Abstract:
An input unit having a keyboard arrangement allowing efficient input of English words, a mobile terminal having the input unit, and a method for inputting a message using the input unit are provided. The input unit includes a plurality of symbol keys to which a plurality of symbols are sequentially allocated; and at least one selection key for selecting symbols allocated to at least second positions in the symbol keys. The input unit includes the selection key, to reduce the number of selections of keys when alphabet letters are input, thereby reducing the message input time and solving the inconvenience associated with inputting letters of the alphabet via the input unit.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to an application entitled “INPUT UNIT, MOBILE TERMINAL HAVING THE SAME, AND METHOD FOR INPUTTING MESSAGE USING THE SAME,” filed in the Korean Intellectual Property Office on Aug. 16, 2007 and assigned Serial No. 2007-0082363, the contents of which are incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a mobile terminal and a method for performing a function thereof. More particularly, the present invention relates to an input unit having a keyboard arrangement allowing for an efficient input of English words, a mobile terminal having the input unit, and a method for inputting a message using the input unit. 
         [0004]    2. Description of the Related Art 
         [0005]    There has been tremendous growth in the mobile terminal in part because of new technologies and functions that stimulate consumer&#39;s interests. Especially, due to development of new technologies, various applications satisfying the intricate needs of users, as well those providing simple applications are being installed in the mobile terminals. 
         [0006]    Accordingly, a user of a mobile terminal can access voice information, letter information, image information, an MP3 (MPEG layer 3), a game, etc. 
         [0007]    In particular, a mobile terminal sometimes provides a short message service (SMS) for transferring a letter (i.e. text) message at any time, regardless of the operational state of a mobile terminal of a counterpart. The SMS is a wireless call service allowing easy transfer of the intention of a user to a counterpart mobile user at a relatively low cost. These days, there has been a gradual increase in the use of multimedia messaging services for allowing transmission and reception of data of various types of formats such as a voice, a music, a still image, and a moving image, as well as a text messages. 
         [0008]    Meanwhile, since the number of keys provided in an input unit of a mobile phone is limited, a plurality of letters is generally allocated to one key of an alphanumeric keypad. 
         [0009]    For example, as illustrated in  FIG. 1 , in an input unit  120  having a key arrangement of four by three, a plurality of alphabet letters are allocated to keys  121  in the order of the alphabet, respectively. Then, the alphabet letters are allocated to nine keys  121  in rows 1 to 3 among the twelve keys. 
         [0010]    However, in the conventional input unit  120 , since the alphabet letters are arranged simply in the order of the alphabet, there is a limit in efficiently inputting English words through the input unit  120 . That is, it is necessary to select a key  121  to input an alphabet letter allocated to the same key  121  up to a maximum of three times. Furthermore, in order to sequentially input an alphabet letter allocated to the same key  121 , it is necessary to wait for a period of time after inputting an alphabet letter at the foremost position in the key  121  or input the next alphabet letter after pressing a direction key, which is bothersome and time-consuming. Vowels are also not at the first position with the exception of the “A” which makes spelling out words even more time consuming. 
         [0011]    For example, since all of the alphabet letters are allocated to the nine keys  121  in  FIG. 1 , the probability of inputting a predetermined alphabet letter using a predetermined key  121  again after inputting the predetermined alphabet letter using the predetermined key is a high value of one ninth (approximately 11%). Especially, since the alphabet letters are arranged in the order of the alphabet, when alphabet letters of high use frequencies are disposed at rear positions in the keys  121 , the number of selections of the keys  121  may increase further. 
         [0012]    Accordingly, since the input unit  120  having the conventional arrangement of alphabet letters is accompanied by several repeated inputs of keys to input the alphabet letters, it is inconvenient and time-consuming to input an English message. In an attempt to bypass the problem of time consuming keying, text messaging has evolved an abbreviated language of its own. However, not everyone understands the meaning of the abbreviated texts, which can result in a miscommunication or additional time spent by a recipient to decipher an abbreviated message. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention has been made at least in part in view of the above problems, and provides advantages as discussed herein below. Accordingly the present invention provides a mobile terminal and a method for performing a function thereof that allow for efficient input of a symbol allocated to a key. 
         [0014]    The present invention provides a mobile terminal and a method for performing a function thereof that can minimize the number of selections of keys when an alphabet letter is input, thereby speeding up the input time and increasing the ease of use. 
         [0015]    In accordance with an exemplary embodiment of the present invention, there is provided an input unit of a mobile terminal having a plurality of keys, the input unit typically comprising: a plurality of symbol keys to which a plurality of symbols are sequentially allocated; and at least one selection key for selecting symbols allocated to at least second positions in the symbol keys and a mobile terminal having the same. It is preferable that the at least one selection key for selecting symbols allocated to at least second positions in the symbol keys is actuated to select a symbol with a number of keystrokes being less than a position number of said at least second positions. 
