Abstract:
A spread spectrum time division duplex user equipment uses frames with time slots for communication. It receives power commands and receives a first communication having a transmission power level in a first time slot. It measures a power level of the first communication as received and determines a pathloss estimate based on in part the measured received first communication power level and the first communication transmission power level. The user equipment sets a transmission power level for a second communication in a second time slot from the user equipment based on in part the pathloss estimate weighted by a quality factor and adjusted by the power commands.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of U.S. application Ser. No. 10/459,035, filed Jun. 11, 2003, which is a continuation of U.S. patent application Ser. No. 09/531,359 filed Mar. 21, 2000, which is incorporated by reference as if fully set forth. 
     
    
     
       BACKGROUND  
         [0002]    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.  
           [0003]    [0003]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  301  communicates with user equipments (UEs)  321 - 323  in its operating area. Communications transmitted from a base station  301  to a UE  321  are referred to as downlink communications and communications transmitted from a UE  321  to a base station  301  are referred to as uplink communications.  
           [0004]    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  361 - 36   n,,  such as fifteen time slots. In such systems, a communication is sent in selected time slots  361 - 36   n  using selected codes. Accordingly, one frame  34  is capable of carrying multiple communications distinguished by both time slot  361 - 36   n  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.  
           [0005]    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  321  or base station  301  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.  
           [0006]    One approach to control transmission power levels is open loop power control. In open loop power control, typically a base station  301  transmits to a UE  321  a reference downlink communication and the transmission power level of that communication. The UE  321  receives the reference communication and measures its received power level. By subtracting the received power level from the transmission power level, a pathloss for the reference communication is determined. To determine a transmission power level for the uplink, the downlink pathloss is added to a desired received power level at the base station  301 . The UE&#39;s transmission power level is set to the determined uplink transmission power level.  
           [0007]    Another approach to control transmission power level is closed loop power control. In closed loop power control, typically the base station  301  determines the signal to interference ratio (SIR) of a communication received from the UE  321 . The determined SIR is compared to a target SIR (SIRTARGET). Based on the comparison, the base station  301  transmits a power command, bTPC. After receiving the power command, the UE  321  increases or decreases its transmission power level based on the received power command.  
           [0008]    Both closed loop and open loop power control have disadvantages. 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 100 cycles per second which is not sufficient for fast fading channels. Open loop power control is sensitive to uncertainties in the uplink and downlink gain chains and interference levels.  
           [0009]    One approach to combining closed loop and open loop power control was proposed by the Association of Radio Industries and Business (ARIB) and uses Equations 1, 2, and 3.  
               T   UE     =         P   BS          (   n   )       +   L             Equation                 1                   P   BS          (   n   )       =         P   BS          (     n   -   1     )       +       b   TPC          Δ   TPC                 Equation                 2                 b   TPC     =     {           1        :                   if                   SIR   BS          〈     SIR   TARGET                         -   1          :                   if                   SIR   BS       〉          SIR   TARGET                       Equation                 3                               
 
           [0010]    T UE  is the determined transmission power level of the UE  32   1 . L is the estimated downlink pathloss. P BS (n) is the desired received power level of the base station  30   1  as adjusted by Equation 2. For each received power command, bTPC, the desired received power level is increased or decreased by Δ TPC . Δ TPC  is typically one decibel (dB). The power command, b TPC , is one, when the SIR of the UE&#39;s uplink communication as measured at the base station  30 , SIR BS , is less than a target SIR, SIR TARGET . Conversely, the power command is minus one, when SIR BS  is larger than SIR TARGET .  
           [0011]    Under certain conditions, the performance of these systems degrades. For instance, if communications sent between a UE  32  and a base station  30  are in a highly dynamic environment, such as due to the UE  32  moving, the path loss estimate for open loop severely degrades the overall system&#39;s performance. Accordingly, there is a need for alternate approaches to maintain signal quality and low transmission power levels for all environments and scenarios.  
