Abstract:
A baseband signal demodulating apparatus in a mobile radio communication system capable of solving the channel increase problem due to the increase of subscribers by sharing correlators and accumulators used in a base station receiver by a time division multiplexing method. The apparatus includes a plurality of front end rake receivers for demodulating oversampled baseband signals to restore the original signal, buses for sharing demodulated signals from the front end rake receivers by the time division multiplexing method, and a despreding section for despreding the signals from the buses.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a base station receiving system in a mobile radio communication system. In particular, the present invention relates to a baseband signal demodulating apparatus and method in a code division multiple access (CDMA) type mobile radio communication system which can share a CDMA signal processor by a time division multiplexing method by constructing the processor for processing input multichannel signals with the minimum number of elements. 
     2. Description of the Related Art 
     Generally, in processing a transmitted/received signal in a base station transmitting/receiving system of a mobile radio communication system, its frequency band may be divided into a baseband, an intermediate frequency band, and a radio frequency band. The baseband is a frequency band occupied by the original information signal which is not modulated. Accordingly, the baseband is a low frequency band which is nearly zero or which includes a direct current (DC) component. The intermediate frequency band is a frequency band wherein the baseband frequency to be transmitted to the outside is heightened by adding thereto a local oscillating frequency, while the radio frequency received from the outside is lowered by subtracting therefrom the local oscillating frequency, by means of a frequency converter provided in the base station. The radio frequency band is a frequency band which is used for radio communications, and generally means the frequencies in the electromagnetic spectrum range of 10 KHz˜300 GHz. 
     Generally, in the CDMA type mobile radio communication system, the respective base station transmitting/receiving system includes a CDMA baseband demodulating apparatus which demodulates a CDMA baseband signal transmitted from a mobile unit to a radio frequency (RF) signal to transmit the radio frequency signal. 
     Such a CDMA baseband demodulating apparatus is disclosed in U.S. Pat. No. 5,654,979 issued to Levin. According to U.S. Pat. No. 5,654,979, the conventional CDMA baseband demodulating apparatus includes an analog-to-digital (A/D) converter for converting an input analog RF signal into a digital signal and repeatedly oversampling the digital signal for a predetermined period, a plurality of rake receivers for processing multipath signals outputted through the A/D converter to produce a single signal and producing an early signal and a late signal, which are respectively earlier and later than an on-time signal, based on the on-time signal determined by selecting one among the oversampled signals that is synchronized with transmitted PN sequence most superior intensity, a plurality of correlators for obtaining correlation values of the on-time signal, early signal, and late signal outputted from the front end of rake receivers in accordance with pseudo-noise codes generated by a pseudo-noise code generator, and an accumulator for accumulating output signals of the correlators. 
     According to the conventional CDMA baseband demodulating apparatus as described above, baseband signal from analog receiver which converts analog RF signal into a digital baseband signal is oversampled 8 times per 1 PN sequence, called “chip” in the A/D convertor  90 . Thereafter, to maintain the synchronization of PN sequence and the received baseband signals, the early signal and late signal, which are earlier and later than the on-time signal by the predetermined number of samples, respectively, are determined based on the on-time signal which is a signal synchronized with the pseudo-noise code. 
     At this time, if each front end rake receiver, which processes signals through one channel, is normally provided with 4 finger sections. The transmitted signal may be received through a direct path, while it may be received through the multipaths caused by reflection or refraction of the signal. The reason why 4 finger sections are employed is to receive multiple of the transmitted signals through the direct and multipaths, and then to select and combine some of them which have the better energy level among the received signals. Accordingly, the respective finger section separately processes an baseband signal transmitted from one transmitter and the same baseband signals received through another paths. 
     Thereafter, the early signal, late signal, and on-time signal are respectively inputted to three decimators in each finger section in the baseband signal demodulating apparatus at the point where on-time signal is synchronized with pseudo-noise code. The respective decimator selects one sample which is synchronized with pseudo-noise sequence among the oversampled signals for a predetermined period. Correlators are connected to respective decimeters one to one receiving early, late, and on-time samples, and correlation values between the output signals of the decimators and the pseudo-noise codes are respectively obtained. The correlation values are accumulated by accumulators connected to the correlators, respectively, for a predetermined period. The accumulated signals are transmitted to a symbol processor for further processing such maximum, deinterleaving, time tracking, decoding, and so on. 
