Abstract:
A code division multiplex transmitting and receiving apparatus has a transmitting apparatus with two coders per channel and terminal units with two matched filters each. One coder and one matched filter employ one spreading code; the other coder and the other matched filter employ another spreading code. The two coded signals output in parallel by the two coders are converted to a single serial signal before being multiplexed. The two matched filters sample alternate chips in the multiplexed signal. The two coders can be supplied with different data signals to double the transmission capacity, or with the same data signal to double the transmission distance. The outputs of the two matched filters are processed separately in the former case and are combined in the latter case.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a code division multiplex transmitting and receiving apparatus and a method of carrying out code division multiplexed communication between a central station and a plurality of receiving apparatuses. 
         [0003]    2. Description of the Related Art 
         [0004]    The following documents are referred to below. 
         [0005]    Patent document 1: Matsuno et al., Japanese Patent No. 3913139, CDMA transmitter, CDMA multiplex transmitter, CDMA receiver, and CDMA communication system, pre-grant publication Mar. 11, 2004 as JP 2004-080385 
         [0006]    Patent document 2: Sasaki et al., JP 2003-317026, Signed product sum computing element and analog matched filter including the same, published Nov. 7, 2003 
         [0007]    Non-patent document 1: Tamai et al., ‘Jisedai hikari akusesu shisutemu COF-PON no kenkyu kaihatsu’ (Research and development of COF-PON: a next-generation optical access system),  Oki Technical Review,  Issue 210, Vol. 74, No. 2, April 2007 
         [0008]    Non-patent document 2: Kashima et al., ‘Ko-QoS maruchi media hikari haishin shisutemu no kenkyu kaihatsu—COF transhiba’ (Research and development of high-QoS multimedia optical distribution system—COF transceiver)  Oki Technical Review,  Issue 200, Vol. 71, No. 4, October 2004 
         [0009]    Non-patent document 3: Sasase, ‘Hikari shisutemu ni okeru hikari fugo bunkatsu tagen setsuzoku gijutsu’ (Optical Code Division Multiple Access Techniques in Optical Communication Systems)  TELECOMFRONTIER,  November 2004 
         [0010]    Code division multiplexing (CDM) is currently employed in mobile access network systems, where it provides the capability to carry high volumes of communication traffic on multiple channels while conserving frequency and time-slot resources. Synchronous CDM systems in which each channel is separately synchronized have the particular advantage of providing stable extraction of an arbitrary channel from the multiplexed signal, as described by Matsuno et al. in the above patent document 1. 
         [0011]    Passive optical network (PON) systems that use CDM for fiber-optic communication between a provider and stationary users are also attracting attention. Known as CDM-on-fiber-PON or COF-PON systems, these systems permit transmission over longer distances than are feasible in more traditional time division multiplex (TDM) optical access systems. COF-PON also has the advantage of being compatible with wavelength division multiplexing (WDM). COF-PON systems are described in non-patent documents 1-3 and patent document 2. 
         [0012]    CDM transmitting and receiving apparatus is thus needed both for COF-PON and other optical access network systems, and for mobile communications. The present apparatus is concerned with apparatus for transmitting and receiving a CDM signal that is sent from the central station to a plurality of terminal units. The transmitting apparatus is often referred to as central office apparatus or optical line termination apparatus in optical access networks and as a base station apparatus in mobile communication systems. The terminal units may be referred to as optical network units in optical access systems, as mobile stations in mobile communication systems, and as subscriber apparatus in both types of systems. 
         [0013]    In conventional CDM communication systems, typified by the system described by Matsuno et al. in patent document 1, the transmission rate per channel is fixed, and the maximum distance from the transmitting equipment to the receiving equipment is also fixed. These system parameters are related to the coding rate, that is, the number of chips into which each data bit is divided when transmitted, and the chip rate, that is, the number of chips transmitted per second. Modifying these parameters to accommodate a subscriber requiring a particularly high data rate or a subscriber located particularly far from the transmitting apparatus is not easy. 
         [0014]    Accordingly, a conventional system must be designed to accommodate the needs of the most distant anticipated subscriber, and the subscriber with the highest anticipated transmission rate, even though most subscribers may have less demanding requirements. As a result, much of the capacity of the system becomes excess capacity that is not used. Furthermore, if an unanticipated new subscriber joins the system and the subscriber&#39;s communication needs exceed the system capabilities in terms of distance or transmission rate, the entire central office apparatus must be modified or replaced. 
