OFDMA baseband clock synchronization

Abstract

A method for synchronizing baseband clocks in an OFDMA wireless microphone system is disclosed. 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 further includes determining a modified reference signal by modifying a reference oscillator based on the tuning value. The method further includes controlling (i) an audio sample clock and (ii) an antenna data clock based on the modified reference signal.

Description

[Technical Field]

[0001] cross reference This application claims priority to U.S. Patent Application No. 16 / 425,694, filed May 29, 2019, the entire contents of which are incorporated herein by reference.

[0002] This application relates generally to timing and frequency synchronization of wireless audio systems and devices that use orthogonal frequency division multiple access (OFDMA) for communication, and specifically to audio devices having an audio sample clock and an antenna data clock. [Background technology]

[0003] Orthogonal frequency division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. The subcarriers are transmitted together to form a wideband, high-speed communication link that can be used for many purposes, including digital television and audio broadcasting, DSL Internet access, wireless networks, powerline networks, and mobile communications.

[0004] In an OFDMA audio system, there may be an access point and one or more subscribers. The subscriber devices must compensate for any frequency offset in their respective antenna data clocks relative to the access point so that frame and symbol timing is maintained. This allows the subscriber devices to properly transmit and receive data with the access point.

[0005] Existing subscriber devices may include a sample rate conversion block that prevents audio distortion caused by inserting or dropping samples, but the sample rate conversion block requires additional resources and introduces latency into the audio path.

[0006] Therefore, there is an opportunity 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, thus reducing the required resources and removing a source of latency in the audio path. Summary of the Invention

[0007] Embodiments of the present disclosure are intended to alleviate some of the above problems by providing a method and system for locking the baseband clocks of subscribers in a wireless microphone system to an access point by generating their respective baseband clocks from a common reference. The access point (also referred to as an "audio transmitter") and each subscriber device (also referred to as an "audio receiver") all include one or more baseband clocks set based on a reference oscillator (i.e., control the audio sample clock and / or data sample clock). Embodiments of the present disclosure may include tuning the reference oscillator of each subscriber device based on the measured phase difference so that the baseband clocks of all subscribers and the access point are synchronized.

[0008] An exemplary method includes receiving a plurality of pilot subcarriers from an audio transmitter, and calculating a timing offset estimate based on the pilot subcarriers. (timing offset estimate) The method also includes determining a tuning value by passing the timing offset estimate through a proportional-integral controller. (tuning value) The method further includes determining a reference oscillator based on the tuning value. (reference oscillator) determining a modified reference signal by modifying the reference signal. The method further includes controlling (i) an audio sample clock and (ii) an antenna data clock based on the modified reference signal.

[0009] These and other embodiments and various permutations and aspects will become apparent and more fully understood from the following detailed description and accompanying drawings that set forth illustrative embodiments that illustrate various ways in which the principles of the present invention may be employed. [Brief explanation of the drawings]

[0010]

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[0011] The following description describes, illustrates, and illustrates one or more specific embodiments of the present invention in accordance with its principles. This description is not provided to limit the present invention to the embodiments described herein, but rather to explain and teach the principles of the present invention in a manner that will enable those skilled in the art to understand these principles and, with that understanding, apply them to practice not only the embodiments described herein but also other embodiments that may be conceived in accordance with these principles. The scope of the present invention is intended to cover all such embodiments that may fall within the scope of the appended claims, either literally or under the doctrine of equivalents.

[0012] It should be noted that in the description and drawings, similar or substantially similar elements may be labeled with the same reference numerals. However, these elements may be labeled with different numerals, for example, where labeling with different numerals facilitates a clearer description. Furthermore, the drawings described herein are not necessarily drawn to scale, and in some cases proportions may be exaggerated to more clearly depict particular features. Such labeling and drawing practices do not necessarily imply any underlying substantive intent. As stated above, the specification is intended to be viewed as a whole, interpreted, and understood by those skilled in the art in accordance with the principles of the present invention taught herein.

[0013] As mentioned above, embodiments of the present disclosure relate to wireless audio systems and device communication using OFDMA, as well as methods and systems for baseband synchronization between devices in a wireless audio system. For a subscriber device to properly communicate with an access point, the subscriber 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 examples, this may be done by frequency and phase locking between the access point's and the subscriber's baseband clocks. The baseband clocks may be used by the access point and subscriber to send and receive data (i.e., audio data, control signals, pilot signals, etc.) via their respective antennas, to sample incoming audio, and for various other purposes.

