Low-Complexity Orthogonal Spectral Signal Construction for Generalized OFDMA Uplink With Frequency Synchronization Errors

In orthogonal frequency-division multiplexing, the total spectral resource is partitioned into multiple orthogonal subcarriers. These subcarriers are assigned to different users for simultaneous transmission in orthogonal frequency-division multiple access (OFDMA). In an unsynchronized OFDMA uplink, each user has a different carrier frequency offset (CFO) relative to the common uplink receiver, which results in the loss of orthogonality among subcarriers and thereby multiple access interference. Hence, OFDMA is very sensitive to frequency synchronization errors. In this paper, we construct the received signals in frequency domain that would have been received if all users were frequency synchronized. A generalized OFDMA framework for arbitrary subcarrier assignments is proposed. The interference in the generalized OFDMA uplink due to frequency synchronization errors is characterized in a multiuser signal model. Least squares and minimum mean square error criteria are proposed to construct the orthogonal spectral signals from one OFDMA block contaminated with interference that was caused by the CFOs of multiple users. For OFDMA with a large number of subcarriers, a low-complexity implementation of the proposed algorithms is developed based on a banded matrix approximation. Numerical results illustrate that the proposed algorithms improve the system performance significantly and are computationally affordable using the banded system implementation     

Multiuser OFDM Using Cyclic Time Shift for Wireless Uplink Broadcasting

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper presents our investigation into reducing the multiple access interference (MAI) introduced by symbol timing misalignment in an OFDMA uplink system. To combat the MAI, we provide a new OFDMA symbol frame employing a simple symbol repetition coupled with a cyclic time shift for typical OFDMA uplink scenarios. Theoretical analysis and computer simulations are used to assess the performance of the proposed OFDMA uplink system. It is found that this scheme can decrease the probability of destroying the orthogonality among the users and provide the MAI-free reception.     

An efficient resource-allocation scheme for spatial multiuser access in MIMO/OFDM systems

Fast adaptive transmission has been recently identified as a key technology for exploiting potential system diversity and improving power-spectral efficiency in wireless communication systems. An adaptive resource-allocation approach, which jointly adapts subcarrier allocation, power distribution, and bit distribution according to instantaneous channel conditions, is proposed for multiuser multiple-input multiple-output (MIMO)/orthogonal frequency-division multiplexing systems. The resultant scheme is able to: 1) optimize the power efficiency; 2) guarantee each user's quality of service requirements, including bit-error rate and data rate; 3) ensure fairness to all the active users; and 4) be applied to systems with various types of multiuser-detection schemes at the receiver. For practical implementation, a reduced-complexity allocation algorithm is developed. This algorithm decouples the complex multiuser joint resource-allocation problem into simple single-user optimization problems by controlling the subcarrier sharing according to the users' spatial separability. Numerical results show that significant power and diversity gains are achievable, compared with nonadaptive systems. It is also demonstrated that the MIMO system is able to multiplex several users without sacrificing antenna diversity by using the proposed algorithm.     

A novel transmitter design for GLSFBC–OFDM–CDMA communication systems

Relying on mutual orthogonality between subcarriers of different users in orthogonal frequency-division multiple access (OFDMA) systems and mutual orthogonality between spreading codes in code-division multiple-access (CDMA) systems, a novel transmitter design is proposed for group layered space–frequency block code (GLSFBC)–OFDM–CDMA communication systems over frequency-selective fading channels. The proposed method is based on a three-level design of user codes: the top level (based on OFDMA) deals with group interference and intersymbol interference (ISI), the middle level (based on space–frequency block coding) results in space–frequency diversity, and the lower level (based on CDMA) handles multiuser interference. The new approach only needs one receive antenna to distinguish multiple users and suppress group interference simultaneously, so the complexity of the receiver decreases remarkably. Simulation results confirm the validity of the proposed technique.     

