Patent Publication Number: US-8989305-B2

Title: Apparatus and method for generating multiband radio frequency signals in maritime wireless communication

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2012-0043841, filed on Apr. 26, 2012, which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to a method and apparatus for generating multiband Radio Frequency (RF) signals in maritime wireless communication and, more particularly, to a method and apparatus for generating multiband RF signals in maritime wireless communication, in which in a maritime wireless communication environment, an RF transmitter compensates for Direct Current (DC) offsets, eliminates image signals, and then generates multiband RF signals. 
     2. Description of the Related Art 
     Maritime communication systems include medium frequency (MF), high frequency (HF) and very high frequency (VHF) band communication equipment and communication equipment using satellites in order to perform worldwide maritime communication. A global maritime distress and safety system (GMDSS) based on a communication system in which pieces of communication equipment having a variety of purposes were integrated with each other was introduced at the end of 1990s. Recently, the International Maritime Organization (IMO) is playing a leading role in establishing an E-navigation strategy that is intended to construct an Intelligent Transportation System (ITS) that can accommodate data and multimedia using a maritime automatic identification system (AIS) communication network. Furthermore, the use of wireless local area (WLAN), World Interoperability for Microwave Access (WiMAX) and Radio Frequency Identification (RFID) technologies, reflecting current radio wave technologies, in harbors or on ships is being considered in order to improve maritime communication, and an AIS using the VHF band in conjunction with satellites attracts attention as a maritime infrastructure communication network. 
     Ships that perform international navigation should be equipped with a variety of types of communication equipment because the ships can communicate with ships on all the seas of the earth. The IMO defined the communication technology and coverage for each of sea areas A1, A2, A3 and A4, as shown in  FIG. 1 . 
     Current maritime communication necessitates a variety of types of communication using a plurality of frequency bands, a plurality of pieces of maritime communication means including a GMSDD are present for the band range from the MF band to the VHF band, and, when an ITS will be constructed in the future, maritime communication is also required for the higher frequency bands such as the Ultra High Frequency (UHF) band the super high frequency (SHF) band (see P. Sharma et al, “High Speed Maritime Mesh Network System for e-Navigation,” E-navigation underway international conference on c-navigation, Feb. 2, 2011.). 
     Accordingly, there is a need for a method and apparatus that can improve conventional technology that uses an independent RF transmitter in each frequency band. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method and apparatus for generating multiband RF signals in maritime wireless communication, in which in a maritime wireless communication environment, an RF transmitter compensates for DC offsets, eliminates image signals, and then generates multiband RF signals so that the multiband RF signals can be sent via two or more frequency bands. 
     In order to accomplish the above object, the present invention provides an apparatus for generating multiband RF signals in maritime wireless communication, including a baseband conversion unit configured to convert RF signals corresponding to a baseband modulation signal having a negative frequency and a baseband modulation signal having a positive frequency into baseband signals, respectively; a phase compensation unit configured to eliminate images of multiband signals so as to correspond to the baseband signals; a DC offset compensation unit configured to compensate the baseband modulation signals for DC offsets using the multiband signals from which the images have been eliminated; and an RF transmission unit configured to generate the RF signals using the signals which have been compensated for the DC offsets and send the RF signals via an antenna. 
     The baseband conversion unit may include RF couplers and down-converters, and convert the RF signals into the baseband signals, respectively, using the RF couplers and the down-converters. 
     The phase compensation unit may eliminate the images of the multiband signals by adjusting the weights of I/Q of RF coupled signals corresponding to the baseband signals. 
     The RF transmission unit may include a negative frequency up-conversion unit configured to up-convert the baseband modulation signal having a negative frequency and a positive frequency up-conversion unit configured to up-convert the baseband modulation signal having a positive frequency; rate matching units configured to perform matching so that results of the conversion of the negative and positive frequency up-conversion units have a same sampling rate; a signal synthesis unit configured to generate positive and negative frequency signals based on outputs of the rate matching unit; and RF mixers configured to convert the positive and negative frequency signals into RF signals. 
     The negative and positive frequency up-conversion units may up-convert the corresponding baseband modulation signals into an intermediate frequency in a digital domain. 
     The baseband modulation signals may correspond to signals that have been digitally modulated using a method such as Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM). 
     In order to accomplish the above object, the present invention provides a method of generating multiband RF signals in maritime wireless communication, comprising, by an apparatus for generating multiband RF signals in maritime wireless communication, converting RF signals corresponding to a baseband modulation signal having a negative frequency and a baseband modulation signal having a positive frequency into baseband signals, respectively; eliminating images of multiband signals so as to correspond to the baseband signals; compensating the baseband modulation signals for DC offsets using the multiband signals from which the images have been eliminated; and generating the RF signals using the signals which have been compensated for the DC offsets and sending the RF signals via an antenna. 
     The converting may include converting the RF signals into the baseband signals, respectively, using the RF couplers and down-converters of the apparatus for generating multiband RF signals. 
     The eliminating may include eliminating the images of the multiband signals by adjusting the weights of I/Q of RF coupled signals corresponding to the baseband signals. 
     The method may further include, before the converting, up-converting the baseband modulation signal having a negative frequency and the baseband modulation signal having a positive frequency; performing matching so that results of the up-converting have a same sampling rate; generating positive and negative frequency signals based on results of the matching; and converting the positive and negative frequency signals into RF signals. 
     The up-converting may include up-converting the baseband modulation signal having a negative frequency and the baseband modulation signal having a positive frequency into an intermediate frequency in a digital domain. 
     The baseband modulation signals may correspond to signals that have been digitally modulated using a method such as QPSK or QAM. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a table illustrating the communication technology and coverage for each sea area that was defined by the IMO; 
         FIG. 2  is a flowchart schematically illustrating a method of generating signals according to an embodiment of the present invention; 
         FIGS. 3 and 4  are reference diagrams illustrating the process of generating basic signals in the method of generating signals according to the embodiment of the present invention; 
         FIGS. 5 to 7  are reference diagrams illustrating the process of defining the frequency responses of basic signals in the method of generating signals according to the embodiment of the present invention; 
         FIG. 8  is a diagram illustrating the configuration of an apparatus for generating multiband RF signals according to an embodiment of the present invention; 
         FIGS. 9 to 12  are diagrams illustrating a single band transmission process according to an embodiment of the present invention; and 
         FIG. 13  is a flowchart illustrating a method of generating multiband RF signals according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail below with reference to the accompanying drawings. Repeated descriptions and descriptions of known functions and constructions which have been deemed to make the gist of the present invention unnecessarily vague will be omitted below. The embodiments of the present invention are provided in order to fully describe the present invention to a person having ordinary knowledge in the art. Accordingly, the shapes, sizes, etc. of elements in the drawings may be exaggerated to make the description clear. 
     A method and apparatus for generating multiband RF signals in maritime wireless communication according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     The method and apparatus for generating multiband RF signals in maritime wireless communication according to the embodiments of the present invention are based on a multiband transmitter (not shown) that generates RF signals in two frequency bands using a general digital-intermediate frequency (IF) transmitter. Here, the digital-IF transmitter is similar to a two-step transmitter, and refers to a transmitter that performs primary step up-conversion into the IF in the digital domain and then secondary step up-conversion into the RF. 
       FIG. 2  is a flowchart schematically illustrating a method of generating signals according to an embodiment of the present invention. Furthermore,  FIGS. 3 and 4  are reference diagrams illustrating the process of generating basic signals in the method of generating signals according to the embodiment of the present invention. 
     Referring to  FIG. 2 , a multiband transmitter (not shown) generates a single signal at any IF in the frequency domain as basic signals S HT   + (f) and S −   HT (f) that have opposite signs at positive and negative frequencies, as shown in  FIGS. 3 and 4 , at step S 11 . 
     Thereafter, the multiband transmitter defines the frequency responses of the basic signals as Equations 1 and 2 at step S 12 . 
     
