KR100797460B1 - Method for controlling data rate in reverse link - Google Patents

Abstract

The present invention relates to a mobile communication system, and more particularly, to a data rate control method in a reverse link. The reverse data rate control method according to the present invention comprises the steps of calculating the transmission energy level required by the terminal by applying the cell interference probability of each terminal; Receiving transmittable data rate information of each terminal; And generating data rate control information of each terminal according to the calculated transmission energy level and data rate information.

Cell interference probability, received energy, total interference

Description

Method for controlling data rate in reverse link}

1 is a flow diagram illustrating a data rate control procedure for the prior art.

2 is a flowchart illustrating an example of a dedicated rate control procedure for a terminal of a base station according to the present invention.

3 is a diagram illustrating an update process of BS_RCV according to the present invention;

4 is a diagram illustrating a rate control information generation procedure using BS_RCV according to the present invention.

5 is a flowchart illustrating another example of a dedicated rate control procedure for a terminal of a base station according to the present invention.

The present invention relates to a mobile communication system, and more particularly, to a data rate control method in a reverse link.

In general, the reverse data transmission is closely related to the total amount of interference received by the base station (hereinafter referred to as ROT). The total amount of interference received by the base station refers to the sum of signal powers of all terminals received by the base station. This will be described with reference to the example of FIG. 1.

1 is a flowchart illustrating a data rate control procedure in the prior art.

When the total amount of interference received by the base station is small, the terminal may transmit data by increasing the data rate of reverse transmission. Otherwise, if the total amount of interference is higher than a certain level, the terminal may reduce the data rate or perform data transmission. Sometimes you have to stop.

In the case of the 1x EV-DO (1x Evolution Data Only) system, as shown in FIG. Generate and transmit to all active terminals (S12). This RA command is transmitted to all terminals in the active set transmitting data in the reverse direction through a common channel called a random access (RA) channel.

The base station compares the measured total received interference amount with a threshold value (S11), when the total amount of received interference is large, that is, when the sum of signal powers of all terminals is equal to or greater than a predetermined threshold, the RA corresponding to the data rate reduction command. Generates a bit and sends it to active terminals.

However, when the total amount of interference received is small, that is, when the sum of signal powers of the terminals is less than or equal to a predetermined threshold, the base station generates and transmits an RA bit corresponding to the data rate increasing command to the active terminals.

That is, all the terminals simultaneously receive the increase command RA bit or at the same time receive the decrease command RA bit.

This RA bit is transmitted from the base station to the terminal for one frame, and the corresponding terminal adjusts the data rate of the next frame using the RA bit (S14).

Prior to the adjustment, when the terminal receives the RA bit, each terminal performs a test for increasing or decreasing the data rate (S13). That is, the fact that terminals are commanded to increase or decrease does not necessarily increase or decrease the data rate.

After receiving the RA bit, the terminals test themselves whether to increase or decrease the data rate. If the test passes, the terminal will either increase or decrease the data rate, otherwise it will maintain the current data rate without increasing or decreasing the data rate.

If the data rate of the current backward transmission frame is low, there is a high probability that the data rate of the next frame increases but the probability of decreasing the data rate is low. On the contrary, if the data rate of the current backward transmission frame is high, the data rate of the next frame is The terminal performs a test on whether the data rate changes so that the probability of increase is low while the probability of decrease is high.

As such, the prior art has been made based only on the total amount of interference received at the base station without the RA bit taking into account the reception status of each user. Thus, this RA bit has been passed to all users in the same increment or decrement instruction.

Therefore, there is a problem in that efficient data transmission cannot be achieved since the terminal must increase or decrease the data rate without considering its channel condition at all.

As a result, this problem is better than the data processing degradation problem of the terminal.

For the base station, even if the terminal receives the RA bit corresponding to the data rate increase or decrease command, since the data rate is increased or decreased only after executing the test itself, the base station will have the effect of changing the total amount of interference as expected by the base station. Unexpected problem occurs.

Accordingly, the present invention has been made in view of the above-mentioned problems of the prior art, and is to provide a reverse link data rate control method that is suitable for improving the data throughput of a terminal.

It is also an object of the present invention to provide a reverse link data rate control method that is suitable for improving the reception rate of a base station.

