Orthogonal Frequency Division Multiplexing Method Research Articles

OFDM and MIMO wireless communication performance measurement using enhanced selective mapping based partial transmit sequences

The great spectral efficiency to mitigate multipath fading of OFDM (orthogonal frequency division multiplexing) made more accepted for broadband communication systems with some drawbacks such as high PAPR (Peak-to-Average Power Ratio), ISI (Inter-Symbol Interference) and ICI (Interiv Carrier Interference). High PAPR forces the operations of the HPA amplifier to work beyond its linear region causing in-band distortion as well as out-band noise and are overcome by using an amplifier with a large dynamic range. In this method, OFDM symbols are partitioned into multiple disjoint sub-blocks which are independently rotated in phase by optimum phase weighting factors. The resultant OFDM signal has less PAPR when compared to that of original OFDM signal in wireless communication. To minimize computational complexity, a modified PTS technique is proposed to enhance PAPR reduction and power amplifier efficiency in OFDM and MIMO (multiple inputs, multiple outputs)-OFDM methods through Selective Mapping based PTS (SMPTS) method combined with PAPR reduction methods.

A comprehensive study on the role of advanced technologies in 5G based smart hospital

In the present scenario, orthogonal frequency division multiplexing (OFDM) waveform technique is playing a significant part in smart hospitals. Still, its impact is not effective in smart hospitals due to the numerous limitations such as loss of bandwidth due to the use of guard band, spectrum leakage, high peak to average power (PAPR), high detection delay, and thus along. Right now, fifth-generation (5G) employment is becoming regularized around the world and the state-of-the-art radio system is expected to meet all the demands of smart hospitals. High spectrum access, massive capacity, high throughput, and low PAPR are the requirements of smart healthcare hospitals. The digital hospital has seen tremendous progression in bandwidth requirements. From transmitting medical images to wearable devices, networks must operate at maximum speeds to safeguard patient care. The selection of competent transmission technologies will play an important role in the regularization of digital hospitals equipped with 5G. In the projected work, we focus on the implementation of novel waveforms such as Non-orthogonal multiple access (NOMA), Universal filter multi-carrier (UFMC), and filter bank multi-carrier (FBMC) system. Several parameters such as power spectrum density, bit error rate, capacity, and PAPR of advanced waveforms and OFDM methods are analyzed and studied.

Channel Estimation with an Interpolation Trained Deep Neural Network

This paper proposes a deep learning-based channel estimation method for orthogonal frequency-division multiplexing (OFDM) systems. The existing OFDM receiver has low estimation accuracy when estimating channel state information (CSI) with fewer pilots. To tackle the problem, in this paper, a deep learning model is first trained by the interpolated channel frequency responses (CFRs) and then used to denoise the CFR estimated by least square (LS) estimation. The proposed deep neural network (DNN) can also be trained in a short time because it only learns the CFR and the network structure is simple. According to the simulation results, the performance of the DNN estimator can be compared with the minimum mean-square error (MMSE) estimator. Furthermore, the DNN approach is more robust than conventional methods when fewer pilots are used. In summary, deep learning is a promising tool for channel estimation in wireless communications.

Non-linear companding scheme for peak-to-average power ratio (PAPR) reduction in generalized frequency division multiplexing

Abstract Due to the evolution of mobile phone technology which enables the use of Internet, the demand for data rates has increased tremendously. Therefore, current 4G-LTE (long-term evolution) technology needs to be replaced in near future. Hence, currently possible solutions for 5G technology are heavily investigated. Generalized frequency division multiplexing (GFDM) is one such technique which is based on multi-carrier filters and uses time-frequency structure to optimize delay and out-of-band emission (OOB). Still GFDM is very much similar to orthogonal frequency division multiplexing (OFDM) with much improved spectral and peak-to-average power ratio (PAPR) performance. This paper presents a detailed description of GFDM technique, along-with its comparison to OFDM method. This paper further investigates the non-linear companding scheme which can be used to further reduce PAPR. Simulation results are presented to obtain (Power Spectral Density) PSD and PAPR performance.

