Abstract:
Provided are multi-carrier (MC)/multi-carrier direct sequence (MC-DS) dual-mode adaptable CDMA apparatus, the method thereof, and a computer program that implements the method. The apparatus can vary the user modulation degree and the transmission repetition degree independently, and convert a spreading scheme between the time-based spreading scheme (MC-DS-CDMA) and the frequency-based spreading scheme (MC-CDMA) in a MC-CDMA system. The apparatus includes: a user signal processing unit for performing symbol modulation, repetition and spreading of bit stream for each user based on a transmission mode suitable for channel environment of each user, and generating spread chip streams for the user; a combining unit for adding up all the spread chip streams for the users; a first interleaving unit for interleaving the chip streams added up in the combining unit and generating a first interleaved stream; and a second interleaving unit for optionally performing a second interleaving on the first interleaved stream.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a multi-carrier (MC)/multi-carrier direct sequence (MC-DS) dual-mode adaptive multi-carrier code division multiple access (MC-CDMA) apparatus, the method thereof, and a computer-readable recording medium for recording a program that implements the method; and, more particularly, to a MC/MC-DS dual-mode adaptive MC-CDMA apparatus that supports two orthogonal code spreading schemes, i.e., a time-based spreading scheme (which is MC-DS-CDMA) and a frequency-based spreading scheme (which is MC-CDMA), simultaneously so that the two schemes can be converted to each other, and changes modulation degrees and the number of transmission repetition independently according to the channel condition of a user in a down-link MC-CDMA system, the method thereof, and a computer-readable recording medium for recording a program that implements the method. 
   In other words, the present invention provides a transmission apparatus that can vary the degree of modulation and the number of symbol repetitions separately according to the channel condition of a user in a MC-CDMA system, and selects one spreading scheme between the time-based spreading scheme (MC-DS-CDMA) and the frequency-based spreading scheme (MC-CDMA), in the level of the system, the method of the apparatus, and a recording medium. 
   DESCRIPTION OF RELATED ART 
   Prior technology related to the apparatus and method of the present invention is ‘A Programmable Transceiver Structure of Multi-rate OFDM-CDMA for Wireless Multimedia Communications,’ published in ‘Proceedings of VTC&#39;01,’ pp. 1942-1946 by Po-Wei Fu and Kwang-Cheng Chen. In the above research, a transceiver signal processing is embodied as programmable software to suggest a structure of a universal transceiver that can be used in various spreading schemes. It is designed suitable for a spreading scheme determined in the system based on how variables are determined in the function blocks. 
   However, since the method of the above research should establish variables differently at each signal processing block based on the corresponding spreading scheme, there is a problem that the signal processing is changed complicatedly and it is not proper to be operated in real-time. That is, if the spreading scheme needs to be changed in the middle of system operation, the process of establishing variables at each signal processing block should be performed with respect to the signals of each user, individually. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a multi-carrier (MC)/multi-carrier direct sequence (MC-DS) dual-mode adaptive multi-carrier code division multiple access (MC-CDMA) apparatus that can vary the user modulation degree and the number of transmission repetition independently, and convert a spreading scheme between the time-based spreading scheme (MC-DS-CDMA) and the frequency-based spreading scheme (MC-CDMA) in a MC-CDMA system, the method of the apparatus, and a computer-readable recording medium for recording a program that implements the method. 
   In accordance with an aspect of the present invention, there is provided a MC/MC-DS dual-mode adaptive CDMA apparatus, comprising: a user signal processing unit for performing symbol modulation, repetition and spreading of bit stream for each user based on a transmission mode suitable for channel environment of each user, and generating spread chip streams for the user; a combining unit for adding up all the spread chip streams for the users; a first interleaving unit for interleaving the chip streams added up in the combining unit and generating a first interleaved stream; and a second interleaving unit for performing a second interleaving on the first interleaved stream, optionally, based on a spreading scheme selection signal which indicates a spreading scheme determined depending on system conditions and outputting a second interleaved stream. 
