Abstract:
A transmitter for a multiple frequency radio communication system provides a capability for filtering spread spectrum signals of a multiple frequency system by using one lowpass filter, and variably controlling the sampling speed when converting the lowpass filtered digital signal into an analog signal. In one embodiment, the transmitter comprises a clock generating section for receiving a predetermined bandwidth selecting control signal and then generating a clock having a speed in proportion to the predetermined bandwidth, a multiplexing section for inserting zero into a signal, spread modulated in a desired bandwidth, the bandwidth being selected from the plurality of frequency bandwidths according to the predetermined bandwidth selecting control signal, to produce an oversampled signal, a lowpass filter for receiving the clock and for lowpass filtering an output signal of the multiplexing section, a digital/analog converter for receiving the clock and then converting an output signal of the lowpass filter into an analog signal at the clock speed, and a switch for receiving the bandwidth selecting control signal and then switching an output of the digital/analog converter to a corresponding one of a plurality of intermediate frequency circuits according to the selected bandwidth.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a radio communication system, and more particularly to a radio communication transmitter providing multiple frequency bandwidths. 
     2. Description of the Related Art 
     Current digital radio communication systems are primarily designed to provide voice service, which can be allocated to one frequency bandwidth since voice service user information is unified and its information rate is fixed. In the next-generation digital radio communication system, however, the user information may vary, comprising data, video and multimedia services, in addition to voice service. Accordingly, in order to efficiently use the limited frequency bands available and expand user capacity, a new multiple frequency bandwidth system capable of efficiently using frequency resources is required, in which one system (e.g. a mobile and fixed radio unit) provides a plurality of frequency bandwidths, so that a narrower bandwidth can be allocated to a service using a lower information rate, and a wider bandwidth can be allocated to a service using a higher information rate. 
     In general, a digital radio communication system must provide multiple frequency bandwidth characteristics to provide data service having varying information and information rates, in addition to voice service. In particular, to provide adequate capacity and variable services in a radio communication system such as a direct sequence code division multiple access (DS-CDMA), it is necessary to provide a multiple frequency bandwidth system. 
     The construction of a transmitter capable of providing multiple bandwidths based on the unitary baseband digital radio communication system in accordance with the prior art is illustrated in FIG. 1. A source coding section  111  compresses sound, data or video information inputted by a user service. Channel coding sections  112  and  113  code the outputs of the source coding section  111  to minimize a bit transferring error during radio communication. Channel coding sections  112  and  113  each use a different channel coding mode, respectively; one channel coding section  112  is a convolutional coder, and the other channel coding section  113  is a turbo coder, the turbo coder being concatenated with a Read-Solomon coder and a convolutional coder. By using two channel coding sections  112 ,  113 , a channel coding mode can be selected based on the user service and the required level of service quality. Accordingly, any one of the channel coding sections  112 ,  113  may be used in accordance with the user service and the required level of service quality. For example, in the case of voice service, channel coding section  112  consisting of only one convolutional coder is used, while in the case where a higher quality data service is required, channel coding section  113  being concatenated with the Read-Solomon coder and the convolutional coder is used. 
     A multiplexer  114  selects the outputs of the channel coding section  112  or the channel coding section  113  by using a control signal of a controller (not shown). A digital modulating section  115  digital modulates the outputs of the multiplexer  114  in accordance with the characteristics of a digital radio communication system. For example, in the case of the DS-CDMA, the digital modulating section  115  carries out a spectrum spread and a data modulation (such as binary or quadrature phase shift keying; BPSK/QPSK). 
     The output of the multiplexer is spread at any one of the specific bandwidths among the multiple bandwidths, depending on the type of user service, in the digital modulating section  115 , and the information related with the spread spectrum is passed through a respective lowpass filter  116 ,  117 ,  118  by a switching section (not shown) in order to improve the efficiency of the bandwidth and reduce the inter symbol interference. In particular, in order to filter the different spread spectrum signals, the multiple bandwidth system utilizes a number of lowpass filters  116 ,  117 ,  118 , with varying frequency bandwidths and operating speeds. Each lowpass filter consists of a digital finite impulse response filter to maximize the efficiency of the frequency bandwidth only, or a pulse shaping digital finite impulse response filter to maximize the efficiency of the frequency bandwidth and reduce the inter symbol interference. A root raised cosine type is most commonly used as the pulse shaping digital finite impulse response filter. 
