not depend on any node density, and that it dominates the aggregate interference. Furthermore, it is shown that increasing primary and underlay receiver densities reduce the outage probability under lower required power thresholds for the primary receivers.

This paper analyzes how transmit power control affects the aggregate interference arising from a Poisson field of underlay cognitive radio (CR) transmitter nodes distributed in a finite area. We consider three per-user, location dependent power control schemes, when each CR transmitter is associated with the nearest CR receiver, where the CR receivers form a Poisson field in the entire 2-dimensional space. The 3 schemes are based on: 1) CR transmitter-receiver distance rc2) rcand a constant cut-off power level 3) rcand a random cut-off power level, respectively. For each of these, the exact moment generating function (MGF) and mean of aggregate interference power are derived. We also investigate the primary system outage due to aggregate interference. Rayleigh fading and exponential path loss links are assumed. Monte-Carlo simulation results validate our analysis and also show that the CR power thresholds and node densities significantly affect the aggregate interference.

This paper investigates the aggregate interference from interweave cognitive secondary nodes spatially distributed in a finite Poisson field. These secondary nodes sense an out-of-band beacon to initiate their transmissions, which can be concurrent with those of the primary system if a sensing error is made. The resulting aggregate interference is analyzed in this paper. For this purpose, general Nakagami-m fading and path-loss are assumed for all relevant channels. Moreover, we incorporate random secondary node transmit powers with any probability distribution. The analysis includes the exact moment generating function (MGF) of the aggregate interference along with the exact outage probability of the primary system. Furthermore, we develop a simple MGF approximation which is valid for severely fading channels and for lower beacon reception threshold to beacon transmit power ratios. Finally, we show that a lower fading severity significantly improves the diversity order of the PR due to more accurate spectrum sensing by the secondary nodes.

Erroneous beacon detection by interweave secondary nodes generates interference on the primary system. This paper analyzes how the aggregate interference behaves when secondary nodes use transmit power control and receiver association schemes. For this purpose, secondary transmitter nodes and receiver nodes are assumed to be distributed over a circular region and over the entire 2-D plane respectively. Two independent Poisson point processes model these distributions. We propose a receiver association scheme where each secondary transmitter attempts to connect to the closest available receiver. If it is not available, the transmitter attempts to connect with the next closest. This process continues until the M-th closest receivers are scanned. If no receiver is available, the transmitter remains silent. Moreover, a per-user transmit power control scheme is considered in which the transmit power is based on the distance between the transmitter and the associated receiver subject to a cut-off power level. All links are assumed to undergo path-loss and Rayleigh fading. The exact moment generating function (MGF) of the aggregate interference, the outage probability of the primary system, and the average probability of concurrent transmission are derived. Validated by simulations, our results show how the aggregate interference is affected by secondary power thresholds, receiver densities, and availability of the secondary receiver.