Assessment of UWAC System Performance Using FBMC Technique

It is well known that Orthogonal Frequency Division Multiplexing (OFDM) systems suffer from intercarrier interference (ICI) in mobile environment due to loss of orthogonality among subcarriers caused by Doppler shifts. There exist many ICI mitigation techniques in the literature to improve the performance of OFDM systems. However, most of the existing ICI mitigation techniques assume the OFDM transmission bandwidth is narrow enough that the frequency offsets on all subcarriers are identical. In a wideband OFDM transmission or a non-contiguous OFDM spanning over large bandwidth, the Doppler shifts on different subcarriers are different, especially in high speed aerial vehicle communication systems. In this paper, we analyze the wideband OFDM system in high mobility environment where the frequency offsets vary from subcarrier to subcarrier. We then propose a novel ICI cancellation scheme to eliminate the ICI effect and offer the wideband OFDM system significantly improved BER performance. Simulation results in AWGN channel and multipath fading channel confirm the effectiveness of the proposed scheme in the presence of frequency offset and time variations in the channel, offering the best BER performance available which matches the BER performance of wideband OFDM system without ICI. To our knowledge, this paper is the first to address the ICI problem of varying frequency offsets across subcarriers in wideband OFDM system.

The performance of Link Adaptation (LA) under the influence of inter carrier interference (ICI), which is cause by carrier frequency offset (CFO) and Doppler frequency spread due to mobility, in orthogonal frequency division multiplexing (OFDM) based wireless systems is analyzed in this work. LA maximizes throughput while maintaining a target bit error rate (BER) or Block Error Rate (BLER) at the receiver using the signal to noise ratio (SNR) fed back from the receiver to the transmitter. Since ICI power is proportional to the signal strength, the implications of such an impairment on LA OFDM systems is not obvious and has not received much attention till now. It is seen that when ICI is not captured in the SNR measured then the LA system fails to meet the target error rate. However, it is shown that with suitable modification to the SNR switching threshold for changing the adaptive modulation can over the problem. It is also noted that ICI severely reduces the spectral efficiency of OFDM systems even when LA is used.

Underwater acoustic communication (UAC) channel is a typical doubly spread channel with large multipath delay spread and large Doppler spread. The large multipath delay spread will lead the severe inter symbol interference (ISI) in the single carrier communication systems. The usual ISI will affect as much as hundreds of symbols, which will need complex equalization. OFDM can effectively reduce the ISI with guard interval. However due to the slow speed of acoustic propagation (about 1500m/s), the slow movement of underwater sensor nodes caused by ocean current will lead large Doppler spread, which will lead severe inter carrier interference (ICI). Channel estimation in this doubly spread channel will encounter the problem that the channel state information of pilot is affected by the neighboring unknown data symbols, which will largely degrade the performance of OFDM system. The paper studies the iterative OFDM receiver to reduce the performance degradation caused by the ICI.

In underwater acoustic communication (UWA), there is a significant presence of multipath and Doppler spread. The integration of orthogonal frequency division multiplexing (OFDM) with index modulation (IM), utilizing subcarrier index modulation to enhance system performance underwater, has garnered considerable interest from researchers. Multiple mode modulation can enhance utilization by transmitting symbols of different modes on different subcarriers. Inspired by the concepts of index modulation and multiple mode modulation, this paper proposes a method that combines multiple mode indexing with the filter bank multicarrier system (MM-FBMC-IM), enabling the transmission of different modulation orders on each subcarrier to increase system flexibility. Simulations demonstrate that under the same spectral efficiency, multiple mode indexing FBMC offers superior bit error rate (BER) performance compared to conventional FBMC and index modulation FBMC.

Underwater acoustic communications are limited by the following channel impairments: time variability, narrow bandwidth, multipath, frequency selective fading and the Doppler effect. Orthogonal Frequency Division Modulation (OFDM) is recognized as an effective solution to such impairments, especially when optimally designed according to the propagation conditions. On the other hand, OFDM implementation requires accurate channel knowledge atboth transmitter and receiver sides. Long propagation delay may lead to outdated channel information. In this work, we present an adaptive OFDM scheme where channel state information is predicted through a Kalman-like filter so as to optimize communication parameters, including the cyclic prefix length. This mechanism aims to mitigate the variability of channel delay spread. This is cast in a protocol where channel estimation/prediction are jointly considered, so as to allow efficiency. The performance obtained through extensive simulations using real channels and interference show the effectiveness of the proposed scheme, both in terms of rate and reliability, at the expense of an increasing complexity. However, this solution is significantly preferable to the conventional mechanism, where channel estimation is performed only at the receiver, with channel coefficients sent back to the transmit node by means of frequent overhead signaling.

