KR100810974B1 - A method of modeling a cdma cellular telephone system, a computer implemented system and an apparatus for enhancing performance of a base station - Google Patents

Abstract

The computer-implemented process of calculating the interference level at each location using the actual received signal level data collected at adjacent locations covering the entire CDMA system determines the base station that is most likely to communicate with the mobile unit at each particular location. Collecting a list of neighbor base stations for each base station at each location throughout the system, determining the transmit power required for each base station to communicate with the mobile unit at each location, and calculating the interference level at each base station, Determine the transmit power required for the mobile unit to communicate with the base station that is most likely to communicate with the mobile unit at that particular location.

Description

A METHOD OF MODELING A CDMA CELLULAR TELEPHONE SYSTEM, A COMPUTER IMPLEMENTED SYSTEM AND AN APPARATUS FOR ENHANCING PERFORMANCE OF A BASE STATION}

1 is a partial view of a CDMA cellular telephone system.

2 is a diagram illustrating drive testing for collecting data in a CDMA cellular telephone system.

3 illustrates a handoff category of a CDMA cellular telephone system.

4 is a probability curve diagram used to evaluate the probability that a base station will provide a received signal having a specific ratio of signal to interference levels and serving a specific location in a CDMA cellular telephone system.

5 is a probability diagram illustrating the probability of receiving a signal from each group of likely base stations at a location within a CDMA cellular telephone system.

6 is a process diagram in accordance with the present invention.

7 is a detailed view of some of the processes of FIG.

8 is a mobile unit adapted to collect data to be used to perform the process of the present invention.

FIELD OF THE INVENTION The present invention relates to cellular telephone systems and, more particularly, to a process for modeling code division multiple access (CDMA) cellular telephone systems.

Currently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) that transmit and receive signals to and from mobile units within their communication area. In a CDMA system, each base station communicates with a mobile unit by making digital transmissions over the same frequency spectrum. For most cellular systems, especially cellular systems with cells in urban areas carrying heavy traffic, each support station may be further divided into two or three sectors, each with its own transmission equipment with 180 or 120 degrees of antenna coverage. Can be. The term base station can be used herein to refer to sectors and cells.

CDMA systems send messages digitally. All transmissions in a CDMA system are on the same frequency spectrum, so the digital signals that make up each message must somehow be recognized from all available transmissions. To achieve this, the digital message is encoded by a series of overlapping digital codes. One of these codes, called a pseudo random noise (PN) code, is applied to all transmissions through the CDMA system. The PN code is used to encode each bit of the original message at the transmitter and to decode the encoded message at the receiver. To recognize a message from a particular base station, each base station begins to encode the transmission using that PN code using some repetitive initial time and a separate time offset (PN offset). Thus, one base station can initiate encoding transmission at its initial time, the second base station can initiate encoding transmission at one unit of offset from its initial time, and the third base station can have two units from the initial time. Encoding transmission can be initiated at an offset, up to a total of 512 offsets.

In addition, each transmission between the mobile unit and the base station is efficiently located on a separate channel by further encoding the transmission with one of a plurality of Walsh codes. Messages encoded by Walsh codes, such as PN codes, can only be decoded by the same Walsh code at the receiver. Thus, the encoding transmission on a particular channel is decoded by applying a mask comprising a Walsh code and a PN code to the reception pattern of information bits starting at the PN offset specified for the particular base station.

The base station typically has 64 Walsh codes that can be used to define the channel on which the mobile unit and the transmission can be established. Certain such channels are pre-assigned to function as control channels. For example, to inform the mobile unit that a particular PN offset was used, each base station constantly broadcasts a PN code using the assigned PN offset on one of the channels (pilot channels) defined by the Walsh code. The mobile unit monitors this pre-allocated pilot channel. When the mobile unit finds an offset that the pilot can decode, the mobile unit determines the initial time with reference to another control channel (synchronous channel) and then identifies the PN offset of the base station. Each system also maintains a paging channel through which an indication that a new message is arriving. A total of nine channels are provided by this and other control functions.

In order for a mobile unit to transmit and receive telephony communications while moving over a wide geographic area, each base station is typically physically located such that its coverage area is adjacent and overlaps with the coverage areas of a plurality of other base stations. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communication with that mobile unit is transmitted (hand off) from one base station to another base station in an area where the coverage of other base stations overlaps.

