Method and apparatus for carrying out high data rate and voice underwater communication

An underwater apparatus for transmitting and receiving high rate data and voice communication including a transmitter, a receiver, and a Doppler frequency shift compensator. The transmitter includes a data source comprising digital data to be transmitted by the apparatus through the water, a serial-to-parallel data processor for splitting the serial digital data into n parallel data channels, a n-channel modulator for receiving the n parallel data channels and for modulating those channels with n pairs of ultrasonic carriers to produce a modulated signal. The transmitter also includes a hydrophone for receiving and transmitting the modulated signal. The receiver includes a hydrophone for receiving a modulated signal. The receiver also includes RF circuitry for amplifying and shaping the received modulated signal, a serial-to-parallel data processor for splitting the amplified and shaped data into n parallel channels, a n-channel demodulator for demodulating the shaped signal and for outputting n channels of digital data, and a parallel-to-serial data processor for receiving n parallel channels of data from the demodulator and for combining those channels into serial data. The Doppler compensator measures the frequency of at least one of two unmodulated signals transmitted as part of the modulated signal and compares the measured frequency with a predetermined frequency. Operation of the demodulator is adjusted to compensate for any deviation between the measured and predetermined frequency.

FIELD OF THE INVENTION 
The present invention relates to means for underwater communication. More 
particularly, the invention relates to a method for carrying out high rate 
underwater communication, and to an apparatus for carrying out high rate 
underwater communication according to said method. 
BACKGROUND OF THE INVENTION 
Performing a reliable underwater communication is a relatively complicated 
task. It is known that electromagnetic waves are significantly attenuated 
when propagating through water. The only frequency band that is used for 
electromagnetic underwater communication is the VLF (Very Low Frequency) 
band, in the range of up to 10 kHz. In this range, high power transmission 
is needed, and use of extremely long antennas is required at both the 
receiving and transmitting ends. Therefore, such use is generally limited 
to submarine communications, and cannot be exploited for personal use. For 
shorter range underwater communication, conventional systems use 
ultrasound acoustic transmission, generally in the frequency range of 20 
kHz-600 kHz. Unfortunately, however, in the acoustic frequency range, the 
water as a communication medium provides practically only a relatively 
narrow bandwidth, which limits the speed of the data transfer through 
water. The ability to reliably transfer data through water with acoustic 
waves is further complicated by the different layers of water density, 
resulting from non-constant speed of sound in water, multipath propagation 
of the signal, fading, and other environmental disturbances. Furthermore, 
it is known that the propagation speed of ultrasonic waves in water is 
significantly lower than the propagation speed of electromagnetic waves in 
air. Therefore, when it is desired to communicate in water between two 
apparatuses, of which at least one is not stationary, or moves at a low 
speed, the Doppler effect adversely affects the signal and the ability to 
reliably interpret the transmitted data at the receiving apparatus. 
It has been found that many conventional types of electromagnetic air 
communication techniques are unable to overcome the abovementioned 
problems, which are typical of underwater communications. 
Wireless apparatus for carrying out communication in water is known in the 
art. Such apparatus is used for example in telemetry systems for 
transferring data that was accumulated during oceanographic researches, or 
in communication devices for divers. Copending Israeli Patent Application 
No. 121561, filed on Aug. 18, 1997, by the same applicant herein, 
discloses an underwater communication apparatus and a communication 
network for divers. Communication devices for divers are also shown in CA 
2,141,619, WO 97/26551, and in U.S. Pat. No. 4,463,452. Other existing 
apparatus, which is capable of transferring data at a relatively low rate, 
generally operates in the range of no more than about 600 bits per second, 
a rate which is in general sufficient for telemetry purposes, but not for 
other purposes which require a significantly higher rate of data transfer, 
such as real time voice or picture transmission. For satisfying these 
requirements, it is desired to provide an underwater modem which is 
capable of transferring data at a much higher transfer rate, at least in 
the range of about 4800 bits per second to 9600 bits per second. 
Moreover, existing apparatus enables underwater communication between two 
locations being at a relatively close range, generally in the range of 
less than 150 meters, and require a direct "line of sight" between the 
transmitting and receiving devices. Such apparatus does not provide means 
for carrying out reliable underwater communication between two sites that 
may be located several kilometers away from one another, and between which 
there is no "line of sight". 
It is an object of the invention to provide a modem which can reliably 
transfer and receive data at a high rate through water. The term 
"underwater modem" or simply "modem", when used herein, refers to an 
apparatus which is capable of transmitting and receiving high-rate data 
through water, unless otherwise specifically stated. By "high-rate" data 
transmission it is meant to indicate a band rate of at least 1200 bps, and 
preferably of at least 4800 bps. 
It is still another purpose of the invention to provide an underwater modem 
which can efficiently overcome fading, multipath, Doppler and 
environmental disturbances. 
