Orthogonal Frequency Division Multiple Access Signals Research Articles

Estimation Techniques for MIMO-OFDM-A Review

In the context of Third-Generation Partnership Project Long-Term Evolution (3GPP LTE), the target data rates are 100 Mb/s in downlink and 50 Mb/s in uplink, and other system features include flexible bandwidths and moderate power consumption of mobile terminals. While the previous 3GPP generations, namely, 3G UMTS and 3.5G HSPA, rely on code-division multiple access (CDMA), LTE adopts orthogonal frequency-division multiple access (OFDMA)-based technologies for its uplink and downlink. Due to high peak-to-average power ratio (PAPR) of OFDMA signals, the single- carrier frequency division multiple access (SC-FDMA), also known as discrete Fourier transform (DFT)-spread OFDMA, has been selected for the uplink transmission in 3GPP LTE systems. SC-FDMA has similar throughput performance as OFDMA but with lower PAPR to increase power efficiency and is less sensitive to frequency synchronization errors, which makes it favorable for mobile terminals.

Dynamic User Grouping-Based NOMA Over Rayleigh Fading Channels

With the development of wireless communications and internet of things (IoT), non-orthogonal multiple access (NOMA) has been considered as one of effective schemes to meet the rapidly growing user access requirements. Two types of NOMA, i.e., NOMA-2000 and power-domain NOMA (PD-NOMA), have been proposed in recent years. The first one is a superposition of a set of orthogonal frequency division multiple access (OFDMA) signals and a set of spread-spectrum multicarrier-code division multiple access (MC-CDMA) signals while the second one is a direct superposition of a MC-CDMA signal and an OFDMA signal. This paper proposes a dynamic user grouping method for classifying users and then compares the performance between NOMA-2000 and PD-NOMA over Rayleigh fading channels. Simulation results show that the PD-NOMA can always exhibit lower bit error rate (BER) than the NOMA-2000 under different signal-to-noise ratios. Under the Rayleigh fading environment, the performance of the PD-NOMA after user grouping is better than that of NOMA-2000.

Cooperative Spectrum Sensing with Coded and Uncoded Decision Fusion under Correlated Shadowed Fading Report Channels.

This article addresses the impact of forward error correction when applied to the report channel transmissions of a centralized decision fusion cooperative spectrum sensing scheme designed to detect idle orthogonal frequency division multiple access (OFDMA) subchannels. The OFDMA signal is transmitted over slow frequency-selective multipath Rayleigh fading channels and sensed using the maximum eigenvalue detection test statistic. The decisions on the OFDMA subchannel occupancy are transmitted to a fusion center over report channels represented by a shadowed fading model combining a three-dimensional spatially correlated shadowing with a slow and flat multipath Rayleigh fading. Binary Bose-Chaudhuri-Hochquenghem (BCH) and Repetition codes are used to protect these decisions. Results show that shadowing correlation severely deteriorates the overall spectrum sensing performance and that error correction may not be able to protect the report channel transmissions. It can be even worse with respect to the system performance especially at low signal-to-noise regimes. In the situations in which error correction is effective, the Repetition code is capable of outperforming the BCH, meaning that the diversity gain may be more relevant than the coding gain when the spectrum sensing decisions are subjected to correlated shadowing.

Downlink resource allocation in OFDMA wireless networks under power amplifier non‐linearity

In this study, the physical layer resource allocation in orthogonal frequency-division multiple access (OFDMA) systems is studied by considering the non-linear effects of power amplifier (PA). The non-linearity produces cross-correlation among the subcarriers and results in deteriorating the OFDMA subcarriers orthogonality. Hence, the signal-to-interference ratio (SIR) of the output signal of PA for OFDMA signal with different power scaling factors for subcarriers is derived. Then, the theoretical SIR is verified by simulation result. Next, the optimisation problem is designed to maximise the total achievable downlink rate of all users by assigning subcarriers and power scaling factors. This non-convex problem is solved by comprehensive learning particle swarm optimisation. The simulation results show that when PA is non-linear, better total rate is achieved by considering non-linearity in resource allocation compared with the conventional resource allocation techniques.

