JP2011114603A - Mobile station and connection destination selection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce inter-symbol interference peculiar to an OFDMA system. <P>SOLUTION: The mobile station 12 includes: a reception quality measurement part 26 which measures reception quality of radio signals to be transmitted from each of a plurality of base stations which adopt an orthogonal frequency division multiple access (OFDMA) system; a propagation delay time estimation part 28 which estimates propagation delay time of the radio signals to be transmitted from each of the plurality of base stations; and a connection destination selection part 30 which selects any of base stations whose reception quality is equal to or more than predetermined quality among the plurality of base stations as a connection destination based on shortness of the propagation delay time to be estimated by the propagation delay time estimation part 28. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a mobile station and a connection destination selection method, and more particularly to a technique for suppressing radio wave interference to neighboring cells.

  In a conventional mobile communication system, a mobile station selects a base station as a connection destination based on the reception quality of radio signals transmitted from neighboring base stations.

  For example, XGP (eXtended Global Platform) adopting OFDMA (Orthogonal Frequency Division Multiple Access) and TDMA / TDD (Time Division Multiple Access / Time Division Duplex) ), The mobile station selects a base station to be connected based on RSSI (Received Signal Strength Indication) of radio signals transmitted from neighboring base stations (see Non-Patent Document 1).

  By the way, in a mobile communication system that employs the OFDMA system, such as XGP, FFT (Fast Fourier Transform) is used for demodulation of the OFDM signal. Specifically, one symbol data is obtained by performing FFT on an OFDM signal for one symbol composed of a redundant signal called a CP (Cyclic Prefix) or GI (Guard Interval) and a data signal. However, if FFT is performed at a timing that crosses the boundary between two adjacent OFDM signals, so-called inter symbol interference (ISI) occurs, and correct demodulation cannot be performed.

  For this reason, the mobile station adjusts the transmission timing of the radio signal to the base station based on the propagation delay time of the radio signal transmitted from the connected base station (see, for example, Patent Document 1). Thereby, the time difference between the reception timing of the radio signal transmitted from the mobile station close to the base station and the reception timing of the radio signal transmitted from the mobile station far from the base station is less than the CP length (allowable delay time), Generation of intersymbol interference is suppressed.

JP 2002-77087 A

“ARIB STD-T95 Version 1.2“ OFDMA / TDMA TDD Broadband Wireless Access System (Next Generation PHS) ””, March 18, 2009, Japan Radio Industry Association

  In the above-described conventional mobile communication system, since the connection destination of the mobile station is determined based on the reception quality of the radio signal, the base station close to the mobile station is not necessarily selected as the connection destination. For example, if the reception quality of a radio signal transmitted from a distant but good-looking base station is higher than the reception quality of a radio signal transmitted from a close but poor-viewing base station, the mobile station Select the station as the connection destination.

  However, in a mobile communication system employing the OFDMA scheme, a mobile station connected to a distant base station may be an intersymbol interference source for neighboring cells. Hereinafter, the reason for this will be described with reference to FIGS.

  FIG. 5 is a diagram showing a conventional mobile communication system 50 that employs the OFDMA scheme. As shown in FIG. 5, the mobile stations 52-1, 52-2, and 52-3 are connected to the base stations 54-1, 54-2, and 54-3, and the base stations 54-1 to 54-1 are connected. The cells 56-1 to 56-3 of 54-3 are assumed to overlap each other. In addition, it is assumed that radio signal transmission / reception timings are synchronized between the base stations 54-1 to 54-3.

  Here, when the mobile station 52-1 moves in the direction indicated by the arrow in FIG. 5, it is necessary to select either the base station 54-2 or the base station 54-3 as a new connection destination (handover destination). There is. At this time, due to the influence of the building 58 located between the mobile station 52-1 and the base station 54-2, the reception quality of the radio signal transmitted from the base station 54-2 close to the mobile station 52-1, If the reception quality of the radio signal transmitted from the base station 54-3 far from the mobile station 52-1 is higher, the mobile station 52-1 selects the base station 54-3 as the handover destination.