It is still another purpose of the invention to provide an underwater modem 
which can eliminate Doppler distortions of the transmitted signal which 
are due to movement of the transmitting modem, the receiving modem, or 
both. 
It is still another object of the invention to provide an underwater modem 
comprising means for correcting errors. 
It is another object of the invention to provide means for using the said 
underwater modem in underwater sound communication. 
Other purposes and advantages of the invention will become apparent as the 
description proceeds. 
SUMMARY OF THE INVENTION 
It has been found by the inventors that the fading and multipath problems 
which significantly affect underwater acoustic communications, resemble 
the fading and multipath problems which affect HF (High Frequency, short 
wave) radio communication. One method, originally developed for overcoming 
the fading and multipath problems in HF communication is the Orthogonal 
Frequency Division Multiplexing (OFDM). However, the OFDM communication 
method has not yet been applied to underwater communication. 
OFDM has been found by the applicants to be the best modulation method for 
overcoming fading and multipath problems underwater. However, the use of 
OFDM in itself is not sufficient for solving all the aforesaid problems of 
underwater communication, and additional means should be provided in order 
to overcome the Doppler effect, to ensure communications even when a line 
of sight between the transmitting and receiving apparatuses does not 
exist, and to assure a reliable (error free) communication. The Doppler 
effect is much more severe in underwater acoustic communications than in 
electromagnetic air communications, as acoustic waves propagate in water 
at a speed of about 1500 m/sec, while electromagnetic waves propagate at 
the speed of light, i.e., 300,000 km/sec in air. Furthermore, the 
propagation speed in water is not constant and depends on the depth, the 
water temperature, and other factors. The modem according to one 
embodiment of the invention provides means for overcoming the fading and 
multipath problems, as well as the signal distortions due to the Doppler 
effects. As will be shown hereinafter, the modem according to the 
invention can reliably transfer data at a rate much higher than the 
transfer rate of any prior art underwater communication apparatus. 
According to a preferred embodiment of the invention, the underwater 
modulator-demodulator (modem) apparatus for transmitting and receiving 
data at a high rate through water, comprises: 
a. A transmitting section comprising: 
data source means, comprising digital data to be transmitted through water; 
serial-to-parallel data processing means, for splitting a serial data into 
n parallel channels; 
n-channel modulator means, for receiving data from said n parallel channels 
and for modulating the same with n pairs of ultrasonic carriers, thereby 
producing a modulated signal; and 
hydrophone means, for receiving said modulated signal from said n channel 
modulator, and for transmitting same through water; and 
b. A receiving section comprising: 
hydrophone means, for receiving a modulated signal from the water, and for 
conveying the same to an RF circuit; 
RF circuitry, for amplifying and shaping the received modulated signal, and 
for conveying the same to serial-to-parallel means; 
serial-to-parallel means, for receiving shaped data from the RF circuit, 
and for splitting the same into n parallel channels; 
n-channel demodulator means, for demodulating said shaped signal conveyed 
to the demodulator from said RF circuit, and for outputting n channels of 
digital data; and 
parallel-to-serial means for receiving n parallel channels of outputted 
data from the demodulator, and for combining the data into serial data. 
Preferably, the n-channel modulator means in the transmitting section is an 
n-channel OFDM modulator means, for receiving data from the n parallel 
channels and for modulating the same with n pairs of orthogonal ultrasonic 
carriers, thereby producing a modulated signal, and the n-channel 
demodulator means at the receiving section is an n-channel OFDM 
demodulator means, comprising n pairs of orthogonal ultrasonic sines for 
demodulating the shaped signal conveyed to the n-channel OFDM demodulator 
from the RF circuit, and for outputting n channels of digital data. 
Preferably, the receiving section and the transmitting section are 
contained within the same case, and the transmitting hydrophone and the 
receiving hydrophone are incorporated within the same hydrophone, and more 
preferably, the hydrophone is a multidirectional hydrophone. However, in 
some applications the receiving section and the transmitting section may 
be contained within separate cases. 
According to one embodiment of the invention, the transmitting section 
further comprises an n-channel differential encoder for receiving data 
from the serial-to-parallel means, for differentially encoding the data on 
each one of said n channels, and for providing the differentially encoded 
data to the n-channel OFDM modulator, and the receiving section further 
comprises an n-channel differential decoder for receiving n channels of 
demodulated data from the demodulator, and for differentially decoding 
said demodulated data. 
Preferably, the transmitting section further comprises a Forward Error 
Correcting (FEC) device for encoding the digital data to be transmitted, 
and the receiving section further comprises a Forward Error Correcting 
(FEC) device for decoding the received encoded data by the use of at least 
one error correcting code, and for outputting the same to the 
parallel-to-serial device of the receiving section. 
Still preferably, the apparatus further comprises means for compensating 
for Doppler effects on the transmitted signal propagating through water. 
