3.1 Underwater communication
Three significant factors are affected in the propagation of underwater acoustic channel such as time-varying multipath propagation, attenuation which improves with frequency of a signal, and low sound speed (1500 m/s). At low range of frequencies, acoustic propagation for underwater is supported in the best way. For long range communication, utilized the low frequency band which automatically decreases the capacity of a channel as the attenuation improves with frequency. However, the bandwidth is highly restricted for long range communication. Underwater communication’s performance is becoming worsened due to the parameters such as the ambient noise or the background noise and Doppler spread which is occurred owing to the random motion between the systems of transmission and reception. In low frequencies, noise domination is higher and it becomes lower by improving the frequency range.
Every physical path is acted like a time-varying low-pass filter in the underwater channel’s impulse response. The Doppler shifting and spreading is introduced by the motion between the receiver and transmitter. The surface waves are initiated the disturbances, internal turbulence owing to ocean current, fluctuations in the sound velocity, and the random signal variations are contributed by the certain other minor phenomena.
With the improvement of frequency, attenuation is increased and noise is decayed which is illustrated in fig.4. It will lead to a variation of a signal-to-noise ratio (SNR) over the bandwidth of a signal. The expression of Signal-to-noise ratio can be mentioned below for a frequency band around the frequency at a distance: see formula 1 in the supplementary files.
Where Sl(f) is the transmitted signal’s power spectral density function, N(f) is the noise function, and A(l,f) is the attenuation function. Underwater acoustic propagation is imposed the fundamental restrictions additionally. The performance of underwater communication is affected significantly by the other system conditions. The bandwidth constraint is the most dominant one that is produced by the transducers of acoustic signals in which the usable bandwidth is restricted below the provided channel’s bandwidth. In underwater channel, the significant components are included absorption loss and spreading loss that involves transmission loss. When communication distance is decreased, transmission loss is reduced and vice versa. For balancing the forecasted transmission loss, more transmission power is needed in long range communication.
Almost, the power is supplied through the battery for systems of underwater communication. So, the communication system is necessary to be efficient in terms of energy conservation. However, the energy consumption is relied on the transmitted bits’ size directly. The efficient communication method is utilized that includes the supporting of high bit rate communication which is one of the best ways in conservation of power.
3.1.1Time varying multipath:
The underwater acoustic channel is characterized the time varying multiple propagation paths. Own distance of travel has included at every path and propagation loss is come into the picture while reflection loss and travel. The cumulative channel impulse response is represented by the below-mentioned equation when each path delay and the attenuation path are represented by ᵀp (t) and αp(t) respectively. See formula 2 in the supplementary files.
The complexity and random characteristics are included in the physical geometry of sea bed and sea surface. It’s unpredictable that the motion of the transducers owing to the sea surface motion. Both parameters of by ᵀp (t) and αp(t) are expressed as random process. For a transmitted signal x(t), the received signal y(t) through the channel can be represented by the below equation.
See formula 3 in the supplementary files.
where, n(t) is the additive background noise. Due to the frequency and time dispersion, the selectivity of time-varying frequency is involved in the channel.
3.1.2 Doppler spread:
Owing to the sea surfaces and currents’ random motions, Doppler spread is raised. The Doppler shift for arrival of each ray is contributed by the relative motions between receiver and transmitter. These rays are the primary source of generating the Doppler shifts where the rays are varied as they have different angles of arrival. The Doppler spread has also included the contribution of these variances. When compared to the overall Doppler spread, the expectation of resultant Doppler spread is larger. The system of receiver should have an ability to balance the resultant Doppler spread.
3.2 Choice of OFDM parameters for underwater communication:
The parameters of OFDM’s selection is included a trade-off between different complex requirements for underwater communication. To design the systems of OFDM, three major parameters are considered such as bit rate, bandwidth, and tolerable delay spread. Bit rate is nothing but communication speed with free of errors and bandwidth is useful for communication of a system. The channel multipath is caused the tolerable delay spread.
Because of the causing of attenuation by absorption, available useful bandwidth is severely restricted for underwater communication. The attenuation is increased with the range of frequency. Low frequency is only utilized for long range communication. Let’s say, 1 kHZ bandwidth is only available for the communication range of 100 km in which the possible bit rate is reduced significantly. For short range communication, the larger bandwidth can get access to the high bit rate but the limiting factor is the transducer which supported the bandwidth. In case of the acoustic bandwidth, a wideband communication is to be considered as the underwater acoustic communication that is the major observation point. With the center frequency’s order, the bandwidth is useful often. According to the narrowband communication in which the lower bandwidth is than the centre frequency in most of the existing principles of radio communication. For radio communication, the current existing techniques of signal processing are adopted and can’t be adopted for communication of acoustic signals that leads to the requirements of complicated signal processing.
When over 1 b/s/Hz is to be reached over the channels, the bandwidth is restricted which implied the requirement for techniques of bandwidth-efficient modulation. The delay spread of channel is inversed to the coherence bandwidth of a channel. The band of frequencies’ measure is indicated over the correlation of fading is highly done. If in case of a channel is frequency selective, narrow bandwidth is preferred to select for the sub-carriers. Owing to the Doppler spread, the coherence time of a channel is dictated the signal’s duration. The Doppler spread and coherence bandwidth are largely impacted the duration of a symbol and sub-carrier spacing.
The coherence bandwidth of a channel is contrary to the delay spread. The correlation of fading is to the bandwidth of frequencies’ measure implemented. The narrower bandwidth is preferred for the frequency selective fading for subcarriers but the wider bandwidth is needed for larger Doppler spreading for subcarriers. Based on both parameters like Doppler spread and coherent bandwidth, the duration of a symbol and the spacing of sub-carrier are demonstrated.
The larger guard time should be selected when compared to the delay spread of channel. For minimization of the SNR loss, the delay spread should be much shorter than the duration of a symbol. The subcarriers’ spacing is very low when the larger value for symbol duration that improves the sensitivity to errors of frequency offset phase noise along with the increment of peak-to-average ratio (PAPR). The complexity would be increased with the development of signal processing. For the available bandwidth, the subcarrier spacing is chosen practically that leads to the sub-carriers’ number is a tradeoff between the coherence time of channel and coherence bandwidth of channel. Based on the required bit rate, the sub-carriers’ number is determined. Here, the bit rate is categorized by the bit rate of sub-carrier.
In order to increase the performance of OFDM systems, channel estimation is required. The optimum selection of parameters of OFDM is resulted with the accurate channel estimation that leads to the efficient communication system.