KR20010074735A - Closed loop power control in a radio communication system - Google Patents

Abstract

In a wireless communication system having one first station and a plurality of second stations, the power of the uplink and downlink channels between the first station and the second station is controlled by each station that transmits a power control command to the other station. Controlled by closed loop method. In response to this command, the receiving station adjusts its output power step by step. By combining a plurality of received power control commands prior to adjusting their output power, the receiving station can emulate the ability to use a power control step size smaller than the minimum, thereby improving performance under certain channel conditions. In one embodiment, when the required power control step size is less than the minimum step size of a particular station, the station processes a group of power control commands to determine whether to adjust the output power to the minimum step size. In an alternative embodiment, the power control step size is fixed when the combination algorithm is used. The present invention is applicable to power control in both the first station and the second station.

Description

CLOSED LOOP POWER CONTROL IN A RADIO COMMUNICATION SYSTEM}

There are two basic types of communication required between a base station (BS) and a mobile station (MS) in a wireless communication system. The first is user traffic, for example voice or packet data. Second is control information necessary to set and monitor various parameters of the transmission channel to enable the BS and MS to exchange the necessary user traffic.

In many communication systems, one of the functions of the control information is to enable power control. Power control of the signal transmitted from the MS to the BS is required in order to minimize the transmit power required by each MS while the BS receives signals from several MSs at approximately the same power level. Power control of the signal transmitted by the BS to the MS is required, in order to reduce interference with other cells and wireless systems, while the MS receives signals from the BS at a low error rate, while minimizing transmission power To do that. In a two-way wireless communication system, power control may be operated in a closed loop or open loop method. In a closed loop system, the MS determines the necessary change in transmit power from the BS, signals this change to the BS, and vice versa. In an open loop system that can be used in a TDD system, the MS measures the signal received from the BS and uses these measurements to determine the necessary change in transmit power.

An example of a combined time and frequency division multiple access system using power control is the Global System for Mobile communication (GSM), where the transmit power of both BS and MS transmitters is controlled in 2 dB steps. do. Similarly, power control implementations in systems using Spread Spectrum Code Division Multiple Access (CDMA) technology are disclosed in US patent (US-A-5 056 109).

In considering closed loop power control, it can be seen that for any given channel condition, there is an optimal power control step size that minimizes the required E _b / N _O (energy / noise density per bit). When the channel changes very slowly, the optimal step size may be less than 1 dB, because such values are sufficient to track the change in the channel while providing minimal tracking error. As the Doppler frequency increases, larger steps provide better performance with the optimum value exceeding 2 dB. However, as the Doppler frequency increases further, the latency (or update rate) of the power control loop becomes so large that the channel cannot be properly tracked, and the point where the optimum step size decreases again, perhaps below 0.5 dB. Leads to Because the reason is that fast channel changes cannot be tracked, what is needed is the ability to follow shadowing, which is generally a slow process.

Since the optimal power control step size can vary dynamically, it can improve performance performance when the BS instructs the MS what value of power control size the MS should use for uplink transmissions to the BS. have. One example of a system that can use such a method is the UMTS Frequency Division Duplex (FDD) standard, where power control is important because of the use of CDMA technology. Although improved performance can be achieved by having a small minimum step size, for example 0.25 dB, this will significantly increase the cost of the MS. However, if the MS does not need to implement the minimum step size, it may not be able to implement the step size requested by the BS.

The present invention relates to a wireless communication system, and further relates to a second station for use in such a system and a method of operating the system. Although the present disclosure describes a system that is of particular relevance to the recent Universal Mobile Telecommunication System (UMTS), it should be understood that such techniques are equally applicable for use in other mobile wireless systems.

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

2 is a flow chart illustrating a method according to the invention for performing power control at a second station.

3 is a graph of power control step size used in dB for a MS traveling at 300 km per hour versus received E _b / N _o in dB required for a bit error rate of 0.01.

4 is a graph of power control step size used in dB for a MS traveling at 1 km per hour versus received E _b / N _o in dB required for a bit error rate of 0.01.