Edge sidelobe suppressor scheme for OFDMA uplink systems

We propose an edge sidelobe suppressor (ESS) for the uplink of orthogonal frequency-division multiple-access (OFDMA) systems, which mitigates the interference introduced by different frequency offsets between a desired user and the other users. The simulation results confirm that the proposed ESS scheme improves system performance by approximately 5-dB signal-to-interference ratio as compared to conventional OFDMA systems, and without increasing the level of transmitter and receiver complexity.     

Interleaved-OFDMA中基于子空间的上行链路载波频偏估计方法

在OFDMA系统中,通过为每个用户分配不同的子载波可以实现并行数据传输.采用Interleaved子载波分配方法可以提高频率分集和系统容量,但是发射机和接收机之间的载波频率偏移会破坏子载波间的正交性,从而导致本用户的载波间干扰(ICI)以及用户间干扰(MUI).本文提出了一种基于子空间的两阶段频偏搜索方法,该方法只采用一个OFDMA符号块就可以实现Interleaved-OFDMA上行链路多个用户频偏的联合估计,并且不需要知道接入的用户个数及用户所占用的子信道,因此适用于随机分配子信道的情况.仿真结果验证了算法的精确度和有效性.     

Opportunistic Beamforming and Scheduling for OFDMA Systems

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity     

Deterministic multiuser carrier-frequency offset estimation for interleaved OFDMA uplink

In orthogonal frequency-division multiple access (OFDMA), closely spaced multiple subcarriers are assigned to different users for parallel signal transmission. An interleaved subcarrier-assignment scheme is preferred because it provides maximum frequency diversity and increases the capacity in frequency-selective fading channels. The subcarriers are overlapping, but orthogonal to each other such that there is no intercarrier interference (ICI). Carrier-frequency offsets (CFOs) between the transmitter and the receiver destroy the orthogonality and introduces ICI, resulting in multiple-access interference. This paper exploits the inner structure of the signals for CFO estimation in the uplink of interleaved OFDMA systems. A new uplink signal model is presented, and an estimation algorithm based on the signal structure is proposed for estimating the CFOs of all users using only one OFDMA block. Diversity schemes are also presented to improve the estimation performance. Simulation results illustrate the high accuracy and efficiency of the proposed algorithm.     

Low-complexity and MAI-robust wireless broadcasting system with return channel

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper deals with our investigations into improving the performance and reducing the complexity of a digital multimedia broadcasting (DMB) system with a return channel when OFDMA is used as an access scheme. To alleviate a multiple access interference (MAI) introduced by a symbol timing misalignment, the OFDMA-based DMB system adopts frequency diversity and cyclic suffix (CS) at the transmitter, namely FD-OFDMA DMB system with CS. When the system is fully loaded, in addition to low complexity at the transmitter, the performance of the FD-OFDMA DMB system with the CS comes close to that of a single-user FD-OFDMA system at the cost of a small loss of throughput.     

Modified Frequency-Domain Equalization for Channel Shortening in Reduced-CP OFDMA Systems

In orthogonal frequency-division multiple access (OFDMA) systems, the cyclic prefix (CP) length needs to be no less than the longest delay spread of the channels of many users, reducing bandwidth efficiency more significantly than in single-user orthogonal frequency division multiplexing systems. In this paper, we address OFDMA downlink data transmission when a short CP is used to mitigate the inefficient bandwidth usage. Previous time-domain equalizers (TEQs) can be used to shorten the channel; however, they tend to increase noise by introducing spectral nulls. A previous work on per-tone equalization (PTEQ) structure involves a multitap frequency-domain equalizer (FEQ) for each tone, and shows better performance than TEQ-based receiver structure. We propose a novel receiver structure with only one-tap FEQ for OFDMA systems with a reduced CP, exploiting the unused subchannels for a user. We formulate an optimization problem to set the FEQ coefficients at the unused subchannels such that the channel is shortened (approximation is involved) and noise is not enhanced at the used subchannels. With the aid of computer simulations, it is demonstrated that the proposed equalization method is superior to the conventional TEQ-based receivers, and is comparable to the previous PTEQ-based receiver in terms of the achievable SNR, error performance, and bandwidth efficiency. Although the throughput curve versus synchronization delay of the PTEQ is smoother than that of the proposed receiver, the proposed method shows proper throughput over a wider range of the delay values than PTEQ receiver when the system parameters are set so that the complexities are comparable.     

