JP2013187833A - Transmission device and transmission frame configuration method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transmission device and a transmission frame configuration method that can reduce influence of inter-symbol interference while avoiding a change of basic parameters such as an OFDM symbol length and a guard interval length and complication of a circuit configuration in a multi-path environment having long delay at a degree at which a typical guard interval length cannot suppress influence of inter-symbol interference.SOLUTION: A transmission frame control unit of a transmission device according to the invention selects a sub-carrier about which the length of a guard interval is 1/N of the length of an OFDM symbol, the frequency of subcarriers constituting the OFDM symbol is N×M times a basis frequency, and N and M are natural numbers, and uses a transmission frame including an extended guard interval extended in an equivalent manner by allocating the same modulation symbol to two or more consecutive OFDM symbols for the selected sub-carrier.

Description

  The present invention relates to a transmission apparatus and a transmission frame configuration method.

  In an orthogonal frequency division multiplexing (OFDM) communication system, a predetermined guard interval (GI) is provided between OFDM symbols. For this reason, the receiving apparatus can receive the OFDM symbol transmitted from the transmitting apparatus without being affected by inter-symbol interference (ISI) and inter-subcarrier interference (ICI) even if there is a delayed wave that falls within the GI. . However, since the GI length cannot be increased indefinitely, ISI becomes a problem when the OFDM symbol propagation delay between the transmission device and the reception device exceeds the GI length.

  Therefore, in order to solve such a problem, when two or more consecutive OFDM symbols are the same, a transmission frame is generated in which the even-numbered OFDM symbol and the GI phase are continuous with the odd-numbered OFDM symbol and the GI phase. A method is known (for example, Patent Document 1). According to such a method, since the OFDM symbol can be used as a GI, an effect equivalent to the case where the GI length is expanded can be obtained.

International Publication No. 2008/038769

  However, in the conventional transmission frame generation method described above, the OFDM symbol and the GI are configured to be symmetric with respect to a boundary between two or more consecutive OFDM symbols (see FIG. 2 in Patent Document 1). However, there is a problem in compatibility with the conventional general-purpose OFDM transmission frame configuration. In addition, there is a problem that the circuit configuration of the transmission apparatus becomes complicated because additional processing such as switching of the position where the GI is added or phase shift of even-numbered OFDM symbols is required (see FIG. 8 of Patent Document 1).

  Therefore, the present invention has been made in view of such a situation, and even in a multipath environment having a long delay such that the effect of intersymbol interference cannot be suppressed with a normal guard interval length, the OFDM symbol length and the guard An object of the present invention is to provide a transmission apparatus and a transmission frame configuration method capable of reducing the influence of intersymbol interference while avoiding changes in basic parameters such as interval length and complication of circuit configuration.

  A first feature of the present invention is that transmission according to an orthogonal frequency division multiplexing system includes a transmission frame control unit that controls a configuration of a transmission frame including a plurality of OFDM symbols and guard intervals obtained by duplicating a predetermined portion of the OFDM symbol. The transmission frame control unit is configured such that the guard interval length is 1 / N of the OFDM symbol length, and the frequency of subcarriers constituting the OFDM symbol is N × M times the base frequency. An extension extended equivalently by selecting a subcarrier in which N and M are natural numbers, and assigning the same modulation symbol to two or more consecutive OFDM symbols for the selected subcarrier. The gist is to use the transmission frame including a guard interval.

  A second feature of the present invention is a transmission frame configuration method in a communication apparatus that transmits a transmission frame including a plurality of transmission symbols and a guard interval obtained by duplicating a predetermined period of the OFDM symbol, and the length of the guard interval. Selecting a subcarrier in which N is 1 / N of the length of the OFDM symbol, the frequency of subcarriers constituting the OFDM symbol is N × M times the base frequency, and N and M are natural numbers; And using the transmission frame including an extended guard interval that is equivalently extended by assigning the same modulation symbol to two or more consecutive OFDM symbols for the selected subcarrier. And

  According to the features of the present invention, even in a multipath environment having a long delay that cannot suppress the effect of intersymbol interference with a normal guard interval length, the basic parameters such as the OFDM symbol length and the guard interval length can be changed and the circuit configuration can be obtained. Thus, it is possible to provide a transmission apparatus and a transmission frame configuration method that can reduce the influence of intersymbol interference while avoiding the complexity of.