According to an aspect of the present invention for achieving the above object, a step of calculating the transmission energy level required by the terminal by applying the cell interference probability of each terminal; Receiving transmittable data rate information of each terminal; And generating data rate control information of each terminal according to the calculated transmission energy level and data rate information.

Preferably, the cell interference probability is reported from each terminal or calculated by itself. In this case, the cell interference probability is reported regularly or when the channel environment of each terminal changes. In particular, the cell interference probability is calculated and reported using the pilot signal received from each base station and the total signal power received from the base station through a pilot signal measurement message (PSMM).

Preferably, the cell interference probability is obtained by using a channel gain of a reverse link between any one terminal and a base station and a channel gain having the largest magnitude among channel gains of a reverse link between a plurality of base stations including the base station. Is saved.

Preferably, the cell interference probability uses the channel gain of the forward link between any one terminal and the base station and the channel gain of the largest one among the channel gains of the forward link with the plurality of base stations including the base station. Obtained by

Preferably, the cell interference probability is obtained by using the total sum of signal powers received by one terminal from base stations and signal power having the largest signal power among the base stations.

Preferably, the cell interference probability is calculated by the base station receiving a forward link channel state value transmitted on a reverse link channel.

Preferably, allocating and providing a received energy required for the data rate of the current transmission frame of each terminal; Calculating the transmission energy level by applying the cell interference probability value to the reception energy for the current transmission frame data rate of each terminal. In this case, the transmission energy level is calculated by subtracting a predetermined portion of the cell interference probability value from the reception energy for the current transmission frame data rate of each terminal, which supports a minimum equal data rate for each terminal. To be adjusted.

Preferably, the data rate information is set to 'increase' when the transmission power capacity of each terminal is greater than or equal to the reference, the bits to be transmitted to the transmission buffer are greater than or equal to the reference, and the currently transmitted data rate is less than or equal to the maximum data rate. .

Preferably, when the transmission power capacity of each terminal is greater than or equal to the reference, the bits to be transmitted to the transmission buffer are greater than or equal to the reference, and the data rate being transmitted is less than or equal to the maximum data rate, the maximum two conditions are satisfied. It is set to 'maintain'.

According to another aspect of the present invention for achieving the above object, a step of calculating the transmission energy level required by the terminal by applying the cell interference probability of each terminal; Receiving transmittable data rate information of each terminal; Calculating total energy of signals received from each terminal at regular intervals; And generating data rate control information of each terminal according to the total energy, the calculated transmission energy level, and the data rate information.

According to another aspect of the present invention for achieving the above object, a step of calculating the transmission energy level required by the terminal by applying the cell interference probability of each terminal; Receiving transmittable data rate information of each terminal; Calculating total energy of signals received from each terminal at regular intervals; Updating a reference data rate value according to the calculated total energy; And generating data rate control information of each terminal according to the total energy, the reference data rate value, the calculated transmission energy level, and the data rate information.

Preferably, the reference data rate value has an initial value having a lowest value among transmission energy levels required for each terminal.

According to another aspect of the present invention for achieving the above object, the step of calculating the data rate information that can be transmitted by each terminal and reporting to the base stations in the active set; Calculating a transmission energy level required for the terminal by applying a cell interference probability of each terminal; Calculating total energy of signals received from each terminal at regular intervals; Updating a reference data rate value according to the calculated total energy; Generating data rate control information of each terminal according to the total energy, the reference data rate value, the calculated transmission energy level, and the data rate information; Receiving the generated data rate control information from the base stations; Combining the data rate control information to adjust the rate of data to be transmitted.

Hereinafter, a configuration and an operation according to an exemplary embodiment of the present invention will be described with reference to the accompanying drawings.

The present invention provides a method for controlling a reverse data rate of each terminal to each terminal in consideration of the channel state of each terminal, the effective data rate of the current frame, and the total amount of interference received by the base station. Suggest. For this purpose, the following parameters should be defined between the base station and the terminal.

1) MS_PRI (MS priority)

The MS_PRI is a parameter for measuring the cell interference probability of each terminal and is calculated by using Equation 1 below. This value is a value calculated by the base station itself or reported from the terminal to the base station on a regular basis or when changing the channel environment of each terminal.

