Ofdma baseband clock synchronization

Abstract

To provide a method for synchronizing baseband clocks in an OFDMA wireless microphone system.SOLUTION: An example method includes a step of receiving a plurality of pilot subcarriers from an audio transmitter. The method also includes a step of determining a timing offset estimate based on the pilot subcarriers. The method further includes a step of determining a tuning value by passing the timing offset estimate through a proportional-integral controller. The method further includes a step of determining a modified reference signal by modifying a reference oscillator based on the tuning value. The method further includes a step of controlling (i) an audio sample clock and (ii) an antenna data clock based on the modified reference signal.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] Cross-reference This application claims priority to U.S. Patent Application No. 16 / 425,694, filed on May 29, 2019, the entire content of which is incorporated herein by reference.

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

Background Art

[0003] Orthogonal frequency division multiplexing (OFDM) is a method of encoding digital data at multiple carrier frequencies. Sub-carriers are transmitted together to form a broadband high-speed communication link. This communication link can be used for many purposes including digital television and audio broadcasting, DSL Internet access, wireless networks, power line networks, and mobile communications.

[0004] In an OFDMA audio system, there can be an access point and one or more subscribers. The subscriber device must correct any frequency offset in each antenna data clock relative to the access point so that frame timing and symbol timing are maintained. This allows the subscriber device to properly transmit and receive data at the access point.

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

[0006] Thus, there is an opportunity for a method and system for OFDMA baseband clock synchronization that does not require an additional sample rate conversion block, and thus reduces the resources required and removes the cause of latency in the audio path, particularly in the context of high-quality audio applications. SUMMARY OF THE INVENTION

[0007] Embodiments of the present disclosure are intended to mitigate some of the above problems by providing a method and system for locking the baseband clock of a subscriber of a wireless microphone system to an access point by generating each baseband clock 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., controlling an audio sample clock and / or a data sample clock). Embodiments of the present disclosure may include synchronizing the reference oscillator of each subscriber device based on the measured phase difference such 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. 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 corrected 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 corrected reference signal.

[0009] From the following detailed description and the accompanying drawings, which illustrate exemplary embodiments showing various ways in which the principles of the present invention can be employed, these and other embodiments, as well as various substitutions and aspects, will become apparent and will be more fully understood.

Brief Description of the Drawings

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Best Mode for Carrying Out the Invention

[0011] The following description describes, illustrates, and exemplifies 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 such a way that those skilled in the art can understand these principles and, in light of that understanding, apply the principles to practice not only the embodiments described herein but also other embodiments that can be conceived in accordance with these principles. The scope of the present invention is intended to cover literally or under the doctrine of equivalents all such embodiments that may be included in the appended claims.

[0012] Note 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, when it facilitates a clearer description, such as by using different numbers. Further, the drawings described herein are not necessarily drawn to scale, and in some cases, the ratios may be exaggerated to more clearly depict certain features. Such labeling and drawing practices do not necessarily imply a fundamental substantive purpose. As described above, this specification is intended to be taken as a whole, interpreted, and understood by those skilled in the art in accordance with the principles of the invention taught herein.

[0013] As described above, embodiments of the present disclosure relate to wireless audio systems and device communication using OFDMA, and methods and systems for baseband synchronization between devices in a wireless audio system. For a subscriber device to communicate properly with an access point, the subscriber device must be able to compensate for any frequency offset relative to the access point so that the timing of frames and symbols is maintained. In some examples, this can be done by locking the frequency and phase between the access point and the subscriber's baseband clock. The baseband clock can be used by the access point and the subscriber to transmit and receive data (i.e., audio data, control signals, pilot signals, etc.) via their respective antennas, to sample the input audio, and for various other purposes.

[0014] Locking of the frequency and phase between the baseband clocks of wireless audio devices may be facilitated by having an access point, and one or more subscribers each 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 further from other subscribers) to maintain lock of frequency and phase to the access point reference. This results in all baseband clocks being generated from a common reference, such that all baseband clocks of a given subscriber are locked to their respective access point references.

[0015] In embodiments of the present disclosure, a subscriber can synchronize its reference frequency (i.e., reference oscillator) based on sample clock timing (i.e., phase) offset measurements between the subscriber and an access point that are 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 gradient when plotted in the frequency domain. This gradient is proportional to the timing offset of the sample clock. Thus, by determining the gradient of the channel phase offset between the transmit signal (from the access point) and the receive signal (at the subscriber), the delay, and thus the timing offset measurement, can be determined. This timing offset measurement can then be used to synchronize the subscriber reference oscillator. The synchronized reference oscillator can then be used to control the subscriber's antenna data clock and audio sample clock, thereby synchronizing the subscriber's baseband clock with that of the access point. Also, by determining the timing offset measurement for each frame, the reference oscillator of the subscriber audio device can be continuously synchronized to maintain "steady state" synchronization. Further, by controlling both the antenna data clock and the audio sample clock of the subscriber audio device based on the reference oscillator, both clocks are synchronized with each other and with the corresponding clocks of the access point.

