Patent Publication Number: US-2023155808-A1

Title: Transmission device, reception device, and base station

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2020/033218, filed on Sep. 2, 2020, and designating the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a transmission device, a reception device, and a base station. 
     2. Description of the Related Art 
     In related art, for performing a synchronization process using a preamble included in a received signal, that is, the auto-correlation property of a synchronization signal by a reception device, there is a method for increasing the symbol length of the synchronization signal to increase the auto-correlation property as a method of improving the synchronization performance. In a case where one ground station accommodates a plurality of terminals, the ground station needs to make synchronization signals orthogonal among the terminals to reduce cross-correlation between terminals. An example of a sequence with a high auto-correlation property is a Zadoff-Chu (ZC) sequence. In a case where individual terminals independently and randomly transmit signals and share one channel like a random access channel (RACH) in an uplink access method, ZC sequences having a high auto-correlation property may be used as a pilot signal for channel estimation. 
     In addition, Non Patent Literature 1, “Panasonic, NTT DoCoMo “Narrow band uplink reference signal sequences and allocation for E-UTRA”, 3GPP TSG-RAN WG1 Meeting #47, R1-063183, Riga, Latvia, Nov. 6-10, 2006 teaches a technology of using, as uplink pilot signals, cyclic-shift Zadoff-Chu (ZC) sequences obtained by cyclic shifting of ZC sequences in order to reduce inter-cell interference mutually received by pilot signals among cells in frame timing synchronization with each other in a multi-cell system. In this manner, Non Patent Literature 1, “Panasonic, NTT DoCoMo “Narrow band uplink reference signal sequences and allocation for E-UTRA”, 3GPP TSG-RAN WG1 Meeting #47, R1-063183, Riga, Latvia, Nov. 6-10, 2006 makes synchronization signals, which are assigned to respective terminals or a plurality of cells, orthogonal to each other, reduces interference of synchronization signals, and achieves multiplexing of terminals. 
     With the technology of the related art, however, when cyclic-shift ZC sequences are used for synchronization signals, a sidelobe due to cross-correlation between terminals is caused by performing cyclic shifting. There is therefore a problem in that estimation error may occur at the terminals. In addition, according to the technology of the related art, because the orthogonality is maintained by using all the symbols of a cyclic-shift ZC sequence, the orthogonality among terminals is lost when channel variation occurs within the symbols of the cyclic-shift ZC sequence. In a case where the number of symbols of the cyclic-shift ZC sequence to be used is reduced as measures against channel variation, there is a problem in that thermal noise tolerance and interference signal tolerance attributable to signal averaging are lowered. 
     The present disclosure has been made in view of the above, and an object thereof is to provide a transmission device capable of improving synchronization performance in an environment in which the states of channels vary in a radio communication system including a plurality of communication areas. 
     SUMMARY OF THE INVENTION 
     In order to solve the above problem and achieve the object, the present disclosure is included in one base station in a radio communication system including communication areas adjacent to each other in which the base station communicates with a plurality of wireless terminals. The present disclosure includes: a modulation unit to generate a data symbol sequence; a synchronization signal generating unit to generate a first symbol sequence constituted by two or more continuous repetitions of reference sequence symbols being a reference, generate a second symbol sequence by performing frequency shifting on the first symbol sequence by using a phase rotation sequence so that the reference sequence symbols become orthogonal for each of the wireless terminals, and generate a synchronization signal; and a synchronization signal adding unit to generate a transmission signal by adding the synchronization signal to the data symbol sequence. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an example of a configuration of a radio communication system according to a first embodiment; 
         FIG.  2    is a diagram illustrating an example of a configuration of a transmission device included in a base station according to the first embodiment; 
         FIG.  3    is a diagram illustrating an example of a structure of a radio frame transmitted by a transmission device of a base station according to the first embodiment; 
         FIG.  4    is a diagram illustrating procedures of generating a frequency shift pattern signal as a synchronization signal by a synchronization signal generating unit of a transmission device according to the first embodiment; 
         FIG.  5    is a diagram illustrating an example of frequency shifting in multiplexing of wireless terminals by a synchronization signal generating unit of a transmission device according to the first embodiment; 
         FIG.  6    is a diagram illustrating examples of a spectrum of an inner product of a second symbol sequence, which is a frequency shift pattern signal generated by a synchronization signal generating unit of a transmission device according to the first embodiment, in units of reference sequence symbols; 
         FIG.  7    is a diagram illustrating examples of a phase with which a frequency spectrum is generated by frequency shifting by a synchronization signal generating unit of a transmission device according to the first embodiment; 
         FIG.  8    is a diagram illustrating an example of a plurality of frequency shift pattern signals connected by a synchronization signal generating unit of a transmission device according to the first embodiment; 
         FIG.  9    is a diagram illustrating a reason for which cross-correlation occurs in a case where a synchronization signal generating unit of a transmission device according to the first embodiment connects a plurality of frequency shift pattern signals; 
         FIG.  10    is a diagram illustrating an example of second frequency shifting to reduce cross-correlation in a case where a synchronization signal generating unit of a transmission device according to the first embodiment connects a plurality of frequency shift pattern signals; 
         FIG.  11    is a diagram illustrating an example of a configuration of a synchronization signal generating unit of a transmission device according to the first embodiment; 
         FIG.  12    is a flowchart illustrating operations of a transmission device according to the first embodiment; 
         FIG.  13    is a diagram illustrating an example of a configuration of a reception device included in a wireless terminal according to the first embodiment; 
         FIG.  14    is a flowchart illustrating operations of a reception device according to the first embodiment; 
         FIG.  15    is a diagram illustrating example of a configuration of processing circuitry in a case where the processing circuitry included in a transmission device according to the first embodiment is implemented by a processor and a memory; 
         FIG.  16    is a diagram illustrating an example of processing circuitry in a case where the processing circuitry included in a transmission device according to the first embodiment is constituted by dedicated hardware; 
         FIG.  17    is a diagram illustrating an example of a method for estimating an interference power by a reception device of a wireless terminal according to a second embodiment; 
         FIG.  18    is a diagram illustrating an example of a configuration of a transmission device included in a base station according to a third embodiment; 
         FIG.  19    is a diagram illustrating an example of a configuration of a spreading sequence generating unit of a transmission device according to the third embodiment; 
         FIG.  20    is a diagram illustrating an example of a configuration of a reception device included in a wireless terminal according to the third embodiment; 
         FIG.  21    is a diagram illustrating an example of a synchronization process performed by a reception device included in a wireless terminal in a case of using a spreading sequence in a comparative example; and 
         FIG.  22    is a diagram illustrating an example of a synchronization process performed by a reception device included in a wireless terminal according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A transmission device, a reception device, a base station, a wireless terminal, a radio communication system, a control circuit, a storage medium, a transmission method, and a reception method according to certain embodiments of the present disclosure will be described in detail below with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a diagram illustrating an example of a configuration of a radio communication system  100  according to a first embodiment. The radio communication system  100  includes base stations  101   a  and  101   b , and wireless terminals  102   a  and  102   b . The base station  101   a  forms a communication area  103   a . The base station  101   b  forms a communication area  103   b . In the description below, the base stations  101   a  and  101   b  may be referred to as base stations  101  when the base stations are not distinguished from one another, the wireless terminals  102   a  and  102   b  may be referred to as wireless terminals  102  when the wireless terminals are not distinguished from one another, and the communication areas  103   a  and  103   b  may be referred to as communication areas  103  when the communication areas are not distinguished from one another. The radio communication system  100  is a system in which a plurality of communication areas  103  are formed, which are adjacent to each other. 
     In the radio communication system  100 , one base station  101  forms one communication area  103 , and the communication area  103  can accommodate a plurality of wireless terminals  102 . Specifically, a base station  101  transmits a signal for each wireless terminal  102 . Each wireless terminal  102  receives the signal transmitted from the base station  101  and performs communication in the communication area  103 . The number of wireless terminals  102  accommodated in the communication area  103  is one or more, and is not limited. Note that other base stations  101  may be accommodated in the communication area  103  of the base station  101 , and the number of base stations  101  accommodated in each communication area  103  is not limited. In the radio communication system  100 , all the base stations  101  are each assumed to be in time synchronization with the other base stations  101 , and transmit signals, that is, radio frames at the same timings. As the method of time synchronization between the base stations  101 , any method may be used. All the base stations  101  implement time synchronization by using a global positioning system (GPS), for example. Two or more first base stations located at a boundary of communication areas  103  adjacent to each other transmit radio frames including different synchronization signals from each other. The synchronization signals included in the radio frames transmitted by the first base stations are also different from synchronization signals included in radio frames transmitted by second base stations that are not located at the boundary of the communication areas  103 . 
