HK1109252B - Weighted open loop power control in a time division duplex communication system - Google Patents

Description

Weighted open loop power control in time division multiplexed communication systems

The present application is a divisional application of the chinese patent application having an application date of 22/3/2000, an application number of 200410043102.8, entitled "weighted open loop power control in a time division multiplexing communication system".

Technical Field

The present invention relates generally to spread spectrum Time Division Duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling transmit power in a TDD communication system.

Background

Fig. 1 illustrates a wireless spread spectrum Time Division Duplex (TDD) communication system. The system has a plurality of base stations 30₁To 30₇. Each base station 30₁With the User Equipment (UE)32 in the area in which it operates₁To 32₃Communication is performed. Will be from the base station 30₁To the UE 32₁Is referred to as downlink communication, and the slave UE 32₁To the base station 30₁Is referred to as uplink communication.

In addition to communicating over different frequency spectrums, spread spectrum TDD systems may communicate over the same frequency spectrum in multiple. A plurality of signals are identified by their respective chip code sequences (codes). In addition, to make more efficient use of spreading, the TDD system shown in FIG. 2 uses a system that is divided into a plurality of time slots 36₁To 36_nFor example, a repeating frame divided into 15 slots 34. In this system, the selected code is utilized, at 36, selected₁To 36_nThe communication is sent within a time slot. Thus, one frame 34 may carry multiple utilization slots 36₁To 36_nAnd code identification. A single code used in a single slot is called a resource unit. One or more resource units are allocated to a communication based on a bandwidth required to support the communication.

Most TDD systems are adapted to control the transmit power level. In a TDD system, many communications may share the same time slot and frequency spectrum. When the UE 32 is₁Or base station 30₁While a communication is being received, all other communications using the same time slot and frequency spectrum may interfere with that particular communication. Increasing the transmit power level of one communication reduces the signal quality of all other communications in that timeslot and spectrum. However, reducing the transmit power level too much can result in poor signal-to-noise ratio (SNR) and Bit Error Rate (BER) at the receiver. To maintain the signal quality of the communication and reduce the transmit power level, transmit power control is employed.

In U.S. patent No. 5,056,109 (gilhouse et al), a method for employing transmit power control in a Code Division Multiple Access (CDMA) communication system is disclosed. The transmitter sends the communication to a particular receiver. After reception, the received signal power is measured. The received signal power is compared to a desired received signal power. Depending on the comparison result, a control bit is sent to the transmitter or the transmission power is increased by a fixed value or decreased by a fixed value. This power control technique is often referred to as a closed-loop control technique because the receiver sends a control signal to the transmitter that controls the transmitter transmit power.

In some cases, the performance of the closed loop system may be degraded. For example, if in a highly dynamic environment, e.g., due to the UE 32₁High dynamic environment due to movement, in UM 32₁And a base station 30₁To transmit communications therebetween, such systems cannot be made fast enoughIs adapted to compensate for such variations. The update rate of closed loop power control in a typical TDD system is 100 cycles per second, which cannot satisfy a fast fading channel.

EPO 462952 a discloses a method for controlling and adjusting the transmit power of signals communicated between a mobile station and a base station within a digital mobile telephone system. To determine the transmit power level, an average of the signal strength and an average of the signal transmission quality are determined. From these two averages, the expected signal strength and signal quality are calculated for the subsequent period, assuming that the transmit power remains the same. The transmit power level in the subsequent period is adjusted according to the expected value.

WO 9845962 a discloses a method for controlling the transmit power level of a mobile terminal in a satellite communications system. The power control method has a closed loop unit and an open loop unit. For the closed loop element, the base station calculates the power setting of the mobile terminal based on the received signal strength from the mobile terminal. The base station takes into account the propagation delay of the satellite system in the power setting determination process. For an open loop unit, the strength of the signal received from the base station in each frame is compared to the strength of the signal received in the previous frame. The transmit power of the mobile terminal is adjusted in inverse proportion to the observed change in signal strength. Therefore, other methods are needed that can maintain signal quality and low transmit power levels. Therefore, other methods are needed that can maintain signal quality and low transmit power levels.