         SUMMARY  
         [0012]    A spread spectrum time division duplex user equipment uses frames with time slots for communication. It receives power commands and receives a first communication having a transmission power level in a first time slot. It measures a power level of the first communication as received and determines a pathloss estimate based on in part the measured received first communication power level and the first communication transmission power level. The user equipment sets a transmission power level for a second communication in a second time slot from the user equipment based on in part the pathloss estimate weighted by a quality factor and adjusted by the power commands. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 illustrates a prior art TDD system.  
         [0014]    [0014]FIG. 2 illustrates time slots in repeating frames of a TDD system.  
         [0015]    [0015]FIG. 3 is a flow chart of combined closed loop/open loop power control.  
         [0016]    [0016]FIG. 4 is a diagram of components of two communication stations using combined closed loop/open loop power control.  
         [0017]    [0017]FIGS. 5-10 depict graphs of the performance of a closed loop, ARIB&#39;s proposal and two (2) schemes of combined closed loop/open loop power control. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. Combined closed loop/open loop power control will be explained using the flow chart of FIG. 3 and the components of two simplified communication stations  50 ,  52  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  52  and the communication station receiving power controlled communications is referred to as the receiving station  50 . Since combined closed loop/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 .  
         [0019]    The receiving station  50  receives various radio frequency signals including communications from the transmitting station  52  using an antenna  56 , or alternately, an antenna array. The received signals are passed through an isolator  60  to a demodulator  68  to produce a baseband signal. The baseband signal is processed, such as by a channel estimation device  96  and a data estimation device  98 , in the time slots and with the appropriate codes assigned to the transmitting station&#39;s communication. The channel estimation device  96  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  98 , the interference measurement device  90 , the signal power measurement device  92  and the transmit power calculation device  94 . The data estimation device  98  recovers data from the channel by estimating soft symbols using the channel information. Using the soft symbols and channel information, the transmit power calculation device  94  controls the receiving station&#39;s transmission power level by controlling the gain of an amplifier  76 .  
         [0020]    The signal power measurement device  92  uses either the soft symbols or the channel information, or both, to determine the received signal power of the communication in decibels (dB). The interference measurement device  90  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  98 , or both.  
         [0021]    The closed loop power command generator  88  uses the measured communication&#39;s received power level and the interference level, I RS , to determine the Signal to Interference Ratio (SIR) of the received communication. Based on a comparison of the determined SIR with a target SIR (SIR TARGET ), a closed loop power command is generated, b TPC , such as a power command bit, b TPC , step  38 . Alternately, the power command may be based on any quality measurement of the received signal.  
         [0022]    For use in estimating the path loss between the receiving and transmitting stations  50 ,  52  and sending data, the receiving station  50  sends a communication to the transmitting station  58 , step  40 . 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  50  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  52  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 of the reference channel, T RS . The interference level, I RS , and reference channel power level, T RS , may be sent in other channels, such as a signaling channel. The closed loop power control command, b TPC , is typically sent in a dedicated channel. The dedicated channel is dedicated to the communication between the receiving station  50  and transmitting station  52 , step  40 .  
         [0023]    The reference channel data is generated by a reference channel data generator  86 . 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  82  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  78 . The amplified communication burst may be summed by a sum device  72  with any other communication burst created through devices, such as a data generator  84 , spreading and training sequence insertion device  80  and amplifier  76 .  
         [0024]    The summed communication bursts are modulated by a modulator  64 . The modulated signal is passed through an isolator  60  and radiated by an antenna  56  as shown or, alternately, through an antenna array. The radiated signal is passed through a wireless radio channel  54  to an antenna  58  of the transmitting station  52 . 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).  
         [0025]    The antenna  58  or, alternately, antenna array of the transmitting station  52  receives various radio frequency signals. The received signals are passed through an isolator  62  to a demodulator  66  to produce a baseband signal. The baseband signal is processed, such as by a channel estimation device  100  and a data estimation device  102 , in the time slots and with the appropriate codes assigned to the communication burst of the receiving station  50 . The channel estimation device  100  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  102 , a power measurement device  110  and a quality measurement device  114 .  