  When connected to the base station 54-3, the mobile station 52-1 advances the transmission timing of the radio signal based on the propagation delay time of the radio signal transmitted from the distant base station 54-3. Thereby, as shown in FIG. 6, the reception timing of the radio signal transmitted from the mobile station 52-1 at the base station 54-3 is the same as that at the base station 54-3 of the radio signal transmitted from the mobile station 52-3. It almost coincides with the reception timing.

  However, a radio signal transmitted from the mobile station 52-1 at an earlier timing reaches not only the base station 54-3 but also the base station 54-2. Therefore, depending on the positional relationship among the mobile station 52-1, the base station 54-2, and the base station 54-3, as shown in FIG. 7, the base station 54-2 of the radio signal transmitted from the mobile station 52-1. In some cases, the reception timing at is earlier than the CP timing by the reception timing at the base station 54-2 of the radio signal transmitted from the mobile station 52-2. In this case, since the base station 54-2 performs FFT at a timing that crosses the boundary between adjacent OFDM symbols for two symbols, noise power due to inter-symbol interference specific to the OFDMA scheme occurs.

  Thus, in a mobile communication system employing the OFDMA scheme, a mobile station connected to a distant base station interferes with inter-symbol interference with communication of other mobile stations connected to a base station closer thereto. May be a source.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a mobile station and a connection destination selection method that can reduce intersymbol interference unique to the OFDMA scheme.

  In order to solve the above problems, a mobile station according to the present invention includes a measurement unit that measures reception quality of a radio signal transmitted from each of a plurality of base stations adopting an orthogonal frequency division multiple access scheme, and the plurality of the plurality of base stations Based on estimation means for estimating a propagation delay time of a radio signal transmitted from each of the base stations, and a short propagation delay time estimated by the estimation means, the reception quality of the plurality of base stations is predetermined. Selecting means for selecting any of the base stations having quality or higher as a connection destination.

  In the present invention, the mobile station has a short propagation delay time of a radio signal transmitted from each of a plurality of base stations adopting the OFDMA scheme (for example, in the order of short propagation delay time, the propagation delay time is a predetermined time or less. Or the like) is selected as a connection destination, based on whether the reception quality of the radio signal is equal to or higher than a predetermined quality. That is, the mobile station selects, as a connection destination, a base station that can be expected to have a communication quality higher than a predetermined quality and that is close to the mobile station. For this reason, according to the present invention, it is possible to reduce the intersymbol interference peculiar to the OFDMA scheme while maintaining the communication of the mobile station.

  Also, in one aspect of the present invention, the selecting means includes a base station having the shortest propagation delay time estimated by the estimating means from among base stations whose reception quality measured by the measuring means is equal to or higher than a predetermined quality. May be selected as the connection destination.

  According to this aspect, it is possible to more reliably reduce the intersymbol interference unique to the OFDMA scheme.

  In one aspect of the present invention, when the base station selected by the selection unit is different from the base station to which the mobile station is connected, the handover to the base station selected by the selection unit may be performed. Good.

  In one aspect of the present invention, when the reception quality of a radio signal transmitted from a base station to which the mobile station is connected is less than the predetermined quality, A handover may be performed.

  Further, in one aspect of the present invention, a new connection may be requested to the base station selected by the selection unit.

  In the aspect of the invention, the estimation unit may estimate a propagation delay time of the radio signal based on reception timing of the radio signal transmitted from each of the plurality of base stations.

  In one aspect of the present invention, the estimating means acquires a distance between the mobile station and each of the plurality of base stations, and based on the distance, a radio signal transmitted from each of the plurality of base stations The propagation delay time may be estimated.

