Optical communication system and optical communication method

An optical communication system includes a signal processing apparatus and a wireless apparatus between which functions of a base station are divided, wherein a periodic symbol sequence including a cyclic prefix appended to a signal of a predetermined size to which an IFFT (Inverse Fast Fourier Transform) has been applied is transmitted between the signal processing apparatus and the wireless apparatus by means of digital RoF (Radio over Fiber) transmission, the signal processing apparatus and the wireless apparatus each include a transmission unit and a reception unit, the transmission unit includes: a compression size determination unit that acquires symbol information relating to a starting position of the symbol sequence and lengths of symbols constituting the symbol sequence, and that determines, based on the acquired symbol information, a compression size for each of symbols that are to be compressed; and a compression unit that compresses the symbol sequence in units of determined compression sizes, and the reception unit includes: an expansion size determination unit that determines an expansion size for each of symbols in the symbol sequence that are to be expanded; and an expansion unit that expands the symbol sequence in units of determined expansion sizes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Stage of International Application No. PCT/JP2016/061374, filed Apr. 7, 2016, which claims the benefit of and priority to Japanese Patent Application No. 2015-101840, filed May 19, 2015. The disclosures of the above applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to digital RoF (Radio over Fiber) transmission technology.

BACKGROUND ART

Conventionally, in order to improve the level of freedom of cell architecture in cellular systems, configurations in which the functions of a base station apparatus are divided between a signal processing unit (hereinafter referred to as “BBU” (BaseBand Unit)) and an RF unit (hereinafter referred to as “RRH” (Remote Radio Head)), and the BBU and the RRH are physically separated, have been considered. In such a configuration, the wireless signals transmitted between the BBU and the RRH are transmitted by means of RoF technology. RoF technologies can be largely divided between analog RoF technologies and digital RoF technologies, depending on the optical transmission method. In recent years, the study of digital RoF technologies having superior transmission quality has flourished, and standardization organizations such as the CPRI (Common Public Radio Interface) and the like are working towards the establishment of specifications (see, e.g., Non-patent Document 1). Additionally, while coaxial cable, optical fiber, and the like can be used as the connecting media between BBUs and RRHs, the transmission distance can be extended, in particular, by using optical fiber to connect BBUs and RRHs.

Herebelow, digital RoF transmission will be explained.

In discussing digital RoF transmission, the following terminology will be defined.

A downlink refers to the communication path of radio waves transmitted from a BBU, via an RRH, to a wireless terminal connected to the RRH.

An uplink refers to the communication path of radio waves transmitted from a wireless terminal connected to an RRH, via the RRH, to a BBU.

On a digital RoF transmission downlink, the following processes are performed. A BBU prepares a digital signal (hereinafter referred to as “IQ data”) separately for the I-axis and Q-axis components of a wireless signal, converts the prepared IQ data into an optical signal, and transmits the converted optical signal to an RRH via an optical fiber. The RRH converts the received optical signal into a wireless signal, and transmits the converted wireless signal to a wireless terminal.

Additionally, on a digital RoF transmission uplink, the following processes are performed. An RRH receives a wireless signal transmitted from a wireless terminal, converts the received wireless signal into an optical signal, and transmits the converted optical signal to a BBU via an optical fiber. The BBU converts the received optical signal into IQ data and demodulates the signal.

FIG. 15is a schematic block diagram illustrating the functional structure of an RRH500during digital RoF transmission.

The antenna501transmits and receives wireless signals. The transmission/reception switching unit502switches the antenna501between transmission and reception. The amplifier503amplifies the signal power of a received wireless signal to a level that allows for signal processing. The down-conversion unit504down-converts the amplified wireless signal into the baseband. The A/D conversion unit505converts the down-converted wireless signal (analog signal) into IQ data, which is a digital signal. The baseband filter unit506performs a filtering process on the IQ data. The framing unit507performs framing by multiplexing the filtered IQ data with a control signal. The E/O conversion unit508converts the framed signal (hereinafter referred to as the “frame signal”) (electrical signal) into an optical signal, and transmits the converted optical signal to the BBU via an optical fiber550.

The O/E conversion unit509converts an optical signal received via the optical fiber550into a frame signal (electrical signal). The deframing unit510extracts a control signal and IQ data from the frame signal. The baseband filter unit511performs a filtering process on the IQ data. The D/A conversion unit512converts the filtered IQ data into an analog signal. The up-conversion unit513up-converts the analog signal. The amplifier514amplifies the power of the analog signal to a predetermined transmission power.

FIG. 16is a schematic block diagram illustrating the functional structure of a BBU600during digital RoF transmission.

The BBU600includes an O/E conversion unit601, a deframing unit602, a modulation/demodulation unit603, a framing unit604, and an E/O conversion unit605.

The O/E conversion unit601converts an optical signal received via an optical fiber650into a frame signal (electrical signal). The deframing unit602extracts a control signal and IQ data from the frame signal. The modulation/demodulation unit603restores a wireless signal by demodulating the IQ data. Additionally, the modulation/demodulation unit603generates IQ data by modulating the wireless signal. The framing unit604performs framing by multiplexing the IQ data with a control signal. The E/O conversion unit605converts the frame signal (electrical signal) into an optical signal and transmits the converted optical signal to the RRH500via the optical fiber650.

