Sounding reference signal beamformer enhancement with timing correction

Abstract

A method for estimating beamforming weights, the method including: receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU); performing channel estimation of the received SRS; calculating a time-domain response for the SRS channel estimate at a level of granularity defined by a bandwidth of the SRS; determining a timing offset at the defined level of granularity; compensating for the timing offset at the defined level of granularity; estimating beamforming weights according to the compensated timing offset; and transmitting the estimated beamforming weights to an O-RAN radio unit (O-RU) via a fronthaul interface between the O-DU and the O-RU.

Description

[Technical Field]

[0001] In some demonstrative embodiments, the subject matter herein relates generally to beamforming in wireless communication systems, and more particularly to sounding reference signal (SRS) beamformer enhancement with timing correction in open radio access network (O-RAN) communication systems. [Background technology]

[0002] Beamforming has been introduced with significant advantages to overcome several communication challenges, such as improving the accuracy of wireless connections, enhancing throughput, increasing the number of parallel connections in a given cell area, and saving energy consumption during transmission. For example, in mmWave transmission, beamforming is particularly beneficial for improving the signal-to-noise ratio (SNR) or signal-to-interference-and-noise ratio (SINR) through direct targeting of user groups, primarily indoor coverage. Furthermore, beamforming can also neutralize interference between user equipment (UE) signals in dense environments.

[0003] Furthermore, massive multiple-input multiple-output (MIMO) antenna systems allow distributed units (DUs) to control signal transmission in desired directions by changing antenna gain and phase. In time division duplexing (TDD), channel reciprocity is used to estimate the direction / channel in the uplink (UL) and the downlink (DL) for beamforming. The Third Generation Partnership Project (3GPP®) New Radio (NR) provides a sounding reference signal (SRS) channel that allows base stations to estimate the uplink channel. The SRS channel is a multiplexed channel, and different users are scheduled with different subcarriers (COMBs), cyclic shifts, and bandwidths (BWs) to optimize SRS resources. The base station can use the SRS channel estimation to obtain optimized beamforming weights.

[0004] Note that a timing advance is a special command (notification) from the gNB / eNB to the UE that allows the UE to adjust its uplink transmission. This helps ensure that all uplink transmissions from the UE are synchronized when received by the base station. However, the UE still has a residual timing error even with the timing advance (TA) from the gNB / eNB due to the granularity of the TA command (e.g., ±8Ts or 16Ts). In 5G New Radio (NR), the TA granularity is generally based on several parameters associated with the uplink carrier (e.g., subcarrier spacing, cyclic prefix, transmission time interval, etc.). Timing errors occurring within the TA granularity cannot be corrected by the TA command. Even with pedestrian mobility, few samples of timing error accumulate over a UE movement of tens of meters. The group delay of a multipath fading channel has the same effect as a timing offset. This residual timing offset introduces frequency selectivity in the UL channel and changes beamforming weights within a resource block (RB).

[0005] The SRS beamformer captures full channel information, enabling efficient MU multiplexing. Using the O-RAN interface, beamforming coefficients are transferred in resource blocks or groups of resource blocks to reduce the fronthaul (FH) bandwidth. The fronthaul interface is part of the 5G NR Layer 1 implementation. It communicates between the O-RAN distributed unit (O-DU) and the O-RAN radio unit (O-RU) and consists of multiple hardware and software components. However, in a high-frequency selective channel and / or with timing offsets at the UE, the beamformer granularity is not sufficient to capture the complete channel information, which reduces beamforming performance. Summary of the Invention [Means for solving the problem]

[0006] Existing methods do not address the limitations on beamformer performance due to the granularity of channel frequency selectivity. This constraint from the O-RAN interface does not exist in conventional technologies and is therefore not taken into account. Therefore, there is a need for a method to overcome the above limitations and restrictions at the O-RAN interface.

[0007] According to one embodiment, there is provided a method for estimating beamforming weights, the method including: receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU), performing channel estimation for the received SRS, calculating a time-domain response for the SRS channel estimate at a level of granularity defined by a bandwidth of the SRS, determining a timing offset at the defined level of granularity, compensating for the timing offset at the defined level of granularity, estimating beamforming weights according to the compensated timing offset, and transmitting the estimated beamforming weights to an O-RAN radio unit (O-RU) via a fronthaul interface between the O-DU and the O-RAN radio unit (O-RU).