     A digital/analog converter  119  converts each of the digital signals filtered from the lowpass filters  116 ,  117 ,  118  into analog signals. The digital/analog converter  119  must have a sampling speed capable of converting a signal spread at a maximum bandwidth among multiple bandwidths of the system into an analog signal. A radio circuit section  120  filters the outputs of the digital/analog converter  119  between an intermediate frequency bandwidth and a radio frequency bandwidth and amplifies and transmits the resultant outputs through an antenna. 
     The prior art, system as described above has several disadvantages. First, because a number of digital lowpass filters (or pulse shaping filters) are used to provide multiple frequency bandwidths, additional power is required by the system (a mobile or fixed radio system) and reducing the size of the system becomes more difficult; second, unnecessary additional power is required for fast sampling when sampling the signal spread at other than the maximum bandwidth; and third, the number of digital/analog converters required makes reducing the size of the system more difficult. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention overcomes the disadvantages of the prior art by accomplishing the following two objectives. One object is to provide a transmitter capable of reducing power consumption and thereby reduce the size of a radio communication system providing multiple frequency bandwidths. 
     Another object of the present invention is to provide a transmitter capable of filtering spread spectrum signals of a multiple frequency system by using one lowpass filter, and variably controlling the sampling speed when converting the lowpass filtered digital signal into an analog signal. 
     In order to achieve the above objects, according to the present invention, a transmitter of a multiple frequency radio communication system providing a plurality of frequency bandwidths comprises: a clock generating section for receiving a predetermined bandwidth selecting control signal and then generating a clock having a speed in proportion to the predetermined bandwidth; a multiplexing section for inserting zero into a signal, spread modulated in a bandwidth selected according to the predetermined bandwidth selecting control signal, to produce an oversampled signal; a lowpass filter for receiving the clock and for lowpass filtering an output signal of the multiplexing section; a digital/analog converter for receiving the clock and then converting an output signal of the lowpass filter into an analog signal at the speed of the clock; and a switch for receiving the bandwidth selecting control signal and then switching an output of the digital/analog converter to a corresponding one of a plurality of intermediate frequency circuits according to the selected bandwidth. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating a transmitter of a multiple frequency bandwidth radio communication system in accordance with the prior art; and 
     FIG. 2 is a block diagram illustrating a transmitter of a multiple frequency bandwidth radio communication system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will now be described in greater detail with reference to the accompanying drawings, in which the same or similar elements are denoted by the same reference numerals in different drawings. A detailed description of known functions and configurations will be omitted as not to make the subject matter of the present invention unclear. 
     Referring now to FIG. 2, a source coding section  211  compresses sound, data or video information inputted by auser service. Channel coding sections  212  and  213  code the outputs of the source coding section  211  to minimize a bit transferring error during radio communication. The channel coding sections  212 ,  213  each use a different channel coding mode; one channel coding section  212  is a convolutional coder, and the other channel coding section  213  is a turbo coder, the turbo coder being concatenated with a Read-Solomon coder and a the convolutional coder. By using two channel coding sections  212 , 213 , channel coding mode can be selected depending on the user service and the required level of service quality. For example, in the case of voice service, the channel coding section  212  consisting of only convolutional coder is used, while in the case where a higher quality data service is required, the channel coding section  213  which is concatenated with the Read-Solomon coder and the convolutional coder is used. 
     A multiplexer  214  selects the outputs of the channel coding section  212  or the channel coding section  213  by using a control signal of a controller (not shown). A digital modulating section  215  digitally modulates the outputs of the multiplexer  214  in accordance with the characteristics of the digital radio communication system. For example, in the case of a DS-CDMA communication system, the digital modulating section  215  carries out a spectrum spread and a data modulation (such as QPSK, BPSK, or the like). 
     A bandwidth selecting control signal  217  is a control signal for selecting a specific frequency bandwidth, on which, in a controller (not shown) providing multiple frequency bandwidths, the user information is transmitted. A zero generator  216  is applied with a desired clock and produces zero for oversampling operation of the lowpass filter at the selected frequency bandwidth. 
