Orthogonal Frequency Division Multiplexing Access Research Articles

Efficient Turbo Inter Leaver Based MIMO OFDM System For Low Complexity Outdoor Communication

An arrangement of multiple antennas in both the receiver and transmitter called Multiple Input and Multiple Output (MIMO). The Orthogonal Frequency Division Multiplexing (OFDM) is enabled in MIMO system for high data wireless communications. Combination of both MIMO and OFDM Access (OFDMA) is a growing technology in next generation communication systems. In this work, the Bit Error Rate (BER) performance of MIMO-OFDMA is analyzed with Orthogonal Space Time Block Code (OSTBC), Maximum Ratio Combining (MRC) and Turbo coding scheme over flat fading channel are named as MIMO-MRC-OFDMA. OSTBC is a transmit diversity scheme, which is utilized for delivering an efficient transmission with high peak data rates that significantly improves the capacity of communication systems. Successively, the MRC diversity solves transmit and receive diversity from an OSTBC. MRC approach is applied in the receiving end for summing and weighing the received signals from the multiple paths. Besides, turbo coding scheme is utilized for error correction in a given code rate. The proposed system performance is evaluated in light of BER by varying the number of receive and transmit antennas such as 2×2, 2×4, 4×2 and 4×4.

A channel state information updating method based on the first-order Gauss Markov model under time-varying underwater acoustic channel

Because of the time-varying characteristic and the large propagation delay in underwater acoustic (UWA) channel, adaptive resource allocation in UWA orthogonal frequency-division multiplexing access (OFDMA) system cannot be performed with the assumption of perfect channel state information (CSI) at the transmitter. Therefore, this paper focuses on the imperfect CSI and proposes a CSI updating method to reduce the impact of imperfect CSI on the performance of resource allocation under time-varying UWA channel. First, we analyze the channel estimation error of OFDM system under time-varying UWA channel and define the per-subcarrier channel temporal correlation (PSCTC). Finally, a CSI updating method based on the first-order Gauss-Markov model (GM-CSI) is proposed for optimizing CSI feedback in the UWA channel with large propagation delay, where the channel estimation error and PSCTC factor are considered as two influencing factors in this model. Simulation results show that the GM-CSI can effectively mitigate the effect of time on CSI effectiveness, and the resource allocation by using GM-CSI has higher effective throughput and lower BER than that of imperfect CSI.

Robust QoE-Driven DASH Over OFDMA Networks

In this paper, the problem of effective and robust delivery of Dynamic Adaptive Streaming over HTTP (DASH) videos over an orthogonal frequency-division multiplexing access (OFDMA) network is studied. Motivated by a measurement study, we propose to explore the request interval and robust rate prediction for DASH over OFDMA. We first formulate an offline cross-layer optimization problem based on a novel quality of experience (QoE) model. Then the online reformulation is derived and proved to be asymptotically optimal. After analyzing the structure of the online problem, we propose a decomposition approach to obtain a user equipment (UE) rate adaptation problem and a BS resource allocation problem. We introduce stochastic model predictive control (SMPC) to achieve high robustness on video rate adaption and consider the request interval for more efficient resource allocation. Extensive simulations show that the proposed scheme can achieve a better QoE performance compared with other variations and a benchmark algorithm, which is mainly due to its lower rebuffering ratio and more stable bitrate choices.

Linear approximation for bandwidth‐power product minimisation for downlink OFDM access in cognitive radio

In this Letter, the author proposes a low complex sub-optimal algorithm for resource allocation in spatial spectrum usage of a cognitive radio network by minimising the bandwidth power product while satisfying secondary user's quality of service and interference constraints of primary users. The author has proposed an approach to solve the problem by using linear programming techniques. The author compares the proposed method with the optimal solution to show its effectiveness.