It is an object of the present invention to enable accurate power control without requiring all mobile stations to implement the same minimum power control step size.

According to a first aspect of the present invention, there is provided a wireless communication system comprising one primary station and a plurality of secondary stations, the system communicating between a first station and a second station. A channel, and one of the first station and the second station (transmitting station) has means for transmitting a power control command to the other station (receiving station) to instruct the other station to adjust the output transmission power in stages. The destination station is provided with combining means for processing a plurality of power control commands to determine whether to adjust its output power.

According to a second aspect of the invention, a first station is provided for use in a wireless communication system having a communication channel between a first station and a second station, the first station being itself transmitted by a second station. Means for stepwise adjusting the output transmit power in response to a power control command of < RTI ID = 0.0 >, < / RTI >

According to a third aspect of the present invention, a second station is provided for use in a wireless communication system having a communication channel between a second station and a first station, the second station being a power control transmitted by the first station. Means are provided for stepwise adjusting the transmit power thereof in response to the command, and combination means for processing the plurality of power control commands to determine whether to adjust their output power.

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 the second station. The method includes transmitting a power control command to another station (receiving station) to instruct one of the first station and the second station (sending station) to step-by-step the power to the other station. The station processes a plurality of power control commands to determine whether to adjust the output transmit power.

The present invention is not provided in the prior art, but is based on the recognition that the emulation of a small power control step size can provide excellent performance by the MS.

Embodiments of the present invention will now be described with reference to the accompanying drawings, as an example.

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

In a UMTS FDD system, data is transmitted in 10ms frames each having 15 time slots. BS 100 sends one power control command (consisting of two bits) per slot, where bit 11 (hereafter referred to as a value of 1 for simplicity) tells MS 110 to increase power. Bit 00 (hereinafter referred to as 0) requests the MS 110 to reduce power. The required power control step size change is notified individually via the control channel.

In a system according to the invention, this behavior is changed when requested to implement a power control step size that is smaller than the minimum size that the MS 110 can do. In such a situation, the MS 110 may take no action unless it receives a series of identical power control commands, thereby emulating the performance of the MS 110 with more precise power control.

For example, consider the case where the requested step size is 0.5 dB and the minimum step size implemented by the MS 110 is 1 dB. MS 110 processes the power control commands in pairs and only changes the output power if both commands are identical. Thus, if the received command is 11, the power is increased, if the command is 00, the power is decreased, and if the command is 10 or 01, the power is no change. Align the comparison with the transmission of the frame to combine the power control commands sent to slot 1 and slot 2 of the particular frame, then to combine the commands sent to slot 3 and slot 4, and then combine in this manner. It may be advantageous to do so.

Similarly, if the requested step size is 0.25 dB and the minimum step size is 1 dB, the MS 110 processes four power control commands at the same time and changes the output power only if all four commands are the same. Thus, power is increased if the received command is 1111, and power is decreased if the received command is 0000, otherwise it does not change. It may also be advantageous to align the comparison with frame transmission to combine the commands sent in slots 1 through 4, then combine the commands sent in slots 5 through 8, and continue combining in this manner. have.

Combining commands received in three or five slots is particularly advantageous for the UMTS embodiment under consideration because it maintains alignment with a frame of fifteen slots. However, this method is not limited to such a system. Consider the general case where the minimum step size implemented by the MS 110 is S and the step size requested by the BS 100 is R. In this case, the power control commands can be combined into groups of G, where G = S / R.

2 illustrates a method of emulating less power control steps than the minimum of MS 110. The method begins at step 202 with the MS 110 determining the number of commands G to be combined to group, and setting the received power control command counter value from i to zero. In step 204, MS 110 receives the power control command and increments the coefficient value i. Next, in step 206, the value of i is compared with G. If i is less than G, the received command is stored and the MS 110 waits to receive the next command. If I is greater than or equal to G, then the required number of power control commands have been received, and MS 110 determines in step 208 whether to adjust its power based on the received power control commands. Once this is done, the counter i is reset to 0 (if i is equal to G), or to 1 (if i is greater than G, which will occur if G is not an integer), or MS 110 ) Then waits to receive a power control command.

