Homogeneous Cellular Networks Research Articles

Deep Reinforcement Learning for Distributed Dynamic MISO Downlink-Beamforming Coordination

We consider a homogeneous cellular network where a multi-antenna base station (BS) in each cell transmits messages to its intended user over a common frequency band. To improve the system capacity of this multi-cell multi-input single-output (MISO) interference channel, one of the state-of-the-art algorithms, namely, downlink-beamforming coordination, allows all BSs to cooperate with one another to mitigate the effect of inter-cell interference. However, most existing algorithms are suboptimal and impractical in a dynamic wireless environment, due to the high computational complexity and the overhead involved in collecting global channel state information (CSI). In this study, we exploit deep reinforcement learning (DRL) and propose a distributed dynamic downlink-beamforming coordination (DDBC) method with partial observability of the CSI. Each BS is able to train its own deep Q-network and employs appropriate beamformer depending on its environment, which is observed through a designed limited-information exchange protocol. The simulation results show that the proposed DRL-based DDBC method, with a considerably lower system overhead, achieves a system capacity that is very close to that of the fractional programming algorithm with global and instantaneous CSI measurements. In addition, this work demonstrates the potential of utilizing DRL to solve DDBC problems in a more practical manner.

Proactive Uplink Interference Management for Nonuniform Heterogeneous Cellular Networks

In homogeneous cellular networks, fractional power control (FPC) is employed to partially compensate the path-loss and, hence, improve uplink ( $U_{L} $ ) signal-to-interference ratio (SIR). However, this scheme is less effective in heterogeneous cellular networks (HetNets) because: (i) except the typical user, all other users with variable $U_{L} $ transmit power (UTP) act as interferers, (ii) FPC leads to high UTP by edge users and, hence, more interference, and (iii) small base stations (SBSs)’ densification further increases network interferences. Leveraging FPC in HetNets, we propose nonuniform SBS deployment ( $\text {NU-SBS}_{\mathcal {D}} $ ) to reduce interference and, thus, increase network performance. According to our $\text {NU-SBS}_{\mathcal {D}} $ model, SBS deployment ( $\text {SBS}_{\mathcal {D}} $ ) near macro base station (MBS) is avoided, whereas MBS coverage edge area is enriched with ultra-dense $\text {SBS}_{\mathcal {D}} $ . $\text {NU-SBS}_{\mathcal {D}} $ model leads to: (i) better SIR reception of MBS coverage edge users, (ii) fewer $\text {SBS}_{\mathcal {D}} $ requirement, and (iii) better SBS coverage in the MBS coverage edge area. Moreover, to make a model more proactive, we also consider reverse frequency allocation (RFA) to further abate both $U_{L} $ and downlink $(D_{L}) $ interferences. The coverage probability expressions are derived for both uniform SBS deployment ( $\text {U-SBS}_{\mathcal {D}} $ ) and $\text {NU-SBS}_{\mathcal {D}} $ while using RFA and FPC. Through simulation and numerical results, we characterize coverage probability for different values of SIR threshold, path loss compensation factor, SBS density, users density, and the distance between the typical user and the associated base station. The proposed $\text {NU-SBS}_{\mathcal {D}} $ model along with RFA leads to reduced network interference as compared with $\text {U-SBS}_{\mathcal {D}} $ and, thus, leverages FPC in HetNets.

Effect of intercell interference on cell boundary users in three‐cell and seven‐cell HetCN

SummaryIndoor and cell‐edge coverage has been a major issue of concern for predeployed traditional macrocell (MC)–based homogeneous cellular network. Moreover, with the extensive increase of mobile users and developments of smart and highly specified devices, user demands and activities have led to huge cellular traffic. The key solutions to these that include network upgradation, overlaying of small cells (SCs), and scaling of resources have turned out to be the major causes for intercell interference (ICI) and energy‐efficiency degradation in heterogeneous cellular networks (HetCNs). In this paper, authors have tried to analyze the downlink performance metrics of cell boundary users with MCs overlaying SCs for three‐cell circular and seven‐cell sectorized networks through frequency reuse (FR) schemes. This paper also discusses the impact of ICI being encountered by users and the effect of SCs on the energy efficiency of the network. The locations for SCs are perceived where user density is large and demands high data rate such as at hot‐spot (HS) areas, railway stations, shopping malls, working farms, and organization. The performance metrics sum rate, average user throughput, and energy efficiency are compared by employing FR‐1 (full spectrum) and FR‐3 (three subbands) among MCs and deployed SCs. For both scenarios, simulation results and analyses depict that without SCs, utilization of FR‐1 results in performance degradation due to ICI effects. However, the downlink performance of cell boundary user and energy efficiency of the network could be enhanced by overlaying SCs near cell boundaries of preexisting MCs along with the allocation of FR‐1.

