Division Multiple Access Cellular Systems Research Articles

Interference-Aware Accurate Signal Recovery in Sub-1 GHz UHF Band Reuse-1 Cellular OFDMA Downlinks

Reuse-1 systems operating in the sub-1 GHz UHF band are limited by substantial co-channel interference (CCI). In such orthogonal frequency division multiple access (OFDMA) cellular systems, the inter-sector or inter-tower interference (ITI) makes accurate signal recovery quite challenging as sub-1 GHz bands only support single-input single-output (SISO) links. Interference-aware receiver algorithms are essential to mitigate the ITI in such low-frequency bands. Such algorithms enable ubiquitous mobile broadband access over the entire homeland, say with >95% geographical coverage with quality of service guarantees. One element of the interference-aware signal recovery is the least-squares-based joint channel estimation scheme that uses non-orthogonal pilot subcarriers. This estimator is then compared with a variant that uses orthogonal pilot subcarriers to bring out the advantage of this joint estimator. It is shown that the proposed joint estimator requires fewer pilots to be well-determined when compared to its under-determined orthogonal counterpart. Moreover, it is easy to implement and does not require any knowledge of channel statistics. This work also derives a compensation factor needed for the interference-aware detector in the presence of inter-carrier interference (ICI) originating from multiple transmitters. Simulation results show that the proposed joint channel estimator outperforms traditional estimators at moderate to high frequency selectivity. The proposed compensation factor to the joint detector is found to be essential for recovering the transmitted signal in the absence of phase-tracking pilots.

Congestion relief in downlink OFDMA cellular system using inter-cell relay

This paper proposes the concept of inter-cell relay for downlink orthogonal frequency division multiple access (OFDMA) cellular systems, which uses multi-hop to relay calls from overloaded cells to light-load neighboring cells. It is shown that when using inter-cell relay, the number of calls in the congestion cell can be significantly increased. The congestion cell is divided into two parts. One is called non-relay area (NRA), in which a call directly communicates with the base station (BS) of a congested cell. The other is called relay area (RA), in which a call communicates with the BS of a neighboring cell through a relay station (RS). The two parts have different user-call densities. By adjusting the densities of two parts, we will maximize the number of supported calls inside a congested cell. The results show the benefits gained from inter-cell relay in congestion relief, which can reduce cell congestion by fully utilizing the available resources in the neighboring cells.

Enabling Signal Space Diversity for MU-MIMO/OFDMA Cellular Systems That Employ Frequency Diversity

Signal space diversity (SSD) is a technique to obtain diversity without loss in spectral efficiency. Despite this, SSD has been minimally studied in multiuser scenarios. In this paper, SSD is operated in the uplink of orthogonal frequency division multiple access (OFDMA) cellular systems that exploit frequency diversity, use the optimum maximum-likelihood detector (MLD) and employ antenna arrays at the receivers. Thus, a multiuser multiple-input-multiple-output (MU-MIMO) system that works in presence of own-cell and co-cell interference (CCI) is taken into account. In order to diminish CCI, fractional frequency reuse (FFR) is used in the cellular system. For a feasible MLD implementation, a matrix structure of the received signals is obtained in order to use the sphere decoder algorithm. An exact closed-form expression to calculate the pairwise error probability between two multidimensional symbols is derived. From this, a bit error rate (BER) upper bound is found for single user scenarios. This BER expression is an accurate lower bound for high diversity orders in multiuser scenarios. The BER asymptotic analysis shows that our approach maintains the SSD multidimensional diversity, the spatial diversity of MIMO and the frequency diversity. This allows to overload the system by minimizing the BER and increasing the spectral efficiency, simultaneously.