     A base station  101  includes a transmission device and a reception device. A wireless terminal  102  also includes a transmission device and a reception device. In the present embodiment, configurations and operations of a transmission device included in a base station  101  and a reception device included in a wireless terminal  102  will be described. 
     First, a transmission device included in a base station  101  will be described.  FIG.  2    is a diagram illustrating an example of a configuration of a transmission device  200  included in a base station  101  according to the first embodiment. The transmission device  200  includes a modulation unit  202 , a synchronization signal generating unit  209 , synchronization signal adding units  203 , transmission filter units  204 , digital-to-analog conversion units  205 , transmission high-frequency units  206 , and transmission antennas  207 . In the example illustrated in  FIG.  2   , the transmission device  200  includes a plurality of sets including a synchronization signal adding unit  203 , a transmission filter unit  204 , a digital-to-analog conversion unit  205 , a transmission high-frequency unit  206 , and a transmission antenna  207 . Note that the transmission device  200  may have a configuration including only one set including a synchronization signal adding unit  203 , a transmission filter unit  204 , a digital-to-analog conversion unit  205 , a transmission high-frequency unit  206 , and a transmission antenna  207 . 
     The modulation unit  202  performs first-order modulation on a data signal  201 , which is a bit sequence, to generate a data symbol sequence. Examples of the modulation method of the first-order modulation include phase shift keying (PSK), frequency shift keying (FSK), and quadrature amplitude modulation (QAM), but are not limited thereto. The modulation unit  202  outputs the generated data symbol sequence to each of the synchronization signal adding units  203 . 
     The synchronization signal generating unit  209  generates a synchronization signal on the basis of a pattern instruction signal  208  input as a control parameter to the transmission device  200 . Specifically, the synchronization signal generating unit  209  generates, as the synchronization signal, a symbol sequence in which an arrangement pattern of transmission symbols on a frequency axis is a frequency pattern instructed by the pattern instruction signal  208 . The synchronization signal generating unit  209  outputs the generated symbol sequence, that is, the synchronization signal to each of the synchronization signal adding units  203 . In the present embodiment, pattern instruction signals  208  indicate frequency patterns to the respective transmission devices  200  included in the radio communication system  100  so that the transmission devices  200  transmit symbol signals, that is, synchronization signals whose frequency patterns are different from each other. Regarding the pattern instruction signals  208  for the respective transmission devices  200  in the radio communication system  100 , a host device or host devices of base stations  101  including the transmission devices  200  output, to the respective base stations  101 , the pattern instruction signals  208  depending on the respective base stations  101 , for example. Detailed configuration and operation of the synchronization signal generating unit  209  will be described later. 
     Each synchronization signal adding unit  203  generates a transmission signal on the basis of the synchronization signal generated by the synchronization signal generating unit  209  and the data symbol sequence generated by the modulation unit  202 . Specifically, the synchronization signal adding unit  203  generates a transmission signal by adding the synchronization signal obtained from the synchronization signal generating unit  209  in units of radio frames to the data symbol sequence obtained from the modulation unit  202 . The synchronization signal adding unit  203  outputs the generated transmission signal to the transmission filter unit  204 . 
     Each transmission filter unit  204  upsamples the transmission signal obtained from the synchronization signal adding unit  203  and limits the bandwidth to generate a baseband signal or a transmission digital signal, which is an intermediate frequency (IF) signal. A Nyquist filter is typically used as a bandlimiting filter used by the transmission filter unit  204  when limiting the bandwidth of a transmission signal, but the bandlimiting filter is not limited thereto. The transmission filter unit  204  outputs the generated transmission digital signal to the digital-to-analog conversion unit  205 . 
     Each digital-to-analog conversion unit  205  converts the transmission digital signal obtained from the transmission filter unit  204  into a transmission analog signal. The digital-to-analog conversion unit  205  outputs the transmission analog signal resulting from the conversion to the transmission high-frequency unit  206 . Each transmission high-frequency unit  206  performs frequency conversion on the transmission analog signal obtained from the digital-to-analog conversion unit  205  to generate a radio frame, which is a signal in a radio frequency band. Each transmission high-frequency unit  206  outputs the radio frame to the transmission antenna  207 . Each transmission antenna  207  radiates the radio frame obtained from the transmission high-frequency unit  206  in a form of a radio wave. 
     The transmission device  200  has a configuration capable of transmitting not only the same synchronization signals as the synchronization signals included in radio frames transmitted from the respective transmission antennas  207 , but also synchronization signals different from the synchronization signals included in the radio frames transmitted from the respective transmission antennas  207 , in accordance with the pattern instruction signals  208 . Alternatively, a transmission device  200  according to a modification may have a configuration in which the synchronization signal adding unit  203  is connected downstream of the transmission filter unit  204 , and a synchronization signal is added to a transmission digital signal resulting from band limitation by the transmission filter unit  204 . In this case, the synchronization signal generating unit  209  generates a synchronization signal with the same sampling rate as that of the transmission digital signal output from the transmission filter unit  204 . 
       FIG.  3    is a diagram illustrating an example of a structure of a radio frame transmitted by the transmission device  200  of the base station  101  according to the first embodiment. As illustrated in  FIG.  3   , a radio frame has a structure in which a synchronization signal  301  is added to a data symbol sequence  302  in units of radio frame. As described above, the synchronization signal  301  is generated by the synchronization signal generating unit  209 , and the data symbol sequence  302  is generated by the modulation unit  202 . 
     The synchronization signal  301  is used for synchronization of radio frames, frequency synchronization, symbol timing synchronization, and the like by a wireless terminal  102  of the reception side. The synchronization signal  301  is a signal with a frequency pattern varying depending on time in units of reference sequence symbols. In addition, the shape of the frequency pattern of the synchronization signal  301  varies depending on the communication area  103  formed by a base station  101  and varies depending on the wireless terminal  102  with which the transmission device  200  communicates. Details thereof will be described later. When the frequency pattern of the synchronization signal  301  varies depending on each base station  101 , each wireless terminal  102  can individually measure, for each base station  101 , the reception electric field intensity of a radio frame transmitted from each base station  101 . 
     A signal with a frequency shift pattern generated by the synchronization signal generating unit  209  (hereinafter referred to as a frequency shift pattern signal) will be described.  FIG.  4    is a diagram illustrating procedures of generating a frequency shift pattern signal as a synchronization signal by the synchronization signal generating unit  209  of a transmission device  200  according to the first embodiment. The synchronization signal generating unit  209  first generates a first symbol sequence  402  obtained by repeating reference sequence symbols  401  and connecting the resulting reference sequence symbols  401  as illustrated in  FIGS.  4 ( a ) and  4 ( b ) . The reference sequence symbols  401  constitute a symbol sequence that is known by a wireless terminal  102  of the reception side, and are expressed by a complex vector having preset amplitude and phase. In the example of  FIG.  4 ( b ) , the first symbol sequence  402  is a symbol sequence obtained by repeating the reference sequence symbols  401  four times. 
     In the example illustrated in  FIG.  4   , the reference sequence symbols  401  constitute a symbol sequence for synchronization including four symbols a1 to a4. Because the reference sequence symbols  401  are also used for synchronization of radio frames bye the wireless terminal  102  of the reception side, it is preferable to apply a combination of a plurality of orthogonal symbol sequences with good auto-correlation property and good cross-correlation property. A Walsh code or a constant amplitude zero auto-correlation (CAZAC) sequence, for example, may be applied to the reference sequence symbols  401 . The synchronization signal generating unit  209  can increase the number of frequency shift pattern signals that are orthogonal to each other by using each of a plurality of orthogonal sequences as the reference sequence symbols  401 . 
     Subsequently, as illustrated in  FIG.  4 ( c ) , the synchronization signal generating unit  209  performs first frequency shifting of multiplying the first symbol sequence  402 , which is obtained by repeating the reference sequence symbols  401  and connecting the resulting reference sequence symbols  401 , by a frequency shift amount f k,m    403 . The synchronization signal generating unit  209  can thus generate a second symbol sequence  404  illustrated in  FIG.  4 ( d ) . The second symbol sequence  404  is a frequency shift pattern signal corresponding to the synchronization signal  301  illustrated in  FIG.  3   . When the first symbol sequence  402  obtained by repeating the reference sequence symbols  401  and connecting the resulting reference sequence symbols  401  is represented by a k  and a symbol sequence constituting the frequency shift pattern signal is represented by P k , the symbol sequence P k  can be calculated by formula (1). 
       Formula 1: 
         P   k   =a   k  exp[2π jf   k,m   k ]  (1)
 