[0008] According to another embodiment, an open radio access network (O-RAN) wireless communication system is provided, including: an O-RAN radio unit (O-RU) configured to receive a sounding reference signal (SRS); and an O-RAN distributed unit (O-DU) configured to perform bidirectional communication with the O-RU via a fronthaul interface, receive the SRS from the O-RU, perform channel estimation for the SRS received from the O-RU, calculate a time-domain response of the SRS channel estimation at a granularity defined by a bandwidth of the SRS, determine a timing offset at the defined level of granularity, compensate for the timing offset at the defined level of granularity, estimate beamforming weights according to the compensated timing offset, and transmit the estimated beamforming weights to the O-RU via the fronthaul interface between the O-DU and the O-RU.

[0009] According to another embodiment, a non-transitory computer-readable medium having stored thereon instructions for causing a processing circuit to perform a process including receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU), performing channel estimation of the received SRS, calculating a time-domain response of the SRS channel estimation at a level of granularity defined by a bandwidth of the SRS, determining a timing offset at the defined level of granularity, compensating for the timing offset at the defined level of granularity, estimating beamforming weights according to the compensated timing offset, and transmitting the estimated beamforming weights to an O-RAN radio unit (O-RU) via a fronthaul interface between the O-DU and the O-RAN radio unit (O-RU).

[0010] The drawings are as follows: [Brief explanation of the drawings]

[0011] [Figure 1A]FIG. 1A shows respective channel response graphs in the time domain before and after timing offset compensation, according to one embodiment. [Figure 1B] FIG. 1B shows channel response graphs in the time domain before and after timing offset compensation, respectively, according to one embodiment.

[0012] [Figure 2A] FIG. 2A shows channel response graphs in the frequency domain before and after frequency selectivity reduction, respectively, according to one embodiment. [Figure 2B] FIG. 2B shows respective channel response graphs in the frequency domain before and after frequency selectivity reduction, according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] In an O-RAN communication system, the DU calculates the beamforming weights for a user based on the SRS channel, and these weights are transferred to the radio via the O-RAN interface. The payload of the beamforming weights is proportional to the number of TRXs and the number of RBs allocated to the user. To reduce overhead on the interface, beamformer estimates are transferred per RB according to the O-RAN standard. However, beamformer variations within an RB are not captured in the current configuration. As mentioned above, due to the granularity of the TA command, the UE may have residual timing error even with timing advance (TA) from the gNB / eNB, which leads to frequency selectivity in the UL channel and changes in beamforming weights within a resource block (RB).

[0014] One embodiment of the present disclosure proposes to reduce channel selectivity so that the granularity of the beamformer does not reduce beamforming performance, by estimating the timing offset in the SRS channel and correcting for the offset.

[0015] According to one embodiment, when an open radio access network (O-RAN) distributed unit (O-DU) receives a sounding reference signal (SRS), the O-DU performs channel estimation for the received SRS and calculates a time-domain response for the SRS channel estimate at a level of granularity defined by the bandwidth of the SRS. FIG. 1A illustrates a channel response graph in the time domain. The graph includes a signal portion and a noise portion. A timing offset is determined based on the timing position of the peak relative to a delay equal to the zero reference point in the time domain. As can be seen from FIG. 1A, the peak is located to the right of the delay equal to the zero reference point, indicating that reception of the SRS is delayed by the timing offset value, as shown. On the other hand, if the peak is located to the left of the delay equal to the zero reference point (not shown), reception of the SRS is early by the timing offset value. This timing error results in frequency selectivity in the UL channel. To address this issue, when the SRS is delayed, the timing error is compensated for by shifting the peak to the left by the determined timing offset value. As shown in FIG. 1B, the peak coincides with a delay equal to the zero reference point after compensating for the timing offset. FIG. 2A shows a channel response graph in the frequency domain. It can be seen that the timing error causes the frequency components to vary more. FIG. 2B shows the corresponding channel response graph in the frequency domain after the frequency selectivity is reduced by timing offset compensation. It can be seen that the frequency components exhibit smaller fluctuations compared to those shown in FIG. 2A. The smaller fluctuation range of the frequency components reduces the frequency selectivity in the UL channel. The O-DU then estimates beamforming weights according to the compensated timing offset and transmits the estimated beamforming weights to the O-RAN radio unit (O-RU) via the fronthaul interface between the O-DU and the O-RU.

