KR20070021242A - Method, system and station for communicating data packets using arq - Google Patents

Abstract

The present invention relates to a system (50) for communicating data packets from a first station (100) to a receiver (400) via at least one of a plurality of second stations (200) connected to a receiver (400). 200 notifies the reception rate of the data packet. The first station 100 determines whether to retransmit the data packet or proceed with the transmission of the next data packet, which decision depends on the receipt of a plurality of positive notifications or a plurality of negative notifications.

Description

Data packet communication method, communication system, first station {METHOD, SYSTEM AND STATION FOR COMMUNICATING DATA PACKETS USING ARQ}

The present invention relates to a communication system, a communication station used within the system, and a method of communicating data packets. The present invention finds particular application in mobile communications such as, for example, data transfer from a mobile station to a fixed node between a plurality of base stations, or to different receivers of a plurality of base stations during soft handover. Have

In cellular mobile communication systems, it is known to use soft handovers that allow a mobile station to communicate with one or more cells temporarily to minimize ongoing communication disruptions when the mobile station moves from one cell to another.

As a retransmission protocol used in a communication system, an ARQ (automatic repeat request) protocol is generally used. In the ARQ protocol, if the packet indicates that the packet was not received successfully by notifying or not notifying, the data packet is notified by the receiving device and retransmitted by the transmitting device.

It is necessary to use the ARQ protocol during soft handover to increase throughput and reduce delay. In this case each base station, which is in communication with the mobile station and is also referred to as the active base station, may send an acknowledgment in response to receipt of the data packet. A data packet may be sent from any active base station to a data packet destination and thus reach its recipient if the data packet is successfully received by at least one base station.

It is an object of the present invention to enable an improved retransmission protocol.

According to a first aspect of the present invention, there is provided a data packet communication method from a first station to a node device connected to a plurality of second stations, the method comprising:

 Sending a first data packet from a first station,

 Receiving the transmitted first data packet at a plurality of second stations,

 Evaluating, according to the first criterion, whether the first data packet was successfully received at each of the second stations;

 Forwarding the first data packet from at least one of the second stations where the data packet was successfully received to the node device,

 At least one of a positive acknowledgment from each second station from which the first data packet was successfully received, and a negative acknowledgment from each second station from which the first data packet was not successfully received. Transmitting,

 Receiving a notification at a first station;

 Comparing at least one of a number of positive notifications received and a first threshold and a number of received negative notifications and a second threshold, wherein at least one of the first and second thresholds is greater than an integer one; Is a large number,

 Using the result of the comparison to determine whether to retransmit the first data packet or the second data packet.

According to a second aspect of the invention, there is provided a communication system comprising a first station and a node device coupled to a plurality of second stations for communicating data packets from the first station to the node device, the first station being a first station. Transmitting means for transmitting the data packet, each of the second stations

 Means for receiving a first data packet,

 Means for evaluating whether the first data packet was successfully received according to the first criterion;

 Means for forwarding a first data packet to the node device if the first data packet has been successfully received;

 Means for sending at least one of a positive notification if the first data packet was successfully received and a negative notification if the first data packet was not successfully received,

 The node apparatus comprises means for receiving a forwarded first data packet,

 The first station

 Means for receiving notifications,

 Means for comparing at least one of a number of positive notifications received and a first threshold and a number of received negative notifications and a second threshold, wherein at least one of the first and second thresholds is greater than an integer one; Is a large number,

 Means for using the result of the comparison to determine whether to retransmit the first data packet or the second data packet.

According to a third aspect of the invention there is provided a first station for use in a communication system comprising a node device coupled to a plurality of second stations for communicating data packets from the first station to the node device, which is

 Transmission means for transmitting the first data packet to a second station,

 Means for receiving a notification from a second station,

 Means for comparing at least one of a number of positive notifications received and a first threshold and a number of received negative notifications and a second threshold, wherein at least one of the first and second thresholds is greater than an integer one; Number,

 Means for using the result of the comparison to determine whether to retransmit the first data packet or the second data packet.

Thus, according to the present invention, the first station does not determine whether to retransmit the data packet or transmit a new data packet based on only one received notification, but before making such a decision one or more of the same type of positive Or request to receive a negative notification. Using multiple second stations increases the probability that one of the notifications will be misleading and lead to a wrong decision compared to the case of using a single second station. By making a decision dependent on a plurality of notifications of the same type received from one or more second stations, the probability of making an incorrect decision is reduced. Incorrect decisions can increase delays and reduce throughput and coverage. Thus, the present invention can improve delay and enable improvement in throughput and coverage.

