Patent Publication Number: US-2018054214-A1

Title: Communication method and communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-159553, filed on Aug. 16, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a communication method and a communication system. 
     BACKGROUND 
     Known is a communication system including: a transmission circuit configured to receive data on a radio signal, and to transmit a serial signal representing the received data; and a reception circuit configured to receive the serial signal (see Japanese Laid-open Patent Publications Nos. 2014-11551, 8-116286, and 2011-188356, for example). The communication system is applied to communications between a baseband unit (BBU) and a remote radio head (RRH) which are included in a radio base station, for example. 
     The transmission circuit encodes the received data according to an encoding method of converting an arbitrary bit string into a bit string in which the number of consecutive bits representing the same value is equal to or less than a threshold value. The encoding method is an 86/10B encoding method, for example. The transmission circuit transmits a serial signal representing the encoded data. 
     Meanwhile, as frequency at which values in the bit string represented by the serial signal change (in other words, toggle frequency) becomes higher, the amount of electric power consumed by the transmission circuit and the reception circuit (for example, electric power consumed due to their switching operation) (in other words, power consumption) becomes larger. 
     There is a case where in a period in which no radio signal is communicated wirelessly (in other words, in a non-communication period), the transmission circuit encodes a predetermined bit string (for example, a bit string including 0s in succession) according to the above-mentioned encoding method, and transmits a serial signal representing the encoded bit string. In this case, in the non-communication period, too, the number of consecutive bits representing the same value in the bit string represented by the serial signal is equal to or less than the above-mentioned threshold value. For this reason, the toggle frequency tends to become higher in the non-communication period as well. Accordingly, the amount of power consumption in the transmission circuit and the reception circuit tends to become larger. 
     Meanwhile, there is an idea that the communication by the communication system is halted in the non-communication period. The halted communication, however, involves a risk of putting the transmission circuit and the reception circuit out of synchronization. With the above problem taken into consideration, it is desirable that the power consumption in the transmission circuit and the reception circuit be reduced. 
     SUMMARY 
     According to an aspect of the invention, a communication method executed by a communication system including a transmission circuit and a reception circuit, the communication method includes encoding, by the transmission circuit, inputted data according to an encoding method of converting a pre-converted bit string into a bit string in which a number of consecutive bits representing a same value is equal to or less than a predetermined threshold value, transmitting a first serial signal representing the encoded data, in a first period in which a radio signal outputted from the transmission circuit is communicated wirelessly, and transmitting a second serial signal representing a bit string which includes a continuous number of consecutive bits representing a same value, in a second period in which the radio signal is not communicated wirelessly, the continuous number being greater than the threshold value, and receiving, by the reception circuit, the first serial signal and the second serial signal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of a configuration of a base station apparatus in a first embodiment; 
         FIG. 2  is a block diagram illustrating an example of a configuration of a BBU and an RRH in  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating an example of a configuration of an IF/circuit in the BBU and an I/F circuit in the RRH in  FIG. 2 ; 
         FIG. 4  is a block diagram illustrating an example of a configuration of a transmission frame processing unit and a serializer in  FIG. 3 ; 
         FIG. 5  is an explanatory diagram illustrating examples of a period in a radio frame in which a downlink communication is performed, and examples of a period in the radio frame in which an uplink communication is performed; 
         FIG. 6  is a table illustrating examples of a relationship between a pre-converted bit string and a post-converted bit string which is established in an encoding method; 
         FIGS. 7A, 7B, and 7C  are explanatory diagrams respectively illustrating examples of a signal which is generated by the I/F circuit in  FIG. 3  in a period in which no radio signal is communicated wirelessly; 
         FIG. 8  is a block diagram illustrating examples of configurations of a reception frame processing unit and a deserializer unit in  FIG. 3 ; 
         FIGS. 9A, 9B, 9C, and 9D  are explanatory diagrams respectively illustrating examples of a signal which is generated by an I/F circuit of a comparative example in a period in which no radio signal is communicated wirelessly; and 
         FIG. 10  is a block diagram illustrating an example of a configuration of a reception frame processing unit and a deserializer of the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, referring to the accompanying drawings, descriptions are provided for embodiments. The below-discussed embodiments are provided as examples. Various modifications and techniques may be applied to the embodiments, even if they are not explicitly discussed below. In the drawings used for the embodiments, components denoted by the same reference signs are the same or similar to each other, unless modification or changes are indicated. 
     First Embodiment 
     As illustrated in  FIG. 1 , a base station apparatus  1  of the first embodiment includes a BBU  10 , and M RRH  20 - 1 ,  20 - 2 ,  20 - 3 , . . . ,  20 -M. BBU stands for baseband unit. RRH stands for remote radio head. The value M is an integer equal to 1 (one) or larger. A RRH 20 - m  is also referred to a RRH  20  as well, when a RRH does not have to be distinguished from each other. The value m is an integer from  1  to M. 
     The base station apparatus  1  makes wireless communications with a terminal device (for example, a user terminal) not illustrated in  FIG. 1 , according to a predetermined wireless communication method. In this example, the wireless communication method is LTE-Advanced. LTE stands for Long Term Evolution. The wireless communication method may be a method (for example, W-CDMA) different from LTE. W-CDMA stands for Wideband Code Division Multiple Access. 
     The base station apparatus  1  may be referred to as an evolved node B (eNB) or an access point. The BBU  10  may be referred to as a REC, a wireless controller, or a wireless control unit. REC stands for Radio Equipment Control or Radio Equipment Controller. The RRH  20  may be referred to as a wireless device, or a wireless unit. RE stands for Radio Equipment. 
