Abstract:
A system and method for constructing a transmitter and a receiver that communicate using multiple orthogonal frequencies which are locked to each other. The set of available frequencies can range from 7, 32 frequencies to a much larger set of frequencies. The transmitted frequencies are separated by a small limited bandwidth (B.W). multiple frequencies are selected from a set of available frequencies and transmitted simultaneously to the receiver. The data transmission is preceded by a calibration sequence where the shift between the transmitted frequency and the frequency measured by fast Fourier transform at the receiver system contain encoders and decoders that convert binary numbers to frequency combinations and the reverse. This transceiver system is capable of transmitting large data rate per unit of a bandwidth e.g. 6 Mbps/3.2 MHZ.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the transmission and reception of digital data using digital signal processors (DSP), in frequency shift keying and multiple binary orthogonal frequency based communication system. More particularly, the invention relates to a system and method for transmitting and receiving digital signals using frequency shift keying (FSK), frequency modulation (FM) and using multiple binary orthogonal frequencies 
     BACKGROUND OF THE INVENTION 
     Most of the modern communication systems generally use frequency modulation techniques to transmit analog and digital information between a transmitter and a receiver. At present the sate of the art in digital data transmission over narrow bandwidth is by using Frequency Modulation (FM) and Frequency Shift Keying (FSK). A detailed description of this technique and its background art and specifications is described in the following paragraphs. 
     U.S. Pat. No. 5,852,636 for Mathieu et al., describes Methods and apparatus for modulation of a Frequency Shift Keying (FSK) carrier in a very narrow band. This patent provides a method and a transmitter system for modulating binary information on a carrier in a very narrow band by Frequency Shift Keying (FSK) a carrier signal. Instead of the direct use of the frequency shift keying technique, a phase reversal keying technique is used to create a carrier with frequency shift characteristics. 
     The transmitter of the above referred patent generates a reference signal and a control signal, the latter having two frequencies to represent the binary data to be transmitted. A carrier signal is generated by modulation using a phase reversal technique. The resulting carrier signal is processed by high order filtering such that the output of the transmitter provides a Frequency Shift Keyed (FSK) carrier at very close frequencies. However, Mathieu fails to disclose a system and method for transmitting and receiving digital signals using Frequency Shift Keying (FSK), Frequency Modulation (FM) and using multiple binary orthogonal frequencies. 
     U.S. Pat. No. 5,358,202 for Tse et al. discloses a Frequency Shift Keying (FSK) demodulator using a phase locked loop and voltage comparator. The U.S. Pat. No. 4,456,985 of R. T. Carsten et al., discloses an arrangement in which full duplex above voice band Frequency Shift Keyed (FSK) data may be transmitted simultaneously with telephone signals over a subscriber&#39;s telephone signals which may be present, and they must be sufficiently high to distinguish them from voice band signals and sufficiently low that they are not unduly attenuated by the telephone line. For example, the Frequency Shift Keyed (FSK) center frequency being changed by +0.500 Hz depending on whether a data 0 or 1 is being transmitted. 
     In consequence, the Frequency Shift Keying (FSK) center frequencies are relatively inaccurately determined and may be subject to change due to the combined effects of component tolerances aging, and temperature changes. 
     With conventional Frequency Shift Keying (FSK) demodulation, the effect of an incorrect center frequency in the incoming Frequency Shift Keying (FSK) data is to produce a distorted mark/space ratio in the demodulated data. In the arrangement discussed above the distortion can be severe because the change in the Frequency Shift Keying (FSK) center frequency can be significant compared with narrow band (+0.500 Hz) which is used for Frequency Shift Keying (FSK) transmission. Such distortion can lead to errors in examining the demodulated data if transitions of the demodulated data are used to determine the times at which this data is sampled. 