In most other kinds of cellular communication systems, the mobile unit communicates with only one base station at a time. However, because CDMA systems all transfer on the same frequency spectrum, the mobile unit can use all the information within that range. However, the mobile unit only decodes the information on the PN offset and Walsh code channel related thereto. The CDMA mobile unit utilizes a receiver capable of simultaneously applying a plurality of decoding masks at different offsets of the full spectrum of information it receives. Currently, the mobile receiver can decode up to six PN offsets simultaneously. In general, however, only three PN offsets are used to decode the message, and the other offsets decode the control information. Since in a CDMA system the mobile unit can receive the same information from multiple different base stations at the same instant, the mobile unit can be used to send a single message sent simultaneously to the mobile unit from multiple different base stations using different PN offsets and Walsh codes. The information can be decoded and the information combined to produce a single output message. Thus, while the signal transmitted from one base station can be faded, the same message can be received from the other base station with moderate strength. This allows the CDMA system to offer the possibility of significantly improved transmission. The situation in which a plurality of base stations and mobiles can communicate at the same time is called "soft handoff."

In order for a system operator to intelligently allocate resources to a cellular telephone system, the operator typically models the system. In order to take advantage of CDMA technology, the operator must be able to accurately model the system. However, since a CDMA system can include multiple base stations, each communicating simultaneously with the same mobile unit, more data is handled and more resources are available than are available to accurately model the system. It uses a probabilistic technique (usually Monte Carlo technique) that uses only a few positions throughout the system and estimates between those positions. This incorrectly allocates assets, resulting in less accurate results.

It is desirable to provide a new process by taking steps that can accurately model the characteristics of a CDMA cellular system to improve the system.

The present invention uses actual received signal level data collected from adjacently spaced locations covering the entire CDMA system to calculate the interference level at each location and to have the highest probability of communicating with the mobile unit at each particular location. Determine a list of neighbor base stations for each base station at each location throughout the system, determine the transmit power required for each base station to communicate with the mobile unit at each location, calculate the interference level at each base station, The mobile unit of each base station is realized by a computer running process that determines the transmit power required to communicate with each base station that is most likely to communicate with the mobile unit at that particular location.

Referring now to FIG. 1, a portion of a CDMA cellular telephone system 10 including a plurality of individual base stations 12 arranged to provide coverage of a service area is shown. Each base station 12 of FIG. 1 is shown with an external boundary 13 indicating the limits of the effective communication range. The borders 13 of other adjacent base stations typically overlap.

Each base station 12 includes at least one cell for transmitting and receiving communication with a mobile unit 15 operating within its service area. In many cases, instead of one cell, the base station is divided into two or three sectors each with communication equipment communicating with a plurality of mobile units in an area defined in part by an antenna pattern angle of 180 ° or 120 °. do. All transmissions between base stations and mobile units in a CDMA system are all digital and are performed on the same “spread spectrum” frequency band of 1.25 MHz. The digital information bits of each message are extended using various levels of encoding information. One such level is called a pseudo random noise (PN) code. Each base station in the system encodes transmission information using the same PN code. Each base station identifies itself by using individual time offsets (commonly referred to as PN offsets) from some repetitive initial time to apply the PN code to any transmission. The interval between the initial times is divided into a total of 512 units. Thus, for example, one base station may initiate an encoding transmission at its initial time, a second base station initiates an encoding transmission at one unit of offset from its initial time, and a third base station may transmit the encoding transmission at an offset of two units May be initiated. Typically, base stations that are physically near each other use PN offsets that are far from each other. Its initial time and various offsets are typically set accurately using circuitry such as global positioning system (GPS) circuitry.

Since each base station sends all messages using the same PN code at the same PN offset, there must be a way for the mobile unit to detect the messages intended for him. To accomplish this, each transmission with the base station is efficiently located on a separate channel by further encoding the transmission with one of a plurality of Walsh codes. Messages encoded with Walsh codes, such as PN codes, are sent and received using masks of the same pattern, so that messages sent using different Walsh codes are orthogonal to the encoding. The transmission on a particular channel is decoded by applying a mask containing the Walsh code and the PN code to the reception pattern of information bits starting at the PN offset specified for the particular channel.