Systems Analysis of a Pattern Reconfigurable Antenna for Capacity Improvement of Cell Edge Users in Cellular Networks

An evaluation of a beam reconfigurable base station transceiver for cellular applications is presented from both systems and antenna design perspective. The proposed technique uses an azimuth beam switching method, which incorporates PIN diodes to provide a fast switching reflecting ground plane for a three sector antenna. Numerical systems analysis has been carried out on a hexagonal homogeneous cellular network to evaluate how the reconfigurable antenna can be used to tradeoff the mean and cell edge capacity by reconfiguring the azimuth beamwidth. The systems analysis shows that a mean cell edge improvement from 15 to 18 Mbit/s is achievable and this improvement is consistent for cell sizes ranging from 500 to 1500 m. The proposed reconfigurable antenna has been designed, manufactured, and characterized, and the measured results are shown to be similar to simulations.

Hybrid blind interference alignment in homogeneous cellular networks with limited coherence time

SummaryThe work presented in this paper is inspired by the user grouping approach of topological blind interference alignment (top‐BIA), which is a semi‐blind IA scheme and the fact that the partially connected networks can be advantageous in terms of degree of freedom (DoF) and sum rate. The hybrid‐BIA scheme proposed in the paper uses top‐BIA to group users that are randomly distributed in a dense small‐cell network and aims to reduce the supersymbol length and overcome the DoF loss of the state‐of‐the‐art hierarchical BIA (h‐BIA) technique. The proposed scheme is very suitable for channels where coherence time is limited and also could attain a good DoF over a small number of symbol extensions. Both the sum‐DoF and the network throughput for hybrid‐BIA is greater than that of h‐BIA. By varying the number of user groups, the paper shows that h‐BIA constitutes a special case of hybrid‐BIA. Finally, the paper demonstrates the effect of changing the number of transmit antennas and number of small cells on the sum‐DoF gain of hybrid‐BIA over that of h‐BIA.

User Association and Resource Allocation Optimization in LTE Cellular Networks

As the demand for higher data rates is growing exponentially, homogeneous cellular networks have been facing limitations when handling data traffic. These limitations are related to the available spectrum and the capacity of the network. Heterogeneous networks (HetNets), composed of macro cells (MCs) and small cells (SCs), are seen as the key solution to improve spectral efficiency per unit area and to eliminate coverage holes. Due to the large imbalance in transmit power between MCs and SCs in HetNets, intelligent user association (UA) is required to perform load balancing and to favor some SCs attraction against MCs. As long term evolution (LTE) cellular networks use the same frequency sub-bands, user equipment may experience strong intercell interference (ICI), especially at cell edge. Therefore, there is a need to coordinate the resource allocation (RA) among the cells and to minimize the ICI. In this paper, we propose a generic algorithm to optimize UA and RA in LTE networks. Our solution, based on game theory, permits to compute cell individual offset and a pattern of power transmission over frequency and time domain for each cell. Simulation results show significant benefits in the average throughput and also cell edge user throughput of 40% and 55% gains respectively. Furthermore, we also obtain a meaningful improvement in energy efficiency.

A novel and realistic hybrid downlink-uplink coupled/decoupled access scheme for 5G HetNets

Cell association in present day heterogeneous networks (HetNets) is still based on the technique used by homogeneous cellular networks despite power and coverage area disparities of network nodes. In ongoing policy, both uplink (UL) and downlink (DL) associations are coupled based on DL characteristics, which introduces UL-DL asymmetry and cell load imbalances. Recently, decoupled cell association, also known as downlink-uplink decoupling (DUDe), has been introduced in 3rd Generation Partnership Project (3GPP) Release 12 to improve uplink performance, load balancing, and cell capacity. In DUDe, characteristics of both DL and UL channels can be considered. By using this concept, various theoretical UL or DL analytical decoupled access models have been proposed without giving their practical realization. In these frameworks, all network users are assumed to use DL-UL decoupled access without considering its practical utility. Existing solutions also ignore noise, which may lead to practical inaccuracies. This paper proposes a novel and realistic hybrid scheme in which coupled or decoupled cell associations can be selected depending upon user location and its advantages. Building upon this innovative approach, it has been established that decoupled access is chosen by few users and accordingly a two-tier analysis framework for these devices has been formulated. Simple closed-form solutions for user performance metrics without ignoring noise have been precisely derived, which relate user performance with HetNet densities. Devised distributions are employed to compare the performance of the decoupled case with the ongoing procedure of coupled access. A practical network design has been proposed, which requires minimum changes to the existing network. Results show that decoupling is viable in the 5G HetNet to achieve fairness, cell load balancing, and performance improvements.

Macro-pico amplitude-space sharing with layered interference alignment

Inter-cell interference leads to severe performance degradation in cellular networks, and the study of multi-user interference channel is the corner stone for solving this problem. Amplitude-space layered interference alignment (IA), as an effective complementation to the vector-space IA, is a promising method to increase the data rate in static interference channels. However, recent studies of layered IA has been focused on analyzing the degrees of freedom (DoF) or the achievable rate under specific channel constraints. In this paper, we propose a layered IA scheme that can work with arbitrary channel coefficients. We develop a layer partitioning method and optimize the active layer assignment through linear programming. An implementation scheme is then introduced with multi-level nested lattice codes where the signal and interference are nested in amplitude space, and the interference from different users is nestedly aligned. The performance of the proposed scheme is finally evaluated in homogeneous and heterogeneous cellular networks with practical settings.