         [0015]    It would be preferable for the transmitting and receiving apparatus to more flexible, so that a long-distance or high-data-rate transmission capability could be provided for particular subscribers without being provided for all subscribers. A flexible system of this type would be far less expensive to operate than a conventional fixed-parameter system. 
         [0016]    Diligent study of this problem by the present inventor has shown that a feasible solution is to generate two parallel data signals per channel, code them in parallel with different codes, convert the two coded signals to a single serial signal, and multiplex the serial signal. 
       SUMMARY OF THE INVENTION 
       [0017]    An object of the present invention is to provide a flexible CDM transmitting and receiving apparatus that can accommodate both terminal units requiring comparatively low-data-rate transmission over comparatively long distances and terminal units requiring comparatively high-data-rate transmission over comparatively short distances with the same equipment configuration. 
         [0018]    The invention provides a CDM transmitting and receiving apparatus including a transmitting apparatus and N terminal units connected to the transmitting apparatus through N respective communication channels, N being an integer greater than one. 
         [0019]    For each communication channel, the transmitting apparatus has a pair of coders operating in parallel and employing different codes to code data to be transmitted on the channel, and a parallel-to-serial converter that converts the resulting pair of parallel coded signals to a single serial coded signal. A multiplexer in the transmitting apparatus multiplexes the N serial coded signals to generate a multiplexed signal which is transmitted to all N terminal units. 
         [0020]    Each terminal unit correlates the multiplexed signal with two different codes, thereby obtaining a pair of parallel correlated signals representing-decoded data. 
         [0021]    For channels requiring high-data-rate transmission, the pair of coders encode different data signals, thereby doubling the channel capacity. In this case, the receiving terminal unit may compare each correlated signal separately with a threshold to obtain a decoded data signal. 
         [0022]    For channels requiring long-distance transmission, the pair of coders encode identical data signals. In this case, the receiving terminal unit may additively combine the two correlated signals and compare the combined signal with a threshold to obtain a decoded data signal, thereby doubling the coding gain. 
         [0023]    Accordingly, when the transmitting apparatus transmits a multiplexed signal to a plurality of terminal units, it can accommodate distant terminal units by sending the same data to both coders, and can accommodate terminal units with heavy traffic loads by dividing the data among the two coders so that each coder codes only half of the data. 
         [0024]    The invention also provides a method of transmitting data from a transmitting apparatus to a terminal unit. The method comprises: 
         [0025]    using two coders operating in parallel with different codes to code the data; 
         [0026]    converting the resulting pair of parallel coded signals to a serial coded signal; 
         [0027]    transmitting the serial coded signal to the terminal unit as part of a multiplexed signal; 
         [0028]    receiving the multiplexed signal at the terminal unit; and 
         [0029]    correlating the multiplexed signal in parallel with the different codes at the terminal unit to obtain a pair of correlated signals representing decoded data. 
         [0030]    This method can also be used to transmit data from a transmitting apparatus to N terminal units, where N is an integer greater than one. The same multiplexed signal is sent to all N terminal units. Each terminal unit uses a different pair of codes to decode the multiplexed signal, 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    In the attached drawings: 
           [0032]      FIG. 1  is a schematic block diagram of a conventional CDM transmitting and receiving apparatus; 
           [0033]      FIG. 2  is a more detailed block diagram of the transmitting apparatus in  FIG. 1 ; 
           [0034]      FIG. 3  is a more detailed block diagram of the receiving apparatus in  FIG. 1 ; 
           [0035]      FIG. 4  is a timing diagram illustrating the operation of the CDM transmitting and receiving apparatus in  FIG. 1 ; 
           [0036]      FIG. 5  is a schematic block diagram of a transmitting apparatus embodying the present invention; 
           [0037]      FIG. 6  is a schematic block diagram of a receiving apparatus embodying the invention; and 
           [0038]      FIGS. 7 and 8  are timing diagrams illustrating the operation of the apparatus in  FIGS. 5 and 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. First, however, a general description of a conventional COF-PON system will be given. 