     In formula (1), f k,m  represents a frequency shift amount that is the same value in each unit of the reference sequence symbols  401 . When k represents an index number of the symbol sequence constituting the frequency shift pattern signal and N represents the sequence length of the frequency shift pattern signal, k is an integer satisfying 1≤k≤N. m represents a parameter determining The amount of frequency by which each set of reference sequence symbols  401  is to be shifted. m is a preset integer. Alternatively, as expressed by formula (2), the symbol sequence P k  can also be obtained by performing different phase rotation on each symbol. 
       Formula 2: 
         P   k   =a   k  exp[ jθ   k,m   /L ]  (2)
 
     In formula (2), θ k,m  represents a phase rotation amount corresponding to frequency shifting, and L represents the number of reference sequence symbols indicating the number of symbols of the reference sequence symbols  401 . Note that 0&lt;m≤L, is satisfied. 
       FIG.  4    illustrates an example of a case where the sequence length L, which is the number of reference sequence symbols  401 , is L=4 and the number of repetitions REPM is REPM=4, and in this case, the sequence length of the frequency shift pattern signal is L×REPM=16. The synchronization signal generating unit  209  can generate the second symbol sequence  404 , which is a frequency shift pattern signal, that is, a synchronization signal through the process as described above, but is not limited thereto. The synchronization signal generating unit  209  may be configured to store all frequency shift pattern signals generated in advance in a memory or the like, select a frequency shift pattern signal indicated by the pattern instruction signal  208  and read the frequency shift pattern signal as a synchronization signal from the memory. 
       FIG.  5    is a diagram illustrating an example of frequency shifting in multiplexing of wireless terminals  102  by the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment. As illustrated in  FIG.  5   , the synchronization signal generating unit  209  can use the reference sequence symbols  401  in common as a symbol sequence for synchronization for a plurality of wireless terminals  102  by selecting frequency shift amounts f k,m    403   a  and  403   b  to be orthogonal between users, that is, between wireless terminals  102 . While two wireless terminals  102 , which are wireless terminals #1 and #2, are specifically illustrated as a plurality of wireless terminals  102  in the example illustrated in  FIG.  5   , the number of wireless terminals  102  may be three or more. In addition, the synchronization signal generating unit  209  can increase the number of synchronization signals, that is, the number of the generated second symbol sequences  404  by selecting the reference sequence symbols  401  for synchronization to be sequences orthogonal to each other and also the frequency shift amounts f k,m    403  to be sequences orthogonal to each other. 
       FIG.  6    is a diagram illustrating examples of a spectrum of an inner product of the second symbol sequence  404 , which is a frequency shift pattern signal generated by the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment in units of the reference sequence symbols  401 .  FIG.  7    is a diagram illustrating examples of a phase with which the frequency spectrum is generated by frequency shifting by the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment.  FIG.  6    illustrates examples in which the spectral position of the frequency spectrum is shifted by the frequency shift amount f k,m    403  of formula (1) illustrated in  FIG.  7    or a phase rotation amount θ k,m    701  of formula (2) illustrated in  FIG.  7   . In  FIG.  7 ,  702    represents a specific example of the phase rotation amount θ k,m    701 . 
     In addition, in  FIG.  6   , a signal power at a frequency position at which a signal component is present is represented by  601 , and a signal power at null frequency, that is, a frequency position at which no signal component is present is represented by  602 . The synchronization signal generating unit  209  can distribute signal components of each frequency shift pattern signal to a specific frequency position and make the signal components orthogonal to each other in the frequency domain by changing a set of m which is a parameter. Thus, the synchronization signal generating unit  209  can generate m kinds of frequency shift pattern signals each with signal components distributed to a specific frequency position, where the frequency positions at which the signal components are present differ from each other. The number of frequency shift pattern signals that are orthogonal in the frequency domain depends on the number of repetitions of the basic reference sequence symbols  401  in the process of generating frequency shift pattern signals, that is, the number of repetitions REPM of the reference sequence symbols  401  for obtaining the first symbol sequence  402  illustrated in  FIG.  4   . 
     The radio communication system  100  uses a plurality of orthogonal frequency shift pattern signals obtained as described above to assign different frequency shift pattern signals as synchronization signals to the transmission devices  200  of the respective base stations  101  each forming a communication area  103 . The assignment of the frequency shift pattern signals to the transmission devices  200  of the respective base stations  101  is performed by a host device or host devices of the base stations  101 , for example. In a case where a transmission device  200  of a base station  101  includes a plurality of transmission antennas  207  as in the example of the configuration illustrated in  FIG.  2   , the radio communication system  100  may assign different frequency shift pattern signals as synchronization signals to the respective transmission antennas  207 . 
     In addition, the radio communication system  100  assigns different frequency shift pattern signals as synchronization signals to transmission devices  200  of the respective base stations  101  of two adjacent communication areas  103 , which are the base stations  101   a  and  101   b  of the two adjacent communication areas  103   a  and  103   b  in the example of  FIG.  1   . In this case, in the radio communication system  100 , synchronization signals that are different frequency shift pattern signals are assigned to all the base stations  101  each forming a communication area  103 , including a base station  101  at the boundary of the communication area  103   a  and the communication area  103   b . In addition, as a result of selecting synchronization signals, which are frequency shift pattern signals, to be assigned to the respective wireless terminals  102  so that the synchronization signals are orthogonal for each wireless terminal  102  by the base station  101 , the wireless terminals  102  can remove overreach interference from another communication area  103 . 
     In addition to the method of using frequency shift pattern signals as synchronization signals, there is a method of repeating a frequency shift pattern signal as a method for improving the synchronization performance. Generally, as the number of symbols is used for a synchronization signal is larger, a sequence is more unique. Thus, the synchronization performance can be improved by increasing the sequence length; however, the maximum length of a frequency shift pattern signal is L×REPM. 
