US20080132267A1

US20080132267A1 - Method, access point and wtru for controlling transmission power levels of uplink/downlink communication in a wireless communication system

Info

Publication number: US20080132267A1
Application number: US11/935,715
Authority: US
Inventors: Christopher Cave; Paul Marinier; Noam Tomczak
Original assignee: InterDigital Technology Corp
Current assignee: InterDigital Technology Corp
Priority date: 2003-11-17
Filing date: 2007-11-06
Publication date: 2008-06-05
Also published as: CA2546170A1; TW200525922A; CN101103551B; TW200947905A; WO2005050893A3; JP2007529166A; TW201328227A; KR100807167B1; TWI255103B; KR20060101560A; EP1692802A2; KR20060101536A; US20050143120A1; NO20062819L; KR101124867B1; TW200620866A; JP4571948B2; EP1692802A4; CN102917450A; TWI393370B

Abstract

Transmission power levels of uplink/downlink communication is controlled in a wireless communication system. A receiving station produces and provides to a transmitting station power control information based upon received signals from the transmitting station. A measured block error rate (BLER_msr) is obtained from the number of erroneous blocks in a sliding window of the last N received data blocks and SIR adjustments are set based upon the BLER_msrand a target BLER (BLER_target). A target SIR of received signals is adjusted according to the SIR adjustments.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 10/991,267, filed on Nov. 17, 2004, which claims priority from U.S. Provisional Application No. 60/520,740 filed on Nov. 17, 2003, which is incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to a wireless communication system employing outer loop power control. More particularly, the invention relates to an enhanced outer loop power control system with a modified jump algorithm.

BACKGROUND

It is essential to reduce unnecessary interference in a wireless communication system where users share a common frequency for transmission or reception of data. Effective power control reduces interference to a reasonable level while maintaining satisfactory signal quality for a given radio link connection.
Power control typically consists of two stages: Outer Loop Power Control (OLPC); and Inner Loop Power Control (ILPC). The OLPC controls a target signal to interference ratio (SIR) to keep the received quality as close as possible to a target quality. The ILPC controls transmission power to keep the received SIR of each dedicated channel (DPCH) as close as possible to a target SIR. In other words, the output of the OLPC is an updated target SIR used for the ILPC.
A typical OLPC measures Block Error Rate (BLER) as an indication of the quality of the received signal. The BLER is the ratio of number of erroneous transport blocks to the total number of transmitted transport blocks. Quality targets for transmitted data are determined based on the BLER, for example a target BLER of 1%. The OLPC sets a target SIR according to the required quality for a given service, such as BLER. A Cyclic Redundancy Check (CRC) is used to determine whether there are errors in a particular transmission. Basically, user data is segmented in transport blocks for transmission and CRC bits are appended to each transport block. This data scheme is used at the receiver to determine if an error occurred.
A known OLPC process, the jump algorithm, controls power by adjusting a target SIR based on the BLER. However, the jump algorithm is still problematic in that calls having high quality requirements experience a BLER significantly above a desired BLER. Moreover, this problem occurs more frequently when short calls transmit their smaller number of transport blocks.
It would be desirable to improve OLPC with a jump algorithm that significantly reduces the frequency of calls experiencing a higher error rate.

SUMMARY

Transmission power levels of uplink/downlink communications are controlled in a wireless communication system in accordance with the present invention. A receiving station produces and provides to a transmitting station power control information based upon received signals from the transmitting station. As data blocks are received, a measured block error rate (BLER_msr) is obtained from the number of erroneous blocks in the last N received data blocks, and a target SIR is adjusted based upon the BLER_msrand a target BLER (BLER_target). This permits the present invention to better adapt the target SIR to increase the probability that a call meets its specified BLER requirement.

BRIEF DESCRIPTION OF THE DRAWING(S)

A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a communication station configured to perform OLPC in accordance with the present invention; and

