OFDMA Baseband Clock Synchronization

Abstract

A method for synchronizing baseband clocks in an OFDMA wireless microphone system is disclosed. An example method includes receiving a plurality of pilot subcarriers from an audio transmitter. The method also includes determining a timing offset estimate based on the pilot subcarriers. The method further includes determining a tuning value by passing the timing offset estimate through a proportional-integral controller. The method still further includes determining a modified reference signal by modifying a reference oscillator based on the tuning value. And the method still further includes controlling (i) an audio sampling clock and (ii) an antenna data clock based on the modified reference signal.

Description

[0001] Cross Reference

[0002] This application claims priority to U.S. Patent Application No. 16 / 425,694, filed on May 29, 2019, the contents of which are incorporated herein in their entirety. Technical Field

[0003] The present application relates generally to wireless audio systems and devices for communicating using Orthogonal Frequency Division Multiple Access (OFDMA), particularly timing and frequency synchronization of audio devices having an audio sampling clock and an antenna data clock. Background Art

[0004] Orthogonal frequency division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. The subcarriers are sent together to form a broadband, high-speed communication link. Such communication links can be used for many purposes, including digital television and audio broadcasting, DSL Internet access, wireless networks, power line networks, and mobile communications.

[0005] In an OFDMA audio system, there may be an access point and one or more users. The user device must compensate for any frequency offset of its corresponding antenna data clock relative to the access point so that frame timing and symbol timing are maintained. This allows the user device to properly receive and transmit data via the access point.

[0006] Existing user devices may include a sample rate conversion block to prevent audio distortion introduced by inserting or dropping samples. However, the sample rate conversion block requires additional resources and introduces latency into the audio path.

[0007] Therefore, an opportunity exists for a method and system for OFDMA baseband clock synchronization that does not require additional sample rate conversion blocks (particularly in the context of high-quality audio applications) and thus reduces required resources and removes a source of delay in the audio path. Summary of the invention

[0008] Embodiments of the present invention are intended to alleviate some of the above-described problems by providing methods and systems for locking the baseband clocks of users in a wireless microphone system to an access point by generating respective baseband clocks from a common reference. The access point (also referred to as an "audio transmitter") and each user device (also referred to as an "audio receiver") all include one or more baseband clocks (i.e., controlling the audio sampling clock and / or the data sampling clock) set based on a reference oscillator. Embodiments of the present invention may include tuning the reference oscillator of each user device based on a measured phase difference so that the baseband clocks of all users and the access point are in synchronization.

[0009] An exemplary method includes receiving a plurality of pilot subcarriers from an audio transmitter. The method also includes determining a timing offset estimate based on the pilot subcarriers. The method further includes determining a tuning value by passing the timing offset estimate through a proportional-integral controller. The method still further includes determining a modified reference signal by modifying a reference oscillator based on the tuning value. And the method still further includes controlling (i) an audio sampling clock and (ii) an antenna data clock based on the modified reference signal.

[0010] These and other embodiments and various arrangements and aspects will become apparent and more fully understood from the following detailed description and accompanying drawings, which set forth illustrative embodiments indicating the various ways in which the principles of the invention may be employed. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a simplified block diagram of a wireless communication system according to some embodiments of the present disclosure.

[0012] Figure 2 According to some embodiments of the present disclosure Figure 1 A simplified signal processing flow chart of an exemplary user device of a system.

[0013] Figure 3 According to some embodiments of the present disclosure Figure 2 Simplified signal processing flow chart of the sampling timing offset estimator.

[0014] Figure 4 is a flowchart illustrating an example method according to some embodiments of the present disclosure. DETAILED DESCRIPTION

[0015] The following description describes, illustrates and demonstrates one or more specific embodiments of the present invention according to the principles of the present invention. This description is not provided to limit the present invention to the embodiments described herein, but to explain and teach the principles of the present invention in such a way that those skilled in the art can understand these principles and, with such understanding, can apply the principles to practice not only the embodiments described herein, but also other embodiments that can be conceived according to these principles. The scope of the present invention is intended to cover all such embodiments that can be attributed to the appended claims literally or under the principle of equivalents.