Digital RoF transmission requires an extremely broad band in the optical fiber section. For example, in an LTE (Long Term Evolution) system, the wireless signals in a 2×2 MIMO (Multiple-Input and Multiple-Output) with a system bandwidth of 20 MHz have a maximum data rate of 150 Mbps in the wireless section. However, in order to transmit these wireless signals at a 15-bit quantization bit rate, a CPRI link of option 3 (2.4576 Gbps) or greater is needed. Therefore, the application of compression technologies to digital RoF transmission is being studied in order to make effective use of the optical band. Compression techniques can be largely divided between lossy compression and lossless compression. Lossy compression includes reduction of the sampling frequency, reduction of the quantization bit rate, or the like. Lossless compression includes a combination of linear predictive coding and entropy coding or the like. For example, when raising the transmission rate in the wireless section, the required transmission band in the optical section will also increase, but the increased speed in the wireless section can be handled without changing the optical transceiver if the required transmission band in the optical section is reduced by compression technology. For example, Non-patent Document 2 discusses MPEG-4 ALS (Moving Picture Experts Group-4 Audio Lossless Coding), which is a lossless compression technique.

FIG. 17is a schematic block diagram illustrating the functional structure of an RRH500awhen incorporating compression technology during multiplexed transmission.

The RRH500aincludes an antenna501, a transmission/reception switching unit502, an amplifier503, a down-conversion unit504, an A/D conversion unit505, a baseband filter unit506, a compression unit701, a framing unit507a, an E/O conversion unit508, an O/E conversion unit509, a deframing unit510, an expansion unit702, a baseband filter unit511a, a D/A conversion unit512, an up-conversion unit513, and an amplifier514.

The compression unit701compresses filtered IQ data. The framing unit507aperforms framing by multiplexing the compressed IQ data with a control signal. The expansion unit702restores the IQ data by decompressing the compressed IQ data. The baseband filter unit511aperforms a filtering process on the restored IQ data.

FIG. 18is a schematic block diagram illustrating the functional structure of a BBU600awhen incorporating compression technology during multiplexed transmission.

The BBU600aincludes an O/E conversion unit601, a deframing unit602, an expansion unit801, a modulation/demodulation unit603a, a compression unit802, a framing unit604a, and an E/O conversion unit605.

The expansion unit801restores IQ data by decompressing compressed IQ data. The modulation/demodulation unit603arestores a wireless signal by demodulating the restored IQ data. Additionally, the modulation/demodulation unit603agenerates IQ data by modulating the wireless signal. The compression unit802compresses the IQ data. The framing unit604aperforms framing by multiplexing the compressed IQ data with a control signal.

Among compression technologies, there are those in which a compression process and an expansion process are performed for every predetermined number of samples. In the following explanation, the units for performing the compression process will be referred to as frames, and the predetermined number of samples will be referred to as the frame size. For example, in compression technologies using linear predictive coding, a predicted value is obtained by multiplying coefficients by a number of sample points that are older than a given sample point and adding the multiplication results, and the error between the predicted value and the given sample point is outputted. If the prediction accuracy is high, then the amplitude value of the error signal will be close to zero. For this reason, the required band in the optical section can be reduced by entropy coding for transmitting data at a lower bit rate for amplitude values that have a higher probability of occurrence. It is to be noted that the coefficients are determined separately for each frame, and calculated so that the prediction error will be small for the IQ data in each frame.

Next, LTE wireless signals will be explained.

In LTE, OFDM (Orthogonal Frequency Division Multiplexing) is used in the downlink. As the time waveform, a signal having a cyclic prefix appended to a signal of a predetermined size that has been subjected to an IFFT (Inverse Fast Fourier Transform) is periodically outputted. On the other hand, in LTE, DFT-S-OFDM (Discrete Fourier Transform-Spread-OFDM) is used in the uplink. In this case also, as with OFDM, a signal having a cyclic prefix appended to a signal of a predetermined size that has been subjected to an IFFT is periodically outputted as the time waveform. In the following explanation, a signal having a cyclic prefix appended to a signal that has been subjected to an IFFT will be referred to as an OFDM symbol, without making a distinction between the downlink and the uplink.

In LTE, a normal cyclic prefix and an extended cyclic prefix are defined. A normal cyclic prefix is shorter than an extended cyclic prefix, and has higher frequency utilization efficiency. For this reason, normal cyclic prefixes are normally used, and in the following description, normal cyclic prefixes will be explained as an example.FIG. 19illustrates the structure of time slots in LTE. In the example shown inFIG. 19, seven OFDM symbols are arranged in a 0.5 ms interval. If the system bandwidth is 20 MHz, then the IFFT size is 2048, the size of the cyclic prefix (CP1) of the first OFDM symbol is 160 points and the size of the cyclic prefix (CP2) of the second to seventh OFDM symbols is 144 points. Therefore, the OFDM symbol length is 2208 points for the first OFDM symbol and 2192 points for the second to seventh OFDM symbols. Thus, the OFDM symbol lengths are not all the same. Non-patent Document 3 describes the structure of LTE frames.

FIG. 20is a diagram illustrating the compression rate for each frame when applying MPEG4-ALS to the data in the I component of a wireless signal.

InFIG. 20, the frame number represents the order in which the frames were compressed. The compression rate is the ratio of the data amount after compression to the original data amount. The wireless signal was OFDM-modulated using 1200 subcarriers with a subcarrier spacing of 15 kHz, modulated by 256 QAM (Quadrature Amplitude Modulation), with cyclic prefixes that were 160 samples (first OFDM symbol) or 144 samples (second to seventh OFDM symbols) long. In other words, it was assumed that the entire wireless band was used for data transmission in an LTE downlink system with a system bandwidth of 20 MHz. The frame size was 548.

