JP2010258791A - Base station device and free channel judgment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a base station device capable of decreasing an erroneous judgment of a free channel due to a margin set to an FFT (fast Fourier transform) timing at the time of OFDM (orthogonal frequency division multiplexing) signal demodulation, and to provide an free channel judgment method. <P>SOLUTION: A base station 12 communicates with a mobile station by OFDMA (Orthogonal Frequency Division Multiple Access) method. The base station 12 includes: an FFT part 30 for conducting the FFT at a plurality of timings comprising a normal timing and one or more delay timings delayed from the normal timing every symbol for the received signals including the OFDM signal transmitted from the mobile station; a receiving level calculation part 36 for respectively calculating a plurality of receiving levels every subchannel on the basis of the result of the FFT conducted at the plurality of timings by the FFT part 30; and a free channel judging part 38 for judging a subchannel with at least one of the plurality of receiving levels calculated by the receiving level calculation part 36 being a predetermined threshold or below as a free channel. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a base station apparatus and an empty channel determination method, and more particularly to a technique for determining whether or not there is an empty subchannel using an OFDMA (Orthogonal Frequency Division Multiple Access) scheme.

  Some wireless communication systems perform assignment control of wireless channels by a CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) system (see, for example, Patent Document 1).

  For example, in wireless LAN (IEEE 802.11) system, PHS (Personal Handy-phone System), and XGP (eXtended Global Platform: next generation PHS) The station determines a free channel by carrier sense, and allocates a radio channel having a reception level equal to or lower than a predetermined threshold (hereinafter referred to as “channel allocation threshold”) to the mobile station.

Japanese Patent Laid-Open No. 10-191430

  By the way, in a wireless communication system that employs the OFDMA method, such as a wireless LAN system or XGP, FFT (Fast Fourier Transform) is used for demodulation of an 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 GI (Guard Interval) or CP (Cyclic Prefix) and a data signal. However, correct demodulation cannot be performed if FFT is performed across the boundary of OFDM signals for two adjacent symbols.

  In general, since there are some errors in the transmission / reception timing of the OFDM signal synchronized between the base station and the mobile station, the base station is not limited to the delayed wave (indirect wave) of the OFDM signal transmitted from the mobile station. Even the arrival timing of the main wave (direct wave) cannot be accurately specified. For this reason, normally, as shown in FIG. 7, FFT timing (FFT window) at the time of OFDM signal demodulation is performed so that FFT is not performed so as to straddle the boundary of the OFDM signal for at least two symbols adjacent to the main wave. A margin (usually less than half of the GI length) is set to advance the timing.

  However, in a base station in which a margin is set in the FFT timing at the time of OFDM signal demodulation, it may be erroneously determined that a subchannel that should be determined as an empty channel is being used in carrier sense.

  FIG. 8 is a diagram illustrating an example of the relationship between the main wave and delay wave of the OFDM signal that has arrived at the base station, and the FFT timing. FIG. 9 is a diagram showing the reception levels of subchannels 0 to 7 obtained from the result of FFT performed at the FFT timing shown in FIG. Here, it is assumed that subchannels 0 to 2, 5 to 7 are empty channels, and the delayed waves shown in FIG. 8 are received by subchannels 3 and 4 assigned to communication with the mobile station.

  As shown in FIG. 8, even if the delay time with respect to the main wave of the delayed wave is equal to or shorter than the GI length, if the delay time is larger than the difference between the GI length and the margin of the FFT timing, the adjacent wave included in the delayed wave FFT is performed across OFDM signals for two symbols, and so-called ISI (Inter Symbol Interference) occurs. In this case, since the reception level of the peripheral subchannels of the subchannel where the delayed wave is received is increased by the leakage power due to ISI, the reception level of the subchannel that should be determined as an empty channel may exceed the channel allocation threshold. is there.

  For example, in the example shown in FIG. 9, the reception levels of the subchannels 1 to 6 exceed the channel assignment threshold. In this case, the base station determines that only subchannels 0 and 7 are empty channels. For this reason, the subchannels 1, 2, 5, and 6 that should be determined as empty channels are not assigned to the mobile station, and the frequency utilization efficiency decreases.