  The connection destination selection method according to the present invention includes a step of measuring reception quality of a radio signal transmitted from each of a plurality of base stations adopting an orthogonal frequency division multiple access scheme and received by a mobile station, and A step of estimating a propagation delay time of a radio signal transmitted from each of the base stations and received by the mobile station, and the reception quality among the plurality of base stations based on the shortness of the estimated propagation delay time Selecting any one of base stations having a quality equal to or higher than a predetermined quality as a connection destination of the mobile station.

It is a figure which shows the structure of the mobile communication system which concerns on embodiment of this invention. It is a figure which shows the radio channel structure in the mobile communication system which concerns on this embodiment. It is a functional block diagram of the mobile station which concerns on this embodiment. It is a flowchart which shows the handover starting determination process of the mobile station which concerns on this embodiment. It is a figure which shows the structure of the conventional mobile communication system which employ | adopts an OFDMA system. FIG. 6 is a diagram illustrating an example of a relationship between a reception signal from a mobile station 52-3, a reception signal from a mobile station 52-1, and an FFT timing in the base station 54-3 illustrated in FIG. FIG. 6 is a diagram illustrating an example of a relationship between a reception signal from the mobile station 52-2, a reception signal from the mobile station 52-1, and an FFT timing in the base station 54-2 illustrated in FIG.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of a mobile communication system 10 according to an embodiment of the present invention. As shown in FIG. 1, the mobile communication system 10 includes a mobile station 12 (only one is shown here) and a plurality of base stations 14 (here, base stations 14-1 to 14-14 located around the mobile station 12). 3 is shown). Here, it is assumed that the cells 16-1 to 16-3 of the base stations 14-1 to 14-3 overlap each other.

  The base station 14 performs radio communication with the mobile station 12 located in the cell 16 of the own station by the OFDMA method and the TDMA / TDD method. Examples of the mobile station 12 include a mobile phone, a communication card, and a portable information terminal with a built-in communication function.

  FIG. 2 is a diagram showing a radio channel configuration in the mobile communication system 10 (vertical axis: frequency, horizontal axis: time). As shown in FIG. 2, in the mobile communication system 10, a TDMA frame having a predetermined period (here, 5 ms) is divided into an uplink subframe (2.5 ms) and a downlink subframe (2.5 ms), and each subframe is further divided. Are divided into a plurality of time slots (here, Slot 1 to Slot 4). Between the base stations 14-1 to 14-3, the transmission / reception timing of the radio signal is synchronized so that the boundary of the TDMA frame (radio signal transmission / reception cycle) falls within a predetermined time (for example, within ± 10 μs) ( Frame synchronization is done). A plurality of OFDMA subchannels (here, Sch1 to Sch18) are defined in a predetermined frequency band.

  The minimum unit of the radio channel that the base station 14 allocates to the mobile station 12 is called a PRU (Physical Resource Unit), and each PRU is either a time slot (Slot1 to Slot4) or a subchannel (Sch1 to Sch18). , Belonging to. In addition, it is determined that each PRU is identified by consecutive PRU numbers (1, 2, 3,...) Starting from 1, for example, in both the uplink subframe and the downlink subframe. It is supposed to be used in upper and lower pairs. That is, PRUs are assigned symmetrically.

  Among these, a PRU belonging to a specific subchannel (here, Sch1) is defined as a CCH (Common Channel) shared by one or more mobile stations 12. Also, each base station 14 is assigned a CCH transmission / reception timing (hereinafter simply referred to as “CCH timing”) that does not overlap each other and is repeated at a predetermined period. Specifically, any one of 80 time slot pairs (4 pairs of upper and lower time slots × 20 frames) included in 20 consecutive frames is allocated to each base station 14 as CCH timing. Each base station 14 transmits / receives various control signals to / from one or more mobile stations 12 located in the cell 16 of the own station at the CCH timing assigned to the own station.

  On the other hand, PRUs belonging to subchannels other than the specific subchannel (here, Sch2 to Sch18) are used as ICH (Individual Channel) individually assigned to each mobile station 12. In principle, one ICH is assigned to each mobile station 12 as an individual control channel and is mainly used for transmission of control information. One or more communication channels are assigned to each mobile station 12 and mainly communication data. There are EXCH (Extra Channel) used for the transmission of the.

