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Orthogonal frequency division multiplexing (OFDM) techniques are attractive for underwater acoustic (UWA) communications due to their robustness against large multipath spread. However, OFDM systems in UWA channels suffer from intercarrier interference (ICI) caused by the Doppler effect. In this paper, we propose an effective mirror-mapping-based I...
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... this section, the ICI self-cancellation schemes using mirror- mapping are detailed. Fig. 1 depicts the system architecture. Com- pared with the plain OFDM scheme, the mirror-mapping-based ICI self-cancellation schemes have two additional modules, namely, the ICI canceling modulation before the inverse fast Fourier trans- form (IFFT) operation at the transmitter and the ICI canceling de- modulation after the FFT operation at ...
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Citations
... Here, in this present paper ICI problem is considered and techniques which counter this problem are discussed. In the literature, various ICI countermeasures are presented by researchers [9][10][11][12][13][14][15]. ICI self cancellation techniques is simple to implement. ...
... The channels r th path gain during the k th number of the OFDM symbol is represented by in Equation (5). If our transmission subcarriers lie in the flat region of the analog filters both at the transmitter and the receiver then we can rewrite the Equation (4) as [11] í µí± í µí±,í µí± = í µí± í µí±,í µí± . í µí°» í µí±,í µí± + í µí± í µí±,í µí± ...
There are many research challenges in underwater acoustic communication environment such as large delay spread, ocean waves, motion of transmitter/receiver, Doppler spread etc. OFDM has potential to combat with many such problems, but it is also deteriorated by Inter Carrier Interference and high peak to average power ratio. Conventional OFDM is spectral inefficiency as it uses cyclic prefixing which consumes approximately 20% of available bandwidth. ICI self cancellation technique performs better for ICI problems. As it transmits redundant data on adjacent subcarriers which makes some subcarriers idle, hence, ICI is reduced at the cost of bandwidth. In this paper, a Wavelet based OFDM with ICI cancellations is proposed to counter the problem of ICI. Use of Wavelets reduces the need for cyclic prefixing thereby making it more spectral efficient and wavelets also help in maintaining orthogonality between subcarriers which further improves its ICI performance. Simulation results show that proposed technique performs better in terms of bit error rate (BER) as compared to conventional OFDM.
The available bandwidth of underwater environment tends to several kilohertz, which considers the main challenges of communications under sea water. On the other hand, the bit‐error‐rate (BER) performance degrades because of several reasons such as multipath propagation, time variabilities of the channel, attenuation, and water temperature. In this paper, we aim to improve the underwater acoustic (UWA) BER system performance by using orthogonal frequency division multiplexing (OFDM) based on fast Walsh‐Hadamard transform (FWHT) instead off fast Fourier transform (FFT). We proposed a low‐complexity equalization and carrier frequency offset (CFO) compensation for UWA‐OFDM–based FWHT using banded‐matrix approximation concept. Simulation results show that the UWA‐OFDM–based FWHT with low‐density parity check (LDPC) codes give a good improvement performance compared with traditional OFDM in UWA system especially in case of estimation errors. In this paper, we modify the structure of the orthogonal frequency division multiplexing (OFDM) by replacing the fast Fourier transform (FFT), and its inverse via fast Walsh‐Hadamard transform (FWHT), and its inverse. Moreover, the proposed structure gives a better bit‐error‐rate (BER) performance than the corresponding FFT. Also, we propose a minimum mean square error (MMSE) equalizer that performs the equalization and the carrier frequency offset (CFO) compensation with lower complexity using the banded matrix approximation. The proposed structure is compared with the corresponding traditional system at the same channel conditions. Also, the low‐density parity check codes (LDPC) are used to improve the underwater acoustic (UWA) OFDM system, which gives better performance than other codes in case of UWA wireless communications. Simulation results show that the proposed structure with the proposed MMSE equalizer can improve the system performance especially in case of estimation errors and the variation of the normalized CFO.
Underwater acoustic wireless communication is considered as one of the most
challenging communication due to low propagation speed, water salinity,
water depth, PH degree, and water temperature. Equalization is one of the
means to improve the system performance. Matched filter (MF), zero forcing (ZF),
and minimum mean square error (MMSE) equalizers can be used for channel equalization. Unfortunately, the performance of the matched filter is destroyed in the
Multiple‐Input Multiple‐Output (MIMO) configurations. In addition, the ZF equalizer suffers from noise enhancement and high complexity because of the direct matrix inversion. On the other hand, the MMSE equalizer requires estimating
the operating signal‐to‐noise ratio (SNR) to work properly. In this paper, we
present a joint low‐complexity regularized ZF equalizer and carrier frequency offset (CFO) compensation scheme to deal with these problems. The proposed equalization algorithm mitigates the noise enhancement problem using a constant regularization parameter, and it can be implemented with low complexity using the banded-matrix approximation. Also, we test the performance of the proposed system at different channel conditions. Simulation results show that the proposed equalizer has the ability to enhance the system performance with lower complexity than those of the other equalization algorithms at different channel conditions.