     According to the conventional apparatus performing the above-described operations, correlators and accumulators, the number of which is the same as that of the decimators, should be provided to process multichannel signals received through the multipaths having the same contents, and the early, late, and on-time signals determined from the signal received through a path of a channel, respectively. Specifically, three correlators and three accumulators as well as a pseudo-noise code generator should be provided for each finger section, thereby complicating the construction of the whole baseband signal demodulating apparatus. 
     Accordingly, if the symbol processor has the same performance and the number of channels is continually increased due to the increase of the subscribers, the number of pseudo-noise code generators, correlators, and accumulators becomes increased as many as the number of channels. As a result, in order to increase the capacity of the conventional CDMA baseband demodulating apparatus, the hardware size of the base station system and the system construction cost should be greatly increased. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a baseband signal demodulating apparatus and method in a mobile radio communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a baseband signal demodulating apparatus and method in a mobile radio communication system which can minimize the hardware size of the system due to the increase of channels by minimizing the number of correlators, accumulators, and pseudo-noise code generators and by sharing them by a time division multiplexing method. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the baseband signal demodulating apparatus in a mobile radio communication system comprises a plurality of front end rake receivers for demodulating oversampled baseband signals to restore the original signal, transmitting means for sharing demodulated signals from the front end rake receivers by a time division multiplexing method, and despreding means for despreding output signals of the transmitting means. 
     In another aspect of the present invention, there is provided a baseband signal demodulating method in a mobile radio communication system comprising the steps of demodulating oversampled baseband signals to restore the original signal, transmitting demodulated signals by a time division multiplexing method, and obtaining correlation values of transmitted signals by despreding the transmitted signals. 
     It is to be understood that both the foregoing general description and the following detailed description are explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 is a block diagram illustrating the construction of the baseband signal demodulating apparatus in a mobile radio communication system according to the present invention. 
     FIG. 2 is a block diagram illustrating the construction of the pseudo-noise code generator module in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIG. 1 is a block diagram illustrating the construction of the baseband signal demodulating apparatus in a mobile radio communication system according to the present invention. 
     Referring to FIG. 1, the baseband signal demodulating apparatus according to the present invention includes an analog-to-digital (A/D) converter  90  of an analogue receiver for converting an analog RF audio signal transmitted from a mobile unit into a digital baseband signal and repeatedly oversampling the digital signal for a predetermined period, a plurality of front end rake receivers  100 ˜ 400  for determining an on-time signal S 2  which is a reference signal among oversampled signals outputted from the A/D converter  90 , producing an early signal S 3  and a late signal S 1 , which are respectively earlier and later than the on-time signal for the predetermined number of sampling signals, and separately processing a baseband signals through a multipath, a dedicated bus  70  for transmitting the on-time signal S 2 , early signal S 3 , and late signal S 1  outputted from the front end rake receivers  100 ˜ 400  through predetermined paths in accordance with the corresponding signals outputted from the front end rake receivers, a pseudo-noise code generator module  700  for generating pseudo-noise codes which are different from one another in accordance with users of mobile units, three correlators  500 ,  510 ,  520  for respectively demodulating the on-time signal S 2 , early signal S 3 , and late signal S 1  outputted from the front end rake receiver  100  using the pseudo-noise codes generated from the pseudo-noise code generator module  700  to restore the original baseband signal, three accumulators  600 ,  610 ,  620  for respectively accumulating output signals of the correlators  500 ,  510 ,  520  for a predetermined period, and a symbol processor  800  for further processing of the signals outputted from the accumulators  600 ,  610 , and  620  to a following viterbi decoder (not illustrated) and transmitting the control signals to the front end rake receivers  100 ˜ 400  and to the pseudo-noise code generator module  700 , respectively, for time tracking. 
     Here, each of the front end rake receivers  100 ˜ 400  comprises 4 front end finger sections  110 ˜ 140 , the number of which can be increased on condition that a technical support is applied thereto. Each front end finger section, as shown in FIG. 1, includes two delays  111  and  112 , connected together in series, for producing the on-time signal S 2 , early signal S 3 , and late signal S 1 , using the oversampled signals inputted thereto, and three decimators  113 ,  114 ,  115  for selecting one of the oversampled signals with respect to the on-time signal S 2 , early signal S 3 , and late signal S 1 . 