[0014] Frequency and phase locking between baseband clocks of wireless audio devices may be facilitated by having an access point, where one or more subscribers all generate their respective baseband clocks from a common reference oscillator. Each subscriber can tune its own reference oscillator based on phase offset measurements from any one of the baseband clocks (and even from other subscribers) to maintain frequency and phase lock with the access point reference. This results in all baseband clocks of a given subscriber being locked to their respective access point reference, since all baseband clocks are generated from a common reference.

[0015] In an embodiment of the present disclosure, a subscriber can tune its reference frequency (i.e., reference oscillator) based on a sample clock timing (i.e., phase) offset measurement between the subscriber and the access point, obtained for each frame during communication between the access point and the subscriber. The timing offset between the transmit sample clock and the receive sample clock in an OFDMA system results in a channel phase slope when plotted in the frequency domain. This slope is proportional to the timing offset of the sample clocks. Therefore, by determining the slope of the channel phase offset between the transmit signal (from the access point) and the receive signal (at the subscriber), a delay and therefore a timing offset measurement can be determined. This timing offset measurement can then be used to tune the subscriber reference oscillator. The tuned reference oscillator can then be used to control the subscriber's antenna data clock and audio sample clock, thereby synchronizing the subscriber's and the access point's baseband clocks. Also, by determining the timing offset measurement on a frame-by-frame basis, the reference oscillator of the subscriber's audio device can be continuously tuned to maintain "steady-state" synchronization. Furthermore, by controlling both the subscriber's antenna data clock and audio sample clock based on the reference oscillator, both clocks are synchronized with each other and with the corresponding clocks of the access point.

[0016] 1 shows an exemplary simplified block diagram of a wireless audio communication system or environment 100 in which the methods and apparatus of this disclosure may be used. The wireless audio communication system may include an access point 110 and multiple subscriber devices 120A-N.

[0017] The 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 examples, the access point 110 may be an automixer, a laptop or desktop computer, or any other device configured to communicate with various other devices (e.g., multiple wireless audio devices), including the subscriber devices 120A-N.

[0018] The access point 110 may be configured to perform various functions or operations, such as those described in this disclosure (and the accompanying drawings). The access point 110 may include various components including, 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 set 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 can be volatile memory (e.g., RAM including non-volatile RAM, magnetic RAM, ferroelectric RAM, etc.), non-volatile memory (e.g., disk memory, flash memory, EPROM, EEPROM, memristor-based non-volatile solid-state memory, etc.), non-alterable memory (e.g., EPROM), read-only memory, and / or mass storage device (e.g., hard drive, solid-state drive, etc.). In some examples, the memory includes multiple types of memory, specifically volatile memory and non-volatile memory. The memory can be a computer-readable medium in which one or more sets of instructions, such as software for operating the methods of the present disclosure, may be embedded. The instructions may embody one or more of the methods or logic 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 in the processor during execution of the instructions.

[0019] 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 caches and servers that store one or more sets of instructions. Furthermore, the terms "non-transitory computer-readable medium" and "computer-readable medium" include any tangible medium that is capable of storing, encoding, or retaining a set of instructions for execution by a processor or that causes a system to perform any one or more of the methods or operations disclosed herein. The term "computer-readable medium," as used herein, is expressly defined to include any type of computer-readable storage device and / or storage disk, and to exclude propagating signals.

[0020] In some examples, the access point 110 can be a base station, a centralized controller, or other computing device configured to communicate with multiple wireless audio subscriber devices simultaneously. For example, the access point can operate in a conference room, and the subscriber devices can be multiple microphones that communicate with the access point to provide a conferencing environment. Other examples are possible as well.

[0021] The access point 110 may include one or more antennas that enable wireless audio communication with one or more subscriber 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, an antenna sample clock may be controlled based on the reference oscillator to control the timing of various data transmissions and receptions.

[0022] The access point 110 may be configured to transmit data to one or more wireless audio subscriber devices 120A-N in various formats and using various communication protocols. For example, the access point 110 may transmit data using an OFDMA scheme in which data is transmitted in frames. Each frame may include multiple subcarriers, some of which are used to transmit data, some of which are pilot subcarriers used to synchronize the access point with subscribers, and some of which are "guard" subcarriers to protect against interference from adjacent channels or subchannels in the frequency spectrum. In one example, a given channel may be divided into 64 different subcarriers. A channel may include 52 data subcarriers, 4 pilot subcarriers, and 8 guard subcarriers. It should be understood that these numbers are used merely as an example and other numbers may be used.