     U.S. Pat. No. 4,486,955 to Mass et al., discloses a Frequency Shift Keying (FSK) demodulator. This demodulator is described as a circuit for detecting differences in frequency between a Frequency Shift Keying (FSK) modulated input signal and a reference signal (REF). The circuit includes a sequence generator means for producing a digital code representative of phase angle between the Frequency Shift Keying (FSK) and reference signal (REF) signals. The digital code (which is preferably a two bit binary code) changes a first predetermined sequence when the frequency of the Frequency Shift Keying (FSK) signal is less than the frequency of the reference signal (REF), and changes in a second predetermined sequence when the frequency of the Frequency Shift Keying (FSK) signal is greater than the frequency of the reference signal (REF). 
     The circuit, as disclosed by Mass et al., includes a first sequence detector means for detecting the occurrence of the first predetermined sequence, and a second sequence detector means for detecting the occurrence of the second predetermined sequence. Each time one of the sequence detector means provides an output indicating that its selected sequence has been detected; it is reset to begin another sequence detection cycle. The outputs from the second sequence detector means during a predetermined period are loaded into first and second integrating shift register means. At the end of the predetermined period, decision means compares the contents of the first and second integrating shift register means, and provides a data output based upon the contents of the first and second integrating shift register means. In this way, the sequence which was detected the most times during the period determines the data output from the decision means. Also, the above patent to Mass et al., discloses a method for estimating the frequency of a time signal by means of a discrete Fourier transformation and interpolation, without analyzing sampled data using over sampling. It also uses zero padding in a multi sample, multi frequency message with high speed to determine the data content of the message. 
     Advantageously, the present invention is applicable for both one carrier Narrow Band Frequency Modulation (NBFM) system and for a multiple carrier system. For multiple carrier transmission systems (each carrier has a mark and a space frequency). Either a Band Pass Filter (BPF) or matched filter schemes are used to separate the frequencies in a multiple carrier parallel system. 
     Further, the present invention provides a Frequency Shift Keying (FSK) demodulator that can be used to demodulate Narrow Band Frequency Shift Keying (NBFSK) data. The Frequency Shift Keying (FSK) provides reduced distortion of the mark/space ratio of the demodulated data in the event of a variable Frequency Shift Keying (FSK) center frequency where a phase locked loop is used to determine frequencies. The demodulator preferably includes a low pass filter for coupling the output voltage of the phase locked loop to the D.C blocking means. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a Multiple (m) carrier Narrow Band Frequency Modulation (NBFM) system using orthogonal frequencies. The mark and space frequencies are orthogonal frequencies before Inverse Fast Fourier Transform (IFFT) is applied to create a time domain signal. Here Fast Fourier Transform (FFT) or matched filters are used at the receiver to separate the outputs of each frequency band. This Narrow Band Frequency Modulation (NBFM) system using orthogonal frequencies increases data capacity and rates between a Transmitter and a Receiver by a ratio equal to the number of different pairs of orthogonal carriers (n) between the transmitter and the receiver by m!n! (m−n)! 
     The present invention also contemplates a communication system for sending and receiving data. The system comprises a transmitter and a receiver. A general binary coded Orthogonal Frequency Data Modulated carrier scheme is provided between the transmitter and the receiver to increase code density and data rate per Hz of bandwidth. A plurality of narrow band carrier frequencies that are orthogonal are transmitted in a binary coded fashion to represent data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration showing  8  frequencies to represent 3 bits; 