InFIG. 20, (a) indicates the compression rate when only the first OFDM symbol is contained in a frame. (b) indicates the compression rate when the first OFDM symbol and the second OFDM symbol are contained in a frame. (c) indicates the compression rate when only the second OFDM symbol is contained in a frame. (d) indicates the compression rate when the second OFDM symbol and the third OFDM symbol are contained in a frame. (e) indicates the compression rate when only the third OFDM symbol is contained in a frame. (f) indicates the compression rate when the third OFDM symbol and the fourth OFDM symbol are contained in a frame. (g) indicates the compression rate when only the fourth OFDM symbol is contained in a frame. (h) indicates the compression rate when the fourth OFDM symbol and the fifth OFDM symbol are contained in a frame. (i) indicates the compression rate when only the fifth OFDM symbol is contained in a frame. (j) indicates the compression rate when the fifth OFDM symbol and the sixth OFDM symbol are contained in a frame.

PRIOR ART DOCUMENTS

SUMMARY OF INVENTION

Problems to be Solved by Invention

As shown inFIG. 20, the compression rate when a compression process is performed without including multiple kinds of OFDM symbols is less than 0.7, while the compression rate when a compression process is performed while including multiple kinds of OFDM symbols always exceeds 0.7. In other words, when performing a compression process while including multiple kinds of OFDM symbols, the compression rate becomes worse than for the case in which the compression process is performed within only one OFDM symbol. This is believed to be due to the fact that the prediction accuracy becomes lower because the frequency components are different and the signal properties differ between OFDM symbols. Thus, with conventional technologies, there is a problem in that the compression rate becomes worse due to compression processes being performed while including multiple kinds of OFDM symbols.

In view of the above-described circumstances, an object of the present invention is to provide a technology that can reduce the worsening of the compression rate.

Means for Solving the Problems

An aspect of the present invention is an optical communication system including: a signal processing apparatus; and a wireless apparatus, in which functions of a base station are divided between the signal processing apparatus and the wireless apparatus, a periodic symbol sequence including a cyclic prefix appended to a signal of a predetermined size to which an IFFT (Inverse Fast Fourier Transform) has been applied is transmitted between the signal processing apparatus and the wireless apparatus by means of digital RoF (Radio over Fiber) transmission, the signal processing apparatus and the wireless apparatus each include a transmission unit and a reception unit; the transmission unit includes: a compression size determination unit that acquires symbol information relating to a starting position of the symbol sequence and lengths of symbols constituting the symbol sequence and determines, based on the acquired symbol information, a compression size for each of symbols that are to be compressed; and a compression unit that compresses the symbol sequence in units of determined compression sizes, and the reception unit includes: an expansion size determination unit that determines an expansion size for each of symbols in the symbol sequence that are to be expanded; and an expansion unit that expands the symbol sequence in units of determined expansion sizes.

In the above-mentioned optical communication system, the transmission unit may further include a compression rate measurement unit that measures a compression rate of each of the symbols, and the compression size determination unit may acquire, as the starting position, a position of a symbol at which a predetermined statistical value of measured compression rates is smallest, and determine the compression sizes using the acquired starting position and information relating to the lengths of the symbols.

In the above-mentioned optical communication system, the transmission unit may further include a symbol information estimation unit that estimates the starting position based on IQ data for a downlink or for an uplink.

An aspect of the present invention is an optical communication method in an optical communication system including a signal processing apparatus and a wireless apparatus between which functions of a base station are divided, the signal processing apparatus and the wireless apparatus each including a transmission unit and a reception unit, a periodic symbol sequence including a cyclic prefix appended to a signal of a predetermined size to which an IFFT (Inverse Fast Fourier Transform) has been applied being transmitted between the signal processing apparatus and the wireless apparatus by means of digital RoF (Radio over Fiber) transmission; the optical communication method including: a compression size determination step, performed by the transmission unit, of acquiring symbol information relating to a starting position of the symbol sequence and lengths of symbols constituting the symbol sequence, and determining, based on the acquired symbol information, a compression size for each of symbols that are to be compressed; a compression step, performed by the transmission unit, of compressing the symbol sequence in units of determined compression sizes; an expansion size determination step, performed by the reception unit, of determining an expansion size for each of symbols in the symbol sequence that are to be expanded; and an expansion step, performed by the reception unit, of expanding the symbol sequence in units of determined expansion sizes.

Advantageous Effects of Invention

Due to the present invention, it is possible to reduce the worsening of the compression rate.

MODES FOR CARRYING OUT THE INVENTION

Herebelow, embodiments of the present invention will be explained with reference to the drawings.

According to the present invention, in an optical communication system including an RRH (wireless apparatus) and a BBU (signal processing apparatus) between which the functions of a base station are divided, the RRH and the BBU acquire information (hereinafter referred to as “OFDM symbol information”) regarding the starting position of a symbol sequence composed of multiple OFDM symbols (symbols), and the length of each OFDM symbol. Additionally, the RRH and the BBU determine a frame size for each OFDM symbol that is to be compressed based on the acquired OFDM symbol information, and perform compression using the determined frame sizes.

Herebelow, a detailed explanation will be given, using multiple embodiments (first embodiment to third embodiment) as examples.