         [0016]    In accordance with another exemplary embodiment of the present invention, there is provided a method for inputting a message using an input unit of a mobile terminal, the method typically comprising: a symbol displaying step of displaying a symbol allocated to a selected symbol key at a first position thereof when one of a plurality of symbol keys, to which a plurality of symbols including the alphabet letters are sequentially allocated, is selected; and a symbol changing step of changing the displayed symbol to one of the symbols allocated to at least second positions of the selected symbol key when a selection key is selected. 
         [0017]    In accordance with still another exemplary embodiment of the present invention, there is provided a mobile terminal including an input unit, a display unit, and a control unit. The input unit typically has a plurality of symbol keys to which a plurality of symbols including alphabet letters are sequentially allocated, and at least one selection key for selecting symbols allocated to at least second positions in the symbol keys. The display unit displays a symbol in correspondence to a key selected by the input unit. The control unit displays a symbol allocated to the first position in a selected symbol key when one of the symbol keys is selected and changes the displayed symbol to one of symbols located at least second positions in the selected symbol key when the selection key is selected. 
         [0018]    According to the present invention, the input unit preferably has selection keys for selecting symbols including alphabet letters at the second and third positions in symbol keys, so that a user can input all alphabet letters by two or less selections of keys. 
         [0019]    In particular, since the input unit sequentially allocates alphabet letters in correspondence to use frequencies of the alphabet letters, a user can efficiently input the alphabet letters, minimizing the number of selections of keys. For example, it can be seen through simulation experiments that the input unit according to the present invention enhances the input efficiency of letters by about 40% as compared with a conventional input unit. 
         [0020]    Furthermore, when alphabet letters are allocated in correspondence to the use frequencies thereof in the order of the alphabet, a user familiar with input units having the existing arrangement of the alphabet letters can also use the input unit according to the present invention without an adverse sentiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The above features and advantages of the present invention will be more apparent from the following detailed description in conjunction with the accompanying drawings, in which: 
           [0022]      FIG. 1  is a plan view illustrating an input unit of a conventional mobile terminal; 
           [0023]      FIG. 2  is a block diagram illustrating a mobile phone according to an exemplary embodiment of the present invention; 
           [0024]      FIGS. 3 to 6  are plan views illustrating exemplary input units having various key arrangements that can be employed in the mobile terminal of  FIG. 2 ; 
           [0025]      FIG. 7  is a plan view illustrating an example of the input unit having a key arrangement of four by three as in  FIG. 3 ; 
           [0026]      FIG. 8  is a plan view illustrating another example of the input unit having a key arrangement of four by three as in  FIG. 3 ; 
           [0027]      FIG. 9  is a graph for comparing the input efficiencies of letters in the input units of  FIGS. 1 and 8 ; 
           [0028]      FIG. 10  is a plan view illustrating an example of the input unit having a key arrangement of three by four as in  FIG. 4 ; 
           [0029]      FIG. 11  is a plan view illustrating an example of the input unit having a key arrangement of three by four as in  FIG. 4 ; 
           [0030]      FIG. 12  is a plan view illustrating an example of the input unit having a key arrangement of two by six as in  FIG. 5 ; 
           [0031]      FIG. 13  is a plan view illustrating an example of the input unit having a key arrangement of six by two as in  FIG. 6 ; and 
           [0032]      FIG. 14  is a flow chart illustrating a message inputting method using an input unit of a mobile phone according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Hereinafter, exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. The same reference symbols are used throughout the drawings to refer to the same or like parts. For the purposes of clarity and simplicity, detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring appreciation of the subject matter of the present invention by a person of ordinary skill in the art. Keys arranged in m rows and n columns are expressed by a matrix of m by n (where, m and n are natural integers of at least 2). A row is expressed by R i , a column is expressed by C j , and the respective key is expressed by row i and column j or a ij  (where, i and j are natural integers of 1 to 6). 
         [0034]    As illustrated in  FIG. 2 , a mobile terminal  10  according to an exemplary embodiment of the present invention includes a control unit  11 , an input unit  13 , a storage unit  15 , a wireless communication unit  17 , and a display unit  19 . 
         [0035]    The mobile terminal  10  shown in the exemplary embodiment of the present invention typically comprises a communication terminal capable of transmitting and receiving a message through wireless communication, and may be a mobile phone, a smart phone, a personal digital assistant (PDA), an MP3 player, a portable multimedia player (PMP), and a mobile broadcast receiver such as a digital multimedia broadcasting (DMB) system. 
         [0036]    Still referring to  FIG. 2 , the control unit  11  is a microprocessor performing the overall control operation of the mobile terminal  10 . The control unit  11  controls performance of a message input mode. 