         FIG. 2  is a graphical illustration showing Bit Error Rate (BER) versus increasing the Bit Rate at a Signal to Noise (SNR)=10 dB in a 1 of 8 system; 
         FIG. 3  is a graphical illustration showing Bit Error Rate (BER) versus Signal to Noise ratio (SNR) at a Bit Rate of 1800 Kbps; 
         FIG. 4  is a graphical illustration showing Bit Error Rate (BER) versus Baud Rate in a 2 of 7 system 
         FIG. 5  is a graphical illustration showing Bit Error Rate (BER) versus Signal to Noise Ratio (SNR) in a 2 of 7 system; 
         FIG. 6  shows a 2 of 7 code transmitter block diagram in accordance with the present invention; 
         FIG. 7  shows a 2 of 32 code transmitter system in accordance with the present invention; 
         FIG. 8  is a receiver block diagram in accordance with the present invention; 
         FIG. 9  shows a receiver system in accordance with the present invention; 
         FIG. 10  is a 2 of 7 Mary Narrow Band Frequency Shift Keying (Mary-NBFSK) code flow chart for use in the present invention; 
         FIG. 11  is a spectrum output of a Fast Fourier Transform (FFT) as used in the present invention; 
         FIG. 12  is a graphical illustration showing Bit Rate versus Bit Error Rate (BER); 
         FIG. 13  is a graphical illustration showing Signal to Noise Ratio (SNR) versus Bit Error Rate (BER); 
         FIG. 14  is a graphical illustration showing Bit Error probability versus Baud Rate; 
         FIG. 15  is a graphical illustration showing Bit Error Rate (BER) versus Signal to Noise Ratio (BER) at a Baud rate of 600 Kbps; 
         FIG. 16  is a graphical illustration showing Bit Error Rate (BER) versus Signal to Noise Ratio (SNR) at a Baud Rate of 1000 Kbps; and 
         FIG. 17  is a graphical illustration showing Bit Error Rate (BER) versus Baud Rate with Signal to Noise Ratio (SNR). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention contemplates a Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) system. Multiple orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) communication systems with multiple orthogonal frequencies locked to the same carrier are used instead of using multiple parallel, individually coded, binary Frequency Shift Keying (FSK) communication system. The Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) has several advantages over multiple binary Narrow Band Frequency Shift Keying (Binary NBFSK), first it has less hardware, second it achieves reduced power consumption and third it has a higher code density/Hz of bandwidth. 
     Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) transmits one or multiple frequencies (carriers) at a time. While in a multiple binary Frequency Shift Keying (FSK) mode, each carrier is transmitted when a modulating bit is high, e.g. 3 carriers for 3 FSK systems. In Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) a number of carrier should be two times the number of bits per symbol for efficient transmission. 
     Referring now to Figures,  FIG. 1  shows a schematic illustration of a simplified example of the Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) system using 1 out of 8 frequencies to represent 3 bits per symbol at a nominal frequency of 10 MHz, and the band spacing between each two successive carriers is 100 KHz. The Mat lab simulation results show the low Bit Error Rate (BER) achieved at high rate and Signal to Noise Ratio (SNR) 
     Table 1 Illustrates Frequency Shift Keying (FSK) modulation techniques where a FSK size column represents the equivalent number of parallel, and separate Frequency Shift Keying (FSK) channels for the communication system. The Mary FSK system column represents the number of frequencies transmitted over the number of orthogonal and locked frequencies available or used. The BW column describes the available BW for each communication system alternative. The efficiency column represents the BW utilization efficiency in bps/Hz, number of bits sent in one second per Hz i.e., number of bits represented by each frequency is the number of bits received each sample and the number of available combinations used to calculate the number of bits that can be represented by a coded frequency message. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Mary FSK 
                   