Disclosure of Invention

In a spread spectrum time division multiplex communication station, the transmission power level can be controlled using the present invention. The first communication station transmits a communication to the second communication station. The second communication station receives the communication and measures the power level it receives. A path loss estimate is determined based in part on the received communication power level and the communication transmit power level. The transmit power level of a communication transmitted from the second communication station to the first communication station is set based in part on the weighted path loss estimate and the long-term path loss estimate.

According to an aspect of the present invention, there is provided a method for using a base station in a code division multiple access wireless time division multiplexing communication system, the method comprising: transmitting a signal indicative of reference channel data at a transmit power level; and receiving a communication having a transmission power level set via measuring an interference level; determining the path loss estimate by subtracting the received communication power level from the transmit power level; determining a long-term average of the path loss estimate; multiplying the determined path loss estimate by a first weighting factor α to produce a weighted path loss estimate; multiplying the long-term average of the determined path loss estimates by (1-a) to produce a weighted long-term path loss estimate; and determining the transmission power level by adding the weighted path loss estimate, the weighted long-term path loss estimate, the measured interference level, a target signal-to-interference ratio, and a constant value.

According to another aspect of the present invention, there is provided a time division multiplexing/code division multiple access base station, comprising: a reference channel data generator and an antenna, the antenna transmitting a signal indicating the reference channel data at a transmission power level; and the antenna is for receiving communications having a transmission power level set by measuring an interference level; determining the path loss estimate by subtracting the received communication power level from the transmit power level; determining a long-term average of the path loss estimate; multiplying the determined path loss estimate by a first weighting factor α to produce a weighted path loss estimate; multiplying the long-term average of the determined path loss estimates by (1-a) to produce a weighted long-term path loss estimate; and determining the transmission power level by adding the weighted path loss estimate, the weighted long-term path loss estimate, the measured interference level, a target signal-to-interference ratio, and a constant value.

According to still another aspect of the present invention, there is provided a time division multiplexing/code division multiple access user equipment comprising: means for transmitting a signal indicative of reference channel data at a transmit power level; and means for receiving a communication having a transmission power level set via measuring the interference level; apparatus for: determining the path loss estimate by subtracting the received communication power level from the transmit power level; determining a long-term average of the path loss estimate; multiplying the determined path loss estimate by a first weighting factor α to produce a weighted path loss estimate; multiplying the long-term average of the determined path loss estimates by (1-a) to produce a weighted long-term path loss estimate; and determining the transmission power level by adding the weighted path loss estimate, the weighted long-term path loss estimate, the measured interference level, a target signal-to-interference ratio, and a constant value.

Drawings

Fig. 1 illustrates a prior art TDD system;

FIG. 2 illustrates time slots within a repeating frame of a TDD system;

FIG. 3 shows a flow chart of a weighted open loop power control process;

fig. 4 is a schematic diagram of components of two communication stations employing a weighted open loop power control method;

FIG. 5 shows a performance graph employing a weighted open-loop power control system, an open-loop power control system, and a closed-loop power control system for a UE moving at 30 kilometers per hour (km/h);

fig. 6 shows performance graphs of the above three systems for a UE moving at 60 km/h.

Detailed Description

Preferred embodiments will now be described with reference to the drawings, wherein like reference numerals refer to like parts throughout. Using the flow chart shown in fig. 3 and two simplified communication stations 110 and 110 as shown in fig. 4112, a weighted open loop power control method will be described. For purposes of the ensuing discussion, the communication station whose transmit power is controlled will be referred to as transmitting station 112, while the communication station receiving the controlled power communication will be referred to as receiving station 110. Since weighted open loop power control may be employed for uplink communications, downlink communications, or both, a transmitter with power control may be located at the base station 30₁、UE 32₁Or at the base station 30₁And UE 32₁. Thus, if uplink and downlink power control is employed, components of the receiving station and components of the transmitting station may be located at the base station 30₁And UE 32₁. For use in estimating the path loss between receiving station 110 and transmitting station 112, receiving station 110 sends a communication to transmitting station 112. This communication may be sent over any one of a plurality of channels. Generally, in the TDD system, a channel for estimating a path loss is referred to as a reference channel, but other channels may be used. If receiving station 110 is base station 30₁The communication is preferably sent over a downlink common channel or Common Control Physical Channel (CCPCH).

