JP2017112575A - Base station, mobile station, and communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a plurality of mobile stations and a base station to perform radio communication by efficiently using frequency resources while avoiding the same frequency interference.SOLUTION: In a mobile body radio communication system forming a plurality of cells, the following communication is performed when a base station and a plurality of mobile stations perform radio communication by sharing one carrier wave. If a mobile station is in a central region of a cell, the base station and the mobile station perform OFDM communication. On the other hand, if a mobile station is in a peripheral region of the cell, the base station communicates with the mobile station by using a system combining CDM and OFDM.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a base station, a mobile station, and a communication method in a mobile radio communication system, and more particularly to a communication method between a base station and a mobile station in a next-generation mobile radio communication system, for example.

  Conventionally, in a mobile radio communication system having a cell configuration, various communication schemes have been proposed and used between a base station and a mobile station. For example, W-CDMA / CDMA2000, which is a communication method adopted in the third-generation mobile radio communication system, identifies users by code division and is used between users (mobile stations) and between cells (base stations). One frequency repetition was achieved by avoiding interference at the same frequency. However, in order to perform high-speed transmission using such a communication method, there is a problem in that it is necessary to assign a plurality of codes to one user, and a load is imposed on signal processing on the receiving side. Furthermore, since signals between users (mobile stations) are not orthogonal, there is a problem that the number of signals that can be multiplexed is reduced due to residual noise.

  On the other hand, in the fourth generation mobile radio communication system (LTE: Long Term Evolution) which has been widely used in recent years, orthogonal frequency division multiplexing (OFDM) is applied to improve orthogonality between users. Collateral. In OFDM, high-speed transmission is realized by parallel transmission using a plurality of subcarriers. The LTE system also performs time division at the same time. Further, in the LTE system, the same frequency band is arranged in different cells, and one frequency repetition is performed on the cell configuration. Incidentally, in OFDM, a unit of information transmission assigned to a user is called a resource block (RB).

  Now, since the multiplexing method in the LTE system is basically frequency division, if the same RB is used in adjacent cells, interference occurs. Therefore, an X2 interface is used to exchange base station information between cells. A resource is applied to reduce interference. Therefore, in the LTE system, it is necessary to rearrange resources as a whole, which not only complicates the control, but basically the same RB cannot be used simultaneously in adjacent cells.

  In order to solve such problems, a reuse partition (RP) scheme has been proposed in order to avoid interference between cells in OFDM. In the RP method, a frequency band common to each cell is allocated in the center of the cell, and one frequency repetition is realized, and different frequency bands are allocated in the interference area between cells (for example, the peripheral area of the cell).

  However, although interference between cells can be avoided in the RP method as described above, another problem arises that division loss occurs because the frequency band is divided.

  Further, as described in detail in Non-Patent Document 1, OFCDM (Orthogonal Frequency / Code Division Multiplexing) combining OFDM and CDM (Code Division Multiplexing) is a fourth generation method for avoiding interference between cells. It has been proposed as a communication method in mobile radio communication systems.

  FIG. 10 is a diagram showing an example of a cell configuration in a mobile radio communication system employing OFCDM. FIG. 10 illustrates a cell configuration in a mobile radio communication system including seven cells 100 to 700.

  As shown in FIG. 10, each cell is configured to be divided into four sectors, and different frequency bands are assigned to each sector. That is, the frequency band F1 is assigned to the sector in the central area of each cell, and the frequency bands F1, F2, and F3 are assigned to the three sectors in the peripheral area, respectively. In this way, although the same frequency band is assigned to each cell, the frequency bands used between adjacent cells are not overlapped, so that interference between cells can be avoided.

“Base-station inter-cell interference reduction technology and fractional frequency repetition technology”, Kazuki Maruta, Atsushi Ota, Masataka Iizuka, Takatoshi Sugiyama, NTT Technology Journal, September 2012, pp. 78-81

  However, since OFCDM proposed in Non-Patent Document 1 applies the communication method to all channels, there is a problem that multi-coding is required for high-speed transmission. Also, in order to spread the band, it is necessary to increase the spreading factor in order to accommodate a plurality of users at the same time. Conversely, if the band is given, it is necessary to reduce the signal transmission rate before spreading. That was also a problem. As a result, the fourth generation mobile radio communication system employs OFDM without spreading.