  An object of the present invention is to provide a base station apparatus and a free channel determination method that can reduce erroneous determination of a free channel due to a margin set at the FFT timing at the time of OFDM signal demodulation.

  In order to solve the above-described problem, a base station apparatus according to the present invention is a base station apparatus that communicates with a mobile station apparatus using an OFDMA scheme, and that receives a received signal including an OFDM signal transmitted from the mobile station apparatus. , FFT means for performing FFT at a plurality of timings consisting of a normal timing for each symbol and one or more delay timings later than the normal timing, and a plurality of receptions based on the results of the FFT performed at the plurality of timings A reception level calculation unit that calculates a level for each subchannel, and an empty channel determination that determines that a subchannel in which at least one of a plurality of reception levels calculated by the reception level calculation unit is a predetermined threshold or less is an empty channel Means.

  According to the present invention, since the base station performs FFT on the received signal including the OFDM signal at a plurality of timings after the normal timing for each symbol, compared to the case where the FFT is performed only at the normal timing, the adjacent 2 The probability of obtaining an FFT result that does not cross the boundary of the OFDM signal for the symbol is increased. The influence of ISI (intersymbol interference) (leakage power to surrounding subchannels) does not appear in the result of FFT that does not cross the boundary of the OFDM signals for two adjacent symbols. For this reason, even if a margin for advancing the timing is set for the FFT timing at the time of OFDM signal demodulation, the base station can reduce erroneous determination of an empty channel due to the margin. In addition, since a free channel that has been erroneously determined to be in use is allocated to the mobile station apparatus, the frequency utilization efficiency is improved.

  Further, the one or more delay timings may be earlier than a timing after the regular timing by a guard interval length of the OFDM signal. In this way, an excessive increase in the number of FFT executions can be prevented.

  The vacant channel determination means may determine that a sub-channel whose minimum value among the plurality of reception levels calculated by the reception level calculation means is equal to or less than the threshold is an vacant channel.

  An idle channel determination method according to the present invention is an idle channel determination method for a base station apparatus that communicates with a mobile station apparatus by an OFDMA method, and is based on a received signal including an OFDM signal transmitted from the mobile station apparatus. , Performing FFT at a plurality of timings including a normal timing for each symbol and one or more delay timings later than the normal timing, and a plurality of reception levels based on the results of the FFT performed at the plurality of timings. For each subchannel, and determining that a subchannel in which at least one of the calculated plurality of reception levels is equal to or less than a predetermined threshold is an empty channel.

It is a figure which shows the structure of the radio | wireless communications system which concerns on embodiment of this invention. It is a figure which shows the channel structure in the radio | wireless communications system which concerns on embodiment of this invention. It is a functional block diagram of the base station which concerns on embodiment of this invention. It is a figure which shows an example of the relationship between the main wave and delay wave of OFDM signal, and several FFT timing. It is a figure which shows the reception level of the subchannels 0-7 obtained from the result of FFT performed at the FFT timing 2 shown in FIG. It is a figure which shows an example of the result of the several reception level (corresponding to several FFT timing) calculated for every subchannel, and an empty channel determination. It is a figure which shows an example of the relationship between the main wave of an OFDM signal, and FFT timing. It is a figure which shows an example of the relationship between the main wave of OFDM signal, a delay wave, and FFT timing. It is a figure which shows the reception level of the subchannels 0-7 obtained from the result of FFT performed at the FFT timing shown 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 wireless communication system 10 according to an embodiment of the present invention. As shown in the figure, the wireless communication system 10 includes a base station 12 and a plurality of mobile stations 14 (here, only mobile stations 14-1 to 14-3 are shown). The base station 12 employs an OFDMA system and a TDMA / TDD (Time Division Multiple Access / Time Division Duplex) system, and can perform multiplex communication with a plurality of mobile stations 14. it can.

  FIG. 2 is a diagram illustrating a channel configuration in the wireless communication system 10. As shown in the figure, in the radio communication system 10, a TDMA frame (5 ms) is divided into an uplink subframe (2.5 ms) and a downlink subframe (2.5 ms), and each subframe has four times each. The slots are equally divided into slots (slot # 1 to slot # 4). In addition, a plurality (for example, 18) of OFDMA subchannels are defined in a predetermined radio band. Note that the minimum unit of a radio channel allocated for communication between the base station 12 and the mobile station 14 is called a PRU (Physical Resource Unit) and belongs to one of eight time slots and one of a plurality of subchannels.