     The RRH 20 - m  forms a cell WA-m. The cell WA-m is an example of a wireless area. The cell WA-m may be referred to as a coverage area, or a communication area. For example, the cell WA-m is a macrocell, a microcell, a nanocell, a picocell, a femtocell, a homecell, a smallcell, a sector cell or the like. The RRH 20 - m  makes wireless communications with terminal devices located within the cell WA-m which is formed by the RRH 20 - m.    
     In this example, the cell WA- 1  is a macrocell, while the cells WA- 2 , . . . , WA-M are smallcells. In this example, at least some of the smallcells WA- 2 , . . . , WA-M are located within the macrocell WA- 1 . All the smallcells WA- 2 , . . . , WA-M may be located outside the macrocell WA- 1 . 
     In this example, in the macrocell WA- 1 , a radio signal is communicated according to the frequency division duplex (FDD) method. In this example, in each of the smallcells WA- 2 , . . . , WA-M, the radio signal is communicated according to the time division duplex (TDD) method. For example, the format of a radio frame to be used for wireless communications according to the TDD method is specified in 3GPP TS36.213. 
     The RRH  20 - m  is communicably connected to the BBU  10  via a communication cable FC-m. In this example, the communication cable FC-m includes an optical fiber. In this example, the BBU  10  and the RRH  20 - m  communicate with each other according to a predetermined communication standard. In this example, the communication standard is CPRI. CPRI stands for Common Public Radio Interface. The communication standard may be ORI. ORI stands for Open Radio Equipment Interface. 
     As illustrated in  FIG. 2 , for example, the BBU  10  includes a BB processing unit  11 , and M I/F circuits  12 - 1 , . . . ,  12 - m , . . . ,  12 -M. BB stands for Baseband. I/F stands for Interface. 
     The BB processing unit  11  generates radio information and IQ data for each of the M I/F circuits  12 - 1 , . . . ,  12 - m , . . . ,  12 -M. In this example, radio information for the I/F circuit  12 - m  is information to be used to control wireless communications in the cell WA-m formed by the RRH  20 - m . In this example, the radio information includes first radio information, second radio information, third radio information and fourth radio information. 
     The first radio information is information on whether to make the wireless communications according to the time division duplex (TDD) method. The second radio information is information on a period timing for the radio signal communication in the radio frame (for example, a timing of the beginning of the radio frame, or a time length from the beginning of the radio frame through a timing of the radio signal communication, or the like). 
     The radio frame is used for the radio signal communication. In this example, each radio frame is an element in a radio signal, and has a predetermined time length (in this example, 10 ms). In other words, each radio signal is formed from multiple radio frames in succession in the time axis. 
     The third radio information is information on a period timing for an uplink communication in the radio frame. The fourth radio information is information on a period timing for a downlink communication in the radio frame. 
     In this example, the uplink communication is a communication from a terminal device to the base station apparatus  1 , while the downlink communication is a communication from the base station apparatus  1  to the terminal device. 
     The IQ data is data on the radio signal. In the example, the IQ data is on the amplitude and phase of the radio signal. For example, the BB processing unit  11  generates the IQ data based on information received from an external apparatus of the base station apparatus  1  (for example, another base station apparatus, an exchange apparatus or the like connected to the base station apparatus  1  via a communication network). 
     The BB processing unit  11  outputs the radio information and IQ data thus generated for the I/F circuit  12 - m  to the I/F circuit  12 - m.    
     The I/F circuit  12 - m  generates control information. The control information is information to be used to maintain the connection between the BBU  10  and the RRH  20 - m , as well as to control communications between the BBU  10  and the RRH  20 - m . The control information may be referred to a control word. In this example, the control information is used to synchronize a communication frame, which is discussed later, between the BBN  10  and the RRH  20 - m.    
     Based on the radio information received from the BB processing unit  11 , the I/F circuit  12 - m  converts an electrical signal representing the IQ data received from the BB processing unit  11  and the generated control information into an optical signal representing them. Thereafter, the I/F circuit  12 - m  transmits the thus-converted optical signal to the RRH  20 - m  via the communication cable FC-m. 
     Via the communication cable FC-m, the I/F circuit  12 - m  receives an optical signal transmitted from the RRH  20 - m.    
     Based on the radio information received from the BB processing unit  11 , the I/F circuit  12 - m  converts the received optical signal into an electrical signal. Thereafter, the I/F circuit  12 - m  outputs the IQ data represented by the post-converted electrical signal to the BB processing unit  11 . 
     The BB processing unit  11  processes the IQ data received from the I/F circuit  12 - m . For example, based on the IQ data, the BB processing unit  11  generates information. Thereafter, the BB processing unit  11  transmits the generated information to the external apparatus of the base station apparatus  1  (for example, the other base station apparatus, the exchange apparatus or the like connected to the base station apparatus  1  via a communication network). 
     The RRH  20 - m  includes an antenna  21 , a radio processing unit  22 , and an I/F circuit  23 . 
     The radio processing unit  22  generates radio information, and outputs the generated radio information to the I/F circuit  23 . For example, the radio processing unit  22  may generate the radio information based on information received from the I/F circuit  23 . 
     The radio processing unit  22  receives radio information via the antenna  21 . The radio processing unit  22  generates IQ data which is represented by the received radio information. Thereafter, the radio processing unit  22  outputs the generated IQ data to the I/F circuit  23 . 
     The I/F circuit  23  generates control information. Based on the radio information received from the radio processing unit  22 , the I/F circuit  23  converts an electrical signal representing the IQ data received from the radio processing unit  22  and the generated control information into an optical signal representing them. Thereafter, the I/F circuit  23  transmits the post-converted optical signal to the BBU  10  via the communication cable FC-m. 
     Via the communication cable FC-m, the I/F circuit  23  receives an optical signal transmitted from the BBU  10 . Based on the radio information received from the radio processing unit  22 , the I/F circuit  23  converts the received optical signal into an electrical signal. Thereafter, the I/F circuit  23  outputs the IQ data represented by the post-converted electrical signal to the radio processing unit  22 . 