Said means for compensating for Doppler effects according to one 
embodiment of the invention comprise, in the modulator of the transmitting 
section, additional means for transmitting at least one unmodulated 
carrier, and in the receiving section, a frequency adjusting device for 
measuring the frequency of said at least one unmodulated carrier, and for 
compensating for any deviation in it. When the modem uses OFDM modulation, 
the frequency adjusting device compensates for Doppler effects by changing 
the frequency of each one of the n pairs of orthogonal sines of the OFDM 
demodulator by the same measured deviation. 
Preferably, at least the n-channel OFDM modulator at the transmitting 
section and the demodulator at the receiving section comprises a DSP, 
incorporated within an integrated circuit. 
The invention further relates to a method for carrying out a high-rate 
underwater communication, comprising performing the following steps: 
(i) transmitting data by: 
a. Providing a serial data in digital form to be transmitted; 
b. Providing means for splitting the serial data into n parallel channels 
and assigning symbols to groups of data bits; 
c. Modulating said symbols by an n-channels OFDM modulation; 
d. Transmitting said OFDM modulated data through a hydrophone, into the 
water; and 
(ii) Receiving said transmitted data in understandable form, by: 
a. Receiving said transmitted OFDM modulated data by a hydrophone; 
b. Demodulating the received signal by an n-channel OFDM demodulator; 
c. Decoding the demodulated data in said n channels; and 
d. Converting the data from said n channels from parallel into serial form. 
The said method performs better when multidirectional hydrophones are used 
at the transmitting and at the receiving ends. Optionally, each end may be 
capable of performing bi-directional communication, and if so, one 
hydrophone may be used for both receiving and transmitting. 
Preferably, the said method is further enhanced by further having the 
following steps: 
a. transmitting at least one additional unmodulated ultrasonic carrier 
(pilot) simultaneously with the OFDM modulated data; and 
b. when receiving said OFDM modulated data and said at least one additional 
unmodulated ultrasonic carrier by the hydrophone, measuring the frequency 
shift of said unmodulated signal from its expected frequency value, and 
compensating for said shift by adjusting the OFDM demodulator of the 
receiving section accordingly. 
The underwater modem according to the invention can also be used for 
communicating sound through water. In such case, in the transmitting 
section of the modem, the data source means comprises a microphone for 
receiving a sound and converting it to an analog electric signal, and a 
voice encoder for receiving said analog electric signal and converting it 
to digital data. The receiving section further comprises a voice decoder 
for receiving the serial data from the parallel to serial means and 
converting the said serial data to an analog electric signal, and 
loudspeaker means for receiving the analog electric signal and converting 
it to sound. The voice encoder at the transmitting section can be a 
vocoder operating in a voice encoding mode, and the voice decoder at the 
receiving section can be a vocoder operating in a voice decoding mode. The 
sound can be, for example, a voice of a diver to be communicated through 
water. 
Still preferably, the duration of each symbol includes a guard time and 
said guard time is used for symbol synchronization at the receiving end.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 schematically shows in block diagram form an underwater modem 
according to a preferred embodiment of the invention. Conveniently, the 
modem 1 comprises both a transmitting section 2 and a receiving section 3. 
While the existence of said two sections in each modem is necessary in 
order to carry out a reliable bi-directional communication, there may be 
cases in which communication be carried out according to the invention 
between two apparatuses, a first one comprising only a transmitting 
section 2, and a second comprising only a receiving section 3. 
Hereinafter, if not specifically stated otherwise, it will be assumed for 
the purpose of this description that each modem comprises both 
transmitting and receiving sections. 
The data source 4 of the transmitting section 2 represents data of any 
kind, in digital form, that must be transmitted by the wireless modem 
through water to a receiving modem located underwater in another location. 
The data from the data source (hereinafter also referred to as "the 
original data") is provided into an FEC (Forward Error Correction) encoder 
5, which combines with it additional bits for error correction, in order 
to provide to the receiving modem the capability of correcting errors 
which occur due to disturbances in the underwater medium, and for 
recovering the original data as generated by the data source. The FEC 
encoder 5 is conventional in its structure, applying any type of error 
correction method known in the art. From the FEC encoder, data combining 
the original data and additional bits (hereinafter generally referred to 
as "the encoded data") is conveyed to an OFDM modulator 6, which modulates 
the digital data by a plurality of low frequency carriers. A modulated 
signal is then conveyed to an RF circuit 7, which, if necessary, transfers 
the frequency spectrum of the modulated signal into the ultrasonic range, 
then amplifies it, and transmits it by means of a hydrophone 17 through 
the underwater medium 100. 