FIG. 2 is a flow diagram of a method for performing OLPC in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereafter, the term “station” includes but is not limited to a user equipment, a wireless transmit/receive unit (WTRU), an access point (AP), mobile station, a base station, fixed or mobile subscriber unit, receiving station, transmitting station, communication station, pager, or any other type of device capable of operating in a wireless environment. Furthermore, each of these terms may be used interchangeably herein.
Referring to FIG. 1, a communication station 100 configured to perform OLPC in accordance with the present invention is shown. It is noted that the present invention may be implemented in the uplink and/or downlink. The communication station 100 receives various radio frequency signals including communications from a transmitting station (not shown in FIG. 1). A received signal is passed through an isolator 110 to a demodulator 120. The demodulator 120 produces a baseband signal from the received signal.
A data estimation device 130 recovers data from the baseband signal. An error detection device 140 detects errors in the recovered data. A processor 150 analyzes the detected errors and determines an error rate, such as the BLER of the received communication. The error rate is input to a target SIR generator 160 which generates the target SIR based on the error rate provided by the processor 150. The target SIR is then input to a power control information generator 170. The power control information generator 170 generates appropriate power control information depending, in part, on what type of ILPC is utilized by the system in which the communication station 100 is operating. For example, UMTS TDD uses open loop ILPC in the downlink whereas other types of wireless systems use closed loop ILPC.
Where closed loop ILPC is used, the power control information generator 170 compares a measured SIR of received frames/blocks versus the SIR target generated by the target SIR generator 160 and generates a target power control (TPC) command. The TPC command provides an indication of whether a transmitter communicating with the communication station 100 should increase or decrease its power. For example, where the measured SIR is less than the target SIR, the TPC command indicates that the transmitter should increase its power and where the measured SIR is greater than the target SIR, the TPC command indicates that the transmitter should decrease its power.
Where open loop ILPC is used, the power control information generator 170 simply outputs the target SIR provided from the target SIR generator 160. In this case, the transmitter communicating with the communication station 100 is responsible for determining how it should modify its power setting to achieve the target SIR.
The power control information (i.e. the SIR target or TPC command) generated by the power control information generator 170 is input to a modulator 180. The modulator 180 modulates the information for transmission to the transmitting station (not shown).
Referring to FIG. 2, an OLPC process 200 in accordance with the present invention is shown. Once the receiving station 100 receives a communication signal (step 210), the received communication signal is processed and the BLER of the sequence of data blocks is measured (step 220).
The measured BLER (BLER_msr) is based upon the last N received blocks, and is a ratio of the number of received blocks having an error (N_e) to the total number of received blocks (N), (that is, N_e/N). N is the width of a sliding window during which the BLER is measured. For example, N may be determined as follows to keep track of recent data blocks:
N=K/BLER_target; Equation (1)
where K is a constant, for example larger than or equal to 1, and BLER_targetis a target BLER value. Since the present invention uses the error rate of the most recent N data blocks, the target SIR increases the probability that a call meets its BLER requirement.
After the BLER for a received communication signal is measured, a target SIR (SIR_target) is adjusted based on the BLER values, (i.e., BLER_msrand BLER_target) (step 230). The target SIR adjustments are based on the error check of received data blocks, such as CRC or Forward Error Correction (FEC). It should be understood by those of the skill in the art that other error checking schemes may be employed without departing from the spirit and scope of the present invention.
If the error checking result of a data block is acceptable, the target SIR (SIR_target) is preferably determined by Equations 2-4:
SIR_target=SIR_target*−STEP_down Equation (2)
STEP_down=(1+BLER_down)×BLER_target×STEP_size Equation (3)
BLER_downmax(−1,1−BLER_msr/BLER_target) Equation (4)
where SIR_target* is a previous target SIR, and STEP_sizeis a parameter that determines convergence speed of the jump algorithm.
If the error checking result of a data block is unacceptable, the target SIR (SIR_target) is determined by Equations 5-7:
SIR_target=SIR_target*+STEP_up Equation (5)
STEP_up=(1+BLER_up)×(1−BLER_target)×STEP_size Equation (6)
BLER_up=min(2, BLER_msr/BLER_target−1) Equation (7)
where SIR_target* is a previous target SIR, and STEP_sizeis a parameter that determines convergence speed of the jump algorithm.
The step size of SIR target adjustment (STEP_size) is dependent upon the difference between the measured BLER (BLER_msr) and the target BLER (BLER_target). By way of example, assume the BLER_targetis set to 1%, and the CRC status of the 100 last blocks is kept in memory to calculate the measured BLER (i.e. N=100). Using a basic STEP_sizeof 1.0 dB, the step actual step size up (in case of a CRC error), STEP_up, could take any of the following values:
1. 0.99 dB, if the first error has occurred within the past 100 blocks or;
2. 1.98 dB if the second error has occurred within the past 100 blocks or;
3. 2.97 dB if the third (or greater) error has occurred within the past 100 blocks.
As a result, the SIR target will increase more aggressively when multiple errors have occurred, ensuring a quicker recovery time.
Similarly, the step size down (in case of a CRC success), STEP_down, could take any of the following values:
1. 0.02 dB, if no errors have occurred in the last 100 blocks.
2. 0.01 dB, if one error has occurred in the last 100 blocks.
3. 0 dB, if more than one error has occurred in the last 100 blocks.
In accordance with Equations 2-7, for high BLER_msr, the BLER_upincreases and, in turn, STEP_upbecomes larger than for lower BLER_msr. As a result, the increase of SIR_targetis larger as BLER_msrincreases. On the other hand, when BLER_downdecreases, STEP_downin turn becomes smaller for high BLER_msrthan for low BLER_msr. As a result, SIR_targetchanges less from the previous value SIR_target*, as BLER_msrincreases.
The STEP_upis increased when multiple SIR_targetincreases have taken place in recent history, and STEP_downis increased when no SIR_targetincrease have taken place in recent history. Additionally, since the amount of SIR adjustment for each data block's BLER change is larger, it converges to the target quality of service much more quickly, and the OLPC can respond to short calls which transmit a small number of transport blocks promptly.
In an alternative embodiment, the convergence to the BLER_targetis improved by altering the formulation of STEP_downof the SIR_targetas follows in Equations 8 and 9:
STEP_down=(2×BLER_target−BLER_msr)×STEP_size Equation (8)
STEP_up=STEP_size−STEP_down Equation (9)
With this approach, when the measured BLER exceeds the target BLER, the SIR falls in a critical region where block errors are likely to occur after a longer time. This compensates for the high BLER having occurred in the past. On the other hand, observing a BLER_msrlower than BLER_targetresults in an increase of the STEP_downand consequently, block errors are likely to occur after a shorter time. This compensates for the low BLER having occurred in the past.
Referring back to FIG. 2, after the new target SIR is determined, depending on whether open loop ILPC or closed loop ILPC is being utilized, the target SIR is sent to a transmitting station or a TPC command is computed and sent to the transmitting station, respectively (step 240). As explained above, the target SIR and TPC command may be collectively referred to as power control information. Then, in step 250, the transmitting station controls power of transmission communication signals based on the power control information provided to the transmitting station.
Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.