     Via the antenna  21 , the radio processing unit  22  receives the radio information which is represented by the IQ data received from the I/F circuit  23 . 
     In this example, the optical signal from the I/F circuit  12 - m  to the I/F circuit  23 , and the optical signal from the I/F circuit  23  to the I/F circuit  12 - m  are wavelength-multiplexed and transmitted. 
     Descriptions are hereinbelow provided for the I/F circuit  12 - m  and the I/F circuit  23 . 
     For example, as illustrated in  FIG. 3 , the I/F circuit  12 - m  includes a transmission frame processing unit  121 , a serializer unit  122 , an optical module  123 , a deserializer unit  124 , and a reception frame processing unit  125 . The serializer unit  122  and the deserializer unit  124  may be referred to as a SerDes circuit. SerDes stands for Serializer/Deserializer. 
     The BBN  10  includes an oscillator  13 . 
     For example, as illustrated in  FIG. 3 , the I/F circuit  23  includes a transmission frame processing unit  231 , a serializer unit  232 , an optical module  233 , a deserializer unit  234 , and a reception frame processing unit  235 . The serializer unit  232  and the deserializer unit  234  may be referred to as a SerDes circuit. 
     In this example, the transmission frame processing unit  121 , the serializer unit  122 , the deserializer unit  234 , and the reception frame processing unit  235  form a DL processing unit  301  configured to process downlink signals. In this example, the transmission frame processing unit  231 , the serializer unit  232 , the deserializer unit  124 , and the reception frame processing unit  125  jointly form a UL processing unit  302  configured to process uplink signals. DL stands for Downlink. UL stands for Uplink. 
     For example, as illustrated in  FIG. 4 , the transmission frame processing unit  121  includes a timing management unit  1211 , a first multiplexer unit  1212 , an encoding unit  1213 , an alternating pattern generating unit  1214 , and a second multiplexer unit  1215 . The serializer unit  122  includes a parallel-serial converter unit  1221 . 
     The first multiplexer unit  1212  multiplexes the IQ data received from the BB processing unit  11  and the control information generated by the I/F circuit  23  to generate a communication frame according to the communication standard. The first multiplexer unit  1212  outputs a parallel signal representing the generated communication frame to the encoding unit  1213 . 
     In this example, the communication frame includes: a header section forming a starting section of the communication frame; and a payload section following the header section. The header section stores the control information. The payload section stores the IQ data. 
     The communication frame may be referred to as a CPRI frame, or a Basic Frame. For example, the header section is 128 (=4×32) bits long. Meanwhile, the payload section is 1920 (=60×32) bits long. The bit count of the header section may be at a value which is different from  128 . The bit count of the payload section may be at a value which is different from  1920 . 
     Based on the radio information received from the BB processing unit  11 , the timing management unit  1211  controls the multiplexing of the control information and the IQ data by the first multiplexer unit  1212 . In other words, based on the radio information received from the BB processing unit  11 , the timing management unit  1211  controls timings at which the control information and the IQ data are outputted from the first multiplexer unit  1212 . 
     For example, as illustrated in  FIG. 5 , a radio frame FR includes 10 sub-frames SF# 0  to SF# 9 . In the TTD method, downlink communications are performed in some periods in the radio frame FR, and uplink communications are performed in the other periods (in other words, the rest) in the radio frame FR. In an example illustrated in  FIG. 5 , downlink communications are performed in periods T 1 , T 3 , T 5 , and uplink communications are performed in periods T 2 , T 4 . 
     Thus, the DL processing unit  301  transmits a communication frame with the control information and the IQ data respectively stored in the header section and the payload section in the periods (in other words, the DL communication periods) T 1 , T 3 , T 5  in which the downlink communications are being performed. Meanwhile, the DL processing unit  301  transmits a communication frame with the control information stored in the header section, and with no IQ data stored in the payload section, in the periods (in other words, the DL non-communication periods) T 2 , T 4  in which no downlink communications are performed. 
     In this example, the DL communication periods T 1 , T 3 , T 5  for the DL processing unit  301  are instances of a first period in which radio signals are communicated wirelessly. In this example, the DL non-communication periods T 2 , T 4  for the DL processing unit  301  are instances of a second period in which no radio signal is communicated wirelessly. 
     The encoding unit  1213  encodes the parallel signal received from the first multiplexer unit  1212  according to a predetermined encoding method. The encoding method is a method for converting an arbitrary bit string into a bit string which includes consecutive bits representing the same value where the number of such bits is equal to or less than a predetermined threshold value. 
     In this example, in the encoding method, relationships between pre-converted bit strings and post-converted bit strings are established in advance, and each bit string is converted based on the relationships. In this example, the bit count of each pre-converted bit string and the bit count of the corresponding post-converted bit string are determined in advance. 
     In this example, the encoding method is an 8B/10B encoding method. For example, in the 86/10B method, the threshold value is 4. For example, as illustrated in  FIG. 6 , relationships between pre-converted bit strings and post-converted bit strings are established in advance. A column “Name” represents names or labels. A column “8 bit” represents pre-converted bit strings. A column “currentRD−” and a column “currentRD+” represent post-converted bit strings. RD stands for Running Disparity. 
     An object bit string is converted such that when a “currentRD+” bit string is used as a bit string immediate before the object bit string, a corresponding “currentRD−” bit string is used as the post-converted bit string obtained by converting the object bit string. Otherwise, an object bit string is converted such that when a “currentRD−” bit string is used as a bit string immediate before the object bit string, a corresponding “currentRD+” bit string is used as the post-converted bit string obtained by converting the object bit string. 
     A bit string to be encoded according to the encoding method (in other words, a pre-converted object bit string), or a bit string obtained according to the encoding method (in other words, a post-converted object bit string) may be referred to as a “word”. 