The receiving section 3 receives modulated ultrasonic data from the 
underwater medium 100 by means of hydrophone 17', which is then provided 
to an input RF circuit 8, which amplifies it, and transfers it downward 
into a low frequency range. From the output of the input RF circuit 8, the 
signal is conveyed to an OFDM demodulator 9, which demodulates the signal, 
and outputs encoded data. The encoded data from the OFDM demodulator is 
conveyed into an FEC (Forward Error Correction) decoder 10. The FEC 
decoder 10 performs a process which is the reverse of the process of the 
FEC encoder 5 in the transmitting modem. The FEC decoder 10 recovers the 
original data to be forwarded, as conveyed by the data source 4 to the FEC 
encoder 5 of the transmitting modem. The FEC decoder 10 contains means for 
analyzing the encoded data, and if it finds that the data has been 
corrupted, for example, when passing through the underwater medium, it 
uses the additional bits added by the FEC encoder 5 to correct errors and 
recover the original data The FEC decoder 10, similar to the FEC encoder 
5, is also conventional in its structure, and is capable of recovering 
errors only to a certain extent. 
When discussing binary systems, it is common to refer to bits, as two 
signal levels are possible, e.g., .+-.A. In OFDM, which applies the M-ary 
technique, there exist more than two signal possibilities, and it is 
common to refer to each possible transmitted signal as a "symbol". 
The symbol duration is defined as the sum of the essential duration T plus 
the guard time .DELTA.T. First, the essential duration T of the symbol and 
the bandwidth B of one modulated carrier of the transmitted signal has to 
be chosen in order to accomplish frequency nonselective (flat) and slow 
fading. The two requirements for achieving a flat and slow fading are: 
EQU B&lt;&lt;1/T.sub.d ; and (1) 
EQU T&lt;&lt;1/.DELTA.f (2) 
wherein B is the bandwidth of the transmitted signal, T.sub.d denotes the 
delay time of the channel, T is the duration of each transmitted symbol, 
and .DELTA.f denotes the Doppler spread. 
Assuming that T=1/B, the following conditions should be met: 
EQU T.sub.d &lt;&lt;T&lt;&lt;1/.DELTA.f (3) 
If it can be assumed that T.sub.d.sbsb.max =2 msec and 1/.DELTA.f.sub.max 
=100 msec, the selected symbol duration T should be in the range of: 
EQU 2 msec&lt;&lt;T&lt;&lt;100 msec (4) 
If a symbol duration of T=10.div.20 ms, which is within this range, is used 
and fulfills this requirement of equation, then this means that the 
bandwidth of one modulated carrier is B=50 Hz.div.100 Hz. It has been 
found that by using OFDM, it is possible to fulfill requirement (4) and 
efficiently use a wide spectrum bandwidth (3 kHz, for example). In such a 
case, the number of carriers N should approximately be: 
EQU N=BW.multidot.T 
wherein BW denotes the total bandwidth available to the whole OFDM signal. 
The applicants, have found that for BW=3 kHz, number of carriers=31, and 
SNR=10 dB, and for a desired BER=10.sup.-3, a maximal bit rate of 3000 bps 
can be achieved in accordance with DQPSK modulation, and with, for example 
(31,16,7) forward error correcting BCH code. According to a preferred 
embodiment of the invention, which is given herein as an example, 31 
carriers are used, although, of course, different numbers of carriers may 
be applied. Moreover, it was found that if the number of carriers 
increases, the maximal bit rate also increases. For example, a bit rate of 
9600 bps may be obtained for about 100 carriers. 
As said, the Doppler effect is a very serious problem in underwater 
acoustic communications. A frequency shift of as much as 20 Hz is quite 
normal in underwater communication. In order to overcome such a severe 
shift, and in order to keep the receiving section of the modem tuned to 
the frequency of the received signal, two additional unmodulated carriers, 
referred to hereinafter as "pilots", are transmitted along with the 
information bearing signal, that, as said, comprises said 31 modulated 
carriers. The frequency of the first pilot is selected to be slightly 
below the lowest frequency modulated carrier, and the second pilot above 
the highest frequency modulated carrier. 
A more detailed block diagram of the OFDM modulator 6 of the transmitting 
section 2 of the modem, according to this particular preferred embodiment 
of the invention, is shown in FIG. 2. As use of coherent demodulation is 
very problematic in underwater applications, due to a very random, rapid, 
and frequent change in the carrier phase, the modem according to the 
invention obviates the need for carrier recovery by using differential 
modulation. More particularly, the data of each of the plurality of 
carriers of the OFDM is modulated by DQPSK modulation (Differential 
Quadrature Phase Shift Key Modulation). 
As shown in FIG. 2, a serial (original) data is inputted into the FEC 
encoder 5, which combines with it additional bits for enabling error 
correction at the receiving modem. From the FEC encoder 5, the data in 
serial form is conveyed to a serial to parallel device 12, essentially a 
shift register, which divides each section of 62 bits of serial data 
(hereinafter, each such 62-bit section will also be referred to as a 
"word") into 31 two-bit symbols of data to be processed in parallel. It 
should be noted here that, in order to improve reliability, and to provide 
better error correction, the allocation of the bits from a word to symbols 
is not sequential, but is performed by taking one bit form the first half 
of a 62-bit word, and a second bit from the second half of the same word. 
