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Usercentric nonfull interference cellular networks: BS cooperation and bandwidth partitioning
EURASIP Journal on Wireless Communications and Networking volumeÂ 2022, ArticleÂ number:Â 108 (2022)
Abstract
This paper presents a usercentric frequency reuse scheme depending on the user classification in the cell in a homogeneous Poisson point process network. Each cell is partitioned into multiple regions delimited by the signaltointerference ratio of the user considered. A given resource block (RB) is assigned to one user in a cell and cannot be shared by another user in this cell. The typical user only uses a fraction of the RB, and the remaining part is left unused to avoid interference with the other cells. The base stations (BS) interfering set is hence determined by the coverage probability of the typical user, and this set can be approximated by a thinned version of the original PPP. Using this, coverage probability and spectral efficiency (SE) for each user types are derived, and a BS cooperation technique is proposed to improve the SE of the cell edge user. Finally, the average network spectral efficiency and fairness among users are analyzed under the developed theoretical framework. Our results show that the usercentric frequency reuse improves the coverage probability and the SE compared to the conventional frequency reuse scheme.
1 Introduction
In interferencedominated downlink cellular networks, the quality of service depends on the relative signal strength received from the serving base station (BS) and all other interfering BS through the socalled signaltointerference ratio (SIR), which in turn depends on the network geometry and channel conditions. In a conventional full interference network, where all BS are active all the time in the same band, users near the cell boundary are known to experience a low SIR [1]. However, in a nonfull interference network, locating close to the cell edge does not necessarily lead to significant performance degradation. A nonfull interference in downlink refers to the situation in which all BS do not contribute to the interference received by a user in a given bandwidth [2]. To help users who are more likely to be affected by interference, operators often divide available bandwidth into smaller frequency bands and use the frequency reuse technique to reduce the cochannel interference, by not allocating the same frequencies to neighboring cells. This concept can be extended by categorizing users based on their locations into cell center users and cell edge users and then allocating different subbands to each [3]. However, the locationbased user classification is a purely geometrical approach and ignores the userâ€™s SIR, which may be a useful metric for classifying users. The SIR is indeed used to know active BS distribution in a nonfull interference network.
A generally used approach in the literature is to consider that the BS activity depends on the presence or not of users in a cell. If there is no user in a cell, the cellâ€™s BS is inactive [2]. Under this scenario, authors of [4] model the interference as a random thinning of the original Poisson point process (PPP) of BS, with an approximated empty cell probability. However, when all BS have users inside their cells, the activation of a BS should depend on whether the user is covered or not and hence on its SIR in the cell. From this viewpoint, a better criterion of BS activation could be obtained by correlating the activity of BS with its coverage region.
In this paper, the BS activity on a given band is conditioned on the SIR experienced by a user leading to the nonfull interference concept. This definition strongly differs from the classical frequency reuse concept which is widely used, in the sense that the interference created by a BS on a given band depends on the coverage probability of the tagged user. This dependency makes the analysis of the coverage probability or average spectral efficiency (SE) rather involved since the considered metric depends on itself. Indeed, in this model, which will be detailed in Sect.Â 2, users are classified according to their SIR with respect to predefined thresholds. A resource block (RB) is allocated to each user class, but a user in a certain class is allowed to use only a fraction of the RB and let the remaining band of RB free that can be used only by the other kind of users in the neighboring cell. This concept naturally leads to a usercentric frequency reuse scheme. We show that the user classification within each cell drives the BS activity. This is because the density of interfering BS for each user type is correlated with the type of the user. We present an analytical framework based on stochastic geometry to cope with the spatial distribution of different types of users. This analytical framework allows quantifying key performance measures, i.e., coverage probability and SE, in the nonfull interference context. Moreover, a BS cooperation technique under the introduced nonfull interference framework is studied to improve the SE of cell edge users without jeopardizing the other scheduled users in the same RB. Finally, a bandwidth allocation among users by considering tradeoff on the average SE of the network and the fairness for all user types is proposed and compared with different bandwidth allocation scenarios. Â In the following, literature related to the main topics of this paper is reviewed: classification of users, BS cooperation and bandwidth partitioning.
1.1 Related work
1.1.1 User classification
Stochastic geometry is a powerful tool to link the performance of users to their location, which implies user classification in largescale cellular networks [3]. However, combining sophisticated user classification methods based on the link qualities and intercell interference is challenging [5]. It is because of the correlation between the received signal and the intercell interference seen by the users in the network. Therefore, most of the existing studies in the literature ignore this correlation and instead sort the users according to their mean desired signal strength (i.e., link distances) such that the kth nearest user is the kth strongest user [6]. In the open literature, user classification schemes mainly focus on distancebased [2, 3, 5, 7,8,9], and SIRbased classification [10,11,12].
For instance, in [5, 7, 8], the ratio of the distance between the typical user and the serving BS to the distance between the typical user and the nearest interfering BS is computed. If the ratio is larger than a threshold, the user is a cell edge user; otherwise, it is a cell center user. Authors in [5] studied a metadistribution of the cell center/edge user under orthogonal and nonorthogonal multiple access. A similar approach for classifying users is also used at 3GPP to analyze the performance of schemes such as soft frequency reuse (SFR) [13]. Inspired by this, base station cooperation techniques have been investigated to enhance the cell edge userâ€™s coverage [2, 3, 9]. However, this approach for incorporating a user as a cell center/edge user is not proper for the nextgeneration cellular networks because of neglecting the effects of fading channel and intercell interference seen by the users, which can make them experience low coverage performance even in the cell center area [14].