The data to be transmitted to the transmitting station 112 over the reference channel is referred to as reference channel data. The reference channel data is generated by a reference channel data generator 56. The reference data is allocated to one or more resource units based on the communication bandwidth requirements. Spreading and training (training) sequence insertion means 58 spreads the reference channel data and time division multiplexes the spread reference data with the training sequence within the appropriate slots and codes that allocate resource units. The obtained sequence is referred to as a communication burst. Then, the communication burst is amplified by the amplifier 60. The summing device 62 sums the amplified communication burst with any other communication bursts generated by various devices such as the data generator 50, the extension and training sequence insertion device 52, and the amplifier 54. The modulator 64 modulates the aggregated communication burst. The modulated signal is transmitted through isolator 66 and then antenna 78 as shown, or through an antenna array, step 38. The transmitted signal reaches the antenna 82 of the transmitting station 112 through the radio channel 80. The manner in which the transmitted communication is modulated may be any manner known to those skilled in the art, such as Direct Phase Shift Keying (DPSK) or Quadrature Phase Shift Keying (QPSK).

The antenna 82, or antenna array, of the transmitting station 112 receives various radio frequency signals. The received signal passes through an isolator 84 to a demodulator 86 to produce a baseband signal. The baseband signal is processed within the time slot using appropriate codes assigned to the communication bursts, for example, using channel estimation device 88 and data estimation device 90. The channel estimation device 88 typically utilizes the training sequence component within the baseband signal to provide channel information, such as a channel impulse response. The channel information is used by the data estimation device 90 and the power measurement device 92. At step 40, the power level R of the processed communication corresponding to the reference channel is measured by the power measurement device 92_TSThe measurements are made and sent to the path loss estimation device 94. The channel estimation device 88 may separate the reference channel from all other channels. If an automatic gain control device or amplifier is used to process the received signal, the measured power level may be adjusted at the power measurement device 92 or at the path loss estimation device 94 to correct the gain of these devices.

To determine the path loss L, the transmitting station 112 also needs a transmit power level T for communication_RS. The transmission power level T can be adjusted_RSTransmitted with the communication data or within a signaling channel. If the power level T is to be adjusted_RSTransmitted with the communication data, the data estimation device 90 decodes the power level and then transmits the decoded power level to the path loss estimation device 94. If receiving station 110 is base station 30₁Preferably via a Broadcast Channel (BCH) from the base station 30₁Transmitting a transmission power level T_RS. At step 42, the power level T is transmitted by transmitting from the transmitted communication in dB_RSMinus the received communication power level R in dB_TSThe path loss estimation device 94 estimates the path loss L between the receiving station 110 and the transmitting station 112. Further, at step 44, long term averaging of path lossValue L₀And (6) updating. In some cases, receiving station 110 does not transmit power level T_RSBut rather a reference for the transmit power level. In this case, the path loss estimation device 94 provides the path loss L and the long-term average value L of the path loss₀The reference level of (2).

Since TDD systems transmit downlink and uplink communications in the same spectrum, the conditions experienced by these communications are similar. This phenomenon is called reciprocity. Because of reciprocity, the uplink will experience the path loss experienced by the downlink, as will the downlink. The transmission power level of a communication sent from transmitting station 112 to receiving station 110 may be determined by adding the estimated path loss to the required received power level. This power control technique is referred to as an open loop power control technique.

Open loop systems have drawbacks. If there is a time delay between the estimated path loss and the transmitted communication, the path loss experienced by the transmitted communication may be different from the calculated path loss. In different time slots 36₁To 36_nIn a TDD system with medium transmit communication, the time slot delay between the receive communication and the transmit communication can degrade the performance of the open loop power control system. To overcome these deficiencies, the quality measurement device 96 within the weighted open loop power controller 100 determines the quality of the estimated path loss at step 46. In step 48, the quality measurement device 96 compares the estimated path loss L to the long-term average L of the path loss₀Weighting is performed to set the transmission power level using the transmission power calculation means 98. As shown in fig. 4, the weighted open loop power controller 100 includes a power measuring device 92, a path loss estimating device 94, a quality measuring device 96, and a transmit power calculating device 98.

