JP2004032518A - Diversity receiving method and reception apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diversity receiving method and a reception apparatus for selecting a suitable antenna without depending only on reception power. <P>SOLUTION: Data modulated to a plurality of subcarriers by orthogonal frequency multiple modulation is received by a plurality of antennas. In each of the received signals, a reception signal of a plurality of use subcarrier is compared with a predetermined threshold, and a suitable antenna is selected based on the comparison result. At this time, a noise level of unused subcarriers is measured and the predetermined threshold above is determined based on the noise level. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a receiving apparatus including an antenna switching unit for diversity reception in a wireless system such as a wireless LAN and a wireless home network, and particularly to an OFDM wireless system using a diversity receiving method using a frequency characteristic inspection unit.
[0002]
[Prior art]
2. Description of the Related Art In recent years, as a packet communication access method of a wireless LAN (Local Area Network), a method using carrier sense multiple access / collision avoidance CSMA / CA (Carrier Sense Multiple Access / Collision Aviation) has become widespread. Further, there is a diversity reception or diversity transmission system that switches a plurality of antennas in order to improve error characteristics. In the diversity scheme, a selection algorithm for selecting an optimum antenna from a plurality of antennas is important. In a conventional diversity system, a method of selecting a signal having a strong received signal strength has been generally used.
[0003]
On the other hand, an orthogonal frequency division multiplexing (OFDM) modulation scheme is known as a modulation scheme suitable for broadband communication. OFDM is a type of multi-carrier transmission system, and has the advantage that the effect of a delayed wave within a time corresponding to the guard interval length can be eliminated by adding a guard interval to each OFDM symbol.
[0004]
FIG. 3 shows a schematic configuration of a receiving apparatus including a conventional antenna switching unit in a wireless system employing the OFDM modulation scheme. The antenna 1 and the antenna 2 are switched by the switching unit 31 and connected to the RF unit 32. This switching is performed according to the result determined by the antenna selection determination unit 35 based on the reception signal strength of each antenna measured by the reception signal strength measurement unit 34 from the output of the RF unit 32. The received signal of the selected antenna is converted into a digital signal by the AD unit 33. The synchronization acquisition unit 37 acquires synchronization based on the digital signal, and performs Fourier transform on the digital signal by an FFT (fast Fourier transform means) 38 to demodulate OFDM. This output is subjected to multipath distortion compensation and the like in the transmission path estimating unit 39, and becomes a demodulated output.
[0005]
Further, when communicating with a plurality of different communication partners, there is another problem that in a CSMA / CA system in which it is impossible to predict from which terminal a radio wave next arrives, a previous reception result cannot be referred to. If the operation of selecting the optimum antenna can be completed within the same burst, the throughput does not decrease. In a wireless LAN, one packet is contained in one burst, and if all the received bits are incorrect, it is regarded as a packet error.
[0006]
Therefore, even if a correct antenna is found in the middle of the burst, a bit error occurs at the beginning of the burst, which is too late. Therefore, it is necessary to find a correct antenna in a section before data starts, that is, in a preamble section.
[0007]
FIG. 5 shows an example of the burst. The synchronization acquisition unit 37 detects the arrival of the preamble, finds the start timing of data with high accuracy, and starts reception. Since the burst needs to be received correctly from the first bit, the antenna switching needs to be completed before the data starts, ie within the preamble.
[0008]
Therefore, transmission diversity for transmitting from an optimal antenna at the time of transmission has been realized. That is, transmission is attempted from the first antenna, and if Ack does not return twice in succession, the antenna is considered to be in a poor communication state and transmission is performed from the second antenna. According to this, the communication error situation is directly evaluated instead of the evaluation of the indirect parameter such as the strength of the received signal, so that an antenna having higher communication reliability can be selected.
[0009]
However, in this case, since data is not transmitted in a section where transmission is attempted, there is a problem that the throughput is reduced. Therefore, switching on the receiving side is more effective in reducing throughput than transmitting.
[0010]
[Problems to be solved by the invention]
With the recent increase in data rates, wireless bandwidth has increased and multipath environments have become selective fading. For this reason, there is often no significant difference in the received power regardless of which antenna is used. In particular, in the selective fading, the error rate differs by many digits even though the received power is not so different, so that performing the antenna selection based only on the received power may select the wrong antenna. Therefore, an antenna selection method depending only on the reception power is not always effective.
[0011]
Therefore, an object of the present invention is to provide a diversity receiving method and a receiving apparatus that can perform more appropriate antenna selection without depending only on received power.