  For this reason, especially when different mobile stations use the same frequency at the cell boundary, the same frequency interference occurs. As described above, in order to directly control between base stations in order to avoid the same frequency interference, an X2 interface is introduced, and measures such as different power control and frequency selection are used for using the same frequency. However, there is still a problem that the utilization rate of the frequency resource is lowered due to the chained influence of the increase in traffic.

  The present invention has been made in view of the above-described conventional example, and has a simple configuration, a base station, a mobile station, and a mobile station capable of efficiently performing communication while reducing frequency interference using frequency resources. The purpose is to provide a communication method.

  In order to achieve the above object, the base station of the present invention has the following configuration.

  That is, it is a base station that operates in a mobile radio communication system composed of a plurality of cells, is arranged in each of the plurality of cells, and performs radio communication by sharing one carrier wave with a plurality of mobile stations. And specifying means for specifying the position of the mobile station existing in the cell in which the base station is located, and when the position of the mobile station specified by the specifying means is in the central region of the cell, A first communication unit that communicates with the mobile station using a method that combines CDM and OFDM when the position of the mobile station specified by the specifying unit is in the peripheral region of the cell; And second communication means for communicating with the mobile station.

  Another aspect of the present invention is a mobile station that operates in a mobile radio communication system including a plurality of cells and performs radio communication with a base station arranged in each of the plurality of cells. When the base station with which the mobile station communicates is located in the central region of the cell, the first communication means for communicating with the base station using the OFDM scheme, and the cell of the mobile station When in the peripheral area, the mobile station is characterized by comprising second communication means for communicating with the base station using a method combining CDM and OFDM.

  According to another aspect of the present invention, there is provided a communication method in which a base station and a plurality of mobile stations share a single carrier and perform radio communication in a mobile radio communication system including a plurality of cells, When the mobile station is in the central region of any one of the plurality of cells, the base station and the mobile station communicate with each other using the OFDM scheme, and the mobile station A communication method characterized by performing communication with the mobile station using a method in which CDM and OFDM are combined in a peripheral region of any one of the cells.

  Therefore, according to the present invention, there is an effect that interference of the same frequency can be avoided with a simple configuration.

It is a figure which shows the cell structure of the mobile radio | wireless communications system which is a typical Example of this invention. It is a block diagram which shows the transmission structure of the base station which comprises the system shown in FIG. It is a block diagram which shows the detailed structure of a signal generator. It is a figure which shows the utilization example of a frequency spectrum. It is a block diagram which shows another detailed structure of a signal generator. It is a block diagram which shows the receiving structure of the base station which comprises the system shown in FIG. It is a block diagram which shows the transmission structure of the mobile station which comprises the system shown in FIG. It is a block diagram which shows the receiving structure of the mobile station which comprises the system shown in FIG. It is a flowchart which shows the process of intra-cell handover. It is a figure which shows an example of the cell structure in the mobile radio | wireless communications system which employ | adopted OFCDM.

  Hereinafter, preferred embodiments of the present invention will be described more specifically and in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram showing a cell configuration of a mobile radio communication system (hereinafter referred to as system) which is a typical embodiment of the present invention. For comparison with the conventional example, the system shown in FIG. 1 includes seven cells 100 to 700 as in the conventional system shown in FIG. 10, and each cell is divided into four sectors. These four sectors are composed of a central area and three peripheral areas. A base station 10 is arranged at the center of the central area. In such a system, the mobile station 800 can be located in one of these cells to perform communication or continue communication while moving.

  In the system shown in FIG. 1, the base station and the mobile station communicate with each other using OFDM in the central area (area 2) of each cell, and OFCDM in which the CDM and OFDM are combined in the peripheral area (area 1). The base station and the mobile station communicate with each other.

  As described above, in the system of this embodiment, information is diffused by CDM in the peripheral region of the cell, which is a region where interference between cells occurs based on OFDM, while only OFDM is used in the central region of the cell. Communicate. In other words, this system proposes a combined multiplexing scheme of OFDM and CDM having a feature of spreading while preserving frequency orthogonality.