   In accordance with another aspect of the present invention, there is provided a MC/MC-DS dual-mode adaptive CDMA method, comprising the steps of: a) performing symbol modulation, repetition and spreading on the bit stream of each user based on the transmission mode suitable for the channel environment of each user; b) adding up all the spread chip streams of the users and performing a first interleaving; c) checking if the spreading scheme selected based on the overall system conditions is a time-based spreading scheme; d) if the selected spreading scheme is a time-based scheme, performing a second interleaving on the first-interleaved signal and outputting a signal; and e) if the selected spreading scheme is not a time-based scheme, outputting the first-interleaved signal. 
   In accordance with another aspect of the present invention, there is provided a computer-readable recording medium for recording a program that implements the method in a radio communication system provided with a processor, comprising the steps of: a) performing symbol modulation, repetition and spreading on the bit stream of each user based on the transmission mode suitable for the channel environment of each user; b) adding up the spread chip streams and performing a first interleaving; c) checking if the spreading scheme selected based on the overall system conditions is a time-based spreading scheme; d) if the selected spreading scheme is a time-based scheme, performing a second interleaving on the first-interleaved signal and outputting a signal; and e) if the selected spreading scheme is not a time-based scheme, outputting the first-interleaved signal. 
   The apparatus and method of the present invention performs the user data rate and the system spreading scheme conversion independently. Also, the apparatus and method control the data rate by generating symbols in the modulation degree and the modulations scheme determined based on the channel conditions of each user, and repeating the symbols as many times as the determined number of transmission repetition. In the present invention, the type of a system spreading scheme is determined based on whether the block interleaving, whose column and row correspond to the spreading code degree N and the number of sub-carrier Ns respectively, is performed with respect to the spread chip stream. 
   The method of the present invention can support two spreading schemes without any additional hardware adding to complexity. It maintains the flexibility of the conventional method that it can be used in the different spreading schemes. In addition, since the method of the present invention can be converted to the other spreading scheme in real-time easily, it can be applied to an adaptive spreading scheme conversion system usefully. This is because the method of the present invention performs the conversion of the spreading scheme after the signals of all users are added up, different from the conventional technology. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a graph showing an example of a system throughput based on the spreading mode and the number of symbol repetitions; 
       FIG. 2  is a block diagram showing a MC/MC-DS dual-mode adaptive multi-carrier code division multiple access (CDMA) apparatus in accordance with an embodiment of the present invention; 
       FIG. 3  is a flow chart describing a MC/MC-DS dual-mode adaptive multi-carrier CDMA method in accordance with an embodiment of the present invention; and 
       FIG. 4  is an exemplary view showing the change in the chip stream, each corresponding to a symbol, after interleaving and serial-to-parallel conversion in each spreading scheme in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
   Generally, multi-carrier code division multiple access (MC-CDMA) is interpreted in a broad sense as a method that spreads the data symbol of each user with an orthogonal code assigned to each user, and transmits a user signal using a multi-carrier, which has different center frequencies. The MC-CDMA system can be divided into various types based on the spreading scheme, that is, how the spread chip stream is carried on a multi-carrier and transmitted. 
   The representative ones are MC-CDMA that is interpreted in a narrow sense and multi-carrier direct sequence (MC-DS) CDMA. In the narrow-meaning MC-CDMA system, a chip stream corresponding to one symbol is transmitted carried on a different carrier. R. Prasad and S. Hara teach that in the MC-DS-CDMA system, a chip stream corresponding to one symbol is transmitted carried on the identical carrier in an article ‘An Overview of Multi-carrier CDMA,’ in Proc. IEEE Int. Symp. Spread Spectrum Techniques and Applications, Mainz, Germany, Sept., 1996, pp.107-114. 
   In short, the narrow-meaning MC-CDMA is a system that spreads a data symbol based on a frequency, while MC-DS-CDMA spreads a data symbol based on time. The two types have advantages and disadvantages depending on the system environment. However, the present invention suggests an apparatus and method that can deal with the system environment adaptively, using the advantages of each system mode, and a recording medium therefor. 
   Regardless of the spreading schemes, a user can perform adaptive transmission individually in all the multi-carrier multiple access systems. That is, a user can enhance the performance and throughput of the overall system by varying the modulation mode and the number of symbol repetition transmission based on the channel condition. 
   For example, a user having serious fading or bad channel condition can compensate for his poor channel environment by lowering the modulation degree, increasing the number of symbol repetitions to thereby reduce the data rate and increase the symbol energy. On the other hand, a user having good channel environment can transmit more data by raising the modulation degree and the number of symbol repetitions to thereby reduce the symbol energy and increase the data rate. 