         [0040]    Referring to  FIG. 1 , the conventional COF-PON system comprises a transmitting apparatus  10  connected through an optical fiber  12  to a passive optical coupler  20 . The passive optical coupler  20  is connected by N branch optical fibers  14 - 1 ,  14 - 2 , . . . ,  14 - j,  . . . ,  14 -N to N terminal units  30 - 1 ,  30 - 2 , . . . ,  30 - j,  . . . ,  30 -N, where N is an integer greater than one and j is an arbitrary integer in the range from one to N. 
         [0041]    Referring to  FIG. 2 , the transmitting apparatus  10  comprises N signal generators  40 - 1 ,  40 - 2 , . . . ,  40 -N which output respective data signals  41 - 1 ,  41 - 2 , . . . ,  41 -N to respective coders  42 - 1 ,  42 - 2 , . . . ,  42 -N. The coders  42 - 1 ,  42 - 2 , . . . ,  42 -N code the data signals  41 - 1 ,  41 - 2 , . . . ,  41 -N by use of respective spreading codes. The coders  42 - 1 ,  42 - 2 , . . . ,  42 -N may use exclusive-OR logic gates to carry out the coding operations. Each coder has a different spreading code, but all of the spreading codes have the same length, and all of the coders  42 - 1 ,  42 - 2 , . . . ,  42 -N operate at the same speed. The resulting coded signals  43 - 1 ,  43 - 2  . . . ,  43 -N are multiplexed by a multiplexer  46  and output as a CDM signal  47  on the optical fiber  12  in  FIG. 1 . 
         [0042]    The first terminal unit  30 - 1 , shown in  FIG. 3 , comprises a splitter  50 , a matched filter  52 , a thresholding unit  54 , and a clock recovery unit  56 . 
         [0043]    The splitter  50  splits the multiplexed signal or CDM signal  47 , which is received on branch optical fiber  14 - 1  in  FIG. 1 , into two identical CDM signals  51 - 1 ,  51 - 2 . 
         [0044]    The clock recovery unit  56  extracts a clock signal  57  from CDM signal  51 - 2 , and supplies the clock signal to the matched filter  52 . 
         [0045]    Operating in synchronization with the clock signal  57 , the matched filter  52  correlates CDM signal  51 - 2  with the spreading code employed by coder  42 - 1  in the transmitting apparatus  10 , and outputs a correlated signal  53 . 
         [0046]    The thresholding unit  54  compares the correlated signal  53  with a preset threshold value to generate a decoded signal  55 . 
         [0047]    The matched filter  52  may be an analog matched filter, in which case the thresholding unit  54  may be a well-known circuit comprising a comparator and a D-type flip-flop. Alternatively, the matched filter  52  may be a digital matched filter, in which case the functions of the thresholding unit  54  may be built into the matched filter  52 , making the thresholding unit  54  unnecessary. 
         [0048]    Each other terminal unit  30 - j  (j=2, . . . , N) has a similar internal structure, in which the matched filter  52  is adapted to correlate the CDM signal  47  with the spreading code employed by the coder  42 - j  in the transmitting apparatus  10 . The different spreading codes enable the CDM signal  47  to carry N multiplexed communication channels simultaneously. The term channel will also be used to denote the terminal unit  30 - j,  the corresponding signal generator  40 - j  and coder  42 - j  in the transmitting apparatus  10 , and the signal paths interconnecting them. 
         [0049]    Although the CDM signal  47  transmitted on the optical fiber  12  and branch optical fibers  14 - 1 , . . . ,  14 -N is an optical signal, the signal generators  40 - 1 , . . . ,  40 -N, coders  42 - 1 , . . . ,  42 -N, multiplexer  46 , matched filter  52 , thresholding unit  54 , and clock recovery unit  56  may be electronic circuits if suitable electrical-to-optical and optical-to-electrical conversion elements (not shown) are provided. 
         [0050]    The coding and correlation operations can be described as follows. 
         [0051]    The spreading codes employed in the transmitting apparatus  10  are chip trains of length M (C 1 , C 2 , . . . , C M ). Each chip has two possible signal levels, which may be denoted ‘1’ and ‘0’ or, more conveniently, ‘1’ and ‘−1’. 
