Overlap Frequency Domain Equalization Research Articles

Mitigation of Phase Noise and Nonlinearities for High Capacity Radio-over-Fiber Links

Radio-over-fiber (RoF) links successfully provide high data rates and bandwidth capacity with a low complexity system architecture, as compared to its counterpart digital-RoF. In addition, the compound of quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexed (OFDM) modulation schemes further enhance the process of these achievements. However, high data rates and bandwidth-capacity-supported RoF links face nonlinearities (NLs), linear distortions (LDs), and phase noise challenges that degrade the reliability of communication networks (CNs). Therefore, in this paper, to suppress NLs, LDs, and phase noise, next generation cloud radio access networks (CRANs) are investigated using RoF links and wavelength division multiplexing (WDM) methodology based on 16, 32, and 64 QAM-OFDM modulation schemes. The receiver of the proposed framework is designed, applying an improved digital signal processing (DSP) system that includes overlap frequency domain equalization (OFDE), a synchronization process, and time domain equalization (TDE). Theoretical and simulation models are organized for estimating the proposed RoF link with the aid of different values of transmission ranges, input power, output power, bit rate, bits per symbol, channel spacing, and the number of users. The fitness of the model matches that of existing approaches.

Hierarchical modulation PAM4 with digital Nyquist pulse-shaped for flexible multi-ONU provisioning in NG-TDM PON

Abstract In this paper, we study the performance of digital Nyquist-shaped and hierarchical modulation four-level pulse amplitude modulation (N-HM-PAM4), which is one of the most cost-effective modulations, in the context of the intensity-modulation and direct-detection (IM/DD) enabled the next generation time division multiplexed passive optical network (NG-TDM PON). In addition, by considering the power fading (which is incurred by the chromatic dispersion (CD)) and linear and nonlinear inter-symbol interference (ISI) (which is incurred by CD and square-law photo-detection) during the transmission, vestigial side band (VSB) optical equalization, overlap frequency-domain equalization (OFDE), and iterative nonlinear ISI digital domain compensation are applied in the system to improve both bandwidth efficiency and receiver sensitivity. Simulation results show how different parameters of the N-HM-PAM4 based NG-TDM PON (i.e., optical modulation index (OMI), iterative cancellation number, power ratio, and the distance between the optical line terminal (OLT) and its optical network units (ONUs)) affect the performance of the system, which is defined as the bit error ratio (BER) and received optical power (ROP) of the ONU.

Wavelet OFDM with Overlap FDE for non-Gaussian channels in precoded NOMA based systems

Non-orthogonal multiple access (NOMA) is considered as a prominent multiple access technique for fifth generation (5G) systems mainly because of its high spectral efficiency (SE). NOMA based system employs superposition coding (SC) and successive interference cancellation (SIC) at the transmitter and receiver side respectively. Classical multi-carrier NOMA adopts orthogonal frequency division multiplexing (OFDM) as a multi-carrier modulation (MCM) technique; however, high peak to average power ratio (PAPR) and cyclic prefix (CP) inclusion makes OFDM an inadequate choice for NOMA. To improve SE of the OFDM-NOMA, wavelet transform (WT) has been proposed for NOMA. This article proposes new physical layer techniques based on OFDM-NOMA and Wavelet OFDM (WOFDM) -NOMA for tactile internet and investigates their performance in the presence of Gaussian and non-Gaussian channels. Overlap frequency domain equalization (OFDE) is proposed as a robust equalization technique to improve the performance of SIC in the presence of Gaussian and non-Gaussian channels for OFDM-NOMA and WOFDM-NOMA. Moreover, to enhance the equalization efficiency of the OFDE, linear precoding (LP) in the form of Walsh Hadamard transform (WHT) is also recommended for OFDM and WOFDM based NOMA transceivers which improves the reliability of the network. Performance of the proposed precoded OFDM-NOMA and precoded WOFDM-NOMA with OFDE is analyzed in the presence of additive white Gaussian noise (AWGN) and impulse noise for Rayleigh fading channel. Link level performance of the presented structures for downlink NOMA is evaluated in terms of bit error rate (BER), PAPR, SE and computational complexity (CC). Furthermore, performance of the proposed architectures is also compared in the presence of impulse noise having different intensities. Simulation results via computer show that the proposed precoded WOFDM-NOMA with OFDE transceiver incorporates low latency through waveform shaping and enhanced reliability through precoding and efficient equalization. Thus, proposed advanced physical layer not only performs better than its counterparts but presence of OFDE enhances SIC performance even in the presence of impulse noise at the expense of slight elevation in CC.