         [0037]    The input unit  13  provides a plurality of keys  12  and  14  for manipulation of the mobile terminal  10  and generates key data in correspondence to a key selected by a user and transfers the key data to the control unit  11 . The input unit  13  includes symbol keys  12  and selection keys  14  that are arranged in a matrix of m by n (where, m and n are natural integers of at least 2). In addition, the input unit  13  may further include function keys such as direction keys and volume keys. Then, a pointing unit such as a keypad and a touch pad and a touch screen can be used as the input unit  13 . 
         [0038]    The symbol keys  12  are keys to which alphabet letters and special symbols are allocated, and a plurality of symbols are sequentially allocated to the symbol keys  12 . The alphabet letters are “A” to “Z” , and the special symbols include at least one of “.”, “,”, and “?”. In the exemplary embodiment of the present invention, two or three symbols are sequentially allocated to the symbol keys  12 . 
         [0039]    Although alphabet letters and special symbols are allocated to the symbol keys  12  as the symbols, numbers, Korean letters, or other letters, for example, may be allocated and displayed. In this case, symbols including the alphabet letters can be selected through selection of an input symbol. 
         [0040]    The selection keys  16 ,  18  are used for selecting symbols allocated to at least second positions in the symbol keys  12 . The selection keys include a first selection key  16  for selecting symbols allocated to the second positions in the symbol keys  12 , and a second selection key  18  for selecting symbols allocated to the third positions in the symbol keys  12 . 
         [0041]    The storage unit  15  typically includes at least one volatile memory device and at least one nonvolatile memory device storing a program necessary for the control of the operation of the mobile terminal  10  and data generated during performance of the program. More particularly, the storage unit  15  also stores a program for executing a message input mode and data generated during performance of the program. 
         [0042]    The wireless communication unit  17  transmits and receives wireless communication signals through communication between terminals. For example, the wireless communication unit  17  modulates a message signal output from the control unit  11 , converts the frequency of the message signal, and transmits a wireless signal through an antenna, or separates a message signal from a wireless signal received through an antenna, converts the frequency of the message signal, and transmits the message signal to the control unit  11 . 
         [0043]    With continued reference to  FIG. 2 , the display unit  19  displays the data stored in the storage unit  15 , including various function menus executed in the mobile terminal  10 , on a screen. The display unit displays a message input screen necessary for execution of a message input mode. A liquid crystal display (LCD) or a touch screen can be used as the display unit  19 . The touch screen performs functions of both a display unit and an input unit. 
         [0044]    More particularly, if one of the symbol keys  12  is selected in the message input mode, the control unit  11  displays a symbol allocated to the first position in the selected symbol key  12  on the display unit  19 . Thereafter, if one of the selection keys  14  is selected, the control unit  11  changes the displayed symbol to a symbol allocated to the at least second position in the selected symbol key  12 , and displays the allocated symbol. Then, if the first selection key  16  is selected, the control unit  11  changes the displayed symbol to a symbol allocated to the second position in the selected symbol key  12 . If the second selection key  18  is selected, the control unit  11  changes the displayed symbol to a symbol allocated to the third position in the selected symbol key  12 . 
         [0045]    Meanwhile, when the input unit  13  includes ten symbol keys  12  and two selection keys  14 , the key arrangement of m by n includes 4 by 3 ( FIG. 3 ), 3 by 4 ( FIG. 4 ), 2 by 6 ( FIG. 5 ), and 6 by 2 ( FIG. 6 ). In  FIGS. 3 to 6 , although “&lt;2 nd &gt;” is displayed on the first selection keys ( 25 ,  35 ,  45 , and  55 ) and “3 rd ” is displayed on the second selection keys  27 ,  37 ,  47 , and  57 , other patterns may or may not be displayed. 
         [0046]    As illustrated in  FIG. 3 , an input unit  20  having the key arrangement of 4 by 3 uses “a 11  to a 33  and a 42 ” as the symbol keys  21  and “a 41  and a 43 ” as the selection keys  23 . Then, a plurality of symbols  29  are sequentially allocated to each of the symbol keys  21 . Numbers of “1 to 9 and 0” are sequentially allocated to “a 11  to a 33  and a 42 ”. In addition, a 41  is used as the first selection key  25 , and a 43  is used as the second selection key  27 . Also, “*” and “&lt;2 nd &gt;” are displayed on the first selection key  25 , and “#” and “&lt;3 rd &gt;” are displayed on the second selection key  27 . 
         [0047]    In the input unit  20  illustrated in  FIG. 3 , although a 41  is used as the first selection key  25  and a 43  is used as the second selection key  27 , a 41  may be used as the second selection key  25  and a 43  may be used as the first selection key  27 . Thus, the location of the selection keys  23  is riot limited to the exemplary embodiment of the present invention and may be determined in different ways, all of which are within the spirit of the invention and the scope of the appended claims. 
         [0048]    As illustrated in  FIG. 4 , the input unit  30  having the key arrangement of 3 by 4 uses “a 14  and a 34 ” as the selection keys  33  and the remaining keys as the symbol keys  31 . Then, a plurality of symbols  39  are sequentially allocated to each of the symbol keys  31 . In this example, numbers of “1 to 9 and 0” are sequentially allocated to “a 11  to a 33  and a 24 ”. Moreover, a 14  is used as the second selection key  37  and a 34  is used as the first selection key  35 . Additionally, “*” and “&lt;2 nd &gt;” are displayed on the first selection key  35  and “#” and “&lt;3 rd &gt;” are displayed on the second selection key  37 . 