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

[0017] The access point 110 can be any suitable computing device and may include a processor, memory, antenna, and / or one or more other signal processing components or computing components. In some examples, the access point 110 can be an automixer, laptop or desktop computer, or any other device configured to communicate with various other devices (e.g., multiple wireless audio devices) including subscriber devices 120A - N.

[0018] Access point 110 can be configured to perform various functions or operations, such as those described in this disclosure (and the accompanying drawings). Access point 110 can include various components, for example, a processor and a memory. Access point 110 can also include a display, a user interface, and / or one or more other electronic components. The processor can include a general-purpose processor (e.g., a microprocessor) and / or a dedicated processor (e.g., a digital signal processor (DSP)). The processor can be any suitable processing device or set of processing devices, such as a microprocessor, a microcontroller-based platform, an integrated circuit, one or more field programmable gate arrays (FPGA), and / or one or more application-specific integrated circuits (ASIC), but is not limited thereto. The memory can be a volatile memory (e.g., a RAM including non-volatile RAM, magnetic RAM, ferroelectric RAM, etc.), a non-volatile memory (e.g., a disk memory, a flash memory, an EPROM, an EEPROM, a memory-stable non-volatile solid-state memory, etc.), a non-modifiable memory (e.g., an EPROM), a read-only memory, and / or a mass storage device (e.g., a hard drive, a 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 into which one or more instruction sets, such as software for operating the method of this disclosure, can be embedded. The instructions can embody one or more of the methods or logics described herein. For example, during the execution of the instructions, the instructions are fully or at least partially present in any one or more of the memory, the computer-readable medium, and / or the processor.

[0019] The terms "non-transitory computer-readable medium" and "computer-readable medium" include a single medium or plurality of media such as a centralized or distributed database, and / or related caches and servers that store one or more sets of instructions. Further, the terms "non-transitory computer-readable medium" and "computer-readable medium" include any tangible medium that can store, encode, or hold a set of instructions for execution by a processor, or cause 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, access point 110 can be a base station, a centralized controller, or other computing device configured to communicate with a plurality of wireless audio subscriber devices simultaneously. For example, the access point can operate in a conference room, and the subscriber devices can be a plurality of microphones that communicate with the access point to provide a conference environment. Other examples are possible as well.

[0021] Access point 110 can include one or more antennas and a reference oscillator that enable wireless audio communication with one or more subscriber devices 120A-N. The reference oscillator can be used to control one or more baseband clocks of the access point. For example, an antenna sample clock can be controlled based on the reference oscillator to control the timing of transmission and reception of various data.

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

[0023] Each pilot sub - carrier can be transmitted at a specific known frequency and can be configured not to transmit audio or control information. Based on the known frequency positions of each pilot sub - carrier, a subscriber receiving the frame can determine the phase offset and thus the timing offset between the access point 110 and the subscriber. This will be described in more detail below.

[0024] The wireless audio subscriber devices 120A - N can be portable wireless audio receivers, microphones, conference systems, speakers, and / or any other device that can be communicatively coupled to the access point 110. For the embodiments disclosed herein, subscriber devices that are each a microphone are referred to for description, but it should be understood that the concepts and features disclosed herein can 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 components and memory components suitable for performing the functions described herein, specifically the signal processing functions described with respect to FIGS. 2 and 3. In particular, with respect to the processors and / or memories of the subscriber devices 120A - N, the disclosure herein regarding the processors and / or memories of the access point 110 is also applicable 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. The reference oscillator is used by the subscriber device to control various baseband clocks that 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 an audio signal received by the microphone is sampled.

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

[0028] FIG. 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] Antenna 202 can be a single antenna or can include multiple antennas. The multiple antennas can be arranged in an array. RF receiver 204 can be configured to detect an OFDMA signal.

[0030] ADC 206 can be configured to receive a signal from the RF receiver. Since ADC 206 is configured to sample the input signal at a specific rate, it may also be called a sampler. The sampling rate is determined based on an antenna data clock, and the antenna data clock 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.

[0031] If the wireless audio subscriber includes two or more antennas, each antenna can have a corresponding RF receiver (204), ADC (206), and FFT block (208). The output of the FFT block of each antenna can be supplied to sample timing offset estimator 210.