   As shown above, a user can obtain desired service quality by selecting and transmitting the most appropriate transmission mode to his system environment. Also, the system can prevent wasteful consumption of transmission resources, such as unnecessary electric power, time, frequencies, and thus it reduces the entire system interference. This means that the system can accommodate additional users, and the throughput of the entire system is enlarged. 
   Besides the control of the individual transmission resources, from a view point of the entire system, time-based symbol spreading may be more advantageous or frequency-based spreading may be more advantageous for the user&#39;s environment. This is apparent in an example showing the system throughput based on the spreading scheme of  FIG. 1  and the number L of symbol repetition transmission. 
     FIG. 1  is a graph showing an example of a system processing capacity based on the spreading mode and symbol retransmission degree. 
   The system environment of  FIG. 1  is as follows: N, which denotes the code length of an orthogonal user code being 16, quadrature phase shift keying (QPSK), Rayleigh fading existing, and the symbol energy to noise ratio (Es/NO) being 18 dB. 
   According to the drawing, if the number K of users is less than 9, the throughput of the frequency-based spreading scheme (MC-CDMA) is larger than that of the time-based spreading scheme (MC-DS-CDMA). If the number K of users is not less than 9, the result goes inversely. From the result, it can be seen that there is a spreading scheme that maximizes the throughput based on the number of users in the system. More improved throughput can be obtained by converting the spreading scheme properly based on the number of entire users, or traffic amount, who access to the system, than just using a single spreading scheme. 
   The present invention using the above described property suggests a dual-mode spreading adaptive MC-CDMA that selects and assigns transmission resources suitable for the channel condition of each user, and in the level of a system, this invention generalizes the conditions of the entire users and selects the most suitable spreading scheme for the system. 
     FIG. 2  is a block diagram showing a MC/MC-DS dual-mode adaptive multi-carrier code division multiple access (CDMA) apparatus in accordance with an embodiment of the present invention. The MC/MC-DS dual-mode adaptive MC CDMA apparatus includes a symbol modulator  201 ,  204  or  207 , a symbol repeater  202 ,  205  or  208 , a spreader  203 ,  206  or  209 , a combiner  210 , a M by N chip interleaver  211 , an N by Ns chip interleaver  212 , a 1:Ns serial-to-parallel converter  213 , an IFFT  214 , a Ns:1 parallel-to-serial converter  215  and a guard time inserting unit  216 . 
   First, a k th  user bit stream is modulated in the symbol modulator  201 ,  204  and  207  based on the modulation degree Q k  (the modulation degree of the k th  user) and the modulation mode MOD k  (the modulation mode of the k th  user, such as BPSK, QPSK, 16QAM, etc.) suitable for the channel condition of the user. Then, each modulation symbol is repeated in the symbol repeater  202 ,  205  or  208  L k  (the number of symbol repetitions of the k th  user) times. 
   Each symbol is multiplexed by an orthogonal code C k  (the orthogonal code of the k th  user) having a length N (code length of an orthogonal user code) and then spread into a chip stream. The spread chip streams of the users are synchronized with the symbol timing in the combiner  210  and added to each other. Subsequently, M by N interleaving is performed on the chip stream having N number of symbols to be transmitted simultaneously in the M by N interleaver to array the chips for the same symbol at an interval of M chips. 
   A system generalizes the overall conditions, including traffic condition, such as the number of users of the system, and the entire system channel condition, and then it determines a spreading scheme based on the conditions. 
   If the system selects a time-based spreading scheme, the N by Ns chip interleaver  212  performs N by Ns (Ns=N×M) interleaving again additionally with respect to the interleaved chip stream Ns being the number of sub-carriers. 
   If the system selects a frequency-based spreading scheme, 1:Ns serial-to-parallel conversion is performed in the serial-to-parallel converter  213  without any additional interleaving. That is, the chip signal corresponding to the length of Ns is converted into a parallel signal. 