[0016] It should be noted that in the specification and drawings, similar or substantially similar elements may be marked with the same element symbols. However, sometimes these elements may be marked with different numbers, such as in situations where such marking promotes a clearer description. In addition, the drawings set forth herein may not be drawn to scale, and in some examples, the scale may have been enlarged to more clearly depict certain features. Such marking and diagram practice may not necessarily imply a basic substantial purpose. As described above, this specification is intended to be considered as a whole and is interpreted and understood by a person skilled in the art according to the principles of the present invention as taught herein.

[0017] As described above, embodiments of the present disclosure relate to wireless audio systems and device communications using OFDMA, and methods and systems for baseband synchronization between devices of wireless audio systems. In order for a user device to properly communicate with an access point, the user device must be able to compensate for any frequency offset relative to the access point so that frame and symbol timing is maintained. In some embodiments, this can be accomplished by frequency and phase locking between the access point and the user baseband clock. The baseband clock can be used to sample input audio and for various other purposes by the access point and the user transmitting and receiving data (i.e., audio data, control signals, pilot signals, etc.) via their respective antennas.

[0018] Frequency and phase lock between baseband clocks of wireless audio devices can be facilitated by having the access point and one or more users all generate their respective baseband clocks from a common reference oscillator. Each user can tune its own reference oscillator based on phase offset measurements from any of the baseband clocks (even from other users) in order to maintain frequency and phase lock with the access point reference. This results in all baseband clocks of a given user being locked to the respective access point reference because all baseband clocks are generated from a common reference.

[0019] In an embodiment of the present disclosure, a user may tune its reference frequency (i.e., reference oscillator) based on a sampling clock timing (i.e., phase) offset measurement between the user and an access point, which is made for each frame during communications between the access point and the user. The timing offset between the transmit sampling clock and the receive sampling clock in an OFDMA system results in a channel phase slope when plotted in the frequency domain. The slope is proportional to the sampling clock timing offset. Thus, by determining the slope of the channel phase offset between the transmitted signal (from the access point) and the received signal (at the user), the delay and, therefore, the timing offset measurement may be determined. This timing offset measurement may then be used to tune the user reference oscillator. The tuned reference oscillator may then be used to control the user's antenna data clock and audio sampling clock, thereby enabling synchronization of the user and access point baseband clocks. And by determining the timing offset measurement for each frame, the reference oscillator of the user's audio device may be continuously tuned to maintain "steady-state" synchronization. Furthermore, by controlling both the antenna data clock and the audio sampling clock of the user audio device based on the reference oscillator, the two clocks will be synchronized with each other and with the corresponding clock of the access point.

[0020] Figure 1 An example simplified block diagram of a wireless audio communication system or environment 100 in which the methods and apparatus of the present disclosure may be used is illustrated.The wireless audio communication system may include an access point 110 and a plurality of user devices 120A-N.

[0021] Access point 110 may be any suitable computing device and may include a processor, memory, an antenna, and / or one or more other signal processing or computing components. In some embodiments, access point 110 may be an automatic mixer, a laptop or desktop computer, or any other device configured to communicate with various other devices including user devices 120A-N (e.g., a plurality of wireless audio devices).