     A second multiplexer  218  is applied with the outputs of the digital modulating section  215  and the zero generator  216 , and is applied with the same clock as the clock inputted into the zero generator  216 , thereby inserting the zero into the outputs of the digital modulating section  215 . The same clock as the clock inputted into the zero generator  216  and the second multiplexer  218  is applied to a lowpass filter  219 , and the filter  219  filters a low bandwidth from the output of the multiplexer  218 . 
     The clock is also applied to a digital/analog converter  220 , and the converter  220  converts the digital signal, of which the low bandwidth is filtered, into an analog signal. At that time, the digital/analog converter  220  must have a sampling speed capable of converting a signal spread to a selected bandwidth into an analog signal. In accordance with the bandwidth selecting control signal  217 , the output of the digital/analog converter  220  is transferred to the intermediate frequency section  225 ,  226 ,  227  by a switch  222 . The intermediate frequency section  225 , 226 ,  227  filter an intermediate frequency low bandwidth among the inputted signals. 
     A linear power amplifier  228  linearly amplifies the outputs of the intermediate frequency lowpass filters. A radio circuit section  229  mixes the outputs of the linear power amplifier  228  with a suboscillating frequency to produce a radio frequency, filters and transmits the radio frequency through an antenna  230 . A clock generator  223  produces a clock at the widest bandwidth spreading speed of multiple bandwidths provided by the system. The output signal of the clock generator  223  is inputted to a clock divider  224  for producing a clock to support another bandwidth which is related with integer times. 
     A third multiplexer  221  selects any one of the clocks outputted from the clock generator  223  and the clock divider  224  in accordance with the bandwidth selecting control signal  217 , and supplies the zero generator  216 , the second multiplexer  218 , the lowpass filter  219 , and the digital/analog converter  220  with the selected clock. 
     The operation of the transmitter according to the present invention is described hereinafter, however, descriptions for a source coding section  211 , a channel coding section  212 , a channel coding section  213 , a multiplexer  214 , a digital modulating section  215 , a radio circuit section  229 , and an antenna  230 , are omitted since they have already described in the Description of Related Art. The below description is adapted to a multiple bandwidth system supporting n frequency bandwidths B 1 , B 2 , . . . , BN in one system, however, the invention is not limited to systems of this type. 
     In a digital radio communication system, the bandwidths B 1 , B 2 , . . . , BN are allocated as integer multiples of the most narrow bandwidth B 1 . For example, when B 1  is 5M, B 2  corresponds to 10M. Band spreading rates R S1 , R S2 , . . . , R SN  correspond to the above bandwidths B 1 , B 2 , . . . BN, respectively. The band spreading rates are also integer multiples of the lowest spreading rate, R S1 . For example, R S2  corresponds to 2R S1 . In the case of the multiple bandwidth DS-CDMA systems, the digital modulating section  215  carries out the data modulation (such as QPSK, BPSK, or the like) and the spreading modulation. The spreading modulation is spread in a specific bandwidth from among the multiple spread bandwidths in line with the user service or the information rate. The second multiplexer  218  and the zero generator  216  oversamples a signal to be inputted to the digital lowpass filter or the pulse shaping lowpass filter  219 . The oversample of the signal inputted to the lowpass filter can enhance the accuracy of the analog signal converted by the digital/analog converter  220 . 
     The digital lowpass filter  219  (or the pulse shaping lowpass filter) designs tap coefficients and tap orders, which determine an impulse response at time range of the filter in accordance with the required specification (passing bandwidth frequency, base frequency, and power attenuation of the passing and baseband) of the filter, to realize the digital finite filter. Where the system provides multiple frequency bandwidths B 1 , B 2 , . . . , BN, the specification of the lowpass filter  219  is identical to each of the above multiple bandwidths. Accordingly, considering the specification and the complexity of the filter independent of the multiple bandwidths, the tap coefficient and the tap order of the filter should be designed identically. Although the tap coefficient and the tap order of the filter are fixed independent of the bandwidth, the operating speed of the filter should be different than the bandwidth, because the bandwidth is determined depending on the operating speed of the lowpass filter  219 , i.e., the speed of the clock applied to the lowpass filter  219 . 
