JP4683359B2 - Packet communication system - Google Patents

Description

  The present invention relates to a wireless packet communication system.

In wireless packet communication, packets, which are collections of data, are formed into frames and transmitted and received as radio waves. In this wireless packet communication, due to fading, when the signal strength is large and the communication state is good, and when the signal strength is small and the communication state is bad, there is a period in which the frame length is large in the transmission characteristics. Affect. In other words, if the frame length is long compared to the average period when the signal strength due to fading appears, it will be encountered once when the signal strength is low during the transmission of one frame. Sometimes the frame becomes an error. On the other hand, if the frame length is short, the amount of information per frame is reduced, but the number of frames that are not encountered when the signal strength is low increases and communication is possible.
Therefore, when there is a periodic change in signal strength due to fading, etc., the error rate changes depending on the length of the frame. To obtain good transmission characteristics, adjust the length of the frame. Can be an effective means.

In order to adjust the length of the frame in order to obtain good transmission characteristics as described above, the waveform of the received electric field strength of the wireless signal, the time length of the frame, and the correctness of the frame are graphed to form one time axis. Monitors that can be simultaneously displayed on top have been proposed in Japanese Patent Laid-Open No. 2001-339473 (Patent Document 1) and US Patent Publication 2001-042222 (Patent Document 2).
According to this monitor, it is possible to visualize the time length of the frame, the error occurrence status, and the received electric field strength in time series. Therefore, it is possible to recognize the level of the received electric field strength at which the frame becomes an error and the average period when the received electric field intensity is large and small, and to estimate an appropriate frame length based on this period.
JP 2001-339473 A US Patent Publication No. 2001-046222

According to the monitor described above, it is possible to recognize an appropriate frame length because it can recognize the level of the received electric field strength at which the frame becomes an error and the average period when the received electric field strength is large and small. . However, the monitor described above has a problem that an appropriate frame length is not automatically determined and must be determined by an observer of the monitor.
The present invention has been made in view of such a problem. In a wireless packet communication system, an appropriate frame length is automatically determined when determining an appropriate frame length to obtain good transmission characteristics. An object of the present invention is to provide a packet communication system that can perform the above.

In order to solve the above-mentioned problems, the present invention, in a wireless packet communication system, received means for recording received electric field strength in a time series while receiving a frame, and an error byte or error bit in the frame. Means for obtaining the time and the received electric field strength at that time, the time of receiving the error byte or the error bit in the frame and the data in which the received electric field strength during frame reception is recorded in time series and the received electric field strength at that time Means for determining a threshold value of received electric field strength at which a frame is normally received based on data; timing means for obtaining a time during which the received electric field strength is continuously held above the threshold; and received electric field strength at or above the threshold value And a packet communication system characterized by comprising: means for determining a frame length based on a continuously held time; It is intended.

In addition, the packet communication system is configured to calculate the error byte or the error byte based on the minimum received electric field strength value during the normal byte or normal bit reception in the period from the start of error frame reception to immediately before the error byte or error bit is received. If the received field strength value at the time of receiving the error bit is smaller, it is determined that the error byte or the error bit is caused by fading. The minimum received electric field strength value until immediately before receiving an error byte or error bit can be determined as the threshold value.
Alternatively, in the packet communication system, the minimum received field strength value during the reception of normal bytes and the minimum received field strength during the reception of error bytes between the start of reception of error frames and the end of reception. If the minimum received field strength value while receiving the error byte is smaller than the minimum received field strength value while receiving the normal byte as a result of the comparison with the value, Until the end of reception, the minimum received electric field strength value while receiving normal bytes can be determined as the threshold value.

  According to the packet communication system of the present invention, a time during which a received electric field strength equal to or higher than a threshold at which a frame can be normally received is continuously maintained is detected, and an appropriate frame length is automatically determined based on the detected time. Can be determined.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

(First embodiment)
First, a first embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing the overall configuration of a packet communication system according to the present invention. That is, the packet communication system according to the present invention includes a wireless unit 1 that controls a wireless line, a signal control unit 2 that controls a signal, a computer 7, and a monitor 8. A communication protocol used in this packet communication system is a frame synchronization method such as HDLC (High Level Data Link Control Procedures). In addition, the transmission speeds on the transmission side and the reception side match.