[0032] Sample timing offset estimator 210 is configured to receive the output of FFT block 208 (i.e., a frame) and determine an estimated value of the timing offset between access point 110 and the subscriber. This will be described in more detail with respect to FIG. 3.

[0033] FIG. 3 shows a simplified process flow diagram 300 of a sample timing offset estimator 210. The sample timing offset estimator 210 takes in a frame 302 as an input from the FFT block 208. It should be understood that the sample timing offset estimator 210 is configured to receive, as inputs, multiple frames corresponding to multiple antennas and process each frame as disclosed herein. The frame 302 includes a plurality of resource blocks 304A - N that are distributed across the entire bandwidth of the frame 302. Each resource block has two pilot subcarriers (e.g., pilot subcarriers 310a and 310b), and each pilot subcarrier has an expected frequency. The sample timing offset estimator 210 first determines the channel estimates for both pilot subcarriers within a 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 sample timing offset estimator 210 repeats this calculation across all resource blocks 304A - N (and their corresponding pairs of pilot subcarriers) to remove the effects of noise and frequency - selective fading and accumulates the resulting channel phase slope values.

[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 of the pilot subcarriers and multiplying the channel estimates within each resource block is repeated using the FFT outputs of the additional antennas, and the resulting channel phase slope values are accumulated in sum from the first antenna.

[0035] (In the frequency spectrum) The gradient of the channel phase is proportional to the timing offset between the access point and the subscriber. By scaling the accumulated channel phase gradient, the timing offset in the samples for 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 the trade-off between fast initial convergence at the offset between the access point 110 and the subscriber and smooth steady-state operation. Assigning a large weight to the latest timing offset speeds up convergence, but the system is more susceptible to the effects of temporary noise and interruptions. Using a small weight for the latest timing offset slows down convergence, but the changes during the steady-state operation of the PI controller become smoother, and thus the system is less susceptible to sudden changes and noise. The PI controller 212 outputs the tuning value used by the reference oscillator 218.

[0037] The reference oscillator 218 captures the tuning signal output by the PI controller. The reference oscillator is modified using the tuning signal 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] When the subscriber device includes a microphone, the audio sample clock 224 collects audio data. By using the same reference oscillator to provide a baseband clock signal to the audio sample clock and by transmitting and receiving data via the ADC 206, the system can reduce latency issues, eliminate the need for sample drops and additions, and provide other operational advantages.

[0040] Figure 4 shows a flowchart of an exemplary method 400 according to an embodiment of the present disclosure. With method 400, a wireless audio subscriber device may be enabled to adjust its reference oscillator to synchronize its baseband clock with an access point. The flowchart of Figure 4 represents machine-readable instructions stored in 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 of the functions described herein. Although an exemplary program is described with reference to the flowchart shown in Figure 4, many other methods for performing the functions described herein may alternatively be used. For example, the order of execution of the blocks may be rearranged or they may be executed sequentially or in parallel, and the blocks may be modified, deleted, and / or combined to perform method 400. Further, since method 400 is disclosed in relation to the components of Figures 1-3, some of the functions of these components are not described in detail below.

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

[0042] In block 406, method 400 may include determining channel estimation values for the pilot subcarriers of a frame. Next, in block 408, the channel estimation values are used to determine the channel phase gradients of pairs of pilot subcarriers within each resource block. In block 410, method 400 includes summing the phase offsets for all pairs of pilot subcarriers across all antennas used by the subscriber.

[0043] In block 412, method 400 may include determining a timing offset estimation value based on the sum of the channel phase gradient values for each resource block and each antenna. This timing offset estimation value is measured in samples.

[0044] In block 414, method 400 may include determining a tuning value for the subscriber reference oscillator based on the timing offset estimation value. This can include passing the timing offset estimation value through a proportional-integral controller that adds weights to the current timing offset and the integral of past timing offsets. This enables the subscriber to achieve fast convergence in some scenarios and at the same time enables smooth steady-state operation.

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

[0046] In block 418, method 400 may include controlling the antenna data clock and the audio sample clock based on a reference oscillator corrected by a 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 resulting from the need to insert or drop audio samples. Method 400 then returns to block 404 and can receive the next frame from the access point. The method is repeated to form a steady-state feedback loop and ensure 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.

[0047] The description or blocks of the process in the figure should be understood as representing one or more executable instructions included in a module, segment, or part of code for implementing a particular logical function or step within the process, and alternative implementations in which functions may be executed out of order, substantially simultaneously, or in the reverse order, depending on the relevant functions, as understood by those skilled in the art, are included within the scope of the embodiments of the present invention.

