Interference-limited Cellular Systems Research Articles

Remarks on a stochastic geometric model for interference-limited cellular communications

A plethora of stochastic geometry (SG) models have been developed for cellular communications, especially in the context of internet of things (IoT) applica tions. A typical assumption in such models is that base stations (BS) are de ployed in the Euclidean plane as a spatial poisson point process (PPP) of some density λ, with each communicating equipment transmitting at some power p. The usual objective of these models is to characterize the cellular coverage prob ability in both the downlink (DL) and uplink (UL) directions. In this article we expose, in the form of four remarks, the peculiar behavior of a baseline stochas tic geometric model of an interference-limited cellular system. Specifically, we reveal that under some assumptions, the coverage probability in both the UL and DL directions for this system is independent of both λ and p, flagrantly contra dicting intuition. The aim of the article is by no means to invalidate the use of SG in modeling communications systems, but rather to point out that such modeling may not be adequate all the time.

Optimal Design and Energy Efficient Binary Resource Allocation of Interference-Limited Cellular Relay-Aided Systems With Consideration of Queue Stability

The topic of subcarrier and power allocation in the downlink of an orthogonal frequency division multiple access decode-and-forward relaying system is presented with the objective of encompassing system stability and interference limitations in one inclusive problem. The introduced model is designed to maximize the overall throughput of the cell-edge users that are served by relay stations. We analyze the stability requirement of buffers in the base station and the corresponding relay stations, and define the rate constraints in order to guarantee queue stability without requiring a priori knowledge of arrival traffic’s statistics. The explained model results in a nonconvex optimization problem, and therefore, we employ a time-shared technique to achieve the closed form solution, which is only applicable when subcarriers can be shared by the users during one time-slot. In the case where the subcarriers are not allowed to be time-shared, we introduce a computationally efficient optimal binary subcarrier and power allocation method, in addition to a power conservative allocation mechanism. Using geometric-programming and monomial approximation techniques, we show that the proposed conservative approach, although nonconvex, can be solved in polynomial time. We also study the impact of adjustable time-slot division and interference-tolerance parameters on improving system performance. The extensive simulation results demonstrate the success of the proposed methods in terms of stabilizing the queues, improving the throughput by 30% and energy efficiency gain by 90%, in comparison with the existing similar models in the literature.

Average spectral efficiency of multi‐modulation cellular systems

Fourth generation systems are proposed to create extended wireless networks by applying frequency reuse techniques based on a cellular scheme. However, the total bandwidth needed depends on the channel bandwidth and the signal-to-interference protection ratio. The procedure to compute the performance of such interference-limited cellular systems is introduced in this study for systems with various modulation schemes (quadrature phase shift keying, 16 and 64 quadrature amplitude modulation), founding the percentage of the cell area that could be managed with each modulation, considering full load sectors. The conditions to share the same subcarriers between two users in next to base stations are also presented. The total bandwidth required to deploy a network with three sectored cells with 10 MHz channel results to be 360 MHz. This is a big issue since the size of the bands is generally 200 MHz, and it could limit the expected performance of these systems.

Throughput analysis of interference-limited MIMO-based cellular systems

Cell-wide mean throughputs on cellular system downlinks using multiple-input/multiple-output (MIMO) techniques is a topic of interest to researchers, cellular operators and equipment manufacturers. In previous work, the authors have tested the validity of the noise model (in the MIMO context) for the co-channel interference (CCI), wherein multipath-averaged CCI powers are added to the thermal noise power that results in an overall interference-plus-noise floor. This approximation simplifies throughput computation with only a minor loss of accuracy, while allowing the cell-wide mean throughput per user to be derived analytically. The key to the analysis is (i) the invocation of the noise model for CCI, and (ii) the demonstration that the overall CCI power has a distribution that is log-normal for any user distance from the base station (BS). This enables us to extend a previous analysis for single-cell (noise-only) MIMO systems to the interference-limited case.We present the analytical method and demonstrate its accuracy.

CSI-Based Resource Allocation for OFDM Downlink Broadband Multimedia Services in Cellular Systems

Using channel state information (CSI) in resource allocation in Orthogonal Frequency Division Multiplexing (OFDM) wireless cellular systems has been recently considered to improve the overall throughput. This paper presents an analytical framework to compare the performance of two resource allocation schemes, using CSI-based and random sub-carrier selection, in terms of their user outage probability and throughput in relation to the system load in frequency-selective fading and interference-limited environments. Analytical and simulation results are in good agreement and indicate that a proper use of CSI can achieve substantially better performance in interference limited cellular systems as compared to the blind approach and thus show the benefits of multi-user diversity.

Downlink throughput statistics in interference-limited cellular systems with multi-element antennas

In this study, we first quantify cell-wide mean throughputs in interference-limited single-input-single-output (SISO) and multi-element antenna (MEA) cellular systems along various system-level design dimensions, including: MEA array size; frequency-reuse factor; antenna pattern (omni-directional or sectorized); degree of coding; allowable constellation size; and transmitor receive-adaptation. Next, we examine the validity of a noise-model for co-channel interference present in MEA links. Using system simulations, we show that this model is valid under all conditions, with varying levels of accuracy.

