Average power reduction techniques for multiple-subcarrierintensity-modulated optical signals

We describe two classes of simple, effective techniques for reducing the average optical power requirement in intensity-modulated optical systems using multiple BPSK or QPSK subcarriers. The first class of techniques involves block coding between the information bits to be transmitted and the symbol amplitudes modulated onto the subcarriers in order to increase the minimum value of the multiple-subcarrier electrical waveform. The second class of techniques involves replacing the fixed DC bias by a bias signal that varies on a symbol-by-symbol basis. These two classes of techniques can be applied separately or in tandem. The reduction in power requirement increases with the number of subcarriers and, with eight subcarriers, can be as high as about 3.6 dB and 3.2 dB with BPSK and QPSK, respectively. The techniques described are applicable as long as all subcarriers originate from a single transmitter and are symbol-synchronized     

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.      

无人机空中OFDM光通信链路性能研究

基于无人机空中光正交频分复用(OFDM)通信模型链路的误码性能理论和仿真研究,综合考虑服从对数正态分布的大气湍流模型和服从瑞利分布的指向误差模型对通信系统的影响,采用高斯-厄米特积分方法得出了通信系统链路误码率的闭合表达式。在此基础上,仿真分析了发射端光束发散角、符号速率、子载波数和接收孔径大小与系统误码率的关系。仿真结果表明,在弱湍流条件下,发射端激光存在可使链路误码性能达到最优的光束发射角,湍流强度的改变对最优光束发散角的影响不大。光OFDM系统能够满足无人机间通信速率为吉比特级的光通信要求,并且通信时接收端的孔径尺寸不宜增大,该结果对通信链路的优化设计有一定的参考意义。     

Optical transmission using PSK-modulated subcarriers at frequencies to 16 GHz

The use of a 1.3 ?m InGaAsP constricted mesa laser to transmit microwave subcarriers at frequencies from 3 GHz to 16 GHz is described. The subcarriers are phase-modulated at 2 Gbit/s and the error rate is measured for transmission over 34 km. The effect of laser intensity noise on the system transmission is described.     

基于多载波复用多模光纤通信系统的设计与分析

分析了多模光纤高频带通区域的传输特性,提出一种基于多载波复用的多模光纤通信系统.对该系统的传输特性进行了分析和仿真,结果表明:系统所用载波数是影响系统性能的重要因素;选取合适载波数,该系统可将10.2Gb/s的数据传输1km、将2.5Gb/s的数据传输4.2km;增大激光器的发送功率可以显著增加系统的传输距离.     

双选择性信道中OFDM信号的均衡

正交频分复用（OFDM）技术是一种多载波调制方式，由于他易于实现并且能够有效对抗频率选择性衰落，因此受到了人们极大的重视，他将数据在相互正交的子载波上并行发送，同时在数据的前端引入循环前缀，可以有效地对抗频率选择性信道，消除符号间的干扰，实现数据的高速传输。但是当他在频率选择性和时间选择性信道条件下，也就是双选择性信道条件传输时，子载波之间的正交性会遭到破坏。文章讨论了双选择性信道条件下，在时域采用自适应均衡器来消除信道时变带来的载波间干扰，保证各个子载波的正交性。仿真结果表明，信号可以被正确的恢复。     

The design and use of an FM-AM radiotelemetry system for multichannel recording of biological data

A multichannel telemetry system for EEG recording has been constructed for study of animal or human behavior correlates under natural, unrestrained conditions. To be useful in research of this kind, the transmitted radio signal must be independent of the environment, so that changes in antenna loading and in signal level cause no artifact. Standard IRIG proportional-bandwidth FM subcarrier channels are used. These subcarriers are generated by twin-T oscillators, modulated in turn by the amplified data signals. The FM subcarriers are then linearly summed and impressed upon a crystal-controlled AM transmitter. The system is also relatively insensitive to major shifts in supply voltage. Wherever possible, fabrication was by means of integrated circuits, thus reducing the bulk of the modules.     