[0022] The access point 110 may be configured to perform various functions or actions, such as those described in the present disclosure (and accompanying drawings). The access point 110 may include various components, for example, a processor and a memory. The access point 110 may also include a display, a user interface, and / or one or more other electronic components. The processor may include a general purpose processor (e.g., a microprocessor) and / or a special purpose processor (e.g., a digital signal processor (DSP)). The processor may be any suitable processing device or group of processing devices, such as, but not limited to, a microprocessor, a microcontroller-based platform, an integrated circuit, one or more field programmable gate arrays (FPGAs), and / or one or more application specific integrated circuits (ASICs). The memory may be a volatile memory (e.g., RAM, including nonvolatile RAM, magnetic RAM, ferroelectric RAM, etc.), a nonvolatile memory (e.g., disk memory, FLASH memory, EPROM, EEPROM, nonvolatile solid-state memory based on memristors, etc.), a constant memory (e.g., EPROM), a read-only memory, and / or a high-capacity storage device (e.g., a hard disk drive, a solid-state disk drive, etc.). In some embodiments, the memory includes multiple types of memory, in particular volatile memory and nonvolatile memory. The memory may be a computer-readable medium having one or more instruction sets (e.g., software for operating the methods of the present disclosure) embedded thereon. The instructions may embody one or more of the methods or logics described herein. For example, the instructions reside completely or at least partially in any one or more of the memory, the computer-readable medium, and / or reside in the processor during execution of the instructions.

[0023] The terms "non-transitory computer-readable medium" and "computer-readable medium" include a single medium or multiple media, such as a centralized or distributed database and / or associated cache memory and servers storing one or more sets of instructions. Furthermore, the terms "non-transitory computer-readable medium" and "computer-readable medium" include any tangible medium capable of storing, encoding, or carrying a set of instructions executed by a processor, or causing a system to perform any one or more of the methods or operations disclosed herein. As used herein, the term "computer-readable medium" is expressly defined to include any type of computer-readable storage device and / or storage disk and to exclude propagating signals.

[0024] In some examples, access point 110 may be a base station, a centralized controller, or other computing device configured to communicate with multiple wireless audio user devices simultaneously. For example, an access point may be operated in a conference room, and the user devices may be multiple microphones communicating with the access point to provide a conference environment. Other examples are also possible.

[0025] The access point 110 may include one or more antennas for enabling wireless audio communication with one or more user devices 120A-N and a reference oscillator. The reference oscillator may be used to control one or more baseband clocks of the access point. For example, the antenna sampling clock may be controlled based on the reference oscillator to control the timing of transmission and reception of various data.

[0026] The access point 110 may be configured to transmit data to one or more wireless audio user devices 120A-N in various formats and using various communication protocols. For example, the access point 110 may use an OFDMA scheme to transmit data, wherein the data is transmitted in frames. Each frame may include a plurality of subcarriers, some of which are used to transmit data, some of which are pilot subcarriers for synchronizing the access point with the user, and some of which are "guard" subcarriers that provide protection from interference from adjacent channels or subchannels in the frequency spectrum. In one example, a given channel may be divided into 64 different subcarriers. The channel may include 52 data subcarriers, 4 pilot subcarriers, and 8 guard subcarriers. These numbers are for example only, and it should be understood that other numbers may also be used.

[0027] Each pilot subcarrier may be transmitted at a specific and known frequency and may be configured not to carry any audio or control information. The known frequency position of each pilot subcarrier may enable a user receiving the frame to determine the phase shift and, therefore, the timing offset between the access point 110 and the user. This will be discussed in more detail below.

[0028] The wireless audio user devices 120A-N may be portable wireless audio receivers, microphones, conferencing systems, speakers, and / or any other device communicatively coupled to the access point 110. The embodiments disclosed herein are described with reference to user devices that are each microphones, however, it should be understood that the concepts and features disclosed herein may also be applied to other types of user devices.

[0029] Each wireless audio user device 120A-N may include one or more antennas, a reference oscillator, a baseband clock for antenna / symbol transmission and reception through the antenna, a baseband clock for audio sampling, and a baseband clock for implementing the functions described herein, particularly with respect to Figure 2 and 3 Appropriate processing and memory components for the described signal processing functions. Specifically, with respect to the processor and / or memory of the user devices 120A-N, the disclosure herein with respect to the processor and / or memory of the access point 110 may also be applied to each user device 120A-N.

[0030] The antenna of each wireless audio user device may operate based on an antenna data clock, which determines the rate at which data is sampled from the antenna. A reference oscillator is used by the user device to control various baseband clocks that may include the antenna data clock. The wireless audio user device also includes a baseband clock for audio sampling, which determines the rate at which incoming audio signals entering the microphone are sampled.