Optical disc writing strategy for analog signal recording

Optical discs are being sought as low-cost, earth-friendly storage candidates in response to increasing data volumes. To increase the recording density of optical disc, analog recording technique such as the Orthogonal Frequency Division Multiplexing (OFDM) method has been investigated. Since bitwise information is recorded on optical discs, it is necessary to create a column of marks corresponding to the analog signal for the OFDM method. The Delta-Sigma Modulation (DSM) is known as an analog to digital conversion method in the area of audio. This method transfers efficiently an analog signal to a binary sequence with a simple circuit. In this paper, the application of the method for analog signal recording and the simulation results of the mark recording process on the optical disc are described. The optical disc recording utilizes temperature rise of a multilayer film stack in recording process that is triggered by light focusing. A simplified, 2.5-dimension, thermal analysis is applied to do the large-scale simulation of recording mark sequence.

Hybrid Zadoff‐Chu and multilateral piecewise exponential companding transform–based PAPR reduction technique in OFDM systems

SummaryThe orthogonal frequency‐division multiplexing (OFDM) is a multicarrier modulation system that is used to transmit the large volume of data to the receiver. Reducing the peak‐to‐average power ratio (PAPR) in OFDM system is one of the demanding and crucial task in recent days. For this reason, various precoding and companding mechanisms are developed in the traditional works, but it remains with the limitations of increased complexity, reduced performance, and nonlinear distortion. The reduction of PAPR is achieved by minimizing the companding distortion with the enhancement of the bit error rate (BER) performance significantly. Then, in order to avoid clipping in OFDM, a multilateral piecewise exponential companding transform (MPECT) method has been utilized rather than using piecewise exponential companding transform (PEC) where PAPR is getting reduced. The OFDM is very sensitive to synchronizing error. To overcome this sensitivity, employ the Zadoff‐Chu sequence to carrier frequency offsets. Zadoff‐Chu matrix transform (ZCMT) has numerous merits among the other ODFM systems such as the improvement in the performance of the channels that are fading away and provides an ideal periodic autocorrelation and a constant magnitude periodic cross correlation. Both of these techniques provide improvement in the ODFM systems. To get more efficiency, this paper aims to develop a hybrid technique by integrating the ZCMT and MPECT techniques for reducing the PAPR in OFDM systems. Further, convolutional encoding is applied for better BER and PAPR. The simulation results of the proposed ZCMT‐MPECT technique are evaluated and compared with the conventional OFDM and other precoding methods.

Low-Complexity Joint Time Synchronization and Channel Estimation for OFDM-Based PLC Systems

Orthogonal frequency division multiplexing (OFDM) plays an important role in power-line communications (PLC), due to its robustness to frequency-selective channels. Nevertheless, it also presents some persistent drawbacks over the years, some of them related to the synchronization and channel estimation and equalization, whose features are a key aspect in the global performance of the medium access technique. This work presents a novel timing synchronization and channel estimation method for OFDM under PLC conditions. Typical joint fine synchronization and channel estimation techniques for OFDM make use of the cross correlation from a transmitted pilot symbol with suitable correlation properties. However, these proposals often present the drawback of having a high computational load, sometimes reducing the feasibility of later practical implementations. Our proposal takes advantage of the properties of complementary pairs of sequences, as they provide good correlation properties, as well as efficient algorithms for generation and correlation. Furthermore, some practical thresholds for synchronization are derived, which adapt the system to the current signal-to-noise ratio and channel conditions. Finally, all the theoretical proposals have been validated by simulation, including an analysis of the performance achieved.

Signal processing methods in telecommunication field applied to optical disc systems

The main topic in the telecommunication field is to increase the capacity of information transformation with a limited channel bandwidth. Since the readout and recording bandwidths are also limited in optical disc systems, many technologies in communication systems should be applied to them. In this study, two examples of such technologies, orthogonal frequency division multiplexing (OFDM) and divergence detection methods, were applied to optical disc systems to achieve a higher recording density. I studied optical disc recording and reproduction using the OFDM method, and showed the sample calculation of an approximately 1.5-fold recording density for a BDXL™ system using 5-band multiplexing and 64-quadrature amplitude modulation (64-QAM). Typically, optical disc systems have additional readout channels for objective lens control signals. A dual-phase partial response (PR) signal detection method is demonstrated using an additional signal called tangential push–pull. The calculation result of the method indicates a smaller normalized two-dimensional signal deviation than that of the conventional PR detection method.