1 is an overall schematic configuration diagram of a wireless communication system according to an embodiment of the present invention. It is a functional block block diagram of the transmitter (super large zone base station 100) which concerns on embodiment of this invention. It is explanatory drawing of the general | schematic basic structure of the transmission frame and OFDM symbol which concern on embodiment of this invention. 4 shows an example of frequency resource allocation in the case of using the transmission frame configuration according to the embodiment of the present invention. It is a figure which shows the example of allocation of the frequency resource which concerns on the example of a change of this invention. It is a figure which shows schematic structure of the transmission frame to which the time window process was applied in the transmitter which concerns on the example of a change of this invention.

  Next, an embodiment of the present invention will be described. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic and ratios of dimensions and the like are different from actual ones.

  Accordingly, specific dimensions and the like should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

(1) Overall Schematic Configuration of Radio Communication System FIG. 1 is an overall schematic configuration diagram of a radio communication system 1 according to the present embodiment. As shown in FIG. 1, the wireless communication system 1 includes a plurality of types of base stations having different cell sizes to be formed, and a mobile station 400. Specifically, the radio communication system 1 includes a super large zone base station 100, large zone base stations 200A and 200B, and macro base stations 300A and 300B. In the radio communication system 1, an orthogonal frequency division multiplexing (OFDM) is used. For example, the wireless communication system 1 is a mobile communication system compliant with Long Term Evolution (LTE) defined in the 3rd Generation Partnership Project (3GPP).

  Unlike a normal base station, the super large zone base station 100 forms a cell C1 having a cell radius of about 100 km. The super large zone base station 100 can be suitably used, for example, to transmit information that is broadcast (or multicast) to a plurality of mobile stations 400 and to ensure communication in the event of a disaster. In the present embodiment, the super large zone base station 100 constitutes a transmission device according to the orthogonal frequency division multiplexing scheme.

  The large zone base station 200A forms a cell having a cell radius smaller than that of the cell C1 (cell radius is about 1 to 7 km). The macro base station 300A forms a cell having a smaller cell radius (a cell radius of about 1 km) than the cell formed by the large zone base station 200A.

  The mobile station 400 transmits a transmission frame transmitted from any of the base stations of the super large zone base station 100, the large zone base stations 200A and 200B, and the macro base stations 300A and 300B, depending on the existing position or the type of information to be received. Receive. In the present embodiment, the mobile station 400 constitutes a receiving device.

(2) Functional Block Configuration of Transmitting Device FIG. 2 is a functional block configuration diagram of the super large zone base station 100 configuring the transmitting device in the present embodiment. As shown in FIG. 2, the super large zone base station 100 includes a scheduler unit 101, an S / P and MUX unit 103, a transmission frame control unit 105, an IFFT unit 107, a GI adding unit 109, and a radio transmission unit 111.

  The scheduler unit 101 performs scheduling of input information to predetermined radio resources (frequency domain and time domain).

  The S / P and MUX unit 103 performs serial-parallel conversion of information input via the scheduler unit 101, performs error correction code processing and modulation processing, and outputs a plurality of subcarriers.

  The transmission frame control unit 105 has a transmission frame configuration including a plurality of OFDM symbols composed of a plurality of subcarriers output from the S / P and MUX unit 103, and a guard interval that duplicates a predetermined portion of the OFDM symbol. Control.