     The encoding unit  1213  outputs the post-encoded parallel signal to the second multiplexer unit  1215 . 
     The alternating pattern generating unit  1214  generates an alternating pattern signal. The alternating pattern signal represents a bit string which includes consecutive bits representing the same value where the number of such bits is a continuous number greater than the above-mentioned threshold value. In this example, the alternating pattern signal represents a bit string including an alternating series of a first bit string with the continuous number of 0s in succession and a second bit string with the continuous number of 1s in succession. 
     In this example, the continuous number is less than an upper limit number to be allowed for the I/F circuit  23  to be synchronized with the I/F circuit  12 - m  based in the received serial signal (in other words, the same sign continuation tolerance). In this example, the continuous number is 400. The alternating pattern generating unit  1214  may be configured to change the continuous number. 
     The alternating pattern generating unit  1214  outputs the generated alternating pattern signal to the second multiplexer unit  1215 . In this example, the alternating pattern signal outputted from the alternating pattern generating unit  1214  is a parallel signal. 
     The second multiplexer unit  1215  multiplexes the encoded signal received from the encoding unit  1213  and the alternating pattern signal received from the alternating pattern generating unit  1214  to generate a transmission frame. 
     Based on the radio information received from the BB processing unit  11 , the timing management unit  1211  controls the multiplexing of the encoded signal and the alternating pattern signal by the second multiplexer unit  1215 . In other words, based on the radio information received from the BB processing unit  11 , the timing management unit  1211  controls timings at which the encoded signal and the alternating pattern signal are outputted from the second multiplexer unit  1215 . 
     In this example, the timing management unit  1211  specifies (or identifies) the DL communication periods and the DL non-communication periods based on the radio information received from the BB processing unit  11 . 
     The timing management unit  1211  controls the second multiplexer unit  1215  such that the encoded signal is outputted in periods corresponding to the header sections within the specified DL communication and non-communication periods. The timing management unit  1211  controls the second multiplexer unit  1215  such that the encoded signal is outputted in a period corresponding to the payload section within the specified DL communication period. The timing management unit  1211  controls the second multiplexer unit  1215  such that the alternating pattern signal is outputted in the period corresponding to the payload section within the specified DL non-communication period. 
     The second multiplexer unit  1215  outputs the parallel signal representing the generated transmission frame to the serializer unit  122 . 
     For example, as illustrated in  FIG. 7A , in the DL non-communication period, no IQ data is stored in the payload section. In  FIGS. 7A, 7B, and 7C , “don&#39;t care” represents an arbitrary value. For example, in the DL non-communication period, as the arbitrary value, a predetermined value (for example, 0 or 1) may be stored in the payload section. 
     For example, as illustrated in  FIG. 7B , the second multiplexer unit  1215  outputs the alternating pattern signal in the period corresponding to the payload section within the DL non-communication period. In other words, in the DL non-communication period, the second multiplexer unit  1215  generates the transmission frame including the alternating pattern signal in its part correspond to the payload section. 
     In this example, the parallel signal outputted from the transmission frame processing unit  121  is 40 bits wide, and its transmission rate is 245.76 Mbps. 
     The parallel-serial converter unit  1221  converts the parallel signal received from the transmission frame processing unit  121  into a serial signal, and outputs the post-converted serial signal to the optical module  123 . For example, as illustrated in  FIG. 7C , the parallel-serial converter unit  1221  outputs the alternating pattern signal in the period corresponding to the payload section within the DL non-communication period. 
     The encoded signal outputted from the parallel-serial converter unit  1221  is an example of a first serial signal. The alternating pattern signal I outputted from the parallel-serial converter unit  1221  is an example of a second serial signal. 
     In this example, the transmission rate of the serial signal outputted from the serializer unit  122  is 9.8304 Gbps. 
     The optical module  123  converts the serial signal received from the serializer unit  122  into an optical signal. Thereafter, the optical module  123  transmits the post-converted optical signal to the RRH  20 - m  via the communication cable FC-m. 
     In this example, as illustrated in  FIG. 3 , an oscillator  13  generates a clock signal with a predetermined frequency, and outputs the generated clock signal to the serializer unit  122 . The serializer unit  122  operates according to timing synchronized with the clock signal received from the oscillator  13 . The serializer unit  122  outputs the clock signal received from the oscillator  13  to the transmission frame processing unit  121  and the BB processing unit  11 . The transmission frame processing unit  121  and the BB processing unit  11  operate according to timing synchronized with the clock signal received from the serializer unit  122 . 
     For example, as illustrated in  FIG. 8 , the deserializer unit  234  includes a serial-parallel converter unit  2341 . The reception frame processing unit  235  includes a timing management unit  2351 , a synchronization code detector unit  2352 , a decoding unit  2353  and a separator unit  2354 . 
     Via the communication cable FC-m, the optical module  233  receives the optical signal transmitted from the BBU  10 . The optical module  233  converts the received optical signal into a serial signal. Thereafter, the optical module  233  outputs the post-converted serial signal to the deserializer unit  234 . 
     The serial-parallel converter unit  2341  coverts the serial signal received from the optical module  233  into a parallel signal. Thereafter, the serial-parallel converter unit  2341  outputs the post-converted parallel signal to the reception frame processing unit  235 . 
     The synchronization code detector unit  2352  detects a synchronization code from a bit string represented by the parallel signal received from the deserializer unit  234 . The synchronization code is a predetermined bit string. 
     In this example, the synchronization code is stored in a predetermined part in the header section of the communication frame. Thus, by detecting the synchronization code, the synchronization code detector unit  2352  detects timing at which the synchronization code is detected (in other words, the position of the synchronization code in the bit string, or the phase of the communication frame). The synchronization code detector unit  2352  informs the timing management unit  2351  of the detection timing. 