         [0052]    For illustrative purposes, it will be assumed that there are only two channels (N=2), and spreading codes are of length four. The spreading code (C 1-1 , C 1-2 , C 1-3 , C 1-4 ) used by coder  42 - 1 , and by the matched filter  52  in first terminal unit  30 - 1 , will be (1, 0, 0, 1), or (1, −1, −1, 1) in the more convenient algebraic notation. The spreading code (C 2-1 , C 2-2 , C 2-3 , C 2-4 ) used by coder  42 - 2 , and by the matched filter  52  in terminal unit  30 - 2 , will be (1, 0, 1, 0), or (1, −1, 1, −1) in algebraic notation. 
         [0053]    Suppose that the data to be transmitted on the first channel are (1, 0, 1 . . . ), or (1, −1, 1 . . . ). When the algebraic notation is used, the coding process can be described as a multiplication process in which each data bit is multiplied by all of the code chip values to produce a string of coded chips. For the first and third data bits, which have values of ‘1’, the result is 
         [0000]      (1×1, 1×−1, 1×−1, 1×1)=(1, −1, −1, 1). 
         [0000]    For the second data bit, which has a value of ‘−1’, the result is 
         [0000]      (−1×1, −1×−1, −1×−1, −1×1)=(−1, 1, 1, −1). 
         [0000]    The data signal (1, −1, 1, 1, . . . ) is accordingly coded to a coded signal  43 - 1  with the following chip values: 
         [0000]      (1, −1, −1, 1, −1, 1, 1, −1, 1, −1, −1, 1, . . . ) 
         [0054]    Similarly if the data to be transmitted on the second channel are (1, 1, 0, . . . ), or (1, 1, −1, . . . ) in algebraic notation, the first and second data bits are coded to 
         [0000]      (1×1, 1×−1, 1×1, 1×−1)=(1, −1, 1, −1), 
         [0000]    the third data bit is coded to 
         [0000]      (−1×1, −1×−1, −1×1, −1×−1)=(−1, 1, −1, 1), 
         [0000]    and the coded signal  43 - 2  has the following chip values: 
         [0000]      (1, −1, 1, −1, 1, −1, 1, −1, −1, 1, −1, 1, . . . ) 
         [0055]    The multiplexer  46  multiplexes the two coded signals  43 - 1 ,  43 - 2  by adding their values, obtaining 
         [0000]      (2, −2, 0, 0, 0, 0, 2, −2, 0, 0, −2, 2, . . . ). 
         [0056]    When the first four chips of the CDM signal  47  have been received by the first terminal unit  30 - 1 , the matched filter  52  correlates them with the spreading code (1, −1, −1, 1) by multiplying the received signal chips by the corresponding code chips and adding the sums, obtaining 
         [0000]      (2×1)+(−2×−1)+(0×−1)+(0×1)=4 
         [0000]    When the eighth chip has been received, the correlated result is 
         [0000]      (0×1)+(0×−1)+(2×−1)+(−2×1)=−4. 
         [0000]    When the twelfth chip has been received, the correlated result is 
         [0000]      (0×1)+(0×−1)+(−2×−1)+(2×1)=4 
         [0057]    By comparing the correlated signal with a suitable threshold such as zero at these timings, outputting a result of ‘1’ when the correlated value exceeds the threshold, and outputting a result of ‘0’ when the correlated value is less than the threshold, the thresholding unit  54  recovers the transmitted data signal (1, 0, 1, . . . ). 
         [0058]    In terminal unit  30 - 2 , a similar correlation operation using the spreading code (1, −1, 1, −1) of the second channel yields correlated values of 4 at the fourth and eighth chips and −4 at the twelfth chip, causing the thresholding unit  54  in terminal unit  30 - 2  to reproduce the data (1, 1, 0, . . . ) which were transmitted on the second channel. 
         [0059]    The reason for the reappearance of the transmitted data in the correlated output signal is as follows. In the general case of an apparatus with N channels, consider an instant at which data bits D 1 , D 2 , D 3 , . . . coded by multiplication by respective code chips C 1 , C 2 , C 3 , . . . used on the first, second, third, . . . channels arrive at the decoder. The chip received at this instant has the value 
         [0000]      (D 1 ×C 1 )+(D 2 ×C 2 )+(D 3 ×C 3 )+ . . . 
         [0060]    When the analog matched filter in the first channel correlates the received signal with the spreading code, it multiplies this received chip value by the code value C 1 , obtaining 
         [0000]      (D 1 ×C 1 ×C 1 )+(D 2 ×C 2 ×C 1 )+(D 3 ×C 3 ×C 1 )+ . . . 