Adaptive modulation scheme for DML-based IM/DD FOFDM systems using overlap frequency-domain equalization

Abstract To further investigate the optical fast orthogonal frequency-division multiplexing (FOFDM) with sub-channel spacing equal to half of the symbol rate, an adaptive modulation scheme for FOFDM is proposed and experimentally demonstrated in low-cost short-reach intensity-modulated and direct-detection (IM/DD) systems using a directed modulated distributed feedback laser (DML). The experimental results and analyses indicate, it is feasible to transmit an adaptively modulated optical FOFDM signal over 20-km and 50-km standard single mode fibers (SSMFs). Compared with a single modulation format of 16-PAM, after 20-km SSMF transmission, the receiver sensitivity of the adaptive modulation scheme can be improved by about 2.3 dB at the bit error ratio (BER) of 2.4 × 10−2. Moreover, after 50-km SSMF transmission, the adaptive modulation scheme can still under 2.4 × 10−2 when received optical power (ROP) is larger than −1 dBm, while the 16-PAM modulation scheme cannot. In addition, an overlap frequency-domain equalization (OFDE) scheme is employed after sub-carrier de-multiplexing at the receiver to enable the effects of inter-symbol interference (ISI) to be effectively suppressed even though a cyclic prefix (CP) is not employed. Experimental results show that the FOFDM systems employing OFDE scheme have almost no performance degradation when compared to employing conventional FDE with an additional CP, but the spectral efficiency (SE) can be significantly improved by 12.37%.

Linear precoded wavelet OFDM‐based PLC system with overlap FDE for impulse noise mitigation

SummaryIn recent years, power lines have gained significant interest for their use in high‐speed communications because of the already deployed power distribution infrastructure. However, to achieve high data rates with reliability in power line communication (PLC), robust signal processing techniques are required to mitigate channel distortion and noise. Orthogonal frequency division multiplexing (OFDM) as a multicarrier modulation technique has been standardized for PLC; however, to further enhance the quality of communication, wavelet OFDM (WOFDM) has been proposed as a suitable choice. In this article, OFDM‐based and WOFDM‐based PLC systems are studied, and overlap frequency domain equalization (OFDE) as a robust and efficient equalization technique is presented. Moreover, to enhance the efficiency of the OFDE, linear precoding (LP) is also suggested for fast Fourier transform and wavelet transform–based filter bank transceivers over the PLC channel. Performance of the proposed LP‐based OFDM‐OFDE and LP‐based WOFDM‐OFDE systems is compared with previously proposed equalization structures in terms of bit error rate, peak‐to‐average power ratio, and computational complexity via computer simulations. Furthermore, the performance of proposed architectures is also compared with classical equalization techniques under impulse noise with different intensities. Results show that not only the proposed LP‐based WOFDM‐OFDE transceiver performs better than the previous equalization models but also the LP‐based OFDM‐OFDE at the expense of slight increment in computational complexity.

Robust TC-OLA Reception With Frequency Domain Equalization for Ultrawideband (UWB) Applications

A variant of time-compression overlap-add (TC-OLA) is proposed as an alternate physical layer scheme for ultrawideband communications. Although the construction of the transmitted signal is identical to other TC-OLA applications, this letter explores block-based receiver structures using frequency domain equalization (FDE) techniques that allow for a longer delay spread relative to the window size than time-domain TC-OLA processing. An important and novel contribution of this method is the reduction or elimination of the cyclic prefix (CP) overhead due to the way that redundancy is added to the transmitted signal by the TC-OLA process. Unlike overlap FDE, the CP overhead is eliminated without the increased processing load required to maintain a practically useful bit error rate floor. For very high delay spread channels, such as CM8, a long effective CP length can be obtained without requiring a much longer fast Fourier transform block length to maintain efficiency.

WDM–CAP–PON integration with VLLC system based on optical frequency comb

In this paper, a wavelength division multiplexing carrier-less amplitude phase modulation passive optical network (WDM–CAP–PON) integration with visible laser light communication (VLLC) system is proposed and experimentally demonstrated. To reduce the cost of WDM system, the optical frequency comb scheme using one Mach–Zehnder modulator (MZM) is utilized and five flat optical combs can be generated. Meanwhile, a blue laser diode (LD) as a VLLC optical source can provide high data rate and long transmission distance. Utilizing overlap frequency domain equalization (OFDE) and negative chirp of MZM, the system performance in both Q-factor and receiver sensitivity can be improved. After 20km standard single mode fiber (SSMF) and 4.5m free space transmission, the experimental results show that 10Gb/s CAP signal can be achieved under 7% forward error correction (FEC) limit of 3.8×10−3.