Regressive group phase weighting in PTS combined with SLM approach for PAPR reduction in OFDM systems

In the recent period, numerous techniques were implemented to increase the performance of orthogonal frequency division multiplexing (OFDM) signals. Among those methods, the partial transmit sequence (PTS) and selected mapping (SLM) shows a significant reduction and improvement in peak to average power ratio (PAPR). Compared to individual SLM and PTS, the combination of SLM-PTS significantly reduces the complexity and PAPR. This scenario focus on improving the SLM-PTS method with the improvement in new regressive group phase weighting approach. This approach exploits in managing the complexity ratio possessed by conventional SLM-PTS, further achieves PAPR reduction in OFDM method. Simulation and experimental outcome confirm that the proposed technique helps to reduce the complexity ratio in a significant manner.

An Integrated Approach Using Chaotic Map & Sample Value Difference Method for Electrocardiogram Steganography and OFDM Based Secured Patient Information Transmission.

This paper presents a patient's confidential data hiding scheme in electrocardiogram (ECG) signal and its subsequent wireless transmission. Patient's confidential data is embedded in ECG (called stego-ECG) using chaotic map and the sample value difference approach. The sample value difference approach effectually hides the patient's confidential data in ECG sample pairs at the predefined locations. The chaotic map generates these predefined locations through the use of selective control parameters. Subsequently, the wireless transmission of the stego-ECG is analyzed using the Orthogonal Frequency Division Multiplexing (OFDM) system in a Rayleigh fading scenario for telemedicine applications. Evaluation of proposed method on all 48 records of MIT-BIH arrhythmia ECG database demonstrates that the embedding does not alter the diagnostic features of cover ECG. The secret data imperceptibility in stego-ECG is evident through the statistical and clinical performance measures. Statistical measures comprise of Percentage Root-mean-square Difference (PRD), Peak Signal to Noise Ratio (PSNR), and Kulback-Leibler Divergence (KL-Div), etc. while clinical metrics includes wavelet Energy Based Diagnostic Distortion (WEDD) and Wavelet based Weighted PRD (WWPRD). The various channel Signal-to-Noise Ratio scenarios are simulated for wireless communication of stego-ECG in OFDM system. The proposed method over all the 48 records of MIT-BIH arrhythmia database resulted in average, PRD=0.26, PSNR=55.49, KL-Div=3.34×10-6, WEDD=0.02, and WWPRD=0.10 with secret data size of 21Kb. Further, a comparative analysis of proposed method and recent existing works was also performed. The results clearly, demonstrated the superiority of proposed method.

New Blind Recognition Method of SCLD and OFDM in Alpha-Stable Noise

This paper deals with modulation classification under the alpha-stable noise condition. Our goal is to discriminate orthogonal frequency division multiplexing (OFDM) modulation type from single carrier linear digital (SCLD) modulations in this scenario. Based on the new results concerning the generalized cyclostationarity of these signals in alpha-stable noise which are presented in this paper, we construct new modulation classification features without any priori information of carrier frequency and timing offset of the received signals, and use support vector machine (SVM) as classifier to discriminate OFDM from SCLD. Simulation results show that the recognition accuracy of the proposed algorithm can be up to 95% when the mix signal to noise ratio (MSNR) is up to ?1 dB.

