KR100736755B1 - Radio communication system, a primary station and secondary station for use in such a radio communication system, and method of operating such a radio communication system - Google Patents

Abstract

The wireless communication system includes means for improving power control of the communication channel for data transmission after interruption of transmission. This control is performed by adjusting the transmit power immediately after the interruption by an offset from the power used before the interruption. The offset may be fixed but may be determined from the transmit power of the period before the interruption. This technique reduces, on average, the time it takes for power control to re-establish, so that if the transmit power level is too low, it is likely that data transmission immediately after interruption will be received in a corrupted state, and if the transmit power level is Too high addresses a problem where data transmission immediately after an interruption would cause additional interference.

Description

A wireless communication system, a first station and a second station for use in such a wireless communication system, and a method of operating such a wireless communication system. AND METHOD OF OPERATING SUCH A RADIO COMMUNICATION SYSTEM}

The present invention relates to a wireless communication system, and more particularly to a first station and a second station and a method of operating such a system for use in such a system. Although the present disclosure has described the system with particular reference to an emerging universal mobile telecommunications system, it should be understood that such techniques are equally applicable to use in other mobile wireless systems.

In a wireless communication system, there are two basic types of communication required between a base station (BS) and a mobile station (MS). The first is the amount of user calls, such as voice or packet data. The second is control information necessary for setting and monitoring various parameters of the transmission channel so that the BS and the MS exchange the required user call volume.

One of the control information functions in many communication systems is to enable power control. Power control of the signals transmitted from the MS to the BS is needed to minimize the transmit power required by each MS while simultaneously allowing the BS to receive signals from other MSs at approximately the same power level. In order for the MS to receive a signal from the BS with a low error rate while minimizing the transmit power, power control of the signal transmitted by the BS to the MS is required, which reduces interference between other cells and the wireless system. Power control in a two-way wireless communication system is normally operated in a closed loop manner, whereby the MS determines the required power change in transmission from the BS and conversely signals this change to the BS. ).

One example of a combined time division frequency division multiple access system using power control is a universal mobile communication system (GSM), where the transmit power of both the BS and the MS transmitter is controlled in 2 dB steps. Similarly, power control implementation in systems using spread spectrum code division multiple access (CDMA) technology is disclosed in US-A-5 056 109.

A problem with this known technique is the fact that it may take considerable time for the power control loop to fully converge when transmission is started or after transmission is stopped. Until such convergence is achieved, there is a possibility that a data transmission is received in a corrupted state if the transmission power level is too low, or that data transmission will cause further interference if the transmission level is too high.

It is an object of the present invention to address the above mentioned problems.

According to a first aspect of the invention, there is provided a wireless communication system comprising a first station and a plurality of second stations, the system having a communication channel between the first station and a second station, the channel having power An uplink and downlink control channel for transmitting control information, including a control command, and a data channel for data transmission, wherein the power control is adapted to adjust the power of the control and data channels in response to the power control command. Means are provided, and means are provided for setting an initial transmit power after a pause of transmission to a transmit power before said pause adjusted by an offset.

According to a second aspect of the invention, there is provided a first station for use in a wireless communication system having a communication channel between a first station and a second station, the channel comprising a power control command for transmitting control information. An uplink and downlink control channel and a data channel for data transmission, wherein power control means are provided to adjust the power of the control and data channels in response to the power control command, and after pausing of transmission Means are provided for setting the initial transmit power of to the transmit power before the pause adjusted by an offset.

According to a third aspect of the invention, there is provided a second station for use in a wireless communication system having a communication channel between a second station and a first station, the channel comprising a power control command for transmitting control information. An uplink and downlink control channel and a data channel for data transmission, wherein power control means are provided to adjust the power of the control and data channels in response to the power control command, and after pausing of transmission Means are provided for setting the initial transmit power of to the transmit power before the pause adjusted by an offset.

