Hybrid Orthogonal Frequency Division Multiplexing Research Articles

Dual high-order QAM-modulated mm-wave signal transmission in the Q-band enabled by simple IM/DD architecture and bandpass delta-sigma modulation.

The intensity-modulation (IM)/direct-detection (DD) systems have been proven effective and low-cost due to their simple system architecture. However, the Mach-Zehnder modulator (MZM) of the IM/DD systems only reserves its driving signal intensity. Therefore, the IM/DD systems are generally unable to transmit vector signals and have a restricted spectrum efficiency and channel capacity. Similarly, the radio-over-fiber (RoF) transmission systems based on IM/DD are limited by their simple architecture and generally cannot transmit high-order quadrature amplitude modulation (QAM) signals, which hinders the improvement of their spectrum efficiency. To address the challenges, we propose a novel, to the best of our knowledge, scheme to simultaneously transmit the dual independent high-order QAM-modulated millimeter-wave (mm-wave) signals in the RoF system with a simple IM/DD architecture, enabled by precoding-based optical carrier suppression (OCS) modulation and bandpass delta-sigma modulation (BP-DSM). The dual independent signals can carry different information, which increases channel capacity and improves spectrum efficiency and system flexibility. Based on our proposed scheme, we experimentally demonstrate the dual 512-QAM mm-wave signal transmission in the Q-band (33-50 GHz) under three different scenarios: 1) dual single-carrier (SC) signal transmission, 2) dual orthogonal-frequency-division-multiplexing (OFDM) signal transmission, and 3) hybrid SC and OFDM signal transmission. We achieve high-fidelity transmission of dual 512-QAM vector signals over a 5 km single-mode fiber (SMF) and a 1-m single-input single-output (SISO) wireless link operating in the Q-band, with the bit error rates (BERs) of all three scenarios below the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. To the best of our knowledge, this is the first time dual high-order QAM-modulated mm-wave signal transmission has been achieved in a RoF system with a simple IM/DD architecture.

A long-reach radio over free space optics (Ro-FSO) system using hybrid orthogonal frequency division multiplexing (OFDM)-multibeam concept with enhanced detection

Abstract This paper focuses on designing of a 10 Gbit/s-10GHz hybrid Orthogonal frequency division multiplexing (FDM) based Radio over Free Space Optics (Ro-FSO) transmission link using optical single sideband modulation format and its performance has been compared using a different number of transmission beams. The proposed link has been simulated and compared using 1-beam, 2-beam, and 4-beam in the system. We show that by using the 4-beam system in the OFDM-Ro-FSO link, a 3000 m range has been achieved reliably, which a notable improvement when compared to previous work is. Also, we show an improved performance of the system by using an enhanced detection mechanism using a Square root module (SRm) at the receiver side.

A reduced PAPR hybrid OFDM visible light communication system

Orthogonal frequency division multiplexing (OFDM) is used by visible light communication (VLC) systems. Its main goal is to lessen inter-symbol interference's negative impacts. As a result, a lot of bandwidth and a high data rate are transmitted. Using OFDM-based VLC systems, nonetheless, have a high peak-to-average power ratio (PAPR). This research presents a new VLC system for indoor applications that reduces the PAPR while maintaining an acceptable bit error rate (BER). To achieve this, a hybrid pre-coding technique-based PAPR reduction solution is proposed; Vander-monde like matrix (VLM) combined with Gaussian matrix (GM). This technique takes advantage of both VLM and GM as a two stages for the signal to simultaneously reduce the PAPR and save the BER from damage. According to the results, the proposed solution (VLM in conjunction with GM) lowers the PAPR to 9.5 dB. Our system performs better when compared to other systems like discrete cosine transform (DCT), GM, VLM, DCT&GM, and original OFDM by 27%, 25%, 11%, 20%, and 34%, respectively.

A Novel OFDM Format and a Machine Learning Based Dimming Control for LiFi

This paper proposes a new hybrid orthogonal frequency division multiplexing (OFDM) form termed as DC-biased pulse amplitude modulated optical OFDM (DPO-OFDM) by combining the ideas of the existing DC-biased optical OFDM (DCO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT). The analysis indicates that the required DC-bias for DPO-OFDM-based light fidelity (LiFi) depends on the dimming level and the components of the DPO-OFDM. The bit error rate (BER) performance and dimming flexibility of the DPO-OFDM and existing OFDM schemes are evaluated using MATLAB tools. The results show that the proposed DPO-OFDM is power efficient and has a wide dimming range. Furthermore, a switching algorithm is introduced for LiFi, where the individual components of the hybrid OFDM are switched according to a target dimming level. Next, machine learning algorithms are used for the first time to find the appropriate proportions of the hybrid OFDM components. It is shown that polynomial regression of degree 4 can reliably predict the constellation size of the DCO-OFDM component of DPO-OFDM for a given constellation size of PAM-DMT. With the component switching and the machine learning algorithms, DPO-OFDM-based LiFi is power efficient at a wide dimming range.