  In the packet communication system, the wireless unit 1 includes a receiving unit 1a having means for receiving wireless signals and a transmitting unit 1b having means for transmitting wireless signals. The signal control unit 2 includes a modem 3 and a frame processing unit 6. The modem 3 includes a demodulator 4 and a modulator 5. The frame processing unit 6 is, for example, a PAD (Packet Assembly Dissembly). Such a frame assembling / disassembling means (not shown), a frame inspection means (not shown), and a frame control means (not shown) are provided. FIG. 2 is a block diagram showing the configuration of the monitor 8. As shown in FIG. 2, the monitor 8 includes a field intensity detector 21, a flag detector 22, a frame determination unit 23, a control circuit (D) 24, a control circuit (E) 25, a counter 26, and memories 27, 28 and 29. I have.

FIG. 3 is a diagram showing an example of a frame used in the packet communication system of the present invention. As shown in FIG. 3, an I frame (information frame: Information Frame) 31 for sending information includes a start flag, address, control, PID (packet identifier: Packet IDentifier), information, FCS (Frame Check Sequence: Frame Check Sequence). , And an end flag, and the information has a variable length. In general, a packet means an information part in the I frame 31.
In the packet communication system, data transparency is guaranteed, and a CRC (Cyclic Redundancy Check Character) is used as an error detection code employed in the frame check sequence.

Next, an operation for automatically determining an appropriate frame length by the packet communication system according to the present invention will be described.
First, in the packet communication system on the transmission side, data input to the computer 7 is assembled into a frame by the frame processing unit 6, modulated by the modulator 5, and transmitted wirelessly by the transmission unit 1b.
The signal of the frame transmitted by radio is received by the receiving unit 1a of the packet communication system on the receiving side. Therefore, the received electric field strength S while the signal is being received is measured by the electric field strength detector 21 at a predetermined sampling interval and detected as a digital value. The received electric field strength S and the count value C1 of the counter 26 at the time when the received electric field strength S is detected are continuously recorded in the memory 27 by the control circuit (D) 24. Here, the counter 26 is a clock counter provided with a clock means that increases a predetermined count number every predetermined time, and the count value output from the counter 26 indicates time or time. For example, if the count value of the counter 26 is incremented by 1 at regular intervals, and the received electric field strength S is detected at a timing when the count value increases by 1, the data recorded in the memory 27 is shown in FIG. It looks like the table 41.

フレーム長Ｌ１は，フェージングによって信号強度が小さくなる周期の平均値に対して十分短いため，通信を可能とするために適切なフレーム長となる。このフレーム長Ｌ１は時間長であるが，例えばこれがバイト長又はビット長に変換され，このバイト長又はビット長の情報が送信側にフィードバックされて，以降の通信のフレーム長として設定される。The electric field strength detector 21 determines whether the value of the received electric field strength S is greater than or less than the threshold value Sk, the count value C2 of the counter 26 when the received electric field strength S is greater than or less than the threshold value Sk, and the threshold value. The count value C3 of the counter 26 when it becomes less than Sk or more is recorded in the memory 27 (see the table 42 in FIG. 4). Here, the threshold value Sk is the minimum received electric field strength at which a frame can be normally received, and a method for determining the threshold value Sk will be described later.
Data recorded in the memory 27 is transferred to a storage device of the computer 7.
Based on the data in the table 42 transferred to the storage device, the computer 7 calculates the time during which the received electric field intensity equal to or higher than the threshold Sk at which the frame can be normally received is continuously held. That is, as shown in Equation 1, the value obtained by subtracting the count value C2 from the count value C3 is regarded as the time T1 during which the received electric field strength at which the frame can be normally received is held.
T1 = C3-C2 (Formula 1)
Next, the computer 7 obtains an average value of the plurality of calculated times T1, and determines the frame length so as to be shorter than the average value of the times T1. For example, as shown in Expression 2, a value obtained by dividing the average value of the time T1 by 10 is set as the frame length L1.

Since the frame length L1 is sufficiently short with respect to the average value of the period in which the signal intensity decreases due to fading, the frame length L1 is an appropriate frame length to enable communication. This frame length L1 is a time length. For example, this is converted into a byte length or a bit length, and this byte length or bit length information is fed back to the transmission side and set as a frame length for subsequent communication.