         [0026]    The power level of the processed communication corresponding to the reference channel, R TS , is measured by the power measurement device  110  and sent to a pathloss estimation device  112 , step  42 . Both the channel estimation device  100  and the data estimation device  102  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  110  or the pathloss estimation device  112 . The power measurement device  110  is a component of the combined closed loop/open loop controller  108 . As illustrated in FIG. 4, the combined closed loop/open loop power controller  108  comprises the power measurement device  110 , pathloss estimation device  112 , quality measurement device  114 , and transmit power calculation device  116 .  
         [0027]    To determine the path loss, L, the transmitting station  52  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  102  interprets the power level and sends the interpreted power level to the pathloss estimation device  112 . If the receiving station  50  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  112  estimates the path loss, L, between the two stations  50 ,  52 , step  44 . In certain situations, instead of transmitting the transmitted power level, T RS , the receiving station  50  may transmit a reference for the transmitted power level. In that case, the pathloss estimation device  112  provides reference levels for the path loss, L.  
         [0028]    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 systems 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. Combined closed loop/open loop power control utilizes both closed loop and open loop power control aspects. If the quality of the path loss measurement is high, the system primarily acts as an open loop system. If the quality of the path loss measurement is low, the system primarily acts as a closed loop system. To combine the two power control aspects, the system weights the open loop aspect based on the quality of the path loss measurement.  
         [0029]    A quality measurement device  114  in a weighted open loop power controller  108  determines the quality of the estimated path loss, step  46 . The quality may be determined using the channel information generated by the channel estimation device  100 , the soft symbols generated by the data estimation device  102  or other quality measurement techniques. The estimated path loss quality is used to weight the path loss estimate by the transmit power calculation device  116 . If the power command, b TPC , was sent in the communication&#39;s data, the data estimation device  102  interprets the closed loop power command, b TPC . Using the closed loop power command, b TPC , and the weighted path loss, the transmit power calculation device  116  sets the transmit power level of the receiving station  50 , step  48 .  
         [0030]    The following is one of the preferred combined closed loop/open loop power control algorithms. The transmitting station&#39;s power level in decibels, P TS , is determined using Equations 4 and 6.  
           P   TS   =P   0   +G ( n )+α L    Equation 4  
         [0031]    P 0  is the power level that the receiving station  50  desires to receive the transmitting station&#39;s communication in dB. P 0  is determined by the desired SIR at the receiving station  50 , SIR TARGET , and the interference level, I RS , at the receiving station  50  using Equation 5.  
           P   0   =SIR   TARGET   +I   RS    Equation 5  
         [0032]    I RS  is either signaled or broadcasted from the receiving station  50  to the transmitting station  52 . For downlink power control, SIR TARGET  is known at the transmitting station  52 . For uplink power control, SIR TARGET  is signaled from the receiving station  50  to the transmitting station  52 . G(n) is the closed loop power control factor. Equation 6 is one equation for determining G(n).  
           G ( n )= G ( n− 1)+ b   TPC Δ TPC    Equation 6  
         [0033]    G(n−1) is the previous closed loop power control factor. The power command, b TPC , for use in Equation 6 is either +1 or −1. One technique for determining the power command, b TPC , is Equation 3. The power command, b TPC , is typically updated at a rate of 100 ms in a TDD system, although other update rates may be used. Δ TPC  is the change in power level. The change in power level is typically 1 dB although other values may be used. As a result, the closed loop factor increases by 1 dB if b TPC  is +1 and decreases by 1 dB if b TPC  is −1.  
         [0034]    The weighting value, α, is determined by the quality measurement device  114 . α is a measure of the quality of the estimated path loss and is, preferably, based on the number of time slots, D, between the time slot of the last path loss estimate and the first time slot of the communication transmitted by the transmitting station  52 . The value of a is from zero to one. Generally, if the time difference, D, 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 closed loop aspect is most likely more accurate. Accordingly, a is set at a value closer to zero.  
