Abstract:
A user equipment (UE) with circuitry configured to determine a pathloss associated with a received signal. The circuitry is configured to receive an adjustment and adjust a value in response to the received adjustment. The circuitry is configured to determine a transmit power level based on multiplying the determined pathloss by a parameter and adding the adjusted value to a result of the multiplying, wherein the parameter is value in the range of 0 to 1. The circuitry is configured to transmit a signal at the determined transmit power level.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/545,665, filed Oct. 10, 2006, which is a continuation of U.S. application Ser. No. 10/435,796, filed May 12, 2003, which issued on Oct. 10, 2006 as U.S. Pat. No. 7,120,188, which is a continuation of U.S. application Ser. No. 09/533,423, filed Mar. 22, 2000, which issued on Aug. 5, 2003 as U.S. Pat. No. 6,603,797; which are incorporated herein by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     This invention generally relates to spread spectrum time division duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling transmission power within TDD communication systems. 
     BACKGROUND 
       FIG. 1  depicts a wireless spread spectrum time division duplex (TDD) communication system. The system has a plurality of base stations  30   1 - 30   7 . Each base station  30   1  communicates with user equipment (UEs)  32   1 - 32   3  in its operating area. Communications transmitted from a base station  30   1  to a UE  32   1  are referred to as downlink communications and communications transmitted from a UE  32   1  to a base station  30   1  are referred to as uplink communications. 
     In addition to communicating over different frequency spectrums, spread spectrum TDD systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective chip code sequences (codes). Also, to more efficiently use the spread spectrum, TDD systems as illustrated in  FIG. 2  use repeating frames  34  divided into a number of time slots  36   1 - 36   n , such as sixteen time slots. In such systems, a communication is sent in selected time slots  36   1 - 36   n  using selected codes. Accordingly, one frame  34  is capable of carrying multiple communications distinguished by both time slot and code. The combination of a single code in a single time slot is referred to as a resource unit. Based on the bandwidth required to support a communication, one or multiple resource units are assigned to that communication. 
     Most TDD systems adaptively control transmission power levels. In a TDD system, many communications may share the same time slot and spectrum. When a UE  32   1  or base station  30   1  is receiving a specific communication, all the other communications using the same time slot and spectrum cause interference to the specific communication. Increasing the transmission power level of one communication degrades the signal quality of all other communications within that time slot and spectrum. However, reducing the transmission power level too far results in undesirable signal to noise ratios (SNRs) and bit error rates (BERs) at the receivers. To maintain both the signal quality of communications and low transmission power levels, transmission power control is used. 
     One approach using transmission power control in a code division multiple access (CDMA) communication system is described in U.S. Pat. No. 5,056,109 (Gilhousen et al.). A transmitter sends a communication to a particular receiver. Upon reception, the received signal power is measured. The received signal power is compared to a desired received signal power. Based on the comparison, a control bit is sent to the transmitter either increasing or decreasing transmission power by a fixed amount. Since the receiver sends a control signal to the transmitter to control the transmitter&#39;s power level, such power control techniques are commonly referred to as closed loop. 
     Under certain conditions, the performance of closed loop systems degrades. For instance, if communications sent between a UE and a base station are in a highly dynamic environment, such as due to the UE moving, such systems may not be able to adapt fast enough to compensate for the changes. The update rate of closed loop power control in TDD is typically 100 cycles per second which is not sufficient for fast fading channels. Accordingly, there is a need for alternate approaches to maintain signal quality and low transmission power levels. 
     SUMMARY 
     A user equipment (UE) is disclosed with circuitry configured to determine a pathloss associated with a received signal. The circuitry is configured to receive an adjustment and adjust a value in response to the received adjustment. The circuitry is configured to determine a transmit power level based on multiplying the determined pathloss by a parameter and adding the adjusted value to a result of the multiplying, wherein the parameter is value in the range of 0 to 1. The circuitry is configured to transmit a signal at the determined transmit power level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art TDD system. 
         FIG. 2  illustrates time slots in repeating frames of a TDD system. 
         FIG. 3  is a flow chart of outer loop/weighted open loop power control. 
         FIG. 4  is a diagram of components of two communication stations using outer loop/weighted open loop power control. 
         FIG. 5  is a graph of the performance of outer loop/weighted open loop, weighted open loop and closed loop power control systems. 
         FIG. 6  is a graph of the three systems performance in terms of Block Error Rate (BLER). 
     