Evaluation of multiple-channel OFDM based airborne ultrasonic communications

Orthogonal frequency division multiplexing (OFDM) modulation has been extensively used in both wired and wireless communication systems. The use of OFDM technology allows very high spectral efficiency data transmission without using complex equalizers to correct the effect of a frequency-selective channel. This work investigated OFDM methods in an airborne ultrasonic communication system, using commercially available capacitive ultrasonic transducers operating at 50kHz to transmit information through the air. Conventional modulation schemes such as binary phase shift keying (BPSK) and quadrature amplitude modulation (QAM) were used to modulate sub-carrier signals, and the performances were evaluated in an indoor laboratory environment. Line-of-sight (LOS) transmission range up to 11m with no measurable errors was achieved using BPSK at a data rate of 45kb/s and a spectral efficiency of 1b/s/Hz. By implementing a higher order modulation scheme (16-QAM), the system data transfer rate was increased to 180kb/s with a spectral efficiency of 4b/s/Hz at attainable transmission distances up to 6m. Diffraction effects were incorporated into a model of the ultrasonic channel that also accounted for beam spread and attenuation in air. The simulations were a good match to the measured signals and non-LOS signals could be demodulated successfully. The effects of multipath interference were also studied in this work. By adding cyclic prefix (CP) to the OFDM symbols, the bit error rate (BER) performance was significantly improved in a multipath environment.

Full-Duplex Airborne Ultrasonic Data Communication Using a Pilot-Aided QAM-OFDM Modulation Scheme.

Orthogonal frequency division multiplexing (OFDM) has been extensively used in a variety of broadband digital wireless communications applications because of its high bandwidth utilization efficiency and effective immunity to multipath distortion. This paper has investigated quadrature amplitude modulation and OFDM methods in air-coupled ultrasonic communication, using broadband capacitive ultrasonic transducers with high- k dielectric layers. OFDM phase noise was discussed and corrected using a pilot-aided estimation algorithm. The overall system data rate achieved was up to 400 kb/s with a spectral efficiency of 2 b/s/Hz. An ultrasonic propagation model for signal prediction considered atmospheric absorption of sound in air, beam divergence, and transducer frequency response. The simulations were compared with the experimental results, and good agreement was found between the two. Two-way communication through air was also implemented successfully by applying three-way handshaking initialization and an adaptive modulation scheme with variable data rates depending on the transmission distance, estimated using received signal strength indication measurement. It was shown that the error-free transmission range could be extended up to 2.5 m using different system transfer rates from 400 kb/s down to 100 kb/s. In full-duplex transmission mode, the overall error-free system data rate achieved was 0.8 Mb/s up to 1.5 m.

Modulation Schemes for Single-Laser 100 Gb/s Links: Multicarrier

We evaluate multicarrier modulation methods for 100 Gb/s single-wavelength data center interconnects. We consider two different orthogonal frequency-division multiplexing (OFDM) techniques: DC-biased OFDM (DC-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM). We also consider two different techniques for bit loading and power allocation: fixed bit loading with preemphasis and optimized bit loading and power allocation. We first present a semianalytical performance and complexity evaluation of these OFDM methods including the effects of linear filtering, clipping, and quantization. We then include the effects of chromatic dispersion and chirp, as well as intensity and shot noises. Performance is quantified in terms of the required average optical power to achieve a target bit-error probability for a given modulator bandwidth. Complexity is quantified in terms of the resolution and sampling rate required of digital-to-analog (DAC) and analog-to-digital (ADC) converters, as well as the number of signal processing operations required. For each OFDM technique, we adjust the clipping ratio to minimize the optical power requirement. For DC-OFDM, taking into account the DAC frequency response reduces the optical power requirement up to 2 dB. ACO-OFDM is more power efficient and requires lower DAC/ADC resolution than DC-OFDM, but ACO-OFDM requires prohibitively high sampling rates owing to its poor spectral efficiency.