Due to noise enhancement, conventional Zero Forcing (ZF) equalizers are not suitable for wireless Underwater Acoustic (UWA) Orthogonal Frequency Division Multiplexing (OFDM) communication systems. Furthermore, these systems suffer from increasing complexity due to the large number of subcarriers, especially in Multiple-Input Multiple-Output (MIMO) systems. On the other hand, the Minimum Mean Square Error equalizer suffers from high complexity. This type of equalizers needs an estimation of the operating Signal-to-Noise Ratio to work properly. In this paper, we propose a Joint Low-Complexity Regularized ZF equalizer for MIMO UWA-OFDM systems to cope with these problems. The main objective of the proposed equalizer is to enhance the system performance with a lower complexity by performing equalization in two steps. The co-channel interference can be mitigated in the first step. A regularization term is added in the second step to avoid the noise enhancement. Simulation results show that the proposed equalization scheme has the ability to enhance the UWA system performance with low complexity.
Underwater acoustic communication (UWA) is a reliable way for data transmission in underwater environment, however, it is quite challenging due to its doubly spread characteristics as delay spread and Doppler spread. In this paper, a discrete-time channel input-output relationship for multiple-input-multiple-output (MIMO) single-carrier block transmissions is derived based on parameterization for amplitude and delay variations with polynomial fitting, where the mathematical model is characterized by a few parameter sets, transforming the estimation problem into estimation of the low-dimensional parameter sets. Then a two-stage sparse approach for delay-spread and Doppler-scale estimation of MIMO UWA channel is proposed, using orthogonal matching pursuit (OMP) algorithm to search on the delay-Doppler scale grid. The results of the proposed approach are further evaluated by simulations.
Underwater acoustic (UWA) communication channel features relatively fast time variation. Spatial modulation (SM) combined with orthogonal frequency division multiplexing (OFDM) transmission can be applied to UWA communications. Channel estimation may induce a performance penalty for SM since only one transmit antenna is activated to carry pilots. Recently, a differential solution has been proposed for SM, which dispenses with channel estimation and restricts the signal-to-noise (SNR) loss to a relatively low level under quasi-static channel conditions. Hence, the application of differential SM (DSM) to OFDM UWA communications is of particular interest. Simulation results show that when applied to UWA communications, DSM has an obvious performance enhancement in bit error rate (BER) over SM.
OFDM is promising for underwater acoustic (UWA) communications due to its potential to combat the large delay spread. However, the performance of the OFDM system is seriously deteriorated by the inter-carrier interference (ICI) caused by the transmitter/receiver motion and ocean waves. In this paper, we propose a novel ICI countermeasure. The key idea is to inactivate partial subcarriers according to index modulation and meanwhile transmit signals with opposite polarity on two adjacent active subcarriers for ICI self cancellation. Simulation results validate that the proposed scheme achieves much better bit error rate (BER) performance than many existing schemes in UWA communications.
In underwater optical wireless communication (UOWC), a channel is characterized by abundant scattering/absorption effects and optical turbulence. Most previous studies on UOWC have been limited to scattering/absorption effects. However, experiments in the literature indicate that underwater optical turbulence (UOT) can cause severe degradation of UOWC performance. In this paper, we characterize an UOWC channel with both scattering/absorption and UOT taken into consideration, and a spatial diversity receiver scheme, say a single-input–multiple-output (SIMO) scheme, based on a light-emitting-diode (LED) source and multiple detectors is proposed to mitigate deep fading. The Monte Carlo based statistical simulation method is introduced to evaluate the bit-error-rate performance of the system. It is shown that spatial diversity can effectively reduce channel fading and remarkably extend communication range.
Fast time-varying, phase variation, and Doppler shift constitute the features of the underwater acoustic (UWA) communication channel. As a consequence, the effect of intercarrier interference (ICI) becomes significantly critical for orthogonal frequency division multiplexing (OFDM) based UWA communication systems. In this paper, an ICI countermeasure, termed two-path transmission, is proposed. The key idea is to employ data repetition within two concatenated OFDM blocks according to subcarrier mirror mapping criterion. Through theoretical analysis and numerical results, it is shown that the proposed two-path transmission schemes can effectively improve the carrier-to-interference power ratio (CIR) in the presence of carrier frequency offset (CFO) and additive white Gaussian noise (AWGN). Furthermore, a recent sea experiment conducted in Taiwan in May 2013 verifies the ICI suppression capability of the proposed schemes in UWA channels.
