HK1117284B - A user device for spread spectrum time division duplex communication - Google Patents

Description

User equipment for spread spectrum time division duplex communication

The application is a divisional application of an application with the application number of 200410059280.X and the invention name of 'outer loop/weighted open loop power control in a time division duplex communication system' filed on 6, 15 and 6 of 2004.

Technical Field

The present invention relates to a spread spectrum Time Division Duplex (TDD) communication system, and more particularly, to a user equipment for spread spectrum time division duplex communication.

Background

Fig. 1 depicts a wireless spread spectrum Time Division Duplex (TDD) communication system. The system has a plurality of base stations 30₁-30₇. Each base station is in its operating area with User Equipment (UE)32₁-32₃Communication is performed. From a base station 30₁To a user equipment 32₁Is referred to as downlink communication, and is provided by a user equipment 32₁Transmitting to a base station 30₁Is referred to as uplink communication.

In addition to communicating over different frequency spectrums, spread spectrum time division duplex communication systems also communicate in multiple paths over the same frequency spectrum. The multiple signals are distinguished by their respective chip sequences (codes). To make more efficient use of spread spectrum, a time division duplex system, such as that shown in fig. 2, uses a repeating frame 34, the repeating frame 34 being divided into a number of time slots 36₁-36_nFor example 16 time slots. In such a system, at selected time slots 36₁-36_nWherein a communication is transmitted using the selected code. Thus, one frame 34 can carry multiple communications that are distinguished by time slots and codes in common. The combination of a single code in a single slot is referred to as a resource unit. One or more resource units are allocated to a communication based on the bandwidth required to support the communication.

Most time division duplex systems automatically control the transmit power level. In a time division duplex system, many communications may share the same time slot and frequency spectrum. When a user device 32₁Or a base station 30₁When a particular communication is received, all other communications sharing the same time slot and frequency spectrum cause interference to the particular communication. Increasing the transmit power level of one communication decreases the signal quality of all other communications in the same time slot and spectrum. However, excessive reduction in the transmission power level results in undesirable signal-to-noise ratio (SNR) and Bit Error Rate (BER) at the recipient. To simultaneously maintain the signal quality and low transmit power level of the communication, transmit power control techniques are employed.

U.S. patent No.5,056,109 (Gilhousen et al) describes one way to apply transmit power control in a Code Division Multiple Access (CDMA) communication system. The transmitter sends a communication to a particular receiver. During reception, the power of the received signal is measured. The power of the received signal is compared to the required received signal power. Based on the comparison, a control code is sent to the transmitter to increase or decrease the transmit power by a fixed amount. Such power control techniques are commonly referred to as closed-loop because the receiver sends a control signal to the transmitter to control the power level of the transmitter.

Under certain conditions, the performance of the closed loop system may be degraded. For example, if a user communicates with a base station in a high-speed environment, such as when the user is moving, such a system may not be able to adapt to, compensate for, and/or compensate for, the changes that occur as quickly as possible. The update rate of closed loop power control in a time division duplex system is 100 cycles per second, which is not fast enough for fast fading channels. Thus, there is a need to maintain signal quality and low transmit power levels through other means.

WO 9845962 discloses a method of controlling the transmit power level in a satellite communications system. The power control method has open and closed loop elements. For closed-loop elements, the base station calculates the functional settings of the mobile terminal based on the strength of the signal received from the mobile terminal. The base station takes into account the satellite system propagation delay in the power setting decision. For the 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 inversely with the observed change in signal strength.

Us patent No.5,542,111 discloses a method of adjusting mobile station transmit power control with long term and short term transmit power control. Long-term power control occurs in the base station at an upper layer forming closed-loop control. The base station transmits a statement of the decision unit to the mobile station. The short term transmit power level is implemented on the lower loop with an identifier of the long term power and a decision unit.

Therefore, additional methods are needed to maintain signal quality and low transmit power levels.

Disclosure of Invention

The present invention provides a user equipment for spread spectrum time division duplex communication, which communicates using frames having time slots, the user equipment comprising: means for receiving a first communication having a transmit power level in a first time slot and measuring a power level of the communication; means for determining a path loss estimate by subtracting the received power level from the measured power level; means for setting a transmit power level to transmit a second communication in a second time slot, wherein the path loss estimate weighted by a first factor and a long term path loss weighted by a second factor are used to determine a transmit power level for transmission of the second communication, wherein the first and second factors are a function of a time interval of the first and second time slots; and means for transmitting the second communication in the second time slot at the set transmit power level; and means for determining a quality a of the path loss estimate between the first and second time slots as a function of a number of time slots D; and wherein the first factor is a and the second factor is 1-a.

