Orthogonal Frequency Division Multiplexing Subcarriers Research Articles

Phase Noise Effects on Phase-Modulated Coherent Optical OFDM

One of the primary drawbacks of orthogonal frequency-division multiplexing (OFDM) is the large peak-to-average ratio (PAPR). It continues to contribute nonlinear degradations after it was introduced into optical fiber transmissions. However, phase-modulated OFDM can reduce the PAPR to the minimum, since its amplitude can be constant. In this paper, we conduct the theoretical and experimental study on the laser phase noise effect in phase-modulated coherent optical OFDM systems. Due to the laser phase noise, the phase transformation at the receiver is prone to errors due to phase wrapping. Consequently, we propose a mean-phase-compensation (MPC) method, which compensate for the mean phase offset for each OFDM symbol that has a Gaussian-distributed histogram. This method can avoid the problematic unwrapping function in phase transformation. We derive the closed-form expressions for the phase noise spreading and the corresponding signal-to-noise ratios (SNRs) based on the Wiener phase noise model. Unlike conventional coherent optical OFDM, the phase noise spreading to each OFDM subcarrier is decided by the reciprocal of the square of the subcarrier frequency, and the SNR can be improved by using a stronger PM modulation. We also discuss the SNR improvement by discarding a few affected subcarriers closing to dc. In a simulation, we demonstrate that our closed-form expressions are accurate. Finally, we experimentally verify our MPC method and our analysis.

Multiuser MIMO-OFDM for Visible Light Communications

Visible light communication (VLC) is emerging as a promising technique to provide ubiquitous wireless connection. In this paper, a multiuser VLC system utilizing multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) is investigated. Since the distances of the multiple transmitter–receiver links are different, their temporal delays are also different, resulting in complex channel gain and phase differences when transformed to the frequency domain. For each subcarrier in OFDM, the corresponding precoding matrix is calculated in the frequency domain to eliminate multiuser interference. Phase information in the frequency domain is first considered, where complex, instead of real, channel matrices are used for precoding, which reduces the channel correlation and achieves better performance. Moreover, minimum dc bias, unified dc bias, and asymmetrically clipped optical OFDM-based schemes are proposed to generate real-valued nonnegative signals for intensity modulation, and their performances are validated via simulations with zero forcing and minimum mean-squared error (MMSE) precoding techniques.

Suppression of impulsive noise in OFDM system using imperfect channel state information

This paper presents the orthogonal frequency division multiplexing (OFDM) system working under Nakagami-m multipath fading channels, which utilizes the impulsive noise excision technique. Its performance is also evaluated under the imperfect channel state information (CSI) constraint. Although the impact of noise reduces due to the noise bucket effect appearing in OFDM systems with longer symbol duration, yet the impulsive noise energy spreads over OFDM subcarriers, which appears as a hazard in the demodulation and detection when the energy of impulsive noise exceeds certain threshold level. Under realistic conditions, the channel estimators cannot provide perfect/ideal channel state information. In this paper, we use the impulsive noise compensation method as a technique for the suppression of impulsive noise in the frequency-domain after OFDM demodulation and channel equalization. Under the aforementioned scenario, the simulation results are demonstrated for the performance evaluation of underlying OFDM communication system, under the Nakagami-m multipath fading channel, which also focuses on the adverse effects of imperfect CSI at the receiver, in terms of bit error rate.

Low PMEPR OFDM Radar Waveform Design Using the Iterative Least Squares Algorithm

This letter considers waveform design of orthogonal frequency division multiplexing (OFDM) signal for radar applications, and aims at mitigating the envelope fluctuation in OFDM. A novel method is proposed to reduce the peak-to-mean envelope power ratio (PMEPR), which is commonly used to evaluate the fluctuation. The proposed method is based on the tone reservation approach, in which some bits or subcarriers of OFDM are allocated for decreasing PMEPR. We introduce the coefficient of variation of envelopes (CVE) as the cost function for waveform optimization, and develop an iterative least squares algorithm. Minimizing CVE leads to distinct PMEPR reduction, and it is guaranteed that the cost function monotonically decreases by applying the iterative algorithm. Simulations demonstrate that the envelope is significantly smoothed by the proposed method.