         [0049]    In the input unit  30  illustrated in  FIG. 4 , although a 14  is used as the second selection key  37 , and a 34  is used as the first selection key  35 , a 14  may be used as the first selection key and a 34  may be used as the second selection key. The location of the selection keys  33  is not limited to the exemplary embodiment of the present invention and may be determined in different ways that are within the spirit of the invention and the scope of the appended claims. For example, as illustrated in the example in  FIG. 11 , a 31  is used as the first selection key  35  and a 34  is used as the second selection key  37 . 
         [0050]    As illustrated in the example in  FIG. 5 , the input unit  40  may have the key arrangement of 2 by 6, using “a 21  and a 26 ” as the selection keys  43  and the remaining keys as the symbol keys  41 . Then, a plurality of symbols  49  are sequentially allocated to each of the symbol keys  41 . Numbers of “1 to 6” are sequentially allocated to “a 11  to a 16 ” and numbers of “7 to 9 and 0” are sequentially allocated to “a 22  to a 25 ”. a 21  is used as the first selection key  45  and a 26  is used as the second selection key  47 . “*” and “&lt;2 nd &gt;” are displayed on the first selection key  45  and “#” and “&lt;3 rd &gt;” are displayed on the second selection key  47 . 
         [0051]    In the example of input unit  40  illustrated in  FIG. 5 , although a 21  is used as the first selection key  45  and a 26  is used as the second selection key  47 , a 21  may be used as the second selection key and a 26  may be used as the first selection key. The location of the selection keys  43  is not limited to the exemplary embodiment of the present invention and may be determined in different ways that is within the spirit of the invention and the scope of the appended claims. 
         [0052]    As illustrated in  FIG. 6 , the input unit  50  having the key arrangement of 6 by 2 uses “a 61  and a 62 ” as the selection keys  53  and the remaining keys as the symbol keys  51 . Then, a plurality of symbols  59  are sequentially allocated to each of the symbol keys  51 . Numbers of “1 to 9 and 0” are sequentially allocated to “a 11  to a 52 ”. In addition, a 61  is used as the first selection key  55  and a 62  is used as the second selection key  57 ; and “*” and “&lt;2 nd &gt;” are displayed on the first selection key  55  and “#” and “&lt;3 rd &gt;” are displayed on the second selection key  57 . 
         [0053]    In the input unit  50  illustrated in  FIG. 6 , although a 61  is used as the first selection key  55  and a 62  is used as the second selection key  57 , a 61  may be used as the second selection key and a 62  may be used as the first selection key. The location of the selection keys  53  is not limited to the embodiment of the present invention and may be determined in different ways. 
         [0054]    In the input units  10 ,  30 ,  40 , and  50  having twelve keys, when ten keys are used as the symbol keys  21 ,  31 ,  41 , and  51  and three or less symbols  29 ,  39 ,  49 , and  59  are sequentially allocated to the symbol keys  21 ,  31 ,  41 , and  51  respectively, thirty symbols  29 ,  39 ,  49 , and  59  can be allocated. The symbols  29 ,  39 ,  49 , and  59  allocated to the symbol keys  21 ,  31 ,  41 , and  51  can include twenty six alphabet letters and four or less special symbols. 
         [0055]    When a plurality of alphabet letters are allocated to the ten symbol keys  21 ,  31 ,  41 , and  51 , at least one of the following elements can be reflected to allocate the alphabet letters. Then, the reflected elements include a random order, the order of the alphabet letters, the use frequencies of the alphabet letters, and the input efficiency of hand manipulation. 
         [0056]    More particularly, “a random order” means that a plurality of alphabet letters are allocated to the symbol keys  21 ,  31 ,  41 , and  51  at random. Also, “the order of the alphabet letters” means that a plurality of alphabet letters are allocated to the symbol keys  21 ,  31 ,  41 , and  51  in the order of the alphabet, and “the use frequencies of the alphabet letters” means that a plurality of alphabet letters are allocated to the symbol keys  21 ,  31 ,  41 , and  51  according to the use frequencies of the alphabet letters. Additionally, “the input efficiency of hand manipulation” means that the symbols are allocated so that the symbol keys  21 ,  31 ,  41 , and  51  and the selection keys  23 ,  33 ,  43 , and  53  can be selected by one hand or both hands. 
         [0057]    For example, a method using the input efficiency of hand manipulation includes allocating the alphabet letters so that a symbol key  21 ,  31 ,  41 , and  51  is selected with one hand of a user and a selection key  23 ,  33 ,  43 , and  53  for selecting the at least second symbol key  21 ,  31 ,  41 , and  51  is selected with the other hand of the user. 
         [0058]    Comparison results about the number of selections of keys during inputs of alphabet letters in an input unit to which alphabet letters are allocated at random without using a selection key as in the conventional technology and an input unit to which alphabet letter are allocated at random and which has at least one selection key are as follows. 
         [0059]    When three symbols are sequentially allocated to a predetermined symbol, the probabilities of selecting the symbols allocated to the first to third positions in the key are one third respectively. 
         [0060]    Also, in the prior art, when the symbol allocated to the first position in the key is to be input, one selection of the key is necessary, when the symbol allocated to the second position in the key is to be input, two selections of the key are necessary, and when the symbol allocated to the third position in the key is to be input, three selections of the key are necessary. Accordingly, in the case of the conventional input unit, as expressed in Formula 1, two selections of a key is arithmetically necessary to input an arbitrary symbol. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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                       × 
                       1 
                     