  The base station 12 performs PRU allocation control for the mobile station 14 by the CSMA / CA method. That is, the base station 12 appropriately determines an empty channel by carrier sense for each time slot, and allocates a PRU having a reception level equal to or lower than a predetermined threshold (channel allocation threshold) to the mobile station 14.

  At the start of communication, the transmission / reception timing of the OFDM signal is synchronized between the base station 12 and the mobile station 14. However, since some errors may be included in the transmission / reception timing of the OFDM signal, it is difficult for the base station 12 to accurately specify even the arrival timing of the main wave of the OFDM signal transmitted from the mobile station 14. Therefore, the base station 12 uses an FFT timing (hereinafter referred to as “regular timing”) at the time of OFDM signal demodulation so that the FFT is not performed so as to straddle the boundary between the OFDM signals of at least two symbols adjacent to the main wave. A margin is set to advance the timing.

  As described above, in a conventional base station in which a margin is set in the FFT timing at the time of OFDM signal demodulation, it may be erroneously determined that a subchannel that should be determined as an empty channel is being used in carrier sense. . This is because the reception level of the peripheral subchannel of the subchannel where the delayed wave is received is the ISI (intersymbol interference) as a result of performing the FFT across the OFDM signals of two adjacent symbols included in the delayed wave of the OFDM signal. This is because only the leakage power due to) increases.

  In this regard, in the present embodiment, when at least one PRU performs carrier sense in a time slot assigned to the mobile station 14, the base station 12 performs a received signal including an OFDM signal transmitted from the mobile station 14. Thus, FFT is executed at a plurality of timings after the regular timing for each symbol. This is to increase the probability of obtaining an FFT result that does not cross the boundary of the OFDM signals for two adjacent symbols as compared with the case where the FFT is performed only at the normal timing. The influence of ISI (leakage power to surrounding subchannels) does not appear in the result of FFT that does not cross the boundary of the OFDM signals for two adjacent symbols. For this reason, the base station 12 can reduce erroneous determination of an empty channel due to the margin set at the regular timing.

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

  FIG. 3 is a functional block diagram of the base station 12. As shown in the figure, the base station 12 includes an antenna 20, a reception RF unit 22, a transmission RF unit 24, a reception baseband unit 26 (buffer memory 28, FFT unit 30, demodulation unit 32), error correction unit 34, reception The level calculation unit 36, the free channel determination unit 38, the channel assignment control unit 40, and the transmission baseband unit 42 are configured.

  The antenna 20 receives a radio signal and outputs the received radio signal (reception signal) to the reception RF unit 22. The antenna 20 transmits a radio signal supplied from the transmission RF unit 24 to the mobile station 14.

  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, and outputs the digitally converted signal to the reception baseband unit 26.

  The transmission RF unit 24 includes a power amplifier, a frequency converter, a band pass filter, and a D / A converter. The transmission RF unit 24 converts the digital signal input from the transmission baseband unit 42 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.

  The reception baseband unit 26 includes a buffer memory 28, an FFT unit 30, and a demodulation unit 32, and performs primary demodulation (FFT) and secondary demodulation (symbol demapping) on the digital signal input from the reception RF unit 22. And so on. The FFT unit 30 and the demodulation unit 32 are configured by, for example, a DSP (Digital Signal Processor).

  The buffer memory 28 is composed of, for example, a semiconductor memory element, and holds the received signal until a plurality of FFTs (described later) performed by the FFT unit 30 for each symbol on the received signal is completed.

  The FFT unit 30 performs FFT on the received signal including the OFDM signal transmitted from the mobile station 14 at a plurality of timings including a normal timing and one or more delay timings later than the normal timing for each symbol. Each subcarrier component of the symbol data is obtained.

  FIG. 4 is a diagram illustrating an example of the relationship between the main wave and delay wave of the OFDM signal transmitted from the mobile station 14 and a plurality of FFT timings. The figure shows an example in which a total of three FFTs are executed for each symbol at FFT timing 1 (regular timing) and FFT timings 2 and 3 (delay timing) later than FFT timing 1.