According to a fourth aspect of the invention, a method is provided for operating a wireless communication system comprising a first station and a plurality of second stations, the system having a communication channel between the first station and a second station, The channel includes an uplink and downlink control channel for transmitting control information including a power control command and a data channel for data transmission, wherein at least one of the first station and the second station is selected from the Power control means for adjusting the power of the control and data channels in response to a power control command, the method comprising: setting an initial transmit power after a pause of transmission to a transmit power before the pause adjusted by an offset; Steps.

The offset may be predetermined. Alternatively, the offset may be determined from the difference between the weighted average transmit power and the last transmit power over a period prior to pausing the transmission (predeterminable), or a power control command applied prior to poise in transmission. It may also be determined from the weighted sum of. In this case, the offset must be quantized to the size of the available power control step before it is applied.

In JP-A-10224294, for example, the use of one or more power control step sizes is known. However, the use in the above quoting is limited only to situations where power control has already been established but delivery conditions are changing rapidly. The citation does not address the problem of obtaining rapid convergence of power control at the beginning of transmission or after transmission is interrupted.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings.

1 is a schematic block diagram of a wireless communication system.

2 illustrates an existing structure for establishing a communication link.

3 illustrates a structure for establishing a communication link with a delayed start for data transmission.

4 is a flow chart illustrating a method of performing a power control operation with variable step sizes.

FIG. 5 is a graph of received signal power P in dB versus time T in ms for different power control algorithms, where the solid line represents the result when no power control and the chain dashed line represents a single step size A graph showing the results at power control and the dotted line showing the results at power control of two levels.

FIG. 6 is a graph of received signal power (P) in dB versus time (T) in ms for another power control algorithm, where the solid line represents the result when no power control and the chain dashed line is of single step size. A graph showing the results at power control and the dotted line showing the results at power control with three levels.

In the drawings, like reference numerals are used to denote like characteristics.

Referring to FIG. 1, a wireless communication system operable in a frequency division duplex mode includes a first station (BS) 100 and a plurality of second station (MS) 110. BS 100 comprises microcontroller (μC) 102, transceiver means (Tx / Rx) 104 connected to antenna means 106, power control means (PC) 107 for changing the transmitted power level, And connection means 108 for connecting to the PSTN or other suitable network. Each of the MSs 110 is a microcontroller (μC) 112, a transceiver means (Tx / Rx) 114 connected to the antenna means 116, and a power control means (PC) 118 for changing the transmitted power level. It includes. Communication from BS 100 to MS 110 occurs on downlink frequency channel 122, while communication from MS 110 to BS 100 occurs on uplink frequency channel 124.

One embodiment of the wireless communication system uses the structure shown in simplified form in FIG. 2 to establish a communication link between the MS 110 and the BS 100. The link is initiated by the MS 110 sending a request for resource 202 (REQ) on the uplink channel 124. If the BS receives the request and has the resources available, the BS 100 sends an acknowledgment 204 on the downlink channel 122 which provides the information needed to establish communication. . After the acknowledgment 204 has been transmitted, two control channels CON, uplink control channel 206 and downlink control channel 208, are established, and data from the MS 110 to the BS 100. Uplink data channel 210 is established for transmission. In some UMTS embodiments, there may be additional signaling between acknowledgment 204 and control and data channel establishment.

In this structure, separate power control loops operate on both uplink 124 and downlink 122 channels, each of which includes an inner and outer loop. The inner loop adjusts the received power to match the target power, while the outer loop adjusts the target power to the minimum level that will maintain the required quality of service (ie, bit error rate). However, this structure allows the channel in which the measurement is made to differ from the newly initiated channel since the initial power level and quality targets are derived from the open loop measurement when transmission is initiated in the control channels 206 and 208 and the data channel 210. It has a problem that it may not be accurate enough because it can have. As a result, when the data channel 210 starts, data transmission is received in a corrupted state if the data transmission power level is too low, or data transmission causes additional interference if the data transmission power level is too high. There is a possibility.