Energy Efficiency Based Joint Computation Offloading and Resource Allocation in Multi-Access MEC Systems

With the rapid growth of computation demands from mobile applications, mobile-edge computing (MEC) provides a new method to meet requirement of high data rate and high computation capability. By offloading the latency-critical or computation-intensive tasks to the edge server, mobile devices (MDs) could save energy consumption and extend battery life. However, unlike cloud servers, resource bottlenecks in MEC servers limit the scalability of offloading. Hence, computation offloading and resource allocation need to be optimized. Toward this end, we consider a multi-access MEC servers system in which Orthogonal Frequency-Division Multiplexing Access (OFDMA) is used as the transmission mechanism for uplink. In order to minimize energy consumption of MDs, we propose a joint optimization strategy for computation offloading, subcarrier allocation, and computing resource allocation, which is a mixed integer non-linear programming (MINLP) problem. First, we design a bound improving branch-and-bound (BnB) algorithm to find the global optimal solution. Then, we present a combinational algorithm to obtain the suboptimal solution for practical application. Simulation results reveal that the combinational algorithm performs very closely to the BnB algorithm in energy saving, but it has a better performance in average algorithm time. Furthermore, our proposed solutions outperform other benchmark schemes.

Adaptive Downlink OFDMA System With Low-Overhead and Limited Feedback in Time-Varying Underwater Acoustic Channel

Because of the time-varying characteristic and the large propagation delay in underwater acoustic (UWA) channel, adaptive resource allocation in UWA orthogonal frequency-division multiplexing access (OFDMA) system cannot be performed with the assumption of perfect channel state information (CSI) at the transmitter. Therefore, this paper focuses on the outdated CSI and proposes an adaptive downlink OFDMA system with low-overhead and limited feedback in time-varying UWA channel. First, a data fitting method is proposed for channel reconstruction, which shows better performance and lower overhead than the traditional group quantization method in simulations. Second, we define the per-subcarrier channel temporal correlation (PSCTC) and long-term statistical mean value as two indicators of outdated CSI, for optimizing CSI feedback in UWA channel with large propagation delay. Finally, a CSI selection method based on channel correlation attenuation (CCA) factor is proposed, where CSI used for feedback is selected between outdated-instantaneous CSI and outdated-average CSI according to the CCA factor in the current observation period. Sea trail results show that the CSI reconstruction based on data fitting method has lower bit error ratio and less overhead than traditional group quantization method. The PSCTC coefficient is related to the feedback delay, and the long-term statistics is stationary, which means that the outdated-average CSI is suitable for channel feedback. The CSI selection method based on CCA factor has better performance than that only uses outdated-instantaneous CSI or outdated-average CSI under different time-varying channel conditions in experimental results.

Joint Resource Allocation and Software-Based Reconfiguration for Energy-Efficient OFDMA-PONs

The widely deployed passive optical network (PON) is a cost-effective way to create a front/backhaul carrier for ubiquitous wireless access connectivity. To hide the difference of frame formats and multiplexing methods, an orthogonal frequency division multiplexing access PON (OFDMA-PON) was proposed. In upstream transmission, multiple optical network units (ONUs) from the same segment or residential area can transmit data through a shared group of orthogonal low-bitrate subcarriers (SCs) during different time slots (TSs) within the entire transmitting period. In downstream transmission, the optical line terminal can send control signaling of reconfiguring ONUs’ parameters so that SCs can be reallocated among segments, thus sleeping segments with empty data load while ensuring service continuity. Though energy savings can be ensured for upstream data transmission by using existing solutions, only frequency and time domains are involved. It is meaningful for us to make the flexible and energy- efficient configuration of transmitting power and modulation level assigned for ONUs in terms of improving energy efficiency. In this paper, we investigate joint resource allocation for energy-efficient upstream data transmission in OFDMA-PONs, with the help of software-based ONU reconfiguration. We formulate the problem, and the upper bound of energy saving is also derived. To solve the problem in a short span of time, we propose a novel heuristic algorithm whose effectiveness has been demonstrated by extensive simulations.