  In FIG. 1, only one mobile station 800 is shown for simplicity of explanation, but it goes without saying that a plurality of mobile stations normally perform radio communication simultaneously. Therefore, in this system, one base station and a plurality of mobile stations can share one carrier wave and simultaneously perform wireless communication.

  FIG. 2 is a block diagram showing a transmission configuration of the base station that constitutes the system shown in FIG.

  As shown in FIG. As a transmission configuration, the base station 10 includes two signal generators 20 and 30, a synthesizer 40 that synthesizes signals from these two signal generators, an amplifier 50 that amplifies the synthesized signal, and frequency conversion of the amplified signal. Including a frequency converter 60 and an antenna 70. Further, a control parameter setting unit 35 is provided between the signal generator 20 and the signal generator 30, and switching between the signal generator 20 and the signal generator 30 is performed according to the received signal strength RSI from the mobile station and the position information POS. I do. Also, the control parameter setting unit 35 designates various control parameters such as non-spreading, spreading and modulation parameters for the two signal generators 20 and 30 according to the position of the mobile station and the radio environment thereof.

  The signal generator 20 generates a user signal in accordance with the OFCDM scheme in which CDM and OFDM are combined when the mobile station is located in the peripheral region of the cell. In order to distinguish the signal generator 20 from the signal generator 30, it may be called a CDM / OFDM signal generator. In contrast, the signal generator 30 generates a user signal according to the conventional OFDM method when the mobile station is located in the central region of the cell. In order to distinguish the signal generator 30 from the signal generator 20, it may be referred to as an OFDM signal generator.

  FIG. 3 is a block diagram showing a detailed configuration of the signal generator 20. FIG. 3 illustrates an example in which two transmission signals TX1 and TX2 are multiplexed in order to simplify the description.

  As shown in FIG. 3, the transmission signal TX1 is spread in the frequency domain by the spread code SC1 and by the sector identification code SI, and is modulated by the modulator (MOD) 21. On the other hand, the transmission signal TX2 is spread in the frequency domain by the spread code SC2 and by the sector identification code SI, and is modulated by the modulator (MOD) 22. The two modulated spread signals are combined by the combiner 23 and serial / parallel converted by the S / P converter 24. The user signal that has become a parallel signal is frequency-mapped by the frequency mapper 25, converted from the frequency domain to the time domain by the inverse fast Fourier transformer (IFFT) 26, and the user signal is serialized again by the P / S converter 27. Become a signal. A cyclic prefix (CP) is inserted into the OFDM signal, and the gain is adjusted by the amplifier 29 and output.

  Note that the two transmission signals may be spread over the entire frequency band handled by the system, or may be spread over a part of the band. When spreading for a part of the band, a zero pad is input to IFFT 26 corresponding to the band that is not spread.

  FIG. 4 is a diagram showing an example of using the frequency spectrum.

In the example shown in FIG. 4 has the full bandwidth of the system is available frequency band by the f ₀ ~f _n. Here, when spreading using the entire band, as shown in (1) of FIG. 4, the two transmission signals are spread like spread signals SS1 and SS2, and the same frequency is used using a part of the bands. In the case of spreading to the band, spread signals SS1 ′ and SS2 ′ are spread as shown in FIG. 4 (2). FIG. 4 (2) shows a state in which two user signals are spread over the bands of frequencies f _{0 to} f ₄ .

  The example shown in FIG. 3 and the examples shown in FIGS. 4 (1) and 4 (2) are diagrams showing a configuration in which two user signals TX1 and TX2 are spread over the same frequency band. However, as shown in (3) of FIG. 4, the two transmission signals TX1 and TX2 may be spread to different frequency bands using a part of each of the entire bands.

  FIG. 5 is a block diagram showing another detailed configuration of the signal generator 20. FIG. 5 illustrates a signal generator configured to spread the two transmission signals TX1 and TX2 in different frequency bands by modifying the configuration illustrated in FIG. Accordingly, the same components as those shown in FIG.