[0016] Timing correction according to one embodiment significantly improves the performance of SRS beamformers in O-RAN communication systems. In two simulations, it has been observed that average gains of approximately 8 dB and 8.5 dB, respectively, can be achieved using timing offset compensation according to one embodiment.

[0017] In the case of a highly dispersive channel, even when there is no residual timing error, i.e., when the main peak of the time-domain channel response graph is at a delay equal to the zero reference point, there are still multiple similar power level peaks with several large delays. These peaks also make the channel highly frequency selective. In this case, correcting the timing error based only on the time offset of the main peak may not be sufficient to reduce the frequency selectivity. In one embodiment, the root-mean-square (RMS) delay spread of the channel's time-domain response is calculated on the SRS channel, and the RMS delay spread is compensated accordingly. In one embodiment, the timing offset of the RMS delay spread is determined, and further, the timing offset compensation includes adjusting the RMS delay spread to a reference point of a delay equal to zero.

[0018] It should be noted that the SRS processing defined above can be performed with oversampling or undersampling according to some embodiments, and in one embodiment, a mixture of both at different stages, e.g., oversampling before performing the inverse discrete Fourier transform / inverse fast Fourier transform (IDFT / IFFT) and undersampling after the discrete Fourier transform / fast Fourier transform (DFT / FFT).

[0019] In a multi-antenna environment, according to one embodiment, a single timing compensation is performed for all antennas. The single timing estimate can be an average or weighted average of timing estimates from all antennas, or an estimate from the channel derived by taking an average or weighted average of the SRS channels across all antennas. In one embodiment, the weighted average is based on the channel conditions. For example, the weighted average is based on the SNR / SINR of the antennas.

[0020] As described above, in one general aspect, a method for estimating beamforming weights is provided, the method including: receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU), performing channel estimation of the received SRS, calculating a time-domain response for the SRS channel estimate at a level of granularity defined by a bandwidth of the SRS, determining a timing offset at the defined level of granularity, compensating for the timing offset at the defined level of granularity, estimating beamforming weights according to the compensated timing offset, and transmitting the estimated beamforming weights to an O-RAN radio unit (O-RU) via a fronthaul interface between the O-DU and the O-RAN radio unit (O-RU).

[0021] Implementations of the method may include one or more of the following features. In the method, according to one embodiment, the SRS is processed by oversampling or undersampling, or by a mixture of oversampling and undersampling at different stages. In the method, according to one embodiment, compensating for the timing offset includes adjusting the peak of the SRS to a reference point of a delay equal to zero in the time domain. In the method, according to one embodiment, a peak of the SRS in the time domain to the right of the delay equal to zero indicates a delay in the reception of the SRS, and a peak of the SRS in the time domain to the left of the delay equal to zero indicates an advance in the reception of the SRS. Furthermore, in the method, according to one embodiment, the delay in the reception of the SRS is due to a timing delay at the user equipment (UE), and the advance in the reception of the SRS is due to a timing advance at the UE. The method further includes calculating a root-mean-square (RMS) delay spread of the time-domain response of the channel calculated on the SRS channel and compensating for the RMS delay spread. According to one embodiment, the method further includes determining a timing offset of an RMS delay spread, and further includes compensating for the timing offset, and further includes adjusting the RMS delay spread to a reference point of delay equal to zero. Furthermore, according to one embodiment, the method is performed in a multi-antenna environment, and the method further compensates for a single timing offset for all antennas, the single timing offset being estimated from all antennas by averaging time estimates from all antennas. Furthermore, according to one embodiment, the method is performed in a multi-antenna environment, and the method further compensates for a single timing offset for all antennas, the single timing offset being estimated from all antennas by estimating from a channel derived by averaging an SRS channel across all antennas.