In the first embodiment, the RRH and the BBU acquire OFDM symbol information (symbol information), determine the frame size for each OFDM symbol that is to be compressed based on the acquired OFDM symbol information, and perform compression using the determined frame sizes. Additionally, the RRH and the BBU determine the frame size of each OFDM symbol that is to be expanded based on the acquired OFDM symbol information, and perform expansion using the determined frame sizes.

FIG. 1is a schematic block diagram illustrating the functional structure of the RRH100in the first embodiment. Additionally,FIG. 2is a schematic diagram illustrating the functional structure of the BBU200in the first embodiment. First, the RRH100will be explained.

The RRH100includes an antenna101, a transmission/reception switching unit102, an amplifier103, a down-conversion unit104, an A/D conversion unit105, a baseband filter unit106, a compression process size determination unit107, a compression unit108, a framing unit109, an E/O conversion unit110, an O/E conversion unit111, a deframing unit112, an expansion process size determination unit113, an expansion unit114, a baseband filter unit115, a D/A conversion unit116, an up-conversion unit117, and an amplifier118.

The antenna101transmits and receives wireless signals with respect to a wireless terminal connected to the RRH100. The transmission/reception switching unit102switches the antenna101between transmission and reception. It is to be noted that the transmission/reception switching unit102is compatible with both FDD (Frequency Division Duplex) and TDD (Time Division Duplex). For example, when the BBU and RRH are connected by a CPRI interface, around 1/16 of the total capacity is used for control signals while 15/16 is used for sending IQ data, and as a control signal, a K28.5 code or the like is transmitted in order to establish a CPRI link.

The amplifier103amplifies the signal power of a received wireless signal to a level that allows for signal processing. The down-conversion unit104down-converts the wireless signal into the baseband. The A/D conversion unit105converts the down-converted wireless signal (analog signal) into IQ data, which is a digital signal. The baseband filter unit106performs a filtering process on the IQ data. Due to this process, OFDM symbols of the wireless signal are generated.

The compression process size determination unit107determines the frame sizes (hereinafter referred to as “compression sizes”) of frames that are to be compressed, based on the acquired OFDM symbol information. In this case, a frame is a unit by which the compression process is performed, and is formed, for example, by arranging the IQ data in a time waveform for a predetermined number of samples.

The compression unit108compresses the OFDM symbols, frame by frame, in units of the compression sizes determined by the compression process size determination unit107.

The framing unit109generates a frame signal by multiplexing the compressed OFDM symbols with a control signal.

The E/O conversion unit110converts the frame signal (electrical signal) into an optical signal and transmits the converted optical signal to the BBU200via an optical fiber150.

The O/E conversion unit111converts an optical signal received via the optical fiber150into a frame signal (electrical signal).

The deframing unit112extracts a control signal and compressed OFDM symbols from a frame signal.

The expansion process size determination unit113determines frame sizes for performing the expansion process (hereinafter referred to as “expansion sizes”) based on frame sizes acquired from the BBU200. As a method for the expansion process size determination unit113to obtain information on the expansion size, it is possible to contemplate a method wherein a compression unit in the BBU200is configured to append a frame size, as a header, in front of compressed data and transmit the header and the compressed data, and the frame size is acquired on the basis of the header. In the case of an LTE system, at minimum, two frame sizes may be sufficient, in which case a single bit may be used as the header. The expansion size and the compression size are the same size.

The expansion unit114expands the compressed OFDM symbols in units of the expansion sizes determined by the expansion process size determination unit113. Specifically, the expansion unit114restores the OFDM symbols by decompressing the compressed OFDM symbols in units of the determined expansion sizes.

The baseband filter unit115performs a filtering process on the restored OFDM symbols.

The D/A conversion unit116converts the filtered signal into an analog signal.

The up-conversion unit117up-converts the analog signal.

The amplifier118amplifies the power of the analog signal to a predetermined transmission power.

The BBU200includes an O/E conversion unit201, a deframing unit202, an expansion process size determination unit203, an expansion unit204, a modulation/demodulation unit205, a compression process size determination unit206, a compression unit207, a framing unit208, and an E/O conversion unit209.

The O/E conversion unit201converts an optical signal received via an optical fiber250into a frame signal (electrical signal). The deframing unit202extracts a control signal and a multiplexed signal from the frame signal.

The expansion process size determination unit203determines expansion sizes based on the frame sizes acquired from the RRH100. As the method for the expansion process size determination unit203to acquire the expansion sizes, it is possible to contemplate a method of acquisition by a process similar to that of the expansion process size determination unit113.

The expansion unit204expands compressed OFDM symbols in units of the expansion sizes determined by the expansion process size determination unit203. Specifically, the expansion unit204restores the OFDM symbols by decompressing the compressed OFDM symbols in units of the determined expansion sizes.

The modulation/demodulation unit205restores the wireless signal by demodulating the restored OFDM symbols. Additionally, the modulation/demodulation unit205outputs the IQ data of the wireless signal to the compression process size determination unit206and the compression unit207.

The compression process size determination unit206determines compression sizes based on acquired OFDM symbol information. The relationship between OFDM symbol length and frame size may be predetermined. In that case, the compression process size determination unit206determines the compression sizes based on the OFDM symbol lengths in the acquired OFDM symbol information and the predetermined frame sizes. For example, the compression process size determination unit206may set frame sizes corresponding to OFDM symbol lengths as the compression sizes.