  The mobile station 12 receives a radio signal transmitted from each of the surrounding base stations 14 (including the connected base station 14). Radio signals transmitted from the base station 14 to the mobile station 12 include BCCH (Broadcasting Control Channel), PCH (Paging Channel), and ICH transmitted via the CCH. There are control signals and communication signals to be transmitted.

  Based on the short propagation delay time of the received radio signal, the mobile station 12 is a base whose reception quality of the radio signal is equal to or higher than a predetermined quality (reception quality required for maintaining communication). One of the stations 14 is selected as a connection destination. That is, the mobile station 12 selects a base station 14 that can be expected to have a communication quality higher than a predetermined quality as a connection destination from the base station 14 that is close to the mobile station 12. For this reason, the mobile communication system 10 can reduce intersymbol interference peculiar to the OFDMA scheme while maintaining the communication of the mobile station 12.

  Below, the structure with which the mobile station 12 is provided in order to implement | achieve the said process is demonstrated concretely.

  FIG. 3 is a functional block diagram of the mobile station 12. As shown in FIG. 3, the mobile station 12 includes an antenna 20, a reception RF unit 22, a reception baseband unit 24, a reception quality measurement unit 26, a propagation delay time estimation unit 28, a connection destination selection unit 30, a handover control unit 32, A transmission baseband unit 34 and a transmission RF unit 36 are included.

  The antenna 20 receives a radio signal transmitted from each of the surrounding base stations 14 (including the connected base station 14), and outputs the received radio signal (received signal) to the reception RF unit 22. The antenna 20 transmits the radio signal supplied from the reception RF unit 22 to the base station 14 being connected or the base station 14 being a connection destination.

  The reception RF unit 22 includes a low noise amplifier, a frequency converter, a band pass filter, and an A / D converter. The reception RF unit 22 amplifies the radio signal input from the antenna 20 with a low noise amplifier, down-converts it to an intermediate frequency signal, further converts it into a digital signal, and outputs it to the reception baseband unit 24 To do.

  The reception baseband unit 24 includes a serial-parallel converter, an FFT (Fast Fourier Transform) unit, a parallel-serial converter, and a demodulation unit (not shown). The reception baseband unit 24 performs serial-parallel conversion, CP removal, primary demodulation (fast Fourier transform), parallel-serial conversion, secondary demodulation (symbol demapping), and the like on the digital signal input from the reception RF unit 22. The received data obtained is output to an upper layer (not shown).

  The reception quality measurement unit 26 measures the reception quality of the radio signal transmitted from each of the neighboring base stations 14 based on the complex symbol sequence obtained by the primary demodulation (fast Fourier transform) in the reception baseband unit 24. To do. The radio signal transmitted from the base station 14 is transmitted via BCCH (broadcast control channel), PCH (simultaneous call channel), and ICH (individual channel) transmitted via CCH (common channel). There are control signals and communication signals. The channel quality detected by the reception quality measuring unit 26 includes RSSI (Received Signal Strength), SINR (Signal to Interference and Noise Ratio), FER (Frame Error Rate). )and so on.

  The propagation delay time estimation unit 28 estimates the propagation delay time of the radio signal transmitted from each of the neighboring base stations 14. The radio signal transmitted from the base station 14 includes a training symbol (known signal) for frame synchronization. Therefore, the propagation delay time estimation unit 28 detects a correlation value between the complex symbol sequence obtained by the primary demodulation (fast Fourier transform) in the reception baseband unit 24 and the training symbol, and a correlation value equal to or greater than a predetermined value. Is detected as the reception timing of the radio signal transmitted from the base station 14. Then, the propagation delay time estimation unit 28 estimates the propagation delay time of the radio signal based on the acquired reception timing of the radio signal.