[0022] As noted above, in another general aspect, an Open Radio Access Network (O-RAN) wireless communication system is provided, including an O-RAN radio unit (O-RU) configured to receive a sounding reference signal (SRS), and an O-RAN distributed unit (O-DU) configured to: perform bidirectional communication with the O-RU via a fronthaul interface, receive the SRS from the O-RU, perform channel estimation for the SRS received from the O-RU, calculate a time-domain response of the SRS channel estimation at a granularity defined by a bandwidth of the SRS, determine a timing offset at the defined level of granularity, compensate for the timing offset at the defined level of granularity, estimate beamforming weights according to the compensated timing offset, and transmit the estimated beamforming weights to the O-RU via the fronthaul interface between the O-DU and the O-RU.

[0023] An implementation of an open radio access network (O-RAN) wireless communication system may include one or more of the following features. In the system, according to one embodiment, the SRS is processed with oversampling or undersampling, or a mixture of oversampling and undersampling at different stages. In the system, according to one embodiment, the O-DU is further configured to compensate for a timing offset by adjusting a peak of a time-domain response of the SRS channel to a reference point of delay equal to zero in the time domain. In a further embodiment, according to one embodiment, a peak of the SRS in the time domain to the right of delay equal to zero indicates a delay in reception of the SRS, and a peak of the SRS in the time domain to the left of delay equal to zero indicates an advance in reception of the SRS. In a further embodiment, according to one embodiment, the delay in reception of the SRS is due to a timing delay at the user equipment (UE), and the advance in reception of the SRS is due to a timing advance at the UE. In the system, according to one embodiment, the O-DU is further configured to calculate a root-mean-square (RMS) delay spread of the SRS channel and calculate a time-domain response of the RMS delay spread of the SRS channel. In a further embodiment, the O-DU is configured to determine a timing offset of an RMS delay spread, and further configured to compensate for the timing offset by adjusting the RMS delay spread to a delay equal to zero. In a multi-antenna environment, according to one embodiment, the O-DU is further configured to compensate for a single timing offset for all antennas, the single timing offset being estimated from all antennas by averaging time estimates from all antennas. In a multi-antenna environment, according to one embodiment, the O-DU is further configured to compensate for a single timing offset for all antennas, the single timing offset being estimated from all antennas by estimating from a channel derived by averaging an SRS channel across all antennas.

[0024] As noted above, in another general aspect, a non-transitory computer-readable medium having stored thereon instructions for causing a processing circuit to perform a process including receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU), performing channel estimation of the received SRS, calculating a time-domain response of the SRS channel estimation at a level of granularity defined by a bandwidth of the SRS, determining a timing offset at the defined level of granularity, compensating for the timing offset at the defined level of granularity, estimating beamforming weights according to the compensated timing offset, and transmitting the estimated beamforming weights to an O-RAN radio unit (O-RU) via a fronthaul interface between the O-DU and the O-RAN radio unit (O-RU).

[0025] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed features, from a study of the drawings, the disclosure, and the appended claims.

[0026] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0027] A single processor, device or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0028] Operations such as obtaining, accessing, analyzing, capturing, comparing, determining, displaying, inputting, obtaining, outputting, providing, storing or storing, calculating, simulating, receiving, alerting, and stopping may be implemented as program code means of a computer program and / or as dedicated hardware.

[0029] The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunications systems.

Claims

1. 1. A method for estimating beamforming weights, comprising: The method comprises: receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU); performing channel estimation of the received SRS; Calculate a time-domain response for the SRS channel estimate at a level of granularity defined by a bandwidth of the SRS; determining a timing offset at said defined level of granularity; Compensating for the timing offset at the defined level of granularity; estimating beamforming weights according to the compensated timing offset; and transmitting the estimated beamforming weights to an O-RAN Radio Unit (O-RU) via a fronthaul interface between the O-DU and the O-RU; A method comprising:

2. The SRS is processed with oversampling or undersampling, or a mixture of oversampling and undersampling at different stages. The method of claim 1.

3. and compensating for the timing offset includes adjusting a peak of the SRS to a reference point of delay equal to zero in the time domain. The method of claim 1.

