Downlink power control for multiple downlink time slots in tdd communication systems

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DESCRIPTION

Downlink transmit power control in multiple downlink time slots in TDD communication systems.

Background

This invention relates generally to spread spectrum time division duplex (TDD) communication systems. More particularly, the present invention relates to a system and method for controlling downlink transmit power in TDD communication systems.

Spread spectrum TDD systems support multiple communications on the same spectrum. Multiple signals are distinguished by their respective chip code sequences (codes). Referring to Figure 1, TDD systems use repetitive frames 34 divided into a series of time slots 37i-37_n, such as fifteen time segments. In such systems, a communication is sent in a time slot selected from the plurality of time slots 37₁-37_n using selected codes. Consequently, a frame 34 is capable of supporting multiple communications differentiated by both a time slot and a code. The combination of a single code in a single time slot is called a physical channel. Based on the bandwidth needed to support a communication, one or more multiple channels are assigned to that communication.

Most TDD systems adaptively control transmit power levels. In a TDD system, many communications can share the same time slot and spectrum. While user equipment (UE) 22 is receiving a downlink transmission from a base station, all other communications using the same time slot and spectrum cause interference in the specific communication. Increasing the transmit power level of one communication degrades the signal quality of all other communications in that time slot and spectrum. However, excessive reduction of the transmit power level results in undesirable signal-to-noise ratios (SNRs) and bit error rates (BERs) at the receivers. In order to maintain both communication signal quality and low transmit power levels, a transmit power control is employed.

The standard approach to a TDD downlink power control is a combination of inner and outer loop control. In this standard solution, the UE transmits physical layer transmit power control (TPC) commands to adjust the base station transmit power. A base station sends a transmission to a particular UE. Upon receipt, the UE measures the signal interference ratio (SIR) in all time slots and compares this measured value with a SIR_OBobjective. This SIR_OBThe objective is generated from the Block Error Rate (BLER) signaled from the base station.

As a result of comparing the measured SIR value with the SIR_Target, the UE transmits a TPC command to the base station. The standard approach enables one TPC command per encoded composite transport channel (CCTrCH). The CCTrCH is a physical channel comprising the combined units of data for transmission over the radio interface to and from the UE or the base station. This TPC command instructs the base station to adjust the transmit power level of the downlink communication. The base station, which is set to an initial transmit power level, receives the TPC command and adjusts the transmit power level in all associated time slots in unison with the CCTrCH.

This approach to TDD downlink power control works well as long as the interference in each time slot is the same. Unfortunately, in most cases, the interference in each time slot is different. A small difference may be acceptable due to the averaging effect of interpolation, but larger differences cause degradation due to thresholding effects at the receiver. This requires the receiver to have a wider dynamic range and unnecessarily high transmit power in some time slots. An adjustment made in the SIR_Target of the base station for all time slots based on the error value can cause an unbalanced rise or fall of the power level. In other words, those time segments in which the power level was lower than the initial value of the base station will be adjusted to even lower levels when the calculated error is greater than the SIR._Target. These low-power time slots can be removed after detection, thereby degrading the transmission. The same is true for those time segments in which the power level was higher than the SIR._Target from the base station. When the detected error rate is less than the SIR_Target, the higher power level time slots will be increased, thus creating interference with other channels in the system.

Consequently, there is a need for a TDD downlink power control approach that individually adjusts the power level of each segment.

Document US 5,898,925 describes the transmission power control of a mobile station in Time Division Multiple Access (TDMA) telecommunications system.

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Summary

The present invention is a method and a system for controlling downlink transmission power levels in a spread spectrum time division communication system having frames with time slots for communication, which it receives at a user equipment ( UE) a downlink communication from a base station and determines an error rate of the received communication. The UE then produces power level adjustments for each of the time slots based in part on the error rate and transmits an uplink communication to the base station, which includes the power level adjustment for each of the time segments. In response to the power level adjustments, a transmit power level is set for each time segment of the downstream communication.

Brief description of the drawing (s)

Figure 1 illustrates time slots in repetitive frames of a TDD system.

Figure 2 illustrates a simplified wireless TDD system.

Figures 3A and 3B illustrate block diagrams of a UE and a base station, respectively.

Figure 4 illustrates a flow chart of a first embodiment.

Figure 5 illustrates a flow chart of a second embodiment.

