An accurate performance analysis of an FFR scheme in the downlink of cellular systems under largeshadowing effect
 Benoit Pijcke^{1, 2, 3},
 Marc Gazalet^{1, 2, 3},
 Marie ZwingelsteinColin^{1, 2, 3}Email author and
 FrançoisXavier Coudoux^{1, 2, 3}
https://doi.org/10.1186/16871499201348
© Pijcke et al.; licensee Springer. 2013
Received: 22 July 2011
Accepted: 15 January 2013
Published: 22 February 2013
Abstract
We address the problem of analyzing the performances of interferencelimited cellular networks in largescale shadowing environments. Focusing on the fractional frequency reuse (FFR) framework, we examine how to optimally assign mobile users in a cell either to the full frequency reuse part or to the orthogonal part of the FFR band. Instead of using traditional Monte Carlo simulations, which do not provide sufficiently accurate results under important shadowing, we propose a fast and accurate numerical method. We consider a fastfading environment and we use the ergodic capacity as the performance measure. Applying a distributed power control and scheduling strategy, we examine both cases where access points have knowledge of partial or fullchannel state information (CSI); for the latter, we also propose an approximated waterfilling strategy. The interest of our method lies in the fact that it allows for a fast and accurate analysis of the performances of FFR. In addition, it takes into account a broad range of shadowing environments.
Keywords
1 Introduction
Since the invention of digital networks, efforts for providing higher capacity and higher quality of service have never stopped. With the emergence of new wireless networks that aim at providing multimedia service everytime and everywhere, the need for higher spectral efficiency has never been so high [1, 2]. In order to achieve these goals, aggressive reuse of the whole frequency spectrum throughout the network is mandatory. As a consequence, today’s systems have to make intensive use of intercell interference mitigation techniques, including power control, opportunistic scheduling, FFR, MIMO... [3–9].
As the term ‘fractional’ implies, FFR attempts to find a compromise between the full frequency reuse scenario, which allocates the entire available bandwidth to each cell but at the expense of a maximum level of intercell interference, and the partial frequency reuse scenario, which provides zero intercell interference by orthogonally sharing the bandwidth among adjacent cells [10]. The idea behind FFR has come from the observation that user terminals (UTs) within a cell are not equivalent from an intercell interference point of view: due to the pathloss effect, celledge users are far more sensitive to it than users located near the center of the cell.
A key aspect in FFR design is for the access points (APs) to appropriately decide, for each UT, whether it has to be scheduled within the full frequency reuse part of the FFR band or within the orthogonal part of the FFR band. In this article, we investigate this decision issue based on the knowledge of the users channel gains.
Generally, the performances of FFR are evaluated by Monte Carlo simulations and/or empirical measurements [8, 11–13]. A recent contribution [14] has tackled the problem by following an analytical approach, however using some simplifying assumptions (very basic power control at the AP, no shadowing effect). Although the Monte Carlo simulation technique is a very common and practical tool, it does not provide sufficiently accurate results in the case where the random phenomenon included in the simulation model exhibits a heavytailed behavior. This is due to the fact that for heavytailed distributions, large amplitudes are associated a very low probability, so they rarely appear in the simulation process, despite their great impact on system performance [15]. In wireless networks, the heavytailed behavior of the interference power originates from the cumulative effect of the shadowing phenomenon in interfering links (sum of lognormal distributions) [16, 17], and this makes accurate and fast simulations very challenging in such environments.
In this article, we propose an accurate and fast numerical method to determine the performances of FFR in terms of ergodic capacity, in a multicell network suffering from path loss, shadowing and Rayleigh fading. In particular, we examine how the UT should be assigned to the FFR band, based on its channel gain. Unlike other studies that assume a fixed shadowing standard deviation (see e.g., [11, 13], where σ_{dB}=8 dB), we consider it as a variable parameter (σ_{dB}∈ [0,10] dB) in order to accurately analyze its influence on the performances of FFR.
The rest of the article is organized as follows. In Section 2, we present the system model. In Section 3, we derive the methodology of the numerical method that we use to provide a fast and accurate simulation of the performances of FFR. Numerical results are analyzed in Section 4.