                     + 
                     
                       
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                       2 
                     
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                         1 
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                       × 
                       3 
                     
                   
                   = 
                   2 
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
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                   1 
                 
               
             
           
         
       
     
         [0061]    On the other hand, in the case of the input unit of the present invention, when the symbol allocated to the first position in the key is to be input, one selection of the key is necessary, and when the symbols allocated to the second and third positions in the key are to be input, two key selections are necessary. In other words, according to the present invention, the inputs of the symbols allocated to the second and third positions in the key are achieved through selections of a symbol key and a selection key. 
         [0062]    Accordingly, in the input unit of the present invention, as expressed in Formula 2, according to the present invention 1.67 selections of keys are necessary to input an arbitrary symbol. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         1 
                         3 
                       
                       × 
                       1 
                     
                     + 
                     
                       
                         1 
                         3 
                       
                       × 
                       2 
                     
                     + 
                     
                       
                         1 
                         3 
                       
                       × 
                       2 
                     
                   
                   = 
                   
                     
                       5 
                       3 
                     
                     = 
                     1.67 
                   
                 
               
               
                 
                   Formula 
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         [0063]    It can be easily seen that the number of selections of keys in the present invention can be reduced as compared with that of the conventional input unit even if the alphabet letters are allocated to the symbol keys at random. This effect basically can be achieved through employment of the selection keys in the input unit of the present invention. 
         [0064]    Further, it can be easily expected that when a plurality of alphabet letters are allocated to ten symbol keys, considering the use frequencies of the alphabet letters and the input efficiency of hand manipulation, the number of key selections can be reduced further as compared with the conventional input unit. Especially, when the alphabet letters are allocated in correspondence to the use frequencies thereof, ten alphabet letters of the first ten highest use frequencies are allocated to the first positions in the symbol keys one by one. The remaining alphabet letters are allocated to the second and third positions of the symbol keys. 
         [0065]    The use frequencies of the alphabet letters may be rather different in the field of literature, religion, politics, society, and science. For example, the use frequencies of the alphabet letters used in “The Cambridge Encyclopedia of the English Language-David Crystal (1995)” is in the order of “E, A, T, I, N, O, R, S, L, H, D, C, M, U, F, P, G, B, Y, W, V, K, X, J, Z, and Q” from the first highest use frequency to the last. However, as mentioned above, they may be rather different according to the fields. 
         [0066]    In other words, in the field of literature, the alphabet letters of the first to fifth highest use frequencies are “C, a, i, t, and n”, in the field of religion, “e, t, i, a, and n”, and in the field of politics, “e, t, i, a, and n”. 
         [0067]    Furthermore, the use frequencies of the alphabet letters used in “The Concise Oxford Dictionary (9 th  edition, 1995), http:www.askoxford.com/asktheexperts/faq/aboutwords/freqency” is in the order of “E, A, R, I, O, T, N, S, L, C, U, D, P, M, H, G, B, F, Y, W, K, V, X, Z, J, and Q”. 
         [0068]    As mentioned above, since the use frequencies of the alphabet letters can be rather different according to offices announcing them or fields, the use frequencies of the alphabet letters disclosed in “The Cambridge Encyclopedia of the English Language-David Crystal (1995)” are used as an example in the exemplary embodiment of the present invention. 
         [0069]    Hereinafter, the input units  20   a ,  20   b ,  30   a ,  30   b ,  40 , and  50  according to the exemplary embodiments of the present invention will be described in detail with reference to  FIGS. 7 to 13 . 
         [0070]      FIGS. 7 and 8  illustrate the input units  20   a  and  20   b  having a key arrangement of 4 by 3 according to the exemplary embodiments of the present invention. 
         [0071]    Referring to  FIG. 7 , in the input unit  20   a , alphabet letters are allocated to symbol keys  21  in the order of the alphabet. That is, “.”, “Q”, and “Z” are sequentially allocated to a 11 , and the remaining alphabet letters are allocated to a 12  to a 33  in the order of the alphabet by three alphabet letters per key. Then, the first selection key  25  is disposed at a 41  and the second selection key  27  is disposed at a 43 . 
         [0072]    Referring to  FIG. 8 , in the input unit  20   b , alphabet letters are allocated to the symbol keys  21 , considering all of the use frequencies of the alphabet letters, the input efficiency of hand manipulation, and the order of the alphabet. 
         [0073]    Ten alphabet letters (E, A, T, I, N, O, R, S, L, and H) of the first ten highest use frequencies are allocated to the first positions in the symbol keys  21  one by one. The alphabet letters of the first ten highest use frequencies may be allocated to the symbol keys  21  in the order of the alphabet. That is, the alphabet letters of the first ten highest use frequencies are allocated to the symbol keys  21  in the order of the alphabet in correspondence to the orders corresponding to the numbers allocated to the symbol keys  21 . For example, the ten alphabet letters of the first ten highest use frequencies are allocated to “a 11  to a 33  and a 42 ” in the order of “A, E, B, I, L, N, O, R, S, and T” respectively. 
         [0074]    Although the ten alphabet letters of the first ten highest use frequencies are allocated to the symbol keys  21  in the order of the alphabet, they may be allocated at random or in the order of use frequencies. 
         [0075]    The remaining alphabet letters may be allocated to the second or third positions in the symbol keys  21 , and may be allocated in the following way. 