The SIR ratio is a more suitable way for classifying the users [10, 11]. In [10], the entire frequency band has been split into two subbands that are used to separate cell center and cell edge users. Moreover, spectrum access strategies such as fractional and soft frequency reuse are proposed to increase the cell edge userâ€™s coverage. In [11], the authors used the instantaneous signaltonoiseplusinterference ratio to classify users and got an approximation of the coverage probability of the typical cell edge user for a PPPmodeled 3tier heterogeneous network. However, since all BS are active in the classical full interference scenario, both aspects, geometricalbased and SIRbased classifications, lead to similar conclusions on average, and a user located at the cell border undergoes severe performance degradation.
Indeed, in all previous works, full interference in a given resource block is implicitly assumed. Even in the case of frequency reuse or SFR, in a given frequency band all BS are assumed to transmit (and hence creating interference) independently of the status, covered or not, of their users. These works ignore the correlation in the BS activity, which is a function of the randomly selected user in the cell in the scheduled resource.
In our previous work [12], an SIRdependent nonfull interference model has been introduced to address the existing analytical modelsâ€™ shortcoming. In our model, the given RB is assigned to a single user in each cell and cannot be shared within the cell. With two user types, the RB is thus divided into two subbands, each being used exclusively depending on whether the user is a cell center or cell edge user, with a portion of the RB remaining unused to reduce interference to the other cell. This work has been extended to N user classes, and the key performance metrics, i.e., the coverage probability and the SE, in a nonfull interference context, have been addressed in [15]. It has been found that these metrics are the solution of fixedpoint equations. However, cell edge users still suffer from performance degradation and cooperation techniques to improve their performance have not been studied in the context of nonfull interference network. Moreover, the fairness induced by the usercentric frequency reuse approach has not been tackled neither.
1.1.2 BS cooperation
The analysis of the performance enhancement of a typical user based on BS cooperation in cellular networks using stochastic geometry has been investigated in several works [2, 3, 16,17,18,19]. In [2], a semiclosedform expression for the typical edge userâ€™s coverage probability has been obtained. It is assumed that each BS serving a cell center user and placed within the socalled cooperation radius from an edge user cooperates and does not transmit any signal at the tagged time slot. A usercentric BS cooperation scheme with the joint transmission has been analyzed in [3], where users are grouped by their relative received signal strengths to the three strongest BS. The typical edge userâ€™s performance in millimeter wave cellular networks under two cooperative schemes, namely fixednumber BS cooperation scheme and fixedregion BS cooperation scheme, has been investigated in [9]. A Poissonâ€“Delaunay triangulationbased BS cooperation approach has been proposed to improve the cell edge usersâ€™ performance in [20]. The authors of the work in [14] proposed a hybrid cooperation scheme for sub6 GHz cellular networks where an edge user chooses its mode of operation with or without cooperation. According to the followup studies, BS cooperation was investigated for the typical link, which represents all other links in a full interference scenario.Â In these works, cooperating BS should abandon scheduled users in their cells by delaying them until the next round of transmissions which may increase theÂ network traffic load.
In contrast, in the nonfull interference network presented in this paper, each BS is allowed to transmit a signal to its served user in the assigned fraction of the scheduled RB, and it is off in the remaining subchannels of the RB. This technique hence allows a BS to help a neighboring cell edge user without postponing the service to its own user. Considering this scenario, an edge user can send a cooperation request to BS which serves different user types. In this paper, a cooperation technique, named optimal point selection (OPS), is applied to improve the cell edge userâ€™s performance under the nonfull interference model. In OPS technique, data are concurrently available at multiple BS; only one BS out of the BS cooperation set sends data to a user [21, 22]. In our model, we aim at improving the performance of the cell edge user without jeopardizing other scheduled users in the same RB and without raising the interference level in the network. It is the opposite of the conventional approach in BS cooperation that focuses on a typical user and drops connected users in other cells to help the tagged typical user [2].
1.1.3 Bandwidth partitioning
Besides user classification and BS cooperation technique, bandwidth partitioning (BWP) is a part of the nonfull interference network. Given existing challenges like restricted spectrum availability and poor spectrum usage, efficient radio resource usage is critical for reaching the high peak data rates. A full frequency reuse factor of one appears to be the most effective solution for meeting SE performance goals. However, BWP introduces a tradeoff between radio resource utilization efficiency and interference management [23]. The authors in [24] proposed a bandwidth allocation model that is able to adapt bandwidth allocation to individual users based on data rate requirements. These analyses rely on precise representations of key performance metrics, such as the success probability, signaltointerference ratio and Shannon throughput. In [25], the author investigated the effect of BWP on the reliability and delay performance. Reliability is measured in terms of simultaneous transmission density that meets certain reliability constraints, whereas the delay is measured in terms of average number of time slots for successful transmission of a packet. On this line, prior works have investigated BWP for the downlink full interference system using various performance metrics, such as user fairness [26] and weighted sumrate maximization [27]. In this paper, we investigate BWP for the nonfull interference scenario where the interference received by users is correlated by their types in the network. Based on this model, we can quantify spectrum sharing and service differentiation in this particular context, namely what performance will be delivered to a user based on its type as well as overall performance. Assuming that an RB is partitioned into N parts, i.e., for N user types, we study different BWP strategies. A riskaverse BWP scheme considering tradeoff on the average spectral efficiency of the network and the fairness for all user types is proposed and compared with different BWP strategies.
1.2 Contributions
The aim of this paper is to define edge users in cellular networks based on active neighbor BS and to propose a SIRdependent usercentric frequency reuse scheme that improves the quality of service to the newly defined edge users. Specifically, this paper studies the nonfull interference cellular networks from the perspective of stochastic geometry. As discussed in Section Iâ€“A, it is challenging to combine sophisticated user classification methods based on link qualities and intercell interference. Despite this, the related works do not explicitly classify users with distinct signal qualities, which is crucial to accurately detecting user performance degradation. This means that these analyses do not capture the correlation in the SIR induced by the fact that the userâ€™s interference set is a function of its coverage probability. The nonfull interference model presented in this paper overcomes the above limitations while enabling the tractable systemlevel study of the typical user performance in downlink nonfull interference networks using stochastic geometry. The contributions of our study are briefly summarized below.