Drawings

Fig. 1 illustrates a prior art time division duplex system.

Fig. 2 illustrates time slots of a repeating frame of a time division duplex system.

Fig. 3 is a flow chart of outer loop/weighted open loop power control.

Fig. 4 is a component diagram of two communication stations applying outer loop/weighted open loop power control.

Fig. 5 is a graph of performance of an outer loop/weighted open loop power control system, a weighted open loop power control system, and a closed loop power control system.

Fig. 6 is a graph of the performance of the three systems described above in terms of block error rate (BLER).

Detailed Description

The preferred embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. The outer loop/weighted open loop power control will be described with reference to the block diagram of fig. 3 and the components of the two simplified communication stations 110, 112 shown in fig. 4. In the following description, a communication station in which transmitter power is controlled is referred to as a transmitting station 112 and a communication station in which communication whose reception power has been controlled is referred to as a receiving station 110. Since outer loop/weighted open loop power control may be used for uplink, downlink, or both types of communication, the power controlled transmitter may be related to the base station 30₁User device 32₁Or both, to the base station 30₁And to the user equipment 32₁. Thus, components of both receiving and transmitting stations are involved in the base station 30 if both uplink and downlink power control is used₁And to the user equipment 32₁。

In step 38, receiving station 110 receives various radio frequency signals including communications transmitted by transmitting station 112 using one antenna 78 or one antenna array. The received signal is passed through an isolator 66 to a demodulator 68 to produce a baseband signal. The baseband signal is processed, for example, by a channel estimation device 70 and a data estimation device 72 in time slots with appropriate coding assigned to the communication of the transmitting station. The channel estimation device 70 typically uses the training sequence component of the baseband signal to provide channel information, such as the channel impulse response. The channel information is used by data estimation means 72, interference measurement means 74 and transmit power calculation means 76. The data estimation device 72 uses the channel information to recover the data from the channel by estimating soft symbols.

Prior to transmission of a communication at transmitting station 112, the data signal in the communication is error encoded by an error detection/correction encoder 110. A typical coding scheme is a Cyclic Redundancy Code (CRC) followed by forward error correction coding, although other types of error coding schemes may also be used.

Using the soft symbols generated by the data estimation means 72, the error detection means 112 detects errors in the soft symbols. In step 39, processor 111 analyzes the detected errors and determines the error rate of the received communication. Based on the error rate, processor 111 determines the amount, if any, that needs to be changed at transmitting station 112 at the target level, e.g., the target signal to interference ratio (SIR), step 40_TARGET) The amount of change in (c). Based on the determined amount, the target adjustment generator 114 generates a target adjustment signal. The target adjustment signal is then sent to the transmitting station in step 41. The target adjustment signal is transmitted to the transmitting station 112 via, for example, a dedicated channel or a reference channel.

One technique for determining the target level adjustment uses an upper and lower bound method. If the determined error rate exceeds the upper limit, the target level is at an unacceptably low level and needs to be increased. A target level adjustment signal is issued to indicate that the target level is to be increased. If the determined error rate is below the lower limit, the target level is at an unnecessarily high level and may be lowered. By lowering the target level, the power level of the transmitting station is lowered to reduce interference with other communications using the same time slot and spectrum. To improve performance, target adjustments are issued as soon as the error rate exceeds an upper limit. As a result, the high error rate is improved quickly and the low error rate is adjusted slowly, e.g., once every 10 seconds. If the error rate is between the upper and lower limits, the target adjustment is not issued and the target level is maintained.

The following description applies the above technique to a system using Cyclic Redundancy Code (CRC) and forward error correction(FEC) coded systems. Each Cyclic Redundancy Code (CRC) code group is used to check for errors. The counter is incremented each time a frame is determined to have an error. Once the number of counters exceeds an upper limit, e.g., 1.5 to 2 times the required block error rate (BLER), a target adjustment is sent to increase the target level. In order to adjust the target signal-to-interference ratio (SIR) of the transmitting station 112_TARGET)，SIR_TARGETIncrease in (SIR)_INC) It is transmitted at a value typically in the range of 0.25dB to 4 dB. If the number of accumulated CRC frames exceeds a predetermined limit, e.g., 1000 code blocks, the value of the counter is compared to the aforementioned lower limit, which may be, for example, 0.2 to 0.6 times the required BLER. If the accumulated number of block errors is below the lower limit, a target adjustment signal SIR for lowering the target level is transmitted_DECWith a typical range of 0.25dB to 4 dB. SIR_DECMay be based on SIR_INCAnd target code group error rate BLER_TARGETAnd then. BLER_TARGETA typical range is 0.1% to 10% depending on the type of service. Equation 1 illustrates determining the SIR_DECThe method of (1).