         [0076]    The six alphabet letters D, C, M, U, F, and P of the second six highest use frequencies are allocated to one of the second or third positions of the symbol keys  21  in column 1 C 1  and the symbol kcys  21  in column 3 C 3  by three alphabet letters per column respectively. Then, since the first selection key 25 is disposed at a 41  and the second selection key  27  is disposed at a 43 , three of the alphabet letters of the second six highest use frequencies may be allocated to the third positions in the symbol keys  21  in column 1 C 1  and the remaining alphabet letters of the second six highest use frequencies may be allocated to the second positions in the symbol keys  21  column 3 C 3 . The alphabet letters of the second six highest use frequencies may be allocated to the symbol keys  21  in the order of the alphabet letters. For example, “C, F, and P” are allocated to the third positions in the symbol keys  21  in column 1 C 1  in the order thereof and “D, M, and U” are allocated to the second positions in column 3 C 3 . 
         [0077]    On the other hand, when the first selection key is located at a 43  and the second selection key is located at a 41 , three of the alphabet letters of the second six highest use frequencies may be allocated to the symbol keys in row 1 and the other three of them may be allocated to the symbol keys in row 3. 
         [0078]    Six alphabet letters G, B, Y, W, V, and K of the third six highest use frequencies are allocated to the second and third positions in the symbol keys  21  in column C 2 . Then, the alphabet letters of the third six highest use frequencies may be allocated to the second and third positions in the symbol keys  21  in column 2 C 2  by three alphabet letters per position respectively. 
         [0079]    Further, the alphabet letters of the third six highest use frequencies may be allocated to the symbol keys  21  in column 2 C 2  in the order of the alphabet. For example, “B, K, and W” are allocated to “a 12,  a 22 , and a 32 ” in the order thereof, and “G, V, and Y” are allocated to “a 12  a 22 , and a 32 ” in the order thereof. 
         [0080]    Four alphabet letters of the fourth four highest use frequencies may be allocated to the unallocated positions in the symbol keys  21  in column 1 C 1  and column 3 C 3  to which the alphabet letters of the second six highest use frequencies are allocated. The alphabet letters of the fourth four highest use frequencies may be allocated to the symbol keys  21  in the order of the alphabet. For example, “J and X” are allocated to the second positions of a 11  and a 21  in the order thereof and “Q and Z” are allocated to the third positions of a 13  and a 23  in the order thereof. 
         [0081]    Special symbols may be allocated to the unallocated positions in the symbol keys  21  that remain after the alphabet letters are allocated. For example, “.” is allocated to the second position of a 13 , “?”, the second position of a 42 , and “,”, the third position of a 33 . 
         [0082]    Here, the alphabet letters of the first to fourth highest use frequencies are classified by designating the numbers of alphabet letters in correspondence to the use frequencies thereof and may be classified differently according to a key arrangement in the input unit. 
         [0083]    The comparison result of  FIG. 9  was obtained through simulation experiments about the numbers of selections of keys of the input unit  20   b  having the arrangement of alphabet letters of  FIG. 8  and the input unit of  FIG. 1  during inputs of English words. 
         [0084]    In the results based on the simulation experiments shown in  FIG. 9 , material Nos. 1 to 7 are used as materials for English words. According to the simulation experiments, the conventional input unit  120  required approximately two selections of keys in order to input an arbitrary symbol. 
         [0085]    On the other hand, the input unit  20   b  according to the embodiment of the present invention required approximately 1.2 selections of keys in order to input an arbitrary symbol. Accordingly, it can be seen that the input unit  20   b  according to the embodiment of the present invention improves the input efficiency by approximately 40 percent as compared with the conventional input unit  120 . 
         [0086]    Material Nos. 1 to 7 were arbitrarily selected from various fields such as news, articles, medical science, history, literature, and humor, including situation comedy, and a sufficient amount of materials to be statistically important were selected as the objects of the materials. 
         [0087]    (1) Material No. 1: the entire script of “Friends Season 1” (257,089 letters) 
         [0088]    (2) Material No. 2: arbitrary excerpts format CNN news articles (129,731 letters) 
         [0089]    (3) Material No. 3: “Manual of Surgery by Alexander Miles and Alexis Thomson” (977,114 letters) 
         [0090]    (4) Material No. 4: “History of the United States by Charles A. Beard and Mary R. Beard” (1,025,076 letters) 
         [0091]    (5) Material No. 5: “Fifteen Thousand Useful Phrases by Greenville Kleiser” (438,257 letters) 
         [0092]    (6) Material No. 6: “Jokes for All Occasions Selected and Edited by One of America&#39;s Foremost Public Speakers” (316,603 letters) 
         [0093]    (7) Material No. 7: “Pride and Prejudice by Jane Austen” (547,495 letters) 
         [0094]    More particularly, comparison results of the numbers of selections during input of an English word in the input unit  20   b  of  FIG. 8  and the input unit  120  of  FIG. 1  are as follows. Here, an English word that is to be input is assumed to be “access”. 
         [0095]    In the following description, “number” represents the number displayed on the symbol keys and “&gt;” represents the right direction key. 
         [0096]    In the input unit  120  of  FIG. 1 , a total of eighteen inputs of keys of “2&gt;222&gt;22233777&gt;777” are necessary. On the other hand, in the input unit  20   b  of  FIG. 8 , a total of eight inputs of keys of “11#1#299” are necessary. Accordingly, it can be easily seen that the input unit  20   b  according to the present invention can reduce the number of selections of keys as compared with the conventional input unit  120 . 
         [0097]      FIGS. 10 and 11  illustrate input units  30   a  and  30   b  having a key arrangement of three by four according to exemplary embodiments of the present invention. 
         [0098]    Referring to  FIG. 10 , in the input unit  30   a , the alphabet letters are allocated to symbol keys  31 , considering all of the use frequencies of the alphabet letters, the input efficiency of hand manipulation, and the order of the alphabet. Especially, considering the symbols allocated to the input unit  30   a , the symbol keys  21  and  23  in row 4 R 4  of the input unit  20   b  of  FIG. 8  are moved to the keys  31  and  33  of row 4 R 4 . The symbols are allocated to the symbol keys  31  in columns 1 to 3 C 1 ,C 2 , and C 3  as in the symbol keys  21  in columns 1 to 3 C 1 ,C 2 , and C 3  of the input unit  20   b  of  FIG. 8 . 