Jamming game for secure orthogonal frequency division multiplexing access systems

In a communication system, security and reliability have an important impact on the quality of services. In this study, the authors investigate power and subcarrier allocation of a transmitter over the downlink of an orthogonal frequency division multiplexing access system with security considerations among users in the presence of a jammer. This study considers the interaction between the jammer and the transmitter as a zero-sum game where the objective function is the transmitter's secrecy sum-rate. To allocate the available resources, they solve the optimisation problem of each player to characterise its optimal strategies. Considering the intractability of the closed form Nash equilibrium for general power regime, they study a matrix game for obtaining maxmin and minmax solutions and demonstrate the validity of inequality m a x m i n ≥ m i n m a x . Moreover, they show the existence of pure Nash equilibrium point for low total power regimes of the transmitter and the jammer and attain an expression for each regime. Finally, they examine the jamming power effect on the secrecy sum-rate and acquire lower and upper bounds of this rate using simulations.

Efficient OFDMA for LiFi Downlink

Light-fidelity (LiFi) relies on visible light communication to provide multiuser access in a small cell. For higher throughput, this paper studies the resource allocation problem for an orthogonal frequency-division multiplexing access scheme for LiFi downlink transmissions and presents a joint design of the bias level, power and subcarrier allocation. Decomposing the original optimization into subproblems, we first develop an efficient algorithm for the bias optimization, along with its performance analysis in terms of convergence properties and the global optimality. We then propose several power and subcarrier allocation strategies, including an optimal algorithm and a near-optimal simplified method, to satisfy different complexity and accuracy requirements. Finally, we present two algorithms to jointly optimize the bias, power, and subcarrier, and analyze the convergence properties in terms of both the objective and the solution. Comprehensive simulation results under practical LiFi system setups illustrate the effectiveness of the proposed algorithms.

Energy efficiency and spectral efficiency tradeoff in downlink OFDMA systems with imperfect CSI

In this paper, we investigate the tradeoff between energy efficiency (EE) and spectral efficiency (SE) in downlink orthogonal frequency division multiplexing access (OFDMA) systems, whilst considering the channel estimation cost and the corresponding effect of imperfect channel state information (CSI) on SE and EE. The problem is formulated as a multi-objective optimization to determine the optimal pilot transmission power, data transmission power and subcarrier assignment, and then transformed into a single-objective optimization problem, which is a non-convex mixed-integer nonlinear programming (NCMINP) and NP-hard. To address it, we propose an efficient algorithm by adopting alternating optimization and convex optimization methods in lower power region as well as approximate conversion and branch-and-bound methods in high power region. Simulation results analyze and validate the performance of EE-SE tradeoff.

Research on modified selected mapping technique for PAPR reduction and reception performance improvement in 8QAM-OFDM optical access systems

BackgroundOrthogonal frequency division multiplexing has been regarded as one of the promising technologies for the optical access systems application. However, high peak-to-average power ratio will cause the signal distortion and enable the used amplifier to operate in nonlinear section.MethodsIn this paper, a modified selected mapping technique is introduced for reducing the peak-to-average power ratio and enhancing reception performance of the downstream 8QAM-OFDM signals in optical access systems.ResultsCompared with the traditional selected mapping technique, the PAPR value of 8QAM-OFDM optical downstream signals with 128 subcarriers is reduced by almost 2 dB using modified SLM when the complementary cumulative distribution function probability value is 10−2. Moreover, the bit error rate performance will be improved, while 10Gbit/s 8QAM-OFDM optical downlink signals are transmitted over single mode fiber with different access length.ConclusionsHence, the proposed scheme becomes more efficient for the future optical orthogonal frequency division multiplexing access systems.

