Comparison of Laguerre-Gaussian and Donut modes for MDM-WDM in OFDM-Ro-FSO transmission system

Radio-over-free-space-optics (Ro-FSO) technology may pave the way towards a ubiquitous platform for seamless integration of radio and optical networks without expensive optical fiber cabling. In this paper, to increase the capacity of Ro-FSO, mode division multiplexing (MDM) of two modes has been capitalized in a three-channel WDM system spaced by 1 nm over a FSO link of 80 km, resulting in a 120 Gbps six-channel Ro-FSO system. The SNR and received power of MDM of two Laguerre-Gaussian modes LG00 and LG01 is compared with respect to MDM of two transverse donut modes. The performance of four-level quadrature amplitude modulation (QAM) for orthogonal frequency division multiplexing (OFDM) of radio subcarriers in the WDM-MDMs system is investigated for mitigation of frequency-selective fading under strong atmospheric turbulence.     

25km光纤链路高精度频率传输实验研究

分析了影响光纤频率传输精度的主要因素,设计并 实现了一种新的基于锁相环(PLL)补偿的双向异频传输方案。首先,为了 确定光纤链路引入的不稳定性,设计了恒温条件下链路稳定性测试方案,通过光电联合测试 得出了链路传输指标;然后,结 合环境温度及PLL对光纤频率传输的影响,设计了环境温度影响下链路时延稳定性测 试方案,并搭建了实验平台;最后, 进行了光纤传输实验,分析了环境温度和阻尼系数变化对传输性能的影响。结果表明,由于 PLL的有效补偿,10MHz高 精度原子钟频率信号经过25km的光纤传输后仍能保持很高的稳定性;本文方案下的链路时 延抖动在环境温度慢变时可以稳定在0.5ns以内,显著降低了实际光 纤应用中传输时延的不稳定性。     

Computing the single-link performance measurement for elastic optical OFDM

Elastic optical networks based on orthogonal frequency-division multiplexing (OFDM) have emerged as the preferred technology for future optical networks because of their good spectral efficiency and flexibility. In OFDM optical networks, multiple subcarriers can be allocated to accommodate both subwavelength and superwavelength traffic. In this work, we developed an algorithm based on the superposition concept in electrical networks using the Kaufman/Delbrouck recursion model to accurately compute the revenue loss in the OFDM-based single link. The algorithm is applicable when there are many call types requesting diverse numbers of contiguous subcarriers offered to a link with numerous subcarriers.     

A 20-channel optical communication system using subcarriermultiplexing for the transmission of digital video signals

Microwave subcarriers have been used to demonstrate an optical communication system suitable for the distribution of digital video channels. The design, performance, and optical link requirements are discussed for a 2-Gb/s subcarrier-multiplexed (SCM) system with 20 microwave subcarriers modulated at 100 Mb/s and using a frequency-shift-keyed format. The system uses the 2-6-GHz bandwidth and represents the first example of an SCM system using more than one octave of bandwidth. The bit error rate was measured for all 20 channels; results for five typical channels are shown and discussed. The performance results show that a digital SCM approach is attractive in an optical link for a multichannel wide bandwidth video distribution system made from standard microwave components     

Optimization of an OFDM Rayleigh fading system

An OFDM cellular system operating in a Rayleigh fading environment is described and its spectral efficiency is estimated in terms of the theoretically achievable average channel capacity (in the Shannon sense) per user. The analysis covers the operation over a downlink cellular time‐varying link and leads to a simple novel closed‐form expression for the optimal number of individual OFDM subcarriers, based on the maximization of the achieved spectral efficiency. Copyright © 2006 John Wiley & Sons, Ltd.     