  As shown in FIG. 4, even when the delay time of the delayed wave with respect to the main wave is equal to or shorter than the GI length, if the delay time is larger than the difference between the GI length and the margin of the normal timing, at the FFT timing 1 (normal timing), FFT is performed across OFDM signals for two adjacent symbols included in the delay wave. For this reason, the influence of ISI appears in the result of the FFT performed at the FFT timing 1 (first FFT). On the other hand, at the FFT timing 2 which is one of the delay timings, the FFT is performed so as not to straddle the adjacent two symbols of the OFDM signal included in the delay wave, so the result (the result of the second FFT) Is not affected by ISI.

  Therefore, when the subchannels 0 to 2, 5 to 7 are vacant channels, and the subchannels 3 and 4 are assigned to communication with the mobile station 14, when the delayed wave shown in FIG. The reception levels of subchannels 0 to 7 obtained from the result of FFT performed at FFT timing 2 are as shown in FIG. That is, the received power of the subchannels 3 and 4 that are used exceeds the channel allocation threshold, but is not affected by ISI (leakage power to the peripheral subchannels), and thus is a peripheral subchannel of the subchannels 3 and 4. The reception levels of subchannels 0 to 2 and 5 to 7 are equal to or less than the channel allocation threshold.

  Needless to say, the greater the number of FFT executions in the FFT unit 30, the higher the probability that an FFT result without the influence of ISI will be obtained. However, each delay timing at which the second and subsequent FFTs are executed is preferably set between the normal timing and the timing after the normal timing by the GI (guard interval) length of the OFDM signal. This is because an excessive increase in the number of FFT executions can be prevented. Further, the FFT unit 30 may perform the FFT only at regular timing when the base station 12 does not perform carrier sense (when the reception level of a subchannel where the base station 12 is not used is not detected). .

  The demodulator 32 concatenates the subcarrier components of the symbol data input from the FFT unit 30 for each subchannel, and further concatenates each mobile station 14. Then, the demodulator 32 performs secondary demodulation on the symbol data connected for each mobile station 14 to obtain received data from each mobile station 14. The reception data obtained in this way is output to an upper layer (not shown) after a bit error is appropriately corrected by the error correction unit 34.

  Based on the input from the FFT unit 30, the reception level calculation unit 36 and the free channel determination unit 38 perform free channel determination by carrier sense.

  The reception level calculation unit 36 calculates a plurality of reception levels for each sub-channel based on the result of FFT (each subcarrier component of symbol data) performed by the FFT unit 30 at a plurality of timings for each symbol. That is, the reception level calculation unit 36 connects each subcarrier component of the symbol data input from the FFT unit 30 for each subchannel every time one FFT is completed. And the reception level calculation part 36 calculates the reception level (one set of reception levels) of each subchannel based on the symbol data connected for every subchannel. For this reason, for example, when three FFT timings (one regular timing and two delay timings) are set, three sets of reception levels are obtained (refer to the inside of the thick frame in FIG. 6).

  When the received signal for one time slot includes an OFDM signal for a plurality of symbols (for example, 19 symbols), the reception level calculation unit 36 performs a plurality of symbols (for example, 19 symbols) as described above. A plurality of sets (for example, three sets) of reception levels obtained may be averaged one time slot at a time, and the average value may be used as the reception level for each of the plurality of sets (for example, three sets).

  The free channel determination unit 38 determines that a subchannel in which at least one of the plurality of reception levels calculated by the reception level calculation unit 36 is equal to or less than the channel allocation threshold is an empty channel. For example, the vacant channel determination unit 38 may determine that a subchannel whose minimum value among the plurality of reception levels calculated by the reception level calculation unit 36 is equal to or less than the channel allocation threshold is an vacant channel.

  FIG. 6 is a diagram illustrating an example of the three reception levels (corresponding to the FFT timings 1 to 3 shown in FIG. 4) calculated by the reception level calculation unit 36 for each sub-channel and the result of free channel determination. For example, when the reception level shown in FIG. 6 is obtained by carrier sense in a certain time slot, the empty channel determination unit 38 determines that at least one of the three reception levels for each subchannel is a channel allocation threshold (here, 3 dBm). ) It is determined whether or not the following is satisfied, and it is determined that a subchannel in which at least one of the three reception levels is a channel allocation threshold is an empty channel. Here, the reception level calculated based on the result of the FFT performed at FFT timing 2 (the second FFT that is not affected by ISI) is reflected in the result of the empty channel determination.