The compression unit207compresses OFDM symbols, frame by frame, in units of the compression sizes determined by the compression process size determination unit206.

The framing unit208generates a frame signal by multiplexing the compressed OFDM symbols with a control signal.

The E/O conversion unit209converts the frame signal (electrical signal) into an optical signal and transmits the converted optical signal to the RRH100via the optical fiber250.

It is to be noted that in the following explanation, the compression unit108and the compression unit207will be referred to simply as compression units when no particular distinction is to be made therebetween. Additionally, in the following explanation, the expansion unit114and the expansion unit204will be referred to simply as expansion units when no particular distinction is to be made therebetween. Additionally, in the following explanation, the compression process size determination unit107and the compression process size determination unit206will be referred to simply as compression process size determination units when no particular distinction is to be made therebetween. Additionally, in the following explanation, the expansion process size determination unit113and the expansion process size determination unit203will be referred to simply as expansion process size determination units when no particular distinction is to be made therebetween.

The compression units perform compression processes in accordance with the determined compression sizes. Additionally, the expansion units perform expansion processes in accordance with the determined expansion sizes. For example, if linear predictive coding is performed by the compression units, the number of samples used for determining coefficients is changed if the frame size (compression size) is changed. The compression units and expansion units may be configured to include compression and expansion circuits for each frame size, and switch between which circuit is to be used in accordance with notified frame sizes.

As the method for the compression process size determination unit206to acquire the OFDM symbol information, it could be acquired from the modulation/demodulation unit205of the BBU200. In that case, the compression process size determination unit107of the RRH100must be notified of the OFDM symbol information. Therefore, if the BBU200and RRH100are connected by a CPRI interface, then the OFDM symbol information can be transmitted by using reserved bits or the like in the CPRI control signal. The compression process size determination unit107acquires the OFDM symbol information from the BBU200by being notified of the OFDM symbol information.

For example, in the case of a TDD LTE system, uplink and downlink communications are switched at a minimum period of 1 ms. Therefore, if the OFDM symbol starting position and the OFDM symbol length information in a downlink 0.5 ms period are known, then it is possible to estimate the OFDM symbol starting position and OFDM symbol length information for the uplink. Normally, the OFDM symbol lengths are fixed for each system, so the OFDM symbol length information could be pre-stored in the compression process size determination units. For example, in an LTE wireless system, if the starting position of an OFDM symbol having a CP (cyclic prefix) length of 160 is known, then the starting positions and OFDM symbol lengths of subsequent OFDM symbols will also be known. For this reason, it is sufficient to acquire just the starting position information for an OFDM symbol having a CP length of 160. Additionally, since the OFDM signals are outputted continuously, it is sufficient to obtain the starting position of an OFDM symbol just once, and there is no need to periodically acquire starting position information.

FIG. 3is a diagram illustrating an operation example of a compression process size determination unit and an expansion process size determination unit.

In the example shown inFIG. 3, multiple OFDM symbols are shown. (a) represents a first OFDM symbol, (b) represents a second OFDM symbol, and (c) represents a third OFDM symbol. By indicating the number of frames in the j-th OFDM symbol as Aj and the frame size of each frame as ai,j(1≤i≤Aj), (a) to (c) can be represented respectively as shown inFIG. 3. Thus, the compression process size determination unit determines the frame sizes ai,jseparately for each OFDM symbol, so as not to include multiple OFDM symbols. In an LTE system, the OFDM symbol lengths of the second to seventh OFDM symbols are the same, so it is possible to have the relationship, ai,2=ai,3=ai,4=ai,5=ai,6=ai,7. In this case, it is sufficient for the compression process size determination units107and206to determine two frame sizes, i.e., one for the first OFDM symbol and one for the second to seventh OFDM symbols. Additionally, the relationship may simply be a1,j=a2,j= . . . =aAj,j. Additionally, the expansion process size determination units113and203may also determine the frame sizes (expansion sizes) in a similar manner.

FIG. 4is a flow chart showing the processing flow for an uplink in the RRH100in the first embodiment.

The antenna101receives a wireless signal (step S101). The antenna101outputs the received wireless signal to the amplifier103via a transmission/reception switching unit102. The amplifier103amplifies the signal power of the wireless signal to a level allowing for signal processing (step S102). The down-conversion unit104down-converts the wireless signal into the baseband (step S103). Thereafter, the A/D conversion unit105converts the down-converted wireless signal into IQ data, which is a digital signal (step S104). The baseband filter unit106performs a filtering process on the IQ data (step S105).

The compression process size determination unit107determines the compression size based on the OFDM symbol information acquired from the BBU200(step S106). The compression unit108compresses the OFDM symbols in units of the compression sizes determined by the compression process size determination unit107(step S107). The framing unit109generates a frame signal by multiplexing the compressed OFDM symbols with a control signal (step S108). The E/O conversion unit110converts the frame signal into an optical signal (step S109). Then, the E/O conversion unit110transmits the optical signal to the BBU200via the optical fiber150(step S110).

FIG. 5is a flow chart showing the processing flow for an uplink in the BBU200in the first embodiment.

The O/E conversion unit201converts an optical signal received via the optical fiber250into a frame signal (electrical signal) (step S201). The O/E conversion unit201outputs the frame signal to the deframing unit202. The deframing unit202extracts a control signal and compressed OFDM symbols from the frame signal (step S202). The expansion process size determination unit203determines the expansion sizes based on frame sizes indicated by the RRH100(step S203).