[0031] Figure 2 According to some embodiments of the present disclosure Figure 1 A simplified signal processing flow chart 200 of an exemplary wireless audio user device of a system of FIG. Any one or more of the user devices 120A-N may include information about Figure 2 Describes the components and functionality.

[0032] Diagram 200 illustrates an antenna 202, a radio frequency (RF) receiver 204, an analog-to-digital converter (ADC) 206, a Fourier transform block (FFT block) 208, a sampling timing offset estimator 210, a proportional-integral controller 212, a reference oscillator 218, a first phase-locked loop 220 and a second phase-locked loop 222, and an audio sampling clock 224.

[0033] Antenna 202 may be a single antenna, or may include multiple antennas. The multiple antennas may be arranged in an array. RF receiver 204 may be configured to detect OFDMA signals.

[0034] ADC 206 may be configured to receive a signal from an RF receiver. ADC 206 may also be referred to as a sampler because the ADC is configured to sample the input signal at a particular rate. The sampling rate is determined based on the antenna data clock, which is determined based on reference oscillator 218, as described below. FFT block 208 is configured to convert the sampled input signal from ADC 206 into the frequency domain.

[0035] In the case where the wireless audio user includes two or more antennas, each antenna may have a corresponding RF receiver (204), ADC (206), and FFT block (208). The output of the FFT block for each antenna may be fed into the sampling timing offset estimator 210.

[0036] The sampling timing offset estimator 210 is configured to receive the output (ie, frame) of the FFT block 208 and determine an estimate of the timing offset between the access point 110 and the user. Figure 3 This is described in more detail.

[0037] Figure 3A simplified process flow chart 300 of the sampling timing offset estimator 210 is illustrated. The sampling timing offset estimator 210 receives a frame 302 as input from the FFT block 208. It should be understood that the sampling timing offset estimator 210 is configured to receive a plurality of frames corresponding to a plurality of antennas as input and process each frame as disclosed herein. The frame 302 includes a plurality of resource blocks 304A-N distributed across the bandwidth of the frame 302. Each resource block has two pilot subcarriers (e.g., pilot subcarriers 310a and 310b or 310c and 310d) each having a desired frequency. The sampling timing offset estimator 210 first determines the channel estimates for the two pilot subcarriers in the resource block and then calculates the channel phase slope by multiplying the channel estimate at subcarrier k+D 310B by the complex conjugate of the channel estimate at subcarrier k 310A. The sampling timing offset estimator 210 repeats the calculation for resource blocks 304A-N (and their corresponding pilot subcarrier pairs) and accumulates the resulting channel phase slope values ​​in order to filter out noise and the effects of frequency selective fading.

[0038] The sampling timing offset estimator 210 is also configured to estimate the sampling timing offset for multiple antennas in the case where the user includes two or more antennas. The FFT output is used for additional antennas to repeat the same process of calculating the channel estimate of the pilot subcarriers and multiplying the channel estimates within each resource block, and accumulating the resulting channel phase slope values ​​as a sum starting from the first antenna.

[0039] The slope of the channel phase (in the frequency spectrum) is proportional to the timing offset between the access point and the user. By scaling the accumulated channel phase slope, the timing offset in the samples can be estimated for frame 302. The sample timing offset estimator 210 then outputs the timing offset estimate.

[0040] The PI controller 212 receives the timing offset estimate. The PI controller includes a weighted integral of previously determined timing offset estimates 214, and a weighted current timing offset estimate 216. The PI controller may adjust the weights based on a trade-off between fast initial convergence on the offset between the access point 110 and the user, and smooth steady-state operation. Heavier weighting of recent timing offsets provides faster convergence, but makes the system more susceptible to transient noise and perturbations. Using smaller weights for recent timing offsets provides slower convergence but smoother changes during steady-state operation of the PI controller, thus making the system less susceptible to rapid changes and noise. The PI controller 212 outputs a tuning value to be used by a reference oscillator 218.

