WO2015017657A1

WO2015017657A1 - Autonomous calibration and control of transmit diversity weights using received signals for a tdd system

Info

Publication number: WO2015017657A1
Application number: PCT/US2014/049149
Authority: WO
Inventors: Phil F. Chen; Eduardo Abreu; Haim Harel
Original assignee: Google Inc.
Priority date: 2013-08-01
Filing date: 2014-07-31
Publication date: 2015-02-05

Abstract

In a diversity transmission TDD system, a calibration table is selectively used to determine diversity parameters for uplink transmissions. In one example, a UE includes two antennas and receives a downlink transmission separately at each antenna. The difference in phase of the two versions of the received downlink transmission is selectively used to determine uplink diversity transmission parameters from prior diversity transmission parameters stored in the calibration table.

Description

AUTONOMOUS CALIBRATION AND CONTROL OF TRANSMIT DIVERSITY WEIGHTS USING RECEIVED SIGNALS FOR A TDD SYSTEM

RELATED APPLICATION (S)

[001] This application claims priority to United States Provisional Patent Application Serial No. 61/861,246, filed August 01, 2013, which is incorporated herein by reference.

FIELD OF THE DISCLOSURE

[002] The present disclosure relates to calibration and control of transmit diversity parameters in a multi-antenna device using a calibration table.

BACKGROUND

[003] Prior patents and publications teach antenna transmit diversity in a sending device such as mobile user equipment (UE) based on a feedback signal from a receiving communication device such as a base station. For example, transmit diversity phases may be determined by the sending communication device based on a feedback signal from the receiving

communication device. The feedback signal may be a power control command (TPC) intended to mitigate fading channels and interference to other users. Another example of feedback is Antenna Selection Feedback in Long Term Evolution (LTE). Antenna Selection (AS) feedback may be also used to determine beamforming phases. Closed-loop ULTD for HSPA and UL MIMO beamforming for LTE Advanced are also proposed in the 3GPP standard, where Precoding weights are sent by a base station as a Precoding Control Indicator (PCI).

[004] All feedback mechanisms are implemented with a round trip delay and their performance is limited by fading and a feedback rate. This imposes a burden on transmit diversity performance in a high mobility

environment, especially for Time Division Duplex (TDD) systems where feedback rates are lower and round trip delay is longer than for Frequency Division Duplex (FDD) systems.

SUMMARY

[005] A UE in an environment with rapidly changing transmission paths may determine diversity parameters for uplink transmission from a calibration table instead of from feedback from the base station. Diversity parameters include, for example, phases and power levels of each separate antenna transmission. In one example, a UE transmits an uplink diversity signal from a first UE antenna and a second UE antenna. The uplink diversity signal is received at a base station and the base station sends a downlink signal back to the UE. The downlink signal is received at the first UE antenna and separately at the second UE antenna. Since the signal is received separately, it generally will be received at different phases at each antenna - downlink phase difference. The difference in phase of the signal received at each antenna may be recorded in the calibration table along with feedback information relating to the received power of the prior uplink signal and the diversity parameters of the prior uplink signal.

[006] When the UE is in a low mobility environment, the transmission path between the UE and the base station will remain substantially constant between the uplink transmission and receipt of the feedback information relating to that uplink transmission. Accordingly, the feedback will have a strong degree of accuracy. However, when the UE is in a high mobility environment, the transmission path will have changed from the time of the uplink transmission and the time of receipt of the feedback relating to that transmission. This change in the transmission path results in a lack of accuracy of the feedback.

[007] The present disclosure describes a system and method that takes advantage of the previously described downlink phase difference in order to populate a calibration table with at least fields for downlink phase difference, uplink diversity transmission phase difference, uplink diversity transmission power, and downlink received power.

[008] When the UE is in a low mobility environment, where the feedback information will accurately provide diversity control, the diversity parameters are obtained based upon the feedback and the calibration table is populated for later use. When the UE is in a high mobility environment, and the calibration table is populated, then the UE determines diversity transmission parameters from the calibration table based on the downlink phase difference. U.S. Patent No. 7,991,365 discloses a system and method for determining the mobility of a UE and is hereby incorporated by reference.