  As shown in FIG. 5, the two transmission signals TX1 and TX2 are respectively shown in (3) of FIG. 4 in the frequency domain by the spreading code SC11 and the sector identification code SI and by the spreading code SC21 and the sector identification code SI, respectively. To spread signals SS1 ″ and SS2 ″. These spread signals are modulated by modulators 21 and 22, respectively, and serial / parallel converted by S / P converters 24A and 24B, respectively. The two user signals that have become parallel signals are frequency mapped by the frequency mappers 25A and 25B, respectively, and converted from the frequency domain to the time domain by inverse fast Fourier transformers (IFFT) 26A and 26B. Zero pads are input to the two IFFTs 26A and 26B in correspondence with bands not used for spreading.

  The two user signals converted into the time domain are converted again into serial signals by the P / S converters 27A and 27B. A cyclic prefix (CP) is inserted into the OFDM signal, synthesized by the synthesizer 28, and the gain is adjusted by the amplifier 29 and outputted.

  In the example shown in (3) of FIG. 4, a part of the two spread signals SS1 ″ and SS2 ″ have overlapping frequency bands, but a spread signal having completely separated frequency bands may be generated.

  FIG. 6 is a block diagram illustrating a reception configuration of a base station configuring the system illustrated in FIG. The example shown in FIG. 6 shows two cases in which the signal from the mobile station is an OFDM signal and the other is an OFCDM signal in which CDM and OFDM are combined.

  In the base station 10, the user signal from the mobile station received by the antenna 70 is frequency-converted by the frequency converter 60, the gain is adjusted by the amplifier 80, and then input to the S / P converter 90. The S / P converter 90 removes the cyclic prefix (CP) inserted by the mobile station and converts the received serial signal into a parallel signal. The parallel signal is converted from the time domain to the frequency domain by a fast Fourier transformer (FFT) 91.

When a spread signal is generated according to the CDM method, a frequency band in which the user signal is spread is selected by the spread designated frequency selection unit 92A for the user signal converted into the frequency domain. For example, the spread signal SS2 ′ in (2) of FIG. 4 is spread in a band of frequencies f _{0 to} f ₄ . In the P / S converter 93A, the user signal is again converted from a parallel signal to a serial signal, demodulated by the demodulator (DEMOD) 94A, and the received signal RX1 from the user is obtained by the despread code ISC11.

  On the other hand, when the signal from the mobile station is an OFDM signal that does not use the CDM method, the user signal converted into the frequency domain is the frequency used for the user signal by the non-spread designated frequency selection unit 92B. A band is selected. Then, the user signal is converted again from a parallel signal to a serial signal in the P / S converter 93B, and demodulated by the demodulator (DEMOD) 94B to obtain a received signal RX2 from the user.

  FIG. 7 is a block diagram showing a transmission configuration of the mobile station constituting the system shown in FIG.

  As shown in FIG. 7, also in the mobile station 800, when a user signal is generated according to the OFCDM scheme combining CDM and OFDM, the transmission signal TX is spread with a spreading code SC and modulated by a modulator MOD810. The modulated transmission signal is converted from a serial signal to a parallel signal by an S / P converter 820, frequency-mapped by a frequency mapper 830, and converted from a time domain to a frequency domain by an inverse fast Fourier transformer (IFFT) 840. Is done. In the case where the spread signal is spread only in a part of the usable band, the zero pad is input to the IFFT 840 in the other band.

  The transmission signal converted to the frequency domain is converted again from a parallel signal to a serial signal in the P / S converter 850, and a cyclic prefix (CP) is inserted into the OFDM signal, and the gain is adjusted by the amplifier 860. Is output. The gain-adjusted signal is frequency-converted by frequency converter 870 and transmitted from antenna 880.

  Note that the transmission configuration for generating an OFDM signal without applying the CDM method is the same as the known SC-FDMA (DFTS-OFDM) transmission method in the uplink in the LTE system, and the description thereof is omitted.

  FIG. 8 is a block diagram showing a reception configuration of a mobile station that constitutes the system shown in FIG.