Performance Analysis of Time of Arrival Estimation on OFDM Signals

This letter characterizes the error performance of realistically modelled orthogonal frequency division multiplexing (OFDM) signals, when their time of arrival has to be estimated in an additive white Gaussian noise channel. In particular, different power distributions on the available sub-carriers of the OFDM signal are considered, and bounds on the corresponding root mean square estimation error (RMSEE) are evaluated. The tools used for such purpose are the widely adopted Cramér-Rao bound and the Ziv-Zakai bound, which is tight in a wide range of signal-to-noise ratio (SNR) values. The presented analysis reveals that, for a given signal bandwidth, a proper power distribution on the OFDM sub-carriers is crucial for achieving a good performance in the low to medium SNR region, where the RMSEE curve exhibits the typical threshold behavior. Moreover, a trade-off between asymptotic and threshold performance is identified, thanks to the adoption of a novel performance figure, which directly describes the threshold RMSEE behavior.

Spectrum Monitoring Using Energy Ratio Algorithm for OFDM-Based Cognitive Radio Networks

This paper presents a spectrum monitoring algorithm for Orthogonal Frequency Division Multiplexing (OFDM) based cognitive radios by which the primary user reappearance can be detected during the secondary user transmission. The proposed technique reduces the frequency with which spectrum sensing must be performed and greatly decreases the elapsed time between the start of a primary transmission and its detection by the secondary network. This is done by sensing the change in signal strength over a number of reserved OFDM sub-carriers so that the reappearance of the primary user is quickly detected. Moreover, the OFDM impairments such as power leakage, Narrow Band Interference (NBI), and Inter-Carrier Interference (ICI) are investigated and their impact on the proposed technique is studied. Both analysis and simulation show that the energy ratio algorithm can effectively and accurately detect the appearance of the primary user. Furthermore, our method achieves high immunity to frequency-selective fading channels for both single and multiple receive antenna systems, with a complexity that is approximately twice that of a conventional energy detector.

Selection of the Best Two-Hop AF Wireless Link under Multiple Antenna Schemes over a Fading Channel

In evaluating the performance of a dual-hop wireless link, the effects of large and small scale fading has to be considered. To overcome this fading effect, several schemes, such as multiple-input multiple-output (MIMO) with orthogonal space time block codes (OSTBC), different combining schemes at the relay and receiving end, and orthogonal frequency division multiplexing (OFDM) are used in both the transmitting and the relay links. In this paper, we first make compare the performance of a two-hop wireless link under a different combination of space diversity in the first and second hop of the amplify-and-forward (AF) case. Our second task in this paper is to incorporate the weak signal of a direct link and then by applying the channel model of two random variables (one for a direct link and another for a relayed link) we get very impressive result at a low signal-to-noise ratio (SNR) that is comparable with other models at a higher SNR. Our third task is to bring other three schemes under a two-hop wireless link: use of transmit antenna selection (TAS) on both link with weak direct link, distributed Alamouti scheme in two-hop link and single relay antenna with OFDM subcarrier. Finally, all of the schemes mentioned above are compared to select the best possible model. The main finding of the paper is as follows: the use of MIMO on both hops but application TAS on both links with weak direct link and the full rate OFDM with the sub-carrier for an individual link provide a better result as compared to other models.

Analytical bit error rate performance evaluation of an orthogonal frequency division multiplexing power line communication system impaired by impulsive and Gaussian channel noise

An analytical approach is presented to evaluate the bit error rate (BER) performance of a power line (PL) communication system considering the combined influence of impulsive noise and background PL Gaussian noise. Middleton class-A noise model is considered to evaluate the effect of impulsive noise. The analysis is carried out to find the expression of the signal-to-noise ratio and BER considering orthogonal frequency division multiplexing (OFDM) with binary phase shift keying modulation with coherent demodulation of OFDM sub-channels. The results are evaluated numerically considering the multipath transfer function model of PL with non-flat power spectral density of PL background noise over a bandwidth of 0.3–100 MHz. The results are plotted for several system and noise parameters and penalty because of impulsive noise is determined at a BER of 10−6. The computed results show that the system suffers significant power penalty because of impulsive noise which is higher at higher channel bandwidth and can be reduced by increasing the number of OFDM subcarriers to some extent. The analytical results conform well with the simulation results reported earlier.