The transmission CDMA system offers many advantages. One of these advantages is that the mobile unit can receive the same message relayed through a plurality of different base stations at the same time. Since all transmissions occur on the same frequency band, the mobile unit receives all the information available within that range, while decoding only the information on the channel about the mobile unit. The CDMA mobile unit uses a receiver that can apply a plurality of different Walsh and PN decoding masks to the entire spectrum of the information received at the same instant. By knowing the desired channel for the receiver to receive, the mobile unit can decode information from one message sent to the mobile unit by a plurality of different base stations at the same time and combine the information to generate one output message. Thus, while a message from one base station fades, the same message can be received from the other base station at an appropriate strength. This allows CDMA systems to provide significantly improved transmission probabilities than other systems.

Despite these advantages, CDMA systems have several problems. One of these problems is caused by the fact that all transmissions occur on the same frequency spectrum. Since all transmissions occur on the same frequency band, the mobile unit actually receives all the transmissions available within that range. Transmissions that are not directed to a particular receiver act as interference that obscures the desired transmission. When the transmission level (preferably, undesirable) at the receiver reaches a level (before decoding) about 14 dB above the desired signal level, it becomes difficult to decode the desired transmission. Prior to decoding, the level of this signal is converted to a level approximately 7 dB above the level of interference after decoding the message to the receiver.

In order to provide high quality transmission, the CDMA system is equipped with a feature that automatically increases or decreases the power level at the base station and the mobile unit in order to maintain the message strength at a typical level of approximately 7 dB above and beyond all interference levels on the channel after decoding. Include.

The mobile unit determines whether the strength of the received signal is strong enough by measuring the rate at which errors occur in the received decoded signal (frame error rate), a factor directly related to the signal to interference ratio. When an error occurs above the limits described above, the mobile unit signals the base station to increase the strength of the signal. The base station does this, but incrementally decreases the signal strength from the higher transmission level until the mobile unit signals again to increase the strength. Thus, when the frame error rate becomes too high, when the signal drops to a level indicating strength below the interference level after decoding, the base station automatically increases the power of the transmitted signal so that the received signal level for interference Increase the quality of the signal.

In a similar manner, the base station measures the strength of the signal received from the mobile unit by monitoring the frame error rate and indicates to the mobile unit whether the transmission strength goes up and down. When a mobile unit contacts a plurality of base stations, the mobile unit receives a signal from each base station indicating whether the transmission strength is rising or falling for that base station. As long as there is one base station signaling to the mobile unit to reduce its transmission strength, the mobile unit ignores any signal to increase the transmission strength and instead responds to a signal that lowers the transmission strength, because This is because a strong signal can sufficiently provide an interference free service for the mobile unit.

In order to improve the operation of the system, it is very useful to evaluate the quality of service in the CDMA service area. To do this correctly, check the data regarding the actual signal level transmitted and received by each base station and any mobile unit at any location throughout the system, so that the transmit power level can be used to provide a quality signal through the system. To determine whether it is necessary to compare the received signal level with the interference level.

All conventional methods of evaluating a CDMA system to determine whether a transmit power level is a system characteristic available for providing a quality signal through the system are actual data obtained at only a few points of the system using a propagation model. The data values were evaluated at locations passing through the system from. Using this derived data, this conventional method evaluates the transmit / receive power for a limited number of arbitrary positions, store the data for these positions, and then transmit and receive power for a second limited number of arbitrary positions. Go to evaluate. This continues until a sufficient number of positions has been evaluated using the estimated values to provide a view of the overall system. This prior art has found its necessity because of the large number of operations and the large amount of data needed to obtain useful results from the actual data for a small number of locations which seem to be unable to use a significantly larger number of locations.

Since the data provided from very few locations is used, it is very difficult to understand the actual system. This result cannot accurately determine how long the model has been run and how many random points are selected. As a result, it is not possible to know exactly whether a problem occurs in the system due to an error in the system layout or its modeling. This effect is that none of the prior art can accurately determine the characteristics of the system that are sufficient to reasonably determine the effect that would be caused by changing the system characteristic differently.

The present invention provides a process to more accurately evaluate the characteristics of a CDMA system, allowing the operator to take action to improve the quality of the provided services.