A vector of one 62-bits word, its bits allocation to 31 symbols, and the 
symbols allocation to separate carriers of the OFDM modulator, is shown 
below: 
Vector of input bits assigned to one 62-bit word: 
______________________________________ 
b.sub.1 
b.sub.2 . . . b.sub.30 
b.sub.31 
b.sub.32 
. . . 
b.sub.60 
b.sub.61 
b.sub.62 
______________________________________ 
Bits allocation to symbols and carriers in the modulator: 
______________________________________ 
carrier 
number 1 2 . . . 30 31 
______________________________________ 
bits b.sub.1 b.sub.2 b.sub.30 
b.sub.31 
bits b.sub.2 b.sub.33 b.sub.61 
b.sub.62 
symbols [b.sub.1,b.sub.32 ] 
[b.sub.2,b.sub.33 ] 
[b.sub.30,b.sub.61 ] 
[b.sub.31,b.sub.62 ] 
______________________________________ 
Then, all of said 31 symbols of a same data word are conveyed to 31 
differential encoders, which perform the following operation: 
##EQU1## 
where n denotes the carrier number (n=1, 2, 3, 4 . . . N), L=4 are the 
four possible symbols {0, 1, 2, 3} (assuming each symbol comprises two 
bits) as defined and shown in the constellation map of FIG. 3. 
A.sub.k.sup.(n) are symbols originated in data source 4, encoded by FEC 
encoder 5, and separated by the serial to parallel device 12, and k 
denotes the symbol running index. The structure of each one of the 31 
differential encoders is schematically shown in FIG. 4. Block 16 indicates 
a 1-bit adder, and 117 indicates a delay of T seconds, wherein T is the 
duration of each symbol. 
From the said 31 differential encoders 15, encoded symbols B.sub.k are 
conveyed in 31 parallel lines 22 to a 31-carrier quadrature OFDM modulator 
unit 21. The structure of the quadrature OFDM modulator is shown in FIG. 
5. Data splitter 20 maneuvers the data from each line 22 to a 
corresponding orthogonal modulator 23. Each modulator comprises two 
multipliers 25 and 25', the first multiplier 25 multiplying the data by a 
cosine sub-carrier, and the second, multiplier 25', multiplying it by an 
orthogonal, sine sub-carrier. Adder 26 adds the result from said two 
multipliers. A similar modulating process is performed in each one of the 
other 30 modulators. The results from all 31 modulators are first combined 
by summing means 27, then combined by adder 29 with two additional 
sub-carriers (pilots), cos.omega..sup.(0) t and cos.omega..sup.(N+1) t, 
and finally provided to a hydrophone 28, which transmits the combined 
signal into the water. If the signal to the hydrophone 28 is not in the 
required transmitting frequency, a frequency shift to the required 
transmitting frequency can be made by any conventional means, before 
conveying the signal to the hydrophone for transmission. The purpose of 
the transmission of said two sub-carriers cos.omega..sup.(0) t and 
cos.omega..sup.(N+1) t is to help to overcome the frequency shift due to 
Doppler effects in water, as will become apparent as the description 
proceeds. Furthermore, it should be noted that, for synchronization 
purposes at the receiving modem, a guard interval is used. The duration of 
the guard interval may vary. However, it should preferably be at least in 
the order of about 10% of the symbol duration T. 
An example of a band spectrum of a modem according to one embodiment of the 
invention is shown in FIG. 13. As shown, the OFDM transmission is carried 
out by 31 modulated carriers cos.omega..sup.(1) t-cos.omega..sup.(31) t, 
and two unmodulated carriers (pilots) cos.omega..sup.(0) t and 
cos.omega..sup.(32) t. The frequencies assignments for each one of said 
carriers according to this example are as follows: 
__________________________________________________________________________ 
cos.omega..sup.(0) t = 200 Hz 
cos.omega..sup.(9) t = 1200 Hz 
cos.omega..sup.(18) t = 2100 Hz 
cos.omega..sup.(27) t = 3000 Hz 
cos.omega..sup.(1) t = 400 Hz 
cos.omega..sup.(10) t = 1300 Hz 
cos.omega..sup.(19) t = 2200 Hz 
cos.omega..sup.(28) t = 3100 Hz 
cos.omega..sup.(2) t = 500 Hz 
cos.omega..sup.(11) t = 1400 Hz 
cos.omega..sup.(20) t = 2300 Hz 
cos.omega..sup.(29) t = 3200 Hz 
cos.omega..sup.(3) t = 600 Hz 
cos.omega..sup.(12) t = 1500 Hz 
cos.omega..sup.(21) t = 2400 Hz 
cos.omega..sup.(30) t = 3300 Hz 
cos.omega..sup.(4) t = 700 Hz 
cos.omega..sup.(13) t = 1600 Hz 
cos.omega..sup.(22) t = 2500 Hz 
cos.omega..sup.(31) t = 3400 Hz 
cos.omega..sup.(5) t = 800 Hz 
cos.omega..sup.(14) t = 1700 Hz 
cos.omega..sup.(23) t = 2600 Hz 
cos.omega..sup.(32) t = 3600 Hz 
cos.omega..sup.(6) t = 900 Hz 
cos.omega..sup.(15) t = 1800 Hz 
cos.omega..sup.(24) t = 2700 Hz 
cos.omega..sup.(7) t = 1000 Hz 
cos.omega..sup.(16) t = 1900 Hz 
cos.omega..sup.(25) t = 2800 Hz 
cos.omega..sup.(8) t = 1100 Hz 
cos.omega..sup.(17) t = 2000 Hz 
cos.omega..sup.(26) t = 2900 Hz 
__________________________________________________________________________ 
The hydrophone, according to a preferred embodiment of the invention, is a 
multidirectional hydrophone which can transmit or receive essentially 
equally to or from all directions. It should be noted here that the use of 
a multidirectional hydrophone, preferably in accordance with OFDM 
transmission, has been found to significantly improve the reliability of 
the transmission, and has been found to best overcome various disturbances 
in the water, such as noise, multipath, and fading. Moreover, such use has 
been shown to allow an effective, reliable transmission, even when no line 
of sight exists between the transmitting and the receiving modems. This is 
indeed surprising since the prior art emphasizes the use of directional 
hydrophones in underwater communication. 