     Thus, there is a method of simply repeating a frequency shift pattern signal in units of L×REPM symbols as illustrated in  FIG.  8   , but in this case, as illustrated in  FIG.  9   , the repetition of the sequence of the frequency shift pattern signal may cause a sidelobe in cross-correlation power distribution of the frequency shift pattern signal and erroneous detection of a synchronization position may be occurred.  FIG.  8    is a diagram illustrating an example in which the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment connects a plurality of frequency shift pattern signals.  FIG.  9    is a diagram illustrating a reason for which cross-correlation occurs in a case where the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment connects a plurality of frequency shift pattern signals.  FIG.  8    illustrates an example in which the frequency shift pattern signals is repeated twice.  FIG.  9 ( a )  illustrates an example of a radio frame in a case where the frequency shift pattern signal is repeated twice.  FIG.  9 ( b )  illustrates a peak position of correlation power with a reference signal used for estimating a synchronization position at the reception side.  FIG.  9 ( c )  illustrates a position as which a sidelobe occurs in correlation power with a reference signal used for estimating a synchronization position at the reception side. In the example of  FIG.  9 ( c ) , there is a match of 50%, which means that a high correlation power is caused. 
     Thus, in the present embodiment, the synchronization signal generating unit  209  can generate a synchronization symbol sequence PS with reduced cross-correlation by performing second frequency shifting so that repeated frequency shift pattern signals are orthogonal to each other among frequency shift pattern signals as illustrated in  FIG.  10   .  FIG.  10    is a diagram illustrating an example of the second frequency shifting to reduce cross-correlation in a case where the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment connects a plurality of frequency shift pattern signals.  FIG.  10 ( a )  illustrates an example in which a frequency shift pattern signal that is the second symbol sequence  404  is repeated twice.  FIG.  10 ( b )  illustrates an example of the second frequency shifting.  FIG.  10 ( c )  illustrates a synchronization symbol sequence PS generated by the second frequency shifting. As a result of the processes illustrated in  FIG.  10   , the synchronization signal generating unit  209  can generate a synchronization symbol sequence PS of L×REPM×NSP symbols using the number of repetitions NSP of a frequency shift pattern signal. 
     In this process, the synchronization signal generating unit  209  has to select the second frequency shifting to maintain the orthogonality between frequency shift pattern signals obtained by the first frequency shifting. A configuration of the synchronization signal generating unit  209  that performs such processes will be described.  FIG.  11    is a diagram illustrating an example of a configuration of the synchronization signal generating unit  209  of the transmission device  200  according to the first embodiment. The synchronization signal generating unit  209  includes a first repetition unit  221 , a first frequency shifting unit  222 , a second repetition unit  223 , a second frequency shifting unit  224 , and a phase offset unit  225 . 
     The first repetition unit  221  connects REPM repetitions of the reference sequence symbols  401  having a sequence length L to generate a first symbol sequence  402  of L×REPM symbols. The first frequency shifting unit  222  performs, for orthogonality among users, that is, among the wireless terminals  102 , the first frequency shifting in units of reference sequence symbols  401  on the first symbol sequence  402  to generate a second symbol sequence  404 . The second repetition unit  223  connects NSP repetitions of the second symbol sequence  404  of L×REPM symbols to generate a frequency shift pattern signal of L×REPM×NSP symbols. The second frequency shifting unit  224  performs, for lower cross-correlation, the second frequency shifting on the frequency shift pattern signal of L×REPM×NSP symbols to generate a synchronization symbol sequence PS. 
     Note that, when arrangement of signal point positions of the synchronization symbol sequence PS is biased, this may cause degradation in the synchronization performance when a signal like a continuous wave (CW) with biased frequency coming from another system is mixed as interference. The phase offset unit  225  therefore adds a phase offset defined in units of symbols for the first frequency shifting to the synchronization symbol sequence PS to eliminate the signal point arrangement bias and reduce degradation in synchronization performance due to an interference wave from another system. The synchronization signal generating unit  209  uses a synchronization symbol sequence PSO obtained by the processes performed by the phase offset unit  225  as a synchronization signal  301 . 
     Note that, in a case where the channel states are stable, such as a case where no signal coming from another system is mixed as interference and the synchronization performance is not degraded, the synchronization signal generating unit  209  may use the second symbol sequence  404  output from the first frequency shifting unit  222  as a synchronization signal or may use the synchronization symbol sequence PS output from the second frequency shifting unit  224  as a synchronization signal. 
     In the case of using the second symbol sequence  404  output from the first frequency shifting unit  222  as a synchronization signal, the synchronization signal generating unit  209  generates a first symbol sequence  402  constituted by two or more continuous repetitions of the reference sequence symbols  401  being a reference, and performs frequency shifting on the first symbol sequence  402  by using a phase rotation sequence so that the reference sequence symbols  401  become orthogonal for each wireless terminal  102  to generate a second symbol sequence  404 . In this case, the synchronization signal generating unit  209  holds one or more sets of reference sequence symbols  401  and one or more phase rotation sequences, and generates the first symbol sequence  402  and the second symbol sequence  404  by using one of the sets of reference sequence symbols  401  and one of the phase rotation sequences for each radio frame period. 
     In the case of using the synchronization symbol sequence PS output from the second frequency shifting unit  224  as a synchronization signal, and the aforementioned phase rotation sequence is referred to as a first phase rotation sequence, for example, the synchronization signal generating unit  209  performs frequency shifting on the second symbol sequence  404  by using a second phase rotation sequence so that second symbol sequences  404  to be repeatedly transmitted become orthogonal to each other to generate a third symbol sequence that is the synchronization symbol sequence PS. In this case, the synchronization signal generating unit  209  holds one or more sets of reference sequence symbols  401 , one or more first phase rotation sequences, and one or more second phase rotation sequences, and generates the first symbol sequence  402 , the second symbol sequence  404 , and the third symbol sequence by using one of the sets of reference sequence symbols  401 , one of the first phase rotation sequences, and one of the second phase rotation sequences for each radio frame period. 
     In the case of using the synchronization symbol sequence PSO output from the phase offset unit  225  as a synchronization signal, the synchronization signal generating unit  209  performs frequency shifting on the third symbol sequence by using a third phase rotation sequence so that the signal point bias of the reference sequence symbols  401  is eliminated to generate a fourth symbol sequence that is the synchronization symbol sequence PSO. In this case, the synchronization signal generating unit  209  holds one or more sets of reference sequence symbols  401 , one or more first phase rotation sequences, one or more second phase rotation sequences, and one or more third phase rotation sequences, and generates the first symbol sequence  402 , the second symbol sequence  404 , the third symbol sequence, and the fourth symbol sequence by using one of the sets of reference sequence symbols  401 , one of the first phase rotation sequences, one of the second phase rotation sequences, and one of the third phase rotation sequences for each radio frame period. 