Wideband Generalized Beamspace Modulation (wGBM) for mmWave Massive MIMO Over Doubly-Selective Channels

Generalized beam modulation (GBM) is a recently proposed spectrum efficiency (SE)-enhanced index modulation (IM) scheme for narrowband millimeter wave (mmWave) massive multiple-input multiple-output (mMIMO) systems with hybrid transceivers. Unlike most existing fully-digital IM techniques whose application to wideband channels is straightforward after adopting the orthogonal frequency division multiplexing (OFDM) technique, extending GBM from the narrowband to wideband channels is quite challenging. The reason is that the per-subcarrier process in hybrid OFDM systems is no longer independent across individuals as all subcarriers share a common analog beamformer. Hence, the designed wideband GBM (wGBM) has to be compatible with hybrid OFDM systems while maintaining the SE-enhanced merit of GBM. Towards this objective, a symbol-based instead of the prevalent subcarrier-based paradigm is applied to guide wGBM design. To avoid the high detection complexity in large-scale OFDM systems, we devise a low-complexity detector by transforming the complicated symbol (block) detection into the manageable subcarrier (sub-block) detection. Finally, a simple first-order Doppler compensator is carefully designed to enhance the robustness of wGBM against Doppler. Both theoretical analyses and numerical simulations have been carried out to demonstrate the superiority of wGBM over existing alternatives in terms of error performance and energy efficiency.

Performance analysis of different intensity modulation techniques over atmospheric turbulent free-space optical channels.

In this paper, we introduce a comprehensive study of the performance of intensity modulation (IM) techniques over gamma-gamma (GG) distributed free-space optical (FSO) channels. We derive closed-form expressions for the average bit error rate (BER) of thermal-noise-limited FSO systems adopting various IM techniques. The effect of atmospheric turbulence is studied. Comparisons are presented in terms of the transmitted peak power to achieve a certain BER. Results show that on-off keying, pulse amplitude modulation, and expurgated pulse-position modulation (EPPM) provide superior performance compared to other IM techniques. In addition, a proposal to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) is introduced by adding another modulation layer of EPPM. A closed-form expression for the average BER of the proposed hybrid OFDM-EPPM technique over GG turbulent FSO channel is derived. Moreover, an expression for the outage probability of the FSO system adopting the proposed OFDM-EPPM technique is presented. Results are investigated and compared to those of traditional OFDM and hybrid OFDM with PPM (OFDM-PPM). It is shown that the hybrid OFDM-EPPM technique outperforms the hybrid OFDM-PPM technique for the same data rate. Moreover, the hybrid OFDM-EPPM technique allows higher data rates with lower PAPR values compared to those of the hybrid OFDM-PPM technique for the same number of time slots.

A hybrid OFDM–CDMA-based robust image watermarking technique

Digital watermarking is a process of embedding hidden information called watermark into different kinds of media objects. It uses basic modulation, multiplexing and transform techniques of communication for hiding information. Traditional techniques used are least significant bit (LSB) modification, discrete cosine transform (DCT), discrete wavelet transform (DWT), discrete Fourier transform (DFT), code division multiple access (CDMA) or a combination of these. Among these, CDMA is the most robust against different attacks except geometric attacks. This paper proposes a blind and highly robust watermarking technique by utilizing the basis of orthogonal frequency division multiplexing (OFDM) and CDMA communication system. In this scheme, the insertion process starts by taking DFT of host images, permuting the watermark bits in randomized manner and recording them in a seed as a key. Then PSK modulation follows inverse DFT (IDFT) that gives watermark information as OFDM symbols. These symbols are spread using spreading codes and then arithmetically added to the host image. Finally, scheme applies inverse DCT (IDCT) to get watermarked host images. The simulation results of the proposed scheme are compared with CDMA-based scheme in DCT domain. The results show that the robustness of the proposed scheme is higher than the existing scheme for non-geometric attacks.