The nonlinearity of high power amplifiers (HPAs) exhibits amplitude and phase distortions, which cause loss of orthogonality among the subcarriers in orthogonal frequency division multiplexing (OFDM) systems, and hence, inter-carrier interference (ICI) is introduced in the transmitted signal. The ICI power is proportional to the amplitude of the signal at the amplifier input and it may cause a considerable bit error rate (BER) degradation. Due to the ICI is a random process and has unknown distribution function, it is difficult to evaluate the theoretical BER when the ICI is taken into account. This paper evaluates the BER analytically using sequence development approach (SDA) for OFDM systems perturbed by the HPA. In comparison to the state-of-the-art technique, the evaluation of the BER using this approach achieves superior approximation at the expense of additional computational complexity.

Peak-to-average power ratio (PAPR) is a hot issue in orthogonal frequency division multiplex (OFDM) technology. High PAPR of OFDM system limits its application in underwater acoustic communications. The DFT-s-OFDM technology has been widely used in long term evolution (LTE) and fifth-generation new radio (5G NR) uplink transmission schemes for wireless communications due to its good PAPR performance. However, it is difficult to adapt to time-varying underwater acoustic channels by using time-multiplexed training. We use the DFT-s-OFDM technology of the sideband tone reservation method to suppress the PAPR of the system. At the same time, for the DFT-s-OFDM system, it is impossible to insert data and pilots in a single symbol block at the same time to track the channels. A low-power training sequence is superimposed with symbols, and the training sequence is transmitted over all time. It is used to enhance the tracking channel capability with a small energy overhead and is used to process time-varying channels. Field experiments were carried in Qiandao Lake in 2021, and the results verify the effectiveness of the proposed scheme.

The impulsive noise and the time-varying channel are two major detrimental factors which greatly constrain the performance of underwater acoustic communications (UACs). Utilizing the joint sparsity of the impulsive noise and the channel impulse response, the paper proposes the generalized approximate message passing (GAMP)-based sparse Bayesian learning (SBL) frameworks for joint impulsive noise mitigation and channel estimation and tracking for orthogonal frequency division multiplexing (OFDM) UACs. Firstly, the SBL framework for the joint estimation is employed. To reduce the computational complexity, the GAMP is introduced into the expectation-maximization (EM) algorithm, and a low-complexity GAMP based SBL framework is formulated without performance degradation. To further estimate and track the impulsive noise and channel state information in the slow time-varying scenarios, we propose a novel GAMP-based temporal SBL framework. The factor graph and GAMP are used to achieve the approximated estimation for the posterior statistics of both the channel state information and the impulsive noise. The algorithm formulates the message passing scheduling for the EM algorithm to solve the joint multiple sparse signal recovery problem. Simulations and sea-trial results demonstrate that the proposed algorithms significantly improve the performance in terms of the mean square error of channel estimation, impulsive noise estimation, bit error rate and computational complexity compared with their corresponding SBL-based counterparts.

The basic idea of this letter is concerned with the large Peak-to-Average Power Ratio (PAPR) problem in multi-carrier Under-Water Acoustic (UWA) communication systems that implement Orthogonal Frequency Division Multiplexing (OFDM). A Tangent Rooting (TanhR) companding technique is presented in this paper to reduce the PAPR in UWA communication systems. This technique achieves a noticeable reduction of the PAPR in UWA systems, while maintaining a good Bit Error Rate (BER) performance . Channel equalisation with Linear Minimum Mean Square Error (LMMSE) and Zero-Forcing (ZF) equalisers is considered in this paper. Simulation results ensure superiority of the proposed technique compared to the Selected Mapping (SLM) and the Partial Transmit Sequence (PTS) techniques from both the PAPR and BER perspectives.

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.