         [0000]    Regardless of whether C 1  is 1 or −1, the product C 1 ×C 1  is always 1, while the products C 2 ×C 1 , C 3 ×C 1 , and so on are 1 and −1 at random. In the correlation process, calculations such as the above are performed simultaneously for all chips in the spreading code, consistently obtaining D 1  but obtaining D 2  and −D 2 , D 3  and −D 3  and so on about equally often. When the results of these simultaneous operations are added to obtain the correlated output, the value D 1  is reproduced with a gain equal to the code length while the other values D 2  and −D 2 , D 3  and −D 3  and so on cancel out to zero, or approximately zero. 
         [0061]    The operation of the conventional CDM transmitting and receiving apparatus is summarized in  FIG. 4 . Waveform A illustrates two bits of an exemplary data signal (1, 0, . . . ) to be transmitted on, for example, the first channel, before coding by the M-chip spreading code (C 1-1 , C 1-2 , . . . , C 1-M ) of that channel (waveform B). The coding process converts the data signal to the chip train indicated as waveform C. At the first terminal unit  30 - 1 , the chip train is sampled in synchronization with the clock signal (waveform D) output by the clock recovery unit  56 , and the matched filter  52  correlates the samples with the spreading code. Waveform E schematically illustrates the correlated signal output by the matched filter  52 . At every M-th clock pulse, the chip values of the spreading code match the values by which the signal was coded at the transmitting apparatus  10 , and a positive or negative peak appears in the correlated signal, as indicated by the arrows. The thresholding unit  54 , operating in synchronization with a bit-rate clock signal (not shown) compares the peak values with a threshold value, outputs a ‘1’ when the peak value exceeds the threshold, outputs a ‘0’ when the peak value is less than the threshold, and thereby recovers the transmitted data as illustrated in waveform F. 
         [0062]    The non-peak parts of the correlated waveform are due to correlation of the spreading code with the data transmitted on all channels, at times at which the spreading code in the matched filter  52  is not aligned with the bit boundaries in the transmitted signal. At these times, the multiplication operations in the correlation process produce essentially random positive and negative results that, when added together, produce a value near zero. 
         [0063]    The strength of the peaks in the correlated waveform F, that is, their amplitude in relation to the amplitude of the other parts of the waveform, depends on the code length M: longer spreading codes produces stronger peaks. The ratio of the peak amplitude to the amplitude of the individual chips before correlation is referred to as the coding gain. 
         [0064]    The number of different channels that can be multiplexed into a single CDM signal of the above type depends on the code length M; longer codes permit more channels. The code length M is accordingly determined when the apparatus is designed. The same code length is used for all channels. 
         [0065]    The data transmission rate is determined by the code length M and the frequency of the clock signal (waveform D) output by the clock recovery unit  56  in the terminal units. All terminal units must operate at the same clock frequency, so they all have the same data transmission rate. 
         [0066]    The maximum distance over which data can be transmitted from the transmitting apparatus  10  to a terminal unit also depends on the code length M. A longer code produces a higher coding gain, making the received data detectable after transmission over a greater distance, despite the greater signal attenuation on the transmission path. Since the code length M is the same for all channels, the maximum transmission distance is the same for all channels. 
         [0067]    When transmitting apparatus  10  has been installed, it may be initially connected to a limited number of terminal units, and further terminal units may be added later. The further terminal units cannot exceed the constraints on the data transmission rate and transmission distance imposed by the fixed code length M. 
         [0068]    A transmitting and receiving apparatus embodying the present invention will now be described with reference to  FIGS. 5 and 6 . This apparatus may be employed in a COF-PON system of the type illustrated in  FIG. 1 , or in other data transmission systems, such as a system in which the transmitting apparatus is a base station transmitting wireless signals to N terminal units, where N is an integer greater than one. 
         [0069]    Referring to  FIG. 5 , the transmitting apparatus comprises a signal generating module  140 , a coding module  142 , a parallel-to-serial (P/S) conversion module  144 , and a multiplexer  146 . 