The following is a preferred weighted open loop power control algorithm. Determining power level P in dB for a transmitting station using equation 1_TS。

P_TS＝P_RS+α(L-L₀)+L₀+ oftenNumerical formula 1

P_RSThe receiving station 110, being in dB, requires a power level to receive the transmitting station's communication. Using the SIR, SIR required by the receiving station 110_TARGETAnd interference level I at receiving station 110_RSDetermining P_RS。

For determining interference level I at receiving station_RSThe communications received from the transmitting station 112 are demodulated by the demodulator 68. The resulting baseband signal is processed, for example, in time slots using channel estimation means 70 and data estimation means 72 and using the appropriate codes assigned to the communication of the transmitting station. The interference measuring means 74 determines the interference level I using the channel information generated by the channel estimation means 70_RS. The channel information may also be used to control the transmit power level of receiving station 110. The channel information is input to the data estimation means 72 and the transmit power calculation means 76. The data estimates produced by the data estimation means 72 and the channel information of the transmit power calculation means 76 are used to control the amplifier 54, the amplifier 54 controlling the transmit power level of the receiving station.

Determination of P using equation 2_RS。

P_RS＝SIR_TARGET+I_RSEquation 2

Or will I_RSFrom receiving station 110 to transmitting station 112, or I_RSFrom receiving station 110 to transmitting station 112. For downlink power control, the SIR is known at the transmitting station 112_TARGET. For uplink power control, SIR is adjusted_TARGETFrom receiving station 110 to transmitting station 112. Using equation 2, equation 1 can be rewritten as equation 3 or equation 4.

P_TS＝SIR_TARGET+I_RS+α(L-L₀)+L₀+ constant equation 3

P_TS＝αL+(1-α)L₀+I_RS+SIR_TARGET+ constant equation 4

L is the pathloss estimate in dB, T_RS-R_TSNearest time slot 36 for estimated path loss₁To 36_n. Long term average L of path loss₀Is a running average of the path loss estimate L. The constant is a correction term. The constant corrects for the difference between the uplink and downlink channels, e.g., compensates for the difference between the uplink and downlink gains. Furthermore, if the transmit power reference level of the transmitting-receiving station is not transmitted, the actual transmit power T is not transmitted_RSThe constant may provide a correction. If the receiving station is a base station 30₁The constants are sent preferentially using layer 3 signaling.

The weight value alpha determined by the quality measurement device 96 is a measure of the estimated path loss quality and is preferably based on the time slot 36 between the time slot n of the last path loss estimate and the first time slot of the communication transmitted by the transmitting station 112₁To 36_nIs measured. The value of α is between 0 and 1. In general, if the time difference between slots is small, the most recent path loss estimate will be quite accurate and set the value of α to be close to 1. Conversely, if this time difference is large, the path loss estimate is inaccurate, and a long-term average path loss measurement is probably not a good estimate of path loss. Therefore, α is set to be close to 0.

Equation 5 is one equation for determining α, but other equations may be used.

α＝1-(D-1)/D_maxEquation 5

D is the time slot 36 between the time slot of the last path loss estimate and the first time slot of the transmitted communication₁To 36_nMay be referred to as a slot delay. If the delay is one slot, then α is 1. D_maxIs the maximum possible delay. A typical value for a frame with 15 slots is 6. If the delay is D_maxOr greater, α approaches 0. In step 48, the transmission power level P determined by means of the transmission power calculation means 98_TSAddingThe burst open loop power controller 100 sets the transmission power of the transmit communication.

Data generator 102 generates data to be transmitted in a communication to be transmitted by transmitting station 112. The spreading and training sequence insertion means 104 spreads and time-multiplexes the communication data with the training sequence in the appropriate time slot and with the code for allocating resource units. The spread signal is amplified by an amplifier 106 and modulated into a radio frequency signal by a modulator 108.

Weighted open loop power controller 100 controls the gain of amplifier 106 to obtain a determined transmit power level P for communication_TS. The communication passes through isolator 84 and is then transmitted by antenna 82.