Coordinated Scheduling and Power Allocation in Downlink Multicell OFDMA Networks

We consider a multicell orthogonal frequency-division multiple-access (OFDMA) wireless network with universal frequency reuse and treat the problem of cochannel interference mitigation via base station coordination in the downlink. Assuming that coordinated access points only share channel quality measurements but not user data symbols, we propose to select the set of cochannel users and the power allocation across tones to maximize the weighted system sum rate subject to per-base-station power constraints. Since this is a nonconvex combinatorial problem, efficient suboptimal algorithms are presented and discussed, each requiring a different level of coordination among base stations and a different feedback signaling overhead. Simulation results are provided to assess the performances of the proposed strategies.      

The effect of carrier phase jitter on the performance of orthogonalfrequency-division multiple-access systems

We investigate the sensitivity to carrier phase jitter of an orthogonal frequency-division multiple-access (OFDMA) system. When all OFDMA carriers have the same power level and jitter spectrum, the degradation caused by the jitter is shown to be equal to the degradation of an OFDM system. Also, traditional FDMA is found to be slightly more robust than OFDMA     

Cross‐layer 1 and 5 user grouping/power allocation/subcarrier allocations for downlink OFDMA/NOMA video communications

Wang et al proposed cross‐layer resource allocation for orthogonal frequency division multiple access (OFDMA) video transmission systems. Unlike Wang et al, we add non‐orthogonal multiple access (NOMA) to the downlink OFDMA video transmission system and propose power allocation for users on each subcarrier (cluster) to minimize sum of video mean square error (MSE) to increase the peak signal‐to‐noise ratio (PSNR), the video quality. For OFDMA/NOMA video communication systems, we propose cross‐layer user clustering to reassign the subcarriers based on sum video distortion minimization and derive the optimal power allocation among NOMA users on the same subcarrier to minimize the sum video distortion. Numerical results show that the proposed scheme outperforms the previous OFDMA cross‐layer scheme by Wang et al by 2.2 to 4.5 dB in PSNR and previous OFDMA NOMA physical layer scheme by Ali et al by 2 to 4.4 dB in PSNR, when SNR = 15 dB, and the number of users is 6 to 12.     

Comparison of Multimedia Delivery Scenarios Using Parallel Wireless Links and Path Diversity

Several scenarios to deliver multimedia streams in wireless multi-hop networks, as in the wireless metropolitan area network (WMAN) environment, are considered and examined. Those scenarios considered in this paper are not new but they are examined in a new environment where the system is benefited from path diversity and cooperative diversity. By simulation, we examine those scenarios in an orthogonal frequency-division multiple access (OFDMA)-based wireless network. The simulation results demonstrate how much path diversity and cooperative diversity improve the performance in average sense. We also discuss open research issues on resource utilization in emerging wireless networks.     

基于扩频与跳频的多带OFDM-UWB多址技术

根据多带OFDM-UWB通信系统的特点,提出一种基于扩频与跳频的多址技术。该多址技术将多载波码分多址和跳频多址相结合,构成一种新颖的多址技术-正交频分多址(OFDMA),实现多载波码分多址技术和跳频多址技术的优势互补,此接收机的复杂性明显小于常规时频码的多用户接收机。仿真结果表明,基于OFDMA多址技术方案的UWB系统在系统误比特率性能上优于常规时频码方案约5dB。     

Multiple-Access Interference Suppression for Interleaved OFDMA System Uplink

In this paper, the issue of multiple-access interference (MAI) suppression for the uplink in an interleaved orthogonal frequency-division multiple-access (OFDMA) system is investigated. In such a system, a carrier frequency offset (CFO) disrupts the orthogonality between the subcarriers and gives rise to MAI among users. Based on the signature vector formulated for each user, we propose a novel detector that performs MAI suppression before CFO compensation and fast Fourier transform (FFT) demodulation. Subspace zero-forcing and minimum mean square error (MMSE) techniques are then developed to suppress MAI. The proposed scheme is shown to become almost MAI free, provided that the CFO estimation is accurate enough. From the obtained simulation results, the proposed scheme is also found to be able to enhance the system performance at low complexity.     