[009] The feedback signal relates to the transmission path at the time of the uplink transmission and the downlink phase difference relates to the transmission path at the time of the subsequent downlink transmission. Since the transmission path is rapidly changing over time, the downlink phase difference provides a more accurate identification of the transmission path. Thus, the diversity parameters saved in the calibration table and corresponding to the downlink phase difference will provide for a more accurate transmission than the diversity parameters provided in the feedback signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates a user terminal (UE) with diversity antennas communicating with a base station for uplink.

[0011] FIG. 2 illustrates a user terminal (UE) with diversity antennas communicating with a base station for downlink.

[0012] FIG. 3 shows a general TDD transmission frame structure.

[0013] FIG. 4 is an exemplary block diagram of a UE with diversity antennas.

[0014] FIG. 5. illustrates an exemplary frame structure of the present disclosure. [0015] FIG. 6 is an exemplary flow chart of calibration and control of transmit diversity phases using received signals.

[0016] FIG. 7 is an example of a calibration table generated from the calibration procedure in FIG. 6.

DETAILED DESCRIPTION

[0017] As illustrated in Figs. 1 and 2, the system 100 of the present disclosure includes a base station 102 (sometimes referred to as a "Node B") and a User Equipment 106 (hereafter referred to as a "UE"). The base state includes an antenna 104 (an example of one antenna is shown and described, though two or more antennas may be used) and is generally fixed in location. The UE includes multiple antennas 108, 110 (an example of two antennas is shown and described, though three or more antennas may be used) and is mobile in location.

[0018] Fig. 1 illustrates the UE 106 transmitting to the base station

102 (uplink). Consistent with diversity transmission (a/k/a beam forming), the UE 106 transmits the same content from each antenna 108, 110, only the different UE 106 antenna transmissions are phase shifted and may be at different power levels in order to create a constructive interference at the location of the base station antenna 104. Fig. 2 illustrates the base station 102 transmitting to the UE 106 (downlink). The base station 102 transmits a single signal that is received separately by each of the UE antennas 108, 110. If the transmission path between the base station 102 and each UE antenna 108, 110 is not the same (or is not shifted by a wavelength or an integer multiple of the wavelength), the signal received by the first UE antenna 108 will be phase shifted and/or power level shifted compared to the second UE antenna 110.

[0019] In a closed-loop or open-loop diversity transmission system, the diversity parameter of the signals being transmitted by the UE 106 starts out as an approximation of the value that produces the strongest signal at the base station and is then updated each transmission cycle in response to a feedback signal from the base station 102. In general terms, the UE will incrementally increase or decrease the phase shift and signal strength Tx based on the feedback signal from the base station 102. The incremental phase shifts work well to obtain the best diversity phase difference if the transmission path remains mostly constant. However, if the transmission path is rapidly changing, incrementally shifting the phase may leave the phase constantly off the target because the incremental adjustments are not fast enough to match the changes in transmission path.

[0020] One example of a rapidly changing transmission path is for a UE 106 that is physically moving (e.g., in a car), though the transmission path may change even if the UE 106 is stationary due to a number of other factors. The degree of change of the transmission path is referred to as "mobility." A diversity transmission system 100 with high mobility has a rapidly changing transmission path; a diversity transmission system 100 with low mobility has a relatively constant transmission path.

[0021] Fig. 3 shows a general Time Division Duplex ("TDD") transmission scheme. A TDD scheme includes a single transmission frequency for both downlink transmissions and uplink transmissions. While a TDD system is used as an example, the system and method may be applicable to other systems. The downlink and uplink transmissions alternate use of the frequency spectrum in order to avoid interfering with each other. The illustration of Fig. 3 shows three frames, n-1, n and n+1. Each frame includes a downlink subframe and an uplink subframe.