Low-Cost Channel Estimation Algorithm for DRM Receiver

—In this paper, we consider a digital radio mondiale (DRM) system using orthogonal frequency division multiplexing (OFDM) with coherent detection at the receiver. The DRM pilots are two-dimensional distribution, linear interpolation and Wiener filter interpolation respectively used in the time and frequency domain as the channel estimation algorithm. We propose a low-complexity improved Wiener filter coefficient algorithm to estimate the OFDM channels. Our proposed algorithm differs from traditional algorithms in two ways. Firstly, we divide the DRM pilots into n groups to estimate the channel responses of corresponding OFDM subcarriers (instead of exploiting the whole pilot at the same time in traditional algorithms). Secondly, we utilize the uniform distribution characteristic of the DRM pilots to reduce the number of Wiener filter coefficients, and this effectively leads to a reduction in computation complexity. In addition, simulation results are also presented to gain further insights.

A low-power photonic quantization approach using OFDM subcarrier spectral shifts.

Photonic analog-to-digital conversion and optical quantization are demonstrated, based on the spectral shifts of orthogonal frequency division multiplexing subcarriers and a frequency-packed arrayed waveguide grating. The system is extremely low-energy consuming since the spectral shifts are small and generated by cross-phase modulation, using a linear-slope high-speed and low-jitter pulse train generated by a mode locked laser diode. The feasibility of a 2, 3 and 4-bit optical quantization scheme is demonstrated.

Exact Average OFDM Subcarrier SINR Analysis Under Joint Transmit–Receive I/Q Imbalance

The subcarrier signal-to-interference-plus-noise ratio (SINR) expression for orthogonal frequency-division multiplexing (OFDM) systems impaired by in-phase/quadrature (I/Q) imbalance is a ratio of correlated random variables. In its original form, a quadruple integral is required to evaluate the exact average subcarrier SINR, which is cumbersome to compute numerically. We show that the quadruple integral can be reduced to a single integral, which can be easily evaluated numerically. Furthermore, closed-form expressions are derived for several important special cases, including transmit (TX)-only I/Q imbalance, receive (RX)-only I/Q imbalance, and joint TX/RX I/Q imbalance with equal levels of I/Q imbalance at the transmitter and at the receiver. In addition, we derive high-input-SNR approximations that provide valuable insights on how the average SINR is affected by the I/Q imbalance levels at the transmitter and the receiver and by the channel correlation between the image OFDM subcarriers. For example, we show that at high input SNR, TX-only I/Q imbalance is more harmful, in terms of average subcarrier SINR, than RX-only I/Q imbalance. Furthermore, we show that an approximate OFDM subcarrier SINR expression, commonly used in the literature, is not accurate when RX I/Q imbalance is present.

Adaptive OFDM Radar Waveform Design for Improved Micro-Doppler Estimation

We analyze the performance of a wideband orthogonal frequency division multiplexing (OFDM) signal in estimating the micro-Doppler frequency of a rotating target having multiple scattering centers. The use of a frequency-diverse OFDM signal enables us to independently analyze the micro-Doppler characteristics with respect to a set of orthogonal subcarrier frequencies. We characterize the accuracy of micro-Doppler frequency estimation by computing the Cramer-Rao bound (CRB) on the angular-velocity estimate of the target. Additionally, to improve the accuracy of the estimation procedure, we formulate and solve an optimization problem by minimizing the CRB on the angular-velocity estimate with respect to the OFDM spectral coefficients. We present several numerical examples to demonstrate the CRB variations with respect to the signal-to-noise ratios, number of temporal samples, and number of OFDM subcarriers. The improvement in estimation accuracy due to the adaptive waveform design is also analyzed numerically. A grid-based maximum likelihood estimation technique is applied to evaluate the corresponding mean-squared error performance.

OFDM-Aided Differential Space–Time Shift Keying Using Iterative Soft Multiple-Symbol Differential Sphere Decoding

Soft-decision multiple-symbol differential sphere decoding (MSDSD) is proposed for orthogonal frequency-division multiplexing (OFDM)-aided differential space-time shift keying (DSTSK)-aided transmission over frequency-selective channels. Specifically, the DSTSK signaling blocks are generated by the channel-encoded source information and the space-time (ST) blocks are appropriately mapped to a number of OFDM subcarriers. After OFDM demodulation, the DSTSK signal is noncoherently detected by our soft-decision MSDSD detector. A novel soft-decision MSDSD detector is designed, and the associated decision rule is derived for the DSTSK scheme. Our simulation results demonstrate that an SNR reduction of 2 dB is achieved by the proposed scheme using an MSDSD window size of N-w = 4 over the conventional soft-decision-aided differential detection benchmarker, while communicating over dispersive channels and dispensing with channel estimation (CE).