     The operations of the transmission device  200  will be explained with reference to a flowchart.  FIG.  12    is a flowchart illustrating the operations of the transmission device  200  according to the first embodiment. In the transmission device  200 , the modulation unit  202  performs first-order modulation on a data signal  201 , which is a bit sequence, to generate a data symbol sequence (step S 101 ). The synchronization signal generating unit  209  generates a synchronization signal on the basis of a pattern instruction signal  208  (step S 102 ). The synchronization signal adding unit  203  generates a transmission signal on the basis of the synchronization signal generated by the synchronization signal generating unit  209  and the data symbol sequence generated by the modulation unit  202  (step S 103 ). The transmission filter unit  204  upsamples the transmission signal and limits the bandwidth to generate a transmission digital signal (step S 104 ). The digital-to-analog conversion unit  205  converts the transmission digital signal obtained from the transmission filter unit  204  into a transmission analog signal (step S 105 ). The transmission high-frequency unit  206  performs frequency conversion on the transmission analog signal to generate a radio frame (step S 106 ). The transmission antenna  207  radiates the radio frame in a form of a radio wave (step S 107 ). 
     Next, a reception device that is included in the wireless terminal  102  and that receives radio frames transmitted from the base station  101  will be described.  FIG.  13    is a diagram illustrating an example of a configuration of a reception device  500  included in a wireless terminal  102  according to the first embodiment. The reception device  500  includes reception antennas  501 , reception high-frequency units  502 , analog-to-digital conversion units  503 , reception filter units  504 , a received synchronization signal generating unit  505 , a synchronization unit  506 , a received signal measuring unit  507 , an interference signal measuring unit  508 , a measurement result storing unit  509 , and a demodulation unit  510 . In the example illustrated in  FIG.  13   , the reception device  500  includes a plurality of sets including a reception antenna  501 , a reception high-frequency unit  502 , an analog-to-digital conversion unit  503 , and a reception filter unit  504 . Note that the reception device  500  may have a configuration including only one set including a reception antenna  501 , a reception high-frequency unit  502 , an analog-to-digital conversion unit  503 , and a reception filter unit  504 . 
     Each reception antenna  501  receives radio frames. The reception antenna  501  outputs a received radio frame to the reception high-frequency unit  502 . Each reception high-frequency unit  502  downsamples the radio frame obtained from the reception antenna  501  to convert the radio frame into an IF signal or a baseband signal, which is an analog signal. The reception high-frequency unit  502  outputs the IF signal or baseband signal resulting from the conversion to the analog-to-digital conversion unit  503 . Each analog-to-digital conversion unit  503  converts the analog signal obtained from the reception high-frequency unit  502  into a digital signal. The analog-to-digital conversion unit  503  outputs the digital signal resulting from the conversion to the reception filter unit  504 . Each reception filter unit  504  limits the bandwidth of the digital signal obtained from the analog-to-digital conversion units  503  so as to remove noise out of the desired signal frequency band. The reception filter unit  504  outputs the radio frame resulting from the bandwidth limitation to the synchronization unit  506 . 
     The received synchronization signal generating unit  505  generates a synchronization pattern signal similar to a synchronization signal generated by a synchronization signal generating unit  209  of a transmission device  200  included in a base station  101 . Note that the received synchronization signal generating unit  505  generates synchronization pattern signals of a plurality of synchronization shift patterns that the synchronization signal generating unit  209  may generate. For example, in a case where the number of kinds of synchronization shift patterns of synchronization signals that the synchronization signal generating unit  209  of the transmission device  200  may generate is four, the received synchronization signal generating unit  505  generates four kinds of synchronization pattern signals. The received synchronization signal generating unit  505  has the function of generating the same signal as the synchronization signal that the synchronization signal generating unit  209  of the transmission device  200  generates, but is different in that the received synchronization signal generating unit  505  generates, as synchronization pattern signals, all synchronization signals that may be received, that is all synchronization signals that the synchronization signal generating unit  209  of the transmission device  200  may generate. The received synchronization signal generating unit  505  generates all the synchronization pattern signals in a manner similar to the method by which the synchronization signal generating unit  209  of the transmission device  200  generates the synchronization signals. The received synchronization signal generating unit  505  may generate synchronization pattern signals by storing all the synchronization pattern signals generated in advance in a memory or the like, and reading the synchronization pattern signals from the memory. The received synchronization signal generating unit  505  outputs the generated synchronization pattern signals to the synchronization unit  506 . 
     The synchronization unit  506  performs a synchronization process, that is, a process of determination on a synchronization signal on the basis of the radio frames obtained from the respective reception filter units  504 , and a plurality of synchronization pattern signals obtained from the received synchronization signal generating unit  505  to achieve synchronization of the radio frames. Specifically, the synchronization unit  506  calculates a correlation power of each of the radio frames obtained from the respective reception filter units  504  and each of the synchronization pattern signals. The synchronization unit  506  performs determination on each of the obtained correlation powers by using a first threshold, selects correlation powers that exceed the first threshold, and detects a timing at which the sum of the selected correlation powers is largest. Subsequently, the synchronization unit  506  uses a second threshold on the sum of correlation powers at the timing of maximum sum to determine whether or not the sum of correlation powers exceeds the second threshold, and determines detection of a synchronization signal included in a radio frame. Thus, the synchronization unit  506  detects a synchronization signal included in a radio frame by using the first threshold, and determines the reception timing of the detected synchronization signal by using the second threshold. 
     Details of the processes of detecting a synchronization signal by the synchronization unit  506  will be explained. When a radio frame is represented by r q (t), an antenna number of the reception antenna  501  that receives the radio frame is represented by q, a symbol period is represented by T s , each symbol sequence constituting a synchronization pattern signal is represented by PSO i,k , and a kind number of the synchronization pattern signal is represented by i, the synchronization unit  506  calculates a correlation power PC q,i (t) of the radio frame received by the reception antenna  501  with the antenna number q with reference sequence symbols P k of the synchronization pattern signal at sample time t as expressed by formula (3). Note that 1≤q≤Q is satisfied. In addition, the synchronization unit  506  calculates a received signal power PR q (t) of the radio frame received by the reception antenna  501  with the antenna number q at sample time t as expressed by formula (4). 
     