         [0035]    Equations 7 and 8 are two equations for determining a, although others may be used.  
         α=1−( D− 1)/( D   max −1)   Equation 7  
         α=max{1−( D− 1)/( D   max-allowed −1), 0}  Equation 8  
         [0036]    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 calculated transmit power level, P TS , determined by a transmit power calculation device  116 , the combined closed loop/open loop power controller  108  sets the transmit power of the transmitted communication.  
         [0037]    Data to be transmitted in a communication from the transmitting station  52  is produced by a data generator  106 . 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 producing a communication burst. The spread signal is amplified by the amplifier  74  and modulated by the modulator  70  to radio frequency.  
         [0038]    The combined closed loop/open loop power controller  108  controls the gain of the amplifier  74  to achieve the determined transmit power level, P TS , for the communication. The power controlled communication is passed through the isolator  62  and radiated by the antenna  58 .  
         [0039]    Equations 9 and 10 are another preferred combined closed loop/open loop power control algorithm.  
           P   TS   =P   0   +K ( n )   Equation 9  
           K ( n )= K ( n− 1)+ b   TPC Δ TPC   +αL    Equation 10  
         [0040]    K(n) is the combined closed loop/open loop factor. As shown, this factor includes both the closed loop and open loop power control aspects. Equations 4 and 5 segregate the two aspects.  
         [0041]    Although the two above algorithms only weighted the open loop factor, the weighting may be applied to the closed loop factor or both the open and closed loop factors. Under certain conditions, the network operator may desire to use solely open loop or solely closed loop power control. For example, the operator may use solely closed loop power control by setting a to zero.  
         [0042]    [0042]FIGS. 5-10 depict graphs  118 - 128  illustrating the performance of a combined closed-loop/open-loop power control system. These graphs  118 - 128  depict the results of simulations comparing the performance of the ARIB proposed system, a closed loop, a combined open loop/closed loop system using Equations 4 and 6 (scheme I) and a combined system using Equations 9 and 10 (scheme II). The simulations were performed at the symbol rate. A spreading factor of sixteen was used for both the uplink and downlink channels. The uplink and downlink channels are International Telecommunication Union (ITU) Channel model [ITU-R M.1225, vehicular, type B]. Additive noises were simulated as being independent of white Gaussian noises with unity variance. The path loss is estimated at the transmitting station  52  which is a UE  32   1  and in particular a mobile station. The BCH channel was used for the path loss estimate. The path loss was estimated two times per frame at a rate of 200 cycles per second. The receiving station  50 , which was a base station  30   1 , sent the BCH transmission power level over the BCH. RAKE combining was used for both the UE  32   1  and base station  30   1 . Antenna diversity combining was used at the base station  30   1 .  
         [0043]    Graphs  118 ,  122 ,  126  depict the standard deviation of the received signal to noise ratio (SNR) at the base station  30   1  of the UE&#39;s power controlled communication as a function of the time slot delay, D. Graphs  120 ,  124 ,  128  depict the normalized bias of the received SNR as a function of the delay, D. The normalization was performed with respect to the desired SNR. Each point in the graphs  118 - 128  represents the average of 3000 Monte-Carlo runs.  
         [0044]    Graphs  118 ,  120  depict the results for an α set at one. For low time slot delays (D&lt;4), scheme I and II outperform closed loop power control. For larger delays (D&gt;4), closed loop outperforms both scheme I and II which demonstrates the importance of weighting the open loop and closed loop aspects.  
         [0045]    Graphs  122 ,  124  depict the results for an α set at 0.5. As shown, for all delays excluding the maximum, schemes I and II outperform closed loop power control. The ARIB proposal only outperforms the others at the lowest delay (D=1).  
         [0046]    Graphs  126 ,  128  depict the results for an a set using Equation 7 with D max  equal to seven. As shown, schemes I and II outperform both closed loop and the ARIB proposal at all delays, D.