Figure 6 illustrates a block diagram of the base station made according to the second embodiment.

Figure 7 illustrates a flow chart of a third embodiment.

Figure 8 illustrates a flow chart of a fourth embodiment.

Figure 9 illustrates a flow chart of a fifth embodiment.

Figure 10 illustrates a flow chart of a sixth embodiment.

Figure 11 illustrates a flow chart of a seventh embodiment.

Detailed description of the preferred embodiment (s)

Preferred embodiments will be described with reference to the drawing figures, in which like numbers represent like elements throughout.

FIG. 2 illustrates a simplified wireless time division duplex (TDD) or spread spectrum code division multiple access (CDMA) communication system 18. System 18 comprises a series of nodes Bs 26, 32, 34, a series of radio network controllers (RNCs) 36, 38, 40, a series of UEs 20, 22, 24 and a core network 46. The plurality of nodes Bs 26, 32, 34 are connected to a plurality of RNCs 36, 38,40, which, in turn, are connected to the core network 46. Each Node B, such as Node B 26, communicates with its associated user equipment 20-24 (UE). Node B 26 has a single site controller (SC) associated with a single base station 30₁ or to multiple 30i base stations ... 30_n.

Although the present invention is intended to operate with one or more UEs, Node Bs and RNCs, for simplicity of explanation, reference will be made hereinafter to the operation of a single UE in combination with its associated Node B and RNCs.

Referring to Figure 3A, the UR 22 comprises an antenna 78, an isolator or switch 66, a modulator 64, a demodulator 68, a channel estimation device 70, a data estimation device 72, a calculation device 76 transmit power, an interference measurement device 74, an error detection device 112, a processor 111, a target set generator 114, a reference channel data generator 56, a data generator 50 and two extension and training sequence insertion devices 52, 58.

The UE 22 receives various radio frequency (RF) signals including communications from the base station 30i over the wireless radio channel using an antenna 78, or alternatively an array of antennas. The received signals are passed through a T / R switch 66 to a demodulator 68 to produce a baseband signal. The baseband signal is processed, such as by a channel estimating device 70 and by a data estimating device 72, in the time slots and with the appropriate codes assigned to the communication of the UEs 22. The device 70 Channel estimation commonly employs the training sequence component of the baseband signal to provide channel information, such as channel impulse responses. The channel information is used by the data estimating device 72, the interference measuring device 74, and the transmit power calculating device 76. The data estimating device 72 retrieves data from the channel by estimating soft symbols using the channel information.

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Before transmission of communication from base station 30₁, the communication data signal is encoded in error using an error detection / correction encoder 112. The error coding scheme is typically a cyclic redundancy code (CRC) followed by a direct error correction coding, although other types of error coding schemes may be employed. As is known to those skilled in the art, the data is typically interpolated in all time slots and in all codes.

Using the soft symbols produced by the data estimation device 72, the error detection device 112 detects errors in the frame. Every time a frame is determined to have an error, a counter is incremented. This counter value becomes the block error rate (BLER). A processor 111 in UE 22 typically determines a target signal-to-interference ratio SIR value based on the measured BLER and determines a signal-to-interference ratio SIRue for all time slots. Building on the SIR_EU processor 111 determines base station transmit power setting by comparing SIR_EU with the SIR_Target. Based on this comparison, a TPC command is generated by the target setting generator 114 for each time segment. Each TPC command is subsequently sent to the base station.

In a first embodiment of the present invention, the target set generator 114 of the UE 22 generates and transmits TPC commands in each time slot of the CCTrCH. The TPC command of each time slot indicates to the base station 30₁ that adjusts the downlink transmit power level in each time slot. The uplink physical channel comprises these TPC commands for each segment associated with the CCTrCH, and this is communicated to the base station for processing. These TPC commands can be transmitted on a single physical uplink channel, or spread over several physical uplink channels.

Referring to Figure 3B, a base station manufactured in accordance with the first embodiment of the present invention is illustrated. The antenna 82 or, alternatively, the base station antenna array 30₁ receives various RF signals including TPC commands. The received signals are passed through a switch 84 to a demodulator 86 to produce a baseband signal. Alternatively separate antennas can be used to transmit or receive functions. The baseband signal is processed, such as by a channel estimating device 88 and by a data estimating device 90, in the time slots and with the appropriate codes assigned to the communication burst of the UE 22. The device Channel estimation 88 commonly uses the training sequence component of the baseband signal to provide channel information, such as channel impulse responses. The channel information is used by the data estimating device 90. The data information is provided, by the processor 103, to the transmit power calculator 98.