       
         
           
             
               
                 
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     As a result, the synchronization unit  506  can obtain a normalized correlation power S i (t) resulting from normalization by the received power as expressed by formula (5). 
     
       
         
           
             
               
                 
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     The synchronization unit  506  determines time t at which the normalized correlation power S i (t) is largest as reception timing of a synchronization signal  301  in a radio frame. The synchronization unit  506  may further set a third threshold, and perform synchronization determination only on normalized correlation powers S i (t) larger than the third threshold. 
     The received signal measuring unit  507  measures reception electric field intensity for each transmission device  200  included in a base station  101  that is the transmission source of a radio frame on the basis of a synchronization signal included in the radio frame. The received signal measuring unit  507  store the measured reception electric field intensity in the measurement result storing unit  509 . The measurement result storing unit  509  stores the reception electric field intensity measured by the received signal measuring unit  507 . 
     The interference signal measuring unit  508  measures an interference signal on the basis of a signal power other than the reception electric field intensity of each transmission device  200  included in a base station  101 . The interference signal measuring unit  508  reads the reception electric field intensity measured and stored by the received signal measuring unit  507  from the measurement result storing unit  509 . 
     The demodulation unit  510  performs a demodulation process on a data symbol sequence  302  corresponding to a data signal from a symbol sequence constituting a radio frame. The demodulation unit  510  outputs a demodulated data signal  511  obtained by the demodulation process. 
     The operations of the reception device  500  will be explained with reference to a flowchart.  FIG.  14    is a flowchart illustrating the operations of the reception device  500  according to the first embodiment. In the reception device  500 , the reception antenna  501  receives a radio frame (step S 201 ). The reception high-frequency units  502  downsamples the radio frame to convert the radio frame into an analog signal (step S 202 ). The analog-to-digital conversion unit  503  converts the analog signal into a digital signal (step S 203 ). The reception filter unit  504  limits the bandwidth of the digital signal (step S 204 ). The received synchronization signal generating unit  505  generates a synchronization pattern signal similar to a synchronization signal generated by a synchronization signal generating unit  209  of a transmission device  200  (step S 205 ). The synchronization unit  506  achieves synchronization of radio frames on the basis of a plurality of synchronization pattern signals obtained from the received synchronization signal generating unit  505  and the radio frames with the bandwidth limited by the reception filter unit  504  (step S 206 ). The received signal measuring unit  507  measures reception electric field intensity for each transmission device  200  included in a base station  101  that is the transmission source of a radio frame on the basis of a synchronization signal (step S 207 ). The interference signal measuring unit  508  measures an interference signal on the basis of a signal power other than the reception electric field intensity of each transmission device  200  included in a base station  101  (step S 208 ). The demodulation unit  510  performs a demodulation process on a data symbol sequence  302  constituting a radio frame (step S 209 ). 
     Next, a hardware configuration of a transmission device  200  included in a base station  101  will be described. In the transmission device  200 , the transmission antenna  207  is implemented by an antenna element. The modulation unit  202 , the synchronization signal generating unit  209 , the synchronization signal adding unit  203 , the transmission filter unit  204 , the digital-to-analog conversion unit  205 , and the transmission high-frequency unit  206  are implemented by processing circuitry. The processing circuitry may be constituted by a processor that executes programs stored in a memory and the memory, or may be dedicated hardware. The processing circuitry is also called a control circuit. 
       FIG.  15    is a diagram illustrating an example of a configuration of processing circuitry  90  in a case where the processing circuitry  90  included in the transmission device  200  according to the first embodiment is implemented by a processor  91  and a memory  92 . The processing circuitry  90  illustrated in  FIG.  15    is a control circuit including the processor  91  and the memory  92 . In the case where the processing circuitry  90  is constituted by the processor  91  and the memory  92 , the functions of the processing circuitry  90  are implemented by software, firmware, or a combination of software and firmware. The software or firmware is described in the form of programs and stored in the memory  92 . The processing circuitry  90  implements the functions by reading and executing the programs stored in the memory  92  by the processor  91 . Specifically, the processing circuitry  90  includes the memory  92  for storing programs that results in execution of processes of the transmission device  200 . The programs are, in other words, programs for causing the transmission device  200  to perform the functions implemented by the processing circuitry  90 . The programs may be provided by a storage medium storing the programs, or may be provided by other means such as a communication medium. 
     The programs are, in other words, programs for causing the transmission device  200  to perform a first step in which the modulation unit  202  generates a data symbol sequence, a second step in which the synchronization signal generating unit  209  generates a first symbol sequence  402  constituted by two or more continuous repetitions of the reference sequence symbols  401  being a reference, and performs frequency shifting on the first symbol sequence  402  by using a phase rotation sequence so that the reference sequence symbols  401  become orthogonal for each wireless terminal  102  to generate a second symbol sequence  404 , and generates a synchronization signal, and a third step in which the synchronization signal adding unit  203  adds the synchronization signal to the data symbol sequence to generate a transmission signal. 
     Note that the processor  91  is a central processing unit (CPU), a processing device, a computing device, a microprocessor, a microcomputer, a digital signal processor (DSP), or the like, for example. In addition, the memory  92  is a nonvolatile or volatile semiconductor memory such as a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable programmable ROM (EPROM), or an electrically EPROM (EEPROM: registered trademark), a magnetic disk, a flexible disk, an optical disk, a compact disc, a mini disc, a digital versatile disc (DVD) or the like, for example. 
       FIG.  16    is a diagram illustrating an example of processing circuitry  93  in a case where the processing circuitry included in a transmission device  200  according to the first embodiment is constituted by dedicated hardware. The processing circuitry  93  illustrated in  FIG.  16    is a single circuit, a composite circuit, a programmed processor, a parallel-programmed processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a combination thereof, for example. Part of the processing circuitry may be implemented by dedicated hardware, and part thereof may be implemented by software or firmware. As described above, the processing circuitry is capable of implementing the above-described functions by dedicated hardware, software, firmware, or a combination thereof. 
     While the hardware configuration of a transmission device  200  included in a base station  101  has been described, the same is applicable to the hardware configuration of a reception device  500  included in a wireless terminal  102 . In the reception device  500 , the reception antenna  501  is an antenna element. The measurement result storing unit  509  is a memory. The reception high-frequency unit  502 , the analog-to-digital conversion unit  503 , the reception filter unit  504 , the received synchronization signal generating unit  505 , the synchronization unit  506 , the received signal measuring unit  507 , the interference signal measuring unit  508 , and the demodulation unit  510  are implemented by processing circuitry. In a manner similar to the case of the transmission device  200 , the processing circuitry may be constituted by a processor that executes programs stored in a memory and the memory, or may be dedicated hardware. 
     As described above, according to the present embodiment, in the radio communication system  100 , the transmission device  200  of each base station  101  generates a synchronization signal to which a frequency shift pattern signal unique to each base station  101  is assigned, and transmits a radio frame including the synchronization signal from the transmission antenna  207 . The transmission device  200  has a function capable of changing the synchronization signal to be included in a radio frame for each transmission antenna  207 . As a result, the reception device  500  of a wireless terminal  102  can perform the synchronization process on radio frames transmitted from the transmission devices  200  of a plurality of base stations  101 , and measure the reception electric field intensity of a radio frame individually for each transmission device  200  that is a transmission source. 
     Note that, in the case where a transmission device  200  of a base station  101  includes a plurality of transmission antennas  207 , the transmission device  200  may assign a frequency shift pattern signal unique to each transmission antenna  207 , and transmit a radio frame including a synchronization signal of the assigned frequency shift pattern signal. In this case, the reception device  500  of a wireless terminal  102  can measure the reception electric field intensities of radio frames transmitted from the transmission devices  200  of a plurality of base stations  101  individually for each of a plurality of transmission antennas  207  included in a transmission device  200  that is the transmission source, and obtain the quality of reception of a radio frame transmitted from each transmission antenna  207 . 
     The radio communication system  100  includes a plurality of communication areas  103 , achieves multiplexing of wireless terminals  102 , has resistance to channel variation, has noise tolerance and interference tolerance, and can therefore achieve high synchronization performance even over channels that vary at high speeds. The transmission device  200  can improve the synchronization performance in an environment in which the states of channels vary in the radio communication system  100  including a plurality of communication areas  103 . 
     Note that the radio communication system  100  of the present embodiment is also applicable to multi-station simultaneous transmission, that is, transmission by a plurality of base stations  101  using the same information and the same frequency. In this case, in a radio frame illustrated in  FIG.  3   , for example, a synchronization signal  301  is further used for measurement of the reception electric field intensity of each of the base stations  101  that perform multi-station simultaneous transmission. The same is applicable to subsequent embodiments. 
     In addition, while the radio communication system  100  of the present embodiment is specifically described with reference to an example in which a base station  101  includes a transmission device  200 , a wireless terminal  102  includes a reception device  500 , and downlink communication from the base station  101  to the wireless terminal  102  is performed, the radio communication system  100  is not limited thereto. As described above, each base station  101  also includes a reception device, and each wireless terminal  102  also includes a transmission device. Thus, when a wireless terminal  102  includes a transmission device  200  and a base station  101  includes a reception device  500 , for example, the present embodiment is also applicable to uplink communication from the wireless terminal  102  to the base station  101 . 
     Second Embodiment 
     In a second embodiment, a method by which a reception device  500  of a wireless terminal  102  estimates an overreach signal power, which is an interference power coming from another communication area  103  will be explained. 
     In the second embodiment, the configurations of the radio communication system  100 , the transmission device  200  included in a base station  101 , and the reception device  500  included in a wireless terminal  102  are similar to those in the first embodiment. The symbols of a frequency shift pattern signal in a received signal r(t) at time t described in the first embodiment are represented by rP i,k (t). In this case, when an inner product value R q,i (t) is to be obtained in units of reference sequence symbols  401  having a sequence length L from synchronization sequence symbols PSO, the inner product value R q,i (t) can be expressed as in formula (6). 
       Formula 6: 
         R   q,i ( t )= rP   q ( t+kT   s )· PSO*   i,k   (6)
 