2 Multicell downlink transmission model
We consider the downlink of a synchronous discretetime 19cell cellular network having the 2D hexagonal layout depicted in Figure 1. We assume a unitgain omnidirectional SISO (single input, single output) antenna pattern, both for the fixed APs and the mobile UTs. The UTs are supposed to be uniformly and independently distributed over the service area. Users belonging to the same cell are considered orthogonal, which gives rise to a wireless channel primarily limited by intercell interference, especially at the cell edges (as we do not consider sectorization in this article, there is no intersector interference^{a}). This model corresponds, e.g., to a TDMA system. In this context, the FFR scheme that we consider is a mixture of the two following patterns for allocating the available frequency spectrum across the network:

a partial frequency reuse pattern, denoted FR3, with reuse factor 3 (N=6 interferers). For this pattern, three groups of APs are active simultaneously on three orthogonal bands;

the full frequency reuse pattern, denoted FR1, where all APs in the network transmit at the same time using the whole frequency band (N=18 intercell interferers).
Here x_{0}(m) represents the information symbol intended to UT_{0} and x_{ n }(m), n≠0, the n th interfering symbol (this symbol is sent from AP n to its respective user UT_{ n }). The coefficient h_{ n }(m) denotes the instantaneous channel gain from AP n to UT_{0}. Each channel is subject to additive white Gaussian noise w_{0}(m), with variance W.
where $\alpha ={(c/(4\mathrm{\Pi f}\left)\right)}^{2}{d}_{0}^{\gamma 2}$, with c being the speed of light, f, the operating frequency, and d_{0}, a reference distance for the antenna farfield; and γ represents the attenuation coefficient. The Rayleigh fading gain H_{f,n} is modeled by an exponential distribution with rate parameter equal to 1, i.e., $\mathbb{E}\left\{{H}_{\text{f},n}\right\}=1$. The shadowing gain H_{s,n} is modeled by a unitmean lognormal distribution parameterized by what is called the shadowing standard deviation σ_{dB} (usually expressed in dB) which captures the importance of the shadowing phenomenon (e.g., a σ_{dB}=0 dB scenario represents a wireless channel subject to fading only). Since r.v.’s H_{f,n} and H_{s,n} are independent of each other, and since $\mathbb{E}\left\{{H}_{\text{f},n}\right\}=\mathbb{E}\left\{{H}_{\text{s},n}\right\}=1$, from (1) we have $\mathbb{E}\left\{{H}_{n}\right\}={H}_{\text{pl},n}$, which reflects the fact that the Rayleigh fading and shadowing components cause the actual gain H_{ N } to fluctuate about its mean value H_{pl,n}.
We now introduce the power control and scheduling strategy that we consider in this article. In what follows, we first assume a distributed environment, i.e., each AP has access to some (local) CSI provided by its respective mobile users only. According to what type of CSI data are fed back from UTs to their serving APs, we consider two different scenarios: a partial (statistical) CSI scheme (for which only path losses are available at the APs), and a fullCSI scheme (APs have knowledge of instantaneous channel gains). Since channel information is available only at the cell level^{c}, we consider a distributed power control and scheduling strategy. A channel inversiontype power control is applied within each cell, based on the mean information (i.e., the path losses H_{pl,n}) available at the APs: the power emitted at each time slot by AP n is ${P}_{n}=\mathbb{E}\left\{\left{x}_{n}{\left(m\right)}^{2}\right\right\}=min\left({P}_{max},{P}^{\ast}/{H}_{\text{pl},n}\right)$, where P_{max} is the maximum power available at the AP, and P^{∗} is the target power at the mobile user. To this power control scenario we associate the resourcefair opportunistic scheduling policy proposed (and proved to be sumrate optimal) in [19]: each cell ranks its users by (say decreasing) order of channel gain and assigns the best users to the available time slots, regardless of channel gains in other slots.
3 Methodology
In Section 3.1, we present the ergodic capacity as the performance measure. In Section 3.2, we consider the partialCSI scheme. We examine the fullCSI scheme in Section 3.3, where we propose an approximated waterfillingintime strategy.
3.1 The ergodic capacity as the performance measure
where I is the total interference power received by UT_{0}, that can be written $I=\sum _{n=1}^{N}{P}_{n}{H}_{n}$. Since no closedform expression exists for the ergodic capacity (3), it must be computed numerically. The traditional approach consists in performing Monte Carlo simulations. However, because the shadowing phenomenon exhibits a heavytailed behavior for large values of the shadowing standard deviation σ_{dB}, efficient simulations in such environments are very challenging. Hence the need for a method to quickly and accurately compute the ergodic capacity that takes into account the importance of the shadowing.