The processor 103 converts the soft symbols produced by the data estimating device 90 into bits and extracts the TPC commands for each time slot associated with the CCTrCH. The transmit power calculating device 98 combines the TPC commands with the SIR_Target to determine the transmission power of each time slot associated with the CCTrCH.

The data to be transmitted from the base station 30₁ are produced by the data generator 102. The data is encoded in error detection / correction by the error detection / correction encoder 110. The error-coded data is spread and time multiplexed with a training sequence by the training sequence inserter 104 into the appropriate time segment (s) and channel code (s). assigned physicists, producing a communication burst (s). The extended signal is amplified by an amplifier 106 and modulated by a radio frequency modulator 108. The gain of the amplifier is controlled by the transmit power calculating device 98 to achieve the determined transmit power level of each time slot. The controlled power communication burst (s) are passed through isolator 84 and radiated by antenna 82.

A flow chart illustrating the downlink power control method according to the first embodiment of the present invention is shown in Fig. 4. UE 22 receives a downlink signal from base station 30₁ (step 401), which is then processed by the UE 22 (step 402). The UE 22 then determines the SIR for each time slot of the CCTrCH and compares it with the SIR._Target (step 403). The UE then generates a TPC command for each time slot (step 404). The TPC commands are transmitted to the base station 30i associated with the UE 22 (step 405), which adjusts the transmission power per time slot of the CCTrCH (step 406).

The use of TPC commands in each time slot provides the communication system with a simple method of equalizing the signal-to-interference ratio (SIR) on all downlink segments. Since the interference level in different time slots is generally different, this method of the first embodiment of the present invention takes this difference into account and generates a separate TPC command for each time slot in order to adjust the power level. of each time slot in the downlink signal.

A second embodiment of the present invention presents an alternative approach for individually balancing the power level adjustment in each time slot, during a downlink transmission, using the time slot interference data from each time slot, i.e. , a measured interference signal code power (ISCP) downlink. This ISCP measurement is performed by the UE 22 from time to time and is determined by an interference rate of change and the degree of interference difference that can be tolerated by the UE 22 without degradation.

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This second embodiment uses the time slice interference data from each time slice to equalize the SIR in different slices to counteract the fact that the interference is different in each slice. As will be explained in more detail below, a TPC command per CCTrCH is used together with interference information from each segment to adjust the transmit power. The difference between the interference in different time slots modifies the values ​​obtained from the TPC commands. Therefore, although the interference in each time slot may be different, the use of the ISCP information maintains approximately the same SIR in all time slots.

The UE 22, in each frame, sends a TPC command that corresponds to the average SIR in all the time slots belonging to the same CCTrCH. Base station 30₁ it then builds an average transmit power per CCTrCH based on the received TPC commands. As will be explained in more detail below, the base station 30i then modifies the average power to obtain the transmit power of each time slot for the CCTrCH based on the relevant interference data and the time slot mapping employed. It should be noted that this alternative approach allows the use of multiple extension factors.

Referring to Figure 6, a base station manufactured in accordance with this second embodiment is illustrated. The base station 30i transmit power calculating device 698 initializes the downlink power control approach of the second embodiment by combining the spread and interference code information to estimate an equivalent power obtained from P TPC commands.

P = (F / NjZjIjZkl / Sjk

Equation 1 where j and k refer, respectively, to the time segment and the physical channel; N is the total number of physical channels at a spread factor of 16 in a segment. Ij represents the interference in time slot j, j = 1, ... N; F is a scaling factor and 1 / Sj_k is the extension factor.

The transmit power calculating device 698 then calculates, using the mapping and time slot interference information stored in the base station database 696, the scaling factor F according to the following equation:

F = NP / (ZjIjZk1 / Sjk) and the transmit power for all physical channels Pj_k according to equation 3:

Pjk = FIj / 1 / Sjk

Power per time segment is defined as:

Pj = FIjZk 1 / Sjk

Equation 2

Equation 3

Equation 4

During steady state operation, the transmit power calculator 698 updates the scaling factor F for each physical channel whenever new interference signal code (ISCP) power measurements I are available for each associated time slot. with the particular downlink CCTrCH. In order for the transmit power calculating device 698 to calculate the scaling factor F, the spread factor of each physical channel is used. The transmit power calculator 698 calculates the transmit power using the I ISCP metering, which is made available to the transmit power calculator 698 periodically or whenever new interference information warrants an update.