Experimental demonstration of SCMA-OFDM for passive optical network

Abstract We introduces a novel architecture for next generation passive optical network (PON) based on the employment of sparse code multiple access (SCMA) combined with orthogonal frequency division multiplexing (OFDM) modulation, in which the binary data is directly encoded to multi-dimensional codewords and then spread over OFDM subcarriers. The feasibility of SCMA-OFDM-PON is verified with experimental demonstration. We show that the SCMA-OFDM offers 150% overloading gain in the number of optical network units compared with the orthogonal frequency division multiplexing access.

Dynamic power and subcarrier allocation for downlink OFDMA systems under imperfect CSI

In this paper, we investigate the joint power and subcarrier allocation for the downlink of orthogonal frequency division multiplexing access systems, with various practical considerations including imperfect estimation of channel state information, a stochastic packet arrival and a time-varying channel. To this end, we formulate the stochastic optimization problem to minimize the time-averaged power consumption, whilst keeping all queues at the base-station stable. The data transmission rate is defined as a function of the transmit power, the assigned subcarrier and the estimation error. With the aid of Lyapunov optimization method, the original problem is transformed into a series of mixed-integer programming problems, which are then solved via the dual decomposition technique. We determine analytical bounds for the time-averaged power consumption and queue length achieved by our proposed algorithm, which depend on the channel estimation error. Moreover, the theoretical analysis and simulation results show that the proposed algorithm reduces the energy consumption at the expense of queue backlog (i.e., achieves a energy-queue tradeoff), and quantitatively strike the energy-queue tradeoff by simply tuning an introduced control parameter V.

A Novel Subspace Based Method for Compensation of Multiple CFOs in Uplink MIMO OFDM Systems

The tri-linear data model for an arbitrary multi-user uplink multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) system comprising orthogonal frequency division multiplexing access users and co-channel (CC) users is formulated in this letter. A novel subspace-based method for multi-access interference cancellation and CC data separation in multi-user MIMO OFDM systems, utilizing the independent manifolds spanned by user’s carrier frequency offset matrix, is proposed in this letter. The system constraints under which the proposed subspace nulling method works are derived, which includes the under-determined cases, where minimum mean square error/zero forcing methods fail.

Multi-user Communication Based OFDM-UWB System under Gaussian and Non-Gaussian Noisy Channel

For a multiuser Ultra Wide Band (UWB) system, the performance is mainly limited by inherent noise and interferences such as Multiple Access Interference (MAI) and Inter Symbol Interference (ISI) which are poorly approximated by a Gaussian distribution because of their impulsive nature. A combination between Orthogonal Frequency Division Multiplexing (OFDM) and Interleave Division Multiple Access (IDMA) has been considered as an efficient technique due to its bandwidth efficiency, and robustness against MAI and ISI. Moreover, OFDM-IDMA technique has the ability to allow frequency diversity and performance improvement with the help of efficient FEC coding technique such as Low Density Parity Check (LDPC) code. Therefore, this paper addresses the performance of LDPC-coded-IDMA-based OFDM-UWB communication system under Gaussian and non-Gaussian noisy channels. Three different models as Symmetric Alpha Stable (SS), Laplacian model and Gaussian Mixture Model (GMM), have been introduced for approximating the non-Gaussian noise which are more realistic for UWB system. Simulation results show that LDPC-coded OFDM-IDMA system is robust against noise and interferences in UWB indoor environment, and achieves good performance under Cauchy (SS) model rather than other non-Gaussian models with simple Chip-By-Chip (CBC) iterative receiver. Furthermore, the proposed system gains a significant performance improvement compared with the un-coded OFDM-IDMA-UWB system under the studied noise models.

Novel OBI noise reduction technique by using similar-OBI estimation in optical multiple access uplink

A novel optical beat interference (OBI) noise reduction technique for optical multiple access uplink using the same wavelength is presented. The OBI noise is estimated by similar-OBI and mitigated by subtracting estimated OBI noise using the digital signal processing technique. To derive similar-OBI, an RF clipping tone is transmitted out of signal band. We numerically and experimentally verified the validity of proposed OBI noise reduction method in orthogonal frequency division multiplexing access (OFDMA) passive optical network (PON) uplink where the OBI noise has been reduced and signal to noise ratio (SNR) has been improved from 4.4 dB to 10.6 dB.