    
    
     DETAILED DESCRIPTION 
     The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. Outer loop/weighted open loop power control will be explained using the flow chart of  FIG. 3  and the components of two simplified communication stations  110 ,  112  as shown in  FIG. 4 . For the following discussion, the communication station having its transmitter&#39;s power controlled is referred to as the transmitting station  112  and the communication station receiving power controlled communications is referred to as the receiving station  110 . Since outer loop/weighted open loop power control may be used for uplink, downlink or both types of communications, the transmitter having its power controlled may be associated with the base station  30   1 , UE  32   1  or both. Accordingly, if both uplink and downlink power control are used, the receiving and transmitting station&#39;s components are associated with both the base station  30   1  and UE  32   1 . 
     The receiving station  110  receives various radio frequency signals including communications from the transmitting station  112  using an antenna  78 , or alternately, an antenna array, step  38 . The received signals are passed thorough an isolator  66  to a demodulator  68  to produce a baseband signal. The baseband signal is processed, such as by a channel estimation device  70  and a data estimation device  72 , in the time slots and with the appropriate codes assigned to the transmitting station&#39;s communication. The channel estimation device  70  commonly uses the training sequence component in the baseband signal to provide channel information, such as channel impulse responses. The channel information is used by the data estimation device  72 , the interference measurement device  74 , and the transmit power calculation device  76 . The data estimation device  72  recovers data from the channel by estimating soft symbols using the channel information. Prior to transmission of the communication from the transmitting station  112 , the data signal of the communication is error encoded using an error detection/correction encoder  117 . The error encoding scheme is typically a circular redundancy code (CRC) followed by a forward error correction encoding, although other types of error encoding schemes may be used. Using the soft symbols produced by the data estimation device  72 , an error detection device  113  detects errors in the soft symbols. A processor  111  analyzes the detected error and determines an error rate for the received communication, step  39 . Based on the error rate, the processor  111  determines the amount, if any, a target level, such as a target signal to interference ration (SIR TARGET ), needs to be changed at the transmitting station  112 , step  40 . Based on the determined amount, a target adjustment signal is generated by the target adjustment generator  114 . The target adjustment is subsequently sent to the transmitting station, step  41 . The target adjustment is signaled to the transmitting station  112 , such as using a dedicated or a reference channel as shown in  FIG. 4 , step  41 . 
     One technique to determine the amount of adjustment in the target level uses an upper and lower threshold. If the determined error rate exceeds an upper threshold, the target level is set at an unacceptably low level and needs to be increased. A target level adjustment signal is sent indicating an increase in the target level. If the determined error rate is below a second threshold, the target level is set at an unnecessarily high level and the target level can be decreased. By reducing the target level, the transmitting station&#39;s power level is decreased reducing interference to other communications using the same time slot and spectrum. To improve performance, as soon as the error rate exceeds the upper limit, a target adjustment is sent. As a result, high error rates are improved quickly and lower error rates are adjusted slowly, such as once per 10 seconds. If the error rate is between the thresholds, a target adjustment is not sent maintaining the same target level. 
     Applying the above technique to a system using CRC and FEC encoding follows. Each CRC block is checked for an error. Each time a frame is determined to have an error, a counter is incremented. As soon as the counter exceeds an upper threshold, such as 1.5 to 2 times the desired block error rate (BLER), a target adjustment is sent increasing the target level. To adjust the SIR TARGET  at the transmitting station  112 , the increase in the SIR TARGET  is sent (SIR INC ), which is typically in a range of 0.25 dB to 4 dB. If the number of CRC frames encountered exceeds a predetermined limit, such as 1000 blocks, the value of the counter is compared to a lower threshold, such as 0.2 to 0.6 times the desired BLER. If the number of counted block errors is below the lower threshold, a target adjustment signal is sent decreasing the target level, SIR DEC . A typical range of SIR DEC  is 0.25 to 4 dB. The value of SIR DEC  may be based on SIR INC  and a target block error rate, BLER TARGET . The BLER TARGET  is based on the type of service. A typical range for the BLER TARGET  is 0.1% to 10%. Equation 1 illustrates one such approach for determining SIR DEC .
 