MS_PRI=

MS_PRI =

는 단말기와 i번째 기지국과의 역방향 링크의 채널 이득(channel gain)을 나타내며,

는 전체 기지국 중에서 채널 이득이 가장 큰 j번째 기지국과 단말기와의 역방향 링크 채널 이득을 나타낸다. 또한,

는 단말기와 i번째 기지국과의 순방향 링크의 채널 이득을 나타내며,

는

와 마찬가지로 전체 기지국 중에서 채널 이득이 가장 큰 j번째 기지국과 단말기와의 순방향 채널 이득을 나타낸다. In Equation 1,

Denotes the channel gain of the reverse link between the terminal and the i-th base station,

Denotes a reverse link channel gain between the j-th base station and the terminal having the largest channel gain among all the base stations. Also,

Denotes the channel gain of the forward link between the terminal and the i < th > base station,

Is

Similarly, it represents the forward channel gain between the j-th base station and the terminal having the largest channel gain among all the base stations.

On the other hand, since the channel gains of the reverse link and the forward link without considering fading are equal to each other, the channel gains averaged to some extent to reduce the fading effect may be considered to be equal to each other although there is a slight error. .                     

"을 만족한다. Thus, Equation 1 is "

"Satisfied.

에 기지국에서 전송하는 전체 전력을 곱하게 되면 이 값은, 어느 하나의 단말기가 i번째 기지국으로부터 수신하는 전체 신호 전력 즉, Ior과 동일하다. Further, assuming that the total power transmitted by all base stations is about the same, multiply the channel gain by the total power transmitted by the base station, i.e.

Multiplying by the total power transmitted by the base station, this value is equal to the total signal power that one terminal receives from the i-th base station, i.

"를 이용하여 상기 MR_PRI를 산출할 수 있다. Therefore, as in Equation 1 above,

"May be used to calculate the MR_PRI.

In Equation 1, Io denotes the total sum of signal powers from all base stations received in the terminal (sum of Ior received from all base stations), and max_Iorj represents the strongest received signal power among all base stations. It means the signal power received from the j-th base station.

The MS_PRI value is inversely proportional to how much different cell interference the terminal can cause on average to other cells.

If the MS_PRI value is large, it is less likely to cause other cell interference in other cells, while if the MS_PRI value is small, the probability of causing other cell interference in other cells is increased.

In other words, the large MS_PRI value indirectly indicates that the terminal is likely to be close to the base station or in a good channel condition. There are three ways to calculate and determine the MS_PRI value.

First, the terminal calculates the MS_PRI using the Io value of the total sum of the signal powers of all the base stations received by the terminal, and the max_Ior value of the largest base station among these base stations. There is a way to send.

Secondly, when the terminal transmits an Ec / Io value for signal Ec power to a corresponding base station through a pilot signal measurement message (PSMM) reported to the base station to the corresponding base station, the received base station receives this value. The MS_PRI value is calculated based on.

Third, if there is a reverse link channel (for example, a Data Rate Control (DRC) channel of the 1x EV DO system) indicating the channel status of the forward link, the base station transmits the forward channel status value transmitted to this channel ( For example, the MS_PRI value may be calculated based on Ec / Nt for the pilot signal power Ec.

2) MS Reverse Control Value (MS_RCV)

The MS_RCV is a parameter for calculating the transmission energy level required for each terminal and is calculated by the following equation. For this purpose, the function f (x) is first defined. Here, this MS_RCV value is expressed in units of dB.

MS_RCV=f(Current_Assigned_Data_Rate)[dB]-MS_PRI[dB]

MS_RCV = f (Current_Assigned_Data_Rate) [dB] -MS_PRI [dB]

Here, "Current_Assigned_Data_Rate" represents a data rate in the current transmission frame, and in f (x), a function related to a reception energy required for normally receiving a data rate of x at the base station. For example, if the "Current_Assigned_Data_Rate" is 9600, f (9600) = 4dB is a function for converting the received energy value calculated in advance for each data rate.

That is, the MS_RCV is a cell interference probability of the corresponding terminal applied to the received energy required for each terminal. By using the MS_RCV, the present invention provides the received energy required by the base station to terminals close to the cell or having a good channel environment. It is possible to use lower transmission energy than satisfying. This results in less cell interference.

In general, the higher the data rate, the greater the reception energy required for each terminal. Therefore, the higher the value of "Current_Assigned_Data_Rate", the larger the value of MS_RCV.