To evaluate any system, data about that system is first collected. This may be the same data collected for use in an AMPS or TDMA system used in the same area as the CDMA system, as long as the data is collected from adjacently separated locations throughout the system. In one embodiment, data is collected at discrete locations by about 100 feet. In another embodiment, data may be accumulated to determine the quality of CDMA services, particularly in the service area. In any case, the specific data used is data indicating the transmission signal strength transmitted from the base station, the reception signal strength transmitted from one location, and the reception location that records adjacently spaced locations throughout the system.

Particularly in a CDMA system, collecting data is drive tested using a specific receiver called a PN scanning receiver that is capable of receiving signals at levels below the total received signal strength and typically on the order of 21 dB. PN scanning receivers are associated with global area position system (GPS) receivers and computers in a test vehicle. The test vehicle drives the roads of system 10 when the scanning receiver automatically generates measurements at regular intervals (typically every 1 to 5 seconds). In each measurement interval, the receiver measures the total signal strength of all received signals and the strength of each pilot signal received from any base station. These values are generally stored in the computer of the test vehicle in accordance with the time and position values provided by the GPS receiver. 8 shows a mobile unit with such data collection.

Once data about the system is collected, this data is used to provide an assessment. Unlike conventional system evaluation methods, the present invention utilizes real data collected at closely spaced locations throughout the system, thereby eliminating the need to estimate the results between widely separated locations from which actual measurements are obtained. . 6 illustrates the steps of the process of the present invention.

Pilot signals generated by all base stations on the pilot channel are transmitted at constant power throughout the system. This allows the mobile unit to compare the strengths of the pilot signals generated by different base stations with each other. In addition, the known transmission level allows the determination of the path loss for any transmission between the base station and the mobile unit that was received at the location. This path loss value, the pilot signal strength received from each distinguishable base station, and the total strength of all signals received at the location are recorded for each location in the system. For this purpose, received pilot signal strengths below some cutoff levels are considered indistinguishable by the mobile unit.

In order to determine which base station serves one location (which is on the active list), a stochastic test is applied according to the invention in the method shown in FIG. Using a probability curve (derived from the data of the system), such as the curve shown in Figure 4, which evaluates the probability that there is a base station providing a received signal at a particular location with a specific signal to interference level (Ec / Io) ratio. In this case, the probability of each individual base station serving the location can be determined. For any location, the Ec / Io ratio can be determined for each base station by dividing the strength of the received pilot signal by the total received signal strength (total interference) at that location before decoding. On average, it is found that pilot signals are received from two base stations at each location. If more than one base station provides a discernible pilot signal at one location, the probabilities for each base station are determined individually. Next, the probability for each possible group of base stations from which a discernible signal is received is multiplied by the probability for each individual base station being received in the group by the probability of all other base stations in that group, and all base stations not in the group not received. It can be obtained by multiplying by probability. This is illustrated for a group of four base stations providing pilot signals discernable by the table of FIG. This table shows the probability of a particular group of base stations servicing the location at each intersection.

For example, in FIG. 5, other pilot signals are illustrated including the strongest pilot A with all interference ratios Ec / Io below 8 μs while each other pilot is progressively weakened and has a lower rate. Value is assigned. In the example graph of FIG. 4, the probability P (X) for each individual base station generating the pilot signal AD is determined and located in the P (X) column of FIG. 5 for the row specifying the particular pilot. do. These values are described at column level pn (X) for one embodiment of the present invention in order to prepare for a situation in which only three channels carrying voice signals can be received at once due to the constraints of actual network implementation. Then, in the second part of the figure, the value of the intersection of the row and column is the final probability for the group of pilots indicated at the beginning of each row and column. In that figure, the accent indicates that the particular pilot is absent.

In this, a list of possible base stations ranked in order of probability is obtained at each location taking into account any one of the base stations that can serve that location. In order to determine the probability that any individual base station will service the coverage area defined by the outer limit 13 of FIG. 1 of the first base station, the probability for each group that includes the first and other base stations is Add up. For example, in FIG. 5, the probability for each group including the base stations A and B can be determined. This sum gives a number for that location. When all locations in the coverage area are summed, the total probability number for each particular base station is realized. Thus, when determining the neighbor list for base station A, the probability that the base station A will service the location along with each other possible base stations (e.g., A and B, A and C, A and D) is determined by the base station ( Together with A) provides a list of neighboring base stations based on the net probability of each other possible base station serving a location.