The expansion unit204restores the OFDM symbols by decompressing the OFDM symbols in units of the expansion sizes determined by the expansion process size determination unit203(step S204). The modulation/demodulation unit205restores the wireless signal by demodulating the restored OFDM symbols (step S205). The modulation/demodulation unit205receives the restored wireless signal (step S206). It is to be noted that the reception in the processing in step S206refers to the modulation/demodulation unit205acquiring a wireless signal by demodulating OFDM symbols.

FIG. 6is a flow chart showing the processing flow for a downlink in the RRH100in the first embodiment.

The O/E conversion unit111converts an optical signal received via the optical fiber150into a frame signal (electrical signal) (step S301). The deframing unit112extracts a control signal and compressed OFDM symbols from a frame signal (step S302). The expansion process size determination unit113determines expansion sizes based on frame sizes indicated by the BBU200(step S303). The expansion unit114restores the OFDM symbols by decompressing the compressed OFDM symbols in units of the frame sizes determined by the expansion process size determination unit113(step S304).

The baseband filter unit115performs a filtering process on the restored OFDM symbols (step S305). The D/A conversion unit116converts the filtered signal into an analog signal (step S306). The up-conversion unit117up-converts the analog signal (step S307). The amplifier118amplifies the power of the analog signal to a predetermined transmission power (step S308). The antenna101transmits the analog signal to a wireless terminal connected to the RRH100(step S309).

FIG. 7is a flow chart showing the processing flow for a downlink in the BBU200in the first embodiment.

The modulation/demodulation unit205outputs the IQ data of a wireless signal to the compression process size determination unit206and the compression unit207(step S401). The compression process size determination unit206determines compression sizes based on acquired OFDM symbol information (step S402). The compression unit207compresses OFDM symbols in units of the compression sizes determined by the compression process size determination unit206(step S403). The framing unit208generates a frame signal by multiplexing the compressed OFDM symbols with the control signal (step S404). The E/O conversion unit209converts the frame signal (electrical signal) into an optical signal (step S405). The E/O conversion unit209transmits the optical signal to the RRH100via an optical fiber250(step S406).

According to the RRH100and the BBU200configured as above, it is possible to reduce the worsening of the compression rate. Herebelow, this effect will be explained in detail.

The RRH100and the BBU200determine the compression sizes for compressing a wireless signal based on acquired OFDM symbol information. Due to this process, the RRH100and the BBU200determine the frame sizes so as not to include OFDM symbols having different frequency characteristics when performing compression processes. Additionally, the RRH100and the BBU200perform compression in units of the determined compression sizes. For this reason, it is possible to reduce the worsening of the compression rate overall. Additionally, since the worsening of the compression rate is reduced, it is possible to make effective use of the transmission band.

In the second embodiment, the RRH and the BBU acquire information on the OFDM symbol starting position based on the compression rate for each OFDM symbol. Additionally, the RRH and the BBU determine the frame sizes in OFDM symbols that are to be compressed based on the acquired information on the OFDM symbol starting position and OFDM symbol length information, and perform the compression processes using the determined frame sizes.

FIG. 8is a schematic block diagram illustrating the functional structure of the RRH100ain the second embodiment. Additionally,FIG. 9is a schematic block diagram illustrating the functional structure of the BBU200ain the second embodiment. First, the RRH100awill be explained.

The RRH100aincludes an antenna101, a transmission/reception switching unit102, an amplifier103, a down-conversion unit104, an A/D conversion unit105, a baseband filter unit106, a compression process size determination unit107a, a compression unit108, a framing unit109, an E/O conversion unit110, an O/E conversion unit111, a deframing unit112, an expansion process size determination unit113, an expansion unit114, a baseband filter unit115, a D/A conversion unit116, an up-conversion unit117, an amplifier118, and a compression rate measurement unit119.

The RRH100ahas a different structure from the RRH100in that a compression process size determination unit107ais provided instead of the compression process size determination unit107, and a compression rate measurement unit119is newly provided. The other features of the RRH100aare the same as those in the RRH100. For this reason, the explanation of the RRH100aas a whole will be omitted, and only the compression process size determination unit107aand the compression rate measurement unit119will be explained.

The compression rate measurement unit119measures the compression rate for each OFDM symbol using the output from the compression unit108.

The compression process size determination unit107adetermines the compression sizes based on the compression rates measured by the compression rate measurement unit119. Specifically, the compression process size determination unit107aacquires, as the OFDM symbol starting position, the position at which the average value, the maximum value, or the like of the measured compression rates is smallest. Furthermore, the compression process size determination unit107adetermines the compression sizes so as not to cross between OFDM symbols having different frequency characteristics, based on the acquired OFDM symbol starting position information and the OFDM symbol length information. It is to be noted that in the second embodiment, the compression process size determination unit107amust acquire the OFDM symbol length information by the method in the first embodiment, or have the information pre-stored.

The BBU200aincludes an O/E conversion unit201, a deframing unit202, an expansion process size determination unit203, an expansion unit204, a modulation/demodulation unit205, a compression process size determination unit206a,a compression unit207, a framing unit208, an E/O conversion unit209, and a compression rate measurement unit210. The BBU200ahas a different structure from the BBU200in that a compression process size determination unit206ais provided instead of the compression process size determination unit206, and a compression rate measurement unit210is newly provided. The other features of the BBU200aare the same as those in the BBU200. For this reason, the explanation of the BBU200aas a whole will be omitted, and only the compression process size determination unit206aand the compression rate measurement unit210will be explained. The processing in the compression process size determination unit206aand the compression rate measurement unit210are the same as the processing in the compression process size determination unit107aand the compression rate measurement unit119.