[0041] The reference oscillator 218 receives a tuning signal output by the PI controller. The tuning signal is used to modify the reference oscillator to reduce the timing offset between the access point 110 and the user.

[0042] The output of reference oscillator 218 is passed to two phase locked loops (PLLs) 220 and 222. First PLL 220 generates a baseband clock frequency signal for audio sampling clock 224. Second PLL 222 generates a baseband clock frequency signal for ADC 206 to control sampling of the antenna.

[0043] Audio sampling clock 224 collects audio data when the user device includes a microphone. Using the same reference oscillator to provide the baseband clock signal for the audio sampling clock and the transmit / receive data through ADC 206 enables the system to reduce latency issues and eliminate the need to drop or add samples, as well as provide other operational benefits.

[0044] Figure 4 A flow chart illustrating an example method 400 according to an embodiment of the present disclosure is illustrated. The method 400 may enable a wireless audio user device to adjust its reference oscillator in order to synchronize its baseband clock with an access point. Figure 4 The flowcharts of represent machine-readable instructions stored in the memory and may include one or more programs that, when executed by the processor, may cause one or more systems or devices to perform one or more functions described herein. Figure 4 The flowchart illustrated in the flowchart describes an exemplary process, but alternatively many other methods for implementing the functions described herein may be used. For example, the order of execution of the blocks may be rearranged or performed sequentially or in parallel with each other, and the blocks may be changed, eliminated, and / or combined to perform the method 400. In addition, since the method 400 is combined Figures 1 to 3 The components in are disclosed, so some functions of those components will not be described in detail below.

[0045] The method 400 begins at block 402. At block 404, the method 400 includes receiving a frame from an access point. Figure 3 As mentioned above, a frame may include a plurality of subcarriers, such as data subcarriers, pilot subcarriers, and the like.

[0046] At block 406, method 400 may include determining channel estimates for pilot subcarriers of the frame. Then, at block 408, the channel estimates are used to determine channel phase slopes for pilot subcarrier pairs in each resource block. At block 410, method 400 includes summing the phase shifts for all pairs of pilot subcarriers and across all antennas used by the user.

[0047] At block 412, method 400 may include determining a timing offset estimate based on a sum of channel phase slope values ​​for each resource block and each antenna. The timing offset estimate is measured in samples.

[0048] At block 414, method 400 may include determining a tuning value for a user reference oscillator based on the timing offset estimate. This may include passing the timing offset estimate through a proportional-integral controller that adds a weight to the integral of the current timing offset and the past timing offset. This enables the user to achieve fast convergence in some situations while also achieving smooth steady-state operation.

[0049] At block 416, method 400 may include modifying a reference oscillator based on the tuning value.The reference oscillator may provide a reference frequency for controlling one or more baseband clocks of the user.

[0050] At block 418, method 400 may include controlling the antenna data clock and the audio sample clock based on the reference oscillator modified by the tuning value. Controlling both the audio sample clock and the antenna data clock based on the same reference oscillator frequency enables the user to reduce latency and avoid having to insert or drop audio samples. Method 400 may then return to block 404 and receive the next frame from the access point. The method may be repeated to form a steady-state feedback loop, thereby ensuring that the audio sample clock and the antenna data clock remain synchronized with the baseband clock of the access point. Method 400 may then end at block 420.

[0051] Any process descriptions or blocks in the figures should be understood to represent modules, segments, or code portions, which include one or more executable instructions for implementing specific logical functions or steps in the process, and alternative implementations are included within the scope of the embodiments of the present invention, where functions may not be performed in the order from that shown or discussed, including substantially simultaneously or in reverse order, depending on the functionality involved, as will be understood by those skilled in the art.

[0052] The present disclosure is intended to explain how to make and use various embodiments according to the present technology without limiting its true, intended and clear scope and spirit. The foregoing description is not intended to be exhaustive or limited to the precise form disclosed. In view of the above teachings, modifications and changes are possible. The embodiments are selected and described to provide the best description of the principles of the described technology and its practical application, and to enable those skilled in the art to utilize the present technology in various embodiments and with various modifications as suitable for the specific purposes covered. When interpreted according to the breadth clearly, legally and fairly granted, as may be amended during the application of this patent application and all equivalents thereof, all these modifications and changes are within the scope of the embodiments as determined by the appended claims.