Relay Architectures for 3GPP LTE-Advanced

The Third Generation Partnership Project's Long Term Evolution-Advanced is considering relaying for cost-effective throughput enhancement and coverage extension. While analog repeaters have been used to enhance coverage in commercial cellular networks, the use of more sophisticated fixed relays is relatively new. The main challenge faced by relay deployments in cellular systems is overcoming the extra interference added by the presence of relays. Most prior work on relaying does not consider interference, however. This paper analyzes the performance of several emerging half-duplex relay strategies in interference-limited cellular systems: one-way, two-way, and shared relays. The performance of each strategy as a function of location, sectoring, and frequency reuse are compared with localized base station coordination. One-way relaying is shown to provide modest gains over single-hop cellular networks in some regimes. Shared relaying is shown to approach the gains of local base station coordination at reduced complexity, while two-way relaying further reduces complexity but only works well when the relay is close to the handset. Frequency reuse of one, where each sector uses the same spectrum, is shown to have the highest network throughput. Simulations with realistic channel models provide performance comparisons that reveal the importance of interference mitigation in multihop cellular networks.

Multistage MMSE PIC Space–Time Receiver With Non-Mutually Exclusive Grouping

The arrival of new data services for wireless mobile communications requires an efficient use of the available bandwidth. Interference-limited cellular systems based on code-division multiple access (CDMA) can benefit from multiuser detection (MUD) and beamforming with antenna array to reduce multiple-access interference. Group-based techniques have been proposed to reduce the complexity of space-time MUD and have been shown to provide a performance-complexity tradeoff between matched filtering and full MUD. In this paper, the intergroup interference, which is a limiting factor in group-based systems, is reduced using multistage parallel interference cancellation after group-based minimum mean square error (MMSE) linear filtering. In addition, the extra resources that are available at the receiver are exploited by sharing users among groups. The proposed receiver is shown to converge, as the number of stages increases, to the full space-time MMSE linear MUD filter. The results show that the new approach provides bit error rate (BER) performance close to the full MUD receiver at a fraction of the complexity.

Multiuser Diversity for Antenna Optimal Combining in Interference-Limited Systems

This letter explores the benefit of exploiting multiuser diversity for improving the performance of antenna optimum combining in the interference-limited cellular system. The asymptotic achievable rate is derived to assess this improvement as well as to reveal the interaction of multiuser diversity and optimum combining. For the overloaded case where the number of antennas is insufficient to suppress all the impact of interferers, it is shown that multiuser diversity considerably improves the achievable rate. For the underloaded case where the number of interferers is smaller than that of antennas, multiuser diversity can also provide additional rate increments so that it is possible to use a fewer number of antennas for optimum combining without sacrificing the system achievable rate.

Analytical expression for uplink C/I-distribution in interference-limited cellular radio systems

The cumulative distribution function (CDF) of the uplink carrier-to-interference ratio (C/I) of an interference-limited cellular radio network is presented. Based on this and on the assumptions of Gaussian inter-cell interference and flat Rayleigh fading channels, the CDF of the bit error ratio for a QPSK-modulated space/time/code division multiple access system deploying a zero-forcing block linear equaliser with receive antenna diversity is derived.

Cellular performance bounds via shotgun cellular systems

This paper considers two-dimensional interference-limited cellular radio systems. It introduces the shotgun cellular system that places base stations randomly and assigns channels randomly. Such systems are shown to provide lower bounds to cellular performance that are easy to compute, independent of shadow fading, and apply to a number of design scenarios. Traditional hexagonal systems provide an upper performance bound. The difference between upper and lower bounds is small under operating conditions typical in modern TDMA and CDMA cellular systems. Furthermore, in the strong shadow fading limit, the bounds converge. To give insights into the design of practical systems, several variations are explored including mobile access methods, sectorizing, channel assignments, and placement with deviations. Together these results indicate cellular performance is very robust and little is lost in making rapid minimally planned deployments.

Dynamic channel allocation in interference-limited cellular systems with uneven traffic distribution

Most recently proposed wireless dynamic channel allocation methods have used carrier-to-interference (C/I) information to increase the system performance. Power control is viewed as essential for interference-limited systems. However, the performance of such systems under an imbalance of load among cells, as may occur often in microcells, is largely unknown. Here, we study a typical interference-limited dynamic channel allocation policy. Calls are accepted if a channel can be assigned that will provide a minimum C/I, and power control and intracell handoffs are used to maintain this level. We focus on the relationship between system performance and the amount of imbalance in load among neighboring cells. Previous studies for systems that do not use C/I information have found that dynamic channel allocation (DCA) outperforms fixed channel allocation (FCA) in all but heavily loaded systems with little load imbalance. We present two principal new results. First, we find that with use of C/I information, the difference in performance between FCA and DCA (in terms of throughput or blocking probability) is increasing with load imbalance. DCA was found to be more effective in congestion control at the cost of a slightly lower call quality. Second, we find that use of power control to maintain a minimum C/I results in two equilibrium average power levels for both DCA and FCA, with DCA using a higher average power than FCA, and that while DCA's power is increasing with load imbalance, FCA's average power is decreasing with load imbalance.