A more detailed schematic block diagram of the receiving section 3 of the 
modem is shown in FIG. 6. The RF circuit 8 of the receiving section 
comprises a preamplifier 40, a local oscillator 42 and mixer 41, a band 
pass filter 43, and an analog to digital converter 44. Data which is 
received in the hydrophone is first transferred to a preamplifier 40, 
which amplifies the signal. From the preamplifier, an amplified signal is 
passed on to a mixer 41. As mentioned hereinbefore, the signal to the 
mixer spans a bandwidth of 3.5 kHz according to the example given above, 
and is positioned, for example, between 40.2 kHz and 43.6 kHz. The mixer 
41 also receives a frequency of 40 kHz, for example, from the local 
oscillator, converting down the bandwidth of the signal to span 
frequencies of between 200 Hz-3600 Hz. Then the signal from the mixer 41 
is conveyed to low pass filter 43 and then to an analog to digital (A/D) 
converter 44, which samples the signal and converts it into a digital 
representation. 
The signal, as said, in digital representation, is then provided into the 
OFDM demodulator 9. The OFDM demodulator 9 comprises a DFT (Discrete 
Fourier Transformer) 45, a symbol synchronizer 46, a frequency adjust 
circuit 47, a decision device and differential decoder 48, and a parallel 
to serial device 49. It should be noted here that the OFDM demodulator 9 
is preferably implemented, according to the invention, by one DSP (Digital 
Signal Processing) circuit, generally available in one integrated chip. 
However, it may be also implemented by other means well known to those 
skilled in the art, for example, by a powerful microprocessor. The signal 
form the A/D converter 44, as said, in digital form, is conveyed in 
parallel to the DFT 45, to the symbol synchronizer 46, and to the 
frequency adjust circuit 47. The symbol synchronizer provides to both the 
DFT 45 and to the decision device and differential decoder 48 a clock 
indicating the beginning and the end of a received symbol. The frequency 
adjust circuit 47 inspects the two sub-carriers that are combined with the 
transmitted signal, for detecting frequency shift, generally due to 
Doppler effects on the signal propagated in water. The frequency adjust 
circuit 47 continuously updates the decision device and differential 
decoder 48 of any frequency shift. The decision device and differential 
decoder 48, after detecting 31 symbols simultaneously, provides 62 bits 
representing the symbols in parallel to the parallel to serial device 49, 
which separates the symbols into bits, which are then combined into two 
words, and outputted to the FEC decoder 10. 
FIG. 7 shows the structure of the DFT 45 in greater detail. The received 
signal, after being sampled by the A/D 44, and converted into a digital 
representation, is conveyed to the DFT on line 50, which is then split 
into sixty-two parallel lines 51, each one of said parallel lines 51 
leading to a corresponding one of sixty-two multipliers 53. The said 
sixty-two multipliers are divided into thirty-one orthogonal pairs, one 
multiplier in each pair is provided with a sin(.omega..sup.(n) 
t+.theta..sub.x), and a second one with a cos(.omega..sup.(n) 
t+.theta..sub.x), wherein n [n=1, 2, 3, . . . 31] indicates the symbol 
number in a 31-symbol word, and .theta..sub.x indicates a phase which is 
not phase-synchronized with sine or cosine entries to other multipliers 53 
of other pairs. The output from each one of said sixty-two multipliers is 
then integrated by a corresponding one of sixty-two integrators 54, each 
of which, as indicated, performs the integration 
##EQU2## 
during a period T of a complete symbol, a period which is indicated to the 
DFT 45 by a clock provided from the symbol synchronizer 46. The two 
outputs from any pair of integrators produce a complex vector 
Q.sub.k.sup.(n), wherein n (n=1, 2, 3, . . . 31=N) indicates the symbol 
location in the word, and k indicates the symbol index. A vector 
Q.sub.k.sup.(1.fwdarw.N), representing all said 31 vectors, is then 
conveyed into the decision device and differential decoder 48. 