Real-time data transmission with high data rate and low bit error rate is realized by underwater multi-carrier acoustic communication system, which plays an important role in AUV. OFDM (orthogonal frequency division multiplexing) is an attractive multi-carrier communication method around the world. According to the requirements of OFDM signal transmission in underwater multi-carrier acoustic communication, the OFDM signal transmitter with FPGA (field programmable gate array) and DSPs (digital signal processor, ADSP-TS101) is designed and implemented. And the methods of interrupts of DMA (direct memory access) and half-full are used to realize data transmission between DSP and FIFO (first in first out) with high speed. And the signal synthesis method, DDWS (direct digital wave synthesis), is adopted to realize producing OFDM signal. From the results, we can see that the transmitter has good steady and can complete the signal transmission in real time.

Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a strong candidate for next generation wireless communication systems. Compared to traditional OFDM, Single Carrier OFDM (SC-OFDM) has demonstrated excellent bit error rate (BER) performance, as well as low peak to average power ratio (PAPR). Similar to other multi-carrier transmission technologies, SC-OFDM suffers significant performance degradation resulting from intercarrier interference (ICI) in high mobility environments. Existing techniques for OFDM can be directly adopted in SC-OFDM to improve performance, however, this improved performance comes at costs such as decreased throughput. In this paper, we analyze the effect of ICI on an SC-OFDM system and propose a novel modulation scheme. The proposed Magnitude-Keyed Modulation (MKM) modulation provides SC-OFDM system immunity to ICI and with an easy implementation it significantly outperforms OFDM, SC-OFDM and MC-CDMA systems with Phase Shift Keying (PSK) modulation and Quadrature Amplitude Modulation (QAM) in severe ICI environment. Analysis also illustrates the proposed SC-OFDM system with MKM modulation maintains low PAPR compared to traditional OFDM and SC-OFDM systems with PSK and QAM modulations. Simulation results for different modulation schemes in various ICI environments confirm the effectiveness of the proposed system.

Communication over doubly selective channels (both time and frequency selective) suffers from significant intercarrier interference (ICI). This problem is severe in underwater acoustic communications. In this paper, a new Partial Fractional Fourier (PFrFT) based orthogonal frequency division multiplex (OFDM) scenario is presented for dealing with such challenges. A band minimum mean square error (BMMSE) weight combining equalizer based on Least Square MINRES (LSMR) iterative algorithm is used in the proposed communication system. Simulation results demonstrate significant BER performance improvements (up to 8dB) over traditional orthogonal based methods and those considering Partial FFT demodulation, and Discrete Fractional Fourier Transform (DFrFT) with only a moderate computational complexity increase.

One of the widely adopted multicarrier modulation technique in wireless communication system is Orthogonal Frequency Division Multiplexing (OFDM), because of its enhanced robustness against channel dispersion. However, Inter Carrier Interference (ICI) caused by time varying wireless channel and Peak to Average Power Ratio (PAPR) due to the presence of a large number of subcarriers with varying amplitude are the critical issues faced in OFDM. One of the efficient ICI self-cancellation scheme is correlative precoder but it introduces increased PAPR. To overcome this shortcoming a new form of circulantly precoded OFDM system is implemented to enhance ICI reduce and also employed with Selective Mapping (SLM) which is effective in PAPR reduction and uncomplicated. BER performance of correlative and circulantly precoded OFDM system is evaluated for LMMSE and OLMMSE equalizers. Simulation results show that circulantly precoded OFDM with SLM can achieve jointly better ICI self cancellation and PAPR reduction than correlative precoding technique over time varying channel.

This paper evaluates the performance of differentially encoded quadrature phase shift keying (DQPSK) modulated orthogonal frequency division multiplexing (OFDM) for underwater acoustic (UWA) communication. The combination of OFDM and the non-coherent detection scheme maintains the receiver design simple, reliable and spectrally efficient. QPSK helps achieve higher data rate as two bits are sent over each sub-carrier. Further differential coding is chosen as it does not require phase coherent reference signal at the receiver, which helps to overcome the detrimental effect of the random phase in the received signal. An added advantage of using DQPSK is that for slow Doppler frequency shift, the phase difference remains unaffected and the data can be retrieved. The performance of DQPSK OFDM is studied over a typical underwater channel through extensive computer simulation and a semi-realistic experiment. DQPSK OFDM scheme is used to transmit an image through the underwater channel in an experimental tank to validate the simulation. Furthermore selection combining technique is also implemented at the receiver to enhance the system reliability and performance. With two hydrophones at the receiving end and diversity combining it is possible to achieve a bit error rate (BER) of 2 × 10−4 with 512 sub-carrier and 2 × 10−5 with 256 sub-carriers respectively at a signal-to-noise ratio (SNR) of 21 dB.