[0022] In Fig. 4 an exemplary block diagram of a UE 106 of the disclosure is provided. The UE 106 includes a first antenna 108 connected to a first Transmit/Receive Switch 112 and a second antenna 110 connected to a second Transmit/Receive Switch 114. The UE further includes a Down Converter 120, a Receiver (Demodulator) 122, a Processor (Modulator) 124, Memory 126, Other Modules 128 (e.g., user interface, etc.), a Beamformer 132 and an Up Converter 138.

[0023] When the UE 106 is receiving a downlink transmission, the switches 112, 114 connect the respective antennas 108, 110 to the Down Converter 120. The Down Converter 120 receives and downconverts the respective downlink signals 116 and 118 to a baseband frequency. The Down Converter 120 is then connected to the Receiver 122, and the Receiver 122 is further connected to the Processor 124. The Receiver 122 demodulates the downlink baseband signals which are phase shifted relative to each other by A9(R1, R2), where Rl is the receive phase of Antenna Al and R2 is the receive phase of Antenna A2. The UE further includes Memory 126 and Other Modules 128. The Memory 126 and Other Modules 128 are connected to the processor 124. The Calibration Table is preferably stored in the Memory 126.

[0024] When an uplink transmission is sent, the processor 124 sends an uplink signal 130 to the Beamformer 132. The Beamformer 132 copies the uplink signal and adapts the copied uplink signal based on the diversity transmission parameter including the uplink phase difference ΔΦ(Τ1, T2), where Tl is the transmission phase of Antenna Al and T2 is the transmission phase of Antenna A2. The uplink diversity signals 134, 136 are then converted to a radio frequency by the Up Converter 138 and sent to the respective Transmit/Receive Switches 112, 114. The Transmit/Receive Switches 112, 114 convey the diversity signals 134, 136 to the respective antennas 108, 110 for transmission. An example of a baseband Beamformer is shown and described, though a radio frequency (RF) Beamformer may be used instead, where a RF Beamformer receives the uplink signal in radio frequency from the Up Converter 138.

[0025] Fig. 5 shows an example of a timeline description of uplink and downlink subframes according to the present disclosure. In this example, a first uplink subframe 202 is transmitted from the UE 106 to the base station 102. The first uplink subframe 202 is transmitted at time to, has a power Tx, and has an uplink phase difference ΔΦ(Τ1, T2)(to). The first uplink subframe 202 is received at the base station 102 at time to+Δ- The base station 102 observes the received power Rx of the first uplink subframe 202 and, at ti, sends a first downlink subframe 204 back to the UE 106. The first downlink subframe 204 includes first uplink transmission feedback FB[A0(T1, T2)(to)] (e.g., a Transmit Power Control (TPC signal), an Antenna Selection (AS) signal, and/or a Precoding Control Indicator (PCI) signal) in addition to other downlink data. At time ti+Δ, the first downlink subframe 204 is received at the UE 106. Since the UE 106 includes two separate antennas 108, 110, the first downlink subframe 204 is received at the first antenna 108 with a first received phase Rl and at the second antenna 110 with a second received phase R2. The first downlink subframe phase difference is represented as A9(R1, R2)(ti).

[0026] The present disclosure describes an example of a system and method that increases the control accuracy for diversity transmission in a high mobility environment. A table of transmission control data (e.g., Tx Power, Rx Power, A9(R1, R2), ΔΦ(Τ1, T2), and frequency) is populated during low mobility transmission. An example of a table of transmission control data is provided in Fig. 7. During high mobility transmission, the uplink diversity parameters are determined based on the data in the table instead of based on a new feedback signal.

[0027] If the UE 106 is in a low mobility environment, then it is known that that the uplink path and downlink path have not materially changed between to and ti+Δ. If two uplink subframes are transmitted over the same transmission path, the two corresponding downlink subframes will be received by the UE with substantially the same downlink phase difference A9(R1, R2) (t_x). Further, if the UE 106 is in a low mobility environment, then the diversity parameters obtained from the feedback FB[A0(T1, T2)(t_x-i)] will still be accurate for the subsequent uplink transmissions at t_x+1, t_x+2, t_x+3...t_x+n. However, if the mobility is high, the feedback FB[A0(T1, T2)(t_x-i)] will be providing diversity parameters that are based on a transmission path a full two time periods old.