[0012]
It is another object of the present invention to provide a diversity receiving method and a receiving apparatus capable of preventing a decrease in throughput by providing an algorithm for instantly selecting an optimum antenna on a receiving side.
[0013]
[Means for Solving the Problems]
A receiving apparatus according to the present invention is a receiving apparatus that employs a diversity receiving method in an orthogonal frequency division multiplexing modulation wireless system, and includes a plurality of antennas and a frequency characteristic inspection for inspecting frequency characteristics of received signals of each of the plurality of antennas. Means, synchronization acquisition means for acquiring synchronization by receiving each of the received signals, and based on the output of the synchronization acquisition means and the output of the frequency characteristic inspection means, a reception signal of an antenna having a good preamble frequency characteristic. Antenna selection judging means for selecting, the frequency characteristic inspection means, a threshold to be compared with the received signal level of a plurality of used subcarriers by orthogonal frequency multiplexing modulation, the unused sub-carrier obtained by the frequency characteristic inspection means The determination is made based on the noise level of the carrier.
[0014]
Since the error rate of the burst is not reflected only by the reception intensity in the preamble section, another measurement criterion is introduced in the present invention. That is, since the error rate is strongly affected by multipath fading, the problem is solved by measuring the frequency characteristics of the preamble section and selecting an antenna having good frequency characteristics. For this purpose, a threshold for determining the received signal level of the used subcarrier is determined based on the noise level of the unused subcarrier. Specifically, for example, a threshold higher than the noise level by a predetermined ratio is set.
[0015]
Further, when the noise level of the lower unused subcarrier is different from the noise level of the upper unused subcarrier, the average noise level of the lower unused subcarrier and the average noise level of the upper unused subcarrier are determined, and the larger one is used as a reference. Can be determined. In this case, for example, a level obtained by adding a predetermined dB value to the larger of the average noise level of the lower unused subcarrier and the average noise level of the upper unused subcarrier can be used as the threshold.
[0016]
In the frequency characteristic inspection means, the energy of each used subcarrier is obtained, and the branch having the smaller number of used subcarriers whose energy is equal to or less than the threshold value is selected.
[0017]
The frequency characteristic inspection unit and the synchronization acquisition unit may be provided for each reception signal of each antenna. Alternatively, an antenna switching means for switching the reception signals of the plurality of antennas is provided, and the frequency characteristic inspection means and the synchronization acquisition means share a single means for the reception signals of the plurality of antennas. You can also. In the latter case, the antenna switching means performs continuous switching during standby, the synchronization acquisition means acquires synchronization during a preamble, and the frequency characteristic inspection means receives each switched reception signal. Inspect the frequency characteristics of the signal. The antenna switching period corresponds to a time that is an integral multiple of the preamble repetition period.
[0018]
A fast Fourier transform unit can be used as the frequency characteristic inspection unit. In this case, the fast Fourier transform means as the demodulation means of the orthogonal frequency multiplex modulation can be shared as the frequency characteristic inspection means.
[0019]
The frequency characteristic inspection means can be constituted by a plurality of filters.
[0020]
A diversity receiving method according to the present invention is a diversity receiving method used in an orthogonal frequency division multiplexing modulation wireless system, wherein data modulated on a plurality of subcarriers by orthogonal frequency multiplex modulation is received by a plurality of antennas, For each signal received by a plurality of antennas, comparing received signals of a plurality of used subcarriers with a predetermined threshold, selecting an appropriate antenna based on the comparison result, and selecting the appropriate antenna Measuring the noise level of an unused subcarrier and determining the predetermined threshold value based on the noise level. In this case, it is preferable that the antenna selection is determined within the preamble and the switching is completed.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0022]
FIG. 1 is a block diagram of a receiving apparatus including an antenna switching unit in a wireless system employing an OFDM modulation scheme according to the present embodiment.
[0023]
The received signals received from the two antennas are frequency-converted and amplified by the RF units 11 and 13, respectively, and further converted into digital signals by the AD converter. These digital signals are input to the frequency characteristic (f characteristic) inspection units 15 and 17 and also to the synchronization acquisition units 16 and 18, respectively.
[0024]
The two synchronization acquisition units 16 and 18 search for the arrival of the signal and wait for the signal. The two frequency characteristic inspection units 15 and 17 monitor the frequency characteristics of the signals received by the respective antennas.
[0025]
The received signals received from the two antennas are input to the switching unit 21. The switching unit 21 selects the signal of the antenna specified by the antenna selection determination unit 20 based on the outputs of both frequency characteristic inspection units 15 and 17. Thereafter, in the same manner as described above, the Fourier transform is performed by the FFT 38 to perform OFDM demodulation, and the transmission path estimating unit 39 performs compensation for multipath distortion and the like, thereby providing a demodulated output. Thereafter, the data is sent to a deinterleave circuit, a Viterbi decoder circuit and the like (not shown).