The activity of a BS within a cell is correlated with the user classification in our novel definition of a nonfull interference network;

An accurate approximation of the user classification probability is derived. This expression represents how the desired signal correlates with the interference set for each type of user using a fixedpoint equation. Besides, we also provide per user type performance metrics, namely coverage probability and SE;

This paper reformulates OPSâ€™s cooperation theory in the framework of our proposed nonfull interference cellular network model. In this way, cell edge users will experience improved performance without compromising the performance of other users;

In order to formulate different methods of BWP, the usercentric frequency reuse model is used. As a network perspective, the objective is to maximize SE under constraints, while minimizing SEâ€™s variance to ensure fairness between types of users. We propose a riskaverse BWP approach, inspired from the portfolio theory [28], to find a meanâ€“variance tradeoff for the network performance;

Our numerical results demonstrate that: (i) in comparison with the conventional frequency reuse scheme, our usercentric frequency reuse boosts network efficiency and allows high SE, (ii) the nonfull interferencebased BS cooperation can help vulnerable users without penalizing other users in other cells, (iii) the proposed tradeoff BWP strategy assigns fair shares to the different types of users based on their performance metrics.
The rest of this paper is organized as follows. SectionÂ 2 describes the system model and the principle of constructing the active BS sets. SectionÂ 3 briefly revisits our previous results on coverage and SE of a typical user in a nonfull interference network. SectionÂ 4 is devoted to type N user performance enhancement using optimal point selection. Moreover, a modern portfolio theorybased model is applied to optimize the network SE considering BWPâ€™s fairness in Sect.Â 5. Simulation results are presented and discussed in Sect.Â 6. Finally, Sect.Â 7 concludes the paper.
1.2.1 Notations
Random variables are denoted in capital letters, while their realizations remain in lowercase. The operators \(\mathbb {P}(\cdot )\) and \(\mathbb {E}[\cdot ]\) denote the probability and the mathematical expectation functions, \(\llbracket n_1,n_2 \rrbracket\) is the interval of all integers between \(n_1\) and \(n_2\), and \(\mathbf {1}(\mathcal {P}) \in \{0,1\}\) is the indicator function, which is one when the proposition \(\mathcal {P}\) is true and is null otherwise. The Gamma function is defined as \(\Gamma (a) \triangleq \int _{0}^{\infty } t^{a1} e^{t} \mathrm {~d} t\) for all \(a>0\). The hypergeometric function is defined as \({ }_{2}F_{1}\left( a, b; c ;z\right) \triangleq \frac{\Gamma (c)}{\Gamma (b) \Gamma (cb)} \int _{0}^{1} t^{b1}(1t)^{cb1}(1z t)^{a} d t\), for \(c>b>0\) and \(a>0\) [29].
2 Methods and problem formulation
2.1 Network modeling
Let us consider an OFDMAbased singletier downlink cellular network where the location of BS is modeled as an homogeneous PPP \(\Phi \equiv \{X_1,X_2,...\}\) with density \(\lambda\) and \(X_i\in\) \(\mathbb {R}^2\). The given set of nucleus at \(X_i\in \Phi\) can divide the space into Poissonâ€“Voronoi regions as
Similar to the works in [12, 30], the location of the users on a given RB is a point process such as \(\Omega \triangleq \left\{ U\left( V_i\right) : X_i \in \Phi \right\}\). Here, U(B) is chosen uniformly at random from the set B independently for different B. Hence, only one randomly chosen user per RB is considered to communicate with its BS. Since by Slivnyakâ€™s theorem, conditioning on a point is the same as adding a point to the PPP, we consider that the nucleus of the typical cell of the point process \(\Phi \cup \{X_{0}\}\) is located at \(X_{0}\) with its typical cell \(V_0\). Note that the typical user from the point process \(\Omega\) describes a uniformly random point process in the typical cell.
The standard powerlaw path loss model with exponent \(\alpha >2\) is considered for signal propagation. We assume that all BS and users are equipped with a single antenna, and users are experiencing an independent block Rayleigh fading channel on each RB. A cell is divided into N classes, and a typical user belongs to a certain class depending on its SIR. Moreover, each class has its proper subband that may be used by the nearest interfering cell but only by the same user type in that interfering cell. In our model, a user is classified to be a type 1 when its SIR on subchannel 1 is larger than a threshold; a user is type 2 if its SIR on subchannel 1 is less than the firstclass threshold and its SIR on subchannel 2 is larger than the second class one, and so on. A type N user has all its SIR lower than the thresholds in all subchannels 1 to \(N1\) and may be in outage in the last remaining subchannel N. Contrary to the type N user, all type k users are covered with a probability equal to one (by definition). The type 1 user is a cell center user while the type N is the cell edge one. By the above construction, the typical user is a type k user according to the relative value of its SIR w.r.t. some thresholds that, in turn, depends on the location of others BS and the channel conditions. FigureÂ 1 illustrates one realization of a PPP network for three user types, with only one user per BS in a given RB. The BS colored in green, red, and black represent the active BS for type 1, type 2, and type 3 users, respectively. The RB is allocated to all considered users within the network, one user in each cell. This feature can take advantage of 4G or 5G systems in which the RB is divided into multiple subcarriers. However and contrary to 4G, type k user uses only subchannel k although the entire RB is associated to this user. Since the entire RB is dedicated to a given type of user, the BS cannot allocate the remaining part of the RB to another user in the cell. This setting leads to a nonfull interference context in each subchannel because only a part of the RB is used in a given cell. The other users in a cell are associated to other RB of the entire frequency band.