  In particular, in this embodiment, the transmission frame control unit 105 has a guard interval length added to the OFDM symbol by the GI adding unit 109 that is 1 / N (N = natural number) of the OFDM symbol length, and Arbitrary subcarriers whose subcarrier frequency constituting the OFDM symbol is N × M times the base frequency (M = natural number) are selected. More specifically, transmission frame control section 105 selects an arbitrary subcarrier whose angular frequency of subcarriers constituting the OFDM symbol is N × M times base angular frequency ω.

  Furthermore, transmission frame control section 105 assigns the same modulation symbol, specifically the same information, to two or more consecutive OFDM symbols for the selected subcarrier.

  By such processing of the transmission frame control unit 105, an extended guard interval in which the guard interval length is equivalently expanded can be configured, and the super large zone base station 100 can use a transmission frame including the extended guard interval. The configuration of the transmission frame according to the present embodiment will be further described later.

  IFFT section 107 performs an inverse Fourier transform (IFFT) on the subcarriers output from S / P and MUX section 103 to generate an OFDM symbol.

  Based on the control from transmission frame control section 105, GI adding section 109 adds a guard interval that duplicates a predetermined portion of the OFDM symbol to the OFDM symbol. In particular, in the present embodiment, the GI adding unit 109 can add an extended guard interval that is longer than a normal guard interval based on the control from the transmission frame control unit 105.

  Radio transmitting section 111 performs D / A conversion, up-conversion, amplification processing, and the like on the transmission frame output from GI adding section 109, and transmits the radio signal subjected to the processing from the antenna.

  The super large zone base station 100 may include a propagation environment determining unit 113 that determines a propagation environment between the super large zone base station 100 and the mobile station 400. Specifically, the propagation environment determination unit 113 determines the propagation environment based on the OFDM symbol (multipath) delay dispersion between the super large zone base station 100 and the mobile station 400.

  In addition, when the propagation environment determination unit 113 is provided, the transmission frame control unit 105 determines that the propagation environment determination unit if the multipath delay spread between the super large zone base station 100 and the mobile station 400 is greater than a predetermined threshold value. If determined by 113, a transmission frame including the above-described extended guard interval can be used.

(3) Configuration of Transmission Frame Next, the configuration of the transmission frame transmitted from the super large zone base station 100 will be described.

3A and 3B are explanatory diagrams of a schematic basic configuration of a transmission frame and an OFDM symbol according to this embodiment. As shown in FIG. 3A, in this embodiment, in order to realize the extended guard interval, the length of the normal guard interval (T _GI in the figure) is the length of the OFDM symbol (T _{symbol in the} figure). ) 1/4 (that is, N = 4). Further, a subcarrier having a frequency (e ^j4ωt in the ^figure ) that is four times (N = 4, M = 1) the base angular frequency ω constituting the OFDM symbol is selected.

For the selected subcarriers, the same modulation symbol (same information) is assigned to two or more consecutive OFDM symbols. By such processing, a transmission frame including an extended guard interval in which the OFDM symbol (T _symbol ) portion is equivalently extended as a guard interval can be configured (upper part of FIG. 3 (a) shown as “OK”). To the third configuration). Note that “NG” is shown in FIG. 3A if T _GI = 1 / N · T _symbol and the condition that the angular frequency of the subcarrier is N × M times the base angular frequency ω are not satisfied. As in the frame configuration, since the phase of the OFDM symbol and the guard interval do not match (continuous), it cannot be used as an extended guard interval that is equivalently extended.

FIG. 3B shows a state where an equivalent extended guard interval can be generated at intervals of 4 subcarriers when T _GI = 1/4 · T _symbol and M = 1.

(4) Frequency Resource Allocation Example FIG. 4 shows an example of frequency resource allocation when the transmission frame configuration described above is used. As shown in FIG. 4, an OFDM signal using the above-described transmission frame configuration is applied only to a specific frequency region B1 in a system band that can be used by the wireless communication system 1.