Next, a method for determining the threshold value Sk will be described.
As described above, when the signal of the frame transmitted by radio is received in the packet communication system on the receiving side, the received electric field strength S while the signal is received and the received electric field strength S when the received electric field strength S is detected. The count value C1 is continuously recorded in the memory 27 in time series. On the other hand, the flag detector 22 takes in serial data obtained by digitally demodulating the frame signal by the demodulator 4 while sequentially performing frame synchronization control, and detects a start flag and an end flag of the frame. The control circuit (E) 25 detects the time when the flag detector 22 detects the start flag of the frame and the time when the flag end flag is detected as the count value of the counter 26. Further, based on the count value, the control circuit (E) 25 records a count value C4 indicating the frame reception start time and a count value C5 indicating the frame reception end time in the memory 28 (see the table 43 in FIG. 4). ). Further, the frame determination unit 23, based on the frame inspection result performed by the frame processing unit 6, the frame arrival number r, correct / incorrect data N (r) indicating whether the frame is normal or error, and the frame Is stored in the memory 29 (see table 44 in FIG. 4). The correct / incorrect data N (r) represents, for example, “0” for normality and “1” for error.
Then, the data recorded in the memory 27, the memory 28, and the memory 29 is transferred to the storage device of the computer 7.

The computer 7 detects an error frame from the correct / incorrect data N (r) of the frame transferred to the storage device, and detects a normal retransmission frame retransmitted for the error frame. Next, the computer 7 compares the error frame with the normal retransmission frame based on the frame data F (r), specifies the error byte in the error frame, and further obtains the time when the error byte is received. The time when the error byte is received can be obtained by the following procedure.
First, it is detected what numbered byte the specified error byte is counted from the beginning of the frame (excluding the start flag). That is, the number of bytes (including the specified error byte) from the beginning of the frame to the specified error byte is detected. The number of bytes from the beginning of the frame to the specified error byte is referred to as error byte number By (n). Here, the subscript n indicates the serial number of the error byte for each frame.
Next, based on data in the table 43 (see FIG. 4) and the table 44 (see FIG. 4), a count value C4 that is the reception start time of the error frame is detected. Based on the count value C4 that is the reception start time of the error frame, the count value Tby that is the reception time per byte, and the error byte number By (n), the error byte is received according to Equation 3. A count value Cby (n) indicating the time is calculated (see FIG. 5). That is, the error byte number By (n) is multiplied by the count value Tby to obtain a time value from the start of frame reception until the error byte is received, and the count value Cby (n) is obtained by adding the count value C4 to this time value. Can be requested.
Here, the count value Tby, which is the reception time per byte, is the count increase number of the counter 26 during the time for receiving the 1-byte signal (Tby = 8 in this embodiment). The count value Cby (n) corresponds to the count value indicated by the counter 26 when an error byte is received.
Cby (n) = By (n) × Tby + C4 (Formula 3)

  By the above-described method, the count value Cby (n) when all error bytes are received is obtained for each error frame. Then, a table 45 (see FIG. 4) that records the count value C4 and count value C5 for each error frame, and a table 46 (FIG. 4) that records the count value Cby (n) at the time of error byte reception for each error frame. Is stored in the storage device of the computer 7. Further, the number of error bytes BY in one frame for each error frame is stored in the storage device of the computer 7.

The computer 7 determines the threshold value Sk based on the data in the table 41, the table 45, and the table 46. In the first embodiment, the received electric field strength when an error byte is received from the minimum received electric field strength value while receiving a normal byte in the period from the start of error frame reception to immediately before receiving the error byte. If the value is smaller, it is determined that the error byte is due to fading. Then, the minimum received electric field strength value from the start of error frame reception to immediately before receiving the first error byte due to fading is set as the threshold value Sk.
A flowchart A of FIG. 6 is a flowchart showing a procedure for determining the threshold value Sk in the present embodiment. Hereinafter, the procedure for determining the threshold value Sk will be described in more detail with reference to the flowchart A.

  In the flowchart A, first, in step 61, the subscript n is set to 1 in order to refer to the count value Cby (1) corresponding to the reception time of the error byte first having an error from the start of error frame reception. Is substituted. Next, in step 62, the count value C4 corresponding to the reception start time of the error frame is substituted for the count value t, and the process proceeds to step 63. In step 63, the received electric field strength S (t) at the count value t is substituted for Smin, and the process proceeds to step 64.