2.2 Proposed userâ€”BS classification
Let us consider the typical BS at \(X_0\) and divide the interfering BS set \(\Phi\) into \(N\ge 2\) complementary subsets \(\{\Phi _k\}_{k=1}^N\) such that \(\Phi =\cup _{k=1}^{N}\Phi _k\) and \(\Phi _k\cap \Phi _l=\emptyset\) for all \(k\ne l\). When \(k\in \llbracket 1,N1 \rrbracket\), \(\Phi _k\) is the subset that gathers the BS serving type k users across the network such that BS in \(\Phi _k\) fully cover their associated users. The BS in \(\Phi _{N}\) are transmitting with the worse link quality, and they may have users in outage.
Let \(X_i\) be the position of BS i, and \(\mathrm {SIR}_i^k\) be the SIR experienced by a randomly selected user in the cell i over the subchannel k. The subset \(\Phi _k\) is
where \(\theta _k> 0\), \(\forall k\in \llbracket 1, N \rrbracket\), are the SIR classification thresholds. For the subset \(\Phi _N\), the \(\mathrm {SIR}_i^N\) can be higher or lower than the threshold \(\theta _N\). Let \(\mathrm {SIR}_0^k\) be the received SIR by the typical user in \(V_0\), on subchannel k, then
where \(R_i\) is the distance between the BS i and the typical type k user, \(H^{k}_{i}\) is the channel gain between the typical user and the BS i in subchannel k. Since separated frequency subbands are allocated to different user types, they do not experience the same instantaneous channel gain, and \(H^k_{i}\) are independent and identically exponentially distributed (i.i.e.d.). The indicator function in (3) ensures that the typical user only experiences interference from BS that serve type k users.
3 Coverage and spectral efficiency without cooperation
This section first revisits the coverage probability of the typical user under a usercentric frequency reuse approach in the nonfull interference network. Next, to fairly evaluate the proposed method, the SE of the network is studied.
3.1 Coverage probability
In interferencelimited wireless networks, the standard coverage probability describes the probability that the SIR of the typical link exceeds a threshold [31]. In our model, we have a set of thresholds \(\{\theta _k\}_{1\le k\le N}\) to successfully demodulate and decode the received signal. Then, the typical user located in the typical cell is covered on downlink if:
The following corollary presents the downlink coverage probability of the typical user for a nonfull interference network with N classes.
Corollary 1
Given \(\{\theta _k\}_{1\le k\le N}\), the coverage probability of a randomly located typical user in a nonfull interference network with N user types is given by
where
and
Proof
The proof follows from the law of total probability and the fact that sets in (4) are disjoint and can be found in [15]. \(\square\)
One important point is that (6) is a fixedpoint equation of \(p_k\), which approximates the probability of a typical user being a type k user [15, TheoremÂ 1], \(\mathcal {P}_{N}\) is the conditional coverage probability of type N user in the nonfull interference network [15, CorollaryÂ 1]. Two other corollaries of importance are deduced from CorollaryÂ 1.
Corollary 2
Given \(\alpha\) and \(\theta _1\), the probability of being the cell center user (\(k=1\) in (6)) is given as
Proof
The proof follows from (6) and using \(\left( \frac{\sqrt{1+x}1}{x}\right) ^{a}=2^{a} \,{ }_{2} F_{1}\left( \frac{a}{2}, \frac{a+1}{2}; a+1 ; x\right)\). \(\square\)
Corollary 3
Given \(\alpha\) and classification thresholds \(\{\theta _k\}_{1\le k\le N}\), we have
Proof
Referring to (4), for \(M=N+1\), we can write
\(\square\)
CorollaryÂ 3 ensures that the increase in the number N of user types cannot decrease the coverage probability. It is important to consider SE as well as coverage Â since the coverage does not take the resource used into consideration. The following is a description of how the proposed classification impacts SE.
3.2 Spectral efficiency
Considering the system setting of [31], where only one type of user is considered with full interference, the average SE per RB for the typical user is \(\mathbb {E}[\ln (1+\mathrm {SIR})]\). In our case, a given RB is divided into N subchannels, each assigned to its corresponding user type. Each type k user transmits over the kth subchannel. Then, the achievable SE of the typical type k user on the subchannel k is defined as
where the expectation is taken over the network geometry and channel gains. Then, the weighted SE in nats/s/Hz of the typical user of type k can be written as
where \(0 \le \omega _k\le 1\) is the fraction of the given RB according to subchannel k, with \(\sum \limits _{k=1}^{N} \omega _k =1\). The term \(\mathcal {R}_k\), \(\forall \, k\in \llbracket 1,N \rrbracket\), has been obtained in [15, TheoremÂ 2] as
where
and
Hence, the average SE \(\overline{\eta }\) of the typical user in the nonfull interference network is \(\overline{\eta }=\sum \limits _{k=1}^{N} p_k \eta _k\).
4 Coverage and spectral efficiency with cooperation
In this section, we investigate a BS cooperation scheme [32] to improve the performance of the typical type N user. Precisely, our proposed scheme exploits the fact that a BS that serves a type k user in another cell is only active on the kth subchannel and remains idle for all the other subchannels and in particular for the Nth subchannel, i.e., the one used by the cell edge user.