[0028] Looking again to the example of Fig. 5, at a time t_n, the UE receives a downlink subframe 206 having feedback FB[A0(T1, T2)(t_n-i)] and a received downlink phase difference A9(R1, R2) (t_n). If A9(R1, R2) (t_n) = A9(R1, R2) (tn-i), the transmission path between the UE and the base station at time t_n is substantially the same as it was at time t_n-i . Here, if the system transmits the next uplink signal at t_n+i based on FB[A0(T1, T2)(t_n-i)], the diversity parameters will be inaccurate to the degree that the transmission path has changed from t_n-i tO tn+i .

[0029] However, if the system transmits the next uplink signal at tn+i based on FB[A (T1, T2)(t_x-i)] from the Calibration Table in Memory 126, the diversity parameters will be inaccurate only to the degree that the transmission path has changed from t_n to t_n+1. This is the case because the saved parameters were not subject to any change of transmission path during their round trip transmission between the UE and base station since the mobility was low at that time.

[0030] Fig. 6 is a flow chart illustrating an exemplary method of the present disclosure. First, the UE sends an uplink transmission to the base station from the UE's two antennas (302). This signal is received at the base station and the base station determines if the power should be increased or decreased (304). The base station then sends a downlink transmission back to the UE (306). The downlink signal includes an indication of whether the uplink transmission power should be increased or decreased. The downlink signal is next received by the two antennas of the UE (308).

[0031] Since the two UE antennas have different locations

(generally opposite ends of the UE), the phase of the downlink signal is different at each antenna at a given time. This difference in phase is calculated by the UE and represented by ΔΘ(Μ, R2) (310).

[0032] Next, the method makes two determinations. First, whether or not the mobility is high; and second, whether or not calibration data is available (312). As discussed above, mobility relates to the rate of change of the signal propagation path, and calibration data is available after calibration data has been recorded in a calibration table. In this example, the calibration table is stored in the Memory 126 at the UE 106.

[0033] If UE mobility is not high or calibration data is not available, the UE determines the next uplink diversity phase difference from the received feedback signals (TPC, AS, or PCI) from the base station (314). Then the method records the Rx Phase Difference [A9(R1, R2)], the Rx Power, the Tx Phase Difference (from feedback), Tx Power, and the Frequency number of the prior transmission in the Calibration Table (316). In one example of the method/system, the calibration table is only updated when the mobility is low.

[0034] If the mobility is high and calibration data is available, the UE determines the next uplink diversity phase difference from the Calibration Table based on the Rx Phase Difference (318). In one example, the data of the Calibration Table is averaged, and the UE uses an average value for the next uplink diversity phase difference. In another example, if calibration data is available, the UE may determine the next uplink diversity phase difference from the Calibration Table based on the Rx Phase Difference in low mobility as well.

[0035] Following the determination of the next uplink diversity phase difference, the UE applies the phase difference and transmits from the diversity antennas (320).

[0036] The accompanying drawings illustrate examples of method and system for diversity transmission. Other types and styles are possible, and the drawings are not intended to be limiting in that regard. Thus, although the description above and accompanying drawings contains much specificity, the details provided should not be construed as limiting the scope of the examples but merely as providing illustrations of some of the presently preferred examples. The drawings and the description are not to be taken as restrictive on the scope of the examples and are understood as broad and general teachings in accordance with the present disclosure. While the present examples of the disclosure have been described using specific terms, such description is for present illustrative purposes only, and it is to be understood that modifications and variations to such examples, including but not limited to the substitutions of equivalent features, materials, or parts, and the reversal of various features thereof, may be practiced by those of ordinary skill in the art without departing from the spirit and scope of the disclosure.

Claims

1. In a diversity transmission system having a calibration table, a method of calibrating transmit diversity parameters, the method comprising the steps of: receiving a signal having feedback information at a first antenna and a second antenna;

determining a phase difference and a mobility of the received signal;

if the mobility is high and calibration data is available from a calibration table, then assigning transmit diversity parameters based on the calibration table;

otherwise, assigning transmit diversity parameters based on the feedback information; and

transmitting a signal from the first antenna and the second antenna using the assigned transmit diversity parameters.