   The Ns-long parallel signal generated through the above spreading scheme goes through Ns point inverse fast Fourier transform (IFFT) in the IFFT unit  214 , and then goes through Ns:1 parallel-to-serial conversion in the parallel-to-serial converter  215 . Here, the Ns-long parallel signal is converted into a serial signal. Finally, a guard time is inserted in the converted serial signal to form a bottom bandwidth signal. 
   As described above, the apparatus and method of the present invention has an advantage that the conversion of the spreading scheme hardly add to the system complexity because the conversion is not performed with respect to each user signal individually, but performed after the signals of all users are added up. 
     FIG. 3  is a flow chart describing a MC/MC-DS dual-mode adaptive multi-carrier CDMA method in accordance with an embodiment of the present invention. 
   At step  301 , a system determines and assigns the modulation degree, the modulation mode, the number of symbol repetitions to each user. That is, a transmission mode is determined suitable for the channel environment of each user. 
   At step  302 , a bit stream of the k th  user is modulated based on the assigned modulation mode MOD k  and the modulation degree Q k , and each modulation symbol is repeated in a predetermined L k  times. 
   At step  303 , each of the modulation symbols is multiplexed by the orthogonal user code having a code length of N, and then it is spread into a chip stream. At step  304 , the spread chip streams of the users are synchronized with symbol timing and added to each other. 
   At step  305 , M by N interleaving is performed on the chip stream having N number of symbols to be transmitted simultaneously, and the chips forming the same symbol are arrayed at an interval of M chips. 
   At step  306 , a spreading scheme is determined based on the general system conditions, including traffic condition, such as the number of users in the system, and the overall channel condition of the users. 
   Subsequently, at step  307 , it is checked what type of a spreading scheme is determined. If the system selects a time-based spreading scheme, additional N:Ns (Ns=N×M) interleaving is performed on the interleaved chip stream, Ns being the number of sub-carriers. Then, at step  309 , if the system selects a frequency-based spreading scheme, 1:Ns serial-to-parallel conversion is performed without additional interleaving. 
   Subsequently, at step  310 , IFFT is performed on the parallel chip stream. At step  311 , Ns:1 parallel-to-serial conversion is carried out again and at step  312 , guard time is inserted in the serial signal. 
     FIG. 4  is an exemplary view showing the change in the chip stream, each corresponding to a symbol, after interleaving and serial-to-parallel conversion in each spreading scheme in accordance with the present invention. The drawing shows the chip stream corresponding to each symbol after interleaving and serial-to-parallel conversion in a case where time-based spreading is selected and in a case where frequency-based spreading is selected, in a system, where N, M and Ns are  4 ,  3  and  12  respectively. In the frequency-based spreading, the chips belonging to a symbol maintain the interval of M chips and they are arrayed in the same parallel signal. In the time-based spreading, chips for the same symbol are arrayed in the N number of consecutive parallel chip stream with the same index. 
   As described above, the apparatus and method of the present invention support the time-based spreading scheme (MC-DS CDMA) and the frequency-based spreading scheme (MC-CDMA) simultaneously that can be converted to each other, and changes the modulation degree and the number of symbol repetitions, independently, according to the user channel conditions in a down-link MC-CDMA system. 
   The present invention provides an apparatus and method that can convert a spreading scheme between the time-based one (MC-DS CDMA) and the frequency-based one (MC-CDMA) and varies the user modulation degree and the number of symbol repetitions, thus reducing the amount of additional signal processing to convert a spreading scheme in a MC-CDMA system 
   The apparatus and method of the present invention can maximize the performance of the adaptive transmission by selecting a proper spreading scheme based on the fading property and the number of users without increasing complexity in hardware in an adaptive MC-CDMA system that varies the modulation degree and the number of symbol repetition of each user. Since the conversion of the spreading scheme is not performed with respect to each user signal individually, but performed after the signals of all users are added up, little complexity is added to the system in the present method. 
   If a frequency-based spreading scheme is selected, since the chip signals of the consecutive symbols are arrayed in a parallel signal at an interval of N chips, symbol-based interleaving can be obtained inside the parallel signal. 
   In addition, since the apparatus and method of the present invention can support two spreading schemes without adding to complexity in hardware, they can be applied to a conventional system that supports only one spreading scheme, flexibly. Also, since the spreading scheme can be converted in real-time, the apparatus and method of the present invention can be applied to the adaptive spreading scheme conversion system. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.