SIR_DEC＝SIR_INC×BLER_TARGET/(1-BLER_TARGET) Equation 1

If the number in the counter is between the upper and lower limits of the predetermined code group, the target adjust signal is not transmitted.

Alternatively, a single threshold may be used. If the error rate exceeds the threshold, the target level is increased. If the error rate is below the threshold, the target level is lowered. In addition, the target level adjustment signal may have several adjustment levels, for example, the value of the target level adjustment signal may be between 0dB and ± 4dB in increments of 0.25dB, depending on the difference between the determined error rate and the requested error rate.

The interference measuring device 74 of the receiving station 110 determines the interference level I in decibels in the channel_RSThe determination is based on channel information or onThe soft symbols generated by the data estimation device 72, or both. Using the soft symbols and channel information, transmit power calculation means 76 controls the transmit power level of the receiving station by controlling the gain of amplifier 54.

In step 41, receiving station 110 sends a communication to transmitting station 112 for estimating the path loss between receiving station 110 and transmitting station 112. The communication may be sent over any of a number of channels. In a time division duplex system, the channel used to estimate the path loss is generally referred to as the reference channel, although other channels may be used to estimate the path loss. If receiving station 110 is a base station 30₁Preferably, the communication is sent over a downlink common channel or a Common Control Physical Channel (CCPCH). The data transmitted to the transmitting station 112 over the reference channel is referred to as reference channel data. The reference channel data may include an interference level I as shown_RSThe interference level I_RSWith other reference data, e.g. transmission power level T_RSAre multiplexed together. Interference level I_RSAnd a reference channel power level T_RSMay be transmitted by other channels, such as a signal channel.

The reference channel data is generated by a reference channel data generator 56. The reference data is allocated one or more resource units according to the bandwidth requirements of the communication. A spreading (spread) and training sequence insertion means 58 spreads the reference channel data and time multiplexes the reference data with the training sequence in the appropriate time slot and the code of the allocated resource unit. The resulting sequence is referred to as a communication burst. The communication burst is then amplified by an amplifier 60. An adder 62 adds the amplified communication burst to communication bursts generated by other means such as the data generator 50, the spreading and training sequence insertion means 52 and the amplifier 54.

The summed communication bursts are modulated by a modulator 64. The modulated signal is transmitted through an isolator 66 as shown by an antenna 78 or through an antenna array. The transmitted signal travels through a wireless radio frequency channel 80 to an antenna 82 of a transmitting station 112. The type of modulation used for the transmit communication may be any 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 including a target adjustment signal. The received signal is passed through an isolator 84 to a demodulator 86 to produce a baseband signal. The baseband signal is processed, for example, by a channel estimation device 88 and a data estimation device 90 in the series of time slots along with the appropriate code of the communication burst assigned to the receiving station 110. The channel estimation device 88 typically utilizes the training sequence component in the baseband signal to provide channel information, such as the channel impulse response. The channel information is used by the data estimation device 90 and the power measurement device 92.

In step 42, the power level R of the processed communication corresponding to the reference channel_TSMeasured by the power measuring device 92 and then sent to a path loss estimation device 94. Both the channel estimation device 88 and the data estimation device 90 are capable of separating the reference channel from the other channels. If an automatic gain control device or amplifier is used to process the received signal, the measured power level is adjusted to correct the gain of the automatic gain control device or amplifier, either at the power measurement device 92 or at the path loss estimation device 94. The power measurement device is a component of the outer loop/weighted open loop control device 100. As shown in fig. 4, the outer loop/weighted open loop control means 100 includes a power measuring means 92, a path loss estimating means 94, a quality measuring means 96, a target updating means 101, and a transmission power calculating means 98.