ㆍMS_PRI에서 MS_PRI는 전술한 바와 같이, 다른 셀에 유발하는 간섭의 확률을 나타내는 값이며, 이 MS_PRI 값이 작을 경우 즉, 다른 셀 간섭을 유발할 확률이 클 경우 MS_RCV의 값이 커진다. Of Equation 3,

MS_PRI in MS_PRI is a value indicating the probability of interference caused in other cells as described above, and the value of MS_RCV increases when this MS_PRI value is small, that is, when the probability of causing other cell interference is large.

는 상기 MS_PRI가 MS_RCV에 영향을 끼치는 정도를 조절해 주는 것으로, 사용자간의 형평성(fairness)을 조절하기 위한 변수이다. 기지국은 이

를 조절하여, 모든 단말기들에게 적정 수준의 데이터 레이트를 보장해주도록 한다. 예를 들어,

=0일 경우 단말기의 채널 상황은 고려하지 않게 되며, 사용자간의 형평성 정도는 최대이다. 반면,

값이 커질수록 각 단말기의 채널 상황이 MS_RCV에 더 많이 영향을 끼친다. Of Equation 3

The MS_PRI controls the degree to which the MS_PRI affects the MS_RCV, and is a variable for controlling the fairness between users. The base station is

By adjusting, all terminals are guaranteed to have an appropriate data rate. E.g,

If = 0, the channel condition of the terminal is not considered, and the degree of equity between users is maximum. On the other hand,

The larger the value, the more the channel status of each terminal affects MS_RCV.

In summary, the higher the data rate being transmitted, the smaller the MS_PRI (the greater the likelihood of causing other cell interference), the larger the value of MS_RCV. The base station calculates and manages MS_RCVs for all terminals in the active state.

3) MS Rate Increase Available Bit (MS_IAB)

The MS_IAB is a parameter for providing effective data rate information that can be transmitted in the next frame of the terminal and has two states of 'increase' and 'unchange' according to the following conditions.

When all of the following conditions are satisfied, the MS_IAB is set to 'increment', and when any one condition is not satisfied, it is set to 'maintain'.

I. When the transmit power margin (capacity of transmit power that can be transmitted) is higher than the standard,

II. When the bit to be sent to the send buffer is above the reference,

III. When the current data rate (Current_Assigned_Data_Rate) is less than the maximum data rate (MAX_data_Rate) set by the system,

Dedicated rate control for the terminal of the base station using the parameters MS_PRI, MS_RCV, MS_IAB will be described with reference to FIG.

2 is a flowchart illustrating an example of a dedicated rate control procedure for a terminal of a base station according to the present invention.

Referring to FIG. 2, the base station updates the MS_PRI periodically or by receiving or directly calculating the MS_PRI value from the terminal when the channel condition of the terminal changes. This MS_PRI value is updated while it is initially set to 0 (S20).

The base station calculates and manages the MS_RCV value for the terminal that is active in its base station by using the MS_PRI value and the data rate transmitted by the terminal, that is, "Current_Assigned_Data_Rate" (S21).

The base station measures the total energy of the signals received at the base station, that is, the total interference amount ROT value, every time having a specific period (S22).

Each terminal transmits an MS_IAB value to the base station in every frame unit (S24).

The base station generates a rate control command (hereinafter referred to as RCC) to control the data rate of each terminal using the MS_RCV value and the MS_IAB (S23), and transmits the RCC to each terminal (S25).

The RCC consists of three commands: an increase command that causes the terminal to increase the data rate, a decrease command that decreases the data rate, and a maintenance command that does not change the data rate.

If the measured ROT of the base station is in a good state (ROT is smaller than ROT_TH1), the MS_RCV is less than an arbitrary threshold (hereinafter, RCV_TH), and the MS_IAB is set to 'increase'. The RCC is set to 'increment' for some terminals and the RCC is set to 'maintain' for the other terminals.

However, when it is determined that the measured ROT of the base station maintains the ROT within the range ROT_TH1 to ROT_TH2 set by the system, the RCC is set to 'maintain' for all terminals.                     

However, if it is determined that the measured ROT of the base station is in a bad state (ROT is larger than ROT_TH2), the RCC is set to 'decrease' for some terminals having MS_RCV exceeding the RCV_TH, and the other terminals are ' Set to 'maintain'.