Multichannel 64-QAM transmission in AM/digital and all-digitalhybrid-fiber-coax based video dial-tone systems

A hybrid-fiber-coax (HFC) system is operated with a combination of 63 AM and 21 64-QAM subcarriers as well as with the 21 64-QAM subcarriers alone. The 64-QAM subcarriers are 6-MHz wide, decorrelated, and of the type intended for compressed digital video. The AM signal levels (over both the optical and coaxial links) are set to produce standard carrier-to-noise ratios (CNRs), composite second order distortion (CSO), and composite triple beat (CTB). The optical modulation depth (OMD) is independently varied and transmission tests are carried out over the optical link alone, as well as over the entire system (including two coaxial amplifiers, or actives). The relative degradation of the system due to the optical link versus the actives is investigated. For an OMD of 24% or less and with the raw bit-error ratio (BER) set to 10^-5 (as anticipated for use with forward error correction (FEC) systems), the performance is limited by the actives. There is an approximate 3.5 dB penalty in the necessary CNR for 64-QAM reception, due to the presence of the AM signals. On the other hand, when the system is operated with all-digital subcarriers, the optical modulation depth may be increased from 24% to 37% with no penalty in the required digital CNR     

Flipped-Exponential Nyquist Pulse Technique to Optimize PAPR in Optical Direct-Detection OFDM Systems

In this paper, we describe a novel technique based on the flipped-exponential (FE) Nyquist pulse method for reducing peak-to-average power ratio (PAPR) in an optical direct-detection orthogonal frequen...     

Optimal Transmission and Limited Feedback Design for OFDM/MIMO Systems in Frequency Selective Block Fading Channels

In this paper, we propose a systematic design framework to deal with the problem of limited CSIT feedback for MIMO-OFDM systems with correlated subcarriers. Based on the framework, we obtain the optimal transmission and CSI feedback strategies given the limited CSI feedback constraint. We propose a MIMO-OFDM design with combined adaptive power control and beam-forming framework for optimizing MIMO-OFDM link capacity with limited feedback in frequency selective fading channels. We derive a computationally efficient algorithm which exploits subcarrier correlation to search for the design of the optimal transmission and feedback strategy. We found that with a small number of bits for CSIT feedback, there is already significant capacity gain in the MIMO-OFDM systems     

副载波多路复用模拟光传输系统中激光器非线性畸变的分析

本文主要分析副载波多路复用模拟光传输系统中激光器的非线性畸变，提出一个激光器非线性失真分析的完整的数学模型，特别考虑了多路副载波情况下激光器的削波影响。认真分析了ＱｕｎＳｈｉ等人的论文，对他们提出的传输系统中ＣＳＯ和ＣＴＢ的计算公式进行了改进，考虑了高阶失真项的影响。理论推导已为实验所得多路光纤ＣＡＴＶ链路中的ＣＳＯ和ＣＴＢ结果所证实。     

A Novel Solution for OFDM Based Relay Systems

In this paper we propose a novel solution for performance enhancement of dual-hop orthogonal frequency division multiplexing (OFDM) based decode-and-forward (DF) relay system, which assumes that relay station uses more than one antenna for downlink communications and implements transmit antenna selection (TAS) on subcarrier basis jointly with ordered subcarrier mapping (SCM). Ordered SCM is technique where subcarriers from the first hop are mapped to corresponding subcarriers on the second hop in accordance to their instantaneous signal-to-noise ratios (SNRs). It is proven to be a mapping scheme that maximizes the achievable ergodic capacity in OFDM based relay systems, while enabling bit error rate (BER) improvement at the same time. In order to assess the level of BER performance improvement in the system proposed in this paper, we analytically derive closed form BER expression of binary phase shift keying modulated OFDM DF relay system with TAS and SCM, assuming a scenario with Rayleigh fading statistics on both hops. Additionally, for further BER reduction, we analyze the combination of TAS with a modified SCM scheme, in which the subcarriers having the lowest SNRs on both hops are omitted. All derived analytical results are completely verified through simulations, showing that significant BER performance improvements are achieved with the novel proposed OFDM relay system, compared to the systems implementing only SCM, or only TAS, which approves that it can be considered as an interesting solution for the next generation of mobile cellular systems.     

Capacity enhancement in coherent optical MIMO (COMIMO) multimode fiber links

In this paper, we study a coherent optical MIMO (COMIEMO) multi-mode fiber link proposed for enhancing the fiber information capacity. We examine the statistical characterization of the equivalent MIMO channel and the improvement in the fiber capacity due to MIMO transmission. It is shown that the equivalent channel behaves similarly to a complex Gaussian MIMO channel, suggesting that the available results on wireless MIMO communication systems can be applied to optical fiber links for capacity enhancement.     