  The channel allocation control unit 40 controls PRU allocation to the mobile station 14 based on the determination result (carrier sense result) of the empty channel by the empty channel determination unit 38. For example, the channel assignment control unit 40 assigns at least one of the PRUs determined to be an empty channel by the empty channel determination unit 38 to the mobile station 14 that requests a new connection.

  The transmission baseband unit 42 includes an IFFT (Inverse Fast Fourier Transform) unit and a modulation unit (not shown), and is input from an upper layer (not shown) according to the PRU assignment result determined by the channel assignment control unit 40. The transmission data addressed to each mobile station 14 is subjected to primary modulation (symbol mapping), secondary modulation (IFFT), or the like.

  According to the embodiment described above, the base station 12 performs FFT at a plurality of timings after the normal timing (FFT timing at the time of OFDM signal demodulation) for each symbol with respect to the received signal including the OFDM signal at the time of carrier sense. As a result, the probability of obtaining an FFT result that does not cross the boundary of the OFDM signals for two adjacent symbols is higher than when performing FFT only at regular timing. For this reason, the base station 12 can reduce erroneous determination of an empty channel due to the margin set at the regular timing. In addition, since a free channel that has been erroneously determined to be in use is allocated to the mobile station 14, the frequency utilization efficiency is improved.

  The present invention is not limited to the above embodiment.

  For example, although not mentioned in the above embodiment, it is more effective to combine the HARQ (Hybrid Automatic Repeat Request) technique with the present invention. Among radio channels that have been erroneously determined to be in use, radio channels that are correctly determined to be free channels by the free channel determination method according to the present invention are incidentally generated ISI. This is because the HARQ gain is particularly easily obtained when receiving the influence of the above (increase in interference power).

  In the above embodiment, the example in which the present invention is applied to the base station adopting the OFDMA scheme and the TDMA / TDD scheme has been shown. However, the present invention is widely applicable to all base stations that communicate with mobile stations by the OFDMA scheme. It is.

  10 wireless communication system, 12 base station, 14 mobile station, 20 antenna, 22 reception RF unit, 24 transmission RF unit, 26 reception baseband unit, 28 buffer memory, 30 FFT unit, 32 demodulation unit, 34 error correction unit, 36 Reception level calculation unit, 38 free channel determination unit, 40 channel allocation control unit, 42 transmission baseband unit.

Claims (4)

A base station apparatus that communicates with a mobile station apparatus by an OFDMA (Orthogonal Frequency Division Multiple Access) method,
FFT (Fast Fourier Transform) is performed on a received signal including an OFDM signal transmitted from the mobile station apparatus at a plurality of timings including a normal timing for each symbol and one or more delay timings later than the normal timing. FFT means;
A reception level calculating means for calculating a plurality of reception levels for each sub-channel based on the result of FFT performed at the plurality of timings;
Empty channel determination means for determining that a sub-channel in which at least one of a plurality of reception levels calculated by the reception level calculation means is equal to or less than a predetermined threshold is an empty channel;
A base station apparatus comprising: The base station apparatus according to claim 1,
The one or more delay timings are earlier than timing after the regular timing by a guard interval length of the OFDM signal,
A base station apparatus. In the base station apparatus according to claim 1 or 2,
The vacant channel determining means determines that a subchannel whose minimum value among the plurality of reception levels calculated by the reception level calculating means is equal to or less than the threshold is an vacant channel.
A base station apparatus. A free channel determination method of a base station apparatus communicating with a mobile station apparatus by an OFDMA method,
Performing FFT on a received signal including an OFDM signal transmitted from the mobile station apparatus at a plurality of timings including a normal timing for each symbol and one or more delay timings later than the normal timing;
Calculating a plurality of reception levels for each subchannel based on results of FFT performed at the plurality of timings;
Determining a subchannel in which at least one of the calculated plurality of reception levels is equal to or less than a predetermined threshold as a free channel;
A free channel determination method comprising:

Images