In the above method, the specific number of the number of terminals to set the RCC to 'increase' or 'decrease' may vary depending on the performance, capacity, and purpose of the operator or system to be actually applied. As one specific example applicable here, the following algorithm is proposed.

First, the parameter BS Rate Control Value (BS_RCV) for obtaining the number of terminals is defined. This BS_RCV has an initial value of the lowest MS_RCV value among the MS_RCVs (calculated at the corresponding base station or reported from the corresponding terminal) having a data rate of 9.6 kbps.

That is, the BS_RCV limits the selection range of the MS_RCV, so that only terminals having MS_RCV above or below a predetermined number can receive data rate control information of increasing or decreasing RCC.

3 is a diagram illustrating an update process of BS_RCV according to the present invention.

Referring to FIG. 3, the base station measures the total amount of interference (ROT) received by the base station in units of time having a specific period. The base station updates BS_RCV using the measured ROT value.

만큼 증가하고, ROT_TH2 이상이면

만큼 감소한다. 그러나, ROT가 ROT_TH1과 ROT_TH2 범위를 유지할 경우, 상기 BS_RCV는 이전 BS_RCV 값을 유지한다. If the measured ROT is less than or equal to ROT_TH1, BS_RCV is

Increment by and over ROT_TH2

Decreases by. However, if ROT maintains the range ROT_TH1 and ROT_TH2, the BS_RCV maintains the previous BS_RCV value.

4 is a diagram illustrating a rate control information generation procedure using BS_RCV according to the present invention.

Referring to FIG. 4, the base station first updates the BS_RCV value through the measurement term ROT value as in FIG. 3.

) < BS_RCV' 이고, 'MS_IAB == '증가' 인 조건을 만족하면, RCC는 '증가'로 셋팅된다. Subsequently, the base station generates an RCC for each terminal by using the MS_RCV value, the BS_RCV value, and the MS_IAB received from the corresponding terminal using the following conditions, and then transmits the generated RCC to each terminal. If, '(MS_RCV +

) <BS_RCV 'and if the condition that' MS_IAB == 'increment' is satisfied, the RCC is set to 'increment'.

However, if 'MS_RCV> BS_RCV', the RCC is set to 'decrease'.

) ≤ BS_RCV' 이면서, 'MS_IAB ≠ '증가'인 경우, 또는 'MS_RCV ≤ BS_RCV ≤(MS RCV +

)'인 경우, 상기 RCC는 '유지'로 셋팅된다. However, '' (MS_RCV +

) ≤ BS_RCV 'and' MS_IAB ≠ 'increment', or 'MS_RCV ≤ BS_RCV ≤ (MS RCV +

) ', The RCC is set to' maintain '.

를 이용하여 통신 환경에 보다 맞는 RCC 할당 범위를 조절할 수 있다. The present invention is the above

You can adjust the RCC allocation range more suited to the communication environment by using.

Reverse rate control using the BS_RCV will be described with reference to the example of FIG. 5.

5 is a flowchart illustrating another example of a dedicated rate control procedure for a terminal of a base station according to the present invention.                     

The base station updates the MS_PRI periodically or by receiving or directly calculating the MS_PRI value from the terminal when the channel condition of the terminal changes. This MS_PRI value is updated while it is initially set to 0 (S40).

The base station calculates and manages the MS_RCV value for the terminal that is active in its base station by using the MS_PRI value and the data rate transmitted by the terminal, that is, "Current_Assigned_Data_Rate" (S41).

The base station measures the total energy of the signals received at the base station, that is, the total interference amount ROT value, every time having a specific period (S42).

The base station updates BS_RCV as in the scheme of FIG. 3 (S43).

The base station generates the RCC of each stager using the MS_RCV, MS_IAB, and the updated BS_RCV.

Each terminal transmits the MS_IAB value to the base station every frame (S45).

The base station generates a rate control command (hereinafter referred to as RCC) to reduce the data rate of each terminal using the MS_RCV value, MS_IAB, BS_RCV (S46), and transmits the RCC to each terminal (S47). .

The terminal receives an RCC from all active base stations (S26, S47), generates a combined RCC from the received RCCs, and controls the data rate of the next frame accordingly (S27, S48). The method of combining RCCs received from all active base stations is as follows.

If all received RCCs are set to 'increment', the combined RCCs are set to 'increment'.