To propose such a method, let us closely examine (3). First of all, we investigate the impact of the power control and scheduling policy on the interference power gain I. Recall that the distributed scheduling policy consists, for each cell, in ranking its users by decreasing order of mean channel gain H_{pl,n}, and in assigning them, in that order, an available time slot [19]. We now make an important assumption over the transmit power. Because of the scheduling policy, each user has a mean channel gain H_{pl,n} contained in a range which (statistically) becomes more and more restricted as the number of users per cell grows. In this article, we only consider a framework in which the number of users per cell is high. So, applying the scheduling policy described previously, it can be expected that the channels between users scheduled at the same time and their serving APs have about the same power gains: H_{pl,n}≈H_{pl,m}, ∀n≠m[16]. Since the power control is based on a constant target (received) power P^{∗}, we can reasonably assume that, at each time slot, each AP transmits at the same power: P_{ n }=P, ∀n. In Appendix 1, we evaluate how this transmit power assumption is impacted by the total number of users.
The interference power thus simplifies to $I=P\sum _{n=1}^{N}{H}_{n}$. We now define the interference power gain—which is denoted G—as being the sum of the channel power gains between the interested user UT_{0} and the N interferers, i.e., $G=\sum _{n=1}^{N}{H}_{n}=\sum _{n=1}^{N}{H}_{\text{pl},n}{H}_{\text{f},n}{H}_{\text{s},n}$. In this expression, G is a function of UT_{0}’s location through the distances r_{ n } (see (2)). However, it has been shown in [20] that the impact of UT_{0}’s position on G is negligible, so G can be approximated by $G\approx \sum _{n=1}^{N}\mathbb{E}\left\{{H}_{\text{pl},n}\right\}{H}_{\text{f},n}{H}_{\text{s},n}$ (the path loss H_{pl,n} of the interfering link between AP n and UT_{0} is then replaced by a mean value). Having introduced this interference power gain, we can further write I=P G. To characterize G’s statistical law, we make use of the closedform expression, parameterized by σ_{dB}, that has been proposed in [16]. In the next place, we evaluate the expression of the power received by UT_{0} (see (3)), i.e., P_{0}H_{0}=P H_{0}. Using (1), the gain H_{0} can be decomposed as H_{0}=H_{pl,0}H_{f,0}H_{s,0}. Since an analytical formula is available in the literature [16] for the probability law ${H}_{\text{fs},0}\triangleq {H}_{\text{f},0}{H}_{\text{s},0}$, we can further write: H_{0}=H_{pl,0}H_{fs,0}.
Fast and accurate calculation of (4) is made possible by the use of available analytical probability laws for both r.v.’s H_{fs,0} and G; this calculation is computed numerically by estimating the sample mean in (4). This makes our method of great interest as compared with traditional timeconsuming Monte Carlo simulations.
where K is a constant. Using (4) in association with (5), we are able to determine a mean network capacity by averaging the position of mobile users in the cell: this capacity corresponds to the average network capacity when there is an infinite number of users in each cell (U→∞). Note that, for the clarity of the illustrations, we express the mean power gain H_{pl,0} in dB; the expression of its associated pdf can be easily derived from (5).
3.2 Partial (statistical) CSI
Again, we can see the great interest of our numerical method: the mathematical expectation (6) can be calculated quickly and accurately—by numerical computation of the mean in (6)—because we have at our disposal, for both frequency reuse patterns FR1 and FR3, analytical expressions for the probability laws of the r.v.’s H_{fs,0} and G, parameterized by the shadowing standard deviation σ_{dB}[16]. Plotting the ergodic capacities for FR1 and FR3, namely ${C}_{\text{erg}}^{\text{FR1}}$ and ${C}_{\text{erg}}^{\text{FR3}}$, as functions of H_{pl,0}, may then exhibit an intersection point, each side of which is applied one of the FR1 or FR3 frequence reuse patterns. This intersection point also helps us classify users in the appropriate FFR band.
Using the ergodic capacity (6) and the pdf (5), we can also determine what the average gain is of swapping from FR1 to FR3 near the cell border, where users suffer from severe interference. This gain can be calculated by comparing the ergodic capacity for the FR1 pattern with the ergodic capacity for the FR3 pattern, which we denote by ${C}_{\text{erg}}^{\text{FR1}}\left({H}_{\text{pl},0}\right)$ and ${C}_{\text{erg}}^{\text{FR3}}\left({H}_{\text{pl},0}\right)$, respectively. Finally, if we consider the FFR area, we can associate to this potential gain the ratio of users concerned with this policy; note that this last ratio grows exponentially as H_{pl,0} diminishes.
3.3 FullCSI: approximated waterfilling strategy
We then consider the fullCSI scheme for which the APs have access to the instantaneous channel gains of their respective users. From the knowledge of the instantaneous channel conditions, we are able to propose a more efficient power allocation strategy in order to optimize the network capacity. We concentrate on what is known as the waterfillingintime strategy, since it is the optimal solution with respect to capacity optimization under an average transmit power constraint when both the AP and the mobile user have perfect and instantaneous knowledge of channel conditions [21].