In an alternative embodiment, instead of combining power control commands into a group of G, MS 110 maintains a running total of the requested power changes, once the total requested power changes are their minimum step size. When it reaches, it changes. For example, if the requested step size is 0.25 dB and the minimum step size is 1 dB, then the sequence 11010111 of the received command results in an increase in power by 1 dB. The MS 110 then subtracts the steps actually implemented from the running sum of the requested power changes. However, such a scheme becomes more complex to implement (since this configuration requires maintaining a running sum of the requested power changes), and it appears to provide only minimal improvements to the performance of the method.

In a variation of this alternative embodiment, the MS 110 uses an easy decision method in maintaining the running sum of the requested power changes, rather than making difficult decisions through each individual power control command. Each power control command is weighted by a function of the amplitude of the signal received for that command, according to a measure of the likelihood that the MS 110 correctly interprets the command before it is added to a running sum. do. For example, sequence 11010111011, once weighted, corresponds to the sequence 0.8 0.3 -0.3 0.4 -0.1 0.5 0.9 0.8 -0.4 0.7 0.5 (in units of 0.25 dB) of the requested power change. This sequence has a running island of 4.1, which performs an ups step of 1 dB and triggers the MS 110 to reduce the running island to 0.1. This modification will provide some improvement in carrying out the method.

Two simulations have been performed to illustrate the effectiveness of the method according to the invention. The simulation examines the performance of the MS 110 with a minimum step size of 1 dB compared to the performance of the MS 110 with a minimum step size of 0.25 dB. Simulation makes a number of ideal assumptions:

There is one slot delay in the power control loop;

No channel coding;

There is complete channel estimation by the receiver;

Equalization at the receiver is performed by a complete RAKE receiver;

No control channel overhead is included in figures of E _b / N _O ;

There is a fixed error rate in the transmission of power control commands.

The channel is modeled as a simple N-path Rayleigh channel.

The first simulation relates to a rapidly changing channel as the MS 110 moves at 300 km per hour in a single path Rayleigh channel with an error rate of 0.01 for power control commands. 3 is a graph of power control step size used in dB versus received E _b / N _O in dB required for an uplink bit error rate of 0.01. While the solid line is the result for the MS 110 with a minimum power control step size of 0.25 dB or less, the dashed line controls power in two or four groups to emulate 0.5 dB and 0.25 dB power control step sizes, respectively. Results are shown for MS 110 with a minimum step size of 1 dB combining bits.

In this situation, the best performance is achieved at small step sizes of less than 1 dB. The emulation of the 0.25 dB and 0.5 dB steps results in an implementation loss of almost about 0.05 dB compared to about 0.6 dB when no emulation is performed, demonstrating the effectiveness of the emulation method. By increasing the error rate of the power control command to 0.1, a general reduction of about 0.2 dB occurs in the received E _b / N _O , but the performance of the MS 110 with the emulated small steps directly implements the small steps. It is similar to the performance of the MS (110).

The second simulation relates to a slowly changing channel as the MS 110 moves at 1 km per hour in a six path Rayleigh channel with an error rate of 0.01 for the power control command. 4 is a graph of power control step size used in dB versus received E _b / N _O in dB required for an uplink bit error rate of 0.01. The lines of the graph are identified in the same way as in FIG.

In this situation, there are some advantages to using a power control step of less than 1 dB. By using the first simulation, the results obtained using the emulated small steps are very similar to the results using the direct implementation of the small steps.

In a further application of this method, when considered advantageous for reasons such as reducing the impact of error in the interpretation of transmitted power control commands (eg by averaging over a longer time period), the value of G is S / It can be set to a value other than R. Therefore, in some circumstances, the MS 110 may choose to use a step size that is larger than the minimum step size that can be implemented.