Enhanced Performance of CAP System Using an Overlap Frequency-Domain Equalization Scheme

In this paper, an overlap frequency-domain equalization (OFDE) scheme is proposed and experimentally demonstrated in an intensity modulation and direct-detection optical communication system with carrierless amplitude and phase (CAP) modulation. Moreover, the negative chirp property of the Mach-Zehnder modulator (MZM) for a CAP32/64/128 system is also investigated. After 100-km standard single-mode fiber transmission, the experimental results show that at the bit error ratio (BER) of 3.8 × 10 -3 , the receiver sensitivities can be improved by 0.6 and 0.4 dB for CAP32 and CAP64 systems by utilizing the OFDE method, respectively. In addition, the receiver sensitivity is improved by 0.5 dB for CAP128 system at the bit error ratio (BER) of 2.4 × 10 -2 . Compared with the FDE based on cyclic prefix (CP), the spectral efficiency of the CAP system with the OFDE scheme can be improved by 12.18%.

Performance Analysis of DWMT Transceiver for ADSL Channel with Overlap FDE in Presence of AWGN, Crosstalk and Impulse Noise

Discrete wavelet Multi-tone (DWMT) was proposed to overcome the problems of Discrete Multi-tone (DMT) for wireline channels. DWMT based system poses more resistance against inter-symbol interference (ISI) because of tight filters and more spectral efficiency due to absence of cyclic prefix as compared to DMT based system. However, no equalization technique has been standardized yet for DWMT transceiver. In this paper, we are proposing Overlap frequency domain equalization (FDE) for DWMT transceiver for ADSL channel in presence of additive white Gaussian noise (AWGN), crosstalk and impulse noise (IN). From simulations results, we can analyze that Bit error rate (BER) performance of Time domain equalization (TDE) is comparable with Overlap FDE but the main advantage lies in lower computational complexity.

DWMT transceiver equalization using overlap FDE for downlink ADSL

Discrete wavelet multitone (DWMT) modulation is a wavelet transform based technique implemented using perfect reconstruction filter banks. It has been recently proposed for various wireline channels such as digital subscriber loops (DSLs) as a solution to the problems posed by a discrete multitone (DMT) transceiver including interblock interference (IBI) and lower spectral efficiency due to the employment of a cyclic prefix (CP) in the guard interval (GI) for DMT symbols. The greater side lobe attenuation offered by wavelet filter banks results in improved spectral containment and lower IBI in DWMT transceivers. However, no standard equalization technique exists for a DWMT based transceiver so as to remove the effect of channel on the transmitted signal in DWMT systems. This paper proposes the application of overlap frequency domain equalization (OFDE) in DWMT modulated systems and compares the bit error rate (BER) performance with time domain equalization (TDE) technique. It is shown through simulation results that minimum mean square error (MMSE) based OFDE can be applied as an equalization technique for a downlink asymmetric DSL (ADSL) channel with lower computational complexity and BER performance comparable to that of TDE.

Experimental demonstration of half cycle 64-QAM Nyquist-SCM direct-detection optical communication system with data-aided estimation and overlap frequency-domain equalization

A half cycle 64-quadrature amplitude modulation (QAM) Nyquist subcarrier modulation (SCM) polarization division multiplexing (PDM) intensity modulation direct detection optical communication system is experimentally demonstrated. At the receiver, training sequences-based channel estimation and an overlap frequency domain equalization method are proposed to enhance the system performance. The experimental results show that the half cycle 64-QAM Nyquist-SCM PDM signal can be transmitted over 43-km standard single-mode fibers with a bit error rate below the forward error coding threshold of 2.4×10−2.

Transmission of a 127 Gb/s PM-QPSK Signal Over a 3350 km SMF-Only Line With Chromatic Dispersion Compensation Using Real-Time DSP [Invited

We experimentally confirmed the effectiveness of chromatic dispersion compensation by overlap frequency-domain equalization in real-time digital signal processing for a 127 Gb/s polarization-multiplexed quadrature phase shift keying (PM-QPSK) signal up to 56,200 ps/nm. We verified a penalty lower than 0.55 dB for accumulated dispersion over a 3350 km single-mode-fiber- (SMF-) only transmission line. We also demonstrated the transmission of a 127 Gb/s PM-QPSK signal in a 50-GHz-spacing 5-channel wavelength division multiplexing configuration over the same SMF-only line with a received Q factor of 10.1 dB.