A low-overhead switched-mode Energy saving strategy for OFDMA-PON downstream transmission

Abstract Green networks and energy saving have become important topics of worldwide concern and attracted intensive attention from both academia and industry. As orthogonal frequency division multiplexing access passive optical network (OFDMA-PON) employs more advanced and energy-consuming digital signal processing (DSP) techniques than other PON systems, it is of critical importance to improve the energy efficiency of OFDMA-PONs. In this paper, we propose a switched-mode energy saving strategy with low overhead for OFDMA-PON downstream transmission. By embedding information in the short training sequences (STSs) of the OFDM frame preamble, the STSs serve a twofold purpose: providing time synchronization and carrying the information with regard to the destination of the frame, hence eliminating the need for extra upper-layer control or information exchange between OLT and ONU. As STS is an inherent part of the OFDM frame, the proposed scheme requires little extra overhead in transmission time and hardware cost. The ONU continues the subsequent processing only when it is the intended receiver, while the unintended ONUs drop the frame and stands by to save energy. In order to achieve a tradeoff between energy saving and delay performance, the scheme switches between single-ONU mode and team mode where either a single ONU or a team of ONUs are the targeted receivers. We verify the feasibility of proposed scheme through experiments and evaluate the performance in terms of energy saving and packet delay by simulations. The results indicate that substantial energy can be saved compared with conventional OFDMA-PON systems.

Experimental Demonstration of an Indoor VLC Positioning System Based on OFDMA

We propose an indoor visible light communication (VLC) and positioning system using the orthogonal frequency division multiplexing access (OFDMA) scheme, which can provide both indoor positioning and communications. Three subcarriers with the maximum received signal intensity with respect to three light-emitting diodes (LEDs) are selected for indoor positioning based on the trilateration algorithm. The experiment results show that the proposed system with quadrature phase shift keying (QPSK) mapping offers a mean positioning error and an error vector magnitude of 1.68 cm and more than 15 dB, respectively.

Efficient subchannel allocation based on clustered interference alignment in ultra-dense femtocell networks

In this paper, we exploit clustered interference alignment (IA) for efficient subchannel allocation in ultra-dense orthogonal frequency division multiplexing access (OFDMA) based femtocell networks, which notably improves the spectral efficiency as well as addresses the feasibility issue of IA. Our problem is formulated as a combinatorial optimization problem which is NP-hard. To avoid obtaining its optimal solution by exhaustive search, we propose a two-phases efficient solution with low-complexity. The first phase groups all the femtocell user equipments (FUEs) into disjoint clusters, and the second phase allocates subchannels to the formed clusters where IA is performed. By doing this, the intra-cluster and inter-cluster interferences are mitigated by clustered IA and subchannel allocation in ultra-dense femtocell networks, respectively. Also, low-complexity algorithm is proposed to solve the corresponding sub-problem in each phase. Simulation results demonstrate that the proposed scheme not only outperforms other related schemes, but also provides a close performance to the optimal solution.

Experimental demonstration of bidirectional NOMA-OFDMA visible light communications.

We propose a non-orthogonal multiple access (NOMA) scheme combined with orthogonal frequency division multiplexing access (OFDMA) for visible light communications (VLC), which offers a high throughput, flexible bandwidth allocation and a higher system capacity for a larger number of users. Bidirectional NOMA-OFDMA VLC is experimentally demonstrated. The effects of power allocation and channel estimation on the bit error rate performance are investigated. The experiment results indicate that accurate channel estimation can eliminate the inter-user interference more effectively. The optimum power allocation ratios for uplink and downlink are both about 0.25 in the case of two users.