                   
                   
                 No of Bits 
                   
               
               
                   
                 system 
                   
                 Baud 
                   
                 represented 
                 No of 
               
               
                   
                 No of used 
                   
                 Rate in 
                 Efficiency 
                 by each 
                 available 
               
               
                 FSK Size 
                 Frequencies 
                 BW 
                 Kpbs 
                 Bps/Hz 
                 Frequency 
                 combination 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 FSK 1 
                 1/2 
                 100 KHz 
                 600 
                 6 
                 1 
                 2 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 100 KHz 
                   
                   
                   
                   
                   
                   
               
               
                 FSK 3 
                 1/8 
                 800 KHz 
                 1800 
                 2.25 
                 3 
                 8 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 450 KHz 
                   
                   
                   
                   
                   
                   
               
               
                 FSK 5   
                  1/32 
                  3.2 MHz 
                 3000 
                 0.9375 
                 5 
                 32 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 750 KHz 
                   
                   
                   
                   
                   
                   
               
               
                 FSK 7   
                  1/128 
                 12.8 MHz  
                 4200 
                 0.328 
                 7 
                 128 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 1.05 MHz  
                   
                   
                   
                   
                   
                   
               
               
                 FSK 4.5  
                 2/8 
                 800 KHz 
                 2700 
                 3.375 
                 4.5 
                 28 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 675 KHz 
                   
                   
                   
                   
                   
                   
               
               
                 FSK 5.5  
                 3/8 
                 800 KHz 
                 3300 
                 4.125 
                 5.5 
                 56 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 825 KHz 
                   
                   
                   
                   
                   
                   
               
               
                 FSK 17.5 
                  5/32 
                  3.2 MHz 
                 10.5 MHz 
                 3.28 
                 17.5 
                 201376 
               
               
                 Channel 
                   
                   
                   
                   
                   
                   
               
               
                 2.625 MHz   
               
               
                   
               
             
          
         
       
     