The decision algorithm of the decision device and differential decoder 48 
is illustrated in FIGS. 8a, 8b, and 9. If the DFT output for carrier 
number n and symbols k-1 and k are vectors Q.sub.k-1.sup.(n) and 
Q.sub.k.sup.(n) respectively, and .DELTA..phi..sub.k.sup.(n) denotes the 
phase difference between Q.sub.k.sup.(n) and Q.sub.k-1.sup.(n), as shown 
in FIGS. 8a and 8b, the values of sin.DELTA..phi..sub.k.sup.(n) and 
cos.DELTA..phi..sub.k.sup.(n) are computed to determine a point 
(sin.DELTA..phi..sub.k.sup.(n), cos.DELTA..phi..sub.k.sup.(n)) on the 
decision plane diagram of FIG. 9, where the decision regions are 
indicated. 
The symbol synchronizer 46 performs a symbol synchronization (often also 
called "timing recovery"), the purpose of which is to recover a clock at 
the symbol rate (or a multiple thereof) from the input to the DFT 45 at 
line 50, representing in digital form the input modulated signal. This 
clock, as hereinbefore noted, determines the boundaries of integration of 
the DFT 45, and is also provided to the decision device and differential 
decoder 48 for determining the symbol timing boundaries. FIG. 10 
illustrates the structure of the symbol synchronizer 46, and FIG. 11 is a 
corresponding timing diagram. A received signal x(t) arriving at line 50 
is inputted to the symbol synchronizer 46. Signal x(t) is in fact S.sub.T 
(t) corrupted by noise and distorted by the channel. It contains a 
modulated symbol of duration T+.DELTA.T. The synchronizer 46 comprises a 
summing component 56 having two inputs, one input is provided with x(t), 
and a second input is provided with x(t) delayed by the delay block 52 by 
a period of T (T is the essential part of the symbol). The output from the 
summing component 56 is provided by line 58 to the average power block 57, 
which measures the average power of the signal provided to it, in purpose 
to find a timing of minimum power. Such search for minimum-power timing is 
carried out by varying the beginning of the integration at block 60, and 
providing the result of the integration into the minimum search block 61. 
The output of said block 61 is fed back to adjust the beginning of the 
integration at block 60. FIG. 11 is a timing diagram illustrating the 
operation of the symbol synchronizer 46. Assuming that the signal x(t) 
comprises a digitally modulated signal of symbols of duration T+.DELTA.T, 
the beginning of the symbols occur at t.sub.k, t.sub.k+1, t.sub.k+2, etc., 
and the symbols include the guarding intervals .DELTA.T, as shown. If the 
processing operation of the symbol synchronizer 46 starts at a random time 
.lambda..sub.k =t.sub.k +.DELTA.t, wherein t.sub.k is the correct 
synchronizing time and .DELTA.t indicates the deviation from said correct 
synchronizing time, then, for an ideal case, with no noise or 
disturbances, P.sub.1 is zero for 0&lt;.DELTA.t&lt;.DELTA.T. For the real case, 
however, the symbol synchronizer 46 seeks the case in which P.sub.1 is 
minimal. In FIG. 11, .lambda..sub.k+1, .lambda..sub.k+2, . . . indicate 
the timing of the beginning of processing of the DFT receiving .DELTA.t 
for synchronization, and .delta..sub.k, .delta..sub.k+1, .delta..sub.k+2 . 
. . indicate the timing of the end of processing of the DSP. As is clear 
to those skilled in the art, after a few symbols, the value of .DELTA.t 
converge to such value that guarantees minimum of P.sub.1. 
FIG. 12 shows that a minimum power occurs during the guard periods 
.DELTA.T. P.sub.1 reaches its minimal power value if and only if the 
system is in full symbol synchronization. As said, the symbol synchronizer 
46 finds the (.DELTA.t), for which the power is minimal. 
The OFDM signal, when transmitted through the underwater medium, may suffer 
a frequency shift .DELTA.F. A first and general reason for a frequency 
shift lies in the frequency accuracy of the transmitter and receiver local 
oscillators. For example, if the accuracy of the local oscillators is 100 
ppm each, then for an RF frequency of 50 kHz, which is the frequency range 
used by the modem for communication, a frequency shift of -5 
Hz.ltoreq..DELTA.F.ltoreq.5 Hz may be expected. Such a frequency shift is 
generally constant and does not change with time, nor does it depend on 
the medium of signal propagation. The second reason for the carrier shift, 
more severe in underwater communication, results from the Doppler effect, 
particularly due to the movement of either the receiving or the 
transmitting modem, or both, or due to the change in density of the water. 