4. 4. The method of claim 3, wherein a peak of the SRS in the time domain to the right of the delay equal to 0 indicates a delay in reception of the SRS, and a peak of the SRS in the time domain to the left of the delay equal to 0 indicates an advance in the reception of the SRS.

5. 10. The method of claim 1, further comprising: calculating a root-mean-square (RMS) delay spread of the time-domain response of the channel calculated on the SRS channel; and compensating for the RMS delay spread.

6. 6. The method of claim 5, further comprising determining a timing offset of the RMS delay spread, wherein compensating for the timing offset comprises adjusting the RMS delay spread to a reference point of delay equal to zero.

7. 2. The method of claim 1, wherein the method is performed in a multi-antenna environment, and the method further compensates for a single timing offset for all antennas, the single timing offset being estimated from all antennas by taking an average or weighted average of time estimates from all antennas.

8. 2. The method of claim 1, wherein the method is performed in a multi-antenna environment, and the method further compensates for a single timing offset for all antennas, the single timing offset being estimated from all antennas by estimating it from a channel derived by taking an average or weighted average of the SRS channels across all antennas.

9. 1. An open radio access network (O-RAN) wireless communication system, comprising: an O-RAN Radio Unit (O-RU) configured to receive a Sounding Reference Signal (SRS); and An O-RAN Distribution Unit (O-DU), comprising: bidirectional communication with the O-RU via a fronthaul interface; receiving the SRS from the O-RU; performing channel estimation of the SRS received from the O-RU; Calculate a time-domain response of the SRS channel estimate with a granularity defined by a bandwidth of the SRS; determining a timing offset at a defined level of granularity; Compensating for timing offsets at a defined level of granularity; estimating beamforming weights with the compensated timing offset; and an O-RAN distributed unit (O-DU) configured to transmit the estimated beamforming weights to the O-RU via a fronthaul interface between the O-DU and the O-RU;

10. The wireless communication system of claim 9 , wherein the SRS is processed with oversampling or undersampling, or a mixture of oversampling and undersampling at different stages.

11. 10. The wireless communication system of claim 9, wherein the O-DU is configured to compensate for the timing offset by adjusting a peak of a time domain response of the SRS channel to a reference point of delay equal to 0 in the time domain.

12. 12. The wireless communication system of claim 11, wherein the peak of the SRS in the time domain to the right of the delay equal to 0 indicates a delay in reception of the SRS, and the peak of the SRS in the time domain to the left of the delay equal to 0 indicates an advance in the reception of the SRS.

13. The O-DU is Calculating the root mean square (RMS) delay spread of the SRS channel; and configured to calculate a time domain response of the RMS delay spread of the SRS channel.

10. The wireless communication system according to claim 9.

14. 14. The wireless communication system of claim 13, wherein the O-DU is configured to determine a timing offset of the RMS delay spread and compensate for the timing offset by adjusting the RMS delay spread to a delay equal to zero.

15. 10. The wireless communication system of claim 9, wherein the wireless communication system operates in a multi-antenna environment and the O-DU is further configured to compensate for a single timing offset for all antennas, the single timing offset being estimated from all the antennas by averaging time estimates from all the antennas.

16. 10. The wireless communication system of claim 9, wherein the wireless communication system operates in a multi-antenna environment, and the O-DU is further configured to compensate for a single timing offset for all antennas, the single timing offset being estimated from all the antennas by estimating it from a channel derived by averaging the SRS channel across all the antennas.

17. A non-transitory computer-readable medium storing instructions for causing a processing circuit to perform a process, the process comprising: receiving a sounding reference signal (SRS) at an open radio access network (O-RAN) distributed unit (O-DU); performing channel estimation of the received SRS; calculating a time domain response of the SRS channel estimate at a level of granularity defined by a bandwidth of the SRS; determining a timing offset at said defined level of granularity; compensating for timing offsets at said defined level of granularity; estimating beamforming weights according to the compensated timing offset; and and transmitting the estimated beamforming weights to an O-RAN radio unit (O-RU) via a fronthaul interface between the O-DU and the O-RU.

18. 20. The non-transitory computer-readable medium of claim 17, wherein the SRS is processed with oversampling or undersampling, or a mixture of oversampling and undersampling at different stages.

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