     The tap coefficient and the tap order of the filter  219  which is designed using integer time functions of the bandwidths B 1 , B 2 , . . . , BN and bandwidth spreading rates R S1 , R S2 , . . . , R SN  are each fixed. The operation of the filter  219  coinciding with the multiple bandwidth spreading rate is described hereinbelow. 
     The zero generator  216  determines the number of zero insertions according to the oversample rate to perform the oversampling operation of the signal inputted to the lowpass filter  219 . After determining the number of zeros to be inserted, the zero generator  216  generates those zeros in a successive fashion. The multiplexer  218  receives the bandwidth selecting control signal along with clocks, so that it inserts the zeros output from the zero generator  216  into the signal output from the digital modulator  215  in a successive fashion, and outputs the resultant signal. In general cases, the oversample rate uses four times the bandwidth spreading rate R S1 , R S2 , . . . , R SN . For example, in the case of oversampling four times, the zero generator  216  sequentially inserts three zeros (0 0 0) between the output bits of the digital modulator  215  through the multiplexer  218  to output the four timed oversample to the lowpass filter  219 . The number of zero insertion, i.e., the oversample rate is controlled by the bandwidth selecting control signal provided from a system controller (not shown), and the bandwidth selecting control signal is also used as a control signal for the clock divider  224  and the switch  222 . 
     The clock generator  223  generates a clock at an oversampling speed of the widest bandwidth spreading rate R S1 , R S2 , . . . , R SN  of the multiple bandwidths provided by the multiple bandwidth DS-CDMA system. The clock divider  224  is another clock signal for supporting another bandwidth being an integer multiple of the output signal of the clock generator  223 . The divided clock speed of the clock divider  224  and the required number of dividers equal the number of multiple bandwidths provided by the multiple bandwidth DS-CDMA system. For example, in the case of a DS-CDMA system providing three frequency bandwidths B 1 , B 2  and B 3  (the maximum bandwidth being B 3 ), the required number of clock dividers  224  is two, and the speed of the outputted clock is determined according to the integer relationship between the bandwidths B 1 , B 2  and B 3  (e.g. B 3  is eight times B 1 , B 2  is four times B 1 ). Accordingly, the output clock of the clock generator  223 , which is used in the widest bandwidth B 3 , is divided by ⅛ for bandwidth B 1 , or ¼ for bandwidth B 2 . The multiplexer  221  receives the bandwidth selecting control signal  217 , and thereby selects one clock signal among the output clocks of the clock generator  223  and the clock divider  224 , and outputs it to the multiplexer  218 , the lowpass filter  219 , the digital/analog converter  220 , and the zero generator  216 . 
     The user information in the bandwidth selected by the bandwidth selecting control signal of the multiple bandwidths is oversampled and inputted into the digital/analog converter  220  through the lowpass filter  219 . In order to operate the digital/analog converter  220 , the clock, which is received from the clock generator  223  when the bandwidth is the widest, or is received from the clock generator  224  when the bandwidth is not the widest, is inputted into the converter  220 . Therefore, the operating speed of the digital/analog converter  220  is not fixed at the maximum clock speed, but is shifted in response to the selected bandwidth. In other words, if the clock speed regarding the selected bandwidth is slow, the clock speed which is applied to the digital/analog converter  220  is slow. Accordingly, it is possible for the digital/analog converter  220  to reduce the power consumed, when compared with the digital/analog conversion at the clock speed corresponding to the maximum bandwidth independent of the bandwidth. The signal outputted from the digital/analog converter  220  is inputted into the switch  222  for switching to an intermediate frequency and a radio frequency circuit in response to the selected bandwidth, because a filter and a circuit of the intermediate frequency and radio frequency bandwidth may be constructed differently from each other depending on the selected bandwidth. The switch  222  is received with the bandwidth selecting control signal, and switches the outputs of the digital/analog converter  220  into the bandwidth intermediate circuit among the bandwidth intermediate frequency section  225 ,  226 ,  227 . The signal filtered through the intermediate frequency filter is amplified through the linear power amplifier  228 , filtered at a radio frequency through the radio circuit section  229 , and transmitted through the antenna  230 . 
     Therefore, the present invention can reduce the power consumed in the baseband lowpass filter and the digital/analog converter. Also, the present invention can provide multiple bandwidths via the lowpass filter and the digital/analog converter. The system&#39;s size can also be decreased as a result. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the invention as described in the appended claims.