SIR DEC =SIR INC ×BLER TARGET /(1−BLER TARGET )  Equation 1
 
     If the count is between the thresholds for the predetermined block limit, a target adjustment signal is not sent. 
     Alternately, 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 is decreased. Additionally, the target level adjustment signal may have several adjustment levels, such as from 0 dB to ±4 dB in 0.25 dB increments based on the difference between the determined error rate and the desired error rate. 
     The interference measurement device  74  of the receiving station  110  determines the interference level in dB, I RS , within the channel, based on either the channel information, or the soft symbols generated by the data estimation device  72 , or both. Using the soft symbols and channel information, the transmit power calculation device  76  controls the receiving station&#39;s transmission power level by controlling the gain of an amplifier  54 . 
     For use in estimating the pathloss between the receiving and transmitting stations  110 ,  112  and sending data, the receiving station  110  sends a communication to the transmitting station  112 , step  41 . The communication may be sent on any one of the various channels. Typically, in a TDD system, the channels used for estimating pathloss are referred to as reference channels, although other channels may be used. If the receiving station  110  is a base station  30   1 , the communication is preferably sent over a downlink common channel or a common control physical channel (CCPCH). Data to be communicated to the transmitting station  112  over the reference channel is referred to as reference channel data. The reference data may include, as shown, the interference level, I RS , multiplexed with other reference data, such as the transmission power level, T RS . The interference level, I RS , and reference channel power level, I RS , may be sent in other channels, such as a signaling channel. 
     The reference channel data is generated by a reference channel data generator  56 . The reference data is assigned one or multiple resource units based on the communication&#39;s bandwidth requirements. A spreading and training sequence insertion device  58  spreads the reference channel data and makes the spread reference data time-multiplexed with a training sequence in the appropriate time slots and codes of the assigned resource units. The resulting sequence is referred to as a communication burst. The communication burst is subsequently amplified by an amplifier  60 . The amplified communication burst may be summed by a sum device  62  with any other communication burst created through devices, such as a data generator  50 , spreading and training sequence insertion device  52  and amplifier  54 . 
     The summed communication bursts are modulated by a modulator  64 . The modulated signal is passed thorough an isolator  66  and radiated by an antenna  78  as shown or, alternately, through an antenna array. The radiated signal is passed through a wireless radio channel  80  to an antenna  82  of the transmitting station  112 . The type of modulation used for the transmitted communication can be any of those known to those skilled in the art, such as direct phase shift keying (DPSK) or quadrature phase shift keying (QPSK). 
     The antenna  82  or, alternately, antenna array of the transmitting station  112  receives various radio frequency signals including the target adjustments. The received signals are passed through an isolator  84  to a demodulator  86  to produce a baseband signal. The baseband signal is processed, such as by a channel estimation device  88  and a data estimation device  90 , in the time slots and with the appropriate codes assigned to the communication burst of the receiving station  110 . The channel estimation device  88  commonly uses the training sequence component in the baseband signal to provide channel information, such as channel impulse responses. The channel information is used by the data estimation device  90  and a power measurement device  92 . 
     The power level of the processed communication corresponding to the reference channel, R TS , is measured by the power measurement device  92  and sent to a pathloss estimation device  94 , step  42 . Both the channel estimation device  88  and the data estimation device  90  are capable of separating the reference channel from all other channels. If an automatic gain control device or amplifier is used for processing the received signals, the measured power level is adjusted to correct for the gain of these devices at either the power measurement device  92  or pathloss estimation device  94 . The power measurement device is a component of an outer loop/weighted open loop controller  100 . As shown in  FIG. 4 , the outer loop/weighted open loop controller  100  comprises the power measurement device  92 , pathloss estimation device  94 , quality measurement device  96 , target update device  101 , and transmit power calculation device  98 . 