  Next, at step 64, the count value t is increased by the count value Tby, and the process proceeds to step 65 where the count value t and the count value Cby (n) are compared. If the count value t is smaller than the count value Cby (n), Smin is compared with the received electric field strength S (t) at the count value t (step 66). As a result, if Smin is equal to or less than S (t), the process directly returns to step 64. If Smin is not equal to or less than S (t), the process proceeds to step 67, and S (t) is substituted for Smin. Return to. While the count value t is smaller than the count value Cby (n), steps 64 to 67 are repeated. In this way, the minimum received electric field strength value Smin in the period from the error frame reception start time (count value C4) to immediately before the error frame error byte reception time (count value Cby (n)) is detected.

In step 65, when the count value t reaches the count value Cby (n), the received electric field strength S (t) at the count value t at that time (equal to the error byte reception time) and immediately before the count value t. Are compared with the minimum received electric field strength value Smin (step 68). In step 68, if Smin is not equal to or less than S (t), the minimum received electric field strength value Smin is substituted for the threshold value Sk (step 69). That is, the error byte received at the count value t is determined to be the first error byte due to fading from the start of error frame reception, and the time when the error byte was received from the start of error frame reception ( The minimum received electric field strength value Smin in the period immediately before the count value Cby (n)) is determined as the threshold value Sk that is the limit received electric field strength at which a signal can be normally received.
When Smin is equal to or less than S (t), the error byte received at the count value t is regarded as an error due to a cause other than fading and is processed in the same manner as a normal byte. In step 70, the subscript n is incremented by 1, and the count value Cby (n) of the next error byte is referred to.

The above steps are repeated until the threshold value Sk is determined. If the threshold value Sk is not determined before n exceeds the number of error bytes BY in one frame, it is determined that there is no error due to fading in the error frame, and the next error frame is referenced (step 71-step 72).
The threshold value Sk is determined by the above procedure. In the flowchart A of the first embodiment described above, one threshold value Sk is determined and the process ends. However, a plurality of threshold values Sk may be obtained from a plurality of error frames. In that case, any one of the maximum value, the minimum value, and the average value of the plurality of threshold values Sk can be determined as the final threshold value Sk.

  An appropriate frame length can be automatically determined by the procedure described above. That is, the minimum received electric field strength at which the frame does not cause an error due to fading is determined as the threshold value Sk, and an appropriate frame length is automatically set based on the time that the received electric field strength equal to or higher than the threshold value Sk is continuously held. Can be determined.

(Second Embodiment)
The procedure for determining the threshold value Sk in the first embodiment is performed in units of bytes, but it is also possible to process them all in units of bits. That is, in the first embodiment, the error is processed in units of bytes, but the threshold value Sk can be determined based on the data in which the errors are processed in units of bits.

Hereinafter, a second embodiment in which processing is performed in units of bits in the procedure for determining the threshold value Sk will be described.
Based on the frame data F (r), the computer 7 compares the error frame with the normal retransmission frame, specifies the error bit in the error frame, and obtains the time when the error bit is received. The time when the error bit is received can be obtained by the following procedure.
First, it is detected how many bits from the head of the frame (excluding the start flag) the identified error bit is. That is, the number of bits from the beginning of the frame to the specified error bit (including the specified error bit) is detected. The number of bits from the beginning of the frame to the specified error bit is referred to as error bit number B (n). Here, the subscript n indicates the serial number of the error bit for each frame. Next, based on data in the table 43 (see FIG. 4) and the table 44 (see FIG. 4), a count value C4 that is the reception start time of the error frame is detected. Based on the count value C4 that is the frame reception start time of the error frame, the count value Tb that is the reception time per bit, and the error bit number B (n), the error bit is received according to Equation 4. A count value Cb (n) indicating the measured time is calculated (see FIG. 5). That is, the error bit number B (n) is multiplied by the count value Tb to obtain a time value from the beginning of the error frame to the error bit, and this time value is added to the count value C4 to indicate the time when the error bit was received. The count value Cb (n) can be obtained.
Here, the count value Tb, which is the reception time per bit, is the count increment of the counter 26 during the time of receiving the 1-bit signal (Tb = 1 in this embodiment). The count value Cb (n) corresponds to the count value indicated by the counter 26 when an error bit is received.
Cb (n) = B (n) × Tb + C4 (Formula 4)

By the method described above, the count value Cb (n) at the time of receiving all error bits is obtained for each error frame and stored in the storage device of the computer 7 (see table 47 in FIG. 4). Further, the number of error bits BN in one frame for each error frame is stored in the storage device of the computer 7.
The computer 7 determines the threshold value Sk by the procedure shown in the flowchart A based on the data in the tables 41, 45, and 47. When processing in bit units, “t ← t + Tby” in step 64 in flowchart A is replaced with “t ← t + Tb”, and “t <Cby (n)” in step 65 is replaced with “t <Cb (n)”. , “N> BY” in step 71 is replaced with “n> BN”.