However, when only one of the received RCCs is set to 'decrease', the combined RCC is set to 'decrease'.

Otherwise, the combined RCC is set to 'maintain'.

As described above, the rate control information according to the present invention enables the dedicated data rate control for each terminal since the RCC is generated in consideration of the reception state of each terminal as well as the total interference received at the base station.

This results in more advanced data transmissions that match the channel conditions of each terminal, resulting in a significant gain in data throughput.

In addition, the base station is able to adjust the load precisely, the large gain in the base station operation aspects.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the spirit of the present invention.

Therefore, the technical scope of the present invention should not be limited to the contents described in the examples, but should be defined by the claims.

Claims (34)

In a mobile communication system including a plurality of base stations and a plurality of terminals, a method for controlling the data rate of the reverse link, Determining, by the base station, a data rate control command for each terminal by controlling a transmission data rate of each terminal in consideration of a channel state of each terminal; And Transmitting the data rate control command to each terminal through a forward control channel to indicate whether to increase, decrease or maintain the current data rate of each terminal; Data rate control method comprising a. The method of claim 1, And receiving status information from each terminal on the reverse control channel indicating whether the data rate of each terminal can be increased. The method of claim 2, And the state information is determined in consideration of at least one of a transmit power that can be transmitted on a reverse packet data channel of each terminal and a bit state to be transmitted in a transmission buffer of each terminal. The method of claim 1, In the determining of the data rate control command, determining the data rate control command based on at least one of a cell interference probability of each terminal and a transmission energy level of each terminal. The method of claim 1, And receiving status information of each terminal related to data transmission of each terminal from each terminal on a reverse control channel. delete delete delete The method of claim 1, The determining of the data rate control command further comprises determining a transmission energy level of each terminal based on a cell interference probability of each terminal. The method of claim 4, wherein And the base station receives the cell interference probability reported from each terminal, or calculates the cell interference probability by itself. delete The method of claim 2, And said status information is used when determining said data rate control command. delete delete delete delete delete delete delete delete delete delete A base station apparatus for controlling a reverse link data rate in a mobile communication system including a plurality of terminals, comprising: A processor for determining, for each terminal, a data rate control command for controlling the data rate of each terminal in consideration of the channel state of each terminal; And A transmitter coupled to said determining means for transmitting a data rate control command to each terminal via a forward control channel, And the data rate control command is sent to indicate whether to increase, decrease or maintain the current data rate of each terminal. The method of claim 23, And a receiver configured to receive, on the reverse control channel, status information indicating whether the data rate of each terminal can be increased from the respective terminals. The method of claim 24, And the state information is determined in consideration of at least one of a transmit power that can be transmitted on a reverse packet data channel of each terminal and a bit state to be transmitted in a transmission buffer of each terminal. A terminal device used in a mobile communication system for controlling a data rate on a reverse link, the terminal device comprising: A receiver for receiving a data rate control command from a base station on a forward link control channel, indicating whether the terminal is to increase, decrease, or maintain a current data rate; And A controller, coupled to the receiving means, for controlling the data rate based on the data rate control command Comprising a terminal device. The method of claim 26, And a transmitter configured to transmit status information on a reverse control channel indicating whether the data rate of the terminal can be increased. The method of claim 27, And the state information is determined in consideration of at least one of a transmit power that can be transmitted on a reverse packet data channel of the terminal and a bit state to be transmitted in the transmit buffer of the terminal. A method of controlling a data rate on a reverse link in a terminal of a mobile communication system, Receiving a data rate control command determined for the terminal to indicate whether to increase, decrease or maintain a current data rate from a base station on a forward link control channel; And Controlling the data rate based on the data rate control command Comprising a data rate control method. The method of claim 29, And transmitting status information on the reverse control channel indicating whether the data rate of the terminal can be increased. The method of claim 30, And the state information is determined in consideration of at least one of a transmit power that can be transmitted on a reverse packet data channel of the terminal and a bit state to be transmitted in the transmit buffer of the terminal. The method of claim 30, Wherein the status information is determined to be 'increase' or 'maintain'. delete The method of claim 30, And the state information is set to 'increase' if the remaining power of the terminal is greater than a threshold and the current data rate is less than the maximum data rate, otherwise it is set to 'maintain'.

Images