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

Claims

1. A method for baseband clock synchronization in a wireless microphone system, comprising: Receiving a plurality of pilot subcarriers from an audio transmitter; Determining a timing offset estimate based on the pilot subcarriers; Determining a synchronization value by passing the timing offset estimate through a proportional-integral controller; Determining a corrected reference signal by correcting a reference oscillator based on the synchronization value; Controlling (i) an audio sample clock and (ii) an antenna data clock based on the corrected reference signal. A method comprising the above steps.

2. The method according to claim 1, further comprising receiving a plurality of frames, each frame including the plurality of pilot subcarriers.

3. The method according to claim 2, further comprising determining a synchronization value for each frame.

4. The method according to claim 3, further comprising: Determining a corrected reference signal for each frame; and Updating the audio sample clock and the antenna data clock for each frame.

5. The step of determining the timing offset estimate includes: Determining a channel phase gradient based on the plurality of pilot subcarriers; and Determining the timing offset estimate based on the channel phase gradient. The method according to claim 1, including the above steps.

6. The step of determining the channel phase gradient includes: Determining a channel estimate for each of the plurality of pilot subcarriers; Determining one or more channel phase gradient values corresponding to one or more pairs of the plurality of pilot subcarriers; and Summing the one or more channel phase gradient values. The method according to claim 5, including the above steps.

7. The step of determining the synchronization value includes applying a weighted coefficient to an integral of (i) the timing offset estimate and (ii) a previously determined timing offset estimate.

8. An audio transmitter configured to transmit a plurality of pilot subcarriers; and One or more audio receivers, each of which: Receives the plurality of pilot subcarriers; Determine a timing offset estimation value based on the plurality of pilot sub-carriers, Determine a tuning value by passing the timing offset estimation value through a proportional-integral controller, Determine a corrected reference signal by correcting a reference oscillator based on the tuning value, Control (i) an audio sample clock and (ii) an antenna data clock based on the corrected reference signal One or more audio receivers configured to A wireless audio microphone system comprising. **Claim 9** The wireless audio system according to claim 8, wherein the audio transmitter is further configured to transmit a plurality of frames, each frame including a respective plurality of pilot sub-carriers, and the one or more audio receivers are further configured to receive the plurality of frames respectively. **Claim 10** The wireless audio system according to claim 9, wherein the one or more audio receivers are further configured to determine the tuning value for each frame respectively. **Claim 11** The one or more audio receivers each Determine the corrected reference signal for each frame, Update the audio sample clock and the antenna data clock for each frame The wireless audio system according to claim 10, further configured as such. **Claim 12** The one or more audio receivers each Determine a channel phase gradient based on the plurality of pilot sub-carriers, and Determine the timing offset estimation value based on the channel phase gradient The wireless audio system according to claim 8, further configured to determine the timing offset estimation value in this way. **Claim 13** The one or more audio receivers each Determine a respective channel estimation value for each of the plurality of pilot sub-carriers, Determine one or more channel phase gradient values corresponding to one or more pairs of the plurality of pilot sub-carriers, and Sum the one or more channel phase gradient values The wireless audio system according to claim 12, further configured to determine the channel phase gradient in this way. **Claim 14** The wireless audio system according to claim 8, wherein each of the one or more audio receivers is further configured to determine the tuning value by applying a weighted coefficient to the integration of (i) the timing offset estimate value and (ii) a previously determined timing offset estimate value.

15. An antenna configured to receive a plurality of pilot subcarriers from an audio transmitter, determine a timing offset estimate value based on the plurality of pilot subcarriers, determine a tuning value based on the timing offset estimate value, determine a corrected reference signal based on the tuning value, and control (i) an audio sample clock and (ii) an antenna data clock based on the corrected reference signal A circuit configured as An audio receiver of a wireless audio system comprising

16. The audio receiver according to claim 15, wherein the antenna is further configured to receive a plurality of frames, each frame includes the plurality of pilot subcarriers, and the circuit is further configured to determine the tuning value for each frame.

17. The circuit is determine the corrected reference signal for each frame, update the audio sample clock and the antenna data clock for each frame The audio receiver according to claim 16, further configured as

18. The circuit is determine a channel phase gradient based on the plurality of pilot subcarriers, and determine the timing offset estimate value based on the channel phase gradient The audio receiver according to claim 15, further configured to determine the timing offset estimate value by

19. The circuit is determine a channel estimate value for each of the plurality of pilot subcarriers, determine one or more channel phase gradient values corresponding to one or more pairs of the plurality of pilot subcarriers, and sum the one or more channel phase gradient values The audio receiver according to claim 18, further configured to determine the channel phase gradient by

20. The audio receiver according to claim 15, wherein the circuit is further configured to determine the tuning value by applying a weighted coefficient to (i) the timing offset estimated value and (ii) an integration of a previously determined timing offset estimated value.

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