     Based on the detected timing, the synchronization code detector unit  2352  performs word alignment on the parallel signal received from the deserializer unit  234 . The word alignment is a process of controlling the phase of the parallel signal such that a bit at the beginning of the bit string (or the “word”) encoded according to the encoding method is transmitted through a predetermined one of multiple signal lines for transmitting the parallel signal. 
     The synchronization code detector unit  2352  outputs the word-aligned parallel signal to the decoding unit  2353 . 
     The decoding unit  2353  decodes the bit string represented by the parallel signal received from the synchronization code detector unit  2352 . The decoding unit  2353  performs the decoding according to a decoding method associated with the encoding method used by the encoding unit  1213 . In this example, the decoding unit  2353  converts the bit string based on the relationships between the pre-converted bit strings and the post-converted bit strings which are determined in the encoding method used by the encoding unit  1213 . 
     In a case where while the decoding unit  2353  is performing the decoding, the bit string of the decoding object is not matched with any one of the post-converted bit strings in the above-discussed relationships, the decoding unit  2353  detects the occurrence of abnormality in the decoding. 
     Based on the radio information received from the radio processing unit  22 , the timing management unit  2351  controls the decoding by the decoding unit  2353 . 
     In this example, the timing management unit  2351  controls the decoding unit  2353  to make the decoding unit  2353  perform the decoding in the periods corresponding to the header sections within the DL communication and non-communication periods. The timing management unit  2351  controls the decoding unit  2353  to make the decoding unit  2353  perform the decoding in the period corresponding to the payload section within the DL communication period. The timing management unit  2351  controls the decoding unit  2353  to make the decoding unit  2353  halt the decoding in the period corresponding to the payload section within the DL non-communication period. 
     While performing the decoding, the decoding unit  2353  outputs the post-decoded parallel signal to the separator unit  2354 . While halting the decoding, the decoding unit  2353  outputs the parallel signal received from the synchronization code detector unit  2352  to the separator unit  2354 . 
     The separator unit  2354  obtains (or extracts or separates) the IQ data and the control information from the data represented by the parallel signal received from the decoding unit  2353 . 
     Based on the radio information received from the radio processing unit  22 , the timing management unit  2351  controls the obtaining of the IQ data and the control information by the separator unit  2354 . In other words, based on the radio information received from the radio processing unit  22 , the timing management unit  2351  controls timings at which the separator unit  2354  obtains the IQ data and the control information. 
     The separator unit  2354  outputs the obtained IQ data to the radio processing unit  22 . 
     In this example, as illustrated in  FIG. 3 , based in the serial signal received from the optical module  233 , the deserializer unit  234  generates a clock signal. In this example, the deserializer unit  234  generates the clock signal based in a timing at which the value represented by the serial signal changes. The generation of the clock signal by the deserializer unit  234  may be referred to as a “clock data recovery.” 
     The deserializer unit  234  may be configured to include an oscillator, to control the phase of a clock signal generated by the oscillator based on the serial signal received from the optical module  233 , and to thereby generate a clock signal synchronized with the serial signal. 
     The deserializer unit  234  outputs the generated clock signal to the reception frame processing unit  235 , the radio processing unit  22 , the transmission frame processing unit  231 , and the serializer unit  232 . The reception frame processing unit  235 , the radio processing unit  22 , the transmission frame processing unit  231 , and the serializer unit  232  operate according to timing synchronized with the clock signal received from the deserializer unit  234 . 
     The DL processing unit  301  is an example of the communication system for the downlink communication. 
     The transmission frame processing unit  121  and the serializer unit  122  are examples of the transmission circuit for the downlink communication. The encoding unit  1213  is an example of the encoding unit for the downlink communication. The serializer unit  122 , the timing management unit  1211 , and the second multiplexer unit  1215  are examples of the communication unit for the downlink communication. 
     The deserializer unit  234  and the reception frame processing unit  235  are examples of the reception circuit for the downlink communication. The deserializer unit  234  is an example of the reception unit for the downlink communication. The decoding unit  2353  and the timing management unit  2351  are examples of the decoding unit for the downlink communication. 
     Next, descriptions are provided for the UL processing unit  302 . The UL processing unit  302  is configured in the same way as the DL processing unit  301 , except that: the UL processing unit  302  processes the uplink signals instead of the downlink signals; and the I/F circuit  23  on the transmission side uses the clock signal generated by the deserializer unit  234 . 
     The transmission frame processing unit  231  and the serializer unit  232  are configured in the same way as the transmission frame processing unit  121  and the serializer unit  122 , respectively. The deserializer unit  124  and the reception frame processing unit  125  are configured in the same way as the deserializer unit  234  and the reception frame processing unit  235 , respectively. 
     For example, as illustrated in  FIG. 5 , in the periods (in other words, the UL communication periods) T 2 , T 4  in which the uplink communications are being performed, the UL processing unit  302  transmits a communication frame with the control information and the IQ data respectively stored in the header section and the payload section. Meanwhile, in the periods (in other words, the UL non-communication periods) T 1 , T 3 , T 5  in which no uplink communications are performed, the UL processing unit  302  transmits a communication frame with the control information stored in the header section, and with no IQ data stored in the payload section. 
     In this example, the UL communication periods T 2 , T 4  for the UL processing unit  302  are instances of a first period in which radio signals are communicated wirelessly. In this example, the UL non-communication periods T 1 , T 3 , T 5  for the UL processing unit  302  are instances of a second period in which no radio signal is communicated wirelessly. 
     The UL processing unit  302  is an example of the communication system for the uplink communication. 
     The transmission frame processing unit  231  and the serializer unit  232  are examples of the transmission circuit for the uplink communication. The encoding unit in the transmission frame processing unit  231  is an example of the encoding unit for the uplink communication. The serializer unit  232 , the timing management unit in the transmission frame processing unit  231 , and the second multiplexer unit in the transmission frame processing unit  231  are examples of the communication unit for the uplink communication. 