[0026]
The antenna selection determining unit 20 determines the best antenna by comparing the outputs of the two frequency characteristic testing units 15 and 17.
[0027]
The synchronization acquisition units 16 and 18 monitor the arrival of the received signal. When the arrival of the received signal is recognized, the timing is transmitted to the antenna selection determination unit 20 to fix the optimum antenna at that time. At the same time, the synchronization acquisition units 16 and 18 perform an OFDM demodulation operation by supplying an appropriate timing signal to the FFT 22 via the timing switching unit 19.
[0028]
Next, a specific configuration of the frequency characteristic inspection unit which is a feature of the present invention will be described in detail. Specific examples of the configuration means include (1) a method using an FFT and (2) a method using a filter.
[0029]
(1) Method using FFT Usually, the number of preambles is as small as 12 out of 52 subcarriers of OFDM, so that the FFT with a small number of points (for example, 16 points) is sufficient. The 16-point FFT has a smaller circuit scale and shorter operation processing time than the 64-point OFDM demodulation FFT. Furthermore, since a 16-point FFT can be configured as a part of a 64-point FFT, one 64-point demodulation FFT can be shared depending on the FFT configuration method. By this sharing, an increase in circuit size can be suppressed.
[0030]
When the FFT is shared, the following two methods are conceivable as division processing methods.
(A) Method by Time-Division Processing One FFT is operated at high speed, and the frequency characteristic inspection unit 15 and the frequency characteristic inspection unit 17 are alternately operated.
(B) Method by Space Division Processing A 64-point FFT for demodulation can be realized by combining a plurality of 16-point FFTs. By utilizing this fact and preparing an interface so that two 16-point FFTs operate in parallel, the demodulation FFT can be shared as frequency characteristic inspection means.
[0031]
(2) Method of Using Filter As shown in FIG. 12, N band filters (BPFs) are prepared and a wireless bandwidth (for example, 17 MHz) is divided into N. It is not always necessary to divide the entire number of subcarriers, but it is sufficient if there is a resolution necessary to determine whether the band has substantially flat frequency characteristics or a large dip occurs. N is set to, for example, about 5 to 10, and the energy in each band is measured to obtain a frequency characteristic.
[0032]
The number of antennas is not limited to two, but may be three or more. In that case, the same number of synchronization acquisition units may be provided as the number of antennas, or one may be shared.
[0033]
Next, a specific example of the frequency characteristic inspection unit will be described.
[0034]
First, an antenna selection algorithm based on a general frequency characteristic test will be described with reference to FIG.
(A) The reception average power Pavg_k of each branch is obtained (Pavg_1, Pavg_2,...).
(B) The reception level of each branch is compared with a predetermined fixed threshold value TH. In the figure, Delta represents the difference (dB) between the received average power and the threshold. When Delta is negative, it is based on the case where the reception intensity is low, which will be described later.
(C) Measure the bandwidth BW_k at which the threshold is interrupted for each branch. (BW_1, BW_2, ...)
(D) Select the antenna that minimizes BW_k.
[0035]
However, if the reception level is determined based on the above-described predetermined threshold TH and the antenna is selected, there is a possibility that an erroneous antenna is selected.
[0036]
Therefore, in the present embodiment, the following operation algorithm is adopted.
[0037]
As shown in FIG. 7, there are unused subcarriers on both sides of the used subcarriers on the frequency axis. Here, a noise level and an interference wave level due to signal leakage of an adjacent channel appear. In this specification, the levels of noise and interference waves are collectively referred to as “noise level”. The noise levels on both sides (upper and lower sides on the frequency axis) of the used subcarrier are not necessarily the same, and can be different noise levels 1 and 2 as shown in FIG. Therefore, the present embodiment employs the following antenna selection algorithm.
(A) The reception average power Pavg_k of each branch is obtained (Pavg_1, Pavg_2,...).
(B) As shown in FIG. 9, the average levels of the lower and upper unused subcarriers are calculated as noise level 1 and noise level 2. In this example, the output of each subcarrier is obtained from the output of the FFT.
(C) For each branch, a threshold TH_k of each branch is obtained based on the noise levels 1 and 2 (TH_1, TH_2,...).
(D) Measure a bandwidth (dip width) BW_k in which a threshold is interrupted (below) for each branch (BW_1, BW_2,...).