Optimal Combination of Base Station Densities for Energy-Efficient Two-Tier Heterogeneous Cellular Networks

In this paper, the optimal BS (Base Station) density for both homogeneous and heterogeneous cellular networks to minimize network energy cost is analyzed with stochastic geometry theory. For homogeneous cellular networks, both upper and lower bounds of the optimal BS density are derived. For heterogeneous cellular networks, our analysis reveals the best type of BSs to be deployed for capacity extension, or to be switched off for energy saving. Specifically, if the ratio between the micro BS cost and the macro BS cost is lower than a threshold, which is a function of path loss and their transmit power, then the optimal strategy is to deploy micro BSs for capacity extension or to switch off macro BSs (if possible) for energy saving with higher priority. Otherwise, the optimal strategy is the opposite. The optimal combination of macro and micro BS densities can be calculated numerically through our analysis, or alternatively be conservatively approximated with a closed-form solution. Based on the parameters from EARTH, numerical results show that in the dense urban scenario, compared to the traditional macro-only homogeneous cellular network with no BS sleeping, deploying micro BSs can reduce about 40% of the total energy cost, and further reduce up to 35% with BS sleeping capability.

On the Tradeoff Between Spectral Efficiency and Energy Efficiency of Homogeneous Cellular Networks With Outage Constraint

In this paper, the tradeoff relationship between the spectral efficiency (SE) and energy efficiency (EE) of homogenous cellular networks in which the BSs are arbitrarily distributed is investigated. The network performance metrics of SE and EE are assessed subject to a downlink transmission outage constraint in interference-limited operational environments. The EE is expressed in closed form as a function of SE, based on which the performance bounds of the network are derived. Unlike the traditional inverse relationship between SE and EE, it is found in this paper that there exists an operational regime for which both the SE and EE increase while satisfying the outage requirement, and the density of base stations (BSs) simultaneously sharing the spectrum is optimal. The difference in the performance achieved for the SE when operating in the EE maximizing mode as compared with the SE maximizing mode strongly depends on the received signal-to-interference ratio (SIR) threshold. In the SE-EE tradeoff regime, the analytical tools from microeconomics theory are applied to determine the optimal BS density with respect to the utility achieved by the network operator via balancing the SE and EE objectives. Numerical results show that, by tuning a preference factor toward either the SE or EE metrics, it is feasible to realize Pareto optimal performance.

Mobility Modeling and Performance Evaluation of Heterogeneous Wireless Networks

The future-generation wireless systems will combine heterogeneous wireless access technologies to provide mobile users with seamless access to a diverse set of applications and services. The heterogeneity in this inter-technology roaming paradigm magnifies the mobility impact on system performance and user perceived service quality, necessitating novel mobility modeling and analysis approaches for performance evaluation. In this paper, we present and compare three mobility models in two-tier integrated heterogeneous wireless systems, the independence model as a naive extension of the traditional cell residence time modeling techniques for homogeneous cellular networks, the basic Coxian model which takes into consideration the correlation between the residence time within different access technologies, and the extended-Coxian model for further improved estimation accuracy. We propose a general stochastic performance analysis framework based on application session models derived from these mobility models, applying it to a 3G-WLAN integrated system as an example. Our numerical and simulation results demonstrate the general superiority of Coxian-based mobility modeling over the independence model. Furthermore, using the proposed modeling and analysis methods, we investigate the impact of different parameters on system performance metrics such as network utilization time, handoff rates, and forced termination probability, for a wide range of user applications.

Modeling Concurrent Search in All-IP Heterogeneous Wireless Networks

In this paper, we propose a novel probabilistic method for evaluating concurrent search in all-Internet protocol (IP) networks. It is expected that the next-generation Internet will consist of a variety of wireless-access networks. To be independent of wireless-access technology and to reduce the cost of mobility management in a heterogeneous environment, we propose integrating IP Paging with concurrent search, which was originally proposed for homogeneous cellular networks. We focus on the case in which the call-arrival process is general and has stationary independent increments. Since a number of calls could arrive simultaneously, the blocking probability is not equal to the probability with the system that is full. Therefore, we resort to renewal theory to accurately derive the blocking probability. Furthermore, we obtain the throughput, the average system size, and the average system delay. Our analytical results are consistent with simulation results.

Analysis of completion of call with on-off data source in wireless networks

In wireless network, call completion probability accounts for users' satisfaction since the admitted ongoing call may be interrupted during hand-off process or even stay in the same cell when dynamically allocating resource to calls because of the loss of resource. We focus on the relationship between call's completion probability and these interruptions and develop an analytical relationship model for homogeneous cellular networks based on probability analysis. Then assuming call's data source is modeled by on-off traffic model, a two dimensional Markov process is established to compute these blocking and dropping probabilities for call's completion probability. The impacts of different new call arrival rate, call's traffic characteristic, user's mobility, call's holding time and call's admission threshold on call's completion are evaluated and compared through numerical examples. These results show that call's completion reaches its maximum value if making no difference between hand-off call and new call in the case of light traffic load. But some resource should be reserved for the hand-off call in high traffic scenario. The analytical model provides a basis for helping to set the call admission threshold.