     In formula (6), i represents a number identifying a frequency shift pattern signal, k represents the number of a symbol, q represents a reception antenna number, and * represents a complex conjugate. A vector of the inner product value R q,i (t) of L symbols obtained from reference sequence symbols P i,k  of L symbols is represented by Rv q,i . By performing fast Fourier transform (FFT) on the inner product value vector Rv q,i , a frequency spectrum SP q,i,m (f) can be obtained by formula (7). 
       Formula 7: 
         SP   q,i,m ( f )= FFT ( Rv   q,i ( t ))  (7)
 
     In formula (7), m represents an identifier associated with a frequency shift amount f k,m . In a case where rP q (t+kT s ) represents a frequency shift pattern signal corresponding to frequency shifting f m , a spectral peak is present at a frequency position shifted by frequency shifting f m  as in  FIG.  6   .  FIG.  6    illustrates a case where the sequence length L of the reference sequence symbols  401  is L=4. In this case, it is desirable to use a sequence that does not have a spectral peak at another frequency as a reference sequence pattern. 
     When the power of an overreach interference wave is represented by IFP, and the power of an interference wave obtained by a frequency spectrum SP q,i,m (f) having a spectral peak at a frequency position shifted by frequency shifting f m  is represented by IFP q,i (f j ), IFP q,i (f) can be expressed as in formula (8). 
     
       
         
           
             
               
                 
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     By obtaining the powers of interference waves at other frequencies f j  in a similar manner by using formula (8) and obtaining an average power as expressed by formula (9), the reception device  500  of the wireless terminal  102  can obtain a coming interference power amount IFP i .  FIG.  17    is a diagram illustrating an example of a method for estimating an interference power by the reception device  500  of the wireless terminal  102  according to the second embodiment.  FIG.  17    illustrates an example of L=4 and N=16. 
     
       
         
           
             
               
                 