When a new I ISCP measurement is performed, the measurement is transferred to base station 301 for calculation of the transmit power of each physical channel. If a new I ISCP measurement is not available, the TPC command of the UE 22 is used to modify P in the standard way, and the transmission power for all physical channels Pjk is calculated from it.

Referring to Fig. 5, a flow chart of a downlink power control according to this second embodiment is illustrated. UE 22 receives a downlink communication from base station 30₁ (step 501). If UE 22 determines that an updated ISCP measurement is necessary, UE 22 performs an ISCP measurement for each time slot in the downlink communication (step 502) and sends the new ISCP measurements to base station 301 (step 504) . Otherwise, the UE 22 generates a TPC command (503) and sends it to the base station (step 504). Base station 301 calculates the scaling factor for all physical channels using the TPC command or the ISCP measurement of UE 22. The transmit power level for each time slot is then calculated by base station 301 (step 505) and the downlink signal is updated accordingly (step 506).

It should be noted that although the second embodiment has been described with the base station storing all the information

ES 2 209 673 T3 necessary and performing all the calculations by itself, instead Node B 26 and RNC 36 can perform this function. Referring to Figure 7, a flow chart illustrates a third embodiment of a downlink power control system in which Node B 26 and RNC 36 are involved. UE 22 receives a downlink communication from the base station 30₁ (step 701). If the UE 22 determines that an updated ISCP measurement is necessary, the UE 22 performs an ISCP measurement for each time slot in the downlink communication (step 702) and sends the new ISCP measurements to the RNC 36 (step 704). Otherwise, the UE 22 generates a TPC command (step 703) and sends it to the RNC 36 (step 704). If the downlink control system is set to cause the RNC 36 to calculate the transmit power, the transmit power for each time slot is calculated by the RNC 36 (step 705) and then sent to Node B. 26 in order to update the downlink signal (step 706) from base station 30₁. If it is established that the Node B 26 is to calculate the transmission power, the RNC 36 transmits the ISCP or TPC command to the Node B 26 (step 707), where the transmission power for each time slot is calculated (step 705 ).

A fourth embodiment for downlink power level control employs time slot interference data similar to that described in the second embodiment above. However, in this approach the time slot interference is calculated from the knowledge of the downlink physical channels assigned by the base station 30₁, and from the load information and path loss of all neighboring base stations up to UE 22, instead of requesting explicit ISCP measurements from UE 22. Each base station, such as base station 30₁, knows all the channel settings assigned to the base station 30₁ specific to UEs 22, as well as other neighboring base stations 30₂...30_n. Obviously, if there is only one base station 30₁, no additional information is required from other base stations. The base station 30i must also know the load and path loss information of all neighboring base stations from there to the UE 22.

When there are multiple base stations, the UE 22 typically measures the primary common control physical channel (PCCPCH) power of the base stations under the control of its base station Node B 26 and all other base stations. The base station 30i uses the known PCCPCH transmission power and the power measurement thereof as received by the UE to estimate the path loss between the UE and each of the neighboring base stations.

Referring back to FIG. 6, the base station database 696 has stored therein loading information specifying the physical channels of the neighboring base station by time slot. This loading information is combined with the PCCPCH. The received signal code power (RSCP) for the particular base station is used to estimate the interference effect of the neighboring base station. From these calculations, the interference in UE 22 can be calculated. For a non-multiple user detection (MUD) UE, interference from its associated base station and interference from neighboring base stations are used to calculate this value. For a UE MUD, the interference generated by the base station associated with the UE is excluded from the UE interference value.

The estimated interference, I (n), using known load information, is calculated by the transmit power calculator 698 as:

I (n) = LP_j(n) L_j(n)

Equation 5

By applying this estimated interference value to Equations 1 through 4, the transmit power calculator 698 calculates the transmit power for each time slot.

Referring to Fig. 8, a downlink power control flow diagram according to this fourth embodiment is illustrated. Base station 30i calculates the estimated interference I for each time slot (step 801) and then calculates the transmit power level for each time slot (step 802) using the above Equations 1 to 5, which updates the signal from downlink from the base station (step 803).

