Interference Management of OFDMA Uplinks

Aiming at the problem of resource allocation strategies limiting the system transmission rate in indoor visible light communication and radio frequency (VLC/RF) systems, a new resource allocation method is proposed, and orthogonal frequency-division multiple access (OFDMA) technology is introduced. The capacity of the communication system is effectively increased. In the VLC/RF system model based on OFDMA, the Lyapunov optimization method is used to transform the time averaging problem into a series of single-slot online problems to reduce its computational complexity, the optimization problem is decomposed into three independent subproblems, and the Lagrange relaxation method and convex optimization theory are used to solve the subproblems to maximize the average transmission rate of the system, and the Lyapunov drift is used to ensure the stability of the system. Simulation verifies that the Lyapunov optimization algorithm does not require iteration, which improves the optimization speed to a high extent. The simulation results show that the proposed resource allocation strategy effectively balances the system queue length and transmission rate, improves the average transmission rate of the system to the greatest extent under the premise of ensuring the stability of the system, and compares with other algorithms from the aspects of system stability and transmission rate, which proves the effectiveness of the Lyapunov optimization algorithm.

IEEE 802.11ax is the emerging standard release of the next-generation of wireless local area networks (WLANs). The core idea is to enhance the area throughput in high-density deployment scenarios through high efficiency (HE) multiple access control (MAC) and physical layer (PHY) technology. Orthogonal frequency-division multiple access (OFDMA) technology has been selected as the key techniques in 802.11ax. However, the existing OFDMA technology has an interference diffusion problem, i.e., multi-user parallel transmission aggravates the interference to the surrounding cells. To solve these problems, we propose a spatial clustering group-based OFDMA protocol (SCG-OFDMA). In the SCG-OFDMA protocol, the concurrent transmission of nodes with close spatial distance can reduce the degree of interference diffusion. Particularly, the SCG-OFDMA protocol and process are designed based on the 802.11ax Multi-User (MU) MAC framework. The computer simulation analysis results demonstrate an improvement in the throughput of 36.62% at most.

In this paper, we propose a joint resource allocation, routing, and connection admission control (CAC) scheme for uplink transmission in orthogonal frequency division multiple access (OFDMA) relay networks with cooperative relaying. For cooperative relaying, relay station can relay uplink data from mobile station (MS) to base station with cooperation of the MS using transmit diversity. Transmit diversity can be achieved by virtual MISO via distributed space---time coding. The proposed scheme jointly allocates OFDMA resources and selects path for each user with CAC to maximize the upink throughput of cooperative OFDMA relay networks. The basic OFDMA resource unit is considered as a resource element which is one subcarrier over one OFDMA symbol. An efficient multi-choice multi-dimensional knapsack (MMKP) algorithm is presented for the proposed scheme. The proposed MMKP algorithm provides a unified framework which is applicable to OFDMA networks with and without cooperative relaying. We evaluate the performance of the proposed scheme with and without cooperative relaying in a hilly terrain with heavy tree density by using OPNET-based simulation. We show that the cooperative relaying improve the uplink system throughput compared with non-cooperative relaying, and the proposed scheme outperforms the conventional link quality-based scheme in both cooperative and non-cooperative relay networks.