4.1 N type user classification
Recalling Fig.Â 1 and the fact that a type N userâ€™s cooperation set consists in the \(N1\) closest BS of class \(k\in \llbracket 1,N1 \rrbracket\) plus its BS on subchannel N, the tagged type N user selects the BS among the cooperation set with the best channel quality, while the remaining \(N1\) BS stay silent in the Nth subchannel. Here, the channel quality is the product of the largescale path loss and the smallscale fading. The user has the opportunity to change its firstly associated BS, based on distance, to another BS with a better channel quality. Let \(\mathcal {C}_{o}= \left\{ X^k\triangleq \arg \min _{X_i} \Vert X_i \Vert : X_i \in \Phi _k, \,1 \le k \le N 1 \right\}\) be the set of BS that are the nearest type k BS to the typical type N user. Hence, the SIR of the typical type N user is \(\mathrm {SIR}_0^{N}= \frac{S_N}{I_{N}}\), where \(I_{N}= \sum \limits _{k = 1}^{N} I_{N,k}\), and
with
The main intermediate stage in analyzing the proposed OPSâ€™s efficiency is to characterize the joint distribution of the distance between the typical user and the set of candidate BS to cooperate. Let \(T_k=\min _{X_i\in \Phi _k}R_i\), for \(1 \le k \le N1\), be the distance of the typical type N user to the nearest BS serving a type k user. Moreover, let \(G_k\), for \(1 \le k \le N1\), be the fading gain between the typical type N user and its nearest type k BS on the Nth subchannel of the RB.
4.1.1 Joint distances distribution
The following lemma gives the joint probability density function (PDF) of \(\varvec{T} = [T_1,\dots ,T_{N1}, R_0]\).
Lemma 1
Conditioning on the distance of a typical type N user from its nearest BS \(R_0\), the PDF of the distance between the typical user and the nearest helping BS which serves a type k user \(T_k\), \(1\le k \le N1\), is
Proof
The void probability of a 2D Poisson process \(\Phi _k\) with density \(\lambda _k=p_k \lambda\) in an area A is \(\exp (\lambda _k A)\). The cumulative distribution function of the distance \(T_k\) from the typical user to the nearest BS in \(\Phi _k\) conditioning on \(T_k>R_{0}\) is
By taking the derivative of (21) with respect to \(t_k\) and knowing that in our model \(\lambda _k=p_k \lambda\), we can derive (20). \(\square\)
Using LemmaÂ 1 and knowing that different types of BS come from disjoint sets, the joint conditional PDF of \(\varvec{T}\) given \(R_0\) is \(\prod _{k=1}^{N1} f_{T_k\mid R_{0}}(t_k \mid r_0, t_k>r_{0})\). Then, unconditioning it by the PDF of \(R_{0}\), i.e., \(f_{R_{0}}(r_{0})=2 \pi \lambda r_{0} e^{ \pi \lambda r_{0}^2}\), gives the joint distances distribution on \(\left\{ T_k>R_{0}, k\in \llbracket 1,N1\rrbracket \right\}\) as
4.1.2 Laplace transform of the interference
According to (18), the Laplace transform of the interference is given by
where
where (a) follows the law of total expectation, i.e., \(\mathbb {E}_{X}[f(X)]=\mathbb {E}_{Y}[\mathbb {E}_{X}[f(X)Y]]\), (b) comes from averaging over the i.i.d. exponential distribution of interfering fading channels \(H^{N}_{i}\) with mean 1, as in [31], and factoring out the term \(\mathbb {E}[\mathbf {1}_{\mathcal {A}_{i}^{k}}]\). Finally by applying the probability generating function (PGFL) of a PPP [33], we have
where \(p_{a_k}\) is the probability that a BS serving a type k user is selected by OPS to help the typical type N user. Hence,
Similarly, for \(k=N\),
Remark
Based on the cooperation set definition, it is possible to have the same BS in the cooperation set of different type N users in the model. Moreover, in Sect.Â 2, we consider that each BS could serve only one user in the relative subchannel of the RB. Hence, if OPS scheme selects the same type k BS for two or more type N users, the selected BS can only serve one type N user, and other type N users must search for another BS in their cooperation set.
According to the above remark, quantifying the probability that one cooperative BS be selected by two or more type N users is a complex problem. However, the density of type N users can be relatively lower than other type of users based on the number of user classes and the classification threshold \(\{\theta _k\}_{k=1}^{N1}\) (see Fig.Â 1). Therefore, it is Â assumed that the common cooperative BS in the cooperation sets of different type N users can be neglected. The numerical results will show that this assumption is reasonable and leads to tight approximation.
The probability that OPS technique selects the first associated type N BS is \(p_{a_N}\), used in (27), and it is the complementary event of all cooperation events in (26). Hence, we can write
The probabilities \(p_{a_k}\) are given by the following theorem.
Theorem 1
Given that OPS is applied to the cooperation set of a typical type N user, the probability that a type k BS is selected to serve the typical type N user is
where \(B^c(0, r_{0})\) denotes the complement of \(B(0, r_{0})\), which is a ball centered at the origin with radius \(r_0\).
Proof
Referring to (26) and knowing that \(\{\{G_{m}\}_{m=1}^{N1}, H^{N}_{0}\}\) are i.i.e.d., by conditioning on \(G_{k}\) and on the distances distribution \(\varvec{T}\), we can write
The proof ends by averaging over \(G_{k}\) and \(\varvec{T}\). \(\square\)
4.1.3 Type N user coverage probability
We derive the coverage probability of the typical type N user using the joint distribution of distances from its cooperation set BS, given inÂ (22).
Theorem 2
Given that OPS is applied to the cooperation set of a typical type N user, the coverage probability of the typical type N user is given by
where \(\mathcal {L}_{I_N}(s)\) is obtained by (23) and
Proof
See AppendixÂ 8.1. \(\square\)
4.1.4 Type N user spectral efficiency
Similar to [15, TheoremÂ 2], SE of the typical type N user under OPS scheme can be written as
4.2 Two user types classification
As a particular case, let us consider the nonfull interference network with two user types in the network. In this case, type 1 and type 2, called cell center and cell edge users, are present in the network. The given RB is divided into cell center and cell edge subchannels. In the following, the closedform expressions for the conditional coverage probability and the spectral efficiency of the typical cell edge user under OPS scheme are derived.