Performance Comparison of Overlap FDE and Sliding-Window Chip Equalization for Multi-Code DS-CDMA in a Frequency-Selective Fading Channel

Recently, overlap frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion which requires no guard interval (GI) insertion was proposed for signal transmission using multi-code direct sequence code division multiple access (DS-CDMA) in a frequency-selective fading channel. Another promising equalization technique is time-domain sliding-window chip equalization (SWCE). In this paper, the bit error rate (BER) performances achievable with overlap FDE and SWCE are compared. It is shown that, by extending the fast Fourier transform (FFT) block size, overlap FDE can achieve almost the same BER performance as SWCE with much less computational complexity than SWCE.

Systematic design of single carrier overlap frequency domain equalization

This paper proposes a systematic design method of overlap frequency domain equalization (FDE) for single carrier (SC) transmission without a guard interval (GI). Based on the analysis of signal-to-interference-plus-noise ratio (SINR) of the equalizer output for each symbol, the authors adaptively determine the block of the overlap FDE, where the block is defined as a set of symbols at the equalizer output with sufficiently low error rate, for a certain fixed sliding window size, which corresponds to a fast Fourier transform (FFT) window size. The proposed method takes advantage of the fact that the utility part of the equalized signal is localized around the center of the FFT window. In addition, the authors also propose to adjust the block size in order to control the computational complexity of the equalization per processed sample associating with the average bit error rate (BER) of the system. Simulation results show that the proposed scheme can achieve comparable BER performance to the conventional SC-FDE scheme with sufficient GI insertion for both the coded and uncoded cases with various modulation levels, while requiring lower computational complexity compared to the SC overlap FDE transmission with the fixed block.

Coherent Optical Single Carrier Transmission Using Overlap Frequency Domain Equalization for Long-Haul Optical Systems

In this paper, we present coherent optical single carrier transmission (COSC) using overlap frequency domain equalization (OFDE) for the chromatic dispersion (CD) compensation. Residual inter-symbol interference is suppressed by time domain equalization with small tap size. In single polarization transmission, 25-Gb/s COSC transmission with OFDE is demonstrated over 800 km to 4320 km of single-mode fiber (SMF) without optical dispersion compensation. The measured results show that the proposed configuration improves the transmission performance by setting adequate overlapped size of FFT windows and that it is suitable for long-haul optical communication systems.

Iterative Overlap FDE for Multicode DS-CDMA

Recently, a new frequency-domain equalization (FDE) technique, called overlap FDE, that requires no GI insertion was proposed. However, the residual inter/intra-block interference (IBI) cannot completely be removed. In addition to this, for multicode direct sequence code division multiple access (DS-CDMA), the presence of residual interchip interference (ICI) after FDE distorts orthogonality among the spreading codes. In this paper, we propose an iterative overlap FDE for multicode DS-CDMA to suppress both the residual IBI and the residual ICI. In the iterative overlap FDE, joint minimum mean square error (MMSE)-FDE and ICI cancellation is repeated a sufficient number of times. The bit error rate (BER) performance with the iterative overlap FDE is evaluated by computer simulation.

BER Performance Analysis of MC-CDMA with Overlap-FDE

Recently, multi-carrier code division multiple access (MC-CDMA) has been attracting much attention as a broadband wireless access technique for the next generation mobile communication systems. Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can take advantage of the channel frequency-selectivity and improve the average bit error rate (BER) performance due to frequency-diversity gain. The conventional FDE requires the insertion of the guard interval (GI) to avoid the inter-block interference (IBI), resulting in the transmission efficiency loss. In this paper, an overlap FDE technique, which requires no GI insertion, is presented for MC-CDMA transmission. An expression for the conditional BER is derived for the given set of channel gains. The average BER performance in a frequency-selective Rayleigh fading channel is evaluated by Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation of the signal transmission.

DS-CDMA HARQ with Overlap FDE

SUMMARY Turbo coded hybrid ARQ (HARQ) is known as one of the promising error control techniques for high speed wireless packet access. However, in a severe frequency-selective fading channel, the HARQ throughput performance significantly degrades for direct sequence code division multiple access (DS-CDMA) system using rake combining. This problem can be overcome by replacing the rake combining by the frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion. In a system with the conventional FDE, the guard interval (GI) is inserted to avoid the inter-block interference (IBI). The insertion of GI reduces the throughput. Recently, overlap FDE that requires no GI insertion was proposed. In this paper, we apply overlap FDE to HARQ and derive the MMSE-FDE weight for packet combining. Then, we evaluate the throughput performance of DS-CDMA HARQ with overlap FDE. We show that overlap FDE provides better throughput performance than both the rake combining and conventional FDE regardless of the degree of the channel frequency-selectivity.