Once a base station capable of serving a location is identified, the transmit signal strength needed to provide a quality signal is calculated for each identified base station. Each base station can adjust its transmitted signal strength to maintain the mobile's quality signal. The quality received signal level is determined by the total received interference measured after decoding versus the Eb / No value that measures the energy per bit received at that location from the intended signal. This calculation is repeated for each group of base stations with a significant probability of servicing a location. In determining the transmit power required to generate a quality signal, the average transmit power may be calculated by weighting the probability of receiving a signal from the base station at the location at the received signal strength. The transmit power determined for each location throughout the system will then be summed for the base station, and the total transmit power will be determined.

The process of calculating the base stations (shown in FIG. 6), the probability of servicing one location, and the required transmit power for each base station, each location in the system (part of the system) where modeling is performed until values are determined for all locations. Persists against. At the end of one calculation, the new values determined during each calculation for each received signal strength needed to provide a quality signal at a location and each transmission strength required for a base station providing the quality signal at that location are 2 Used for counting times. That is, an increase in the value of the received signal at the location and the value of the signal transmitted from the base station is used to determine a new total received signal value at each location. The increase in the total received signal value can be determined for each position by adding the incremental increase in the individual received signal value to the previously calculated total value.

The new total received signal strength at each location is used to calculate Ec / Io along with the received signal strength of the individual pilot signal and to determine a new probability of serving the location in the manner described above. Then, the probability for each group of possible base stations is calculated by the method described above. Finally, the new transmit signal strength from the individual base station needed to provide the quality signal at that location is calculated by determining the received signal strength to provide the mobile Eb / No needed for that location. For many locations, the transmission strength for a particular base station can be increased again, because the increase in transmission strength required by the first calculation raises the total received signal strength at most locations, resulting in a large number of transmissions from many base stations. This is because the received signal strength needs to be increased to maintain Eb / No necessary for quality service.

At some point in the modeling process, the increase in the level of signal strength transmitted by all base stations and the increase in the interference level at each location in the system will be the same so that an additional number of calculations will have a minor impact on the interference of the system. . To determine when this will happen, when any number of calculations for all locations have been completed, it is tested to determine the change in total transmission signal strength since the initial number of calculations. If the level is less than the predetermined level selected for a particular system to determine when the change is too small to be a problem, then the modeling is considered to be over. This is sometimes referred to as convergence in this specification. The value determined for each required transmit power, the base station defined by the transmit power, and the group of base stations or base stations most likely to serve any location in the system are determined by the number of last calculations before the same test is met.

With these values for each location throughout the modeled CDMA system, a neighbor list can be prepared for each sector. Using the accumulated data for each location to determine the probability of another base station serving the location, the probability of any base station and another particular base station in a particular coverage area serving a location within the coverage area is Can be found at each location.

This is done by determining the probability for each probable base station group of all base stations that can serve a location in the coverage area of the group, including the base station with the coverage area and other base stations of interest. The probability for each group is multiplied by the probability for each base station in the group serving one location by the probability that a serviceable base station not in the group will not serve the location. This is shown to the right of the table of FIG. 5. The probability of each group containing two base stations is added to provide a probability of servicing the location of the two base stations. Then, the probability of the base station and other specific base stations serving the location of the coverage area of the first base station is summed for all locations in the coverage area. Similarly, the probability of each such pairing of any base station serving any particular location and each other base station is determined and summed across the system to any other base station that can serve the base station and the coverage area. Probability list for each coverage area by the base station is generated. The result is the sum of the probabilities for the base station and each other base station serving any location that the base station serves. Until a sufficient number of neighboring base stations is selected for the performance of the system, selecting a list in which the sum of the probabilities starts from the highest and moves toward the incremental sum of the probabilities, each location serviced for each base station Will provide a neighbor list containing only other base stations that are most likely to actually provide a service.