수중 채널 환경에서 전송량 증대를 위한 다중반송파 시스템에 관한 연구

Studies applying multiple carrier method such as OFDM(Orthogonal Frequency Division Multiplexing) or FMT(Filtered Multi-Tone) to Underwater acoustic communication(UAC) system are actively under way as UAC is utilized in the various fields and the demand of high speed data transmission increases. In the existing OFDM method, the use of virtual carrier, which is inserted not to affect the adjacent channel in the frequency domain, and the cyclic prefix, which is used to reduce the impact of Inter Symbol Interference and Inter Channel Interference, decrease the throughput. In particular, the length of cyclic prefix to be used becomes longer under water since underwater has a rapidly changing channel characteristic, and the data throughput diminishes because it has to allocate more subcarrier on virtual carrier. This study therefore suggests FMT-OFDM system, a combination of OFDM and FMT, for the purpose of enhanced throughput in the underwater channel environment. Besides, in this study, channel is modeled based on data measured in real sea and the performance is analyzed after setting system parameters. 논문 15-40-06-26 The Journal of Korean Institute of Communications and Information Sciences '15-06 Vol.40 No.06 http://dx.doi.org/10.7840/kics.2015.40.6.1193 1193 ※ 본 연구결과는 국토해양부의 “수중 광역 이동통신 시스템 기술개발” 과제에서 수행된 연구결과 중 일부임을 밝히며, 연구비 지원에 감사드립니다. First Author : Hoseo University Department of Information and Telecommunication Eng., minsang@hoseo.edu, 정회원 ° Corresponding Author : Hoseo University Department of Oceanic IT Eng., taehoim@hoseo.edu, 정회원 * Hoseo University Department of Information and Telecommunication Eng., 정회원 ** Sangmyung University Department of Information and Telecommunication Eng., 정회원 *** Korea Research Institute of Ships and Ocean Eng.(KRISO), 종신회원 논문번호:KICS2015-05-153, Received May 21, 2015; Revised June 16, 2015; Accepted June 16, 2015 The Journal of Korean Institute of Communications and Information Sciences '15-06 Vol.40 No.06 1194 그림 1. 실측데이터 측정지역 Fig. 1. Measurement area map I. 