Application of OFDM Technology in 4G Mobile Network

OFDM (Orthogonal Frequency Division Multiplexing) is a special kind of multi-carrier digital modulation technique. Each OFDM subcarrier exists orthogonality and great use of spectrum resources. To a certain extent, the problem of shortage of spectrum resources will be eased in the future. OFDM has a good ability to resist inter symbol interference and fight against frequency selective fading, and be able to adapt to the future wireless high-speed data transmission without distortion. Each channel modulation and demodulation with FFT and IFFT is easy to implement and meet the need of the business in the wireless communication system. The paper introduces the basic principle of OFDM technology, describes the advantages of OFDM and the defects needed to be overcome. Moreover, we analyzes the characteristics of 4G technology and discusses the application of OFDM in 4G mobile network.

Direct QAM-OFDM Encoding of an L-band Master-to-Slave Injection-Locked WRC-FPLD Pair for 28 × 20 Gb/s DWDM-PON Transmission

The 560 Gb/s (20 Gb/s × 28 channels) DWDM-PON with 16 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) transmission over 25-km SMF by using the master-to-slave injection-locked weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) pair is demonstrated. After master-to-slave injection-locking, the BER of the 16-QAM OFDM data stream under back-to-back and 25-km transmissions can be improved from 3.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> and from 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. With OFDM subcarrier pre-leveling, the BER of 16-QAM OFDM data transmitted by the master-to-slave injection-locked WRC-FPLD over 25-km transmission is further improved from 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> , concurrently enabling the 28 channel transmissions at 20 Gb/s with BER below FEC-limit of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The back-to-back transmission of pre-leveled 64-QAM OFDM data at 30 Gb/s under is also demonstrated to achieve FEC limited BER at receiving power of -6 dBm by using the main of the master-to-slave injection-locked slave WRC-FPLD. The partially coherent injection-locking of maser-to-slave WRC-FPLD pair is comparable with the coherent DFB injection-locked WRC-FPLD, which is capable of serving as the universal transmitters with competitive performance of a power penalty as low as 0.5 dB between WRC-FPLD-to-WRC-FPLD and DFB-to-WRC-FPLD.

Suppressing the relaxation oscillation noise of injection-locked WRC-FPLD for directly modulated OFDM transmission.

By up-shifting the relaxation oscillation peak and suppressing its relative intensity noise in a weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) under intense injection-locking, the directly modulated transmission of optical 16 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data-stream is demonstrated. The total bit rate of up to 20 Gbit/s within 5-GHz bandwidth is achieved by using the OFDM subcarrier pre-leveling technique. With increasing the injection-locking power from -12 to -3 dBm, the effective reduction on threshold current of the WRC-FPLD significantly shifts its relaxation oscillation frequency from 5 to 7.5 GHz. This concurrently induces an up-shift of the peak relative intensity noise (RIN) of the WRC-FPLD, and effectively suppresses the background RIN level to -104 dBc/Hz within the OFDM band between 3 and 6 GHz. The enhanced signal-to-noise ratio from 16 to 20 dB leads to a significant reduction of bit-error-rate (BER) of the back-to-back transmitted 16-QAM-OFDM data from 1.3 × 10(-3) to 5 × 10(-5), which slightly degrades to 1.1 × 10(-4) after 25-km single-mode fiber (SMF) transmission. However, the enlarged injection-locking power from -12 to -3 dBm inevitably declines the modulation throughput and increases its negative throughput slope from -0.8 to -1.9 dBm/GHz. After pre-leveling the peak amplitude of the OFDM subcarriers to compensate the throughput degradation of the directly modulated WRC-FPLD, the BER under 25-km SMF transmission can be further improved to 3 × 10(-5) under a receiving power of -3 dBm.