Bidirectional hybrid OFDM based free-space/wireless-over-fiber transport system

We propose and demonstrate a successful application of orthogonal frequency division multiplexing (OFDM), reflective semiconductor optical amplifier and polarization multiplexing techniques to facilitate bidirectional hybrid OFDM based integrated free-space optics and wireless transport system. The downlink 12 Gbps OFDM data-stream and 79 community antenna television (CATV) channels are transmitted over 50 km single mode fiber (SMF) as well as 12 m and 10 m free-space link, respectively. The uplink 10 Gbps/5 GHz and 5 Gbps/3.5 GHz OFDM data streams are communicated over separate 50 km SMF along with 8 m wireless links. We have used 32-quadrature amplitude modulation (QAM) and 16-QAM modulation formats for downlink and uplink, correspondingly. The performance of the system is observed by evaluating carrier-to-noise ratio (CNR), composite second-order (CSO) and composite-triple-beat (CTB) for the CATV signal, and constellations diagrams, bit error rate (BER), signal-to-noise ratio (SNR), and error vector magnitude (EVM) values for the OFDM signals. After transmission over 50 km SMF as well as 10 m free-space, the resulting CNR, CSO and CTB values are > 51 dB, 23.3 dB, respectively. 4 dBm and 3.2 dBm power penalties at BER of $$10^{ - 8}$$, below 13% EVM and > 21.3 dB/ > 22.2 dB SNR values are observed for the uplink 10 Gbps/5 GHz and the 5 Gbps/3.5 GHz OFDM data signals, respectively, over 50 km SMF as well as 8 m wireless is observed.

Hybrid OFDM receiver assisted by a variable frequency comb

A physically assisted orthogonal frequency division multiplexing (OFDM) receiver is described and characterized. In contrast to recent reports that utilize two physically distinct frequency combs with Verniered frequency pitch, the new receiver topology relies on a single frequency-toggled frequency comb. Dual-comb photonic front end was replaced by a single comb split into two switched paths to achieve spectral decomposition. To demonstrate new receiver operation, hybrid RF-photonic architecture for discrete Fourier transform (DFT) was constructed and used to decompose a wideband RF signal. The receiver demodulated a 4-QAM OFDM channel using 50 carriers from a single frequency comb. OFDM channel, spanning 3-7.9GHz RF band, was encoded using 100MHz-separated subcarriers. OFDM receiver performance was quantified by measuring its error vector magnitude (EVM).

Improved Performance Investigation of 10 Gb/s–10 GHz 4-QAM Based OFDM-Ro-FSO Transmission Link

Abstract In this work, a 10 Gb/s–10 GHz hybrid orthogonal frequency division multiplexing (OFDM)-radio-over-free space optics (Ro-FSO) transmission link using 4-level quadrature amplitude modulation (QAM) has been modeled and its performance is investigated under different atmospheric conditions and system parameters. A link distance of 4000 m m at 10 Gb/s was achieved using the proposed link. Also, an improved performance by using a square root module (SRm) at the receiver end has been demonstrated. The results show a significant improvement in SNR and total received power using the proposed detection technique.

Fast-OFDM With Index Modulation for NB-IoT

In this letter, a hybrid orthogonal frequency-division multiplexing (OFDM)-based modulation technique for narrowband Internet of Things (NB-IoT) is introduced and analyzed. The technique combines fast-OFDM with index modulation in order to maximize bandwidth and power efficiency for IoT applications. The ideal number of active subcarriers to maximize spectral efficiency is derived. The one-dimensional constellation used in fast-OFDM is also optimized to enhance error performance of the proposed system. Numerical results indicate that the proposed system outperforms other OFDM systems based on index modulation in the relatively low signal-to-noise ratio (SNR) region, while it provides additional design options for trading off power efficiency and spectral efficiency in the higher SNR region.

Hybrid DFT-Spread OFDM-Digital Filter Multiple Access PONs for Converged 5G Networks

In this paper, we propose hybrid discrete Fourier transform (DFT)-spread orthogonal frequency division multiplexing (OFDM)-digital filter multiple access (DFMA) passive optical networks (PONs) for the first time to the best of our knowledge. In this case, digital filtering based on digital signal processing is applied to individual DFT-spread OFDM signals from various optical network units, and a single fast Fourier transform operation and its following data recovery processes are implemented in a pipelined approach in the optical line terminal. The proposed networks maintain all salient features associated with previously reported hybrid OFDM–DFMA PONs. More importantly, they additionally reduce the upstream signal peak-to-average power ratios by less than 2 dB. As a direct result, in comparison with the hybrid OFDM–DFMA PONs, the proposed PONs increase the upstream system power budget by more than 3 dB and reduce the minimum required digital-to-analog converter/analog-to-digital converter quantization bits by at least 1 bit.