多天线OFDMA信号盲解码算法研究

论文提出了多天线异步情况下基于独立分量分析的OFDMA (Orthogonal Frequency Division Multiple Access)信号盲解码算法。所提算法把接收到的OFDMA信号看成用户信息在IFFT (Inverse Fast Fourier Transform)矩阵上的投影，可以在传输延迟及信道未知情况下，把接收信号建模成盲源分离的形式，完成子载波分离和信息盲解码。所提基于快速独立分量分析盲解码算法不需要去除循环前缀，相比传统OFDMA基于MMSE信道估计和FFT解码的算法，增加了接收信号能量，提高了解码性能。理论分析和仿真结果表明，这种不需要训练序列的盲解码算法比传统解码算法具有更好的性能。 Multi antenna asynchronous OFDMA (Orthogonal Frequency Division Multiple Access) signal de-coding algorithm based on independent component analysis is proposed in this paper. According to the characteristics that OFDMA signal can be regarded as the subspace projection of the user signal in the IFFT (Inverse Fast Fourier Transform) matrix, the proposed method can be used for subcarrier separation and decoding without channel information and transmission delay by making the received signal as a form of blind source separation. The proposed blind decoding method based on fast independent component analysis does not need to remove the cyclic prefix, which increases the energy of the received signal. As such, the decoding performance is improved. Compared with the traditional MMSE decoding method based on FFT, theoretical analysis and simulations show that the proposed method has better performance.

On the CFO estimation for interleaved OFDMA uplinks

This study presents an eigen-filter-based carrier-frequency-offset (CFO) estimation algorithm for orthogonal-frequency-division-multiple-access (OFDMA) uplinks. The proposed approach takes the periodic time samples of the OFDMA signal as the input of the filter that intends to eliminate the CFO signatures of all users. The wieght vector of the filter is attained from the null space of the input vectors under the criterion of minimum output power. Based on the zeros of the filter response, the proposed approach estimates the CFOs in a polynomial rooting way. This study analytically derives the mean square error (MSE) of the CFO estimates as well.

Linear Precoding of OFDMA Signals to Minimize Their Instantaneous Power Variance

The large instantaneous power fluctuations of orthogonal frequency division multiple access (OFDMA) signals necessitate significant power backoff to minimize the effects of nonlinear power amplifier distortion. Is there an optimum linear block precoding that significantly reduces the instantaneous power variance of OFDMA signals? For contiguous and block-interleaved non-contiguous frequency mappings we find that there is a power variance-minimizing linear block precoding which requires a slightly lower power backoff than that for DFT precoding (which in the form of SC-FDMA, is used for the uplink signaling in the 3GPP-LTE standard). The optimum precoding is not unique, and in its simplest form it imposes only mild linear filtering and a moderate noise enhancement penalty if the receiver uses linear equalization.

Next-generation OFDMA-based passive optical network architecture supporting radio-over-fiber

In this paper, we propose a novel architecture for next-generation orthogonal frequency division multiple access (OFDMA)-based passive optical networks (PON's), referred to as ROFPON. Besides carrying local broadband OFDMA data, ROFPON seamlessly supports radio-over-fiber (RoF) transports between the central office and multiple remote antennas at end users without using costly WDM lasers. We analytically and experimentally study the receiver sensitivity to OFDMA signals and the radio frequency (RF) signal's performance. By corroborating simulation results with experimental results, we discuss the determination of crucial system parameters, such as the optimal broadband-to-radio power ratio, and the exploitation of a notch filter for removing RF interference. Experimental results show that the integrated 10 Gb/s OFDMA and three 20 MHz RF signals are successfully transported both downstream and upstream over a 20 km single-mode-fiber PON. Finally, experimental results demonstrate that QPSK-encoded WiMAX format RF signals are transmitted/relayed upstream with E-O-E conversion at each optical network unit (ONU), and received error-free at the optical line terminal after cascading 32 ONU's.