[0023] Each pilot subcarrier may be transmitted at a specific known frequency and may be configured not to carry audio or control information. The known frequency location of each pilot subcarrier allows a subscriber receiving the frame to determine the phase shift and therefore the timing offset between the access point 110 and the subscriber, as will be described in more detail below.

[0024] The wireless audio subscriber devices 120A-N may be portable wireless audio receivers, microphones, conferencing systems, speakers, and / or any other devices that may be communicatively coupled to the access point 110. Although the embodiments disclosed herein are described with reference to subscriber devices that are each microphones, it should be understood that the concepts and features disclosed herein may also be applied to other types of subscriber devices.

[0025] Each wireless audio subscriber device 120A-N may include one or more antennas, a reference oscillator, a baseband clock for antenna / symbol transmission and reception via the antenna, a baseband clock for audio sampling, and processing and memory components suitable for performing the functions described herein, particularly the signal processing functions described with respect to Figures 2 and 3. In particular, with respect to the processor and / or memory of the subscriber devices 120A-N, the disclosure herein regarding the processor and / or memory of the access point 110 also applies to each subscriber device 120A-N.

[0026] The antenna of each wireless audio subscriber device may operate based on an antenna data clock that determines the rate at which data is sampled from the antenna. A reference oscillator is used by the subscriber device to control various baseband clocks, which may include the antenna data clock. The wireless audio subscriber device also includes a baseband clock for audio sampling that determines the rate at which audio signals arriving at the microphone are sampled.

[0027] 2 is a simplified signal process flow diagram 200 of an exemplary wireless audio subscriber device of the system of FIG. 1, in accordance with some embodiments of the present disclosure. Any one or more of the subscriber devices 120A-N may include the components and functionality described with respect to FIG.

[0028] 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 sample timing offset estimator 210, a proportional-integral controller 212, a reference oscillator 218, first and second phase-locked loops 220 and 222, and an audio sample clock 224.

[0029] The antenna 202 may be a single antenna or may include multiple antennas, which may be arranged in an array. The RF receiver 204 may be configured to detect OFDMA signals.

[0030] The ADC 206 may be configured to receive a signal from an RF receiver. The ADC 206 is sometimes referred to as a sampler because it is configured to sample the input signal at a specific rate. The sampling rate is determined based on an antenna data clock, which is determined based on a reference oscillator 218, as described below. The FFT block 208 is configured to convert the sampled input signal from the ADC 206 into the frequency domain.

[0031] If a wireless audio subscriber includes more than one antenna, each antenna may have a corresponding RF receiver 204, ADC 206, and FFT block 208. The output of each antenna's FFT block may be provided to a sample timing offset estimator 210.

[0032] The sample timing offset estimator 210 is configured to receive the output (i.e., frame) of the FFT block 208 and determine a timing offset estimate between the access point 110 and the subscriber, as will be described in more detail with respect to FIG.

[0033] Figure 3 shows the sample timing offset estimator. (sample timing offset estimator)2 shows a simplified process flow diagram 300 of the sample timing offset estimator 210. The sample timing offset estimator 210 takes a frame 302 as input from the FFT block 208. It should be understood that the sample timing offset estimator 210 is configured to receive multiple frames corresponding to multiple antennas as input and process each frame as disclosed herein. The frame 302 includes multiple 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), each having an expected frequency. The sample timing offset estimator 210 first calculates channel estimates for both pilot subcarriers in the resource block 304A-N. (channel estimate) and then multiplying the channel estimate at subcarrier k+D 310B by the complex conjugate of the channel estimate at subcarrier k 310A to determine the channel phase gradient (channel phase slope) The sample timing offset estimator 210 repeats this calculation across all resource blocks 304A-N (and their corresponding pilot subcarrier pairs) to remove the effects of noise and frequency selective fading, and calculates the resulting channel phase gradient value (channel phase slope values) Accumulate.

[0034] The sample timing offset estimator 210 is also configured to estimate the sample timing offset across multiple antennas if the subscriber includes more than one antenna. The same process of calculating the channel estimates for the pilot subcarriers and multiplying them by the channel estimates in each resource block is repeated using the FFT outputs of the additional antennas, and the resulting channel phase slope values ​​are accumulated with the sum from the first antenna.

