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Autonomous Uplink Intercell Interference Coordination in LTE Systems with Adaptively-tuned Interference Limits
Abstract and Figures
The inter-cell interference (ICI) problem in OFDMA wireless systems is a major impediment to attain high rates particularly for cell-edge users in reuse-1 systems. Interference mitigation techniques combat ICI by using proper resource allocation. Centralized allocation is not practical, particularly in heterogeneous networks, as it requires intensive signaling about interference and channel state information that may not always be practically available. This paper presents a framework for autonomous uplink inter-cell interference coordination in OFDMA wireless systems based on imposing an interference limit for each resource block in each cell. This framework gives a suboptimal closed form autonomous power allocation that can be applied autonomously at each terminal. We also propose two semi-autonomous heuristic and optimal adaptive schemes that use the overload indicator signal in LTE to tune the interference limits in order to achieve fairness among cells by alleviating the interference seen by the overloaded cells. Simulations show that the new suboptimal scheme exhibits the same performance as our initial scheme in the paper "Autonomous uplink inter-cell interference coordination in OFDMA-based wireless systems" which imposes an overall interference limit per cell. However, the two main advantages of the new framework are the ability of each terminal to calculate its power independently and that the adaptive interference limit schemes can be applied to adjust the level of interference seen by each cell. Simulations also show that the adaptive schemes achieve fairness among cells by increasing the rates of the overloaded cells at the expense of slightly reducing the rates of the lightly loaded cells.
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... This is because the statistical properties of the Poisson-Voroni tessellations are not well established yet as they are hard to derive. In [1] we proposed an autonomous uplink power allocation for OFDMA wireless systems. The allocation can be applied autonomously by each terminal in the system, and it does not require exchanging information between cells. ...
... In [1], we proposed an autonomous uplink power allocation where each cell s aims to maximize the sum of the signal to leakage and noise ratio (SLNR), which is the ratio between the received power and the leakage and noise power, of its terminals over all RBs as shown in (1). The leakage power of the cell is the interference produced by the cell on its neighbors. ...
... This paper presents a stochastic analysis for an OFDMA wireless system using our autonomous uplink power allocation scheme in [1] by modeling the terminals and the BSs with independent spatial PPPs. We derive a theoretical formula for the probability of coverage of a typical terminal on any given RB in an interference limited system given that we know the distance between this terminal and its serving BS, and its uplink transmission power on that RB. ...
Stochastic geometry has recently become a very attractive tool to model wireless systems because it provides a clear insight in the statistical performance of systems. In this paper, we present a stochastic analysis of the autonomous uplink power allocation scheme we proposed in [1]. This scheme hinges on defining interference limits to the amount of interference produced by each cell. Since the probability of coverage is a major metric to evaluate the performance of wireless systems, we develop herein a theoretical formula for the probability of coverage in an OFDMA wireless system using that proposed uplink power allocation. The provided theoretical formula is shown to be a good approximation to the actual probability of coverage at low interference limits, while at high interference limits it serves as a lower bound to the probability of coverage. The developed formula helps predicting the performance of the system at different interference limits.
This paper proposes a comprehensive study of uplink scheduling in multi-cell OFDMA networks. We first focus on two scenarios for the homogeneous case, one without and one with a Cloud-RAN (C-RAN), and explore how to design efficient practical uplink schedulers for those scenarios. To compute the best achievable performance (BAP) under complete information, we study a centralized multi-cell scheduler. To this end, we formulate an MINLP problem and show how to solve it quasi-optimally. Then, we study the performance of an existing practical local benchmark scheduler (LBM) in terms of goodput and losses. We compare its performance to BAP and show that LBM only yields 44% of BAP. To reduce this performance gap, we propose two practical enhancements for LBM, one per scenario. The enhanced scheduler for the first scenario yields 51% of BAP (70% for the second). To reduce the gap further, we propose a new scheduler inspired by soft-frequency reuse (SFR). Its performance is 69% (resp. 83%) of BAP. It outperforms LBM by 56% for the scenario without C-RAN (84% with C-RAN). We finally extend our SFR-based scheduler to heterogeneous networks and show that it outperforms LBM by 53% for the scenario without C-RAN (96% with C-RAN).