  In the case of LTE, in the downlink, there is a limitation that the number of assigned subcarriers is an integral multiple of 12 subcarriers (1 resource block (1RB)). Therefore, super large zone base station 100 (transmission frame control unit 105) determines the power density of the subcarriers in specific frequency region B1 including the selected subcarrier as subcarriers in non-specific frequency region B2 other than specific frequency region B1. The power density may be higher.

  Alternatively, the subcarrier transmission in the non-specific frequency region B2 may be stopped while the power density of the subcarrier in the specific frequency region B1 is increased. Furthermore, the transmission frame configuration method described above may be combined with DFT-spread OFDM used in the LTE uplink.

(5) Operation / Effect According to the super large zone base station 100 described above, T _GI is 1 / N of T _symbol , and the angular frequency constituting the OFDM symbol is N × M times the base angular frequency ω. For a subcarrier, a transmission frame including an extended guard interval that is equivalently extended by assigning the same modulation symbol to two or more consecutive OFDM symbols is used.

That is, by appropriately setting the T _GI , that is, the guard interval length, the phase continuity condition between OFDM symbols when the same modulation symbol is allocated to consecutive OFDM symbols is periodically satisfied in the frequency domain. By utilizing such features, changes in basic parameters such as OFDM symbol length and guard interval length and complexity of the circuit configuration can be avoided.

  In addition, according to such a transmission frame configuration, the guard interval can be easily extended, so that it is reliable even in a multipath environment having a long delay that the influence of inter-symbol interference (ISI) cannot be suppressed with the normal guard interval length. The effect of ISI can be reduced. In particular, as in the above-described embodiment, ISI that becomes noticeable when an OFDM signal is received from a super large zone base station having a cell radius as long as 100 km can be effectively reduced.

(6) Other Embodiments As described above, the contents of the present invention have been disclosed through one embodiment of the present invention. However, it is understood that the description and drawings constituting a part of this disclosure limit the present invention. should not do. From this disclosure, various alternative embodiments will be apparent to those skilled in the art.

  FIG. 5 is a diagram illustrating an example of frequency resource allocation according to the modification of the present invention. As shown in FIG. 5, the super large zone base station 100 (transmission frame control unit 105) performs communication between a specific frequency region B1 that includes the selected subcarrier and a non-specific frequency region B2 that does not include the subcarrier. A transmission frame is provided in which a guard band GB having a predetermined bandwidth is provided. By providing such a guard band GB, interference between subcarriers from adjacent subcarriers can be reduced.

  Further, from the viewpoint of further reducing the interference between subcarriers, the power density of the subcarriers in the non-specific frequency region B2 adjacent to the guard band GB may be lower than the power density of the subcarriers in the specific frequency region B1. Good. Also in the specific frequency region B1, in the transmission frame that does not include the extended guard interval described above, the power density of the subcarrier is reduced, and communication with mobile stations located around the base station is not performed without using frequency repetition. Corresponding subcarriers may be used.

  Further, transmission frame control section 105 is the same as the first subcarrier group in which the same modulation symbol is assigned to the first predetermined number of consecutive OFDM symbols and the second predetermined number of OFDM symbols smaller than the first predetermined number. When the second subcarrier group to which the modulation symbols are assigned and the single subcarrier in which the same modulation symbols are not consecutive are mixed, the subcarrier allocation position of the second subcarrier group is higher than the allocation position of the single subcarrier. It is preferable to configure a transmission frame that is close to one subcarrier group.

  Specifically, a subcarrier to be transmitted by concatenating 3 symbols (first predetermined number), a subcarrier to be transmitted by concatenating 2 symbols (second predetermined number), and a subcarrier to be transmitted without being concatenated are specified frequency region B1 In order to reduce inter-subcarrier interference as described above, a second subcarrier group that is transmitted by linking two symbols is arranged around the first subcarrier group that is transmitted by linking three symbols. Is preferred. The specific frequency region B1 and the non-specific frequency region B2 may be used by other types of base stations. For example, the specific frequency region B1 may be used by the super large zone base station 100, and the non-specific frequency region B2 may be used by the macro base stations 300A and 300B. Further, the area of the guard band GB may not be unused, but may be used for communication with the mobile station 400 in a state where the power density is suppressed.