To determine the path loss L, the transmitting station 112 also requires a transmit power level T for communication_RS. Transmission power level T of communication_RSMay be transmitted with the communication data or by a signal channel. If the transmission power level T_RSTransmitted together with the communication data, the data estimation means 90 translates the power level and converts the translated power levelTo the path estimation device 94. If receiving station 110 is a base station 30₁Then the transmission power level T_RSPreferably via a wireless communication from the base station 30₁Is transmitted over the broadcast channel. In step 43, the transmit power level T is adjusted by the communication to be transmitted_RSSubtracting the power level R of the received communication_TSThe path loss estimation device 94 estimates the path loss L between the two communication stations 110, 112. In addition, in step 44, a long-term estimate L of the path loss₀Is updated. An example of a long-term path loss estimate is a long-term average. Long term average L of path loss₀Is the average of the path loss estimates. In some cases, receiving station 110 may transmit a transmit power level reference instead of transmit power level T_RS. Thus, the path loss estimation device 94 provides a reference level for the path loss L.

Since time division duplex systems transmit downlink and uplink communications within the same frequency spectrum, the conditions experienced by these communications are similar. This phenomenon is called reciprocity (reciprocity). Due to reciprocity, the path loss experienced by the downlink will also be experienced by the uplink, and vice versa. By applying an estimated path loss to the target level, the transmit power level of the communication transmitted by transmitting station 112 to receiving station 110 is determined.

If there is a time delay between the estimated path loss and the transmitted communication, the path loss experienced by the transmitted communication may differ from the calculated path loss. In a time division duplex system, if communication is in different time slots 36₁-36_nThe delay between transmitted, received and transmitted communications can degrade the performance of the open loop power control system. To overcome these disadvantages, the weighted open loop power control system determines the quality of the estimated path loss using a quality measurement device 96 and accordingly estimates the path loss L and the long term average L of the path loss L in step 45₀And (4) weighting.

To further enhance the performance of the outer loop/weighted open loop, the target levelIt is adjusted. Processor 103 converts the soft symbols generated by data estimator 90 into bits and extracts target adjustment information, such as SIR_TARGETAnd (6) adjusting. In step 46, the target update means 101 adjusts the target level using the target adjustment. The target level may be the SIR at the receiving station 110_TARGETOr a target received power level.

In step 47, transmission power calculation means 98 compares the adjusted target level with weighted path loss estimate L and long-term average value L of the path loss estimates₀The transmit power level of the transmitting station is determined in combination.

Data to be transmitted in a communication from transmitting station 112 is generated at data generator 102. The data is error detection/correction encoded by the error detection/correction encoder 110. The error-coded data is spread and time-multiplexed at the appropriate time slot with a training sequence generated by the training sequence insertion means 104 and the code of the assigned resource unit, thereby generating a communication burst. The spread spectrum signal is amplified by an amplifier 106 and modulated to radio frequency by a modulator 108. The gain of the amplifier is controlled by the transmit power calculation means 98 to achieve the determined transmit power level. The power controlled communication burst passes through an isolator 84 and is then transmitted by the antenna 82.

The following is an algorithm for outer loop/weighted open loop power control. Transmitting power level P of transmitting station in decibel unit_TSDetermined by equation 2.

P_TS＝SIR_TARGET+I_RS+α(L-L₀)+L₀+ CONSTANT VALUE equation 2

SIR_TARGETHaving an adjustment value according to the received target adjustment signal. For the downlink, SIR_TARGETIs known at the transmitting station 112. SIR for power control of the uplink_TARGETFrom receiving station 110 to transmitting station 112. Additionally, the adjusted SIR may also be communicated_TARGETMaximum and minimum values of. Adjusted SIR_TARGETIs limited to a value between a maximum value and a minimum value. I is_RSIs a measure of the interference power level of the receiving station 110.

L is the nearest slot 36 for which the path loss is estimated₁-36_nIs estimated as the path loss in decibels, i.e., T_RS-R_TS。L₀The long-term average of the path loss in decibels is the dynamic average of the estimated value L of the path loss. CONSTANT VALUE is a correction term. The CONSTANT VALUE corrects for differences between uplink and downlink channels, e.g., compensates for differences in gain between uplink and downlink. In addition, if the transmission power reference level of the receiving station is transmitted instead of the actual transmission power T_RSCONSTANTVALUE may provide a correction. If receiving station 110 is a base station, the CONSTANT VALUE is preferably sent via a Layer 3 message.

The weight value α is a quality measure of the estimated path loss, preferably based on the time slot 36 between the final path loss estimation time slot n and the initial time slot in the communication sent by the transmitting station 112₁-36_nN. The value of α is between 0 and 1. In general, if the time difference between slots is small, the nearest path loss estimate will be quite accurate, with a value close to 1. Conversely, if the time difference is large, the path loss estimate may not be accurate and the long term average of the path loss measurements is likely to be a better estimate of the path loss. Accordingly, α is set to a value closer to 1.