  The propagation delay time estimation unit 28 acquires the distance between the mobile station 12 and each of the surrounding base stations 14, and based on the distance, the propagation delay of the radio signal transmitted from each of the surrounding base stations 14 is obtained. The time may be estimated. The distance between the mobile station 12 and the base station 14 is calculated from, for example, the position of the mobile station 12 and the position of the base station 14 acquired based on GPS information or the like.

  Based on the reception quality measured by the reception quality measurement unit 26 and the propagation delay time estimated by the propagation delay time estimation unit 28, the connection destination selection unit 30 connects any of the surrounding base stations 14 to the connection destination. Choose as. Specifically, the connection destination selection unit 30 determines whether the propagation delay time estimated by the propagation delay time estimation unit 28 is short (for example, whether the propagation delay time is equal to or less than a predetermined time in the order of short propagation delay time). Etc.), any one of the base stations 14 whose reception quality measured by the reception quality measurement unit 26 is equal to or higher than a predetermined quality is selected as a connection destination. The connection destination selection unit 30 selects the base with the shortest propagation delay time estimated by the propagation delay time estimation unit 28 from the base stations 14 whose reception quality measured by the reception quality measurement unit 26 is equal to or higher than a predetermined quality. It is desirable to select the station 14 (the closest to the mobile station 12 among the base stations 14 that can be expected to have a communication quality higher than a predetermined quality) as the connection destination.

  The handover control unit 32 requires the handover of the mobile station 12 when the reception quality of the radio signal transmitted from the base station 14 to which the mobile station 12 is connected is less than a predetermined quality, that is, to maintain communication. In this case, the mobile station 12 performs handover to the base station 14 selected by the connection destination selection unit 30 (transmits a connection request message to the base station 14 selected by the connection destination selection unit 30). Control each part. As a result, the mobile station 12 is handed over to a nearby base station 14 that is expected to have a communication quality higher than a predetermined quality, so that the probability that the mobile station 12 becomes an intersymbol interference source to neighboring cells can be reduced.

  Further, the handover control unit 32 is configured such that the base station 14 selected by the connection destination selection unit 30 is different from the base station 14 to which the mobile station 12 is connected even when the mobile station 12 is not necessarily handed over. In that case, each unit of the mobile station 12 may be controlled to perform a handover to the base station 14 selected by the connection destination selection unit 30. In this way, if the mobile station 12 advances the handover activation timing in advance, the probability that the mobile station 12 becomes an intersymbol interference source to neighboring cells can be further reduced.

  The transmission baseband unit 34 includes a series-parallel converter, an IFFT (Inverse Fast Fourier Transform) unit, a parallel-serial converter, and a modulation unit (not shown). The transmission baseband unit 34 performs primary modulation (symbol mapping), serial-parallel conversion, transmission data addressed to the connected base station 14 or the connected base station 14 input from an upper layer (not shown), Secondary modulation (inverse fast Fourier transform), addition of CP, parallel serial conversion, and the like are performed, and the obtained digital signal is output to the transmission RF unit 36.

  The transmission RF unit 36 includes a power amplifier, a frequency converter, a band pass filter, and a D / A converter. The transmission RF unit 36 converts the digital signal input from the transmission baseband unit 34 into an analog signal, then up-converts it to a radio signal, amplifies it to a transmission output level with a power amplifier, and then supplies it to the antenna 20 .

  Next, an example of the connection destination selection operation of the mobile station 12 will be described.

  FIG. 4 is a flowchart showing the handover activation determination process of the mobile station 12 executed at a predetermined cycle (for example, 20 frame cycle). Here, as shown in FIG. 1, it is assumed that the mobile station 12 connected to the base station 14-1 can receive radio signals transmitted from the base stations 14-2 and 14-3.

  As shown in FIG. 4, the mobile station 12 measures SINR (one of the reception qualities) of radio signals transmitted from each of the base stations 14-1 to 14-3 located in the vicinity (S100). Further, the mobile station 12 estimates the propagation delay time of the radio signal transmitted from each of the base stations 14-1 to 14-3 (S102).