OFDM (Orthogonal Frequency Division Multiplexing) is a well-known multicarrier modulation technique that allows high-rate data transmissions over multipath broadband wireless channels. By usingOFDM, a high-rate data stream is split into a number of lower-rate streams that are simultaneously transmitted on different orthogonal subcarriers. Thus, the broadband channel is decomposed into a set of parallel frequency-flat subchannels; each one corresponding to an OFDM subcarrier. In a single user scenario, if the channel state is known at the transmitter, the system performance can be enhanced by adapting the power and data rates over each subcarrier. For example, the transmitter can allocate more transmit power and higher data rates to the subcarriers with better channels. By doing this, the total throughput can be significantly increased. In a multiuser scenario, different subcarriers can be allocated to different users, which constitutes an orthogonal multiple access method known as OFDMA (Orthogonal Frequency Division Multiple Access). OFDMA is one of the principal multiple access schemes for broadband wireless multiuser systems. It has being proposed for use in several broadband multiuser wireless standards like IEEE 802.20 (MBWA: http://grouper.ieee.org/groups/802/20/), IEEE 802.16 (WiMAX: http://www.ieee802.org/16/, 2011) and 3GPP-LTE (http://www.3gpp.org/). This chapter focuses on the OFDMA broadcast channel (also known as downlink channel), since this is typically where high data rates and reliability is needed in broadband wireless multiuser systems. In OFDMA downlink transmission, each subchannel is assigned to one user at most, allowing simultaneous orthogonal transmission to several users. Once a subchannel is assigned to a user, the transmitter allocates a fraction of the total available power as well as a modulation and coding (data rate). If the channel state is known at the transmitter, the system performance can be significantly enhanced by allocating the available resources (subchannels, transmit power and data rates) intelligently according to the users’ channels. The allocation of these resources determines the quality of service (QoS) provided by the system to each user. Since different users experience different channels, this scheme does not only exploit the frequency diversity of the channel, but also the inherent multiuser diversity of the system. In multiuser transmission schemes, like OFDMA, the information-theoretic system performance is usually characterized by the capacity region. It is defined as the set of rates 6

This chapter presents a brief overview of the OFDMA technique used in 4G WiMAX (Worldwide Interoperability for Microwave Access) and 4G LTE (Long-Term Evolution) cellular system. It is shown that OFDMA is an extension of FDMA, where each frequency band is placed at the null of the adjacent frequency band. This is governed by the well-known “Fourier transform,” so that adjacent frequency bands are orthogonal to each other. OFDMA is a full-duplex communication system. The communication link is maintained in both directions in the time domain known as time division duplex (TDD). Numerous illustrations are used to bring students up to date in key concepts, underlying principles, and practical applications of “Fourier transform,” spectrum, and orthogonal properties of spectrum, leading to OFDMA. Construction of OFDMA channels from the FCC-allocated band is presented to illustrate the concept.

Being the predominant air interface of next-generation wireless standards, orthogonal frequency division multiple access (OFDMA) is well known for its flexibility in allocating subcarriers to different mobile users according to their different fast channel variations. Numerous research studies have demonstrated that OFDMA can bring substantial capacity gain when the subcarriers are optimally allocated. Nonetheless, practical systems can hardly afford optimal subcarrier allocation, because frequent re-optimization performed at the same timescale as fast fading variation would lead to excessively high computational and signaling costs. As a result, most practical systems settle for low-complexity schemes that operate far from the optimum, thus making them unable to enjoy the large capacity gain predicted by theoretical studies. To address this problem, we propose a novel alternative, termed the slow adaptive OFDMA, to drastically reduce the computational and signaling costs. The proposed scheme adapts subcarrier allocation at a much slower timescale than that of channel fading variation, yet achieves similar system capacity and quality of service (QoS) levels as the optimal fast adaptive OFDMA. Moreover, it possesses several attractive features. First, neither prediction of channel state information nor specification of channel fading distribution is needed for subcarrier allocation. As such, the algorithm is robust against any mismatch between actual channel state/distributional information and the one assumed. Secondly, although the optimization problem arising from our proposed scheme is non-convex in general, based on recent advances in chance-constrained optimization, we show that it can be approximated by a certain linear program with provable performance guarantees. In particular, we only need to handle an optimization problem that has the same structure as the fast adaptive OFDMA problem, yet we are able to enjoy lower computational and signaling costs. Last but not the least, instead of relying on standard but abstract linear program solvers such as the interior-point method to solve the aforementioned linear program, we can exploit its special structure and design a provably efficient algorithm for it. The proposed algorithm not only has a transparent engineering interpretation but is also easy to implement at the base stations of practical systems.