The graphs 82, 84 shown in fig. 5 and 6 illustrate the performance of a weighted open loop system using equation 4. Equation 5 is used to calculate alpha. Graphs 82, 84 show simulation results comparing the performance of a weighted open loop system, an open loop system, and a closed loop system for controlling the transmit power level of transmitting station 112. This simulation gives the performance of these systems in a fast fading channel under stand-by conditions. In this example, the receiving station is a base station 30₁The transmitting station is UE 32₁. For simulation, the UE 32₁Is a mobile station. Simulated base station 30₁Two antenna diversity is used for reception, each antenna having a 3-finger rake receiver. This simulation can approximate the true channel and SIR in the presence of Additive White Gaussian Noise (AWGN) based on the medium-large sequence of burst type 1 fields. The simulation process employs International Telecommunications Union (ITU) common type B channels and QPSK modulation. It is assumed that the interference level is known explicitly. Regardless of the channel coding method. A constant is added to L₀Are all set to 0dB.

Graph 82 in fig. 5 illustrates, for various power control techniques, utilization of UE 32 moving at 30 kilometers per hour₁The energy in dB (Es/No) of the transmitted complex symbol required for 1% BER is maintained for various slot delays D. As shown, the performance of weighted open loop and open loop is better than that of closed loop at lower slot delayAnd (4) performance. For higher slot delays, the performance of the weighted open loop is better than the performance of the open and closed loops. As shown in the graph 84 of FIG. 6, if the UE 32₁Moving at 60km/h would produce similar results.

Claims (9)

1. A method for using a base station in a code division multiple access wireless time division multiplexed communication system, the method comprising:

transmitting a signal indicative of reference channel data at a transmit power level; and

receiving a communication having a transmission power level set via measuring an interference level;

determining a path loss estimate by subtracting the received communication power level from the transmit power level;

determining a long-term average of the path loss estimate;

multiplying the determined path loss estimate by a first weighting factor α to produce a weighted path loss estimate; multiplying the long-term average of the determined path loss estimates by (1-a) to produce a weighted long-term path loss estimate; and

the transmission power level is determined by adding the weighted path loss estimate, the weighted long-term path loss estimate, the measured interference level, a target signal-to-interference ratio, and a constant value.

2. The method of claim 1 wherein the first weighting factor represents the quality of the path loss estimate.

3. The method of claim 1 wherein the path loss estimate is determined by subtracting a received power level of the signal indicative of reference channel data from a transmit power level transmitted on a broadcast channel, the method further comprising transmitting the broadcast channel.

4. A time division multiplexing/code division multiple access base station comprising:

a reference channel data generator and an antenna, the antenna transmitting a signal indicating the reference channel data at a transmission power level; and

the antenna is configured to receive communications having a transmission power level set by measuring an interference level;

determining a path loss estimate by subtracting the received communication power level from the transmit power level;

determining a long-term average of the path loss estimate; multiplying the determined path loss estimate by a first weighting factor α to produce a weighted path loss estimate;

multiplying the long-term average of the determined path loss estimates by (1-a) to produce a weighted long-term path loss estimate; and

the transmission power level is determined by adding the weighted path loss estimate, the weighted long-term path loss estimate, the measured interference level, a target signal-to-interference ratio, and a constant value.

5. The TDMA base station according to claim 4 wherein said first weighting factor represents quality of said path loss estimate.

6. The time division multiplexing/code division multiple access base station of claim 4 wherein the path loss estimate is determined by subtracting a received power level of the signal indicative of reference channel data from a transmit power level transmitted on a broadcast channel over which the antenna is used to transmit.

7. A time division multiplexing/code division multiple access user equipment comprising:

means for transmitting a signal indicative of reference channel data at a transmit power level; and

means for receiving a communication having a transmission power level set via measuring an interference level;

apparatus for:

determining a path loss estimate by subtracting the received communication power level from the transmit power level;

determining a long-term average of the path loss estimate;

multiplying the determined path loss estimate by a first weighting factor α to produce a weighted path loss estimate;

multiplying the long-term average of the determined path loss estimates by (1-a) to produce a weighted long-term path loss estimate; and

the transmission power level is determined by adding the weighted path loss estimate, the weighted long-term path loss estimate, the measured interference level, a target signal-to-interference ratio, and a constant value.

8. The time division multiplexing/code division multiple access user equipment of claim 7 wherein the first weighting factor represents the quality of the path loss estimate.

9. The tdm/cdma user equipment of claim 7 wherein the pathloss estimate is determined by subtracting a received power level of the signal indicative of reference channel data from a transmit power level transmitted on a broadcast channel, the means for transmitting the signal at the transmit power level being used to transmit the broadcast channel.