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     As described above, according to the present embodiment, in the radio communication system  100 , because the reception device  500  of the wireless terminal  102  estimates interference power amount in units of a short sequence length L of the reference sequence symbols  401 , the phase variation due to channel variation and the influence of amplitude variation can be minimized, which has an effect of improving the accuracy of estimation of an interference power amount. 
     Third Embodiment 
     In a third embodiment, a synchronization method in a case where a synchronization symbol pattern used in the first and second embodiments as a spreading factor will be explained. 
     In the third embodiment, because a reference sequence length, which is a short sequence length of the reference sequence symbols  401 , is repeatedly used for the spreading sequence, in addition to interference reduction among users, that is among the wireless terminals  102 , as a result of achieving user multiplexing by the first frequency shifting explained in the first embodiment, a matched filter length in calculation of reception correlation can be achieved with the reference sequence length. 
     Assume a case where spreading and user multiplexing are performed by using a spreading factor on preset first-order modulated symbols. Examples of the modulation method of the first-order modulation include PSK, FSK, and QAM, but are not limited thereto. 
       FIG.  18    is a diagram illustrating an example of a configuration of a transmission device  200   a  included in a base station  101  according to the third embodiment. The transmission device  200   a  is different from the transmission device  200  of the first embodiment illustrated in  FIG.  2    in that the synchronization signal generating unit  209  and the synchronization signal adding unit  203  are removed and that a spreading sequence generating unit  219  and modulated signal spreading units  213  are added. 
     The spreading sequence generating unit  219  generates a spreading sequence depending on each transmission antenna  207  on the basis of a pattern instruction signal  208  input as a control parameter to the transmission device  200   a . The spreading sequence generating unit  219  outputs the generated spreading sequence to the modulated signal spreading units  213 . 
     Each modulated signal spreading unit  213  generates a transmission signal on the basis of the spreading sequence generated by the spreading sequence generating unit  219  and the data symbol sequence generated by the modulation unit  202 . Specifically, the modulated signal spreading unit  213  spreads the data symbol sequence obtained from the modulation unit  202  in units of radio frames by using the spreading sequence obtained from the spreading sequence generating unit  219  to generate a transmission signal. The operations of the transmission filter units  204  and subsequent components are similar to those in the first embodiment. 
     Note that the spreading sequence generated by the spreading sequence generating unit  219  can be implemented by the same structure as a synchronization signal generated by the synchronization signal generating unit  209  described in the first embodiment. 
       FIG.  19    is a diagram illustrating an example of a configuration of the spreading sequence generating unit  219  of the transmission device  200   a  according to the third embodiment. The spreading sequence generating unit  219  includes a first repetition unit  231 , a first frequency shifting unit  232 , a second repetition unit  233 , a second frequency shifting unit  234 , and a phase offset unit  235 . 
     The first repetition unit  231  connects REPM repetitions of the reference sequence symbols  401  having a sequence length L to generate a first symbol sequence of L×REPM symbols. The first frequency shifting unit  232  performs, for orthogonality among users, that is, among the wireless terminal  102 , the first frequency shifting in units of reference sequence symbols  401  on the first symbol sequence to generate a second symbol sequence. The second repetition unit  233  connects NSP repetitions of the second symbol sequence of L×REPM symbols to generate a frequency shift pattern signal of L×REPM×NSP symbols. The second frequency shifting unit  234  performs, for lower cross-correlation, the second frequency shifting on the frequency shift pattern signal of L×REPM×NSP symbols to generate a synchronization symbol sequence. The phase offset unit  235  adds a phase offset defined in units of symbols for the first frequency shifting to the synchronization symbol sequence to eliminate the signal point arrangement bias and reduce degradation in synchronization performance due to an interference wave from another system. The spreading sequence generating unit  219  uses the synchronization symbol sequence obtained by the processes performed by the phase offset unit  235  as a spreading sequence. 
     Note that the spreading sequence generating unit  219  can use, as the spreading sequence, a sequence output from the first frequency shifting unit  232 , a sequence output from the second repetition unit  233 , or a sequence output from the second frequency shifting unit  234 . In any of these cases, the spreading sequence generating unit  219  sets the number of symbols of the sequence to be used as the spreading sequence to L×REPM×NSP symbols. As a result, the modulated signal spreading unit  213  spreads one symbol to a maximum of L×REPM×NSP chips. As described above, in the third embodiment, the reference sequence symbols  401  used by a transmission device  200  included in a base station  101  of the first and second embodiments are used as a spreading sequence. 
     Next, a reception device that is included in the wireless terminal  102  and that receives radio frames transmitted from the base station  101  will be described.  FIG.  20    is a diagram illustrating an example of a configuration of a reception device  500   a  included in a wireless terminal  102  according to the third embodiment. The reception device  500   a  is different from the reception device  500  of the first embodiment illustrated in  FIG.  13    in that the received synchronization signal generating unit  505 , the synchronization unit  506 , and the interference signal measuring unit  508  are removed and that a received spreading sequence generating unit  515  and a synchronization unit  506   a  are added. 
     The received spreading sequence generating unit  515  generates a spreading sequence similar to the spreading sequence generated by the transmission device  200 . The received spreading sequence generating unit  515  outputs the generated spreading sequence to the synchronization unit  506   a.    
     The synchronization unit  506   a  performs a synchronization process, that is, a process of determination on a synchronization signal on the basis of received frames obtained from the respective reception filter units  504  and the spreading sequence obtained from the received spreading sequence generating unit  515  to establish synchronization of radio frames. 
       FIG.  21    is a diagram illustrating an example of a synchronization process performed by a reception device included in a wireless terminal in a case of using a spreading sequence as a comparative example.  FIG.  22    is a diagram illustrating an example of a synchronization process performed by the reception device  500   a  included in a wireless terminal  102  according to the third embodiment. In a case where direct spreading is performed on a data symbol sequence of first-order modulated symbols illustrated in  FIG.  21 ( a ) , multipliers and adders whose numbers correspond to the number X of chips after direct spreading are necessary for in-phase addition and correlation detection as illustrated in  FIGS.  21 ( b )  and  21 ( c ). Thus, the circuit size becomes large in order to achieve a very large spreading factor, which may not be realistic. 
     In contrast, in the present embodiment, even when the number of chips is large, a small circuit size can be achieved.  FIG.  22    illustrates a concept of the synchronization process performed by the synchronization unit  506   a  in the present embodiment. A radio frame that is a received signal to be processed by the synchronization unit  506   a  is represented by rc(t). The synchronization unit  506   a  obtains an inner product of chips of a radio frame rc(t) and reference sequence symbols a(t) of a sequence length L, and obtains a spectrum of a frequency spectrum P(f,k,j) as expressed by formula (10). 
     
       
         
           
             
               
                 
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     In formula (10), k represents an integer satisfying REPM×NSP≥k≥1, and j represents an identifier indicating a chip position of a radio frame. In this process, in a case where the spreading sequence used by the transmission device  200   a  that is the transmission source of the radio frame is a spreading sequence with its spectral peak position shifted to f1 by a first frequency spectrum, a second frequency spectrum, and the phase offset process, the maximum value of the spectral powers should appear at the point f1. The synchronization unit  506   a  performs calculation of a spectral power REPM×NSP times to obtain a sum P sum (j) of the spectral powers as expressed by formula (11). 
     
       
         
           
             
               
                 
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     In formula (11), i represents an identifier indicating the frequency position at which a power peak of a frequency spectrum appears. The synchronization unit  506   a  repeats, REPM×NSP times which corresponds to the spreading sequence length, the process of shifting by one chip and obtaining the sum of spectral powers. Thereafter, the synchronization unit  506   a  changes the value of j, similarly obtains a sum of spectral powers, and determines, as a synchronization position, a point j at which the maximum power of a spectral peak is obtained. 
     The synchronization unit  506   a  calculates a correlation power with each of frequency pattern signals, performs determination on each of the obtained correlation powers by using a first threshold, selects only correlation powers that exceed the first threshold, and detects a timing at which the sum of the selected correlation powers is largest. Subsequently, the synchronization unit  506   a  determines detection of a synchronization signal by determining whether or not the sum of correlation powers exceeds a second threshold by using the second threshold on the sum of correlation powers at the timing of maximum sum. The synchronization unit  506   a  can determine a point J obtained by such detection determination as a start position of a spreading sequence. 
     While multipliers and adders whose numbers correspond to the number X of chips after direct spreading are necessary in the method of the comparative example, the sequence length L is sufficient for the matched filter length necessary at reception in the present embodiment because the spreading sequence is constituted by repetitions of reference sequence symbols a(t) having a short sequence length L. Thus, in a case where the spreading rate is sufficiently high, the reception device  500   a  can sufficiently reduce the reception circuit size, and can achieve a very large number of spreading sequences that cannot be achieved by a matched filter type of the related art. 
     As described above, according to the present embodiment, in the radio communication system  100 , the transmission device  200   a  of the base station  101  and the reception device  500   a  of the wireless terminal  102  use the synchronization symbol pattern used in the first and second embodiments as a spreading factor. In this case as well, the radio communication system  100  can produce the same effects as those in the first and second embodiments. In addition, in the radio communication system  100 , the reception device  500   a  of the wireless terminal  102  can prevent the circuit size from increasing. 
     While the radio communication system  100  of the present embodiment is specifically described with reference to an example in which a base station  101  includes a transmission device  200   a , a wireless terminal  102  includes a reception device  500   a , and downlink communication from the base station  101  to the wireless terminal  102  is performed, the radio communication system  100  is not limited thereto. As described above, each base station  101  also includes a reception device, and each wireless terminal  102  also includes a transmission device. Thus, when a wireless terminal  102  includes a transmission device  200   a  and a base station  101  includes a reception device  500   a , for example, the present embodiment is also applicable to uplink communication from the wireless terminal  102  to the base station  101 . 
     A transmission device according to the present disclosure produces an effect of enabling improvement in synchronization performance in an environment in which the states of channels vary in a radio communication system including a plurality of communication areas. 
     The configurations presented in the embodiments above are examples, and can be combined with other known technologies or with each other, or can be partly omitted or modified without departing from the gist.