[0035] The slope of the channel phase (in the frequency spectrum) is proportional to the timing offset between the access point and the subscriber. By scaling the accumulated channel phase slope, the timing offset in samples for the frame 302 can be estimated. The timing offset estimate is then output by the sample timing offset estimator 210.

[0036] The PI controller 212 receives the timing offset estimate. The PI controller includes a weighted integral of a previously determined timing offset estimate 214 and a weighted current timing offset estimate 216. The PI controller can adjust the weights based on a trade-off between fast initial convergence on the offset between the access point 110 and the subscriber and smooth steady-state operation. A larger weight for the most recent timing offset results in faster convergence, but the system is more susceptible to transient noise and interruptions. A smaller weight for the most recent timing offset results in slower convergence, but smoother changes during the PI controller's steady-state operation, making the system less susceptible to sudden changes and noise. The PI controller 212 outputs a tuning value used by the reference oscillator 218.

[0037] The reference oscillator 218 takes the tuning signal output by the PI controller and uses the tuning signal to correct the reference oscillator to reduce the timing offset between the access point 110 and the subscriber.

[0038] The output of the reference oscillator 218 is passed to two phase-locked loops (PLLs) 220 and 222. The first PLL 220 generates a baseband clock frequency signal for the audio sample clock 224. The second PLL 222 generates a baseband clock frequency signal for the ADC 206 to control the sampling of the antenna.

[0039] The audio sample clock 224 collects audio data if the subscriber device includes a microphone. By using the same reference oscillator to provide the baseband clock signal for the audio sample clock and for sending and receiving data through the ADC 206, the system can mitigate latency issues, eliminate the need to drop and add samples, and provide other operational benefits.

[0040] FIG. 4 illustrates a flow diagram of an exemplary method 400 according to an embodiment of the present disclosure. Method 400 may enable a wireless audio subscriber device to adjust its reference oscillator to synchronize its baseband clock with an access point. The flow diagram of FIG. 4 represents machine-readable instructions stored in a memory, which may include one or more programs that, when executed by a processor, cause one or more systems or devices to perform one or more functions described herein. Although the exemplary program is described with reference to the flow diagram illustrated in FIG. 4, many other ways of performing the functions described herein may alternatively be used. For example, the order of execution of the blocks may be rearranged or performed sequentially or in parallel with one another, and blocks may be modified, eliminated, and / or combined to perform method 400. Furthermore, because method 400 is disclosed with reference to the components of FIGS. 1-3, the functionality of some of these components will not be described in detail below.

[0041] The method 400 begins at block 402. At block 404, the method 400 includes receiving a frame from an access point. As described above with respect to FIG. 3, the frame may include multiple subcarriers, such as data subcarriers, pilot subcarriers, etc.

[0042] At block 406, the 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 gradients for pairs of pilot subcarriers in each resource block. At block 410, the method 400 includes summing the phase shifts for all pairs of pilot subcarriers across all antennas used by the subscriber.

[0043] At block 412, the method 400 may include determining a timing offset estimate based on a sum of channel phase gradient values ​​for each resource block and each antenna, the timing offset estimate measured on a sample-by-sample basis.

[0044] At block 414, the method 400 may include determining a tuning value for a subscriber reference oscillator based on the timing offset estimate. This may include passing the timing offset estimate through a proportional-integral controller, which adds weights to the current timing offset and an integral of the past timing offset. This may allow the subscriber to achieve fast convergence in some scenarios while also allowing smooth steady-state operation.

[0045] At block 416, the method 400 may include modifying a reference oscillator based on the tuning value. The reference oscillator may provide a reference frequency used to control one or more baseband clocks of a subscriber.

[0046] At block 418, the method 400 may include controlling the antenna data clock and the audio sample clock based on the reference oscillator modified by the tuning value. By controlling both the audio sample clock and the antenna data clock based on the same reference oscillator frequency, the subscriber can reduce latency and avoid problems that arise from having to insert or drop audio samples. The method 400 may then return to block 404 to receive the next frame from the access point. The method may repeat to form a steady-state feedback loop, ensuring that the audio sample clock and the antenna data clock remain synchronized with the access point's baseband clock. The method 400 may then end at block 420.

[0047] The process descriptions or blocks in the figures should be understood as representing modules, segments, or portions of code that contain one or more executable instructions for implementing specific logical functions or steps within the process; alternative implementations in which functions may be performed out of order from the illustrated or described order, including substantially simultaneously or in reverse order, depending on the functionality involved, as will be understood by those skilled in the art, are within the scope of embodiments of the invention.