2. The method of calibrating transmit diversity parameters of claim 1, further comprising the step of recording the assigned transmit diversity parameters in the calibration table.

3. The method of calibrating transmit diversity parameters of claim 1, wherein the assigned transmit diversity parameters are based upon average values from the calibration table.

4. The method of calibrating transmit diversity parameters of claim 1, wherein both the step of assigning parameters based on the calibration table and the step of updating the calibration table with new feedback data are performed.

5. The method of calibrating transmit diversity parameters of claim 1, wherein the calibration table includes parameters for: Rx power, Tx power, A9(R1, R2), and ΔΦ(Τ1,Τ2).

6. The method of calibrating transmit diversity parameters of claim 5, wherein the calibration table further includes a parameter for frequency number.

7. The method of calibrating transmit diversity parameters of claim 1, wherein mobility relates to the consistency of the transmission path between two transmission devices of the time division duplex system.

8. The method of calibrating transmit diversity parameters of claim 1, wherein the phase difference of the received signal is the difference between the phase of the received signal at the first antenna at a time t_n and the phase of the received signal at the second antenna at the time t_n.

9. The method of calibrating transmit diversity parameters of claim 1, wherein the calibration table is only updated when the mobility is low.

10. A mobile device operating in transmit diversity system, the mobile device comprising:

first and second antennas adapted to receive a transmission from a second device;

a receiver adapted to determine a phase difference and mobility of the received signal;

memory storing a calibration table containing data relating to diversity transmission;

a processor adapted to selectively assign diversity transmission

parameters based on the calibration table when both the mobility is high and the calibration table is available;

wherein the processor is further adapted to selectively load diversity transmission parameters into the calibration table when not assigning diversity parameters based on the calibration table.

11. The mobile device operating in transmit diversity system of claim 10, wherein the assigned transmit diversity parameters are based upon average values from the calibration table.

12. The mobile device operating in transmit diversity system of claim 10, wherein the processor is adapted to both update the calibration table with new feedback data and to assign parameters based on the calibration table.

13. The mobile device operating in transmit diversity system of claim 10, wherein the calibration table includes parameters for: Rx power, Tx power, ΔΘ(Μ, R2), and ΔΦ(Τ1, T2).

14. The mobile device operating in transmit diversity system of claim 13, wherein the calibration table further includes a parameter for frequency number.

15. The mobile device operating in transmit diversity system of claim 10, wherein mobility relates to the consistency of the transmission path between two transmission devices of the time division duplex system.

16. The mobile device operating in transmit diversity system of claim 10, wherein the phase difference of the received signal is the difference between the phase of the received signal at the first antenna at a time and the phase of the received signal at the second antenna at the same time.

17. The mobile device operating in transmit diversity system of claim 10, wherein the calibration table is only updated when the mobility is low.

18. A method of selecting diversity transmission parameters for an uplink transmission by a mobile device having a first antenna and a second antenna, the method comprising the steps of:

recording diversity parameters in a calibration table at the mobile device; receiving a downlink transmission at the first antenna of the mobile device and separately at the second antenna of the mobile device;

determining the phase difference between the downlink transmission as received by the first antenna and the downlink transmission as received by the second antenna;

determining that the mobile device is in a high mobility environment; and sending an uplink diversity transmission from the first antenna and the second antenna, the uplink diversity transmission having diversity parameters selected from the calibration table.

19. The method of selecting diversity transmission parameters for an uplink transmission by a mobile device having a first antenna and a second antenna of claim 18, wherein the step of recording diversity parameters in a calibration table at the mobile device further comprises:

recording a phase difference between a prior downlink transmission as received by the first antenna and the prior downlink transmission as received by the second antenna in the calibration table.

20. The method of selecting diversity transmission parameters for an uplink transmission by a mobile device having a first antenna and a second antenna of claim 18, wherein the step of recording diversity parameters in a calibration table at the mobile device is only performed when the UE is determined to be not in a high mobility environment.