(E) Select an antenna that minimizes BW_k.
[0038]
As a method of obtaining the threshold value TH in step (c), the following method can be considered. As shown in FIG. 10, TH is calculated by selecting the larger PImax of the noise levels 1 and 2 and adding the required CIRreq to this.
[0039]
TH = PImax + CIRreq (1)
Here, the unit of TH, PImax, and CIRreq is [dB]. The required CIRreq is, for example, about 20 dB or 30 dB, and can be predetermined experimentally or empirically in each system.
[0040]
As described above, by considering the noise level in determining the threshold value TH, if the noise level is high, the number of used subcarriers exceeding the threshold value is reduced correspondingly. As a result, even if the signal level of the used subcarrier is large, it is possible to make it difficult to select a branch having a substantially poor reception state due to noise.
[0041]
Note that, as shown in FIG. 11, the determination of the upper limit value THmax of TH may be added after obtaining TH by the above equation (1). That is, when the obtained TH is larger than the upper limit value THmax, TH is fixed to the upper limit value THmax. The upper limit THmax is such a value that if a higher TH is set, the number of subcarriers having a signal level exceeding the TH becomes extremely small and is considered to be practically ineffective.
[0042]
Further, if the reception intensity is very weak, a reception error occurs even if the frequency characteristics are good, so that an antenna having a reception intensity equal to or less than a certain value may not be selected. That is, a branch whose Pavg_k is less than the required minimum received power Pavg_min is excluded from the options. If all the branches are less than Pavg_min, the branch having the maximum Pavg_k is selected.
[0043]
By the way, there are cases where the synchronization acquisition units 16 and 18 cannot detect the signal detection and the FFT timing with sufficient accuracy due to multipath distortion. On the contrary, a case is also assumed in which only one of the two synchronization acquisition units can perform signal detection. Therefore, usually, one synchronization acquisition unit is prepared for each antenna. However, if synchronization acquisition means having sufficiently high performance is adopted, it is possible to secure sufficient reception reliability even by preparing one for any antenna. FIG. 2 shows the configuration of such a receiving apparatus. In the figure, the same components as those shown in FIG. 3 are denoted by the same reference numerals. In contrast to the configuration of FIG. 3, in FIG. 2, the frequency characteristic inspection unit 25 receiving the output of the AD conversion unit 33 and the antenna selection determination unit 26 based on the outputs of the frequency characteristic inspection unit 25 and the synchronization acquisition unit 37 are appropriately The switching unit 31 is controlled to select a proper antenna. The timing control unit 27 performs control for synchronizing the output of the antenna selection determination unit 26 and the output of the synchronization acquisition unit 37. In the configuration of FIG. 2, since the signal of the preamble portion is divided by the switching of the antenna, it is necessary to accurately produce the synchronization acquisition unit 37 so that the synchronization acquisition unit 37 can properly synchronize.
[0044]
The synchronization acquisition unit 37 is configured based on a correlation circuit. As shown in FIG. 13, since the preamble is made of a repetitive waveform of a known waveform, during standby, as shown in FIG. 4, the switching unit 31 is switched for each repetition unit of the preamble to synchronize with the operation of the correlation circuit. Accordingly, a correlation value can be obtained even if the preamble is divided. The antenna switching cycle corresponds to, for example, a time that is an integral multiple of the preamble repetition cycle.
[0045]
As described above, if the switching unit is disposed immediately after the antenna and only one RF unit is prepared, it is possible to reduce the size and power consumption of the device.
Since the transmission diversity function described in the conventional example can be used independently of the present invention, it may be used together with the present invention.
[0046]
The preferred embodiment of the present invention has been described above. However, various modifications and changes can be made without departing from the scope of the claims.
[0047]
【The invention's effect】
According to the present invention, in the diversity receiving method in the orthogonal frequency division multiplexing modulation wireless system, when selecting a branch, it is not necessary to simply determine only the levels of received signals of a plurality of used subcarriers, By taking the level into consideration, a more appropriate antenna can be reliably selected even when channels become crowded. Further, by determining the antenna selection within the same burst, it is possible to prevent a decrease in throughput. In particular, when performing communication with a plurality of different communication partners, it is suitable for use in a CSMA / CA system in which it is impossible to predict from which terminal a radio wave will arrive next.
[Brief description of the drawings]
FIG. 1 is a block diagram of a receiving apparatus including an antenna switching unit in a wireless system employing an OFDM modulation scheme according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating a configuration of a receiving device according to the embodiment of the present invention.
FIG. 3 is a block diagram illustrating a schematic configuration of a receiving apparatus including a conventional antenna switching unit in a wireless system employing an OFDM modulation scheme.
