Abstract:
The invention controls transmission power levels in a spread spectrum time division duplex communication station. A first communication station transmits a communication to a second communication station. The second station receives the communication and measures its received power level. Based on, in part, the received communication&#39;s power level and the communication&#39;s transmission power level, a path loss estimate is determined. A quality of the path loss estimate is also determined. The transmission power level for a communication from the second station to the first station is based on, in part, weighting the path loss estimate in response to the estimate&#39;s quality.

Description:
BACKGROUND 
     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. 
     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 equipments (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 fifteen 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  36   1 - 36   n  and code. The use 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  32   1  and a base station  30   1  are in a highly dynamic environment, such as due to the UE  32   1  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 a typical TDD system is 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 
     The invention controls transmission power levels in a spread spectrum time division duplex communication station. A first communication station transmits a communication to a second communication station. The second station receives the communication and measures its received power level. Based on, in part, the received communication&#39;s power level and the communication&#39;s transmission power level, a path loss estimate is determined. The transmission power level for a communication from the second station to the first station is set based on, in part, weighting the path loss estimate and a long term pathloss estimate. 
    
    
     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 weighted open loop power control. 
     FIG. 4 is a diagram of components of two communication stations using weighted open loop power control. 
     FIG. 5 depicts a graph of the performance of a weighted open loop, open loop and closed loop power control system for a UE moving at 30 kilometers per hour (km/h). 
     FIG. 6 depicts a graph of the three systems&#39; performance for a UE moving at 60 km/h. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. 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 weighted open loop power control may be used for uplink, downlink or both types of communications, the transmitter having its power controlled may be located at a 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 located at both the base station  30   1  and UE  32   1 . 
     For use in estimating the path loss between the receiving and transmitting stations  110 ,  112 , the receiving station  110  sends a communication to the transmitting station  112 . The communication may be sent on any one of various channels. Typically, in a TDD system, the channels used for estimating path loss 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 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 through an isolator  66  and radiated by an antenna  78  as shown or, alternately, through an antenna array, step  38 . 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 the 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. 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&#39;s burst. 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  40 . The channel estimation device  88  is 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 the pathloss estimation device  94 . 
     To determine the path loss, L, the transmitting station  112  also requires the communication&#39;s transmitted power level, T RS . The 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  in dB, from the sent communication&#39;s transmitted power level, T RS  in dB, the pathloss estimation device  94  estimates the path loss, L, between the two stations  110 ,  112 , step  42 . Additionally, a long term average of the pathloss, L 0 , is updated, step  44 . 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 path loss, L, and the long term average of the path loss, L 0 . 
     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 desired received power level, a transmission power level for a communication from the transmitting station  112  to the receiving station  110  is determined. This power control technique is referred to as open loop power control. 
     Open loop systems have drawbacks. 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, a quality measurement device  96  in a weighted open loop power controller  100  determines the quality of the estimated path loss, step  46 . The quality measurement device  96  also weights the estimated path loss, L, and long term average of the pathloss, L 0 , to set the transmit power level by transmit power calculation device  98 , step  48 . As illustrated in FIG. 4, the weighted open loop power controller  100  consists of the power measurement device  92 , pathloss estimation device  94 , quality measurement device  96 , and transmit power calculation device  98 . 
     The following is one of the preferred weighted open loop power control algorithms. The transmitting station&#39;s power level in decibels, P TS , is determined using Equation 1. 
     
       
           P   TS   =P   RS +α( L−L   0 )+ L   0 +CONSTANT VALUE  Equation 1 
       
     
     P RS  is the power level that the receiving station  110  desires to receive the transmitting station&#39;s communication in dB. P RS  is determined by the desired SIR, SIR TARGET , at the receiving station  110  and the interference level, I RS , at the receiving station  110 . 
     To determine the interference level, I RS , at the receiving station, received communications from the transmitting station  112  are demodulated by a demodulator  68 . The resulting 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 the transmitting station&#39;s communications. The channel information produced by the channel estimation device  70  is used by an interference measurement device  74  to determine the interference level, I RS . The channel information may also be used to control the transmit power level of the receiving station  110 . The channel information is input to a data estimation device  72  and a transmit power calculation device  76 . The data estimation produced by the data estimation device  72  is used with the channel information by the transmit power calculation device  76  to control the amplifier  54  which controls the receiving station&#39;s transmit power level. 
     P RS  is determined using Equation 2. 
     
       
           P   RS   =SIR   TARGET   +I   RS   Equation 2 
       
     
     I RS  is either signaled or broadcasted from the receiving station  110  to the transmitting station  112 . For downlink power control, 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 . Using Equation 2, Equation 1 is rewritten as either Equations 3 or 4. 
     
       
           P   TS   =SIR   TARGET   +I   RS +α( L−L   0 )+ L   0 +CONSTANT VALUE  Equation 3 
       
     
     
       
           P   TS   =αL+ (1−α) L   0+I   RS   +SIR   TARGET +CONSTANT VALUE  Equation 4 
       
     
     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. The long term average of the pathloss, L 0 , is a running average of the path loss estimates 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 is a base station  30   1 , the CONSTANT VALUE is preferably sent via Layer  3  signaling. 
     The weighting value, α, determined by the quality measurement device  94 , 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 a is from zero to one. Generally, if the time difference between the time slots is small, the recent path loss estimate will be fairly accurate and a 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 zero. 
     Equations 5 and 6 are two equations for determining a, although others may be used. 
     
       
         α=1−( D −1)/( D   max −1)  Equation 5 
       
     
     
       
         α=max{1−( D −1)/( D   max-allowed 31 1), 0}  Equation 6 
       
     
     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 maximal 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 , the long term average pathloss measurement, L 0 , is considered the better estimate for the pathloss and α=0. Using the transmit power level, P TS , determined by a transmit power calculation device  98 , the weighted open loop power controller  100  sets the transmit power of the transmitted communication, step  48 . 
     Data to be transmitted in a communication from the transmitting station  112  is produced by a data generator  102 . The communication data is spread and time-multiplexed with a training sequence by the spreading and training sequence insertion device  104  in the appropriate time slots and codes of the assigned resource units. The spread signal is amplified by the amplifier  106  and modulated by the modulator  108  to radio frequency. 
     The weighted open loop power controller  100  controls the gain of the amplifier  106  to achieve the determined transmit power level, P TS , for the communication. The communication is passed through the isolator  84  and radiated by the antenna  82 . 
     FIGS. 5 and 6 depict graphs  82 , 84  illustrating the performance of a weighted open loop system using Equation 4. Equation 5 is used to calculate α. These graphs  82 ,  84  depict the results of simulations comparing the performance of a weighted open loop, an open loop and a closed loop system controlling the transmission power level of the transmitting station  112 . The simulations address the performance of these systems in a fast fading channel under steady-state conditions. In this example, the receiving station is a base station  30 , and the transmitting station is a UE  32   1 . For the simulation, the UE  32   1  was a mobile station. The simulated base station  30   1  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 be accurately known with no uncertainty. Channel coding schemes were not considered. The CONSTANT VALUE and L 0  were set at 0 db. 
     For each of the power control techniques, FIG. 5, graph  82  shows the energy for a transmitted complex symbol in decibels (Es/No) required to maintain a BER of 1% for various time slot delays, D, with the UE  32   1  moving at 30 kilometers per hour (km/h). As shown, at lower time slot delays, both weighted open loop and open loop outperform closed loop. For higher time slot delays, weighted open loop outperforms both open loop and closed loop. As shown in FIG. 6, graph  84 , similar results occur if the UE  32   1  is traveling at 60 km/h.