Analysis of Relay-Assisted OFDMA Cellular Systems with Different Frequency Reuse Techniques

Co-channel interference (CCI) degrades performance of cellular networks. Therefore, this paper addresses CCI problem in relay-assisted (R-A) orthogonal frequency division multiple access cellular systems. Analytical treatments are conducted. The network performance improvements through reducing CCI effects are evaluated using different four interference mitigation models. These models represent R-A strict fractional frequency reuse (FFR) technique with frequency reuse factor (FRF) \(=\) 3, R-A strict FFR technique with FRF \(=\) 4, R-A sectored FFR technique and R-A soft frequency reuse (SFR) technique. Each model contains two different configurations for further network performance improvement. The first configuration assumes three relay stations (RSs) per cell (RPC) while the other one proposes 6 RPC. The best RSs locations in terms of SIR are proposed. Moreover, closed-form expressions for worst cases CEU’s SIR, cell centre user’s SIR and inner radius are implemented. These expressions are used to compare different models using various performance evaluation metrics. The results demonstrate that the two configurations in R-A sectored FFR and R-A strict FFR models provide equal CEU’s SIR. The reason of this result can be attributed to applying equal power transmission strategy. Moreover, the two configurations of R-A SFR network attain different CEU’s SIR values. This result can be accredited to the SFR power control factor. The work outcomes attain much higher CEU’s SIR improvement. Therefore, the outage probability is decreased. Thus, more users can be covered by the network. Accordingly, the total network cost is reduced. So, this treatment improves the network performance while keeping lower network cost.

Downlink Subcarriers Required Analysis for Two-Hop OFDMA Cellular System

In Orthogonal frequency division multiple access (OFDMA) cellular system, the user at the different location of the cell requires different number of subcarriers to satisfy the user’s data rate requirement. In this paper, based on the Interference to Signal Ratio (ISR), we analyze the average number of downlink subcarriers required at inner and outer region for the two-hop OFDMA cellular system. Firstly we propose a two-hop OFDMA cellular system model. Then based on this model, we calculate the Cumulative Distribution Function of ISR at inner and outer region respectively, and obtain the theoretical analysis results for the average number of subcarriers required at two regions of the reference cell. Finally, with the help of the numerical calculation and Monte-Carlo simulations, we analyze the impact of the various parameters on the average number of subcarriers required of the two-hop cellular system. And the results are compared with the average number of subcarriers required of the one-hop cellular system.

Multi-user Performance of Relay-Aided MC-CDMA Downlink System Employing Transmitter Preprocessing Based on Feedback of Vector Quantized CSI

The achievable bit-error-rate (BER) performance of a relay-aided downlink (DL) multi-carrier code division multiple access (MC-CDMA) cellular system, where multi-user interference (MUI) and inter-relay interference (IRI) are the dominant channel impairments is investigated in this paper, principally from a transmitter signal processing perspective. Specifically, in this paper, we investigate the considered system by employing three cooperation strategies. The MUI and IRI are mitigated with the aid of the transmitter preprocessing (TP) operated at the base station (BS) and relays, respectively. The TP operated at the BS that suppresses the MUI at the relays is based on the maximum-signal-to-interference-plus-noise ratio approach, while that operated at the relays that mitigates the IRI relies on the classical transmit minimum-mean-square-error principle. Further, the TP in our work is constructed with the aid of vector-quantized channel state information (VQ-CSI) acquired via feedback channels. Our simulation study shows that relay-aided cooperative DL MC-CDMA system employing TP, results in better achievable BER, as the TP helps in achieving the relay-diversity by mitigating the DL-MUI and IRI at the relays and DMSs, respectively. Furthermore, our study demonstrates that the resultant BER performance of the VQ-CSI based TP remains close to that achieved with the perfect CSI based TP. Hence, we advocate that the feedback of the VQ-CSI to the transmitter for conceiving the preprocessing can be considered as an efficient approach, particularly in a frequency division duplex type wireless system.

Self-organized dynamic resource allocation using fraction frequency reuse scheme in long term evolution networks

Inter-Cell Interference (ICI) from neighboring cells is the major challenge that degrades the performance of Orthogonal Frequency Division Multiple Access (OFDMA) cellular mobile systems, particularly for cell edge users. An efficient technique to mitigate ICI is the interference coordination. The most commonly ICI Coordination (ICIC) technique is the Fractional Frequency Reuse (FFR), which effectively mitigates ICI by applying different reuse factors to Users’ Equipments (UEs) situated in different regions in each cell. This paper presents a novel Self-Organized FFR Resource Allocation (SORA) scheme that automatically selects the optimal RA to inner and outer regions of the cell, based on coordination between neighboring evolved NodeBs (eNBs), through a message passing approach over Long Term Evolution (LTE)-X2 interfaces. The performance of the proposed scheme is evaluated using MATLAB and compared with different combinations of RA as well as with frequency reuse-1 and reuse-3 schemes. Simulation results show that the proposed scheme improves the cell-edge performance and achieves high degree of fairness among UEs compared to reference RA schemes.