Enhanced channel-estimation technique for MIMO-OFDM systems

In multi-input-multi-output orthogonal frequency-division multiplexing systems, conventional channel-estimation techniques using comb-type training symbols give relatively large mean squared errors (MSEs) at the edge subcarriers. To reduce the MSEs at these subcarriers, a cyclic comb-type training structure is proposed. In the proposed cyclic training structure, all types of training symbols are transmitted cyclically at each antenna. At the receiver, the channel frequency responses that are estimated using each training symbol are averaged with weights obtained from the corresponding MSEs. Computer simulations showed that the proposed cyclic training structure gives more signal-to-noise ratio gain than the conventional training structure.     

Cancellation technique to reduce intercarrier interference in OFDM

Orthogonal frequency division multiplexing (OFDM) systems are sensitive to frequency errors, which cause intercarrier interference among subcarriers. A new cancellation method to reduce the effects of frequency offset errors is presented. The main feature is to map each data symbol a_k which is to be transmitted onto a pair of non-adjacent subcarriers, with weightings +1 and -1, rather than to a single subcarrier. The carrier-to-interference ratio gain of the proposed method over normal OFDM varies between 10 to 30 dB. This method also offers a frequency diversity effect in a multipath fading channel     

异步MIMO-OFDM中基于预处理矩阵的迭代检测算法

针对异步发射MIMO-OFDM链路,论文提出了一种基于预处理矩阵的迭代并行多天线干扰消除方法。该方法在信号发射前,通过预处理矩阵将信号扩展到所有子载波上,从而降低部分子载波深衰落对扩展前原始信号的影响。在接收端,上次迭代的判决错误在干扰重建时被预处理矩阵扩展,缓解了迭代干扰抵消过程中的误差传播。仿真结果表明,在4发2收场景下,误码率为10-3时,5次迭代后信噪比相比于传统的迭代并行多天线干扰消除方法改善约4.5 dB。     

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.     

Microwave/millimeter-wave frequency subcarrier lightwavemodulations based on self-sustained pulsation of laser diode

A method of subcarrier lightwave modulation based on the self-sustained pulsation (SSP) of laser diodes is presented. The tunable microwave/millimeter-wave frequency subcarrier is generated by the laser diode itself under proper DC biases. The subcarrier is frequency-modulated when the DC bias is superimposed with a small AC signal. No external modulation is needed. A theoretical analysis based on a double-section laser diode reveals that Hopf bifurcation is the origin of SSP. Theoretical calculations also show that the modulator has the potential of generating subcarriers up to 100 GHz by proper control of the device parameters V-channeled substrate inner-stripe (VSIS) GaAlAs laser diodes were used to generate 1-7-GHz SSP subcarriers. A preliminary subcarrier multiplexing (SCM) experiment demonstrated the transmission of 1-GHz modulation and video signals on a readily FM-modulated 6.3-GHz SSP subcarrier     

Spectrally shaped generalized multitone direct sequence spread spectrum

In this paper, a new modulation scheme that is called spectrally shaped generalized multitone direct sequence spread spectrum (SSG-MT-DS-SS) is proposed. This scheme enables moderate shaping of the transmitted spectrum without the use of spectral-shaping filters; instead, the spectrum is shaped by appropriate selection of the parameters of each subcarrier. These parameters are amplitude, data rate, processing gain, and the frequency separation between adjacent subcarriers. Since these parameters are selectable, this scheme is a completely generalized MT-DS-SS approach. Analysis and simulation results show that by properly choosing these parameters, the level of the spectral sidelobes can be reduced, while slightly changing the width of the mainlobe and simultaneously obtaining a good error-probability performance, on both additive white Gaussian noise and dispersive fading channels. For even a small number of subcarriers (e.g., 3-5), spectral sidelobes can be reduced by 3-6 dB from those of the conventional MT-DS-SS spectrum, with little or no degradation in error-probability performance. For larger numbers of subcarriers, additional sidelobe reductions are attainable. In addition, the SSG-MT-DS-SS system has a smaller peak-to-average power ratio than the conventional MT-DS-SS when the amplitudes of each subcarrier are not identical.     