         [0070]    The signal generating module  140  comprises N signal generating units  140 - 1 ,  140 - 2 , . . . ,  140 - j,  . . . ,  140 -N including respective pairs of signal generators  140 - 1   a,    140 - 1   b,    140 - 2   a,    140 - 2   b,  . . . ,  140   -ja,    140 - jb,  . . . ,  140 -Na,  140 -Nb that output respective data signals d 140 - 1   a,  d 140 - 1   b,  d 140 - 2   a,  d 140 - 2   b,  . . . , d 140 - ja,  d 140 - jb,  . . . , d 140 -Na, d 140 -Nb. Each signal generating unit serves one communication channel. 
         [0071]    The coding module  142  comprises N coding units  142 - 1 ,  142 - 2 , . . . ,  142 - j,  . . . ,  142 -N including respective pairs of coders  142 - 1   a,    142 - 1   b,    142 - 2   a,    142 - 2   b,  . . . ,  142 - ja,    142 - jb,  . . . ,  142 -Na,  142 -Nb that receive the data signals d 142 - 1   a,  d 142 - 1   b,  d 142 - 2   a,  d 142 - 2   b,  . . . , d 142 - ja,  d 142 - jb,  . . . , d 142 -Na, d 142 -Nb and output respective pairs of parallel coded signals c 142 - 1   a,  c 142 - 1   b,  c 142 - 2   a,  c 142 - 2   b,  . . . , c 142 - ja,  c 142 - jb,  . . . , c 142 -Na, c 142 -Nb. Each coding unit serves one communicational channel, so there are two coders per channel. 
         [0072]    The parallel-to-serial conversion module  144  comprises N parallel-to-serial converters  144 - 1 ,  144 - 2 , . . . ,  144 - j,  . . . ,  144 -N. For each integer j from 1 to N, the j-th parallel-to-serial conversion module  144 - j  receives the pair of parallel coded signals c 142 - ja,  c 142 - jb  output from the j-th coding module  142 - j  and converts them to a single serial coded signal PC 145 - j.  N serial coded signals PC 145 - 1 , PC 145 - 2 , . . . , PC 145 -N are thereby obtained. 
         [0073]    The multiplexer  146  multiplexes the N serial coded signals PC 145 - 1 , . . . , PC 145 -N to generate a single CDM signal  147  for transmission to N terminal units. 
         [0074]    Referring to  FIG. 6 , the first terminal unit  130 - 1  comprises a pair of splitters  150 ,  152 , a first matched filter  154 , a second matched filter  156 , a clock recovery unit  158 , and a received signal processing section  160 . The received signal processing section  160  comprises a further pair of splitters  162 ,  164 , an adder  166 , and three thresholding units  168 ,  170 ,  172 . 
         [0075]    Splitter  150  splits the CDM signal  147  received from the transmitting apparatus  100  in  FIG. 5  into two identical CDM signals  150 - 1 ,  150 - 2 . Splitter  152  splits CDM signal  150 - 1  into two further identical CDM signals  151 - 1 ,  151 - 2 . 
         [0076]    The clock recovery unit  158  extracts a complementary pair of clock signals  159 - 1 ,  159 - 2  from CDM signal  150 - 2 . ‘Complementary’ means that the two clock signals  159 - 1 ,  159 - 2  are mutually offset by one-half clock cycle, that is, by π radians in phase. These clock signals  159 - 1 ,  159 - 2  have a frequency equal to half the chip rate of the CDM signal  150 - 2 . The clock recovery unit  158  also generates clock signals  159 - 3 ,  159 - 4 ,  159 - 5  with frequencies equal to the bit rate of the CDM signal  150 - 2 . 
         [0077]    Operating in synchronization with clock signal  159 - 1 , the first matched filter  154  correlates CDM signal  151 - 1  with the spreading code employed by coder  142 - 1   a  in the transmitting apparatus  100 , and outputs a first correlated signal  155 . 
         [0078]    Operating in synchronization with clock signal  159 - 2 , the second matched filter  156  correlates CDM signal  151 - 2  with the spreading code employed by coder  142 - 1   b  in the transmitting apparatus  100 , and outputs a second correlated signal  157 . 
         [0079]    Splitter  162  splits the first correlated signal  155  into two identical first correlated signals  163 - 1 ,  163 - 2 . Splitter  164  splits the second correlated signals  157  into two identical second correlated signals  165 - 1 ,  165 - 2 . The adder  166  combines first correlated signal  163 - 2  and second correlated signal  165 - 2  by adding them together to produce a combined correlated signal  167 . 