FIG. 4 is a timing chart showing antenna switching timing in the embodiment of the present invention.
FIG. 5 is a timing chart illustrating a demodulation operation in the embodiment of the present invention.
FIG. 6 is an explanatory diagram of an antenna selection algorithm based on a general frequency characteristic test.
FIG. 7 is an explanatory diagram for determining a threshold value in FIG. 6 in the embodiment of the present invention.
FIG. 8 is an explanatory diagram for supplementing FIG. 7;
FIG. 9 is a diagram illustrating an example of a method of measuring the average level of unused subcarriers on the lower side and the upper side in FIG. 8;
FIG. 10 is a diagram illustrating an example of a method of obtaining a threshold value in FIG. 6 according to the embodiment of the present invention.
FIG. 11 is a diagram illustrating another example of a method of obtaining the threshold value in FIG. 6 according to the embodiment of the present invention.
FIG. 12 is a diagram illustrating a configuration when a filter is used in the configuration of the frequency characteristic inspection unit.
FIG. 13 is a diagram illustrating a configuration of a preamble.
[Explanation of symbols]
11, 13 RF unit, 12, 14 analog-to-digital converter (AD), 15, 17 frequency characteristic inspection unit, 16, 18 synchronization acquisition unit, 19 timing switching unit, 20 antenna selection determination unit 21, ... Switching unit, 22 ... FFT (fast Fourier transform means), 23 ... Transmission channel estimation unit

Claims (13)

A receiving apparatus adopting a diversity receiving method in an orthogonal frequency multiplexing modulation wireless system,
Multiple antennas,
Frequency characteristic inspection means for inspecting the frequency characteristic of the received signal of each of the plurality of antennas,
Synchronization acquisition means for acquiring synchronization by receiving each of the received signals;
Antenna selection determining means for selecting a received signal of an antenna having a good preamble frequency characteristic based on the output of the synchronization obtaining means and the output of the frequency characteristic inspecting means,
The frequency characteristic inspection means determines a threshold value to be compared with the received signal levels of a plurality of used subcarriers by orthogonal frequency multiplexing modulation based on the noise level of unused subcarriers obtained by the frequency characteristic inspection means. A receiving device characterized by the above-mentioned. 2. The receiving apparatus according to claim 1, wherein an average noise level of a lower unused subcarrier and an average noise level of an upper unused subcarrier are obtained, and the threshold is determined based on a larger one. 3. The threshold obtained by adding a predetermined dB value to the larger of the average noise level of the lower unused subcarrier and the average noise level of the upper unused subcarrier, as a threshold. Receiving device. 4. The receiving apparatus according to claim 1, wherein the frequency characteristic inspection means obtains energy of each used subcarrier, and selects a branch having less used subcarriers having energy equal to or less than the threshold. . 2. The receiving apparatus according to claim 1, wherein said frequency characteristic inspection means and said synchronization acquisition means are provided for each reception signal of each antenna. An antenna switching unit for switching the reception signals of the plurality of antennas, wherein the frequency characteristic inspection unit and the synchronization acquisition unit share a single unit for the reception signals of the plurality of antennas. The receiving device according to claim 1. The antenna switching means continuously performs switching during standby, the synchronization obtaining means obtains synchronization during a preamble, and the frequency characteristic inspection means determines the frequency characteristic of the received signal of each switched reception signal. 7. The receiving device according to claim 6, wherein the receiving device is inspected. The receiving device according to claim 6, wherein the antenna switching period corresponds to an integral multiple of a preamble repetition period. 2. The receiving apparatus according to claim 1, wherein the frequency characteristic inspection means has a fast Fourier transform means. 10. The receiving apparatus according to claim 9, wherein said frequency characteristic inspection means shares a fast Fourier transform means as an orthogonal frequency multiplex modulation demodulation means. 2. The receiving apparatus according to claim 1, wherein the frequency characteristic inspection means includes a plurality of filters. A diversity receiving method used in an orthogonal frequency multiplexing modulation wireless system,
Receiving data modulated on a plurality of subcarriers by orthogonal frequency multiplexing modulation on a plurality of antennas,
For each signal received by the plurality of antennas, comparing received signals of a plurality of used subcarriers with a predetermined threshold, based on the comparison result, selecting an appropriate antenna,
Measuring the noise level of an unused subcarrier when selecting the appropriate antenna, and determining the predetermined threshold based on the noise level. 13. The diversity receiving method according to claim 12, wherein the antenna selection is determined within a preamble and switching is completed.

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