       
         
           
             
               
                 
                   
                     
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     At step S 13 , the multiband transmitter generates signals in each of which there is a frequency response at either a negative frequency or a positive frequency, as shown in  FIGS. 5 and 6 , when the basic signals whose frequency responses have been defined at step S 12  are added to the frequency response of a source signal. Here, the cases in which the basic signals whose frequency responses have been defined at step S 12  are added to the frequency response of the source signal are represented by Equation 3:
 
 S   + ( f )= S ( f )+ S   HT   + ( f )
 
 S   − ( f )= S ( f )+ S   HT   − ( f )  (3)
 
     At step S 14 , the multiband transmitter up-converts the RF signals, as shown in Equations 4 and 5, in order to send information over the signals in each of which there is a frequency response at either a negative frequency or a positive frequency, as shown in  FIG. 7 .
 
 u   + ( t )= Re[u   1 ( t )] cos ω b   t−Im[u   1 ( t )] sin ω b   t+j{Re[u   1 ( t )] sin ω b   t+Im[u   1 ( t )] cos ω b   t}   (4)
 
 v _( t )= Re[v   1 ( t )] cos ω b   t−Im[v   1 ( t )] sin ω b   t−j{Re[v   1 ( t )] sin ω b   t+Im[v   1 ( t )] cos ω b   t}   (5)
 