     Also, the pseudo-noise code generator module  700 , as shown in FIG. 2, includes a pseudo-noise code generator  711 , a plurality of pseudo-noise code buffers  731  and  732  for respectively storing a masked PN code produced utilizing the pseudo-noise code outputted from the pseudo-noise code generator  711  and code mask, and a multiplexer  751  for selectively outputting the masked PN code stored in the pseudo-noise code buffers  731  and  732  when it is needed to do so. 
     Referring again to FIG. 1, the RF signal transmitted from a certain mobile unit is inputted to the A/D converter  90  and is converted into a digital baseband signal. The digital audio signal is then oversampled  8  times per bit in A/D converter ( 90 ) of the analog receiver. Here, the number of oversampling times may be varied according to the design condition of the system. 
     The oversampled signal from the A/D converter  90  is then processed in the respective front end rake receiver. However, according to the preferred embodiment of the present invention, the operation of only one front end rake receiver  100  is explained. 
     The oversampled signal of the digital baseband signal inputted to the front end rake receiver  100  is then inputted to the decimator  115  as the late signal S 1 . Simultaneously, the oversampled signal passes through the first delay  111  used for tracking the CDMA signal, and then inputted to the decimator  114  as the on-time signal S 2 . At the same time, the oversampled signal passes through the first delay ill and the second delay  112  in turn, and then inputted to the decimator  113  as the early signal S 3 . Specifically, the signals inputted to the respective decimators  113 ˜ 115  are defined as the early signal S 3  and late signal S 1  which are earlier and later than the on-time signal S 2 , which is synchronized with the pseudo-noise code, for a predetermined number of samples, respectively. The on-time signal S 2 , early signal S 3 , and late signal S 1  are respectively converted into the signals having their original period determined before being oversampled by the respective decimators  500 ,  510 , and  520  at the time point when on-time signal is synchronized with the pseudo-noise code. The respective front end finger section  110 ˜ 140  provided in the front end rake receiver transmits the on-time signal S 2 , early signal S 3 , and late signal S 1  to the correlators  500 ,  510 , and  520  through dedicated bus lines  76  which are classified according to the kinds of the signals, i.e., the on-time signal, early signal, and late signal. At this time, the respective front end finger section  110 ˜ 140  transmits the on-time signal S 2 , early signals S 3 , and late signals S 3 , and thus the system can produce the on-time signals S 2 , early signals S 3 , and late signals S 3 , the number of which corresponds to the number of the whole front end finger sections. 
     Here, the dedicated bus lines  70  are shared by the time division multiplexing method having a period determined corresponding to the number of the whole front end finger sections. In other words, the signals outputted from the whole front end finger sections are processed in parallel by dividing the use period of the dedicated bus lines  70  into the number of the decimators which process the whole on-time signals S 2 , early signals S 3 , and late signals S 3 . 
     Accordingly, a pseudo-noise code buffers  731   a ˜ 731   n , which receive the pseudo-noise codes from the pseudo-noise code generator  711 , transfer the masked PN codes which correspond to the respective front end rake receivers to the correlators  500 ,  510 , and  520  through the multiplexer  751 , thereby obtaining the correlation values. The correlation values outputted from the correlators  500 ,  510 , and  520  are inputted to the accumulators  600 ,  610 , and  620  to be accumulated for the predetermined period. Thereafter, output signals of the respective accumulators  600 ,  610 , and  620  are transmitted to the symbol processor  800  through the bus  80 . 
     The symbol processor  800  performs combining, time tracking, and so on with signals provided from the respective accumulators  600 ,  610 , and  620  to the following viterbi decoder to perform the decoding process, and transmits control signals and late signals to the respective front end finger sections and the pseudo-noise code generator module  700  to compensate for the synchronization error. 
     As described above, according to the present invention, the physical size of the base station receiver can be greatly reduced. This effect may not show if the number of channel is very small, but it will become great if the number of channel is very large. Also, since a plurality of front end rake receivers can receive the necessary pseudo-noise codes from three correlators, three accumulators, a pseudo-noise code generator and pseudo-noise code buffers which are storage elements in the unit of a bit, the time required for synchronization can be reduced when a new input signal is allocated to the respective front end rake receivers. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the baseband signal demodulating apparatus and method in a mobile radio communication system of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.