Group-orthogonal multicarrier CDMA

In the presence of frequency-selective multipath fading channels, code-division multiple access (CDMA) suffers from multiuser interference (MUI) and intersymbol interference (ISI); but when properly designed, it enjoys multipath diversity. Orthogonal frequency-division multiple access (OFDMA) is MUI-free, but it does not enable the available channel diversity without employing error-control coding. On the other hand, coded OFDMA may achieve lower diversity than a CDMA system employing the same error-control codes. In this paper, we merge the advantages of OFDMA and CDMA to minimize MUI effects, and also enable the maximum available diversity for every user. In our group orthogonal multicarrier CDMA (GO-MC-CDMA) scheme, groups of users share a set of subcarriers. By judiciously choosing group subcarriers, we guarantee that every user transmits with maximum diversity. MUI is only present among users in the same group, and is suppressed via multiuser detection, which becomes practically feasible because we assign a small number of users per group. Performance is analyzed, and simulations are carried out to illustrate the merits of GO-MC-CDMA relative to existing alternatives.     

Hierarchical modulation based cooperative relaying over a multi-cell OFDMA network

The relay-assisted cooperative technique is a promising solution for guaranteeing quality of service and improvement in channel capacity. This paper proposes a hierarchical modulation based cooperative relaying method via a fixed relay station (FRS) to improve the performance of cell edge users for orthogonal frequency division multiple access (OFDMA) systems over a multi-cell environment. The grafting of both FRS and hierarchical modulation onto an OFDMA based cooperative system provides opportunities to reduce the interferences, i.e. inter-cell and inter-relay interferences, and to achieve cooperative diversity in a multi-cell environment, simultaneously. Under conditions of severe inter cell interference caused by high carrier collisions, the proposed scheme leads to an improvement in both channel capacity and bit error rate for cell boundary users.     

Antijamming performance of space-frequency coding in partial-band noise

Space-frequency coded (SFC) orthogonal frequency-division multiple access (OFDMA) system is considered under partial band noise jamming (PBNJ). Analytical expressions for the bit error probability (BEP) are derived for OFDMA with and without SFC in a frequency-selective fading environment. It is shown that SFC increases the resistance of OFDMA against PBNJ and reduces the BEP considerably.     

Efficient Resource Allocation for OFDMA Multicast Systems With Spectrum-Sharing Control

This paper considers the important problem of efficient allocation of available resources (such as radio spectrum and power) in orthogonal frequency-division multiple-access (OFDMA)-based multicast wireless systems. Taking the maximization of system throughput as the design objective, three novel efficient resource-allocation schemes with reduced computational complexity are proposed under constraints on total bandwidth and transmitted power at the base station (BS). Distinct from existing approaches in the literature, our formulation and solution methods also provide an effective and flexible means to share the available radio spectrum among multicast groups by guaranteeing minimum numbers of subcarriers to be assigned to individual groups. The first two proposed schemes are based on the separate optimization of subcarriers and power, where subcarriers are assigned with the assumption of uniform power distribution, followed by water filling of the total available transmitted power over the determined subcarrier allocation. In the third scheme, which is essentially a modified genetic algorithm (GA), each individual of the entire population represents a subcarrier assignment, whose fitness value is the system sum rate computed on the basis of the power water-filling procedure. Numerical results show that with a flexible spectrum-sharing control mechanism, the proposed designs are able to more flexibly and fairly distribute the total available bandwidth among multicast groups and, at the same time, achieve a high system throughput.