       FIG. 2  shows a 1 out of 8 Coded Multiple Frequency Transceiver (CMFT) system and indicates that, the Bit Error Rate (BER) occurs after bit rate reach approximately 3×600 Kbps. The capacity of the individual Narrow Band Frequency Shift Keying (NBFSK) system is 600 Kbps. 
     For the 1 out of 8 Coded Multiple Frequency Transceiver (CMFT) system, the Bit Error Rate (BER) disappears after the Signal to Noise Ratio (SNR) equals approximately 7.5 dB at bit rate of 1800 Kbps as shown in  FIG. 3 . This Bit Error Rate (BER) curve versus Signal to Noise Ratio (SNR) is dependent on the bit rate at which data is transmitted. The advantage of the system in accordance with the present invention over a multiple binary Narrow Band Frequency Shift Keying (FSK) system is the reduction in hardware and power consumption as a result of a Fast Fourier Transform (FFT) process that is applied to eight carriers in the system at the receiver instead of 3 Fast Fourier Transform (FFT) processes used for 3 binary Frequency Shift Keying (FSK) systems. In addition high data code density is achieved for a given bandwidth. 
     This Coded Multiple Frequency Transceiver (CMFT) system allows the user to transmit data using only 4.8 Hz as a bandwidth with a data rate around 2.4 Mbps with a Bit Error Rate (BER) approximately equal to 0 and with frequency separation of 600 KHz or they can reach a data rate of 4 Mbps with Bit Error Rate (BER) approximately equal to 0 but bandwidth will be increased to 8 MHz while frequency separation is increased to 1 MHz. 
     The 2 of 7 Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) system enhances the code density per Hz over that achieved in the 1 of 8 system. This technique enhances the 1 of 8 Frequency Shift Keying (FSK) system to increase Bit rate without increasing Bit Error Rate (BER) and with a narrower band. This is done by sending two frequencies instead of one frequency where the transmitter and the receiver have a look up table to represent every 4 bits by two frequencies of the 7 frequencies used. 
     Further, at the receiver, the frequencies transmitted are known by obtaining the Power Spectral Density (PSD) from which the receiver determines the 2 frequencies transmitted in a 2 of 7 message. Then from the look up table found in the receiver the data that has been transmitted is determined. 
     As illustrated in  FIG. 4 , A Mat lab code is used to calculate and draw the Bit Error Rate (BER) for different values of Baud Rate. The results of this simulation describe the relation between Bit Error Rate (BER) and Baud Rate for a 2 of 7 system. Also Mat lab code is used to calculate the Bit Error Rate (BER) for different values of the Signal to Noise Ratio (SNR).  FIG. 5  is the result of this simulation on the 2 of 7 system. 
       FIG. 6  shows a transmitter block diagram  100 , including the steps used to manipulate the 2 of 7 Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) wherein, a first block indicates reading data from the file to be transmitted. A second block  104  indicates making the source coding of the data to decrease the amount of data required to be transmitted. Then a channel coding block  106  is provided for channel enhancement. Further, a fourth block  108  is indicative of taking every successive 4 bits and assign 2 frequencies from the available 7 frequencies to represent the 4 digits using a look up table (Not Shown) so that the frequencies can be transmitted. The 2 of 7 code represents more than 4 binary Frequency Shift Keying (FSK) channels. Only 4 Frequency Shift Keying (FSK) channels are represented by the 2 of 7 code, in order to be able to omit codes corresponding to two adjacent frequencies out of available combinations. 
     As shown in the block diagram of the transmitter in  FIG. 7 , data is read (represented by the first block  102 ) and coded by a source code (represented by the second block  104 ) to decrease the amount of data transmitted. The data is then passed onto a channel encoder (represented by the third block  106 ) to enhance Bit Error Rate (BER). Every group of bits is represented by a bigger group of bits in the encoder to have the ability to detect and correct most of the errors that occur during a transmission process. Also in this stage, the calibration sequence (the output of the calibration sequence generator represented by a fourth block ( 110 )) is added to the data transmitted. The calibration sequence corrects the frequency shift which occurs during the transmission process. 
     Further, a fifth block  112  assigns every 10 bit combination for example to 2 out of 32 frequencies by a look up table (Not Shown) stored in the transmitter. The data is then ready for transmission where the technique is based on sending two different frequencies of 32 available frequencies to represent all relations of the 9 bits (as an example the bits 010110101. The 9 bits are represented by the frequencies F3 and F22 where receiving these two frequencies means receiving 010110101). Then the two frequencies are generated and passed over a Fast Fourier Transform (FFT) block (Not Shown) and the output is entered to the power amplifier  114  to be amplified. The output is sent to the antenna  116  for transmission. 
       FIG. 8  shows a receiver block diagram  200  which receives the signal (represented by a block  118 ). A Power Spectral Density (PSD) technique is used to determine the frequencies received and a look up table is used and the receiver allocates the data that has been sent (represented by a second block  120 ). The channel decoding (represented by a third block  122 ) is done to correct errors that may have happened. The source decoding (represented by a fourth block  124 ) by a source decoder is used to obtain the original data and compare with the sent data is done to calculate the Bit Error Rate (BER). 