Claims

1. A method for baseband clock synchronization in a wireless microphone system, the method comprising: receiving a plurality of pilot subcarriers from an audio transmitter; determining a channel estimate for each of the plurality of pilot subcarriers; determining one or more channel phase slope values ​​corresponding to one or more pairs of the plurality of pilot subcarriers based on channel estimates for the one or more pairs of the plurality of pilot subcarriers; summing the one or more channel phase slope values ​​to determine a timing offset estimate; determining a tuning value for a reference oscillator based on the timing offset estimate, wherein determining the tuning value comprises passing the timing offset estimate to a proportional-integral controller that applies a weighting to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate; determining a modified reference signal by modifying the reference oscillator based on the tuning value; and An audio sampling clock and an antenna data clock are controlled based on the modified reference signal.

2. The method according to claim 1, further comprising: A plurality of frames are received, each frame comprising the plurality of pilot subcarriers.

3. The method of claim 2, further comprising determining the tuning value for each frame.

4. The method according to claim 2, further comprising: determining the modified reference signal for each frame; and The audio sampling clock and the antenna data clock of each frame are updated. 5 . The method of claim 1 , wherein determining the tuning value comprises determining the tuning value based on passing the timing offset estimate through a proportional-integral controller.

6. A wireless audio microphone system, comprising: An audio transmitter configured to: transmitting a plurality of pilot subcarriers; and One or more audio receivers, each audio receiver configured to: receiving the plurality of pilot subcarriers; determining a channel estimate for each of the plurality of pilot subcarriers; determining one or more channel phase slope values ​​corresponding to one or more pairs of the plurality of pilot subcarriers based on channel estimates for the one or more pairs of the plurality of pilot subcarriers; determining a timing offset estimate based on a sum of the one or more channel phase slope values; determining a tuning value by passing the timing offset estimate to a proportional-integral controller that applies a weighting to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate; determining a modified reference signal by modifying a reference oscillator based on the tuning value; and An audio sampling clock and an antenna data clock are controlled based on the modified reference signal.

7. The wireless audio microphone system of claim 6, wherein the audio transmitter is further configured to transmit a plurality of frames, each frame comprising the plurality of pilot subcarriers, and wherein the one or more audio receivers are each further configured to receive the plurality of frames.

8. The wireless audio microphone system of claim 7, wherein the one or more audio receivers are each further configured to determine the tuning value for each frame.

9. The wireless audio microphone system of claim 8, wherein each of the one or more audio receivers is further configured to: determining the modified reference signal for each frame; and The audio sampling clock and the antenna data clock of each frame are updated.

10. An audio receiver of a wireless audio system, the audio receiver comprising: an antenna configured to receive a plurality of pilot subcarriers from an audio transmitter; and A circuit system configured to: determining a channel estimate for each of the plurality of pilot subcarriers; determining one or more channel phase slope values ​​corresponding to one or more pairs of the plurality of pilot subcarriers based on channel estimates for the one or more pairs of the plurality of pilot subcarriers; determining a timing offset estimate based on a sum of the one or more channel phase slope values; determining a tuning value by passing the timing offset estimate to a proportional-integral controller that applies a weighting to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate; determining a modified reference signal by modifying a reference oscillator based on the tuning value; and (i) an audio sampling clock and (ii) an antenna data clock are controlled based on the modified reference signal.

11. The audio receiver of claim 10, wherein the antenna is further configured to receive a plurality of frames, each frame comprising the plurality of pilot subcarriers, and wherein the circuitry is further configured to determine the tuning value for each frame.

12. The audio receiver of claim 11, wherein the circuit system is further configured to: determining the modified reference signal for each frame; and The audio sampling clock and the antenna data clock of each frame are updated.

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