     The deserializer unit  124  and the reception frame processing unit  125  are examples of the reception circuit for the uplink communication. The deserializer unit  124  is an example of the reception unit for the uplink communication. The decoding unit in the reception frame processing unit  125 , and the timing management unit in the reception frame processing unit  125  are examples of the decoding unit for the uplink communication. 
     Next, referring to  FIG. 5 , descriptions are provided for an example of how the base station apparatus  1  operates. 
     First of all, descriptions are provided for how the base station apparatus  1  operates for the downlink communication in the period T 1 . 
     The BB processing unit  11  in the BBU  10  generates the IQ data. The I/F circuit  12 - m  generates the control information. 
     Thereafter, the transmission frame processing unit  121  in the I/F circuit  12 - m  encodes the IQ data generated by the BB processing unit  11 . Furthermore, the transmission frame processing unit  121  encodes the control information generated by the I/F circuit  12 - m.    
     Subsequently, the transmission frame processing unit  121  generates the transmission frame including the encoded code signal in the periods corresponding to the header section and the payload section. Furthermore, the serializer unit  122  in the I/F circuit  12 - m  converts the parallel signal representing the transmission frame generated by the transmission frame processing unit  121  into the serial signal. 
     After that, the optical module  123  in the I/F circuit  12 - m  converts the serial signal converted by the serializer unit  122  into the optical signal. Subsequently, the optical module  123  transmits the post-converted optical signal to the RRH  20 - m  via the communication cable FC-m. 
     Thereby, via the communication cable FC-m, the optical module  233  in the I/F circuit  23  receives the optical signal transmitted from the BBU  10 . Thereafter, the optical module  233  converts the received optical signal into the serial signal. 
     Subsequently, the deserializer unit  234  in the I/F circuit  23  converts the serial signal converted by the optical module  233  into the parallel signal. Thereafter, in the periods corresponding to the header section and the payload section, the reception frame processing unit  235  in the I/F circuit  23  decodes the bit string represented by the parallel signal converted by the deserializer unit  234 . Thereby, the reception frame processing unit  235  obtains the IQ data. 
     Via the antenna  21 , the radio processing unit  22  in the RRH  20 - m  transmits the radio signal represented by the IQ data obtained by the reception frame processing unit  235 . 
     Next, descriptions are provided for how the base station apparatus  1  operates for the uplink communication in the period T 1 . 
     In the period T 1 , the radio processing unit  22  in the RRH  20 - m  receives no radio signal. Accordingly, the radio processing unit  22  generates no IQ data. The I/F circuit  23  generates the control information. Thereafter, the transmission frame processing unit  231  in the I/F circuit  23  encodes the control information generated by the I/F circuit  23 . 
     Subsequently, the transmission frame processing unit  231  generates the transmission frame including the encoded code signal in the period corresponding to the header section, and the alternating pattern signal in the period corresponding to the payload section. Thereafter, the serializer unit  232  in the I/F circuit  23  converts the parallel signal representing the transmission frame generated by the transmission frame processing unit  231  into the serial signal. 
     Thereafter, the optical module  233  in the I/F circuit  23  converts the serial signal converted by the serializer unit  232  into the optical signal. After that, the optical module  233  transmits the post-converted optical signal to the BBU  10  via the communication cable FC-m. 
     Thereby, via the communication cable FC-m, the optical module  123  in the I/F circuit  12 - m  receives the optical signal transmitted from the RRH  20 - m . Thereafter, the optical module  123  converts the received optical signal into the serial signal. 
     Subsequently, the deserializer unit  124  in the I/F circuit  12 - m  converts the serial signal converted by the optical module  123  into the parallel signal. Thereafter, in the period corresponding to the header section, the reception frame processing unit  125  in the I/F circuit  12 - m  decodes the bit string represented by the parallel signal converted by the deserializer unit  124 . Furthermore, in the period corresponding to the payload section, the reception frame processing unit  125  in the I/F circuit  12 - m  does not decode the bit string represented by the parallel signal converted by the deserializer unit  124 . 
     Next, descriptions are provided for how the base station apparatus  1  operates for the downlink communication in the period T 2 . 
     In the period T 2 , the BB processing unit  11  in the BBU  10  generates no IQ data. The I/F circuit  12 - m  generates the control information. The transmission frame processing unit  121  in the I/F circuit  12 - m  encodes the control information generated by the I/F circuit  12 - m.    
     Thereafter, the transmission frame processing unit  121  generates the transmission frame including the encoded code signal in the period corresponding to the header section, and the alternating pattern signal in the period corresponding to the payload section. After that, the serializer unit  122  in the I/F circuit  12 - m  converts the parallel signal representing the transmission frame generated by the transmission frame processing unit  121  into the serial signal. 
     Thereafter, the optical module  123  in the I/F circuit  12 - m  converts the serial signal converted by the serializer unit  122  into the optical signal. After that, the optical module  123  transmits the post-converted optical signal to the RRH  20 - m  via the communication cable FC-m. 
     Thereby, via the communication cable FC-m, the optical module  233  in the I/F circuit  23  receives the optical signal transmitted from the BBU  10 . Thereafter, the optical module  233  converts the received optical signal into the serial signal. 
     Subsequently, the deserializer unit  234  in the I/F circuit  23  converts the serial signal converted by the optical module  233  into the parallel signal. Thereafter, in the period corresponding to the header section, the reception frame processing unit  235  in the I/F circuit  23  decodes the bit string represented by the parallel signal converted by the deserializer unit  234 . Furthermore, in the period corresponding to the payload section, the reception frame processing unit  235  in the I/F circuit  23  does not decode the bit string represented by the parallel signal converted by the deserializer unit  234 . 