Orthogonal Frequency Division Multiple Access (OFDMA) gains increasing attention for broadband, high data rate fixed/mobile communications. As the basic modulation technique in OFDMA is orthogonal frequency division multiplexing (OFDM), it is severely affected by carrier frequency offset (CFO). The CFO causes loss of orthogonality among the subcarriers. In this paper, we propose a method for joint estimation of CFO and cell ID. Cell ID represents the number of the preamble selected by the base station in the downlink. The proposed method is based on the preamble suggested for OFDMA mode of WMAN where the preamble is loaded on every third subcarrier. The proposed method is an extension of Schimdl and Coxpsilas method. We also propose a method for estimation of CP length. Van de Beekpsilas method is adapted for the estimation of CP length in OFDMA mode of WMAN. Simulations are conducted for AWGN and frequency selective channels to demonstrate the performance of the proposed methods.

Performance analysis of colour coded multiple access (CCMA) orthogonal frequency division multiplexing (OFDM) visible light communication (VLC) System is discussed in this paper. First, characteristics of optical OFDM are studied. Second, CCMA is achieved using three laser diodes (LD): red, green and blue LDs. Finally, OFDM-modulated signals are optically modulated with red, green and blue LDs (RGB LDs). Then these three optical signals are combined together to form white light which illuminates as well as provides high-speed data transmission. The performance of the proposed CCMA OFDM VLC system is studied using constellation diagrams, signals and spectrum comparisons. The system is simulated for the data rate of 10 Gbps.

Orthogonal frequency division multiple access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) are the two major multiple access schemes for 4G wireless communications. In long term evolution (LTE) the downlink multiple access scheme is based on OFDMA and the uplink multiple access scheme is based on SC-FDMA. In this letter, we derive the fundamental performance difference between OFDMA and SC-FDMA, and present the general performance comparison between them. Analytical results show that OFDMA performance upper bounds SC-FDMA in the sense of capacity. The conclusion is further confirmed by numerical results.

In this paper, cyclic autocorrelation function (CAF) is used for signal type identification. Orthogonal frequency division multiple access (OFDMA) signals with different cyclic prefix lengths are generated by using software defined radio nodes. According to test results, by using the OFDMA frame structure, it is possible to distinguished between OFDMA signals, hence signal parameters can be blindly differentiated. Additionally, jamming signal and OFDMA signals can also be distinguished from each other. Real-time measurement results are given to demonstrate the performance of the investigated approach.

Opportunistic non-contiguous OFDMA scheduling framework for future B5G/6G cellular networks

Statistical characterization of complex baseband signals for multicarrier multiuser systems is studied. In particular, we derive rigorously the asymptotic distribution of uplink orthogonal frequency division multiple access (OFDMA) baseband signals for different subcarrier allocation schemes. This allows us to establish some useful statistical properties of the power process of the corresponding baseband signals. We show that, in most scenarios, the power process converges to a /spl chi//sup 2/ process. This result allows us to characterize the peak-to-average power ratio (PAPR) for OFDMA signals, a parameter considered critical to real system implementation. Specifically, we are able to compare, both qualitatively and quantitatively, the PAPR for different subcarrier allocation schemes. We show first that the contiguous subcarrier allocation and equally spaced interleaving share identical PAPR. Further, we show that random interleaving, which is implemented in the existing OFDMA standard, has a larger PAPR than that of an equally spaced interleaving scheme. The complementary cumulative distribution function of PAPR for random interleaving is shown to be greater than that of equally spaced interleaving by a factor that is identical to the number of users in the OFDMA system.

Radio resource management for user-relay assisted OFDMA-based wireless networks

QoS scheduling of uplink resources in OFDMA networks

Orthogonal Frequency Division Multiple Access (OFDMA) is a promising technique which can provide high capacity in the wireless environments. Also cooperative communications are effectively used to achieve cooperative diversity for high reliability. In this paper, we propose the pair-based OFDMA frame structure in which cooperative users transmit their blocks in turns for achieving cooperative diversity in OFDMA systems. In order to guarantee the fairness of users we also propose a dynamic resource allocation scheme based on cooperative communications. Through the proposed scheme, user fairness can be achieved by sharing the remained capacity of users near the base station (BS) with users who are farther from the base station. For this purpose, the center-located users select their partner whose channel suffers the lowest SNR to BS among their neighbor set. Based on the partner selection results, subchannels are assigned to cooperative and non-cooperative users. Through simulations in the IEEE 802.16e system environments, it is shown that the proposed scheme outperforms the non-cooperation based scheme in terms of user fairness.