Abstract:
An orthogonal code hopping multiple access communication system divides channels according to hopping patterns of the orthogonal codes allotted to the respective channels. The communication system includes a transmitter for modulating input digital signals using an orthogonal code hopping multiple access technique and transmitting the modulated digital signals, and a receiver for receiving the digital signals using the orthogonal code hopping multiple access technique and restoring the digital signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a CDMA (Code Division Multiple Access) communication system, and more particularly to an orthogonal code hopping multiple access (OCHMA) communication system which divides channels according to the hopping patterns of the orthogonal codes. 
     2. Description of the Related Art 
     FIGS. 1A and 1B are block diagrams illustrating the construction of a conventional CDMA communication system using orthogonal codes, wherein FIG. 1A shows a transmitter and FIG. 1B shows a receiver. Here, the orthogonal code may be a Walsh code, a Hadamard code or a Gold code. 
     Referring to FIG. 1A, an orthogonal code generator (OCG)  10  generates orthogonal codes OC(o)-OC(m) for the respective digital signals Do-Dm output from signal sources  9   a - 9   c.  Mixers  11   a - 11   c  mix the digital signals Do-Dm with the corresponding orthogonal codes OC(o)-OC(m), and a summer  12  sums the mixed signals output from the mixers  11   a - 11   c.  A pseudo-noise sequence generator (PNSG)  13  generates a pseudo-noise sequence (PNS) or a pseudo-random sequence (PRS). A mixer (or multiplier)  14  multiplies an output signal of the summer  12  by the PNS, and a modulator  15  modulates an output signal of the mixer  14  into a RF (Radio Frequency) signal. A power amplifier  16  amplifies the RF signal and transmits the amplified RF signal through an antenna  17 . 
     Referring to FIG. 1B, an RF amplifier  21  amplifies the RF signal received via antenna  20 . Demodulator  22  demodulates the RF signal output from the RF amplifier  21  in sync with a sync signal transmitted from the transmitter of FIG. 1A. A mixer (or multiplier)  24  multiplies the demodulated signal output from the demodulator  22  by the PNS generated from an PNSG  23 . An orthogonal code generator  28  generates orthogonal codes OC(o)-OC(m) identical to those in the transmitter. Mixers (or multipliers)  25   a - 25   c  multiply the output signal of the mixer  24  by the corresponding orthogonal codes OC(o)-OC(m). Integrators  26   a - 26   c  integrate the output signals of the mixers  25   a - 25   c  to restore the original digital signals Do-Dm and transfer the restored digital signals to the respective signal destinations  27   a - 27   c.    
     As appreciated from the foregoing, in order to divide the channels, the conventional CDMA communication system allocates the unique orthogonal codes belonging to a specified set of the orthogonal codes to the respective channels and repeatedly multiplies the transmission digital signals by the allocated orthogonal codes. That is, the transmitter of FIG. 1A repeatedly multiplies the allotted orthogonal codes for the bit duration of the digital signal to be transmitted, to spread the digital signal so that the spread digital signal may have wider spectrum than the original digital signal. In addition, the transmitter multiplies the transmission signals by the PNS to scramble the transmission signals so that the other terminals may not restore the scrambled transmission signals. The receiver of FIG. 1B executes a reverse process of the transmitter. That is, the receiver generates the PNS identical to that used in the transmitter and multiplies the received RF signal by the PNS. The receiver then further multiplies the RF signal by the orthogonal codes identical to those used in the transmitter, repeatedly, and integrates the signals for the bit duration of the digital transmission signal, thereby restoring the original digital signals. 
     As is apparent from the foregoing description, when the digital signals have not been previously encrypted, the conventional CDMA communication system multiplies the digital signals by an agreed PNS between the transmitter and the receiver so as to encrypt the digital signals. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an orthogonal code hopping multiple access communication system which divides channels in accordance with hopping patterns of orthogonal codes allotted to the respective channels. 
     To achieve the above object, the CDMA communication system comprises a transmitter for modulating input digital signals using an orthogonal code hopping multiple access technique and transmitting the modulated digital signals, and a receiver for receiving the digital signals using the orthogonal code hopping multiple access technique and restoring the digital signals. 
     According to an embodiment, the transmitter comprises a first hopping orthogonal code generator for generating orthogonal codes according to a hopping pattern prescribed between the transmitter and the receiver; a plurality of mixers each connected to the first hopping orthogonal code generator, for multiplying the digital signals by the corresponding orthogonal codes; a summer connected to outputs of the mixers, for summing the output signals of the mixers; a modulator for modulating an output signal of the summer; an antenna; and a power amplifier for amplifying an output signal of the modulator and transmitting the amplified signal through the antenna. 
     The receiver comprises an antenna; a radio frequency (RF) amplifier for amplifying an RF signal received through the antenna; a demodulator for demodulating an output signal of the RF amplifier; a second hopping orthogonal code generator for generating orthogonal codes according to a hopping pattern identical to the hopping pattern in the transmitter; a plurality of mixers for multiplying an output signal of the demodulator by the corresponding orthogonal codes; and a plurality of integrators for integrating output signals of the mixers to restore the digital signals. 