(Third embodiment)
Next, a third embodiment using another method in the method for determining the threshold value Sk of the first embodiment will be described. In the third embodiment, the minimum received electric field strength value during reception of normal bytes and the minimum received electric field strength value during reception of error bytes between the start of reception of error frames and the end of reception. Are compared. As a result of comparison, if the minimum received field strength value while receiving an error byte is smaller than the minimum received field strength value while receiving a normal byte, an error due to fading exists in the error frame. It is judged. Then, the minimum received electric field strength value while receiving normal bytes is determined as the threshold value Sk.
Hereinafter, the method for determining the threshold value Sk of the third embodiment will be described in more detail with reference to the flowchart B (see FIG. 7), the flowchart C (see FIG. 8), and the flowchart D (see FIG. 9).

  A flowchart B in FIG. 7 is a flowchart showing a procedure for detecting the minimum received electric field strength value Smin in the normal byte reception period in the error frame. The flow of this flowchart B will be described below. In the flowchart B, first, in step 81, 1 is assigned to the subscript n in order to refer to the first error byte of the error frame. Next, at step 82, the count value C4 corresponding to the reception start time of the error frame is substituted for the count value t, and the process proceeds to step 83. In step 83, the received electric field strength S (t) at the count value t is substituted for Smin, and the process proceeds to step 84.

  In step 84, the count value t is increased by the count value Tby, and the process proceeds to step 85, where the count value t and the count value Cby (n) are compared. When the count value t is smaller than the count value Cby (n), Smin is compared with the received electric field strength S (t) at the count value t (step 86). If it is determined in step 86 that Smin is equal to or less than S (t), the process directly returns to step 84. If Smin is not equal to or less than S (t), the process proceeds to step 87 where the received electric field strength S (t ) Is substituted for Smin, and the process returns to step 84.

  If the count value t reaches the count value Cby (n) in step 85, it is determined whether n is equal to BY (the number of error bytes in one frame) (step 88). If n is not equal to BY, step 84 to step 89 are repeated until n becomes equal to BY while increasing n by 1 in step 89.

  In step 88, if n is equal to BY, the process proceeds to step 90, and the count value t is increased by the count value Tby before proceeding to step 91. In step 91, the count value t is compared with the count value C5 corresponding to the reception end time of the error frame. If the count value t is less than or equal to the count value C5, Smin is compared with the received electric field strength S (t) at the count value t (step 92). If it is determined in step 92 that Smin is equal to or less than S (t), the process directly returns to step 90. If Smin is not equal to or less than S (t), the process proceeds to step 93 where the received electric field strength S (t ) Is substituted for Smin, and the process returns to step 90. Step 90 to step 93 are repeated until the count value t reaches the count value C5.

  According to the flow of the flowchart B described above, the minimum received electric field strength value Smin in the normal byte reception period in the error frame is detected. If the count value t exceeds the count value C5 in step 91, the process proceeds to step 94 of the flowchart C (see FIG. 8).

  A flowchart C in FIG. 8 is a flowchart showing a procedure for detecting the minimum received electric field strength value SEmin in the error byte reception period in the error frame. The flow of this flowchart C will be described below.

In the flowchart C, first, in step 94, 1 is assigned to the subscript n in order to refer to the count value Cby (1) corresponding to the first error byte reception time of the error frame. In step 95, the count value Cby (n) is substituted for the count value t, and the process proceeds to step 96, where the received electric field strength S (t) at the count value t is substituted for SEmin.
Next, at step 97, n is incremented by 1, and the routine proceeds to step 98 where n and BY are compared. When n is not larger than BY, the routine proceeds to step 99, where the count value Cby (n) is substituted for the count value t. Next, the routine proceeds to step 100, where SEmin is compared with the received electric field strength S (t) at the count value t. If SEmin is equal to or less than S (t), the process directly returns to step 97. If SEmin is not equal to or less than S (t), the process proceeds to step 101, and the received electric field strength S (t) at the count value t becomes SEmin. After being substituted, the process returns to step 97.