Power allocations in an interference-limited wireless network for global maximization of the weighted sum throughput or global optimization of the minimum weighted rate among network links are not only important but also very hard optimization problems due to their nonconvexity nature. Recently developed methods are either unable to locate the global optimal solutions or prohibitively complex for practical applications. This paper exploits the d.c. (difference of two convex functions/sets) structure of either the objective function or constraints of these global optimization problems to develop efficient iterative algorithms with very low complexity. Numerical results demonstrate that the developed algorithms are able to locate the global optimal solutions by only a few iterations and they are superior to the previously-proposed methods in both performance and computation complexity.
Orthogonal Frequency Division Multiplexing Access (OFDMA) has been increasingly deployed in various emerging and evolving cellular systems to reduce interference and improve overall system performance. However, in these systems Inter-Cell Interference (ICI) still poses a real challenge that limits the system performance, especially for users located at the cell edge. Inter-cell interference coordination (ICIC) has been investigated as an approach to alleviate the impact of interference and improve performance in OFDMA-based systems. A common ICIC technique is interference avoidance in which the allocation of the various system resources (e. g., time, frequency, and power) to users is controlled to ensure that the ICI remains within acceptable limits. This paper surveys the various ICIC avoidance schemes in the downlink of OFDMA-based cellular networks. In particular, the paper introduces new parameterized classifications and makes use of these classifications to categorize and review various static (frequency reuse-based) and dynamic (cell coordination-based) ICIC schemes.
Consider the problem of joint uplink scheduling and power allocation. Being
inherent to almost any wireless system, this resource allocation problem has
received extensive attention. Yet, most common techniques either adopt
classical power control, in which mobile stations are received with the same
Signal-to-Interference-plus-Noise Ratio, or use centralized schemes, in which
base stations coordinate their allocations.
In this work, we suggest a novel scheduling approach in which each base
station, besides allocating the time and frequency according to given
constraints, also manages its uplink power budget such that the aggregate
interference, "Noise Rise", caused by its subscribers at the neighboring cells
is bounded. Our suggested scheme is distributed, requiring neither coordination
nor message exchange.
We rigorously define the allocation problem under noise rise constraints,
give the optimal solution and derive an efficient iterative algorithm to
achieve it. We then discuss a relaxed problem, where the noise rise is
constrained separately for each sub-channel or resource unit. While
sub-optimal, this view renders the scheduling and power allocation problems
separate, yielding an even simpler and more efficient solution, while the
essence of the scheme is kept. Via extensive simulations, we show that the
suggested approach increases overall performance dramatically, with the same
level of fairness and power consumption.
Reverse link (or uplink) performance of cellular systems is becoming increasingly important with the emergence of new uplink-bandwidth intensive applications such as Video Share, where end users upload video clips captured through their mobile devices. In particular, it is important to design the system to provide good user throughput in most of the coverage area, including at the cell edge. Soft fractional frequency reuse (FFR) is one of the techniques for mitigating inter-cell interference in cellular systems, leading to overall spectral efficiency enhancements and/or cell edge throughput improvements. We propose a novel algorithm that dynamically creates efficient soft FFR patterns on the uplink of orthogonal frequency division multiple access (OFDMA) based cellular systems; this allows the system to ?automatically? adapt to user traffic distribution and system layout. Our algorithm is based on systematically ascending towards a local maximum of the system-wide sum of user utilities, which depend on user throughputs. We show that this can be done in a semi-autonomous fashion: each sector does its resource allocation independently, with only an infrequent periodic exchange of interference costs between neighboring sectors. The proposed algorithm, called Multi-sector Gradient for Uplink (MGR-UL), allocates in-sector resources (power, frequency, time-slots to each user) in a way that simultaneously takes into account both the benefit to its ?own? users' utility and the cost of creating interference to neighboring sectors; along with that each sector estimates the cost of interference to itself. Extensive simulation results show that significant performance benefits (up to 69% in total throughput in some typical scenarios) can be achieved with respect to a baseline approach. Simulations also show the automatic formation of soft FFR patterns.