One partial solution known to this problem is that BS 100 measures the received power level of the request 202 and transmits uplink data 210 to MS 110 via an acknowledgment 204. To indicate the appropriate power level. This improves the problem, but there may still be an error due to a temporary separation between the request 202 and the start of uplink data transmission 210.

3 illustrates a solution to this problem, i.e., the start of the uplink data transmission 210 by a time 302 sufficient for the power control to converge sufficiently to allow satisfactory reception of the data transmission by the BS 100. FIG. This is delayed. A delay of one or two frames (10 or 20 ms) may be sufficient, but a longer delay 302 may be allowed if necessary. The additional overhead in transmitting extra control information in control channels 206 and 208 is reduced Eb / No (per bit) for user data received by BS 100 over data channel 210. Balanced by energy / noise index). Delay 302 may be predetermined or dynamically determined by either MS 110 (which may monitor downlink power control information to detect convergence) or BS 100.

4 is a flow chart illustrating another solution to the problem, wherein the size of the power control step is variable. Since the power control error will probably be maximum when transmission starts or after an idle period, the optimal power control step size will be larger than the size used during normal operation.

The method begins with a start of transmission of control channels 206 and 208 and data channel 210 (or start of retransmission of the channels after an interruption state) (402). The difference between the received power and the target power is then determined 404. The power control step size is then tested 406 to determine if the size of the step is greater than the minimum value. If greater, the power control step size is adjusted 408 before the power is adjusted 410. The change in size of the steps may be deterministic and may be based on previous power control adjustments or some quality measure. Thereafter, the power control loop is repeated starting at 404.

In one embodiment, it is desirable to set the power control step size initially to a large value to incrementally reduce the size of the step until it reaches a value set for normal operation (which may be a cell or application specification). . The size ratio between successive steps is preferably less than or equal to 2 to allow the possibility of correcting errors in transmission or by other factors. The magnitude of the power control step may vary in both uplink 124 and downlink 122 channels.

As an example, consider an initial sequence of power control step sizes in (dB): 3.0, 2.0, 1.5, 1.0, 0.75, 0.75, 0.5, 0.5, 0.25. 0.25 dB is the minimum step size. Using this sequence with power control signals every 1 ms, an initial error of up to 10 dB is corrected within a half frame (5 ms) compared to 2.5 frames using a minimum power control step size of 0.25 dB normally used. Can be. Although the size of the steps is described symmetrically herein (ie, the same step size is applicable to increasing or decreasing power), it is not always appropriate (e.g. in US-A-5 056 109). It is known. In a similar example that is simpler to implement, an initial step size (eg 2 dB) is used for a predetermined number of power control commands and then the step size is reduced (eg 1 dB).

The choice of initial step size and rate of change can be predetermined or determined dynamically. For example, if the power level adjustment signaled in acknowledgment 204 is large, then the initial step size may be increased. In another example, a large step size may be appropriate if the MS 110 can determine by other means whether the MS 110 has a moderately high speed relative to the BS 100.

Fixed power control adjustments may be applied at the beginning of transmission. This may even be performed before any valid power control command is received, but its magnitude and direction may be predetermined or dynamically determined using information such as, for example, the rate of change of channel attenuation derived from receiver measurements. In some channel conditions this application leads to an improvement in performance. Increasing power in this way is particularly suitable when transmission is re-started after interruption, and the state of the power control loop (eg, current power level) from before interruption can be maintained. Interruption occurs during a time period in which one or more of the control and data channels are not transmitted or received (or not both transmitted and received), but the logical connection between BS 100 and MS 110 remains intact. A pause or gap in transmission. This interruption is usually intentional because the MS 110 or BS 100 has no data to transmit or wants to perform some other function, such as scanning an alternate channel, or because of a temporary loss of signal. It can happen unintentional.