4.2.1 Cooperation probability
The probability that a BS serving a cell center user be selected by OPS to cooperate in the second (cell edge) subchannel of the scheduled resource is
where \(G_{1}\) is the cooperation gain in the second subchannel, \(H^{2}_{0}\) is the gain between the tagged BS and its cell edge user, \(T_1\) is the distance between the typical cell edge user and the nearest cooperative BS, and \(R_{0}\) is the distance between the tagged BS and the celledge user. The probability \(p_a\) can be computed with the following lemma.
Lemma 2
The probability that a typical BS serving a cell center user be called for a cooperation is
Proof
See AppendixÂ 8.2. \(\square\)
4.2.2 Type 2 user coverage probability
The coverage probability of a typical cell edge user when OPS is applied can be formalized as follows.
Theorem 3
Given two coverage thresholds \(\theta _1\) and \(\theta _2\), the coverage probability of a typical cell edge user when OPS is applied in the nonfull interference network can be calculated as
where \(\mathcal {M}_i\) and \(\mathcal {Q}_i\), \(i\in \{1,2,3\}\), are
Proof
See AppendixÂ 8.3. \(\square\)
In the next section, the performance of widely studied bandwidth allocation strategies is investigated under the theoretical nonfull interference framework developed in this paper. In particular, the tradeoff between SE and fairness of the identified strategies is investigated.
5 Bandwidth allocation
From (13), the networkâ€™s spectral efficiency \(\varvec{\eta }=[\eta _1,...,\eta _{N}]\) is a function of individual BWP \(\varvec{\omega }=[\omega _1,...,\omega _{N}]\). Hence, the bandwidth allocation strategy plays an important role in the performance achieved.
5.1 Maxmean BWP strategy
The first BWP strategy tries to maximize the mean SE and is expressed as
The optimal solution is simply given by \(\omega _k = 1\) for the index k corresponding to the largest value of \(\mathcal {R}_k\), which is a function of the set of classification thresholds, and \(\omega _k = 0\) otherwise [15]. However, this allocation does not take into account fairness among different users; it only aims at maximizing the sum of SE of all users. It can be preferable to share the RB among all the covered users, i.e., \(\omega _k>0\) for all k.
5.2 Uniform BWP strategy
The simplest partition policy, which does not need any other information from the system, is the equal partitioning method where the RB is equally divided into N subchannels, i.e., \(\omega _k=1/N\), \(\forall k\). In this case, the weighted SE of the typical user is [15 Subsection IIIC]
5.3 SIRproportional BWP strategy
The third strategy is the adaptive bandwidth partition, where the available bandwidth is shared among the users according to the SIR distribution and traffic load [10]. Hence, in the SIRproportional model, we have \(\omega _k= p_k\), and the mean SE is [15 Subsection IIIC]
5.4 Maxmin BWP strategy
The fourth used strategy is named maxmin criterion proposed in [34]. It aims at gaining the worstcase performance by maximizing the lowest rate among all user types as
The solution is achieved when all SE are equal, i.e., \(\eta _i=\eta _j,\forall i,j\in \llbracket 1,N \rrbracket\). Hence, the partition \(\omega _{k}\) allocated to the kth user is directly given by
The maxmin criterion gives a kind of fairness since each user has the same spectral efficiency, butÂ in the same time, this strategy leads to a poor global sumrate. The tradeoff between the fairness and the maximum sumrate in the network can be seen as an attempt to control the variance of the SE allocated to users in the network [35, 36]. In the following, we study a meanâ€“variance tradeoffbased BWP with a given value of the tradeoff level.
5.5 Meanâ€“variance tradeoffbased BWP strategy
In this section, instead of maximizing the average SE, we introduce the concept of risk to ensure maximum network SE subject to a certain level of fairness among all user types in the network. In particular, we use the expected exponential utility risk model [28, 35], which is defined as follows:
where \(\beta\) is an appropriately chosen constant known as the risk sensitivity parameter. The above function is the aversion for the risk when \(\beta < 0\) and riskseeking if \(\beta > 0\). Expanding the Maclaurin series of the \(\log\) and \(\exp\) functions indicates that (43) catches higherorder moments of BWP. Concretely, in small risks, the exponential utility function is provided as [35]:
where \(\mathbb {E}[\varvec{\eta }]\triangleq \sum _{k=1}^{N} p_k \eta _k\), \(\text {Var}[\varvec{\eta }]\triangleq \sum _{k=1}^{N} p_k \left( \eta _k\mathbb {E}[\varvec{\eta }]\right) ^2\). The variance term controls the variability of the SE among user types and can be used to control the fairness among users. Therefore, the bandwidth partitioning problem based on a given level of fairness can be formulated as the following optimization problem:
The objective function in (45) has a positive Hessian w.r.t. \(\varvec{\omega }\) for \(\beta >0\) and a negative Hessian for \(\beta <0\). Hence, in the riskaverse setting, the utility function is concave w.r.t. \(\varvec{\omega }\). Moreover, the feasible region has the convexity property since the constraints \(C_1\) and \(C_2\) are linear constraints. Therefore, for a given \(\{\theta _k\}_{1\le k\le N}\) and \(\alpha\), (45) is a convex optimization problem.