     Next, descriptions are provided for how the base station apparatus  1  operates for the uplink communication in the period T 2 . 
     In the period T 2 , the radio processing unit  22  in the RRH  20 - m  receives the radio signal, and generates the IQ data representing the received radio signal. The I/F circuit  23  generates the control information. Subsequently, the transmission frame processing unit  231  in the I/F circuit  23  encodes the IQ data generated by the radio processing unit  22 . The transmission frame processing unit  231  encodes the control information generated by the I/F circuit  23 . 
     After that, the transmission frame processing unit  231  generates the transmission frame including the encoded code signal in the periods corresponding to the header section and the payload section. Furthermore, the serializer unit  232  in the I/F circuit  23  converts the parallel signal representing the transmission frame generated by the transmission frame processing unit  231  into the serial signal. 
     Subsequently, the optical module  233  in the I/F circuit  23  converts the serial signal converted by the serializer unit  232  into the optical signal. The optical module  233  transmits the post-converted optical signal to the BBU  10  via the communication cable FC-m. 
     Thereby, via the communication cable FC-m, the optical module  123  in the I/F circuit  12 - m  receives the optical signal transmitted from the RRH  20 - m . Thereafter, the optical module  123  converts the received optical signal into the serial signal. 
     Subsequently, the deserializer unit  124  in the I/F circuit  12 - m  converts the serial signal converted by the optical module  123  into the parallel signal. Thereafter, in the periods corresponding to the header section and the payload section, the reception frame processing unit  125  in the I/F circuit  12 - m  decodes the bit string represented by the parallel signal converted by the deserializer unit  124 . Thereby, the reception frame processing unit  125  obtains the IQ data. After that, the BB processing unit  11  in the BBU  10  processes the IQ data obtained by the reception frame processing unit  125 . 
     The base station apparatus  1  operates in the period T 3 , T 5  in the same way as in the period T 1 . Furthermore, the base station apparatus  1  operates in the period T 4  in the same way as in the period T 2 . 
     As discussed above, in the first periods in which the downlink communication signals are communicated wirelessly, the I/F circuit  12 - m  of the first embodiment transmits the first serial signal representing the data encoded according to the encoding method. The encoding method is the method for converting an arbitrary bit string into a bit string which includes consecutive bits representing the same value where the number of such bits is equal to or less than a predetermined threshold value. Furthermore, in the second periods in which no downlink communication signals are communicated wirelessly, the I/F circuit  12 - m  transmits the second serial signal representing a bit string which includes consecutive bits representing the same value where the number of such bits is the continuous number greater than the threshold value. 
     Similarly, in the first periods in which the uplink communication signals are communicated wirelessly, the I/F circuit  23  of the first embodiment transmits the first serial signal representing the data encoded according to the encoding method. Furthermore, in the second periods in which no uplink communication signals are communicated wirelessly, the I/F circuit  23  transmits the second serial signal representing the bit string which includes the consecutive bits representing the same value where the number of such bits is the continuous number greater than the threshold value. 
     For example, as illustrated in  FIG. 9A , in a second period in which no communication signal is communicated wirelessly, an I/F circuit of a comparative example generates a communication frame with predetermined values (0s in this example) stored (or padded) in the payload section. Thereafter, as illustrated in  FIG. 9B , in the second period in which no communication signal is communicated wirelessly, the I/F circuit of the comparative example encodes the header section and the payload section which are included in the generated communication frame. 
     Furthermore, as illustrated in  FIG. 9C , in the second period in which no communication signal is communicated wirelessly, the I/F circuit of the comparative example converts the encoded code signal into a serial signal. Thereafter, the I/F circuit converts the post-converted serial signal into an optical signal, and transmits the post-converted optical signal. 
       FIG. 9D  illustrates an example of the serial signal in a period corresponding to the payload section within the second period in which no communication signal is communicated wirelessly. As illustrated in  FIG. 9D , in the comparative example, in the period corresponding to the payload section within the second period in which no communication signal is communicated wirelessly, the number of consecutive bits representing the same value is equal to or less than the threshold value. 
     In contrast to this, in the first embodiment, in the period corresponding to the payload section within each second period in which no communication signal is communicated wirelessly, the number of consecutive bits representing the same value is the continuous number greater than the threshold value. 
     Accordingly, the I/F circuit  12 - m  and the I/F circuit  23  of the first embodiment are capable of reducing the toggle frequency in the second period. This makes it possible to reduce the amount of power consumption in the second period. For example, it is possible to reduce the amount of power consumption due to the switching operations of the I/F circuit  12 - m  and the I/F circuit  23  of the first embodiment to approximately 53% of the amount of power consumption due to the switching operation of the I/F circuit of the comparative example. 
     In the second period, too, the communication of the control information is maintained. This makes it possible to maintain the connection between the I/F circuit  12 - m  and the I/F circuit  23 . It is accordingly possible to maintain the synchronization between the communication frames of the I/F circuit  12 - m  and the I/F circuit  23 . 
     Furthermore, in the first embodiment, the second serial signal represents the bit string including the alternating series of the first bit string with the continuous number of 0s in succession and the second bit string with the continuous number of 1s in succession. 
     This makes it possible to reduce the toggle frequency in the second period. Accordingly, it is possible to reduce the amount of power consumption in the second period. 
     Furthermore, the I/F circuit  12 - m  and the I/F circuit  23  of the first embodiment are capable of specifying the second period based on the radio information. 
     In the case where the radio signals are communicated according to the TDD method, the periods in which no downlink radio signals are communicated wirelessly, and the periods in which no uplink radio signals are communicated wirelessly are provided. 
     The timing of the second period in the radio frame corresponds to the timing of the period in the radio frame in which the radio signals are communicated. The timing of the second period in the radio frame corresponds to the timing of the period in which the uplink communication is performed. The timing of the second period in the radio frame corresponds to the timing of the period in which the downlink communication is performed. 