In a rapidly changing fading channel, the channel attenuation following the pause of transmission is likely to be uncorrelated with the channel attenuation immediately before the pause. In such a case, it can be discussed that the initial transmit power optimum after the gap will be equal to the average value of the power (ignoring other slow fading effects such as shadowing). This fact will then minimize the difference between the initial and temporary optimal values resulting from the channel variation. In practice, in some devices, the transmit power after the gap is determined from a weighted average of power over some extended period before the gap. Appropriate average periods may depend on specific conditions, but may be approximately 20 slots (ie, 20 power control cycles). Additional offset or fixed power adjustments are optionally applied for this initial power level. Such an optimal offset value for a particular situation can be determined empirically.

In alternative apparatus the initial power is determined from the weighted sum of the power control commands rather than the measurement of the transmit power. In such an apparatus, the power change (in dB) that needs to be applied after the transmission gap can be calculated iteratively by, for example, the following method.                 

ΔP (t) = P _off + K ₁ × (ΔP (t-1) -P _off ) -K ₂ × PC (t) × PS (t)

Here, ΔP (t) is the power change to be applied after the gap, calculated repeatedly at time t, during active transmission.

ΔP (0) may be initialized to zero.

P _off is an additional power offset (may be zero).

K ₁ and K ₂ are empirically determined constants, which may be the same, with 0 ≦ K ≦ 1 being preferred. This constant value may be chosen to reflect the effective average period used to calculate the power change.

PC (t) is the power control command applied at time t.

PS (t) is the power control magnitude used at time t.

ΔP (t) is the substantial difference between the current power and the weighted average power and must be quantized to the size of the power control step available before it can be used.

In one example of an embodiment where the selection of the step size is dynamically determined, the received power control bit code is used to determine the size of the step. When the MS 110 starts receiving a power control command, the MS 110 uses the size of the largest step available and, until the step size is reduced, until such a step that the opposite power control command is received. Keep using the size. This next step size is used until the sign of the power control command is reversed, at which time the step size is reduced again. This process continues until the minimum step size is reached.

5 is a graph illustrating the effect of this method on a system with two step sizes available. The graph shows how the received signal power P (in dB) changes over time T for a target power of 0 dB. The solid line shows the received signal power without power control. The change in received power may be due, for example, to the operation of the MS 110. The chain dashed line shows the received power using a single step sized power control of 1 dB. The dotted line shows the received power using the power control according to the method.

In this way, if the use of power control starts at nearly 4 ms, a larger step size of 2 dB is used. Initially, the received power is less than the target power, so all power control commands request an increase in power and the 2dB step size continues to be used. As a result, at nearly 6 ms, the received power exceeds the target power. Once this is done, the sign of the power control command is reversed for the power reduction request and the effect is to reduce the step size to a standard step size of 1 dB. This step size is then used in response to subsequent power control commands.

It is clear from FIG. 5 that by using the described method, the received power can reach the target value more quickly than is possible in a single step size. Once the target value is reached, accurate power control is maintained by reducing the size of the step to a standard step size. Such a method allows effective handling not only when convergence is achieved quickly, but also when the initial error is large or the channel is changing rapidly.                 

The method can also be used with a larger number of available step sizes. FIG. 6 is the same as the example of FIG. 5 except that the dashed lines show received power using three levels of power control available at 4 dB, 2 dB and 1 dB. Initially a 4 dB step size is used, resulting in power reaching the target value faster than in the previous example. When the sign of the power control command is reversed for a power reduction request, the step size is reduced to 2 dB. For power up requests, when the power control command is inverted again, the step size is reduced to a standard step size of 1 dB and this is maintained.

A variation of the mentioned method is to continue using the larger step size for one slot after the sign of the power control command has changed, which helps to correct any overshoot. However, this is unlikely to have a significant impact on the average performance of the method.

Combinations of the mentioned techniques can be readily used to provide improved results.