[0048] This disclosure is intended to describe how to make and use various embodiments in accordance with the present technology, rather than limiting the true, intended, and fair scope and spirit of the technology. The foregoing description is not intended to be exhaustive or to be limited to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiments have been chosen and described to provide the best illustration of the principles of the described technology and its practical application, and to enable those skilled in the art to utilize the technology in various embodiments, with various modifications suited to the particular uses contemplated. All such modifications and variations are within the scope of the embodiments, as determined by the appended claims, as may be amended during the pendency of this patent application, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

1. 1. A method for baseband clock synchronization in a wireless microphone system, comprising: receiving a frame from an audio transmitter, the frame including a plurality of resource blocks, each of the plurality of resource blocks including a pair of pilot subcarriers and a plurality of data subcarriers; determining a channel estimate for a pair of pilot subcarriers within each of the plurality of resource blocks; determining a plurality of channel phase gradient values ​​based on the channel estimates for the pairs of pilot subcarriers in each of the plurality of resource blocks; summing the plurality of channel phase gradient values ​​to generate a channel phase gradient; determining a timing offset estimate based on the channel phase gradient; determining a tuning value based on the timing offset estimate; modifying a reference oscillator based on the tuning value to generate a modified reference signal; controlling a clock based on the modified reference signal; A method comprising:

2. receiving a plurality of frames; determining the tuning value for each frame; determining the modified reference signal for each frame based on the tuning value for each frame; 2. The method of claim 1, further comprising: updating the clock each frame.

3. The method of claim 2 , wherein the clocks include one or more of an audio sample clock and an antenna data clock.

4. determining the tuning value comprises: determining the tuning value by passing the timing offset estimate through a proportional-integral controller; The method of claim 1 , comprising:

5. 2. The method of claim 1, wherein the step of determining the tuning value comprises applying a weighting factor to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate.

6. an audio transmitter configured to transmit a frame comprising a plurality of resource blocks, each of the plurality of resource blocks comprising a pair of pilot subcarriers and a plurality of data subcarriers; One or more audio receivers, each comprising: receiving the frame; determining a channel estimate for a pair of pilot subcarriers in each of the plurality of resource blocks of the frame; determining a plurality of channel phase gradient values ​​based on the channel estimates for the pairs of pilot subcarriers in each of the plurality of resource blocks; summing the plurality of channel phase gradient values ​​to generate a channel phase gradient; determining a timing offset estimate based on the channel phase gradient; determining a tuning value by applying a weighting factor to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate; modifying a reference oscillator based on the tuning value to generate a modified reference signal; Controlling a clock based on the modified reference signal one or more audio receivers configured to A wireless audio microphone system comprising:

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

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

9. each of the one or more audio receivers: determining the modified reference signal for each frame based on the tuning value for each frame; Update the clock every frame 9. The wireless audio microphone system of claim 8, further configured to:

10. 7. The wireless audio microphone system of claim 6, wherein the one or more audio receivers are each configured to determine the tuning value by passing the timing offset estimate through a proportional-integral controller.

11. an antenna configured to receive a frame from an audio transmitter, the frame including a plurality of resource blocks, each of the plurality of resource blocks including a pair of pilot subcarriers and a plurality of data subcarriers; a reference oscillator configured to generate a reference signal; determining a channel estimate for a pair of pilot subcarriers within each of the plurality of resource blocks; determining a plurality of channel phase gradient values ​​based on the channel estimates for the pairs of pilot subcarriers in each of the plurality of resource blocks; summing the plurality of channel phase gradient values ​​to generate a channel phase gradient; determining a timing offset estimate based on the channel phase gradient; determining a tuning value by applying a weighting factor to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate; modifying a reference oscillator based on the tuning value to generate a modified reference signal; Controlling a clock based on the modified reference signal The circuit is configured as follows: An audio receiver for a wireless audio system comprising:

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

13. The circuit determining the modified reference signal for each frame based on the tuning value for each frame; Update the clock every frame 13. The audio receiver of claim 12, further configured to:

14. 12. The audio receiver of claim 11, wherein the circuitry is configured to determine the tuning value by passing the timing offset estimate through a proportional-integral controller.

15. 12. The audio receiver of claim 11, wherein the clocks include one or more of an audio sample clock and an antenna data clock.

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