The determination of whether to resend may be based on the number of positive notifications or negative notifications or both. Different thresholds may be applied for the number of positive and negative notifications.

In one embodiment, the number of notifications for the initial transmission of data packets and one or more retransmissions of the same data packet is calculated and used to determine whether to retransmit the data packet or send a new data packet. do.

In an embodiment of the present invention, a positive notification is sent by the second station to indicate the successful reception rate of the first data packet, the number P of positive notifications received at the first station is counted, and the threshold T _P (T _P > 1). ). If P <T _P , retransmit the first data packet from the first station, and if P> T _P , send the second data packet from the first station.

In another embodiment of the present invention, a negative notification is sent by the second station to indicate a failed reception rate of the first data packet, the number N of negative notifications received at the first station is counted, and the threshold T _N (T _N &_gt; Compared with 1). If N &gt; T _N , retransmit the first data packet from the first station; if N &lt; T _N, then transmit the second data packet from the first station.

1 is a schematic block diagram of a communication system;

2 is a flow chart of a communication method of a data packet.

The invention will be described by way of example with reference to the accompanying drawings.

Referring to FIG. 1, a radio communication system 50 is shown that includes a plurality of second stations 200 that are connected to a first station 100 and a destination node apparatus 400. The first station 100 may be, for example, a mobile communication equipment and the second station 200 may be a base station in a mobile communication network. The term "node device" refers to any device connected to each of a plurality of second stations 200 to which the first data packet will be directed.

The first station 100 includes a transmitter 110 for sending a data packet on an uplink 310. The transmitter 110 has an output connected to the antenna 120 via a connecting means 130, which may for example be a circulation device or a changeover switch. The connecting means 130 is also connected to an input of a receiver 140 which receives a downlink 320 signal from the antenna 120. The transmitter 110 and the receiver 140 are connected to control means (μC) 150, which may be, for example, a processor for processing signals received from the second station 200. The control means 150 is connected or combined with storage means 160 for storing one or more thresholds and one or more counts of acknowledgments.

The second station 200 each includes a receiver 240 that receives a signal on the uplink 310 transmitted by the first station 100. The receiver 240 has an input connected to the antenna 220 via a connecting means 230, which can be, for example, a circulation device or a changeover switch. The connecting means 230 is also connected to the output of the transmitter 210 which transmits the downlink signal via the antenna 220. The transmitter 210 and receiver 240 may be, for example, a processor that processes data packets received from the first station 100 and obtains a notification signal sent on the downlink 320 by the transmitter 210. Control means (μC) 250 are connected.

The node device 400 includes a receiver 410 that receives a data packet transmitted by the second station 200. It also includes additional elements such as storage, transmission or processing means that act on the received data packet. These are not shown in FIG. 1 because they are not an important part of the present invention.

The operation of the radio communication system 50 will be described with reference to the flowchart of FIG. 2. In FIG. 2, the steps on the left side of the flowchart relate to the steps performed by the first station 100, and the steps on the right relate to steps performed by each of the second stations 200.

In step 510 the first station 100 transmits a first data packet via its transmitter 110 to each second station 200. Each second station 200 receiving the first data packet performs steps 520 through 550.

In step 520 a first data packet is received at its second station 200 via its receiver 240.

In step 530 the control means 250 evaluates whether the first data packet was successfully received according to the first criterion. The term "received successfully" is used to imply that the information bits contained in the data packet can be correctly determined by the receiving equipment. Such success may be the result of applying techniques such as error correction or detection, some selective retransmission of information bits, or a combination of one or more transmissions of data packets or information bits. The first criterion may take any of a variety of forms and may, for example, depend on the type of information carried by the data packet. Some examples of criteria are: cyclic redundancy checks should be implemented; All information bits must be error free; Certain information bits must be error free; Only a certain number of errors should exist; All information bits must satisfy the reliability threshold. Those skilled in the art will understand that other criteria may be used in place of or in conjunction with the examples described above.

 If successful, the control means 250 in step 540 forwards the first data packet to the node device 400 receiving the data packet via its receiver 410. In some embodiments control means 250 may deliver some or all packets that are not correctly received to node device 400.