         [0099]    The first and second selection keys  35  and  37  are on the right side of the input unit  30   a  according to the embodiment of the present invention, in which case the first and second selection keys  35  and  37  can be easily manipulated with the right hand of a user. 
         [0100]    Although the selection keys  33  are disposed in column C 4  in  FIG. 10 , the present invention is not limited thereto. That is, as illustrated in  FIG. 11 , the selection keys  33  may be disposed on both sides of row 3 R 3 . 
         [0101]    Referring to  FIG. 11 , the input unit  30   b  uses a 31  and a 34  as the selection keys  33  and uses the remaining keys as the symbol keys  31 . Here, a plurality of symbols is sequentially allocated to the symbol keys  31 . The numbers of “1 to 9 and 0” are sequentially allocated to “a 11  to a 24 , a 32  and a 33 ” respectively. In addition, a 31  is used as the first selection key and a 34  is used as the second selection key  37 . 
         [0102]    Especially, considering all of the use frequencies of the alphabet letters, the input efficiency of hand manipulation, and the order of the alphabet, a plurality of symbols may be sequentially allocated the symbol keys  31  in the following way. 
         [0103]    Ten alphabet letters E, A, T, I, N, O, R, S, L, and H of the first ten highest use frequencies are allocated to the first positions in the symbol keys  31  one by one. The alphabet letters of the first ten highest use frequencies may be allocated to the symbol keys  31  in the order of the alphabet. That is, the alphabet letters of the first ten highest use frequencies are allocated to the symbol keys  31  in the order of the alphabet in correspondence to the numbers allocated to the symbol keys  31 . For example, “A, E, H, I, L, N, O, R, S, and T” are allocated to “a 11  to a24, a 32 , and a 33 ” in the order thereof respectively. 
         [0104]    Ten alphabet letters D, C, M, U, F, P, G, B, Y, and W of the second ten highest use frequencies are allocated to the symbol keys  31  in columns 1 and 2 C 1 , and C 2  and the symbol keys in columns 3 and 4 C 3  and C 4  by five alphabet letters, respectively. Then, since the first selection key  35  is located at a 31 , the five alphabet letters of the second ten highest use frequencies are allocated to the second positions in the symbol keys  31  in columns 3 and 4 C 3  and C 4 . Further, since the first selection key  37  is located at a 34 , the remaining five alphabet letters of the second ten highest use frequencies are allocated to the third positions in the symbol keys  31  in columns 1 and 2 C 1  and C 2 . The alphabet letters of the second ten highest use frequencies may be allocated to the symbol keys  31  in the order of the alphabet. 
         [0105]    For example, “B and G” are sequentially allocated to the third positions in the symbol keys  31  in column 1 C 1 , “C, M, and W”, to the third positions in the symbol keys  31  in column 2 C 2 , “D, P, and Y”, to the second positions in the symbol keys  31  in column 3 C 3 , and “F and W”, to the second positions in the symbol keys  31  in column 4 C 4 . 
         [0106]    On the other hand, when the first selection key is located at a 34 , five alphabet letters of the second ten highest use frequencies may be allocated to the second positions in the symbol keys in columns 1 and 2. When the second selection key is located at a 31 , the remaining alphabet letters of the second ten highest use frequencies may be allocated to the third positions in the symbol keys in columns 3 and 4. 
         [0107]    The remaining six alphabet letters V, K, X, J, Z, and Q of the third six highest use frequencies are allocated to the unallocated positions in the symbol keys  31 . The alphabet letters of the third six highest use frequencies may be allocated to the symbol keys  31  in the order of the alphabet. For example, “J and K” are allocated to the unallocated second positions of a 11  and a 22 , “Q and V”, to the unallocated third positions of a 13  and a 14 , and “X and Z”, to the unallocated second positions of a 21  and a 22 . 
         [0108]    Special symbols may be allocated to the positions of the symbol keys  31  that are not allocated even after all the alphabet letters are allocated. For example, “.” is allocated to the second position of a 32 , “?”, to the third position of a 33 , and “,”, to the second position of a 23 . 
         [0109]    As illustrated in the example of  FIG. 12 , alphabet letters are allocated to symbol keys  41  in an input unit  40  having an arrangement of keys of two by six according to an embodiment of the present invention, considering the use frequencies of the alphabet letters, the input efficiency of hand manipulation, and the order of the alphabet. 
         [0110]    Ten alphabet letters E, A, T, I, N, O, R, S, L, and H of the first ten highest use frequencies are allocated to the first positions in the symbol keys 41 one by one. The alphabet letters of the first ten highest use frequencies are allocated to the symbol keys  41  in the order of the alphabet. That is, the alphabet letters of the first ten highest use frequencies are allocated to the symbol keys  41  in the order of the alphabet in correspondence to the numbers allocated to the symbol keys  41 . For example, “A, E, H, I, L, N, O, R, S, and T” are allocated to “a 11  to a 16  and a 22  to a 25 ” one by one in the order thereof. 
         [0111]    Ten alphabet letters D, C, M, U, F, P, G, B, Y, and W of the second ten highest use frequencies are allocated to the symbol keys  41  in columns 1 to 3 C 1 , C 2 , and C 3  and the symbol keys  41  in columns 4 to 6 C 4 , C 5 , and C 6  by five alphabet letters, respectively. Then, since the first selection key  45  is located at a 21 , the five alphabet letters of the second ten highest use frequencies are allocated to the second positions in the symbol keys  41  in columns 4 to 6 C 4 , C 5 , and C 6 . 
         [0112]    Furthermore, since the second selection key  47  is located at a 26 , the remaining five alphabet letters of the second ten highest use frequencies are allocated to the third positions in the symbol keys  41  in columns 1 to 3 C 1 , C 2 , and C 3 . The alphabet letters of the second ten highest use frequencies may be allocated to the symbol keys  41  in the order of the alphabet. For example, “B, C, D, P, and U” are allocated to “a 11 , a 12 , a 13 , a 22 , and a 23 ” in the order thereof one by one. “F, G, M, W, and Y” are allocated to “a 14 , a 15 , a 16 , a 24 , and a 25 ” in the order thereof one by one. 
         [0113]    On the other hand, when the first selection key is located at a 26 , five alphabet letters of the second ten highest use frequencies may be allocated to the second positions in the symbol keys  41  in columns 1 to 3. When the second selection key is located at a 21 , the remaining alphabet letters of the second ten highest use frequencies may be allocated to the third positions in the symbol keys  41  in columns 4 to 6. 