     To determine the path loss, L, the transmitting station  112  also requires the communication&#39;s transmitted power level, T RS . The communication&#39;s transmitted power level, T RS , may be sent along with the communication&#39;s data or in a signaling channel. If the power level, T RS , is sent along with the communication&#39;s data, the data estimation device  90  interprets the power level and sends the interpreted power level to the pathloss estimation device  94 . If the receiving station  110  is a base station  30   1 , preferably the transmitted power level, T RS , is sent via the broadcast channel (BCH) from the base station  30   1 . By subtracting the received communication&#39;s power level, R TS , from the sent communication&#39;s transmitted power level, T RS , the pathloss estimation device  94  estimates the path loss, L, between the two stations  110 ,  112 , step  43 . Additionally, a long term average of the pathloss, L 0 , is updated, step  44 . The long term average of the pathloss, L 0 , is an average of the pathloss estimates. In certain situations, instead of transmitting the transmitted power level, T RS , the receiving station  110  may transmit a reference for the transmitted power level. In that case, the pathloss estimation device  94  provides reference levels for the pathloss, L. 
     Since TDD systems transmit downlink and uplink communications in the same frequency spectrum, the conditions these communications experience are similar. This phenomenon is referred to as reciprocity. Due to reciprocity, the path loss experienced for the downlink will also be experienced for the uplink and vice versa. By adding the estimated path loss to a target level, a transmission power level for a communication from the transmitting station  112  to the receiving station  110  is determined. 
     If a time delay exists between the estimated path loss and the transmitted communication, the path loss experienced by the transmitted communication may differ from the calculated loss. In TDD where communications are sent in differing time slots  36   1 - 36   n , the time slot delay between received and transmitted communications may degrade the performance of an open loop power control system. To overcome these drawbacks, weighted open loop power control determines the quality of the estimated path loss using a quality measurement device  96 , step  45 , and weights the estimated path loss accordingly, L, and long term average of the pathloss, L 0 . 
     To enhance performance further in outer loop/weighted open loop, a target level is adjusted. A processor  103  converts the soft symbols produced by the data estimation device  90  to bits and extracts the target adjustment information, such as a SIR TARGET  adjustment. A target update device  101  adjusts the target level using the target adjustments, step  46 . The target level may be a SIR TARGET  or a target received power level at the receiving station  110 . 
     The transmit power calculation device  98  combines the adjusted target level with the weighted path loss estimate, L, and long term average of the pathloss estimate, L 0 , to determine the transmission power level of the transmitting station, step  47 . 
     Data to be transmitted in a communication from the transmitting station  112  is produced by data generator  102 . The data is error detection/correction encoded by error detection/correction encoder  117 . The error encoded data is spread and time-multiplexed with a training sequence by the training sequence insertion device  104  in the appropriate time slots and codes of the assigned resource units producing a communication burst. The spread signal is amplified by an amplifier  106  and modulated by modulator  108  to radio frequency. The gain of the amplifier is controlled by the transmit power calculation device  98  to achieve the determined transmission power level. The power controlled communication burst is passed through the isolator  84  and radiated by the antenna  82 . 
     The following is one outer loop/weighted open loop power control algorithm. The transmitting stations&#39;s transmission power level in decibels, P TS , is determined using Equation 2.
 