In step 550 the control means 250 initiates the transmission of the notification by the transmitter 210. In one embodiment a positive notification is sent if the first data packet is successfully received and a negative notification is sent if the first data success is not successfully received. In another embodiment, only positive notification is sent if the first data packet is successfully received. In another embodiment, only the negative notification is sent if the first data packet is not successfully received. In another embodiment, only negative notification is sent if the first data packet is not forwarded to the node device 400.

In step 560 the first station 100 receives a notification sent by each of the second stations 200 via its receiver 140.

In step 570 control means 150 counts the received notification and stores the number in storage means 160. In one embodiment the positive notification is counted. In another embodiment, negative notifications are counted. In other embodiments, both positive and negative notifications are counted separately.

In step 580 the control means 150 compares the counted number of notifications with a threshold, wherein the threshold is greater than an integer one. The threshold may be stored and predetermined in the storage means 160 or may be received by the receiver 140 from the second station 200 or elsewhere. If both positive and negative notifications have been counted, a separate threshold may be used for each type of notification. Optionally, common thresholds may be used for both positive and negative notifications.

Depending on the value of the corresponding threshold or each count associated with the corresponding threshold, the flow returns to step 510 where the first data packet is retransmitted, or a new data packet is selected and a process is sent to the new packet. The flow proceeds from step 510 to step 590 of repeating.

If the positive notification is counted, the counted number of positive notifications P is the first threshold T _P (T _P). > 1). Retransmission of the first data packet in step 510 occurs when P &lt; T _P , and transmission of a new data packet occurs when P &gt; T _P. Thus, for example, if there are five second stations 200 sending notifications and T _P = 3, the first data packet is actually successfully transmitted by one or more second stations 200 so that the node device Although forwarded to 400, the first data packet will be retransmitted if only one or two positive notifications are received.

If the negative notification is counted, the number N of counted negative notifications is compared with the second threshold value T _N (T _N > 1). Retransmission of the first data packet in step 510 occurs when N &gt; T _N , and transmission of a new data packet occurs when N &lt; T _N. Thus, for example, if there are five second stations 200 sending notifications and T _N = 2, then only one or two negative notifications are received, the second station having three first data packets A new data packet will be sent under the assumption that it has been successfully received by 200 and delivered to node device 400.

In another embodiment, the number of positive notifications received in response to the initial transmission and retransmission of the first data packet is calculated and compared by the control means 150 with a third threshold value T _P '(T _P '> 1). do. Then if the calculated count is less than T _P ', retransmission of the first data packet occurs again, otherwise a new data packet is sent. T _P 'may be the same as or different from T _P.

In another embodiment, the number of negative notifications received in response to the initial transmission and retransmission of the first data packet is calculated and compared by the control means 150 with a third threshold value T _N '(T _N '> 1). do. Then if the calculated count exceeds T _N ', retransmission of the first data packet occurs again, otherwise a new data packet is sent. T _N ′ may be the same as or different from T _N.

In another embodiment, the sent notification may notify the received data packet prior to the most recently received data packet. This selection can improve efficiency by enabling new data to be transmitted while waiting for notification of previously transmitted data. In addition, the notification may notify one or more data packets. This choice can reduce interference by reducing the number of notifications sent.

In other embodiments, the period of time for which notifications are counted may be limited. In this case, when the negative notification is counted, if the number of counts exceeds the fifth threshold T _N '' (1 <T _N ''<T _N ), the first data packet is retransmitted at the time limit expires or When the positive notification is counted, the first data packet is retransmitted at the time point when the time limit expires if the count number does not exceed the sixth threshold T _p '' (1 <T _P ''<T _P ). This selection requires a long time to complete the count or the reception rate by the second station 200 is too low so that the first data packet received by the second station 200 becomes almost positive or negative notification. This can reduce delays in the environment.

Optionally, the control means 150 may apply a signal reliability criterion to the received notification when determining whether to retransmit the first data packet or the second data packet. The selection to apply the signal reliability criteria can be used to increase the reliability of the retransmission decision. Such signal reliability criteria may take into account, for example, received signal amplitude, signal to interference ratio, number of bit errors or bit error ratio.