The optimization problem formulated in (45) is a standard nonlinear programming problem that can be solved using the Karushâ€“Kuhnâ€“Tucker (KKT) conditions [37]. The Lagrangian function expresses as follows:
where \(\mu\) and \(\xi _k\) are the KKT multipliers. Subsequently, the KKT conditions give
Solving the system of equations inÂ (47) yields the optimal BWP with adequate fairness. The system can be solved using the waterfilling algorithm proposed in [38]. From the above results, we can write the waterfilling value \(\omega _i\) as a function of the Lagrange multiplier \(\mu\), i.e., \(\omega _i=f_i(\mu )\), where
Considering that \(\omega _i\) must be nonnegative (\(C_1\)), the waterfilling value can be represented as follows:
Finally, the Lagrange multiplier \(\mu\) is obtained using the primal feasibility (\(C_2\)) as
From (50), no closedform expression for \(\omega _i\) as a function of \(\mu\) independent from \(\{\omega _k, \, \forall \ k \in \llbracket 1,N \rrbracket \backslash \{i\}\}\) can be obtained. However, for a given network realization, the solution can be iteratively estimated via a bisection search algorithm like in [38]. Bisection search is a simple method with very high performance. However, the starting points of the algorithm have to be chosen carefully. The following corollary bounds the possible value of the Lagrangian multiplier \(\mu\).
Corollary 4
Given \(p_k\) and \(\eta _k\) for all \(k\in \llbracket 1,N \rrbracket\), the waterfilling value \(\mu\) should satisfy:
Proof
The proof follows from (49) and the fact that \(\beta\) takes a negative value in the riskaverse model, which limits the Â argument of the logarithm function to be positive and less than 1. \(\square\)
Each of the BWP strategies addressed in this section has its own merit in fairness. To compare the fairness gain of the different BWP strategies, we use the Jainâ€™s index, since it is a widely used metric in literature [39, 40]. The definition of Jainâ€™s index is formulated as follows.
Definition 1
Given a set of achievable SE \({\eta _k : k =1, . . . , N}\), the corresponding Jainâ€™s index is
where \(\mathcal {F}(\varvec{\omega })\) gives a continuous value in the range of \(\left[ \underset{k}{\min } \{p_{k}\}, 1\right]\).
6 Results and discussion
Simulations are conducted in a PPP network. The coverage probability and SE are evaluated at the typical user for 100,000 network realizations. The standard powerlaw path loss model with exponent \(\alpha >2\) is considered for signal propagation. We assume that all BS and users are equipped with a single antenna, and users are experiencing an independent block Rayleigh fading channel on each RB. We consider an interferencelimited scenario with \(\alpha =4\), i.e., without thermal noise at the receiver. We investigate the average SE per unit RB where a given RB is divided into N subchannels. At each realization, the BS locations are generated as a PPP of unit intensity in an area of \([30, 30]\times [30, 30]\), and user density is considered large enough to have at least one user per cell.
FigureÂ 2 shows the simulation and analytical results of the cell edge coverage probability in the case of two user types (36) with/without OPS in the nonfull interference network with a unique classification threshold. From the figure, the coverage probability of the cell edge user is enhanced under OPS compared to the noncooperative scenario. When the unique threshold increases, both techniques converge because the number of BS serving a type 1 user in the network decreases. Moreover, simulation and analytical results match, which validates the theoretical findings of Sect.Â 4.
FigureÂ 3 compares the average SE for the cell center/edge user classification in the nonfull interference scenario versus the type 1 user target threshold \(\theta _1\) under different BWP strategies. Moreover, the achieved average SE is compared with the conventional frequency reuse technique with reuse factor \(\Delta\) presented in [31]. The figure depicts that the maxmean BWP outperforms other strategies, since giving all the bandwidth to the best user is optimal regarding the network SE performance. Moreover, the curve corresponding to the maxmean SE first increases and then decreases after almost 8Â dB, because for small values of \(\theta _1\), almost all users are in the cell center. On the other hand, large values of \(\theta _1\) make all users be cell edge users, and it tends to the full interference case, i.e., frequency reuse with \(\Delta =1\). Moreover, it can be clearly observed that when \(\theta _1\) increases, the average SE under SIRproportional policy (40) decreases while SE increases under equal partitioning (39). It can be seen that equal BWP eventually outperforms SIRproportional BWP by increasing \(\theta _1\) above 5 dB. This is because, contrary to equal BWP, SIRproportional BWP gives more bandwidth to cell edge users when \(\theta _1\) increases, that is less efficient for the global SE. Indeed, in SIRproportional policy, the typical user benefits from a fraction of resources that depends on the SIR and suffers from the same fraction of the interference. However, SIRproportional policy still has a higher SE than static frequency reuse with \(\Delta =2\). Besides, the figure compares the meanâ€“variance BWP (45) according to the risk sensitivity level \(\beta\) with other BWP policies. For \(\beta =1.5\), the meanâ€“variance BWP is a weak riskaverse policy and attempts to maximize the network SE by giving more bandwidth to the cell center users. Hence, the SE achieved by meanâ€“variance BWP is identical to the maxmean strategy for small \(\theta _1\) and then decreases when \(\theta _1\) increases. This is because the number of cell center users decreases by increasing the threshold value, and the policy tends to limit the increase in the variance in the rate allocation. On the other hand, if \(\beta =10\), the policy is even more riskaverse and tends to ensure fairness among users in the Jainâ€™s index sense. It also means that by decreasing \(\beta\), the SE achieved by the meanâ€“variance strategy tends to the one obtained with the maxmin BWP (41). Maxmin BWP consists in giving the same amount of rate among user types and hence minimizing the variance of the allocation and SE increases by increasing \(\theta\).