         [0080]    Operating in synchronization with clock signal  159 - 5 ,. the first thresholding unit  168  compares the first correlated signal  163 - 1  with a first threshold and generates a first received data signal  169 . 
         [0081]    Operating in synchronization with clock signal  159 - 3 , the second thresholding unit  172  compares the second correlated signal  165 - 1  with a second threshold and generates a second received data signal  173 . 
         [0082]    Operating in synchronization with clock signal  159 - 4 , the third thresholding unit  170  compares the combined correlated signal  167  with a third threshold and generates a third received data signal  171 . 
         [0083]    The first and second thresholds may be identical. 
         [0084]    The thresholding units  168 ,  170 ,  172  may be adapted to output a value of ‘1’ when the received signal exceeds the relevant threshold and a value of ‘0’ when the received value is less than the threshold. 
         [0085]    The other terminal units have similar internal structures, except that their matched filters are adapted to correlate the multiplexed signal with the spreading codes used by the other coders in the transmitting apparatus  100 . 
         [0086]    The matched filters  154 ,  156  in  FIG. 6  are analog matched filters, but the invention may also be practiced with digital matched filters that output binary correlated signals that simply indicate the sign of the correlated result. These binary correlated signals may take values of ‘1’ and ‘−1’, for example, or ‘1’ and ‘0’. If digital matched filters of this type are used, the thresholding units  168 ,  170 ,  172  may be omitted, or may be replaced by latch circuits that simply latch the correlated signals in synchronization with a bit clock signal. The adder  166  may be replaced with a logic gate such as an AND gate. 
         [0087]    The transmitting apparatus  10  in  FIG. 5  can operate in one of two selectable modes on each channel. In the first mode, the two signal generators of the signal generating unit of the channel generate different data signals. In the second mode, the two signal generators generate identical data signals. 
         [0088]    The operation of the transmitting and receiving apparatus will now be described with reference to  FIGS. 7 and 8 , which illustrate the coding and decoding operations in the first channel. It will be assumed that the first channel operates in the first mode. 
         [0089]    Waveform A in  FIG. 7  shows exemplary data d 140 - 1   a  (1, 0, . . . ) output by signal generator  140 - 1   a.  Waveform B shows exemplary data d 140 - 1   b  (0, 1, . . . ) output by signal generator  140 - 1   b.  The output circuits of the signal generators  140 - 1   a,    140 - 1   b  may be biased so that the actual output signal levels are a positive voltage and an equal but opposite negative voltage. The data signal levels will also be denoted ‘1’ and ‘−1’ instead of ‘1’ and ‘0’. 
         [0090]    In coding unit  142 - 1 , coder  142 - 1   a  multiplies each bit of data signal d 140 - 1   a  by a predetermined spreading code (C a1-1 , C a1-2 , . . . , C a1-M ) of length M, indicated as waveform C, and coder  142 - 1   b  multiplies each bit of data signal d 140 - 1   b  by a different spreading code (C b1-1 , C b1-2 , . . . , C b1-M ) of the same length M, indicated as waveform D. Data signal d 140 - 1   a  is thereby encoded to the coded signal c 142 - 1   a  indicated as waveform E, while data signal d 1401   b  is simultaneously encoded to the coded signal c 142 - 1   b  indicated as waveform F. Parallel-to-serial converter  144 - 1  converts these two parallel coded signals c 142 - 1   a,  c 142 - 1   b  to a serial signal PC- 145 - 1  in which the chips output by coder  142 - 1   a  are interleaved with the chips output by coder  142 - 1   b,  as indicated in waveform G. 
         [0091]    In first terminal unit  130 - 1 , the first matched filter  154  samples the CDM signal, which includes serial signal PC- 145 - 1 , in synchronization with clock signal  159 - 1 , which has waveform H, thereby sampling the chips of coded signal c 142 - 1   a.  The second matched filter  156  samples the CDM signal in synchronization with clock signal  159 - 1 , which has waveform I, thereby sampling the chips of coded signal c 142 - 1   b.  The clock recovery unit  158  adjusts the timing of clock signals  159 - 1  and  159 - 2  so that their sampling edges (the rising edges indicated by arrows in the drawing) are located within the relevant chip intervals in waveform G. 