For example, signal reliability criteria can be applied when counting notifications. In this case, the notification is counted only if the signal reliability of the notification meets the signal reliability criteria.

The confidence criterion may include obtaining a specific gravity coefficient, where a notification having a high expected reliability is assigned a high specific gravity coefficient and a notification having a low expected reliability assigned a low specific gravity coefficient. The counting of the notification can then include the sum of the specific gravity coefficients, which can be non-integer values. The result of the summation is then compared with the appropriate one of the thresholds to determine whether to retransmit the first data packet.

In another embodiment, the signal reliability degrees of a plurality of notifications of the same type, positive or negative, can be combined, for example by summing, the combined reliability being compared with a confidence threshold for that type.

In other embodiments, any threshold described herein may depend on the confidence frequency of the notification. For example, the confidence threshold for the positive notification may depend on the confidence frequency of the negative notification, or the confidence threshold for the negative notification may depend on the confidence frequency of the positive notification.

In a variation of the present invention, even when it is determined whether a first data packet retransmission or a second data packet transmission is determined for a single notification of each type of notification, the confidence threshold for one type of notification is dependent on the reliability count of the other type of notification. Can depend

Optionally, any threshold may be transmitted to one of the second stations 200, or any other place, to the first station 100 and stored in the storage means 160. This selection can be used to apply thresholds for prevailing system conditions such as loading.

Optionally, any threshold may be the same as its function as the number or replacement of the second station 200. This selection can be used to ensure an increase in the number of positive notifications required before the next data packet is transmitted as the positive notifications increase because of the probability of negative reads that are misread. Similarly, this selection can be used to ensure an increase in the number of negative notifications required before retransmission occurs as the probability of a correct reception rate increases. In order to count the number of second stations 200 with which the first station is communicating, the control means 150 may monitor the signal received from the second station 200.

Optionally, the notification may be sent more than once by the second station 200. This selection can be used to reduce the impact of misreading notifications.

The function of the second station 200 may be provided by distributed or aggregated means.

The articles used in front of the elements in the specification and claims do not exclude a plurality of elements. Also, the term "comprising" does not exclude devices or steps not mentioned.

Reference numerals in parentheses in the claims are for the purpose of understanding and not limitation.

From the present specification, those skilled in the art will understand that other changes are possible. Such changes may include features that may be used in place of, or in addition to, features already known to those skilled in the art of communication and already described herein.

Claims (45)