  Next, the mobile station 12 selects one of the base stations 14-1 to 14-3 whose measured SINR is equal to or greater than a predetermined value (a SINR threshold required for maintaining communication). Then, the base station 14 with the shortest propagation delay time is selected as the connection destination from the selected base stations 14 (S106).

  Here, when the base station 14 selected as the connection destination is different from the connected base station 14-1 (S108: N), that is, when there is a base station 14 closer to the connected base station 14-1. The mobile station 12 activates handover to the base station 14 selected as the connection destination (S110). On the other hand, when the base station 14 selected as the connection destination is the same as the connected base station 14-1 (S108: Y), the mobile station 12 ends this process without starting the handover.

  According to the mobile communication system 10 described above, the mobile station 12 selects a base station 14 that can be expected to have a communication quality of a predetermined quality or higher as a connection destination from the base stations 14 that can be expected. For this reason, the intersymbol interference peculiar to the OFDMA scheme can be reduced while maintaining the communication of the mobile station 12.

  The present invention is not limited to the above embodiment.

  For example, the present invention can be applied not only to the scene where the mobile station 12 performs handover, but also to the scene where the mobile station 12 performs a new connection. In this case, the mobile station 12 may transmit a connection request message for requesting a new connection to the base station 14 selected by the connection destination selection unit 30.

  10, 50 mobile communication system, 12, 52 mobile station, 14, 54 base station, 16, 56 cells, 20 antenna, 22 reception RF unit, 24 reception baseband unit, 26 reception quality measurement unit, 28 propagation delay time estimation unit , 30 connection destination selection unit, 32 handover control unit, 34 transmission baseband unit, 36 transmission RF unit, 58 building.

Claims (8)

Measuring means for measuring the reception quality of radio signals transmitted from each of a plurality of base stations adopting the orthogonal frequency division multiple access method;
Estimating means for estimating a propagation delay time of a radio signal transmitted from each of the plurality of base stations;
Based on the short propagation delay time estimated by the estimation unit, a selection unit that selects, as a connection destination, one of the plurality of base stations, the base station having the reception quality equal to or higher than a predetermined quality,
A mobile station characterized by comprising: The mobile station according to claim 1, wherein
The selection unit selects, as a connection destination, a base station having the shortest propagation delay time estimated by the estimation unit from among base stations whose reception quality measured by the measurement unit is equal to or higher than a predetermined quality.
A mobile station characterized by that. The mobile station according to claim 1 or 2,
When the base station selected by the selection unit is different from the base station to which the mobile station is connected, a handover to the base station selected by the selection unit is performed.
A mobile station characterized by that. The mobile station according to claim 1 or 2,
When the reception quality of a radio signal transmitted from the base station to which the mobile station is connected is less than the predetermined quality, a handover to the base station selected by the selection unit is performed.
A mobile station characterized by that. The mobile station according to claim 1 or 2,
Requesting a new connection to the base station selected by the selection means;
A mobile station characterized by that. In the mobile station in any one of Claim 1 to 4,
The estimating means estimates a propagation delay time of the radio signal based on a reception timing of the radio signal transmitted from each of the plurality of base stations;
A mobile station characterized by that. In the mobile station in any one of Claim 1 to 4,
The estimation means acquires the distance between the mobile station and each of the plurality of base stations, and estimates a propagation delay time of a radio signal transmitted from each of the plurality of base stations based on the distance,
A mobile station characterized by that. Measuring reception quality of radio signals transmitted from each of a plurality of base stations adopting an orthogonal frequency division multiple access scheme and received by a mobile station;
Estimating a propagation delay time of a radio signal transmitted from each of the plurality of base stations and received by the mobile station;
Selecting one of the plurality of base stations as the connection destination of the mobile station, based on the estimated short propagation delay time;
The connection destination selection method characterized by including.

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