Hybrid OFDM-Digital Filter Multiple Access PONs

Hybrid orthogonal frequency division multiplexing (OFDM) digital filter multiple access (DFMA) passive optical networks (PONs) are, for the first time, proposed and extensively investigated, where digital signal processing (DSP) based software-reconfigurable digital filtering is employed for each individual OFDM signal from various optical network units (ONUs), and a single fast Fourier transform operation and its following data recovery processes are implemented in a pipelined approach in the optical line terminal. As a direct result of combining both OFDMA and DFMA PON characteristics, the proposed hybrid OFDM-DFMA PON still maintains highly desirable features associated with DFMA PONs, more importantly, it also offers a number of unique advantages including great relaxation of digital filter DSP complexity and considerable reduction in transceiver cost, significantly enhanced upstream PON performance and its robustness and flexibility, as well as excellent backward compatibility with existing OFDM-based 4G networks. In this paper, an analytical hybrid OFDM-DFMA PON model is developed, and numerical simulations are performed to verify the validity of the proposed technique and also to examine the upstream hybrid OFDM-DFMA PON performance characteristics over representative 25-km standard single mode fibre IMDD PON systems. It is shown that in comparison with the DFMA PONs, the hybrid OFDM-DFMA PON can improve the upstream ONU EVM performance by >10 dB for relatively large received optical powers, and also increases the differential ONU launch power dynamic range by as large as 16 dB.

Hybrid DCO-OFDM, ACO-OFDM and PAM-DMT for dimmable LiFi

This paper proposes a hybrid orthogonal frequency division multiplexing (OFDM) modulation termed as hybrid DC-biased asymmetrically-clipped pulse-amplitude-modulated OFDM (HDAP-OFDM) for providing power efficient communication and dimming control for light fidelity (LiFi) schemes. The proposed HDAP-OFDM is a combination of three OFDM formats: DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT). In HDAP-OFDM, the lower-index subcarriers carry DCO-OFDM, while the remaining higher subcarriers carry ACO-OFDM on the odd-index subcarriers and PAM-DMT on the even-index subcarriers. Mathematical analysis shows that the required DC bias for HDAP-OFDM is a function of the required dimming levels and the root mean square of the three of its components. Furthermore, an algorithm is presented to simulate the dimming control of HDAP-OFDM. Simulations results indicate that by adjusting the proportion of subcarriers for the three OFDM/DMT elements, HDAP-OFDM can have a wide dimming range, a low peak to average power ratio, and high power efficiency. Results also show that at a bit error rate of 10−3 and a dimming range of 3%–97%, HDAP-OFDM exhibits 8 dB more optical and electrical power efficiency than existing asymmetrical hybrid optical OFDM.

Underwater pipelines panoramic image transmission and refinement using acoustic sensors

The natural resources require continuous monitoring of pipelines to avoid algae blooms, corrosion, leakages, damages at joints for better management. Monitoring is performed through Wireless Sensors Network in collaboration with various communication methods using image sensors, but they face limitations like time consumption, color variations, blurred data, bit or data losses. To overcome the limitations, the paper proposes a Panoramic Image Transmission and Refinement Technique that uses acoustical sensors, Hybrid-Orthogonal Frequency Division Multiplexing (Hybrid-OFDM) and Mosaicing technique, targets to sense, capture and forward data to the surface station. It smooths the data using Random Sample Consensus (RANSAC) algorithm. The proposed method is tested and validated through MATLAB simulations. The obtained results support technique’s efficiency with respect to less energy consumption, communication, maximum delivery of data at low cost.

2 × 20 Gbps - 40 GHz OFDM Ro-FSO transmission with mode division multiplexing

Radio-over-Free-Space-Optics (Ro-FSO) is a promising technology for future wireless networks. In this work, we have designed a hybrid orthogonal frequency division multiplexing (OFDM) Ro-FSO system for transmission of two independent channels by mode division multiplexing. Two independent 40 GHz radio signals are optically modulated at 20Gbps by mode division multiplexing of two laser modes LG00 and LG10 and transmitted over a free-space link of 20 km to 100 km. The performance of proposed Ro-FSO system is also evaluated under the effect of strong atmospheric turbulences.

Power Allocation in OFDM Cognitive Radio System in Presence of Relay

This paper proposes power allocation algorithm in orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) system using decode and forward (DF) relay. It is reported in the literature that both classical i.e. uniform power loading scheme and water filling methods that allocate power on OFDM subcarriers based on the channel gain are not effective for cognitive radio network (CRN) as the schemes introduce large interference to primary user (PU). To this aim, the present work proposes a simple, computationally efficient yet effective power loading scheme that maximizes the transmission capacity of CR while keeping the interference introduced to the PU below acceptable limits. To meet the objective, proposed power allocation to subcarriers not only considers channel gains but also the relative distances between CR subcarriers and PU band. This capacity performance is further improved significantly using DF relay, where our research direction focuses on optimal sharing of power between CR and relay using maximal ratio combiner (MRC) at destination receiver. Performance for the proposed power allocation schemes are compared with OFDM based hybrid power allocation as well as joint subcarrier and power adaptation methods reported in the literature. Numerical results show that the relative gain on CR's capacity are 3.7 times and 2.3 times, respectively compared to the optimal and hybrid OFDM power allocation schemes, while for interference introduced to PU is set at 3 times 10-6 (in watt). Finally, a few related research problems are also highlighted as future scope of works.