  The mobile station 800 frequency-converts the OFCDM signal obtained by combining CDM and OFDM from the base station received by the antenna 880 by the frequency converter 870, adjusts the gain by the amplifier 890, and then goes to the S / P converter 891. Entered. The S / P converter 891 removes the cyclic prefix (CP) inserted by the mobile station, and converts the received serial signal into a parallel signal. The parallel signal is converted from the time domain to the frequency domain by a Fast Fourier Transform (FFT) 892.

For the user signal converted into the frequency domain, a frequency band in which the user signal is spread is selected by the spread designation frequency selection unit 893. For example, the spread signal SS2 ′ shown in (2) of FIG. 4 is spread in a band of frequencies f _{0 to} f ₄ . The user signal is converted again from a parallel signal to a serial signal in the P / S converter 894 and demodulated by a demodulator (DEMOD) 895. Finally, despreading is performed by the sector identification code SI and the despreading code ISC, and the received signal RX from the user is obtained.

  The configuration for receiving the OFDM signal generated without applying the CDM method is the same as the known OFDM transmission method in the downlink in the LTE system, and thus the description thereof is omitted.

  Next, communication between a base station and a mobile unit in a system using the apparatus configuration and communication method described above will be described.

  As shown in FIG. 1, for example, when the mobile station 800 exists in the center region of the cell 700, the inter-cell interference is small. Therefore, the base station 10 and the mobile station 800 of the cell 700 are well-known in the conventional LTE system. Communication with each other is started using OFDM and SC-FDMA. In this case, the base station 10 can specify that the mobile station 800 is located in the central region of the cell 700 based on the reception intensity of the signal from the mobile station 800, GPS position information, and the like. That is, the base station 10 selects the signal generator 30 by the control parameter setting unit 35 according to the received signal strength RSI from the mobile station 800 and the position information POS, and generates a user signal.

  On the other hand, when it exists in the peripheral region of the cell 700, the inter-cell interference is large. Therefore, the base station 10 and the mobile station 800 of the cell 700 use the OFCDM method combining the CDM and OFDM described in this embodiment. To start communication with each other. Also in this case, the base station 10 can specify that the mobile station 800 is located in the peripheral region of the cell 700 based on the reception strength of the signal from the mobile station 800, GPS position information, and the like. That is, the base station 10 selects the signal generator 20 by the control parameter setting unit 35 according to the received signal strength RSI from the mobile station 800 and the position information POS and generates a user signal.

  As shown in FIG. 1, for example, when the mobile station 800 starts communication in the central region or the peripheral region of the cell 700 and then moves to the peripheral region or the central region while continuing the communication, the following flowchart is performed. Communication is performed according to the processing shown.

  FIG. 9 is a flowchart showing an intra-cell handover process according to this embodiment.

  At the start of communication, first, in step S10, the base station 10 checks whether the position of the mobile station 800 is in the peripheral area (area 1) or the central area (area 2).

  Here, if it is determined that the mobile station 800 is in the central region (region 2), the process proceeds to step S20, and communication is performed by generating a signal using the OFDM signal generator 30. In step S30, it is checked whether or not to end communication. Here, when the communication is terminated, the process is terminated, but when the communication is continued, the process proceeds to step S40 to check the position of the mobile station 800. Here, when it is determined that the position of the mobile station 800 is in the central region (region 2), the processing returns to step S20 and performs the above-described processing. On the other hand, if it is determined that the position of the mobile station 800 is in the peripheral area (area 1), that is, the mobile station 800 has moved from the central area to the peripheral area while continuing communication, Proceed to S50.

  In step S50, the signal generator operated by the control parameter setting unit 35 that also functions as a switch is switched from the OFDM signal generator 30 to the CDM / OFDM signal generator 20, and then the process proceeds to step S60. In this way, an intra-cell handover from region 2 to region 1 is executed.