Secure OFDM-PON System Based on Chaos and Fractional Fourier Transform Techniques

We propose a novel method to simultaneously improve the physical layer security and the transmission performance of the orthogonal frequency division multiplexing (OFDM) passive optical network system by using chaos and fractional Fourier transform (FrFT) techniques. The designed 3-D chaotic sequences are used to form the training sequence for time synchronization, to perform the OFDM subcarriers masking, and to control the fractional order of the FrFT operation. The analyses show that the whole key space size of the proposed scheme could be beyond 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sup> , and the peak-to-average-power-ratio of the transmitted OFDM signal can be decreased by about 0.5 dB. Furthermore, we successfully demonstrate an 8.18 Gbps 16-quadrature-amplitude- modulation (QAM)-OFDM data transmission experiment with chaotic and FrFT operations over 25 km single mode fiber. The results show that the proposed scheme could effectively enhance the system security and the transmission performance without additional bandwidth requirement.

Sidelobe Suppression And Papr Reduction For Cognitive Radio Mimo-Ofdm Systems Using Convex Optimization Technique

Orthogonal Frequency Division Multiplexing (OFDM)is deployed to overcome the interference. However, OFDM has a relatively large OOB emissions. In spectrum sharing approaches such as dynamic spectrum access networks, the OOB power levels of secondary transmissions should be kept below a certain level, in order not to interfere with primary transmissions.. The difficulties such as sidelobes and PAPR caused by OFDM is reduced by convex optimizationand PTS technique respectively.In this technique each OFDM subcarrier is multiplied with a real-valued weight that is determined in order not to interfere with adjacent users. The problem with the SW technique is involving a very complex optimization. We propose a heuristic approach called convex optimization . It can achieve considerable sidelobe suppressionwhile requiring significantly less computational resources than the optimal solution. Implementation results prove that it can be introduced for real-time transmissions. Optimizing the subcarrier weights and SINR is complex, for which we use the technique of convex optimization. For reducing the PAPR we use Partial Transmit Sequence (PTS) technique.

System performance of Rayleigh fading channel in the presence of adjacent channel interference

In this paper, the authors study the system performance of adaptive transmission techniques in the presence of adjacent channel interference (ACI). The authors obtain closed-form expressions for system capacity when employing different adaptive transmission policies with and without diversity schemes. The simulation results of spectrum efficiency in a WiMAX network employing 256 orthogonal frequency division multiplexing subcarriers are obtained for the analytical expressions derived for selection combining diversity under various adaptation policies. Analytical results show that the optimal power and rate adaptation policy provides the highest spectrum efficiency. Both analytical and simulation results are found to be in close agreement with each other, which shows that the obtained analytical results works well with the scenario of ACI. Also, maximal ratio combining scheme yields the highest spectrum efficiency when subjected to ACI.

Wireless Information and Power Transfer in Multiuser OFDM Systems

In this paper, we study the optimal design for simultaneous wireless information and power transfer (SWIPT) in downlink multiuser orthogonal frequency division multiplexing (OFDM) systems, where the users harvest energy and decode information using the same signals received from a fixed access point (AP). For information transmission, we consider two types of multiple access schemes, namely, time division multiple access (TDMA) and orthogonal frequency division multiple access (OFDMA). At the receiver side, due to the practical limitation that circuits for harvesting energy from radio signals are not yet able to decode the carried information directly, each user applies either time switching (TS) or power splitting (PS) to coordinate the energy harvesting (EH) and information decoding (ID) processes. For the TDMA-based information transmission, we employ TS at the receivers; for the OFDMA-based information transmission, we employ PS at the receivers. Under the above two scenarios, we address the problem of maximizing the weighted sum-rate over all users by varying the time/frequency power allocation and either TS or PS ratio, subject to a minimum harvested energy constraint on each user as well as a peak and/or total transmission power constraint. For the TS scheme, by an appropriate variable transformation the problem is reformulated as a convex problem, for which the optimal power allocation and TS ratio are obtained by the Lagrange duality method. For the PS scheme, we propose an iterative algorithm to optimize the power allocation, subcarrier (SC) allocation and the PS ratio for each user. The performances of the two schemes are compared numerically as well as analytically for the special case of single-user setup. It is revealed that the peak power constraint imposed on each OFDM SC as well as the number of users in the system play key roles in the rate-energy performance comparison by the two proposed schemes.