When the process is repeated a sufficient number of times to generate a neighbor list for each location in the system, the process uses the same original data and moves to determine the required transmit power at each location throughout the system. This is necessarily done in the same way as calculating the transmit power for the base station. That is, the base level total interference in the absence of the received signal at each base station is used to determine the decoding transmission level required to provide a quality signal at the base station. This value plus the path loss to any location provides the mobile unit with the required transmission strength at that location. Moreover, the received signal strength needed at any base station from each location multiplied by the probability (predetermined) of the base station serving the location provides a new received signal level at each base station from each location. Summing these values gives increased interference levels at each base station. This requires the calculation of a new increased received signal strength for the quality signal. For many locations, the transmission strength can be increased again because the required reception strength is increased at the base station for the number of calculations. As a result, it is necessary to continue the number of calculations for determining the mobile transmission strength at various locations. The operation may be repeated many times (as in the previous operation of determining the transmission strength of the forward channel) until it approaches a certain level of interference at each base station. Once this is accomplished, the operator can use all the data necessary to determine the quality of service throughout the system and to improve the quality at any location in the system.

It should be noted that very fast and accurate convergence is obtained by measuring the interference level in the sector until the change in the interference level is below some minimum value. This is in contrast to conventional systems, which indicate when the correct values for a system are available.

Although the present invention has been described based on the preferred embodiments, those skilled in the art will appreciate that various modifications and changes can be made without departing from the spirit and scope of the invention. Accordingly, the invention should be measured by the following claims.

Claims (12)

A method of modeling a CDMA cellular telephone system, At each of a plurality of adjacently spaced locations that define all locations in some regions of the CDMA system where the mobile unit is likely to communicate with the base stations, indicating the received signal level of each location throughout the region and the transmit power of each base station. Determining base stations with probability of communicating with the mobile unit using data associated with Determining, by each base station, transmission power required to communicate with the mobile unit at each location throughout the region; Determine a decoding transmission level required to provide a quality signal from each base station, multiply the probability that the base station provides service to the location, the decoding transmission level determined for each location, and determine the results determined from the multiplied value; Calculating sums of interference at each of the positions, Repeating the previous steps until the values of the transmit powers for each base station converge; Determining, by the mobile unit at each location, the transmit power required to communicate with each base station that is most likely to communicate with the mobile unit at each particular location; Repeating the above steps of determining transmit power for each mobile unit until the total interference level at each base station converges. A computer-implemented system that models the features of a cellular telephone system, Using data collected from adjacently spaced locations throughout the area of the cellular telephone system being modeled, the base stations are most likely to serve each location, and each base station is most likely to serve each location. Means for determining a list of neighboring base stations for; Means for determining the transmit power required for each base station to service each location using data collected at adjacently spaced locations throughout the area of the cellular telephone system being modeled; Transmission required by the mobile unit at each location to communicate with each base station capable of serving the location using data collected at closely spaced locations throughout the area of the cellular telephone system being modeled Means for determining power; Means for determining the transmit power required for each base station to service each location, Means for determining transmit power using the total base level received signal power at all locations where the base station is likely to provide service; Means for determining transmit power using the total received signal power at all locations where the base station is likely to provide service to generate a quality received signal from all locations where the base station is likely to provide service. Means for determining a received signal level sufficient to provide a quality signal for a total base level received signal at a location, and means for determining a transmit power at a base station to generate the quality signal at a location; And means for re-determining the transmit power using the total received signal power to generate a quality receive signal at all locations where the base station will serve until the transmit power converges. An apparatus for improving the performance of a base station of a CDMA cellular network, a) means for identifying a base station having a probability of communicating with a mobile unit at each of a plurality of locations of the cellular network; b) means for determining an interference level at each base station; c) means for evaluating the transmit power required for each base station to communicate with the mobile unit at each location; d) means for evaluating the transmit power required for the mobile unit at each location to communicate with the base station most likely to communicate at that particular location; e) means for repeating the steps performed by the a-d means repeatedly to converge at base station transmit power, Means for determining the minimum transmit power required for each mobile unit to communicate with each base station, Means for repeating the steps performed by said steps a through e means to converge on the transmit power for each mobile unit Device comprising a. 4. The apparatus of claim 3, wherein the means for determining an interference level at each base station comprises: means for determining a decoding transmission level required for providing a quality signal at each base station, a decoding transmission level determined for each location, and a base station at that location. Means for multiplying the probability of providing a service and means for summing results determined from the multiplied value. 4. The apparatus of claim 3, further comprising means for evaluating the probability that one of the plurality of base stations will provide service to the mobile unit over other base stations. delete delete delete delete delete delete delete

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