     Thus, the I/F circuit  12 - m  and the I/F circuit  23  make it possible to detect the second period with high accuracy. 
     Moreover, in the first embodiment, the continuous number is less than the upper limit number. The upper limit number for the I/F circuit  23  is a number to be allowed for the I/F circuit  12 - m  to be synchronized with the I/F circuit  23  based in the received serial signal. The upper limit number for the I/F circuit  12 - m  is a number to be allowed for the I/F circuit  23  to be synchronized with the I/F circuit  12 - m  based in the received serial signal. 
     Thereby, the I/F circuit  23  is synchronized with the I/F circuit  12 - m  based on the serial signal received in the second period of the downlink. This makes it possible to maintain the synchronization between the I/F circuit  23  and the I/F circuit  12 - m  in the second period of the downlink as well. Similarly, the I/F circuit  12 - m  is synchronized with the I/F circuit  23  based on the serial signal received in the second period of the uplink. This makes it possible to maintain the synchronization between the I/F circuit  23  and the I/F circuit  12 - m  in the second period of the uplink as well. 
     Moreover, the I/F circuit  23  of the first embodiment decodes the data represented by the serial signal which is received in the first period in which the downlink radio signals are communicated wirelessly. In addition, the I/F circuit  23  does not decode the data represented by the serial signal which is received in the second period in which no downlink radio signals are communicated wirelessly. 
     Similarly, the I/F circuit  12 - m  of the first embodiment decodes the data represented by the serial signal which is received in the first period in which the uplink radio signals are communicated wirelessly. In addition, the I/F circuit  12 - m  does not decode the data represented by the serial signal which is received in the second period in which no uplink radio signals are communicated wirelessly. 
     This makes it possible to inhibit the occurrence of abnormality in the decoding. The abnormality in the decoding represents that the I/F circuit  12 - m  or the I/F circuit  23  fails to decode the data represented by the received serial signal. 
     Second Embodiment 
     Next, descriptions are provided for a base station of a second embodiment. The base station of the second embodiment is different from the base statin of the first embodiment in that information on whether the payload section is made from the alternating pattern signal is stored in the header section. The following descriptions are provided focusing on what makes the base station of the second embodiment different from the base station of the first embodiment. Components of the second embodiment which are described using the same reference signs as those of the first embodiment are identical to or substantially the same as the components of the first embodiment. 
     The control information generated by the I/F circuit  12 - m  of the second embodiment includes flag information. The flag information indicates whether the payload section to be included in the communication frame together with the header section storing the flag information is formed from an unencoded alternating pattern signal. 
     For example, as illustrated in  FIG. 10 , the reception frame processing unit  235  of the second embodiment includes a timing management unit  2351 A and a separator unit  2354 A instead of the timing management unit  2351  and the separator unit  2354  included in the reception frame processing unit  235  of the first embodiment. 
     The separator unit  2354 A obtains (or extracts or separates) the IQ data and the control information from the data represented by the parallel signal received from the decoding unit  2353 . 
     Based on the radio information received from the radio processing unit  22 , the timing management unit  2351 A controls the obtaining of the IQ data and the control information by the separator unit  2354 A. In other words, based on the radio information received from the radio processing unit  22 , the timing management unit  2351 A controls timings at which the separator unit  2354 A obtains the IQ data and the control information. 
     The separator unit  2354 A outputs the obtained IQ data to the radio processing unit  22 . In addition, the separator unit  2354 A outputs the obtained control information to the timing management unit  2351 A. 
     The timing management unit  2351 A controls the decoding by the decoding unit  2353  based on the control information received from the separator unit  2354 A instead of based on the radio information received from the radio processing unit  22 . 
     In this example, in a case where the flag information included in the control information is information indicating the encoded state, the timing management unit  2351 A controls the decoding unit  2353  to make the decoding unit  2353  perform the decoding in the period corresponding to the payload section to be included in the communication frame together with the header section storing the flag information. The information indicating the encoded state is the flag information indicating that the payload section to be included in the communication frame together with the header section storing the flag information is formed from the encoded code signal (in other words, is not formed from the alternating pattern signal). 
     In this example, in a case where the flag information included in the control information is information indicating the unencoded state, the timing management unit  2351 A controls the decoding unit  2353  to make the decoding unit  2353  halt the decoding in the period corresponding to the payload section to be included in the communication frame together with the header section storing the flag information. The information indicating the unencoded state is the flag information indicating that the payload section to be included in the communication frame together with the header section storing the flag information is formed from the unencoded alternating pattern signal. 
     The separator unit and the timing management unit included in the reception frame processing unit  125  are configured in the same way as the separator unit  2354 A and the timing management unit  2351 A, respectively. 
     As discussed above, the base station apparatus  1  of the second embodiment provides the same working and effect as the base station apparatus  1  of the first embodiment. 
     In addition, the header section included in the communication frame of the second embodiment stores information on whether the payload section to be included in the communication frame together with the header is formed from the serial signal representing the unencoded data (in this example, the alternating pattern signal). Furthermore, the timing management unit  2351 A specifies the second period based on the header section. 
     This makes it possible for the I/F circuit  12 - m  and the I/F circuit  23  to recognize whether the payload section included in the communication frame is formed from the serial signal representing the unencoded data. Accordingly, it is possible to inhibit the I/F circuit  12 - m  and the I/F circuit  23  from failing to decode the payload section (in other words, it is possible to inhibit the occurrence of abnormality in the decoding). 
     The I/F circuit  12 - m  and the I/F circuit  23  of the second embodiment are capable of inhibiting the occurrence of abnormality in the decoding based on factors not related to the radio information even in a case where no IQ data is stored in the payload section. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.