Subcarrier allocation and bit loading algorithms for OFDMA-based wireless networks

Orthogonal Frequency Division Multiple Access (OFDMA) is an emerging multiple access technology. In this paper, we consider OFDMA in the context of fixed wireless networks. This paper addresses the problem of assigning subcarriers and bits to point-to-point wireless links in the presence of cochannel interference and Rayleigh fading. The objective is to minimize the total transmitted power over the entire network while satisfying the data rate requirement of each link. We formulate this problem as a constrained optimization problem and present centralized algorithms. The simulation results show that our approach results in an efficient assignment of subcarriers and transmitter power levels in terms of the energy required for transmitting each bit of information. However, centralized algorithms require knowledge of the entire network topology and channel characteristics of every link. In a practical scenario, that would not be the situation and there is a need for distributed rate allocation algorithms. To address this need, we also present a distributed algorithm for allocating subcarriers and bits in order to satisfy the rate requirements of the links.     

一种降低相干光OFDM系统非线性效应的方法

相干光正交频分复用(Coherent optical-orthogonal frequency division multiplexing,简称CO-OFDM)可有效降低光纤色散和偏振模色散对系统的影响,是近年来光传输领域研究热点之一.但由于OFDM高的峰均值功率比(PAPR)和较窄的载波间隔(几十兆赫兹),CO-OF...     

Joint channel estimation and turbo equalisation for MIMO-OFDM-IM systems

Multiple-Input Multiple-Output (MIMO) communications are frequently employed to improve the transmitted data rate and the link quality. Index modulated orthogonal frequency division multiplexing (OFDM-IM) improves the error rate performance and the peak-to-average power ratio (PAPR) compared with those of the conventional OFDM system due to the activation of partial subcarriers. The MIMO OFDM-IM can transmit additional information bits via the indices of active subcarriers. Also, in order to reduce the transmission power of the OFDM system, the MIMO OFDM-IM scheme can be employed to approach the demanded data transmission rate and the error rate performance. Multiple-input multiple-output orthogonal frequency division multiplexing index modulation (MIMO-OFDM-IM) is an effective multicarrier transmission scheme and can be proposed as an alternative to conventional MIMO-OFDM system. In this scheme, OFDM-IM is combined with MIMO transmission to take the benefits of these two techniques. In this paper, we propose a joint channel estimation and turbo equalisation receiver for MIMO-OFDM-IM system. Some simulation examples are given to demonstrate the effectiveness of the proposed receiver.     

Transmit power adaptation for multiuser OFDM systems

In this paper, we develop a transmit power adaptation method that maximizes the total data rate of multiuser orthogonal frequency division multiplexing (OFDM) systems in a downlink transmission. We generally formulate the data rate maximization problem by allowing that a subcarrier could be shared by multiple users. The transmit power adaptation scheme is derived by solving the maximization problem via two steps: subcarrier assignment for users and power allocation for subcarriers. We have found that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user with the best channel gain for that subcarrier and the transmit power is distributed over the subcarriers by the water-filling policy. In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method, we also propose a simple method where users with the best channel gain for each subcarrier are selected and then the transmit power is equally distributed among the subcarriers. Results show that the total data rate for the proposed transmit power adaptation methods significantly increases with the number of users owing to the multiuser diversity effects and is greater than that for the conventional frequency-division multiple access (FDMA)-like transmit power adaptation schemes. Furthermore, we have found that the total data rate of the multiuser OFDM system with the proposed transmit power adaptation methods becomes even higher than the capacity of the AWGN channel when the number of users is large enough.     