Although the above description refers to data transmission on the uplink channel 124, the technique is equally applicable to data transmission or bidirectional transmission on the downlink channel 122.

Embodiments of the present invention are described using spread spectrum code division multiple access (CDMA) techniques, for example as used in UMTS embodiments. However, it should be understood that the present invention is not limited to CDMA systems only. Similarly, although embodiments of the invention have been described assuming frequency division duplex, the invention is not limited to use in such a system. It is also applicable to other duplex methods such as, for example, time division duplex (although power control rates in such systems are normally limited to once per transmission burst).

Other variations will be apparent to those skilled in the art upon understanding the invention. Such modifications may involve other features already known in the wireless communication system and its components, and other features that may be used in place of or in addition to those already described herein.

The terms described in the singular form in the specification and claims do not exclude the possibility that a large number of such elements are present. Moreover, the term "comprising" does not exclude the possibility of the presence of elements or steps other than those listed.

The invention is applicable to a range of wireless communication systems, for example UMTS.

Claims (20)

In a wireless communication system comprising a first station and a plurality of second stations, The system has a communication channel between the first station and the second station, the channel including an uplink and downlink control channel for transmitting control information including a power control command and a data channel for data transmission. Power control means is provided to adjust power of the uplink control channel, the downlink control channel and the data channel in response to the power control command at at least one of the first station and the second station; Setting means for setting the initial transmission power after pause of to the transmission power before the pause adjusted by an offset, the power control means being provided before the pause in transmission and by the setting means; The uplink control channel in response to the power control command after the initial transmission power is set; Link control channel, and a wireless communication system to adjust the power of the data channel. 2. The wireless communication system of claim 1, wherein means for determining the offset from a difference between a weighted average transmit power and a last transmit power over a predetermined period prior to pausing of the transmission is provided. 2. The wireless communication system of claim 1, wherein means is provided for determining the offset from a weighted sum of power control commands applied prior to pausing of the transmission. The offset according to claim 3, wherein the offset from the weighted sum of the power control commands according to the equation ΔP (t) = P _off + K ₁ (ΔP (t-1) -P _off ) -K ₂ PC (t) PS (t) Is provided, wherein ΔP (t) is an offset calculated at time t of the last power control command before the pause, ΔP (t-1) is a previously determined offset, and P _off is an additional Power offset, PC (t) is the power control command applied at time t, PS (t) is the size of the power control step applied at time t, K ₁ and K ₂ are constant , ΔP (0) is set to 0 at the beginning of transmission or immediately after a gap, Means are provided for quantizing an offset [Delta] P (t) by an integer product of the minimum power control step size supported by the first or second station transmitting the communication channel. system. The wireless communication system of claim 4, wherein the additional power offset (P _off ) is equal to zero. A first station for use in a wireless communication system having a communication channel between a first station and a second station, The channel includes an uplink and downlink control channel for transmitting control information including a power control command and a data channel for data transmission, the uplink control channel and the downlink control in response to the power control command. Power control means is provided for adjusting the power of the channel and the data channel, and setting means for setting the initial transmission power after the pause of transmission to the transmission power before the pause adjusted by an offset is provided. The control means controls the power of the uplink control channel, the downlink control channel, and the data channel in response to the power control command before pausing in transmission and after setting of the initial transmission power by the setting means. Adjusted, the first bureau. 7. The first station of claim 6, wherein means is provided for determining an offset from the difference between the weighted average transmission power and the last transmission power over a predetermined period before pausing of the transmission. 7. The first station of claim 6, wherein means is provided for determining the offset from a weighted sum of power control commands applied prior to pausing of the transmission. 