FIG. 10is a flow chart of the processing flow for an uplink in the RRH100ain the second embodiment. It is to be noted that inFIG. 10, the processes that are the same as those inFIG. 4are indicated by the same reference symbols as inFIG. 4, and their explanations will be omitted.

The RRH100aperforms an OFDM symbol starting position information acquisition process (step S501). The OFDM symbol starting position information acquisition process will be described below. The compression process size determination unit107adetermines the compression sizes so as not to cross between OFDM symbols having different frequency characteristics, based on the OFDM symbol starting position information acquired in the process of step S501, and OFDM symbol length information (step S502). Thereafter, the processes of step S107and subsequent steps are executed.

FIG. 11is a flow chart showing the flow for an OFDM symbol starting position information acquisition process. InFIG. 11, an example for the case of an LTE system having a system bandwidth of 20 MHz will be explained. In LTE having a system bandwidth of 20 MHz, OFDM symbols are transmitted at a period of 0.5 ms (15360 samples). For this reason, it is sufficient for the compression process size determination unit107ato be able to acquire information regarding the lead position of the 15360 samples.

First, the compression process size determination unit107asets initial values so that i=0, ia=0, and a provisional minimum value is ∞ (step S601). In this case, i represents the frame number, iarepresents the estimated value of the OFDM symbol starting position, and the provisional minimum value represents the minimum value of the compression rate. The compression rate measurement unit119measures the compression rate (step S602). The compression process size determination unit107adetermines whether or not the measured compression rate is smaller than the provisional minimum value (step S603). If the measured compression rate is smaller than the provisional minimum value (step S603—YES), then the compression process size determination unit107asets iato the value of i, and sets the provisional minimum value to the measured compression rate (step S604). Thereafter, the compression process size determination unit107aincrements the value of i by 1 (step S605). The compression process size determination unit107adetermines whether or not the value of i is equal to or greater than a preset imax(step S606). In this case, in the case of LTE having a system bandwidth of 20 MHz, imaxis 15360.

If the value of i is equal to or greater than the preset imax(step S606—YES), the compression process size determination unit107aacquires, as the OFDM symbol starting information, the estimated value iafor the OFDM symbol starting position, among all of the estimated values iafor the OFDM symbol starting position, for which the compression rate characteristics are the most favorable (step S607). In this case, the OFDM symbol starting position for which the compression rate characteristics are the most favorable is the position at which the average value, the maximum value, or the like of the compression rate is the smallest.

Additionally, in step S606, if the value of i is not equal to or greater than the preset imax(step S606—NO), then the RRH100arepeatedly runs the process of step S602and subsequent steps.

Additionally, in step S603, if the measured compression rate is not smaller than the provisional minimum value (step S603—NO), then the compression process size determination unit107aincrements the value of i by 1 (step S605).

According to the RRH100aand the BBU200aconfigured as above, it is possible to obtain effects similar to those of the first embodiment.

Additionally, the RRH100aand the BBU200aacquire OFDM symbol starting position information based on the compression rates for compression processes performed by the compression unit108and the compression unit207. Furthermore, the RRH100aand the BBU200adetermine the compression sizes based on the acquired OFDM symbol starting position information and the OFDM symbol length information. For this reason, more highly precise compression is possible. Additionally, it is possible to correct the OFDM symbol starting position even if it has become displaced under the influence of the wireless propagation environment, processing delays in the BBU/RRH, or delays in a fiber between the BBU200aand the RRH100a.

The compression rate measurement unit119and the compression rate measurement unit210may reduce the amount of computation by using the results of analyses performed by the compression unit instead of measuring the compression rate. Specifically, the compression rate measurement unit119and the compression rate measurement unit210estimate the information amount, i.e., the compression rate, in each frame, by using an auto-correlation coefficients and PARCOR (Partial Auto-Correlation) coefficients obtained during linear predictive analysis. Due to this process, it is possible to reduce the processing amount needed for entropy coding.

In the third embodiment, the RRH and the BBU acquire OFDM symbol information from the IQ data on the downlink or the uplink. Additionally, the RRH and the BBU determine the frame sizes in OFDM symbols that are to be compressed based on the acquired OFDM symbol information, and perform the compression processes using the determined frame sizes.

FIG. 12is a schematic block diagram illustrating the functional structure of the RRH100bin the third embodiment. Additionally,FIG. 13is a schematic block diagram illustrating the functional structure of the BBU200bin the third embodiment. First, the RRH100bwill be explained.

The RRH100bincludes an antenna101, a transmission/reception switching unit102, an amplifier103, a down-conversion unit104, an A/D conversion unit105, a baseband filter unit106, a compression process size determination unit107b, a compression unit108, a framing unit109, an E/O conversion unit110, an O/E conversion unit111, a deframing unit112, an expansion process size determination unit113, an expansion unit114, a baseband filter unit115, a D/A conversion unit116, an up-conversion unit117, an amplifier118, and an OFDM symbol information estimation unit120.

The RRH100bhas a different structure from the RRH100in that a compression process size determination unit107bis provided instead of the compression process size determination unit107, and an OFDM symbol information estimation unit120is newly provided. The other features of the RRH100bare the same as those in the RRH100. For this reason, the explanation of the RRH100bas a whole will be omitted, and only the compression process size determination unit107band the OFDM symbol information estimation unit120will be explained.