  If it is determined in step S10 that the mobile station 800 is in the peripheral region (region 1), the process proceeds to step S60, and signal generation is performed using the CDM / OFDM signal generator 20 to perform communication. . In step S70, it is checked whether or not to end communication. Here, when the communication is terminated, the process is terminated, but when the communication is continued, the process proceeds to step S80 and the position of the mobile station 800 is checked. Here, when it is determined that the position of the mobile station 800 is in the peripheral area (area 1), the process returns to step S60 to perform the above-described process. On the other hand, when it is determined that the position of the mobile station 800 is in the central area (area 2), that is, the mobile station 800 has moved from the peripheral area to the central area while continuing communication, Proceed to S90.

  In step S90, the signal generator operated by the control parameter setting unit 35 that also functions as a switch is switched from the CDM / OFDM signal generator 20 to the OFDM signal generator 20, and then the process proceeds to step S20. In this way, an intra-cell handover from region 1 to region 2 is executed.

  Therefore, according to the embodiment described above, multiplexing is performed only by OFDM in the central region of a cell where there is no or very small interference of the same frequency, and CDM and OFDM are combined in the peripheral region of a cell where interference of the same frequency is large. Multiplexing can be performed in the same manner. For this reason, when the mobile station is in the center area of the cell, the OFDM system having high applicability to high-speed transmission can be used, while in the peripheral area where there is a possibility of interference at the same frequency between cells, Communication that avoids interference is possible because it is spread and identified by a code.

  In the embodiment described above, since frequency mapping is performed after spreading the user signal and the signal is assigned to a subcarrier (subcarrier), it is necessary for the conventional CDMA system to receive and demodulate this signal. The use of the existing rake receiver is unnecessary. This also has the advantage that the device configuration is simplified.

  Furthermore, in the embodiment described above, different user signal components are superimposed on each subcarrier, but the power of the spread signal can be reduced, so that the influence of interference can be sufficiently reduced. Is possible. Furthermore, an orthogonal system can be realized by demodulating the spread signal and canceling the signal superimposed on the subcarrier.

10 base stations, 20 CDM / OFDM signal generators, 21-22 modulators,
26 Inverse Fast Fourier Transform (IFFT), 30 OFDM signal generator,
70 antenna, 91 fast Fourier transform (FFT),
100-700 cells, 800 mobile stations

Claims (5)

A base station that operates in a mobile radio communication system composed of a plurality of cells, is arranged in each of the plurality of cells, and performs radio communication by sharing one carrier wave with a plurality of mobile stations,
A specifying means for specifying a position of a mobile station existing in a cell in which the base station is located;
When the position of the mobile station specified by the specifying means is in the center area of the cell, first communication means for communicating with the mobile station using an OFDM scheme;
And a second communication means for communicating with the mobile station using a combination of CDM and OFDM when the position of the mobile station specified by the specifying means is in the peripheral region of the cell. Base station characterized by   If it is determined by the specifying means that the mobile station is moving from the central area of the cell to the peripheral area of the cell while continuing communication, communication with the mobile station is performed in the first area. When it is determined that the mobile station is moving from the peripheral area of the cell to the central area of the cell while continuing communication, the mobile station switches from the communication means to the second communication means. The base station according to claim 1, further comprising switching means for switching the communication of the second communication means from the second communication means to the first communication means. The second communication unit includes a spreading unit that spreads a plurality of user signals over the entire band that can be used in the mobile radio communication system or a part of the whole band,
3. When the plurality of user signals are spread over a part of the entire band, the plurality of user signals are spread over the same frequency band or a frequency band at least partially overlapping. Base station described in. A mobile station that operates in a mobile radio communication system composed of a plurality of cells and performs radio communication with a base station arranged in each of the plurality of cells,
A first communication means for communicating with the base station using an OFDM scheme when the mobile station is in a central area of a cell in which a base station with which the mobile station is located is disposed;
A mobile station comprising second communication means for communicating with the base station using a method combining CDM and OFDM when the mobile station is in a peripheral region of the cell. In a mobile radio communication system composed of a plurality of cells, a base station and a plurality of mobile stations share a single carrier and perform radio communication,
When the mobile station is in the central region of any one of the plurality of cells, the base station and the mobile station communicate using the OFDM method,
A communication method characterized by performing communication with a mobile station using a method combining CDM and OFDM when the mobile station is in a peripheral region of any one of the plurality of cells.

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