Effect of Multiple Fingers on Receiver Performance in CDMA Forward Link

Rake receiver can be implemented in any communication system, which has possibility of multipath components. Rake receiver can take advantage from these multipath components and combined all multipath components to achieve optimum result. The bit error rate (BER) performance of the Code Division Multiple Access (CDMA) cellular system based on IS-95 standard in the presence of an additive white Gaussian noise (AWGN) and interference has been investigated in this paper. The performance of the RAKE receiver is analysed considering of different number of fingers. The performance is evaluated under noise and interference for the CDMA forward link. Rake receiver is used in CDMA-based (Code Division Multiple Access) systems and can combine multipath components, which are time-delayed versions of the original signal transmission. Combining is done in order to improve the signal to noise ratio at the receiver and so reduced the BER. The BER has been studied for different numbers of Rake fingers.

Coordinative Stochastic Resonance Filtering Based Inter-cell Interference Suppression in FrFT-OFDMA Cellular Systems

In fractional Fourier transform (FrFT) based orthogonal frequency division multiple access cellular systems, the inter-cell inference combined with multiple linear frequency modulated (LFM) signals degrades the system performance at the receiver. A novel coordinative stochastic resonance (CSR) filtering method, which is described by an overdamped bistable dynamical system, is proposed to improve the signal-to-interference-plus-noise ratio (SINR), which is defined to characterize CSR in optimal FrFT domain. Simulation results show that the output SINR is effectively improved by the use of CSR filter, which is based on the underlying mechanism that interfering LFM signals cooperate with noise in part to promote particles motion periodically with larger amplitude in a symmetric double well.

Achieving Pareto Optimal Power Tracking Control for Interference Limited Wireless Systems via Multi-Objective H_2/H_infty Optimization

To track a desired signal-to-interference-plus-noise-ratio (SINR) that allows for higher system throughput and better link quality in an interference limited wireless communication system, a feedback power controller is designed to obtain the optimal SINR tracking control. In this paper, we propose the multi-objective (MO) H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> optimal power control for interference limited wireless systems that provides quality solutions for both objectives. The considered multi-objective H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> power tracking control is complicated in nature and an equivalent formulation that minimizes the upper bounds of both objectives is proposed. Furthermore, the resulting constraints for the MO H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> power control problem is transformed as three linear matrix inequalities (LMIs), leading to a LMIs-constrained MO problem (MOP). By combining the LMI toolbox in MATLAB with an evolutionary searching algorithm, a set of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> solutions called Pareto optimal solutions can be obtained and a particular solution can be selected based on the tradeoff determined by the system designer. Finally, to illustrate our design procedure, we provide numerical simulations for a typical interference limited wireless system, i.e., a direct-sequence code division multiple access (DS-CDMA) cellular system. Simulation results demonstrate that the optimal tradeoff indeed can be achieved by the proposed MO H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> power control.

Hybrid link adaptation for inter-carrier interference adjustment in orthogonal frequency division multiple access cellular uplink systems

In the uplink of orthogonal frequency division multiple access (OFDMA) systems, rate adaptation may not be exploitable mainly because of severe inter-carrier interference (ICI). In this study, the authors propose a novel hybrid link adaptation that simultaneously uses power adaptation for ICI adjustment and rate adaptation for capacity improvement in the uplink of OFDMA systems. The proposed hybrid link adaptation determines a maximum allowable transmission power according to the channel gain and velocity of each user, enabling all users to have a guaranteed loss of carrier to interference ratio (CIR). Moreover, the proposed hybrid link adaptation adjusts the transmission rate according to CIR of each user, while keeping the maximum allowable transmission power unchanged. The performance of the proposed hybrid link adaptation is analysed in OFDMA cellular uplink systems. Finally, the analytic results are verified by computer simulation.