     The present invention is directed to an apparatus for generating multiband RF signals based on Equations 4 and 5. The apparatus for generating multiband RF signals compensates for DC offsets that occur in the RF signals up-converted at step S 14 , and eliminates images that occur in the respective bands. 
     Next, an apparatus for generating multiband RF signals according to an embodiment of the present invention will be described in detail with reference to  FIG. 8 . 
       FIG. 8  is a diagram illustrating the configuration of the apparatus for generating multiband RF signals according to the embodiment of the present invention. 
     Referring to  FIG. 8 , the apparatus for generating multiband RF signals includes negative and positive frequency up-conversion units  100   a  and  100   b , rate matching units  200   a  and  200   b , a signal synthesis unit  300 , digital-analog converters (“DACs”)  400   a  and  400   b , RF mixers  500   a  and  500   b , a baseband conversion unit  600 , analog-digital converters (ADCs)  700 , a phase compensation unit  800 , and a DC offset compensation unit  900 . 
     The negative and positive frequency up-conversion units  100   a  and  100   b  primarily up-convert a baseband modulation signal I1, Q1 having a negative frequency and a baseband modulation signal I2, Q2 having a positive frequency into an IF W IF  in the digital domain. Here, the baseband modulation signals that the negative and positive frequency up-conversion units  100   a  and  100   b  receive correspond to signals that have been digitally modulated using a method such as QPSK or QAM. 
     The rate matching units  200   a  and  200   b  perform the process of performing matching so that the baseband modulation signals I1, Q1 and I2, Q2 have the same sampling rate when the baseband modulation signals I1, Q1 and I2, Q2 have different sampling rate sampling rates. Here, the process of performing matching corresponds to a general matching method, and thus a description thereof will be omitted here. 
     The signal synthesis unit  300  generates positive and negative frequency signals by synthesizing the output signals of the rate matching units  200   a  and  200   b.    
     The DACs  400   a  and  400   b  convert the positive and negative frequency signals into analog signals. 
     The RF mixers  500   a  and  500   b  convert the outputs of the DACs  400   a  and  400   b , that is, the analog signals, into RF signals, and output the RF signals to the outside via an antenna. 
     The baseband conversion unit  600  includes RF couplers and down-converters, and converts the results of the conversion of the RF mixers  500   a  and  500   b , that is, the RF signals, into baseband signals. 
     The ADCs  700   a  and  700   b  convert the outputs of the baseband conversion units  600 , that is, the baseband signals, into digital signals. 
     The phase compensation unit  800  eliminates the images of multiband signals by adjusting the weights of I/Q of RF coupled signals corresponding to the outputs of the ADCs  700 , that is, the digital signals. 
     The DC offset compensation unit  900  eliminates portions corresponding to DC offsets from the positive and negative frequency signals using the outputs of the phase compensation unit  800 , that is, the multiband signals from which the images have been eliminated. Thereafter, the DC offset compensation unit  900  inputs the signals from which the portions corresponding to the DC offsets have been eliminated to the DACs  400   a  and  400   b.    
     As described above, the present invention converts two signals so that they correspond to different frequencies, thereby converting two multi-signals using an oscillator that uses only a single frequency so that they correspond to the IF. 
     Furthermore, the present invention has the advantage of allowing a single signal to be present in both the negative and positive frequency areas without up-converting two signals in the negative and positive frequency areas, and the advantage of reducing the sampling rate for each given signal bandwidth and spaced frequency to ½ of that of conventional technology that performs conversion to correspond to different frequencies. 
     Next, a single band transmission process according to an embodiment of the present invention will be described in detail with reference to  FIGS. 9 to 12 . 
       FIGS. 9 to 12  are diagrams illustrating a single band transmission process according to an embodiment of the present invention. 
     For example, in order to send a single baseband modulation signal via a single band, it is necessary to reduce the sampling rate of a baseband signal, such as that shown in  FIG. 9 , into ½ and alternately input I1, Q1 and I2, Q2 samples to the apparatus for generating multiband RF signals. 