For example, for a relative velocity of 0.5 m/sec between the receiving or 
the transmitting modem, a Doppler frequency shift of about 16 Hz is 
expected. 
As mentioned hereinbefore, two pure carriers (pilots), one below the lowest 
modulated carrier, and one above the highest modulated carrier, are 
transmitted from the transmitting modem, along with the other n modulated 
carriers. The frequencies of these pilots are tracked at the frequency 
adjust circuit 47 by two Phase Locked Loop (PLL) devices, each of which is 
tuned to one pilot frequency. When a shift is detected in said 
frequencies, an indicative frequency shift is provided to each plurality 
of sin e.omega..sup.(n) and cosine.omega..sup.(n) components at the DFT 
45. Such shift in frequency therefore realigns the bandwidth of the 
receiving modem for any frequency shift which may occur to the signal due 
to Doppler effect or due to propagation effects in the water. 
As seen, the invention provides a modem which can reliably transfer data at 
a high rate through water. The use of OFDM modulation and a 
multidirectional hydrophone significantly improve the capability and 
quality of the data transfer, and provide a significant improvement to the 
efficient range of transmission. Furthermore, the modem according to the 
invention is provided with means for efficiently overcoming severe 
frequency shifts of the transmitted signal due to Doppler effects. It 
should be noted here that the use of OFDM is preferable for transferring 
data through water according to a preferred embodiment of the invention, 
as this modulation method enables simultaneous transmission of data over a 
plurality of narrow bandwidth channels, wherein each channel is minimally 
prone to amplitude and phase distortions due to its narrow bandwidth. 
However, there are other modulation methods, other than OFDM that can also 
be used, which also enable the simultaneous transfer of data over a 
plurality of narrow band channels. The use of those other modulation 
methods using a plurality of parallel narrow band channels for the high 
rate transfer of data through water is also within the scope of the 
invention. 
The underwater modem of the invention can also be used for sound 
communication. In order to reliably communicate sound through water by 
digital techniques, a minimum bit rate of 2400 bits per second is 
required. As mentioned, this data rate is well within the capability of 
the modem of the invention. 
FIG. 14 illustrates in block diagram form the structure of a sound modem 
according to a preferred embodiment of the invention. FIG. 15 illustrates 
in block diagram form the structure of the transmitting section of a sound 
modem according to a preferred embodiment of the invention. The sound 
modem, comprises in the transmitting section 2, a microphone 77 for 
receiving a sound, converting it to an electric signal, and conveying it 
into a voice encoder 144. The voice encoder 144 converts the elctrical 
signal representing the sound into a bit stream of data, which is in turn 
conveyed into the FEC encoder 5, for encoding the bit stream of data for 
error correction purposes as before. Then ,the encoded bit stream of data 
is transmitted by the transmitting section 2 in the same manner as 
described hereinbefore for the digital data modem. The transmitted data is 
received by a receiving unit 3, and detected as hereinabove described for 
the digital data modem. Therefore, a bit stream of data, presumably 
identical to the bit stream that the voice encoder 144 conveyed to the FEC 
encoder 5 in the transmitting section 2, is conveyed from the FEC decoder 
10 of the receiving unit 3 into a voice decoder 11. The voice decoder 11 
converts the bit stream into an analog sound signal, which is in turn sent 
to a speaker 78. Speaker 78 converts the electric sound signal to voice. 
Speaker 78 may be, for example, earphones of a diver, and the sound may be 
a voice of a diver to be communicated to another diver. 
For the voice encoder 144 of the transmitting section 2 and the voice 
decoder of the receiving section 3, the same component known in the art as 
a "Vocoder" may be used. This component generally has two modes of 
operation: a first mode in which it operates as a voice decoder, and a 
second mode in which it operates as a voice encoder. For example, a 
vocoder of the type AC 4802AE2-C by Audiocodes can be used both in the 
receiving and the transmitting sections. 
While some embodiments of the invention have been described by way of 
illustration, it will be apparent that the invention can be carried out in 
practice with many modifications, variations and adaptations, and with the 
use of numerous equivalents or alternative solutions that are within the 
scope of persons skilled in the art, without departing from the spirit of 
the invention or exceeding the scope of the claims. For example, the 
number of carriers, as selected above to be, for example, 31, can vary and 
is a function of the transfer rate and error correcting consideration. In 
order to achieve a higher data transfer rate, it is preferably to use a 
greater number of carriers. Of course, as the number of carriers 
increases, a more complicated FEC has to be used. According to the 
invention, it can be assumed that a bit rate of up to about 100 kbs is 
practically achievable. Furthermore, although the use of a 
multidirectional hydrophone is preferable, this is not a limitation, as 
the modem can also function with a directional hydrophone. The bandwidth, 
and the frequencies of the plurality of carriers, are also selective and 
may depend on design considerations. The use of a DSP, as well, is 
optional, and different alternatives may be used.