A method of communicating data packets from a first station 100 to a node apparatus 400 connected to a plurality of second stations 200,  Transmitting a first data packet from the first station 100,  Receiving the transmitted first data packet at the plurality of second stations 200;  Evaluating, according to a first criterion, whether the first data packet was successfully received at each of the second stations 200;  Forwarding the first data packet from at least one of the second stations 200 to which the data packet was successfully received, to the node apparatus 400;  Positive acknowledgment from each second station 200 in which the first data packet was successfully received, and from each second station 200 in which the first data packet was not successfully received. Sending at least one of a negative acknowledgment,  Receiving the notification at the first station 100,  -Performing at least one of a comparison of the number of received positive notifications with a first threshold and a comparison of the number of received negative notifications with a second threshold,    At least one of the first and second thresholds is a number greater than an integer one,  Determining whether to retransmit the first data packet or to transmit a second data packet using the result of the comparison. Data packet communication method. The method of claim 1, Sending at least one of the first and second thresholds to the first station 100. Data packet communication method. The method according to claim 1 or 2, At least one of the first and second thresholds depends on the number of the second stations 200 in communication with the first station 100. Data packet communication method. The method according to any one of claims 1 to 3,  Calculating a number of positive notifications received in response to a first transmission and one or more retransmissions of the first data packet and comparing the calculated number with a third threshold value;    Calculating at least one number of negative notifications received in response to a first transmission and at least one retransmission of the first data packet and comparing the calculated number with a fourth threshold, wherein    At least one of the third and fourth thresholds is a number greater than integer 1,  Using the result of the comparison to determine whether to retransmit the first data packet or to transmit a second data packet. Data packet communication method. The method of claim 4, wherein Sending at least one of the third and fourth thresholds to the first station 100. Data packet communication method. The method of claim 4, wherein At least one of the third and fourth thresholds depends on the number of the second stations 200 in communication with the first station 100. Data packet communication method. The method according to any one of claims 4 to 6, The third threshold is equal to the first threshold or the fourth threshold is equal to the second threshold Data packet communication method. The method according to any one of claims 1 to 7, Applying a signal reliability criterion to the received notification when determining whether to retransmit the first data packet or to transmit a second data packet. Data packet communication method. The method of claim 8, Applying the signal reliability criterion includes summing a degree of reliability of notifications of the same type and comparing the summation value with a confidence threshold for the type. Data packet communication method. The method of claim 9, At least one of the first and second thresholds depends on the signal reliability Data packet communication method. The method of claim 9, The type of notification includes positive and negative notifications and the reliability threshold for one of the types of notifications depends on other types of signal reliability degrees. Data packet communication method. The method according to any one of claims 1 to 11, The notification includes a positive notification, The method is Determining the number P of positive notifications received at the first station 100; Comparing the number P with a threshold value T _P (T _P > 1), Retransmitting the first data packet from the first station 100 if P <T _P ; If P ≥ T _P , further comprising transmitting a second data packet from the first station 100 Data packet communication method. The method of claim 12, Applying a signal reliability criterion when determining the number P; Data packet communication method. The method according to any one of claims 1 to 11, The notification includes a negative notification, The method is Determining the number N of negative notifications received at the first station 100; Comparing the number N with a threshold value T _N (T _N > 1), Retransmitting the first data packet from the first station 100 if N> T _N ; If N ≦ T _N , further comprising transmitting a second data packet from the first station 100. Data packet communication method. The method of claim 14, Applying a signal reliability criterion when determining the number N; Data packet communication method. The method according to any one of claims 1 to 15, The first station 100 is a mobile station and the second station 200 is a base station, The method is performed during soft handover of the mobile station between the base stations. Data packet communication method. A first station (100) and a node device (400), wherein the first station (100) and the node device (400) communicate data packets from the first station (100) to the node device (400). In the communication system 50 is connected to a plurality of second stations 200, The first station 100 comprises a transmitting means 110 for transmitting a first data packet, Each of the second stations 200  Means (240) for receiving said first data packet,  Means (250) for evaluating whether the first data packet was successfully received according to a first criterion;  Means (250) for forwarding the first data packet to the node device (400) if the first data packet has been successfully received;  Means for transmitting at least one of a positive notification when the first data packet was successfully received and a negative notification when the first data packet was not successfully received, The node apparatus 400 comprises means 410 for receiving the forwarded first data packet, The first station 100  Means (140) for receiving said notification,  Means (150) for comparing at least one of a number of positive notifications received and a first threshold and a number of negative notifications received and a second threshold,    At least one of the first and second thresholds is a number greater than an integer one,  Means (150) for using the result of said comparison to determine whether to retransmit said first data packet or to transmit a second data packet. Communication system (50). The method of claim 17, The first station 100 further comprises means 140 and 150 for receiving the transmitted value of at least one of the first and second thresholds. Communication system (50). The method of claim 17 or 18, The first station 100 selects means 140 for selecting at least one of the first and second threshold values depending on the number of the second stations 200 in communication with the first station 100. More including 150) Communication system (50). The method according to any one of claims 17 to 19, The first station 100  Calculating the number of the positive notifications received in response to the first transmission and one or more retransmissions of the first data packet and comparing the