  According to the flow of the flowchart C described above, the minimum received electric field strength value SEmin in the error byte reception period in the error frame is detected. In step 98, if n is larger than BY, the process proceeds to step 102 of the flowchart D (see FIG. 9).

The flowchart D of FIG. 9 is a process of determining the threshold value Sk based on the minimum received electric field strength value SEmin in the period in which the error byte was received and the minimum received electric field strength value Smin in the period in which the normal byte was received. It is a flowchart which shows a flow. In step 102 of the flowchart D, SEmin and Smin are compared. If SEmin is smaller than Smin, it is determined that there is an error byte generated by fading in the error frame, and Smin is substituted for threshold value Sk (step 103). If SEmin is not smaller than Smin, it is determined that there is no error byte generated by fading in the error frame, and the next error frame is referred to (step 104). The above steps are repeated until the threshold value Sk is determined.
In the flowchart of the third embodiment described above, one threshold value Sk is determined and the process ends. However, a plurality of threshold values Sk may be obtained from a plurality of error frames. In that case, any one of the maximum value, the minimum value, and the average value of the plurality of threshold values Sk can be determined as the final threshold value Sk.

  FIG. 10 is a diagram illustrating the relationship between the threshold value of the received electric field strength, the received electric field strength, and the frame error.

It is a block diagram which shows the whole structure of one Embodiment by this invention. It is a block diagram which shows one Embodiment of the monitor part of the packet communication system in this invention. It is a figure which shows an example of the frame structure used in this invention. It is a figure which shows the example of the table which recorded each data in this invention. It is explanatory drawing of the method of calculating a count value when an error byte and an error bit are received in an error frame. It is a flowchart which shows the flow of the process which determines threshold value Sk in the 1st Embodiment of this invention. It is a flowchart which shows the flow of the process which detects the minimum received electric field strength value Smin in the receiving period of a normal byte in the 3rd Embodiment of this invention. It is a flowchart which shows the flow of the process which detects the minimum received electric field strength value SEmin in the receiving period of an error byte in the 3rd Embodiment of this invention. It is a flowchart which shows the flow of the process which determines threshold value Sk based on SEmin and Smin in the 3rd Embodiment of this invention. It is a figure which shows the relationship between the received electric field strength, the threshold value of received electric field strength, and the frame error.

Explanation of symbols

1: wireless unit, 1a: receiving unit, 1b: transmitting unit, 2: signal control unit, 3: modem 4: demodulator, 5: modulator, 6: frame processing unit, 7: computer 8: monitor 21: electric field strength Detector: 22: Flag detector, 23: Frame determiner 24: Control circuit (D), 25: Control circuit (E), 26: Counter (clock counter)
27, 28, 29: Memory, 31: I frame 41, 42, 43, 44, 45, 46, 47: Example of table stored in computer storage device

Claims (3)

In a wireless packet communication system, means for recording received electric field strength in a time series while receiving a frame, means for obtaining an error byte or error bit in a frame and a received electric field strength at that time, reception frame based on the received field strength time of receiving the error byte or an error bit in the recorded data and the frame in time series and the data of the received field strength at that time while receiving a frame is received correctly Means for determining a threshold value of the electric field strength, timing means for obtaining a time during which the received electric field strength equal to or higher than the threshold value is continuously held, and a frame based on the time during which the received electric field strength equal to or higher than the threshold value is continuously held. A packet communication system comprising: means for determining a length; Means for determining the threshold value, the minimum received signal strength value while receiving a normal byte or normal bit period until immediately before receiving the error byte or an error bit from the start of reception of an error frame, the error bars site or If the received field strength value at the time of receiving the error bit is smaller, it is determined that the error byte or the error bit is caused by fading. 2. The packet communication system according to claim 1, wherein a minimum received electric field strength value until immediately before receiving an error byte or error bit is determined as the threshold value. The means for determining the threshold value includes a minimum received electric field strength value during reception of normal bytes and a minimum received electric field strength during reception of error bytes between the start of reception of error frames and the end of reception. If the minimum received field strength value while receiving the error byte is smaller than the minimum received field strength value while receiving the normal byte as a result of the comparison with the value, 2. The packet communication system according to claim 1, wherein a minimum received electric field strength value during reception of normal bytes is determined as the threshold value until reception ends.

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