In cellular wireless communication systems, transmitted power is
regulated to provide each user an acceptable connection by limiting the
interference caused by other users. Several models have been considered
including: (1) fixed base station assignment where the assignment of
users to base stations is fixed, (2) minimum power assignment where a
user is iteratively assigned to the base station at which its signal to
interference ratio is highest, and (3) diversity reception where a
user's signal is combined from several or perhaps all base stations. For
the above models, the uplink power control problem can be reduced to
finding a vector p of users' transmitter powers satisfying p⩾I(p)
where the jth constraint p<sub>j</sub>⩾I<sub>j</sub>(p) describes
the interference that user j must overcome to achieve an acceptable
connection. This work unifies results found for these systems by
identifying common properties of the interference constraints. It is
also shown that systems in which transmitter powers are subject to
maximum power limitations share these common properties. These
properties permit a general proof of the synchronous and totally
asynchronous convergence of the iteration p(t+1)=I(p(t)) to a unique
fixed point at which total transmitted power is minimized
The inter-cell interference (ICI) problem in OFDMA-based wireless systems becomes a major impediment to attaining high rates particularly for cell-edge users in reuse-1 systems. Interference mitigation techniques attempt to combat ICI by using proper resource allocation schemes. Using centralized algorithms is not practical, particularly in HetNet environments, as these algorithms require intensive signaling, interference, and channel state information that may not always be practically available. In this paper, we present a framework for autonomous uplink ICIC in OFDMA-based wireless systems. The framework is based on imposing an overall interference limit for each cell. We use the proportional fair algorithm for resource block assignment and the power allocation problem is formulated as a maximization of the sum of the signal to leakage and noise ratio (SLNR) subject to the cell interference on other cells below a certain threshold. We propose a suboptimal closed form method, and an iterative allocation using Newton's method. These schemes do not need coordination between the cells where the resource allocation can completely be performed autonomously. Simulations show that relaxing the interference constraints improves the performance of the two proposed algorithms which exhibit better performance than the trivial equal power allocation. Comparison with a centralized scheme that uses global information shows good performance with acceptable degradation in the spectral efficiency which decreases as the interference limit increases.
In this paper we propose a heuristic inter-cell coordination scheme for scheduling users in an opportunistic multi-cell network. The scheme is mainly designed to be implemented in the uplink of networks that utilize the third generation partnership project long term evolution (3GPP LTE) technology. The objective of the scheme is to coordinate the uplink transmission in neighboring cells such that inter-cell interference is mitigated and the aggregate utility of the mobile users is maximized. In addition to the heuristic approach we formulate the inter-cell scheduling coordination as an integer programming problem. While the optimal solution cannot be realized in practice due to the computational complexity, it provides good insights of how well different algorithms perform. Numerical results show that the proposed coordination method delivers near-optimal performance in the exemplified cases.
This paper proposes a joint proportionally fair scheduling and dynamic power spectrum adaptation algorithm for wireless multicell networks. The proposed system allows multiple base-stations in a multicell network to be coordinated by exchanging interference pricing messages among each other. The messages summarize the effect of intercell interference, and they are functions of transmit power spectra, signal-to-noise ratios, direct and interfering channel gains, and the proportional fairness variables for each user. The use of interference pricing allows the transmit power spectra and user schedule within each base-station to be optimized jointly, while taking into consideration both the intercell interference and the fairness among the users in multiple cells. This paper proposes two power spectrum optimization methods, one based on the Karush-Kuhn-Tucker (KKT) condition of the optimization problem, and another based on the Newton's method. The proposed methods can achieve a throughput improvement of 40%-55% for users at the cell edge as compared to a conventional per-cell optimized system, while maintaining proportional fairness.
Heterogeneous networks (het-nets) - comprising of conventional macrocell base
stations overlaid with femtocells, picocells and wireless relays - offer
cellular operators burgeoning traffic demands through cell-splitting gains
obtained by bringing users closer to their access points. However, the often
random and unplanned location of these access points can cause severe near-far
problems, typically solved by coordinating base-station transmissions to
minimize interference. Towards this direction, the 3rd generation partnership
project Long Term Evolution-Advanced (3GPP-LTE or Rel-10) standard introduces
time-domain inter-cell interference coordination (ICIC) for facilitating a
seamless deployment of a het-net overlay. This article surveys the key features
encompassing the physical layer, network layer and back-hauling aspects of
time-domain ICIC in Rel-10.
For wireless cellular communication systems, one seeks a simple
effective means of power control of signals associated with randomly
dispersed users that are reusing a single channel in different cells. By
effecting the lowest interference environment, in meeting a required
minimum signal-to-interference ratio of ρ per user, channel reuse is
maximized. Distributed procedures for doing this are of special
interest, since the centrally administered alternative requires added
infrastructure, latency, and network vulnerability. Successful
distributed powering entails guiding the evolution of the transmitted
power level of each of the signals, using only focal measurements, so
that eventually all users meet the ρ requirement. The local per
channel power measurements include that of the intended signal as well
as the undesired interference from other users (plus receiver noise).
For a certain simple distributed type of algorithm, whenever power
settings exist for which all users meet the ρ requirement, the
authors demonstrate exponentially fast convergence to these settings