Channel estimation for MC-CDMA uplink transmissions with combined equalization

Combined equalization has recently been proposed to enhance the error rate performance of conventional multicarrier code-division multiple-access (MC-CDMA) systems. This technique applies pre-equalization at the transmitter in conjunction with post-equalization at the receiver, thereby splitting the overall equalization process into two separate parts. In this way, efficient power allocation over the available subcarriers is possible at the transmitter, while leaving the interference cancellation task at the receiver. In this paper, we consider the uplink of an MC-CDMA system employing combined equalization. As the users transmit from different locations, the uplink signals arrive at the base station after passing through different multipath channels and the goal is to estimate the pre-equalized channel frequency response of each user. This is pursued following two different approaches. The first operates in the frequency-domain and treats the channel gains over adjacent subcarriers as independent unknown parameters. The second operates in the time-domain and achieves better performance by reducing the number of unknown parameters. Both schemes are based on maximum-likelihood reasoning and require knowledge of the transmitted symbols. Numerical examples are given to highlight the effectiveness of the proposed methods.     

Subcarrier multiplexing by chaotic multitone modulation

Subcarrier multiplexing by chaotic multitone modulation is investigated. Optical chaotic light can be achieved by injecting multiple subcarriers into a self-pulsating laser diode. Synchronization between two identical chaotic systems (drive and response) can be implemented provided that the conditional Lyapunov exponents are all negative. By adding amplitude modulation (AM) signals to each subcarrier, the two systems become asymptotically synchronized. The AM signals in each subcarrier can be recovered by the introduction of a filtering process where the Lyapunov exponent of the synchronized error function matches the cutoff frequency of a first order low pass filter.     

System capacity for millimeter-wave radio-over-fiber distributionemploying an optically supported PLL

We consider the performance of a hybrid radio-over-fiber distribution system with a remote phase-locked loop (PLL) providing the local oscillator for upconversion to millimeter (mm)-wave frequencies. The reference signal for the PLL is transmitted together with digitally modulated subcarriers over a fiber link, allowing centralization of radio processing. Through analysis and simulation, we identify the relationship between the phase noise requirement for the PLL and the data capacity of the fiber link. For a practical link, we provide an upper limit on system capacity for transmission of 16 quadrature amplitude modulation (QAM) subcarriers     

Power Allocation Schemes in OFDM-Based Femtocell Networks

Orthogonal frequency division multiplex-based femtocell networks in downlink with two-tier interference are considered in this paper. In order to satisfy the quality of service of macrocell users, a capacity maximization problem with the total and the upper power constraints by allocating power to subcarriers for a user in femtocell systems becomes important. In this paper, the optimal power allocation is derived by using the Lagrangian technique. Based on the analysis of the Karush-Kuhn-Tucker conditions, subcarriers can be classified into two sets with different power allocation strategies according to the upper power constraint. One set of subcarriers is allocated with upper power, and the other set of subcarriers is processed by using the waterfilling approach. A direct linear search scheme is presented to achieve the optimal performance by finding the threshold over the channel states of all subcarriers to determine the two sets. In order to reduce the computational load, a reduced complexity scheme is designed for the optimal solution by utilizing the relationship of the allocated power in the two sets of subcarriers. Unlike the schemes in iterative manners, a fast power allocation scheme with a near-optimal performance is also developed. Simulation results reveal that the proposed schemes outperform the existing scheme. The efficacy of the proposed schemes is demonstrated in the simulations.     

Performance of a GaAlAs laser diode used for multichannel video transmission employing multiplexed UHF subcarriers

Good quality transmission of three television channels employing multiplexed UHF subcarriers at 770, 800 and 830MHz using a commercially available laser diode is demonstrated. Both the third-order intermodulation products and crossmodulation products of the laser are found to be better than ?65dB. An SNR per channel of 52dB has been obtained.     

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。