We now closely analyze the complexity of the optimization problem (7)–(9). By carefully examining (7), we highlight an important issue: the waterfilling strategy in cell 0 obviously depends upon the waterfilling procedures performed in other (interfering) cells. Indeed, the power P_{ n } emitted by AP n results from the waterfilling strategy of cell n, and it therefore depends on the instantaneous channel gain H_{fs,n} between AP n and UT_{ n }. However, since we have chosen for a distributed power control and scheduling policy, a cell only has access to local information, namely, the actual channel gains of its own users. As such, the optimization problem (7)–(9) thus reveals too complex to solve and the waterfilling strategy needs to be approximated.
where L and J are both functions of H_{pl,0}. As mentioned before, our method here also allows for a fast and accurate (numerical) computation of the ergodic capacity (17) because the probability laws of H_{fs,0} and G can be expressed analytically [16].
where the ergodic capacities C_{erg}(H_{pl,0}) and ${C}_{\text{erg}}^{\text{WF}}\left({H}_{\text{pl},0}\right)$ can be computed easily, respectively, from (6) and (17) (computation of the optimal value ${C}_{\text{erg}}^{\text{WF}}\left({H}_{\text{pl},0}\right)$ is intractable and meaningless in a realistic scenario). And, as for the partialCSI scheme, we are interested in determining a potential point of intersection of the capacity curves ${C}_{\text{erg}}^{\text{wf,FR1}}\left({H}_{\text{pl},0}\right)$ and ${C}_{\text{erg}}^{\text{wf,FR3}}\left({H}_{\text{pl},0}\right)$, for both the frequency reuse patterns FR1 and FR3, and for different values of the shadowing standard deviation σ_{dB}. This intersection point also allows us to assign mobile users either to the FR1 band or to the FR3 band on the basis of their path losses.
4 Numerical results
In this section, we present numerical results related to the analysis of the FFR framework under different frequency reuse scenarios and for different values of the shadowing standard deviation. In Section 4.1, we first describe the simulation parameters. In Section 4.2, we compare results of the ergodic capacity as a function of the path loss under both the FR1 and FR3 patterns and we compute the potential gain of applying the FFR technique in different wireless channels. In Section 4.3, we improve the ergodic capacity results by performing the approximated waterfilling policy described in Section 3.2.
4.1 Simulation parameters
We use the following simulation parameters. We consider a system functioning at 1 GHz. We fix the cell radius to R=700 m, the farfield distance to d_{0}=10 m, and the attenuation coefficient to γ=3.2, which corresponds to a typical urban environment, as described by the COST231 reference model [22]. We consider a wireless channel that is subject to a largescale shadowing phenomenon that may vary in a [0,10] dB range. The maximum power available at the access point is P_{max}=5·10^{−3} W and the target received power is P^{∗}=10^{−12} W. The additive white Gaussian noise has variance W=10^{−15} W. As stated previously (see Section 3.1), the number of users per cell is considered to be infinite.
4.2 PartialCSI scheme
because in the FR1 scenario, the whole frequency band is used while this resource is divided in three orthogonal bands in the FR3 scenario. Next, we see that all capacity curves slightly take down on the left side of a fixed gain denoted ${H}_{\text{pl},0}^{\text{d}}$. This results from the maximum power constraint applied at the APs: ${H}_{\text{pl},0}^{\text{d}}$ is the demarcation point between the fully power controlled area (right side) and the area where APs emit at their maximum (saturated) power (left side); it follows immediately that ${H}_{\text{pl},0}^{\text{d}}={P}^{\ast}/{P}_{\text{max}}$.