     Then the negative and positive frequency up-conversion units  100   a  and  100   b  of the apparatus for generating multiband RF signals generate a baseband modulation signal, such as that shown in  FIG. 9 , into signals, such as 1A of  FIG. 10  and 1B of  FIG. 11 . Here, if the intermediate frequency W IF  is set to a small value so as to correspond to the sampling rate, a signal can be disposed near DC, likes signals such as 1A of  FIG. 10  and 1B of  FIG. 11 . 
     The signals generated as described above, that is, signals such as 1A of  FIG. 10  and 1B of  FIG. 11 , are converted by the DACs  400   a  and  400   b , as shown in  FIG. 12 . 
     That is, when the apparatus for generating multiband RF signals according to the embodiment of the present invention generates a single-band signal, the spectrum of a baseband signal such as that shown in  FIG. 9  is divided into the positive frequency component of  FIG. 10  and the negative frequency component of  FIG. 11 , and, when the signal synthesis unit  300  adds the components to each other, the sampling rate can be reduced to ½ of the bandwidth of a baseband signal such as that shown in  FIG. 9 . 
     Furthermore, when a signal such as that shown in  FIG. 12  is input to the multiband transmitter, there is no DC in the signal such as that shown in  FIG. 12 , and thus the effect of reducing the burden of compensating for DC offsets can be achieved. 
     Next, a method of generating multiband RF signals will be described in detail with reference to  FIG. 13 . 
       FIG. 13  is a flowchart illustrating the method of generating multiband RF signals according to an embodiment of the present invention. 
     Referring to  FIG. 13 , the apparatus for generating multiband RF signals primarily up-converts received baseband modulation signals I1, Q1 and I2, Q2 into an IF W IF  in the digital domain at step S 100 . Here, the received baseband modulation signals correspond to signals that have been digitally modulated using a method such as QPSK and QAM. 
     The apparatus for generating multiband RF signals performs the process of performing matching so that the baseband modulation signals I1, Q1 and I2, Q2 have the same sampling rate when the baseband modulation signals have different sampling rates at step S 200 . 
     The apparatus for generating multiband RF signals generates positive and negative frequency signals by synthesizing the results of the performance of step S 200 , that is, the baseband modulation signals having the same sampling rate, at step S 300 . 
     The apparatus for generating multiband RF signals converts the positive and negative frequency signals into analog signals and converts the analog signals into RF signals at step S 400 . 
     The apparatus for generating multiband RF signals converts the RF signals obtained at step S 400  into baseband signals at step S 500 . 
     The apparatus for generating multiband RF signals converts the baseband signals obtained at step S 500  into digital signals and then eliminates the images of the multiband signals by adjusting the weights of I/Q of RF coupled signals corresponding to the digital signals at step S 600 . 
     At S 700 , the apparatus for generating multiband RF signals eliminates portions corresponding to DC offsets from the multiband signals from which the images have been eliminated at step S 600 . 
     At step S 800 , the apparatus for generating multiband RF signals generates the RF signals of step S 400  using the signals which have been compensated for the DC offsets at step S 700  and outputs the generated RF signals, that is, multiband RF signals, to the outside via an antenna. 
     In the method and apparatus for generating multiband RF signals in maritime wireless communication according to the embodiments of the present invention, an RF transmitter, in a maritime wireless communication environment, compensates for DC offsets, eliminates image signals, and then generates multiband RF signals, thereby achieving the following effects. 
     First, the present invention up-converts a plurality of band signals discontinuously distributed over a plurality of bands into single-IF signals prior to being input to a transmitter, generates signals and then inputs the signals to the transmitter, thereby enabling multiband signals to be sent using only one transmitter. 
     Second, the present invention can reduce the sampling rate which is used to send signals having a given bandwidth to ½ of that of conventional technology when multiband signals are up-converted into the intermediate frequency. 
     Third, the present invention can be applied to a single band transmission process, and divides a spectrum corresponding to a specific signal into a plurality of spectra and then regenerates signals in the single band transmission process, thereby reducing the sampling rate which is used for input to a transmitter to ½. 
     Fourth, the present invention can eliminate the images of multiband signals by adjusting the weights of I/Q of RF coupled signals corresponding to digital signals, and can also compensate for DC offsets. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.