서 론 최근 해양 생태계 연구 및 자원탐사, 인명구조, 군 사적 목적 등 사람이 직접 작업할 수 없는 환경에서 AUV(Autonomous Unmmanded Vehicle)를 활용한 원격 작업이 증가함에 따라 수중음향통신 기술의 중요 성이 대두되고 있다 . 따라서 향후 수중에서도 원활한 원격 작업을 위해 음파를 사용한 고속의 데이터 서비 스가 요구될 것이며, 이러한 요구를 만족하기 위해 OFDM(Orthogonal Frequency Division Multiplexing) 과 FMT(Filtered Multi-Tone)와 같은 다중반송파 전 송방식을 수중에 적용하기 위한 연구가 활발히 진행 중이다 . 일반적으로 수중 채널 환경은 수온과 수심, 염분 등 의 영향으로 인하여 장소와 계절에 따라 급격하게 변 화하고, 수중생물 및 해면과 해저면, 해양 생물 등에 의해 발생하는 다중경로 페이딩의 영향으로 신호의 심볼 간 간섭이 증가하여 통신성능의 저하를 초래한 다. 따라서 극심한 채널변화를 갖는 수중 채널 환경에 서 단일반송파 전송방식을 사용하게 되면 주파수 선 택적 채널이 되어 이를 보상하기 위해 복잡한 등화기 의 설계가 필요하지만 다중반송파 방식을 사용하면 각 부반송파 단위로 데이터 심볼의 채널 등화를 수행 하기 때문에 간단한 등화기를 사용하여 심볼 복원이 가능해진다 . 그러나 실제 수중 채널 환경에서 상관대역폭은 불 과 수십~수백 Hz로 작기 때문에 OFDM을 적용하기 위해 각 부반송파의 대역폭을 상관대역폭보다 작게 설계하게 되면 부반송파의 개수증가로 FFT 사이즈가 증가하여 시스템의 복잡도가 증가하게 되고, 극심한 채널변화를 갖는 수중 채널 환경은 부반송파 간 인접 채널에 영향을 증가시키므로 인접 채널간의 영향을 줄이기 위해 사용하는 가상 부반송파(virtual carrier) 개수의 증가를 초래하여 데이터 전송량이 감소된다. 따라서 수중 채널 환경에서 신뢰성 있는 고속의 데이 터 통신 및 전송량 증대를 위해 기존의 OFDM 시스 템과 FMT 시스템을 결합한 FMT-OFDM 시스템을 제안한다. FMT-OFDM 시스템은 전체 사용 주파수 대역에서 multi-band를 생성하고 각 sub-band에 OFDM을 수행 하는 방식으로 OFDM 시스템과 부반송파의 개수는 동일하지만 sub-band 내의 OFDM 부반송파의 개수는 감소하여 FFT 사이즈를 감소시키고 FMT의 사용으로 부반송파 간의 인접 채널 간섭이 감소하므로 가상 부 반송파의 개수를 감소시켜 데이터 전송량을 증대시킬 수 있다. 본 논문의 II장에서는 측정환경 설명과 수중 채널 환경에 적합한 FMT-OFDM 시스템 연구를 위해 실해 역에서 측정한 데이터를 분석하였으며, III장에서는 FMT-OFDM 시스템의 구조도에 대해 설명한다. IV장 에서 FMT-OFDM 시스템의 성능 검증을 위한 시뮬레 이션 결과를 보이고, 끝으로 V장에서 본 논문의 결론 을 맺는다. II. 측정 환경 및 수중 채널 분석 그림 1은 측정데이터 측정 장소인 거제도 앞바다와 송신측과 수신측의 위치(35°1'48.43N 128°37'28.3 1E와 35°2'3.64N 128°38' 14.35E)를 나타낸 그 림이다. 그림 2에는 본 측정에 사용한 (a)선박과 (b)선박 내 부에 설치한 측정 장비, (c)송신 센서와 (d)수신센서가 보인다. 거제도 앞바다는 수심이 약 40m이며 송수신기의 입수 깊이는 해면 반사파와 해저면 반사파의 반사 거 리를 동일하도록 전체 수심의 중간 지점인 20m에 위 치시켰고 송수신 선박간의 거리에 따른 수중 채널 특 성의 변화를 알아보기 위해 선박과 수신 선박 간의 거 리를 100m에서 1500m까지 다르게 하여 데이터를 송 수신 하였다. 본 논문에서는 수중 채널 환경에 적합한 FMT-OFDM 시스템의 파라미터 선정과 채널 모델링 을 위해 실측데이터 분석을 하였으며, 측정데이터 분 석의 신뢰성을 높이기 위해 각 거리별로 9~15회 반복 하여 데이터를 수집하였다 . 송신신호는 BPSK 변조 방식을 사용하여 1023개의 PN 코드를 반복해서 전송 하였고, 데이터의 전송률은 5kbps였으며, 25kHz의 반 송주파수로 데이터를 전송하였다. 그림 3은 거제도 천 논문 / 수중 채널 환경에서 전송량 증대를 위한 다중반송파 시스템에 관한 연구 1195 (a) 선박 (b) 선박 내부의 측정장비