Equations 3 and 4 are equations for determining the value of alpha.

α＝1-(D-1)/(D_max-1) equation 3

α＝max{1-(D-1)/(D_max-allowed-1), 0 equation 4

D is a time slot 36₁-36_nIn a transmitting communicationThe number of slots between the final path loss estimation slot and the initial path loss estimation slot is called the slot delay. If the delay is one slot, α is 1. D_maxIs the largest possible delay, which is typically 7 for a frame having 15 slots. If the delay is D_maxThen α is 0. D_max-allowedIs the maximum allowed slot delay with open loop power control. If the delay exceeds D_max-allowedThen open loop power control is effectively turned off by setting alpha to 0. Transmission power of transmission communication is determined by transmission power calculating means 98_TSTo set.

Fig. 5 and 6 compare the performance of weighted outer loop/open loop, open loop and closed loop systems. The simulations in fig. 5 and 6 differ slightly from the algorithm of the outer loop/weighted open loop model. In this model, the target SIR for each block is updated. If a block error is detected, the SIR is increased_TARGET(ii) a If no block error is detected, the SIR is reduced_TARGET. The outer loop/weighted open loop system uses equation 2. Equation 3 is used to calculate α. The simulation compares the system in controlling a user device 32₁Performance at the transmit power level of. For simulation, each block is filled with 16-bit cyclic redundancy codes, each block being four frames. When an error of at least two original bits occurs in one code group, a code group error is declared. For the uplink communication channel, one time slot is allocated per frame. The target for the block error rate is 10%. Updating SIR every 4 frames_TARGET. The above simulations show that these systems operate for a user equipment UE32 moving 30 km per hour₁The performance of (c). The simulated base station uses two antenna diversity for reception, one three-finger RAKE receiver per antenna. The simulation approximates a true channel, and the SIR estimate is based on the middle segment (midamble) sequence of the type 1 burst segment in Additive White Gaussian Noise (AWGN). The simulation used an International Telecommunications Union (ITU) common type B channel and Quadrature Phase Shift Keying (QPSK) modulation. The interference level is assumed to be without uncertainty. The channel coding scheme is not considered. L is₀Set to 0 dB.

Plot 120 in FIG. 5 is shown for 10^-1Required BLER of_S/N_OThe expected performance as a function of the time delay between the uplink time slot and the most recent downlink time slot. The delay is expressed by the number of slots. E_SIs the energy of the composite symbol. FIG. 5 illustrates: when the gain/interference uncertainty is ignored, the performance of the combined system is nearly equal to the weighted open loop system. The combined system outperforms the closed loop system for all delays.

In the presence of gain and interference uncertainties, the transmit power level of an open loop system is either too high or too low relative to the nominal value. In plot 122 in FIG. 6, a gain uncertainty of-2 dB is used. Figure 6 shows BLER as a function of delay. To obtain a 10^-1BLER, initial reference SIR of each system_TARGETIs set to its corresponding nominal value derived from fig. 5. Fig. 6 illustrates that both the combined system and the closed-loop system achieve the required BLER in the presence of gain uncertainty. The performance of a weighted open loop system drops significantly.

Claims (3)

1. A user equipment for spread spectrum time division duplex communication communicating using frames having time slots, the user equipment comprising:

means for receiving a first communication having a transmit power level in a first time slot and measuring a power level of the communication;

means for determining a path loss estimate by subtracting the received power level from the measured power level;

means for setting a transmit power level to transmit a second communication in a second time slot, wherein an adjusted target level, the path loss estimate weighted by a first factor, and a long term estimate of path loss weighted by a second factor are used to determine a transmit power level for transmission of the second communication, wherein the first and second factors are a function of a time interval of the first and second time slots; and

means for transmitting the second communication in the second time slot at the set transmit power level; and

means for determining a quality a of the path loss estimate between the first and second time slots as a function of a number of time slots D; and

wherein the first factor is a and the second factor is 1-a.

2. The user equipment of claim 1, wherein the maximum slot delay is D_maxAnd α is determined by the following formula: α ═ 1- (D-1)/(D)_max-1)。

3. The user equipment of claim 1, wherein the maximum allowed slot delay is D_max-allowedAnd the determined mass α is determined by: α ═ max {1- (D-1)/(D)_max-allowed-1)，0}。