Biorthogonal Frequency Division Multiple Access Cellular System with Angle Division Reuse Scheme

In conventional OFDMA cellular systems, mobile stations (MSs) suffer from large ICI in fully loaded cellular environments with full cell frequency reuse, especially at the cell-edge. The fundamenta...

Resource allocation for energy efficient cellular systems

The need for more throughput in wireless cellular networks has been increasing in recent years. It has led to an increase in operational costs due to higher energy use as operators deploy more cell sites or increase transmission power at existing ones to satisfy demand. Energy costs are a major expense, and reducing them is a priority. This article presents a scheduler which aims to solve the problem of energy efficient resource allocation in orthogonal frequency division multiple access (OFDMA) cellular systems. The suggested approach is to make the resource scheduling process also consider energy costs as well as allow it to manipulate these by exploiting time/frequency vs energy efficiency trade-offs that are present in the system. The energy efficient score-based scheduler (EESBS) is a novel scheduler which takes energy costs into account when allocating resource blocks (RBs) to users. This allows it to promote energy efficiency in the system alongside throughput and fairness maximization. One of the means it has to manipulate users' expended energy is the bandwidth expanded mode (BEM). BEM is a technique that allows the scheduler to decrease a user's energy consumption by allocating it more RBs and maintaining a constant data rate. This is possible when the energy consumption is dominated by the energy used for data communication as opposed to control channel overhead transmission. Time compression mode (TCoM) is a technique that is complementary to BEM. It allows for energy savings through a reduction of the number of allocated RBs to a user when the energy consumption is dominated by the transmission of signaling traffic. Both BEM and TCoM need to be employed by an energy-aware scheduler like EESBS in order to extract the maximum performance gains. A realistic framework modeling future cellular systems is established to test the performance of the proposed techniques. Within this framework, EESBS generates an average energy saving of 29% over a frequency selective proportional fair (FsPF) benchmark. EESBS coupled with BEM or TCoM achieves a saving of 38% over the same benchmark. These savings are achieved with no detriment to user satisfaction in terms of achieved data rate.

Improved Multiple-Scenario Radio Network Dimensioning for WCDMA

In wideband code division multiple access (WCDMA) cellular systems, the coverage radius of a cell depends on its current capacity level. As a result, existing WCDMA radio network dimensioning approaches require that coverage and capacity planning be carried out jointly in an iterative manner in order to obtain the minimum site count needed while fulfilling both coverage and capacity requirements. This requires relatively long computational time, particularly when there are many scenarios or what-if cases to be considered. To overcome this problem, we propose an alternative radio network dimensioning approach where coverage planning and capacity planning can be carried out separately to reduce computational time. Besides, a portion of the values calculated in the initial iteration is preserved in a lookup graph, allowing future what-if analysis to be accomplished rapidly. Simulation results show that, unlike the existing approach, the planning and what-if analysis times of the proposed dimensioning approach are independent of the number of sce-narios considered. Lastly, we present a few case studies and show that the proposed dimensioning method can give the same prediction accuracy as the existing method.

Dynamic resource allocation in relay‐assisted OFDMA cellular system

ABSTRACTThis study investigates a relay‐assisted orthogonal frequency division multiple access cellular system with joint consideration of direct and relaying paths. In this system, a novel implementation adopting full‐duplex relaying is proposed for joint relay‐destination selection, subcarrier and power allocation. This new implementation can be shown to significantly improve spectrum efficiency as compared with the conventional half‐duplex relaying orthogonal frequency division multiple access. In addition, the proposed scheme enables flexible controllability on the tradeoff between system capacity and user fairness. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Optimum processing gain for a hybrid direct sequence/slow frequency hopping code division multiple access cellular system over Rayleigh fading channels: the direct sequence/slow frequency hopping code division multiple access case