A Novel Passive Optical Network Architecture Supporting Seamless Integration of RoF and OFDMA Signals

In this letter, we propose a novel passive optical network (PON) architecture supporting radio-over-fiber signals and orthogonal frequency-division multiple access (OFDMA) PON signals. The architecture transports upstream multiple remote antenna's wireless signals using only one upstream wavelength. In addition, the windowed-orthogonal frequency-division-multiplexing technique is employed to mitigate interference during the integration process. Experimental results shows that 10-Gb/s OFDMA and three radio-frequency signals at 2.1 GHz are successfully transmitted over 20-km single-mode fiber in a 32-optical-network-unit PON.

Spectrum Sensing of OFDMA Systems for Cognitive Radio Networks

To fully exploit wireless radio resource and, thus, increase spectrum efficiency, cognitive radios shall sense wireless environments and identify interference to allow opportunistic transmissions for secondary systems. Based on Chen in their work about a terminal architecture for cognitive radio networks, by further obtaining transmission information with a rate-distance nature that is extended from an overlay concept, secondary systems can even leverage busy duration of primary systems to enhance the opportunity to use spectrum under derived tolerable interference to the primary orthogonal frequency-division multiple access (OFDMA) system. Our proposed novel multistate (i.e., existence, activity, and data transmission rate) spectrum sensing can thus effectively acquire a set of cognitive information for OFDMA mobile communication systems under a general system model. Through the received signal strength indicator (RSSI), fundamental symbol rate, cyclic property of the OFDMA signal, and the control and management signal, we can precisely determine the existence and activity of the primary (OFDMA) system within a subband via Neyman-Pearson (NP) criterion. With <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">priori</i> knowledge of the frame structure of potential primary systems, communication parameters, including data transmission rate and resource allocation state, can be extracted by analyzing the frame header. Through appropriate parametric adjustments, our sensing procedure can be extended to state-of-the-art OFDMA systems.

Radio-Over-Fiber System for TDD-Based OFDMA Wireless Communication Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report on a fiber-optic transport system for next-generation wireless-communication systems utilizing 4 <formula formulatype="inline"><tex>$\times$</tex></formula> 4 multiple-input multiple-output (MIMO) orthogonal-frequency-division-multiple-access (OFDMA) technology. Our system supports time-division-duplex (TDD)-based wireless signals operating at 3.775 GHz. To accommodate the TDD-based MIMO signals over a single strand of optical fiber, we utilize nine-channel coarse wavelength-division-multiplexed optical channels: one for link delay measurement and TDD control signal transmission, four for downlink, and the others for uplink. The system first measures the propagation delay between the central base station (CBS) and the remote antenna (RA) and sends the result to delay modules to compensate for the delay added by the transmission link. This procedure makes the CBS and RA emit the downstream signals simultaneously into the air and, consequently, helps avoid the performance degradation caused by the propagation delay of the radio-over-fiber system. The system then sends the MIMO signals together with TDD control signals to the RA. Our experimental demonstration is carried out with 1-Gb/s OFDMA signals having pilot, control, and data channels. For downstream, the error vector magnitudes (EVMs) are measured to be <formula formulatype="inline"><tex>$≪ -$</tex></formula>30 dB after a 3.9-km transmission over conventional single-mode fiber when the antenna output power is set to be 24 dBm. For upstream, the best EVMs are measured to be <formula formulatype="inline"><tex>$≪ -$</tex></formula>35 dB. Both the downstream and upstream performance is limited by the shot and thermal noise of the receiver when the signal power is low, whereas nonlinear distortions of electrical amplifiers start to degrade the system performance as the signal power increases. We also measure the crosstalk between channels. It is measured to be less than <formula formulatype="inline"><tex>$-$</tex> </formula>42 dB for all channels, which is found to be caused by board-to-board interference at the RA. </para>