calculated number with a third threshold;    Means for performing at least one of calculating the number of said negative notifications received in response to a first transmission and one or more retransmissions of said first data packet and comparing said calculated number with a fourth threshold value (150) ),    At least one of the third and fourth thresholds is a number greater than integer 1,  Means (150) for using the result of said comparison to determine whether to retransmit said first data packet or to transmit a second data packet. Communication system (50). The method of claim 20, A first station (100) further comprising means (140, 150) for receiving the originated value of at least one of the third and fourth thresholds; Communication system (50). The method of claim 20 or 21, The first station 100 selects (140) at least one of the third and fourth threshold values depending on the number of the second stations 200 in communication with the first station 100; More including 150) Communication system (50). The method according to any one of claims 17 to 22, Means 150 for applying signal reliability criteria to the received notification when determining whether to retransmit the first data packet or to transmit a second data packet. Communication system (50). The method of claim 23, Applying the signal reliability criterion includes summing reliability degrees of notification of the same type and comparing the summed value with a confidence threshold for the type. Communication system (50). The method of claim 24, At least one of the first and second thresholds depends on the signal reliability Communication system (50). The method of claim 24, The type of notification includes positive and negative notifications and the reliability threshold for one of the types of notifications depends on other types of signal reliability degrees. Communication system (50). The method according to any one of claims 17 to 26, The second station 200 comprises means 250, 210 to send a positive notification, The first station 100  Means (150) for determining the number of received positive notifications (P), Means (150) for comparing said number P to a threshold value T _P (T _P > 1), Means (150, 110) for retransmitting the first data packet if P &lt; T _P and transmitting a second data packet if P &gt; T _P; Communication system (50). The method of claim 27, The first station 100 further comprises means 150 for applying a signal reliability criterion in determining the number P. Communication system (50). The method according to any one of claims 17 to 26, The second station 200 comprises means 250, 210 to send a negative notification, The first station 100  Means (150) for determining the number N of received negative notifications, Means (150) for comparing said number N to a threshold value T _N (T _N > 1), - if N> T _N is the first retransmission data packet and if the N T ≤ _N further includes a means (150, 110) for sending a second data packet Communication system (50). The method of claim 29, The first station 100 further comprises means 150 for applying a signal reliability criterion when determining the number N. Communication system (50). First station 100 used in a communication system 50 including the node device 400 coupled to a plurality of second stations 200 for communicating data packets from the first station 100 to the node device 400. ), The first station 100  Transmission means (110) for transmitting a first data packet to the second station (200);  Means (140) for receiving a notification from the second station (200),  Means (150) for comparing at least one of a number of positive notifications received and a first threshold and a number of negative notifications received and a second threshold,    At least one of the first and second thresholds is a number greater than an integer one,  Means (150) for using the result of said comparison to determine whether to retransmit said first data packet or to transmit a second data packet. First station 100. The method of claim 31, wherein Means for receiving the originated value of at least one of the first and second thresholds (140, 150) First station 100. The method of claim 31 or 32, Means (140, 150) for selecting at least one of said first and second threshold values depending on the number of said second stations (200) in communication with said first station (100). First station 100. The method according to any one of claims 31 to 33.  Calculating the number of the positive notifications received in response to the first transmission of the first data packet and comparing the calculated number with a third threshold;    Means (150) for performing at least one of calculating the number of said negative notifications received in response to a first transmission of said first data packet and comparing said calculated number with a fourth threshold,    At least one of the third and fourth thresholds is a number greater than integer 1,  Means (150) for using the result of said comparison to determine whether to retransmit said first data packet or to transmit a second data packet. First station 100. The method of claim 34, wherein Means (140, 150) for receiving the sent value of at least one of the third and fourth thresholds; First station 100. The method of claim 34 or 35, Means (140, 150) for selecting at least one of said third and fourth threshold values depending on the number of said second stations (200) in communication with said first station (100). First station 100. The method according to any one of claims 34 to 36, The third threshold is equal to the first threshold or the fourth threshold is equal to the second threshold First station 100. The method according to any one of claims 31 to 37, Means 150 for applying signal reliability criteria to the received notification when determining whether to retransmit the first data packet or to transmit a second data packet. First station 100. The method of claim 38, Applying the signal reliability criterion includes summing reliability degrees of notification of the same type and comparing the summed value with a confidence threshold for the type. First station 100. The method of claim 39, At least one of the first and second thresholds depends on the signal reliability First station 100. The method of claim 39, The type of notification includes positive and negative notifications and the reliability threshold for one of the types of notifications depends on other types of signal reliability degrees. First station 100. The method according to any one of claims 31 to 41,  Means (150) for determining the number of received positive notifications (P), Means (150) for comparing said number P to a threshold value T _P (T _P > 1), Means (150, 110) for retransmitting the first data packet if P &lt; T _P and transmitting a second data packet if P &gt; T _P; First station 100. The method of claim 42, Means (150) for applying a signal reliability criterion when determining the number P; First station 100. The method according to any one of claims 31 to 41,  Means (150) for determining the number N of received negative notifications, Means (150) for comparing said number N to a threshold value T _N (T _N > 1), - if N> T _N is the first retransmission data packet and if the N T ≤ _N further includes a means (150, 110) for sending a second data packet First station 100. The method of claim 44, Means 150 for applying a signal reliability criterion when determining the number N; First station 100.

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