The OFDM symbol information estimation unit120estimates the OFDM symbol starting position and the OFDM symbol length information by using the uplink signal.

The compression process size estimation unit107bacquires OFDM symbol information estimated by the OFDM symbol information estimation unit120. Furthermore, the compression process size determination unit107bdetermines the compression size based on the acquired OFDM symbol information.

The BBU200bincludes an O/E conversion unit201, a deframing unit202, an expansion process size determination unit203, an expansion unit204, a modulation/demodulation unit205, a compression process size determination unit206b,a compression unit207, a framing unit208, an E/O conversion unit209, and an OFDM symbol information estimation unit211. The BBU200bhas a different structure from the BBU200in that a compression process size determination unit206bis provided instead of the compression process size determination unit206, and an OFDM symbol information estimation unit211is newly provided. The other features of the BBU200bare the same as those in the BBU200. For this reason, the explanation of the BBU200bas a whole will be omitted, and only the compression process size determination unit206band the OFDM symbol information estimation unit211will be explained.

The OFDM symbol information estimation unit211estimates the OFDM symbol starting position and the OFDM symbol length information by using the downlink signal.

The compression process size determination unit206bacquires OFDM symbol information estimated by the OFDM symbol information estimation unit211. Furthermore, the compression process size determination unit206bdetermines the compression size based on the acquired OFDM symbol information.

FIG. 14is a flow chart of the processing flow for an uplink in the RRH100bin the third embodiment. It is to be noted that inFIG. 14, the processes that are the same as those inFIG. 4are indicated by the same reference symbols as inFIG. 4, and their explanations will be omitted.

The OFDM symbol information estimation unit120estimates the OFDM symbol starting position and OFDM symbol length information by using the uplink signal (step S801). As a method for estimating the OFDM symbol starting position and the OFDM symbol length information, there is a method of applying an FFT transform to the IQ data and measuring the EVM (Error Vector Magnitude). In this case, the OFDM symbol information estimation unit120shifts the FFT window one point at a time, and estimates the OFDM symbol starting position or the cyclic prefix length (OFDM symbol length) information by the positions or periods at which the EVM after an FFT is smallest. Alternatively, the OFDM symbol information estimation unit120may estimate the OFDM symbol starting position or the cyclic prefix length (OFDM symbol length) by using an autocorrelation of the uplink signal, so as to make use of the periodicity of the cyclic prefix. It is to be noted that the OFDM symbol length information may be acquired by the method of the first embodiment, or may be pre-stored in the OFDM symbol information estimation unit120.

According to the RRH100band the BBU200bconfigured as above, it is possible to obtain effects similar to those of the first embodiment.

Additionally, the RRH100band the BBU200bcan correct the OFDM symbol starting position even if it has become displaced under the influence of the wireless propagation environment, processing delays in the BBU/RRH, or delays in a fiber between the BBU200band the RRH100b. Additionally, the RRH100band the BBU200bcan estimate the OFDM symbol starting position and the OFDM symbol length information by using the downlink signal and the uplink signal respectively, without requiring any additional information in order to estimate the OFDM symbol starting position and the OFDM symbol length information.

It is to be noted that the various above-mentioned processes relating to the processing in the RRH100, the RRH100a, the RRH100b, the BBU200, the BBU200a.and the BBU200bof the present invention may be performed by recording, onto a computer-readable recording medium, programs for executing the processing in the RRH100, the RRH100a, the RRH100b, the BBU200, the BBU200a, and the BBU200b,and reading and running the programs recorded on said recording medium on a computer system. It is to be noted that the “computer system” as referred to herein may include an OS (Operating System) and/or hardware such as peripheral devices. Additionally, the “computer system”, if using a WWW (World Wide Web) system, may include a homepage-providing environment (or display environment). Additionally, “computer-readable recording medium” refers to writable non-volatile memories such as flexible disks, magneto-optic disks, ROMs (Read Only Memories), and flash memories, portable media such as CD (Compact Disc)-ROMs or the like, or memory apparatus such as hard disks that are internally provided in computer systems.

Furthermore, the “computer-readable recording medium” includes media that hold a program for a certain period of time, such as volatile memories (e.g. DRAMs (Dynamic Random Access Memories)) inside computer systems serving as servers or clients when the program is transmitted over a network such as the internet or over communication lines such as telephone lines. Additionally, the above-mentioned program may be transmitted from a computer system that stores the program in a memory apparatus or the like, to another computer system, via a transmission medium or by transmission waves in a transmission medium. In this case, the “transmission medium” that transmits the program refers to a medium having the function of transmitting information, including networks (communication networks) such as the internet or communication lines (communication cables) such as telephone lines. Additionally, the above-mentioned program may be for implementing just some of the aforementioned functions. Furthermore, the above-mentioned program may be a so-called difference file (difference program) that can be implemented by combining the aforementioned functions with a program that is already recorded in a computer system.

While embodiments of the present invention have been described in detail by referring to the drawings above, the specific structure is not limited to these embodiments, and other designs or the like within a range not departing from the gist of the present invention are included.

INDUSTRIAL APPLICABILITY

The present invention may, for example, be applied to digital RoF transmission. According to the present invention, it is possible to reduce the worsening of the compression rate.

DESCRIPTION OF REFERENCE SIGNS