     The first and second hopping orthogonal code generators each comprising an orthogonal code generator for generating the orthogonal codes according to the hopping pattern; and a hopping controller connected to the orthogonal code generator, for generating the hopping pattern and providing it to the orthogonal code generator. 
     In an alternative embodiment, each the first and second hopping orthogonal code generators comprise a memory for storing the orthogonal codes, and for outputting the orthogonal codes according to the hopping pattern; and a hopping controller connected to the memory, for generating the hopping pattern and providing it to the memory. 
    
    
     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: 
     FIGS. 1A and 1B are block diagrams showing the construction of a conventional CDMA communication system using orthogonal codes; 
     FIGS. 2A and 2B are block diagrams showing the construction of an orthogonal code hopping multiple access communication system according to an embodiment of the present invention; 
     FIGS. 3A and 3B are block diagrams showing the construction of an orthogonal code hopping multiple access communication system according to another embodiment of the present invention; and 
     FIG. 4 is a timing diagram showing the hopping patterns of the orthogonal codes according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to preferred embodiments of the present invention. Throughout the drawings, it is noted that the same reference numerals will be used to designate like or equivalent elements having the same function. A detailed description of known functions and constructions necessarily obscuring the subject matter of the present invention has been omitted in the present application for clarity. 
     FIGS. 2A and 2B are block diagrams showing the construction of an orthogonal code hopping multiple access (OCHMA) communication system according to an embodiment of the present invention, in which FIG. 2A shows a transmitter and FIG. 2B shows a receiver. 
     Referring now to FIG. 2A, a hopping orthogonal code generator  30  generates orthogonal codes according to a hopping pattern, and includes a hopping controller  31  for controlling the hopping pattern of the orthogonal codes and an OCG  32  for generating the orthogonal codes according to the hopping pattern controlled by the hopping controller  31 . Mixers  11   a - 11   c  mix the digital signals Do-Dm output from the signal sources  9   a - 9   c  with the corresponding unique orthogonal codes OC(Ho)-OC(Hm) generated according to a specified hopping pattern, and a summer  12  sums the digital output signals of the mixers  11   a - 11   c.  A PNSG  13  generates the PNS and a mixer (or multiplier)  14  multiplies the output signal of the summer  12  by the PNS. A modulator  15  modulates an output signal of the mixer  14  into the RF signal, and power amplifier  16  amplifies the RF signal and transmits it through antenna  17 . 
     Referring to FIG. 2B, an RF amplifier  21  amplifies the RF signal received from the transmitter via antenna  20 . Demodulator  22  demodulates the amplified RF signal output from the RF amplifier  21 . A PNSG  23  generates the PNS identical to that used in the transmitter and mixer  24  multiplies the demodulated signal by the PNS. A hopping orthogonal code generator  40  generates orthogonal codes OC(Ho)-OC(Hm) according to the hopping pattern identical to that used in the transmitter, and includes an orthogonal code generator  42  and a hopping controller  41 . Mixers (or multipliers)  25   a - 25   c  multiply the signal output from the mixer  24  by the corresponding orthogonal codes OC(Ho)-OC(Hm), respectively. Then, integrators  26   a - 26   c  integrate the output signals of the mixers  25   a - 25   c  to restore the digital signals Do-Dm and transfer them to the respective signal destinations  27   a - 27   c.    
     FIGS. 3A and 3B are diagrams illustrating the construction of an OCHMA communication system according to another embodiment of the present invention. It is noted that FIGS. 3A and 3B are identical to the construction of FIGS. 2A and 2B, except for orthogonal code hopping generators  50  and  60 . As illustrated, orthogonal code hopping generator  50  comprises a hopping controller  51  for controlling the hopping patterns of the orthogonal codes and a ROM (Read Only Memory)  52  for outputting orthogonal codes stored therein according to the hopping patterns controlled by the hopping controller  51 . Similarly, the orthogonal code hopping generator  60  comprises a hopping controller  61  for controlling the hopping pattern of the orthogonal code and a ROM  62  for outputting orthogonal codes stored therein under the control of hopping controller  61 . 
     FIG. 4 is a timing diagram showing the hopping patterns of the orthogonal codes according to an embodiment of the present invention. As illustrated, a first orthogonal code (1ST OC) hops three times for every bit duration so that the orthogonal code may be relatively shorter as compared to the bit duration. A second orthogonal code (2ND OC) has the hopping time identical to the bit duration, and a third orthogonal code (3RD OC) hops every two bit streams so that the hopping time is n times the bit duration (where n is an integer). 
     As described above, the OCHMA communication system of the invention can uniformly distribute the power density in a certain frequency band as compared with the conventional CDMA communication system, even without multiplying the pseudo-nose sequence, and only the receiver properlperceiving the hopping pattern can restore the digital signals. Thus, the communication system has an encryption function. Furthermore, by multiplying the digital signals by the pseudo-noise sequence, the present invention can reinforce the encryption function and secure more uniform power density.