Again it is to be noted that node B 26 and RNC 36 can also perform the function of storing all required information and calculating the estimated interference and transmit power for each time slot. Referring to Fig. 9, a downlink power control flow diagram according to this fifth embodiment is illustrated. The RNC 36 calculates an estimated interference I for each time slot (step 901). If the system is configured such that node B 26 calculates the transmit power, the RNC 36 sends the estimated interference I to node B 26 (step 902), where the transmit power is calculated for all physical channels ( step 903), and the base station downlink signal is updated (step 904). Otherwise, the RNC 36 calculates the transmit power for each segment (step 903).

Since physical channels are allocated by the RNC before the actual physical transmission, it is possible for a Node B to calculate the expected UE interference for the frame being transmitted in real time. The real-time interference calculation allows the correct transmit power for each time segment of the frame being transmitted.

A sixth embodiment of the present invention uses the combination of measured and estimated interference approaches described above to control downlink power. In this approach, the base station 30i combines weighted interference values ​​for both the estimated interference and the measured interference in order to

ES 2 209 673 T3 calculate the transmit power per time slot of the CCTrCH. For a UE MUD, the relevant interference (affecting detection performance) in each time slot is indicated as

I_D(n) = ZPj (n) Lj (n) Equation 6 lodojU) where Pj (n) is the transmission power of a base station j at time n in a certain segment, P₀, the transmit power of the base station being 30₁ from the EU. Lj (n) indicates the corresponding path loss. For a non-MUD UE, the relevant interference is indicated as:

I_D(n) = ZPj (n) Lj (n) Equation 7 todoj

However, the measured interference I_D(n) will be reported by the UE as an ISCP measurement. Equations 6 and 7 are merely illustrative of this interference present in the communication system:

Estimated interference is reported as:

I (n) = LPj (n) Lj (n)

Equation 8

Where the sum is made in all interferers whose load and path loss with respect to the UE are known. Similar to the fifth embodiment, the charging information is known to the base station 30₁ for everything j. Any unknown interference from a load UE is indicated as the residual interference I_F(neither_F(n) = I (n) -I_D(n). From each of these interference values, the transmit power device 698 combines them to generate a more accurate interference power value to be used in estimating the downlink transmit power for each time slot. , defined by Equations 1 to 4. The power value is defined as:

I = aI_F + βΐ + yI_D, α + β + γ = 1

Equation 9 where the coefficients α, β and γ are determined by system or even by segment according to measurement delays, or by the existence of strange base stations.

A flow chart of the downlink power control system according to the sixth embodiment is illustrated in Fig. 10. Base station 301 receives a communication from UE 22 that includes an ISCP I interference measurement_D for each time segment (step 1001). The transmit power calculating device 698 then calculates an estimated interference value I using information stored in the base station database 696 (step 1002). A residual interference value I is then calculated_F by the transmit power calculating device (step 1003). The transmit power calculating device then combines the three interference values ​​ID, I, IF (step 1004) and calculates the transmit power for each time slot of the downlink communication (step 1005).

Similar to the previous embodiments, the RNC 36 and Node B 26 can calculate the transmit power for each time slot as described above in a seventh embodiment. Referring to Figure 11, a flow chart of this embodiment is illustrated. RNC 36 receives a communication from UE 22 that includes an ISCP I interference measurement._D for each time segment (step 1101). The RNC 36 then calculates an estimated interference value I using information stored in the RNC 36 (step 1102) and a residual interference value I_F (step 1103). The RNC 36 then combines the three interference values ​​I_D, I, I_F (step 1104) and calculates the transmit power for each time segment of the downlink communication using Equations 1 to 4 (step 1106) and sends the same to base station 30₁ via Node B 26 (step 1107). If the downlink power control system is configured to allow Node B 26 to calculate the transmit power for each time slot, the RNC 36 sends the combined interference value I to Node B 26 (step 1105), the which calculates the transmit power for each time slot (step 1106) and sends this data to the base station (step 1107).

The benefit of providing a system that uses a measured ISCP value and an estimated interference value to calculate the transmit power for each time segment of the downlink communication is twofold: 1) the system provides flexibility in power calculation transmission in a case in which the required information is not known; and 2) the system provides a more accurate estimate of the interference present in the communication system.