  In addition, in order to suppress out-of-band radiation (leakage power to adjacent subcarriers), a normal OFDM signal applies time window processing for controlling the amplitude of the transmission symbol on the time axis to each OFDM symbol. Yes. FIG. 6 shows a schematic configuration of a transmission frame to which time window processing is applied.

  When time window processing is applied in the transmission frame configuration according to the above-described embodiment, the time window multiplication interval is regarded as a guard interval, so that the reception quality of the OFDM signal is degraded depending on the shape of the applied time window. there's a possibility that.

  Therefore, when the super large zone base station 100 (transmission frame control unit 105) executes time window processing, the sum of the respective time points (that is, arbitrary time points) is 1 for the coefficient of the window function used for the time window processing. It is preferable to construct such a transmission frame. That is, the time window to be applied is equivalent to the case where the time window is not multiplied by the time region A1 in which the rear end of the OFDM symbol 1 and the guard interval added before the OFDM symbol 2 shown in FIG. Is preferably set.

  Alternatively, when performing the time window process, the transmission frame control unit 105 applies the window process to each OFDM symbol for the OFDM symbol configured by the subcarrier selected based on the above-described conditions, The transmission signal may be generated by synthesizing the time signal subjected to the time window processing.

  Specifically, the IFFT is applied separately to the portion corresponding to the specific frequency region B1 and the portion corresponding to the non-specific frequency region B2, and a time signal is generated. As described above, whether to apply window processing to each OFDM symbol or whether to apply window processing to each OFDM symbol is switched.

  In the embodiment described above, the cell radius of the super large zone base station 100 is about 100 km, the cell radius of the large zone base stations 200A and 200B is about 7 km, and the cell radius of the macro base stations 300A and 300B is Although described as being about 1 km or less, in the case of such a wireless communication system 1, the base station and the mobile station 400 may execute the following operations and processes.

  First, because the upstream communication between the super large zone base station 100 and the mobile station 400 is not realistic due to limitations on transmission power, the super large zone base station 100 is dedicated to paging (such as text or voice messages) or broadcasts. It is preferable to use for purposes such as reporting an area where the mobile station 400 can be used. Further, when realizing uplink communication, it is conceivable to form a communication path in a multi-hop manner by a plurality of mobile stations or base stations.

  In addition, as one method of extending the uplink communicable range, a specific uplink radio resource block is prohibited between cells by prohibiting use by the large zone base stations 200A and 200B and the macro base stations 300A and 300B. You may make it reduce interference. Alternatively, the transmission power density may be improved by concentrating power on the subcarriers by limiting the number of subcarriers used to be equivalent to one radio resource block. Furthermore, the communicable range may be expanded by applying the repeated transmission of the subcarrier.

  When base stations of different types as described above coexist, subcarriers used for each type of base station may be fixed in advance. Alternatively, the large zone base stations 200A and 200B and the macro base stations 300A and 300B may use all subcarriers including the subcarriers used by the super large zone base station 100. Further, such subcarrier allocation may be determined according to the position of the base station. For example, a macro base station located near the super large zone base station 100 has strong interference from the super large zone base station 100, and therefore, a higher capacity is obtained without using the subcarriers used by the super large zone base station 100. It is considered possible. Furthermore, the vertical antenna directivity (directing the vertical beam far away, or the directivity near the super large zone base station 100 so that all the subcarriers can be used even in the macro base station located near the super large zone base station 100. It is also possible to achieve segregation by making the gain low).

  In the above-described embodiment, base stations of different types are mixed, but the present invention can also be applied to the case of only the super large zone base station 100. Further, in the above-described embodiment, an example in which the super large zone base station 100 configures the transmission device has been described, but another type of base station or mobile station 400 may configure the transmission device.