Mean ergodic capacities $\mathbb{E}\left\{{\mathit{C}}_{\mathbf{\text{erg}}}^{\mathbf{\text{FR1}}}\right\}$ and $\mathbb{E}\left\{{\mathit{C}}_{\mathbf{\text{erg}}}^{\mathbf{\text{FR3}}}\right\}$ for the partialCSI scheme and for different values of the shadowing standard deviation σ _{ dB }
FFR area  10% users  1% users  

σ _{dB}  ${H}_{\text{pl},0}^{\ast}$  $\mathbb{E}\left\{{C}_{\text{erg}}^{\text{FR1}}\right\}$  $\mathbb{E}\left\{{C}_{\text{erg}}^{\text{FR3}}\right\}$  gain  users  $\mathbb{E}\left\{{C}_{\text{erg}}^{\text{FR1}}\right\}$  $\mathbb{E}\left\{{C}_{\text{erg}}^{\text{FR3}}\right\}$  gain  $\mathbb{E}\left\{{C}_{\text{erg}}^{\text{FR1}}\right\}$  $\mathbb{E}\left\{{C}_{\text{erg}}^{\text{FR3}}\right\}$  gain 
[dB]  [dB]  [bits/Hz]  [bits/Hz]  [%]  [%]  [bits/Hz]  [bits/Hz]  [%]  [bits/Hz]  [bits/Hz]  [%] 
0  −108.92  0.737  0.794  7.730  30.8  0.639  0.726  13.61  0.605  0.701  15.9 
1  −108.92  0.736  0.793  7.706  31.0  0.638  0.725  13.58  0.604  0.700  15.8 
2  −108.99  0.725  0.777  7.224  30.2  0.634  0.714  12.63  0.600  0.689  14.8 
3  −109.26  0.702  0.744  6.070  27.4  0.627  0.692  10.33  0.595  0.668  12.3 
4  −109.72  0.670  0.700  4.440  22.4  0.621  0.666  7.11  0.591  0.643  8.9 
5  −110.26  0.637  0.655  2.841  16.2  0.615  0.640  4.00  0.585  0.618  5.6 
6  −110.88  0.600  0.607  1.273  8.4  0.605  0.611  1.01  0.577  0.591  2.4 
7  N.A.  N.A.  N.A.  
8  N.A.  N.A.  N.A.  
9  N.A.  N.A.  N.A.  
10  N.A.  N.A.  N.A. 
We come to the conclusion that, for the partialCSI scheme, the FFR technique may increase the average ergodic capacity for users located near the cell border, in light or moderate shadowing environments. But, more importantly, besides this capacity increase, what really makes FFR of great interest is the fact that it may concern a non negligible proportion of mobile users, especially for small values of the shadowing standard deviation.
4.3 FullCSI scheme
From these observations, we can conclude that applying FFR is of no interest with respect to network capacity when APs perfectly know their local channel conditions (fullCSI scheme), though other simulation scenarios may lead to different conclusions.
5 Conclusion and future study
In this article, we investigated the problem of evaluating the performances of FFR in largescale shadowing environments. Instead of using Monte Carlo simulations or empirical measurements, we proposed a numerical method to easily compute the ergodic capacity in a fastfading environment. Using a distributed power control and scheduling policy, we examined both cases where APs have access to partial or fullCSI. We showed that our method allows for a fast and accurate analysis of the performances of FFR. In addition, it takes into account a broad range of shadowing environments.
The results presented in this article only hold for the simulation conditions specified; they may differ for other simulation scenarios. So a future article will pertain to applying the proposed method to other types of network (e.g., ad hoc networks, …) and using different simulation conditions (user distribution in the cell, transmit or target power, attenuation coefficient, …). Another perspective is to consider sectorization, which induces intersector interference and leads to totally different models for the power gains as well as for the interference. Yet another perspective concerns the analysis of the network performances in a slow fading environment using other performance measures, like the outage capacity or the outage probability.
Appendix 1
Validity of the assumption over the transmit power
In this Appendix, we analyze our assumption over the transmit power. First we investigate how the total number of users impacts the transmit power within one cell and we evaluate our assumption in the case where all cells have the same number of users. Then we examine the case where the total number of users varies among cells.
Same number of users per cell
Maximum values of the standard deviation ${\mathit{\sigma}}_{{\mathit{P}}_{\mathit{u}}}$ , as a proportion of the [0,1]dynamics of P _{ u, } for a total of U = 10, U = 100, and U =1000 users in a cell
U=10  U=100  U=1000  

$max\left\{{\sigma}_{{P}_{u}}\right\}$  16.3 %  5.7 %  1.8 % 
Variability in the number of users per cell
Proportion of scheduled users for which Δ <10 % and Δ <1 %
Δ  <10 %  <1 % 
proportion of scheduled users  93 %  42 % 
Appendix 2
A lower bound for the ergodic capacity ${C}_{\text{erg}}^{\text{WF}}\left({H}_{\text{pl},0}\right)$
Endnotes
^{a}Taking sectorization into consideration would indeed lead to a totally different interference model (see e.g., [24]).^{b}Since we focus on power gains only, the term power will then be omitted in subsequent paragraphs.^{c}Note that considering a fullCSI scheme at the network level would not be a realistic scenario because of the enormous amount of information that a central unit would have to manage.^{d}The notation [x]^{+} means max(0,x).^{e}In what follows, we write P_{0}(H_{fs,0}) instead of P_{0} to emphasize the fact that, for a fixed value of H_{pl,0}, P_{0} depends on the instantaneous channel state H_{fs,0}.
Declarations
Authors’ Affiliations
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