         [0114]    The remaining six alphabet letters V, K, X, J, Z, and Q of the third six highest use frequencies are allocated to the unallocated positions in the symbol keys  41 . Then, the alphabet letters of the third six highest use frequencies may be allocated to the symbol keys  41  in the order of the alphabet. For example, “J, K, Q, V, X, and Z” are allocated to the unallocated second positions of a 11  and a 16  in the order thereof one by one. 
         [0115]    Special symbols may be allocated to the positions of the symbol keys  41  that are not allocated even after all the alphabet letters are allocated. For example, “.” is allocated to the second position of a 22 , “?”, to the second position of a 23 , and “,”, to the second position of a 25 . 
         [0116]    As illustrated in  FIG. 13 , alphabet letters are allocated to symbol keys  51  in an input unit  50  having an arrangement of keys of six by two according to an exemplary embodiment of the present invention, considering the use frequencies of the alphabet letters, the input efficiency of hand manipulation, and the order of the alphabet. 
         [0117]    Ten alphabet letters E, A, T, I, N, O, R, S, L, and H of the first ten highest use frequencies are allocated to the first positions in the symbol keys  51  one by one, and are allocated in the same way as the allocation manner of  FIG. 3  in which case detailed description of the allocation method according to the exemplary embodiment of the present invention will be omitted. 
         [0118]    Ten alphabet letters D, C, M, U, F, P, G, B, Y, and W of the second ten highest use frequencies are allocated to the symbol keys  51  in columns 1 and 2 C 1  and C 2  by five alphabet letters per column, respectively. Then, since the first selection key  55  is located at a 61 , the five alphabet letters of the second ten highest use frequencies are allocated to the second positions in the symbol keys  51  in column 2 C 2 . Further, since the second selection key  57  is located at a 62 , the remaining five alphabet letters of the second ten highest use frequencies are allocated to the third positions in the symbol keys  51  in column 1 C 1 . The alphabet letters of the second ten highest use frequencies may be allocated to the symbol keys  51  in the order of the alphabet. For example, “B, D, G, P, and W” are allocated to “a 11 , a 22 , a 31 , a 41 , and a 51 ” in the order thereof one by one. “C, F, M, U, and Y” are allocated to “a 12 , a 22 , a 32 , a 42 , and a 52 ” in the order thereof one by one. 
         [0119]    On the other hand, when the first selection key is located at a 62 , five alphabet letters of the second ten highest use frequencies may be allocated to the second positions in the symbol keys  51  in column 1. When the second selection key is located at a 61 , the remaining alphabet letters of the second ten highest use frequencies may be allocated to the third positions in the symbol keys  51  in column 2. 
         [0120]    The remaining six alphabet letters V, K, X, J, Z, and Q of the third six highest use frequencies are allocated to the unallocated positions in the symbol keys  51 . Then, the alphabet letters of the third six highest use frequencies may be allocated to the symbol keys  51  in the order of the alphabet. For example, “J, K, Q, V, X, and Z” are allocated to all and a 32  in the order thereof one by one. 
         [0121]    Special symbols may be allocated to the positions of the symbol keys  51  that are not allocated even after all the alphabet letters are allocated. For example, “.” is allocated to the second position of a 41 , “,”, to the third position of a 42 , and “?”, to the second position of a 51 . 
         [0122]    Hereinafter, a method for inputting a message using the input unit  13  of the mobile terminal  10  according to an exemplary embodiment of the present invention will be described in detail with reference to  FIGS. 1 to 13 . 
         [0123]    Now referring to the flowchart of  FIG. 13 , if a message input mode is selected in correspondence to selection of a key by a user (S 71 ), the control unit  11  executes the message input mode and displays a message input screen on the display unit  19  (S 73 ). A user can select input symbols necessary for input of a message through selection of keys. The input symbols that can be selected by a user include alphabet letters, numbers, and special symbols. In the exemplary embodiment of the present invention, alphabet letters are selected as the input symbols for inputting an English word. 
         [0124]    Thereafter, if one of the plurality of symbol keys  12  in the input unit  13  is selected through input of a key by a user (S 77 ), an alphabet letter allocated to the first position in the selected symbol key  12  is displayed (S 79 ). 
         [0125]    Thereafter, the control unit  11  determines whether a symbol key  12  or one of selection keys  14  is selected (S 81 ). After the determination in the step S 81 , if a key is determined to be selected, the control unit  11  determines the type of the selected key (S 83 ). The selected key may be one of the symbol key  12 , the first selection key  16 , and the second selection key  18 . 
         [0126]    If the first selection key  16  is determined to be selected in the step S 83 , the control unit  11  changes the alphabet letter displayed in the step S 79  to the second alphabet letter in the symbol key  12  selected in the step S 77  and displays the changed alphabet letter (S 85 ). 
         [0127]    If the second selection key  18  is determined to be selected in the step S 83 , the control unit  11  changes the alphabet letter displayed in the step S 79  to the third alphabet letter in the symbol key  12  selected in the step S 77  (S 85 ). 
         [0128]    Further, if the symbol key  12  is determined to be selected in the step S 83 , the control unit  11  displays the first alphabet letter in the selected symbol key  12  again next to the alphabet letter displayed in the step of S 79 . 
         [0129]    After performing the steps S 85  and S 87 , the control unit  11  performs the step S 81  again. If none of the symbol key  12  and the selection keys  14  is determined to be selected in the step S 81 , the control unit  11  determines whether the message input mode is to be completed (S 89 ). The control unit  11  can repeat the steps S 79  to S 87  until the message input mode is completed. 
         [0130]    Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be understood that many variations and modifications of the basic inventive concept herein described, which may appear to those skilled in the art, will still fall within the spirit and scope of the exemplary embodiments of the present invention as defined in the appended claims. For example, although input units having ten symbol keys and two selection keys are disclosed in the exemplary embodiments of the present invention, the number of symbol keys may be either more than or less than ten.