         [0092]    Clock waveforms H and I are reproduced in  FIG. 8 . The first matched filter  154 , operating in synchronization with waveform H, generates a first correlated signal  155  having a positive peak P 1  followed at one bit interval by a negative peak Q 1 , as shown in waveform J. The second matched filter  156 , operating in synchronization with waveform I, generates a second correlated signals  157  having a negative peak P 2  followed at one bit interval by a positive peak Q 2 , as shown in waveform K. 
         [0093]    The first thresholding unit  168 , operating in synchronization with clock signal  159 - 5  (not shown in  FIG. 8 ), and compares the first correlated signal  163 - 1  (waveform J) with the first threshold to derive the first received data signal  169  (waveform L), which reproduces the data in data signal d 140 - 1   a.  The second thresholding unit  172 , operating in synchronization with clock signal  159 - 3  (not shown in  FIG. 8 ), and compares the second correlated signal  165 - 1  (waveform K) with the second threshold to derive the second received data signal  173  (waveform M), which reproduces the data in data signal d 140 - 1   b.    
         [0094]    The first terminal unit  130 - 1  processes both the first received data signal  169  (waveform L) and second received data signal  173  (waveform M) as received data, but does not use the third received data signal  171  output by the third thresholding unit  170 . 
         [0095]    Operating in the first mode, the first channel in the novel transmitting and receiving apparatus has the combined data transmission capacity of two channels in the conventional CDM transmitting and receiving apparatus, as can be seen by comparing  FIGS. 7 and 8 , in which four bits are transmitted and received on one channel, with  FIG. 4 , in which four bits are transmitted and received on two channels. 
         [0096]    Operation in the second mode is the same as described above except that in the transmitting apparatus, the two signal generators  140 - 1   a,    140   b  generate identical data signals, and in the terminal unit, the third received data signal  171  output by the third thresholding unit  170  is processed as received data. 
         [0097]    Because the two signal generators  140 - 1   a,    140 - 1   b  generate identical data signals, the two matched filters  154 ,  156  in the terminal unit generate identical correlated signals. The two correlated signals  163 - 2 ,  165 - 2  received by the adder  166  are accordingly identical. In the combined correlated signal  167  output by the adder  166 , the peaks have absolute values twice as great as the peaks P 1 , Q 1 , P 2 , Q 2  in waveforms J and K in  FIG. 8 . 
         [0098]    The data can accordingly be received successfully even if the CDM signal  147  is attenuated during transmission by twice as much as in the first mode. Roughly speaking, this means that data can be transmitted twice as far in the second mode as in the first mode. The data transmission rate is the same as in the conventional CDM transmitting and receiving apparatus. 
         [0099]    The first and second received data signals  169 ,  173  output by the first and second thresholding units  168 ,  172  are not used in the second mode. 
         [0100]    In a COF-PON system, the novel transmitting and receiving apparatus described above provides the capability to accommodate both nearby terminal units requiring high data transmission rates and distant terminal units not requiring such high data transmission rates. 
         [0101]    The invention is not limited to the embodiment described above. In one variation of this embodiment, the first correlated signal  155 , the second correlated signal  157 , or the combined correlated signal  167  is supplied to the clock recovery unit  158  for use in generating the clock signals  159 - 3 ,  159 - 4 ,  159 - 5  supplied to the thresholding units. 
         [0102]    In another variation, a delay circuit is added to delay the first correlated signal  155  output from the first matched filter  154  by one chip period, to align the peaks in the first correlated signal  155  with the peaks in the second correlated signals  157  output from the second matched filter  156 . 
         [0103]    In still another variation, the CDM signal  151 - 1  input to the first matched filter  154  is delayed by one chip period for the same purpose. In this case, the same clock signal may be supplied to both matched filters  154 ,  156 . 
         [0104]    In yet another variation, splitters  162 ,  164 , the adder  166 , and the third thresholding unit  170  are omitted in terminal units operating in the first mode, and splitters  162 ,  164 , the first thresholding unit  168 , and the second thresholding unit  172  are omitted in terminal units operating in the second mode. 
         [0105]    In a further variation, the transmitting apparatus also includes channels having only a single signal generator, a single coder, and no parallel-to-serial converter. In these channels, the coded signal is input directly to the multiplexer. These channels can be used for terminal units that are located comparatively nearby and do not require a high data transmission rate. 
         [0106]    Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.