Claims

1. A base station (BS) for use in a wireless communication system, the BS comprising:

a receiver configured to receive a signal;

a demodulator configured to produce a baseband signal from a received signal;

a data estimation device configured to recover data from the baseband signal;

an error detection device configured to detect errors in the recovered data;

a processor configured to analyze the detected errors and to determine an error rate of the received communication;

a target signal to noise ratio (SIR) generator configured to generate a target SIR based on the error rate; and

a power control information generator configured to generate power control information including a step size.

2. The BS of claim 1, wherein the error rate determined is a block error rate (BLER) of the received communication.

3. The BS of claim 1, further comprising:

the power control information generator configured to generate power control information based an inner loop power control setting.

4. The BS of claim 1, further comprising:

a memory configured to store power adjustment data; and

the processor configured to adjust the step size based on the stored power adjustment data.

5. The BS of claim 1, further comprising:

a forward error correction (FEC) decoder configured to perform error correction on the baseband signal.

6. The BS of claim 1, wherein the power control information generator computes a target power control (TPC) command based on the target SIR.

7. The BS of claim 1, further comprising:

the processor configured to generate a new target SIR based on a cyclic redundancy check (CRC) check of the received data blocks.

8. A method for power control, implemented in a base station (BS) of a wireless communication system, the method comprising:

receiving a signal;

generating a baseband signal from the received signal;

recovering data from the baseband signal using a data estimation device;

detecting errors in the recovered data;

determining an error rate of the received communication;

generating a target SIR based on the error rate; and

generating power control information based on the target SIR.

9. The method of claim 8, wherein the error rate determined is a BLER of the received signal.

10. The method of claim 8, further comprising:

detecting errors in the recovered data using a forward error correction (FEC) decoder.

11. The method of claim 8, further comprising:

generating the power control information based an inner loop power control setting.

12. The method of claim 8, further comprising:

tracking a number of errors occurring over a predetermined period of time; and

adjusting the step size based on the number of errors over the predetermined period of time.

13. The method of claim 8, further comprising:

measuring the SIR of a received signal;

comparing the measured SIR to the target SIR; and

transmitting a transmit power control (TPC) command requesting an increase in transmit power in the case that the measured SIR is greater than the target SIR.

14. The method of claim 8 further comprising:

generating a new target SIR based on a cyclic redundancy check (CRC) of the received data blocks.