fig1 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 &# 34 ; the original data &# 34 ;) 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 &# 34 ; the encoded data &# 34 ;) 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 &# 39 ;, 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 &# 34 ; symbol &# 34 ;. the symbol duration is defined as the sum of the essential duration t plus the guard time δt . 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 : wherein b is the bandwidth of the transmitted signal , t d denotes the delay time of the channel , t is the duration of each transmitted symbol , and δf denotes the doppler spread . if it can be assumed that t d . sbsb . max = 2 msec and 1 / δf max = 100 msec , the selected symbol duration t should be in the range of : if a symbol duration of t = 10 ÷ 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 ÷ 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 : 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 - 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 &# 34 ; pilots &# 34 ;, 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 fig2 . 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 fig2 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 &# 34 ; word &# 34 ;) 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 : ______________________________________carriernumber 1 2 . . . 30 31______________________________________bits b . sub . 1 b . sub . 2 b . sub . 30 b . sub . 31bits b . sub . 2 b . sub . 33 b . sub . 61 b . sub . 62symbols [ 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 fig3 . a 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 fig4 . 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 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 fig5 . data splitter 20 maneuvers the data from each line 22 to a corresponding orthogonal modulator 23 . each modulator comprises two multipliers 25 and 25 &# 39 ;, the first multiplier 25 multiplying the data by a cosine sub - carrier , and the second , multiplier 25 &# 39 ;, 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 ), cosω . sup . ( 0 ) t and cosω . 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 cosω . sup . ( 0 ) t and cosω . 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 fig1 . as shown , the ofdm transmission is carried out by 31 modulated carriers cosω . sup . ( 1 ) t - cosω . sup . ( 31 ) t , and two unmodulated carriers ( pilots ) cosω . sup . ( 0 ) t and cosω . sup . ( 32 ) t . the frequencies assignments for each one of said carriers according to this example are as follows : __________________________________________________________________________cosω . sup . ( 0 ) t = 200 hz cosω . sup . ( 9 ) t = 1200 hz cosω . sup . ( 18 ) t = 2100 hz cosω . sup . ( 27 ) t = 3000 hzcosω . sup . ( 1 ) t = 400 hz cosω . sup . ( 10 ) t = 1300 hz cosω . sup . ( 19 ) t = 2200 hz cosω . sup . ( 28 ) t = 3100 hzcosω . sup . ( 2 ) t = 500 hz cosω . sup . ( 11 ) t = 1400 hz cosω . sup . ( 20 ) t = 2300 hz cosω . sup . ( 29 ) t = 3200 hzcosω . sup . ( 3 ) t = 600 hz cosω . sup . ( 12 ) t = 1500 hz cosω . sup . ( 21 ) t = 2400 hz cosω . sup . ( 30 ) t = 3300 hzcosω . sup . ( 4 ) t = 700 hz cosω . sup . ( 13 ) t = 1600 hz cosω . sup . ( 22 ) t = 2500 hz cosω . sup . ( 31 ) t = 3400 hzcosω . sup . ( 5 ) t = 800 hz cosω . sup . ( 14 ) t = 1700 hz cosω . sup . ( 23 ) t = 2600 hz cosω . sup . ( 32 ) t = 3600 hzcosω . sup . ( 6 ) t = 900 hz cosω . sup . ( 15 ) t = 1800 hz cosω . sup . ( 24 ) t = 2700 hzcosω . sup . ( 7 ) t = 1000 hz cosω . sup . ( 16 ) t = 1900 hz cosω . sup . ( 25 ) t = 2800 hzcosω . sup . ( 8 ) t = 1100 hz cosω . sup . ( 17 ) t = 2000 hz cosω . 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 fig6 . 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 . fig7 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 ( ω . sup . ( n ) t + θ x ), and a second one with a cos ( ω . sup . ( n ) t + θ x ), wherein n [ n = 1 , 2 , 3 , . . . 31 ] indicates the symbol number in a 31 - symbol word , and θ 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 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 k . sup . ( 1 → 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 fig8 a , 8b , and 9 . if the dft output for carrier number n and symbols k - 1 and k are vectors q k - 1 . sup . ( n ) and q k . sup . ( n ) respectively , and δφ k . sup . ( n ) denotes the phase difference between q k . sup . ( n ) and q k - 1 . sup . ( n ), as shown in fig8 a and 8b , the values of sinδφ k . sup . ( n ) and cosδφ k . sup . ( n ) are computed to determine a point ( sinδφ k . sup . ( n ), cosδφ k . sup . ( n )) on the decision plane diagram of fig9 where the decision regions are indicated . the symbol synchronizer 46 performs a symbol synchronization ( often also called &# 34 ; timing recovery &# 34 ;), 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 . fig1 illustrates the structure of the symbol synchronizer 46 , and fig1 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 t ( t ) corrupted by noise and distorted by the channel . it contains a modulated symbol of duration t + δt . 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 . fig1 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 + δt , the beginning of the symbols occur at t k , t k + 1 , t k + 2 , etc ., and the symbols include the guarding intervals δt , as shown . if the processing operation of the symbol synchronizer 46 starts at a random time λ k = t k + δt , wherein t k is the correct synchronizing time and δt indicates the deviation from said correct synchronizing time , then , for an ideal case , with no noise or disturbances , p 1 is zero for 0 & lt ; δt & lt ; δt . for the real case , however , the symbol synchronizer 46 seeks the case in which p 1 is minimal . in fig1 , λ k + 1 , λ k + 2 , . . . indicate the timing of the beginning of processing of the dft receiving δt for synchronization , and δ k , δ k + 1 , δ 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 δt converge to such value that guarantees minimum of p 1 . fig1 shows that a minimum power occurs during the guard periods δt . p 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 ( δt ), for which the power is minimal . the ofdm signal , when transmitted through the underwater medium , may suffer a frequency shift δf . 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 ≦ δf ≦ 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 eω . sup . ( n ) and cosineω . 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 . fig1 illustrates in block diagram form the structure of a sound modem according to a preferred embodiment of the invention . fig1 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 &# 34 ; vocoder &# 34 ; 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 .