  As described above, the present invention naturally includes various embodiments that are not described herein. Therefore, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

1 ... Wireless communication system
100 ... Super large zone base station
101 ... Scheduler part
103 ... S / P and MUX section
105 ... Transmission frame controller
107… IFFT
109… GI addition part
111 ... Wireless transmitter
113 Propagation environment judgment unit
200A, 200B ... large zone base station
300A, 300B ... Macro base station
400 ... Mobile station
C1 ... cell

Claims (9)

A transmission apparatus according to an orthogonal frequency division multiplexing system including a transmission frame control unit that controls a configuration of a transmission frame including a plurality of OFDM symbols and a guard interval obtained by duplicating a predetermined portion of the OFDM symbol,
The transmission frame control unit
The length of the guard interval is 1 / N of the length of the OFDM symbol, the frequency of subcarriers constituting the OFDM symbol is N × M times the base frequency, and N and M are natural numbers. Select a career
A transmission apparatus using the transmission frame including an extended guard interval that is equivalently extended by assigning the same modulation symbol to two or more consecutive OFDM symbols for the selected subcarrier. A propagation environment determination unit that determines a propagation environment between the transmission device and a reception device that receives the transmission frame;
The transmission frame control unit includes the extended guard interval when the propagation environment determination unit determines that the delay dispersion of the OFDM symbol between the transmission device and the reception device is greater than a predetermined threshold. The transmission apparatus according to claim 1, wherein a transmission frame is used.   2. The transmission frame control unit according to claim 1, wherein the power density of subcarriers in a specific frequency region including the selected subcarrier is set higher than the power density of subcarriers in a non-specific frequency region other than the specific frequency region. The transmitting device described.   The transmission frame control unit is configured to transmit the transmission frame provided with a guard band having a predetermined bandwidth between a specific frequency region including the selected subcarrier and a non-specific frequency region not including the subcarrier. The transmission device according to claim 1, which is configured.   The transmission apparatus according to claim 4, wherein the transmission frame control unit makes a power density of subcarriers in the non-specific frequency region adjacent to the guard band lower than a power density of subcarriers in the specific frequency region.   The transmission frame control unit is identical to a first subcarrier group in which the same modulation symbol is assigned to a first predetermined number of consecutive OFDM symbols and a second predetermined number of OFDM symbols smaller than the first predetermined number. When the second subcarrier group to which the modulation symbols are assigned and the single subcarrier in which the same modulation symbols are not consecutive are mixed, the subcarrier allocation position of the second subcarrier group is more than the allocation position of the single subcarrier. The transmission apparatus according to claim 1, wherein the transmission frame is in a state of approaching the first subcarrier group.   When the transmission frame control unit executes time window processing for controlling the amplitude of the transmission symbol on the time axis, the sum of the coefficients of the window function used for the time window processing is 1 at an arbitrary time point. The transmission apparatus according to claim 1, constituting the transmission frame. When the transmission frame control unit performs time window processing for controlling the amplitude of the transmission symbol on the time axis,
The window processing is applied to each of a plurality of OFDM symbols for the OFDM symbols constituted by the selected subcarriers,
The transmission apparatus according to claim 1, wherein the transmission signal is generated by synthesizing the time signals subjected to the time window processing. A transmission frame configuration method in a communication device that transmits a transmission frame including a transmission symbol and a plurality of guard intervals obtained by duplicating a predetermined period of the OFDM symbol,
The length of the guard interval is 1 / N of the length of the OFDM symbol, the frequency of subcarriers constituting the OFDM symbol is N × M times the base frequency, and N and M are natural numbers. Selecting a carrier;
Using the transmission frame including an extended guard interval that is equivalently extended by assigning the same modulation symbol to two or more consecutive OFDM symbols for the selected subcarrier. .

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