26-05-2012, 10:15 AM
Communication with an underwater ROV using ultrasonic transmission
[attachment=23080]
Abstract
Communication with underwater remote operated vehicles (ROV) is usually done by using umbilical cables. These sometimes cause problems with the control of the vehicle, as well as its use in areas where fouling of the cable can take place. This paper describes the prototype of an ultrasonic communications system that can transmit colour, still and video, pictures from such an ROV.
1. Introduction
Over the years, much research has been carried out to obtain a reliable high data rate underwater acoustic communication system. However, the underwater acoustic channel is an unforgiving wireless communication medium. The strong amplitude and phase fluctuations cause multipath fading. Due to limit capacity of bandwidth, maximum data rate on the available bandwidth should be used.
2. Data Sequence
The data sequence is in packet form. The sequence contains synchronisation packets, gap packet, adaptive threshold packets and information data packets. These packets are used to allow synchronisation and noise reduction. Each packet is formed by 48 frequencies within 43kHz to 53kHz, which represent 48 bits with a duration of 5.12ms.
The sequence begins with the Linear Frequency Modulation (LFM) signal packet which is used to synchronise the receiver to the start of the data. The details of LFM signal will be discussed in the next section. Then a packet of the gap signal follows the LFM packet so that synchronisation can be performed in this period. Eight adaptive threshold packets (ATP) are transmitted for receiver-training purposes. The training packets act as a reference of the transmitted signal block so that channel estimation can be calculated from the reference packets. Also, the long training sequence can be sufficient for the system convergence. 800 information data packets (IDP) follow the adaptive packets. At the end of the data sequence, is also a gap signal packet which can minimise the effect of multipath fading between two signal blocks. The time duration for one signal block is (1+1+8+800+1)*5.12ms = 4.15s. Fig. 1 shows the data sequence.
3. Synchronisation
For many current systems, a Linear Frequency Modulation (LFM) signal is used for frame synchronisation. The most significant property of the linear frequency modulation signal is its symmetry in time and frequency. In general, the expression for a linear frequency modulation signal, also referred to as a ‘chirp’ is mentioned in [11, Rihazek] as:
4. Multicarrier Modulation
Multicarrier modulation divides a channel into a set of parallel independent subchannels [14]. The SNR of each subchannel is measured and a suitable number of bits is then assigned to each channel. There are two reasons for choosing Multicarrier Modulation (MCM) in the system. According to [12], the MCM signal can be processed in a receiver without the enhancement of noise or interference that is caused by linear equalisation of a single-carrier signal. Another is that the long symbol time used in MCM produces a much greater immunity to impulse noise and fast fades. According to [12] [13, Proakis], the input data is b/s.
Conclusion
In this paper, a multicarrier modulation underwater acoustic system has been developed and demonstrated the ability of real time underwater acoustic communication. The system performed at a data rate of 10kbps over 1km.
Current work in developing the system involves using error correction techniques, adaptive filtering to achieve a higher data rate and reliability. The error rate can also be improved by implementing channel equalisation and channel coding.
[attachment=23080]
Abstract
Communication with underwater remote operated vehicles (ROV) is usually done by using umbilical cables. These sometimes cause problems with the control of the vehicle, as well as its use in areas where fouling of the cable can take place. This paper describes the prototype of an ultrasonic communications system that can transmit colour, still and video, pictures from such an ROV.
1. Introduction
Over the years, much research has been carried out to obtain a reliable high data rate underwater acoustic communication system. However, the underwater acoustic channel is an unforgiving wireless communication medium. The strong amplitude and phase fluctuations cause multipath fading. Due to limit capacity of bandwidth, maximum data rate on the available bandwidth should be used.
2. Data Sequence
The data sequence is in packet form. The sequence contains synchronisation packets, gap packet, adaptive threshold packets and information data packets. These packets are used to allow synchronisation and noise reduction. Each packet is formed by 48 frequencies within 43kHz to 53kHz, which represent 48 bits with a duration of 5.12ms.
The sequence begins with the Linear Frequency Modulation (LFM) signal packet which is used to synchronise the receiver to the start of the data. The details of LFM signal will be discussed in the next section. Then a packet of the gap signal follows the LFM packet so that synchronisation can be performed in this period. Eight adaptive threshold packets (ATP) are transmitted for receiver-training purposes. The training packets act as a reference of the transmitted signal block so that channel estimation can be calculated from the reference packets. Also, the long training sequence can be sufficient for the system convergence. 800 information data packets (IDP) follow the adaptive packets. At the end of the data sequence, is also a gap signal packet which can minimise the effect of multipath fading between two signal blocks. The time duration for one signal block is (1+1+8+800+1)*5.12ms = 4.15s. Fig. 1 shows the data sequence.
3. Synchronisation
For many current systems, a Linear Frequency Modulation (LFM) signal is used for frame synchronisation. The most significant property of the linear frequency modulation signal is its symmetry in time and frequency. In general, the expression for a linear frequency modulation signal, also referred to as a ‘chirp’ is mentioned in [11, Rihazek] as:
4. Multicarrier Modulation
Multicarrier modulation divides a channel into a set of parallel independent subchannels [14]. The SNR of each subchannel is measured and a suitable number of bits is then assigned to each channel. There are two reasons for choosing Multicarrier Modulation (MCM) in the system. According to [12], the MCM signal can be processed in a receiver without the enhancement of noise or interference that is caused by linear equalisation of a single-carrier signal. Another is that the long symbol time used in MCM produces a much greater immunity to impulse noise and fast fades. According to [12] [13, Proakis], the input data is b/s.
Conclusion
In this paper, a multicarrier modulation underwater acoustic system has been developed and demonstrated the ability of real time underwater acoustic communication. The system performed at a data rate of 10kbps over 1km.
Current work in developing the system involves using error correction techniques, adaptive filtering to achieve a higher data rate and reliability. The error rate can also be improved by implementing channel equalisation and channel coding.