9. The offset according to claim 8, wherein the offset from the weighted sum of the power control commands according to the equation ΔP (t) = P _off + K ₁ (ΔP (t-1) -P _off ) -K ₂ PC (t) PS (t) Is provided, wherein ΔP (t) is an offset calculated at time t of the last power control command before the pause, ΔP (t-1) is a previously determined offset, and P _off is an additional Power offset, PC (t) is the power control command applied at time t, PS (t) is the size of the power control step applied at time t, K ₁ and K ₂ are constant , ΔP (0) is set to 0 at the beginning of transmission or immediately after a gap, Means are provided for quantizing an offset [Delta] P (t) by an integer product of the minimum power control step size supported by the first station transmitting the communication channel. 10. The first station of claim 9, wherein the additional power offset (P _off ) is equal to zero. A second station for use in a wireless communication system having a communication channel between a second station and a first station, The channel includes an uplink and downlink control channel for transmitting control information including a power control command and a data channel for data transmission, the uplink control channel and the downlink control in response to the power control command. Power control means is provided for adjusting the power of the channel and the data channel, means for setting the initial transmission power after pause of transmission to the transmit power before the pause adjusted by an offset, and the power control The means adjusts the power of the uplink control channel, the downlink control channel, and the data channel in response to the power control command before pausing in transmission and after setting of the initial transmission power by the setting means. 2nd bureau. 12. The second station of claim 11, wherein the offset is predetermined. 12. The second station of claim 11, wherein means is provided for determining the offset from a difference between a weighted average transmit power and a last transmit power over a predetermined period prior to pausing of the transmission. 12. The second station of claim 11, wherein means is provided for determining the offset from a weighted sum of power control commands applied prior to pausing of the transmission. 15. The second station of claim 13 or 14, wherein means are provided for quantizing the offset to an available power control step size. 16. The offset according to claim 15, wherein the offset from the weighted sum of the power control commands according to the equation ΔP (t) = P _off + K ₁ (ΔP (t-1) -P _off ) -K ₂ PC (t) PS (t) Is provided, wherein ΔP (t) is an offset calculated at time t of the last power control command before the pause, ΔP (t-1) is a previously determined offset, and P _off is an additional Power offset, PC (t) is the power control command applied at time t, PS (t) is the size of the power control step applied at time t, K ₁ and K ₂ are constant , ΔP (0) is set to 0 at the beginning of transmission or immediately after a gap, And a means for quantizing an offset [Delta] P (t) by an integer product of the minimum power control step size supported by the second station transmitting the communication channel. 17. The second station of claim 16, wherein the additional power offset (P _off ) is equal to zero. A method of operating a wireless communication system including a first station and a plurality of second stations, the system having a communication channel between the first station and the second station, the communication channel including a power control command to control An uplink and downlink control channel for transmitting information and a data channel for data transmission, wherein at least one of the first station and the second station comprises: the uplink control channel in response to the power control command; 10. A method of operating a wireless communication system, comprising: power control means for adjusting power of the downlink control channel and the data channel; The method comprising setting an initial transmit power after a pause of transmission to a transmit power before the pause adjusted by an offset, The power control means adjusts the power of the uplink control channel, the downlink control channel, and the data channel in response to the power control command before a pause in transmission and after the setting of the initial transmission power, How a wireless communication system works. 19. The offset according to claim 18, wherein the offset from the weighted sum of the power control commands according to the equation ΔP (t) = P _off + K ₁ (ΔP (t-1) -P _off ) -K ₂ PC (t) PS (t) Is provided, wherein ΔP (t) is an offset calculated at time t of the last power control command before the pause, ΔP (t-1) is a previously determined offset, and P _off is an additional Power offset, PC (t) is the power control command applied at time t, PS (t) is the size of the power control step applied at time t, K ₁ and K ₂ are constant , ΔP (0) is set to 0 at the beginning of transmission or immediately after a gap, Means for quantizing an offset [Delta] P (t) by an integer product of the minimum power control step size supported by a second station transmitting said communication channel. 20. The method of claim 19, wherein the additional power offset (P _off ) is equal to zero.

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