In this paper, the optimum value of the processing gain of a hybrid direct sequence/slow frequency hopping code division multiple access cellular system, when operating in a Rayleigh fading environment, is examined. The optimization is based on the maximization of the average spectral efficiency achieved, expressed in bits per second per Hertz, which is estimated in terms of the achievable average channel capacity (in the Shannon–Hartley theorem sense) per user during the operation over a broadcast cellular time-varying link. The analysis leads to a simple theoretical novel closed-form expression for the optimal processing gain that is related with the system's parameters but becomes independent of hopping frequencies assigned to each system's cell. Finally, the expression derived can be useful for the practical design of a direct sequence/slow frequency hopping code division multiple access cellular system and for an initial quantitative analysis. Numerical results are presented to illustrate the presented analysis. Copyright © 2011 John Wiley & Sons, Ltd.

A Novel OFDMA Cellular System Based on Multiple FrFT Angles Reuse Scheme

In conventional Orthogonal Frequency Division Multiple Access (OFDMA) cellular systems, the fundamental cause of inter-cell interference (ICI) is that the signals from serving Base Station (BS) and interference BSs, are modulated by same exponential bases, at same subcarrier. In this letter, an effective OFDMA cellular mobile system based on multiple fractional Fourier transform (FrFT) angle reuse scheme is proposed. In this system, FrFT angle is regarded as new resource, which can be optimally allocated to each BS of cellular system. Because chirp bases, generated by different FrFT with optimally allocated angle, are mutually approximately orthogonal at same subcarrier, ICI can be perfectly suppressed. Both theory and simulations show the essential advantages over conventional OFDMA cellular systems for ICI suppression and bit error rate.

BER Performance Analysis of CDMA Reverse Link under AWGN Channel

bit error rate (BER) performance of the Code Division Multiple Access (CDMA) cellular system based on IS-95 standard in the presence of an additive white Gaussian noise (AWGN) and interference has been investigated in this paper. The performance is evaluated under two types of decision feedback receivers for the CDMA reverse link. These two feedback receivers are: (a) Hard decision Viterbi decoder in which coded bit is estimated based on Hamming Distance method and (b) Soft decision Viterbi decoder in which Euclidean Distance method is used for coded bit estimation. The comparison of these two techniques of decision feedback receivers of CDMA is done under AWGN channel. The performance of CDMA system is shown in graphs between BER versus Energy per bit to Noise Ratio i.e. Eb/No ratio. KeywordsBER, Chip Rate, Chip period, Eb/No, PN codes.

Multi‐group linear turbo equalization with intercell interference cancellation for MC‐CDMA cellular systems

AbstractIn this paper, we investigate multi‐group linear turbo equalization using single antenna interference cancellation (SAIC) techniques to mitigate the intercell interference for multi‐carrier code division multiple access (MC‐CDMA) cellular systems. It is important for the mobile station to mitigate the intercell interference as the performance of the users close to cell edge is mainly degraded by the intercell interference. The complexity of the proposed iterative detector and receiver is low as the one‐tap minimum mean square error (MMSE) equalizer is employed for mitigating the intracell interference, while a simple group interference canceller is used for suppressing the intercell interference. Simulation results show that the proposed iterative detector and receiver can mitigate the intercell interference effectively through iterations for both uncoded and coded signals. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Joint Centralized Power Control and Cell Sectoring for Interference Management in CDMA Cellular Systems in a 2D Urban Environment

The interference reduction capability of cell sectoring and power control algorithms have been considered separately as means to decrease the interference in Code Division Multiple Access (CDMA) cellular systems. In this paper, we present Switched-Beam (SB) and Rotatable Equal Sectoring (RES) techniques for CDMA cellular systems in a 2D urban environment. In the SB technique by using a number of fixed, independent, or directional antennas we increase the downlink capacity of the CDMA systems. Also in the RES method, the equal sectors of the base stations are rotating together to decrease the inter-cell and intra-cell interferences. Also in this paper we use centralized power control to overcome the near-far problem. Simulation results indicate that the proposed techniques considerably increase the capacity of the CDMA cellular systems compared to ordinary Equal Sectoring (ES) method.