子载波个数对非对称限幅光正交频分复用误码率的影响

误码率(BER)是非对称限幅光正交频分复用(ACO-OFDM)的一项重要性能指标,子载波个数是影响这一性能指标的重要因素。通过理论推导及仿真证明的方法,分析了子载波个数对光域单边带ACO-OFDM信号在单模光纤中传输时系统误码率的影响。结果表明,在总传输速率一定时,改变子载波个数从128个到16384个,所需要的总带宽改变不超过0.775%。并在此基础上,通过实验证明了不同的传输速率下ACO-OFDM系统均存在使系统误码率最低的最优子载波个数,实验结果表明该最优子载波个数随传输速率的增加而增加。     

QoS-Driven Jointly Optimal Subcarrier Pairing and Power Allocation for Decode-and-Forward OFDM Relay Systems

In this paper, we investigate the quality-of-service (QoS) driven subcarrier pairing and power allocation for two-hop decode-and-forward (DF) OFDM relay systems. By integrating the concept of effective capacity, our goal is to maximize the system throughput subject to a given delay-QoS constraint. Based on whether the destination can receive the signal transmitted by the source, we consider two scenarios, i.e. OFDM DF relay systems without diversity and OFDM DF relay systems with diversity, respectively. For OFDM DF relay systems without diversity, we demonstrate that the jointly optimal subcarrier pairing and power allocation can be implemented with two separate steps. For OFDM DF relay systems with diversity, we propose an iterative algorithm to achieve jointly optimal subcarrier pairing and power allocation. Furthermore, we find that the analytical results show different conclusions for the two types of OFDM relay systems. For OFDM relay systems without diversity, the optimal power allocation depend on not only the channel quality of subcarriers but also the delay QoS constraints, while the optimal subcarrier pairing just depends on the channel quality of subcarriers. For OFDM relay systems with diversity, both the optimal subcarrier pairing and power allocation depend on the channel quality of subcarriers and the delay QoS constraints. Simulation results show that our proposed scheme offers a superior performance over the existing schemes.     

A Nash game approach for OSNR optimization with capacity constraint in optical links

This paper develops a Nash game towards optimizing optical signal-to-noise ratio (OSNR) in the presence of link capacity constraint. In optical wavelength-division multiplexing (WDM) networks, all wavelength-multiplexed channels in a link share the optical fiber. In order to limit nonlinear effects, the total power launched into a fiber has to be below a nonlinearity threshold. This can be regarded as the optical link capacity constraint. We formulate an extended OSNR Nash game that incorporates this underlying system constraint. The status of an optical link is considered directly in the cost function. Sufficient conditions for existence and uniqueness of the Nash equilibrium (NE) solution are given. Two iterative algorithms for channel power control are proposed to compute the NE solution: a parallel update algorithm (PUA) and a relaxed PUA (r-PUA). Their convergence is studied under different conditions, both theoretically and numerically.     

Joint estimation and compensation of transceiver IQ imbalance for PDM CO-OFDM system

In this paper, a frequency-domain mathematical model is deduced for polarization-division multiplexing(PDM) coherent optical orthogonal frequency division multiplexing(CO-OFDM) system with transceiver in-phase and quadrature(IQ) imbalances. A novel training symbol structure is designed in which the mirror-subcarriers of pilot subcarriers were modulated with zero signal so that the channel distortion with transceiver IQ imbalances can be estimated. It proves that the channel distortion and transmitter IQ imbalances cannot be separated using the training symbols;therefore, the channel equalization method was used to recover the signals. Simulation results show that at 2 dB transmitter amplitude imbalances, 15° transmitter phase imbalances, 100 Gbit/s transmission rate, 1 040 km standard signal-mode fiber link, and 1×10^-3 bit error rate, 23.5 dB optical signal-to-noise ratio(OSNR) is necessary for the proposed method. However, the compared schemes cannot achieve effective transmission.     

Coherent optical MIMO (COMIMO)

We present a multiple-input multiple-output (MIMO) optical link based on coherent optics and its ability to exploit the inherent information capacity of multimode fiber. A coherent implementation differs from previous work in optical MIMO by allowing the system to tolerate smaller modal delay spreads, because of a much larger carrier frequency, and yet maintain the necessary diversity needed for MIMO operation. Furthermore, we demonstrate the use of MIMO adaptive equalization to mitigate intersymbol interference when exceeding the bandwidth-length product of the link. The impact of phase noise is studied with numerical simulation.