FigureÂ 4 compares the network average SEâ€“coverage probability tradeoff for SIRproportional BWP, equal BWP and frequency reuse (with \(\Delta =N\)) with/without BS cooperation technique, i.e., OPS scheme, and for unique threshold value \(\theta\) ranging in \(\{5, 2, 10\}\)Â dB. Whatever the BWP scheme considered, the coverage probability increases, whereas the average SE decreases when N increases, but with different trends according to the threshold \(\theta\). However, the curves corresponding to the SIRproportional and equal BWP policies are above the conventional frequency reuse. Furthermore, for \(\theta \in \{5, 2\}\)Â dB, the average SE achieved with SIRproportional BWP is higher because it allocates more bandwidth to the type 1 user, which has a higher achievable spectral efficiency than other user types in the network. In contrast, equal BWP equally allocates bandwidth regardless of the density of the different user types in the network. On the other hand, for \(\theta = 10\)Â dB and \(N=2\), equal BWP has a higher SE than SIRproportional BWP. This is due to the fact that a high threshold value shrinks the cell center region, and the SIRproportional approach allocates more bandwidth to the cell edge user, which has a lower SE than the cell center user. But, when N increases, SIRproportional achieves again higher SE than equal BWP. This is because the negative effect of equally dividing the available bandwidth is greater than the densitydependent allocation based on the user type. Finally, OPS strategy improves the tradeoff front by increasing the SE of the type N user using a BS selection diversity scheme, in particular for moderatetohigh threshold values.
FigureÂ 5 illustrates the Jainâ€™s indexâ€“average SE tradeoff for different strategies investigated in this work. The meanâ€“variance BWP characterizes the tradeoff between the network SE and the fairness among user types, when \(\beta\) value decreases from \(0.01\) to \(40\). As the network SE decreases, the fairness measured with the Jainâ€™s index increases and this index decreases when SE increases. The maxmean BWP has the maximum SE but with the lowest fairness in bandwidth partitioning. By sacrificing SE, SIRproportional and equal BWPs achieve higher fairness regarding the maxmean strategy. The maxmin BWP offers the fairest bandwidth sharing, in the Jainâ€™s index sense, while having the lowest SE. One can remark that the frequency reuse technique allows to achieve a larger network SE than the one obtained with the maxmin policy with a Jainâ€™s index equal to 1. This is because the allocated bandwidth does not depend on the position of the typical user in the cell so the usertype fairness is one. The fairness measured is among user types, and it does not mean that the fairness among users would be one with the frequency reuse technique since users do not experience the same rate. The meanâ€“variance fairness criterion with a given risk level \(\beta\) allows exploring the feasible operational point based on the desired SE and level of fairness among different user types in the cellular network.
7 Conclusion
This paper has presented a complete characterization of a typical userâ€™s downlink coverage probability and spectral efficiency in a nonfull interference homogeneous PPP network, i.e., when the statistic of the interference depends on the classification of the users. To help cell edge users in the network, an optimal point selection cooperation scheme was applied, and semiclosedform expressions of performance metrics based on the typical userâ€™s received SIR level have been derived. Then, to maximize the network average spectral efficiency, different bandwidth allocation schemes among user types have been expressed and evaluated. Finally, using the Jainâ€™s index, the fairness of the bandwidth allocation schemes has been quantified. Moreover, portfolio theory has been firstly used in our context to assess the tradeoff between SE and fairness. Numerical results demonstrated that the usercentric resource allocation approach outperforms the conventional frequency reuse approach, which is BScentric. We intend to follow the usercentric frequency reuse in nonPoisson models to consider spatial repulsion among BS deployment in future work.
Availability of data and materials
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Abbreviations
 BS:

Base station
 BWP:

Bandwidth partitioning
 i.i.e.d.:

Independent and identically exponentially distributed
 OPS:

Optimal point selection
 PPP:

Poisson point process
 RB:

Resource block
 SE:

Spectral efficiency
 SFR:

Soft frequency reuse
 SIR:

Signaltointerference ratio
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Appendix
Appendix
1.1 Proof of Theorem 2
By using the Bayes rule and recalling (17) and (18), we get
Next, we expand the term \(\Xi =\mathbb {P}(S_{N} < \theta _N I_N , X_{0} \in \Phi _{N})\) as
where
1.2 Proof of Lemma 2
Using the definition of \(p_a\) in (34) and by conditioning on the joint distance distribution of \(T_1\) and \(R_{0}\)
where (a) comes from \((1+x)^{1}=\sum _{k=0}^{\infty }(1)^{k} x^{k}\). From the joint distances distribution in (22), the joint PDF of \(T_1\) and \(R_{0}\) is
where \(r_0\in [0,\infty )\) and \(t_1\in [r_0, \infty )\). Hence, we have
where (a) comes from change of variables \(\frac{r_{0}}{t_{1}}=u\) and \(v=r_{0}\), (b) comes by letting \(\lambda \pi \left( 1p_c \left( 1 \frac{1}{u^{2}}\right) \right) v^{2}=y\). Finally, by applying
and substituting (58) in (57), we can derive (35).
1.3 Proof of Theorem 3
Applying TheoremÂ 2 for \(N=2\) leads to
We can derive \(\mathcal {M}_1\) as follows:
where by substituting \(\mathcal {L}_{I_{2}}(\theta _2 r_{0}^{\alpha })\) from (23) into (61), we have
The expressions of \(\mathcal {M}_i, i=2,3\) and \(\mathcal {Q}_j, j= 1,2, 3\) can be obtained following the same steps as the one used for \(\mathcal {M}_1\), by computing
and the proof is complete.
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Mardani, M., Mary, P. & Baudais, JY. Usercentric nonfull interference cellular networks: BS cooperation and bandwidth partitioning. J Wireless Com Network 2022, 108 (2022). https://doi.org/10.1186/s13638022021891
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DOI: https://doi.org/10.1186/s13638022021891