     A receiver system  126  for use in the present invention is shown in  FIG. 9 . The receiver system  126  receives the data using a heterodyne receiver (represented by a first block  128 ). The output is passed over an Inverse Fast Fourier Transform (IFFT) block to get a Power Spectral Density (PSD) (represented by a second block  130 ) in the base band. The output is connected to the calibration path  132  and used to determine the received calibration sequence and from the calibration memory the frequency offset is determined. 
     The input to the receiver path  134  is passed on to the 2 received frequencies block  136  first to obtain the value of the 2 frequencies received and correct the frequencies using the frequency offset (output of the calibration path). Then, the output is passed over the decode data modulated in message block  138  to obtain the transmitted bits from a look up table (Not Shown) stored on the receiver. The output is passed over both the channel decoder ( 140 ) and source decoder ( 142 ) and then data is stored in memory ( 144 ). 
       FIG. 10  shows a flow chart for the Mat lab code that simulates the Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) system. The transmitter and the receiver generate 7 successive frequencies to read the data. Further, every successive 4 bits represented uses 2 frequencies from the available frequencies using a look up table. The output signal is passed over a Fast Fourier Transform (FFT) to get the Power Spectral Density (PSD). Using the Power Spectral Density (PSD), a program obtains the two center frequencies and, from the look up table the program determines the received data. The data received is compared with the transmitted data to calculate the Bit Error Rate (BER). The program plots the Bit Error Rate (BER) versus Signal to Noise ratio (SNR) and Baud Rate to determine the performance of the system. 
     The 2 of 32 Multiple Orthogonal locked frequencies Narrow Band Frequency Shift Keying (Mary-NBFSK) enhances the 1 of 8 or 1 of n Frequency Shift Keying (FSK) system to increase the Bit Rate without increasing Bit Error Rate (BER) and with a narrow band. This is done by sending two frequencies from 32 frequencies instead of one frequency from 8 frequencies where the transmitter and receiver have a look up table to represent every 6 bits by two frequencies of the 32 available frequencies used. 
     The transmitter reads the data before transmission and finds the 2 corresponding frequencies from the look up table then transmit them. 
     The receiver performs Fast Fourier Transform (FFT) to get the Power Spectral Density (PSD) and determine the 2 frequencies that have been sent as shown in  FIG. 11  and from the look up table it determines the 6 bits of data and stores them. 
     Using this technique will allow users to transmit their data using 3.2 MHz as a band width and by a rate of data up to 6 Mbps with Bit Error Rate (BER) approximately equal to 0 with carrier frequency separation 100 KHz. The benefit is a higher data rate per Hz of bandwidth. 
     A Mat lab code is also used to calculate and draw the Bit Error Rate (BER) for different values of data (Baud) rate  FIG. 12  shows the result of this simulation which describes the relation between Bit Error Rate and Baud rate. 
     A Mat lab code is also used to calculate the Bit Error Rate (BER) for different values of the Signal to Noise ration (SNR).  FIG. 13  illustrates the result of this simulation. 
     A Digital Signal Processing approach at the receiver is the Fast Fourier Transform (FFT) applied to individual pattern samples. In this technique, each pattern is processed alone at the receiver end, by applying the Fast Fourier Transform (FFT) on the bit samples. They are about 166 sample at baud rate 600 Kbps and sampling rate of 100 MSPS. To obtain high accuracy when applying Fast Fourier Transform (FFT), zero-padding is used in the Fast Fourier Transform (FFT). The center frequency is the frequency at the peak value of the Fast Fourier Transform (FFT) resulting spectrum. 
     The Bit Error Rate (BER) output from this third technique is about 0 up to 1600 Kbps, after that a Bit Error Rate (BER) is as illustrated in  FIG. 14 , but this technique was done without adding noise. After adding white Gaussian noise, the Bit Error Rate (BER) depends on the Signal to Noise Ratio, as shown in  FIG. 15 . It is sufficient to use Signal to Noise Ratio (SNR) above 8 dB to prevent Bit-Error occurrence at a bit rate of 1000 Kbps, see  FIG. 16 . As shown in the  FIGS. 16 and 17 , Bit Error Rate (BER) versus both Signal to Noise Ratio (SNR), and Baud Rate is plotted 
     The receiver uses Zero-padding on the received frequency to obtain the decision in the Digital Signal Processor (DSP) system whether the received bit is 0 or 1. The Fast Fourier Transform (FFT) is applied to the received sample padded with 10000−166=9834 zeros. 
     The advantages of the zero-padding techniques are: Fewer numbers of operations compared to the Power Spectral Density (PSD) technique applied to the received message. The Power Spectral Density (PSD) technique calculates the Power Spectral Density (PSD) on the spectrum output of a Fast Fourier Transform (FFT), in order to determine the center frequency. 
     The decision of the current received pattern is independent of the previous received pattern, while in the Power Spectral Density (PSD) the current received bit decision process depends on the previous calculated received bit center frequency which may be affected by noise. Thus the current decision is affected. 
     Also the zero padding technique is memory-less because current bit decision does not depend on the previous received bits. i.e. the Digital Signal Processor (DSP) does not need to store the whole message, as in the Power Spectral Density (PSD) only technique.