The foregoing detailed description relates to a system for transmitting a power control command to the MS 110 separately from the BS 100 instructing the MS 110 to set a power control step size. However, the present invention is suitable for use within the scope of other systems. In particular, there is a variable power control step size and can be used in any system where the BS 100 instructs the MS 110 to use a specific value for this step. The invention may also be used in systems in which the power control step size is fixed, or at least while the power control step size emulation method is used. Instead of instructing the MS 110 to use a particular step size, the step size to be used may also be determined by negotiation between the BS 100 and the MS 110.

Moreover, although the foregoing description relates to the emulation of the power control step size by the MS 110, such a method can be equally widely used for the BS 100 to control downlink transmission power.

By reading this specification, other variations will be apparent to those skilled in the art. Such variations may include other features that are already known in the wireless communication system and may be used instead of or in addition to the features already described herein.

In the present specification and claims, although expressions referring to single elements are used, this does not exclude the presence of a plurality of such elements. Moreover, the word "comprising" does not exclude the presence of elements or steps other than those described.

The invention is applicable to the scope of wireless communication systems, for example UMTS.

Claims (16)

A wireless communication system comprising one primary station and a plurality of secondary stations, the system comprising a communication channel between the first station and a second station, the first station and One of the second stations (transmitting station) has means for transmitting a power control command to the other station (receiving station) to instruct the other station to adjust its output transmission power in stages, the receiving station And combining means for processing a plurality of power control commands to determine whether to adjust their output power. A first station for use in a wireless communication system having a communication channel between a first station and a second station, wherein the first station outputs its own step by step in response to a power control command sent by the second station. Means for adjusting the transmit power, wherein a combination means for processing a plurality of power control commands to determine whether to adjust the output power is provided. A second station for use in a wireless communication system having a communication channel between a second station and a first station, wherein the second station transmits its own step by step in response to a power control command sent by the first station. Means for adjusting power, provided for combining means for processing a plurality of power control commands to determine whether to adjust its output power. 4. The apparatus of claim 3, wherein means are provided for selecting one of a plurality of available power control step sizes in response to a command sent by the first station, wherein the combining means is provided with the minimum step size required. A second station for use in a wireless communication system, characterized in that it operates when less than the step size. 5. The wireless communication system of claim 4, wherein means are provided for processing a group of power control commands together, the size of the group being determined by the minimum available step size and the required step size. Second station for use. 6. The second station of claim 5, wherein the size of the group is equal to the ratio between the minimum available step size and the required step size. 4. The wireless device of claim 3, wherein the combining means operates in response to a command sent by the first station to process a group of power control commands together, the size of the group being predetermined. Second station for use in a communication system. 8. The second station of claim 7, wherein the power control step size is predetermined. A method of operating a wireless communication system comprising a first station and a plurality of second stations, the system comprising a communication channel between the first station and a second station, the method comprising: the first station and the second station; One of the stations (sending stations) sending a power control command to the other station (receiving station) to instruct the other station to adjust its power step by step, wherein the destination station transmits its output. A method of operating a wireless communication system, processing a plurality of power control commands to determine whether to adjust power. 10. The apparatus of claim 9, wherein the transmitting station instructs the receiving station to use a particular power control step size, the receiving station combining power control commands if the required step size is less than the minimum available step size. A method of operating a wireless communication system. 11. The wireless communication system of claim 10, wherein the destination station processes the group of power control commands together and determines the size of the group according to the minimum available step size and the required step size. How to operate. 12. The method of claim 11, wherein the size of the group is equal to the ratio between the minimum available step size and the required step size. 10. The wireless communication system according to claim 9, wherein the destination station processes the group of power control commands together in response to a command sent by the transmitting station, and the size of the group is predetermined. How to operate. 14. The method of claim 13, wherein the power control step size is predetermined. 15. The wireless communication of any of claims 9-14, wherein the transmission on the channel occurs in a frame and the group of power control commands has a predetermined position with respect to the start of each frame. How to operate the system. 16. The method of claim 15, wherein the size of the group is accurately divided by the number of power control commands sent in a frame.