Research on Improvement of Spectrum Efficiency of Spread Spectrum OFDM Communication Scheme for Cruising Sensor Network

Cruising sensor nodes have an attractive application in monitoring liquid-based industrial environments. A cruising sensor node, equipped with various sensors, is self-propelled so that they can “cruise” to specified area to gather scientific data. The cruising sensor network consists of an array of such cruising nodes to perform collaborative monitoring task over a given area. A reliable and high-speed underwater acoustic (UWA) communication scheme is necessary for node-to-node communication. Most of the industrial liquid reservoirs are confined spaces, including large oil tanks and nuclear storage ponds. The communication in confined underwater space will suffer more severe multipath interference caused by numerous reflections from boundaries. Conventional orthogonal frequency-division multiplexing (OFDM) technique has poor performance in this channel. Instead, spread spectrum orthogonal frequency-division multiplexing (SS-OFDM), which applies spread spectrum technique into OFDM, has better performance in such severely multipath fading underwater channels. However, the spread spectrum process causes a huge waste in the spectrum efficiency. In order to enhance the transmission efficiency, a mapping sequence spread spectrum OFDM (MSSS-OFDM) method is proposed in this paper. The simulation and experimental results show that our scheme is a robust and spectrum efficient communication method for confined underwater space.

One-Shot Blind CFO and Channel Estimation for OFDM With Multi-Antenna Receiver

In this paper, we design a new blind joint carrier frequency offset (CFO) and channel estimation method for orthogonal frequency-division multiplexing (OFDM) with multiantenna receiver. The proposed algorithm requires only one received OFDM block and thus belongs to the category of one-shot estimation methods. Other advantages of the proposed algorithm include 1) it supports fully loaded data carriers and is thus spectral efficient; 2) the channel from the transmitter to each receive antenna can be blindly estimated with only a scaling ambiguity; and 3) the algorithms outperforms the existing methods. Moreover, we derive the Cramer-Rao Bounds (CRB) of joint CFO and channel estimation in closed forms. Numeral results not only show the effectiveness of the proposed algorithm but also demonstrate its closed performance to CRB.

Multicarrier Spread Spectrum Communication Scheme for Cruising Sensor Network in Confined Underwater Space

A cruising sensor network works as an online monitoring system for industrial liquid environments such as oil tanks and nuclear storage ponds. The cruising sensor network consists of a few submerged nodes which can actuate themselves to execute tasks like “cruising.” This paper presents a multicarrier spread spectrum scheme, which is designed for communications between the nodes and the control station. Although underwater acoustic communication has been widely researched due to the growing demands from ocean development and marine research, communication in confined underwater space is an unexplored domain as most research focuses on communication in spacious water areas. The occasions that the cruising sensor network works in are confined. The main distinction in confined underwater spaces is the existence of strong multipath arrivals reflected by the boundaries, which can cause more severe intersymbol interference (ISI). We propose a communication scheme which applies spread spectrum in orthogonal frequency-division multiplexing (OFDM) to address severe frequency selective fading channels. This scheme can be robust in confined underwater channels when coupled with turbo code. The simulation and experimental results prove the feasibility and reliability of this scheme. It is demonstrated that significantly better performance is achieved than that of the conventional OFDM method.

An Improved Technique of Channel Estimation for OFDM in DVB-T2

Orthogonal Frequency Division Multiplexing (OFDM) with pilot assisted channel estimation is widely applied in wireless communication systems due to its high rate transmission capability and its robustness to multi-path fading and delay. In this paper an improved channel estimation method for OFDM in second generation Digital Video Broadcasting for Terrestrial (DVB-T2) is presented based on a pilot arrangement in time-varying frequency-selective fading channels. A new method which is based on an improved Domain Transform Least Square Estimation method (DTLSE) and a Two Dimensional Interpolation method (2-DI) have been proposed. The proposed methods give a good BER performance compared to other considered methods .The estimation matrix size for obtaining Channel Impulse Response (CIR) depends only on the length of the channel not on the number of pilot subcarriers nor the size of OFDM symbol, it does not require knowledge of channel statistics and works for any pilot pattern. The Minimum Mean Squared Error (MMSE) channel estimation was considered as the reference method that gives the best BER performance but suffers from a high computational load. The proposed method is compared with the two other methods: the improved DTLSE and MMSE. The proposed method proves to give a reduction in complexity compared to the MMSE technique with a comparable performance but better than the DTLSE one. General Terms Pilot-assisted channel estimation, wireless systems, DVB-T2.

Frequency domain based LS channel estimation in OFDM based Power line communications

This paper is focused on low voltage power line communication (PLC) realization with an emphasis on channel estimation techniques. The Orthogonal Frequency Division Multiplexing (OFDM) scheme is preferred technology in PLC systems because of its effective combat with frequency selective fading properties of PLC channel. As the channel estimation is one of the crucial problems in OFDM based PLC system because of a problematic area of PLC signal attenuation and interference, the improved LS estimation technique is proposed. We investigate and evaluate proposed frequency domain LS channel estimation method for OFDM based power line communication system. Also performance comparing with existing pilot based estimation algorithms towards proposed method in terms of their computational complexity, error correction, and suitability conditions is made.