a.  P   TS =SIR TARGET   +I   RS +α( L−L   0 )+ L   0 +CONSTANT VALUE  Equation 2
 
     The SIR TARGET  has an adjusted value based on the received target adjustment signals. For the downlink, the initial value of SIR TARGET  is known at the transmitting station  112 . For uplink power control, SIR TARGET  is signaled from the receiving station  110  to the transmitting station  112 . Additionally, a maximum and minimum value for an adjusted SIR TARGET  may also be signaled. The adjusted SIR TARGET  is limited to the maximum and minimum values. I RS  is the measure of the interference power level at the receiving station  110 . 
     L is the path loss estimate in decibels, T RS −R TS , for the most recent time slot  36   1 - 36   n  that the path loss was estimated. L 0 , the long term average of the path loss in decibels, is the running average of the pathloss estimate, L. The CONSTANT VALUE is a correction term. The CONSTANT VALUE corrects for differences in the uplink and downlink channels, such as to compensate for differences in uplink and downlink gain. Additionally, the CONSTANT VALUE may provide correction if the transmit power reference level of the receiving station is transmitted, instead of the actual transmit power, T RS . If the receiving station  110  is a base station, the CONSTANT VALUE is preferably sent via a Layer 3 message. 
     The weighting value, α, is a measure of the quality of the estimated path loss and is, preferably, based on the number of time slots  36   1 - 36   n  between the time slot, n, of the last path loss estimate and the first time slot of the communication transmitted by the transmitting station  112 . The value of α is between zero and one. Generally, if the time difference between the time slots is small, the recent path loss estimate will be fairly accurate and α is set at a value close to one. By contrast, if the time difference is large, the path loss estimate may not be accurate and the long term average path loss measurement is most likely a better estimate for the path loss. Accordingly, α is set at a value closer to one.
 
a. Equations 3 and 4 are equations for determining α.
 
b. α=1−( D− 1)/( D   max −1)  Equation 3
 
c. α=max {1−( D− 1)/( D   max-allowed −1),0}  Equation 4
 
     The value, D, is the number of time slots  36   1 - 36   n  between the time slot of the last path loss estimate and the first time slot of the transmitted communication which will be referred to as the time slot delay. If the delay is one time slot, α is one. D max  is the maximum possible delay. A typical value for a frame having fifteen time slots is seven. If the delay is D max , α is zero D max-allowed  is the maximum allowed time slot delay for using open loop power control. If the delay exceeds D max-allowed , open loop power control is effectively turned off by setting α=0. Using the transmit power level, P TS , determined by a transmit power calculation device  98  the transmit power of the transmitted communication is set. 
       FIGS. 5 and 6  compare the performance of the weighted outer loop/open loop, open loop and closed loop systems. The simulations in  FIGS. 5 and 6  were performed for a slightly different version of the outer loop/weighted open loop algorithm. In this version, the target SIR is updated every block. A SIR TARGET  is increased if a block error was detected and decreased if no block error was detected. The outer loop/weighted open loop system used Equation 2. Equation 3 was used to calculate α. The simulations compared the performance of the systems controlling a UE&#39;s  32   1  transmission power level. For the simulations, 16 CRC bits were padded every block. In the simulation, each block was 4 frames. A block error was declared when at least two raw bit errors occur over a block. The uplink communication channel is assigned one time slot per frame. The target for the block error rate is 10%. The SIR TARGET  is updated every 4 frames. The simulations address the performance of these systems for a UE  32   i  traveling at 30 kilometers per hour. The simulated base station used two antenna diversity for reception with each antenna having a three finger RAKE receiver. The simulation approximated a realistic channel and SIR estimation based on a midamble sequence of burst type 1 field in the presence of additive white Gaussian noise (AWGN). The simulation used an International Telecommunication Union (ITU) Pedestrian B type channel and QPSK modulation. Interference levels were assumed to have no uncertainty. Channel coding schemes were not considered. L 0  was set at 0 db. 
     Graph  120  of  FIG. 5  shows the performance as expected in terms of the required E S /N O  for a BLER of 10 −1  as a function of time delay between the uplink time slot and the most recent downlink time slot. The delay is expressed by the number of time slots. E S  is the energy of the complex symbol.  FIG. 5  demonstrates that, when gain/interference uncertainties are ignored, the performance of the combined system is almost identical to that of weighted open loop system. The combined system outperforms the closed loop system for all delays. 
     In the presence of gain and interference uncertainties, the transmitted power level of the open loop system is either too high or too low of the nominal value. In graph  122  of  FIG. 6 , a gain uncertainty of −2 dB was used.  FIG. 6  shows the BLER as a function of the delay. The initial reference SIR TARGET  for each system was set to its corresponding nominal value obtained from  FIG. 5 , in order to achieve a BLER of 10 −1 .  FIG. 6  shows that, in the presence of gain uncertainty, both the combined and closed loop systems achieve the desired BLER. The performance of the weighted open loop system severely degrades.