Method and arrangement for determining a clock offset between at least two radio units

Abstract

A method for determining a clock offset between local clocks of at least a pair of radio units, comprising: performing bidirectional transmission between the at least a pair of radio units using a signal including a selected frequency; determining phase information for signals received at the radio units; determining a phase difference based on the phase information; determining at least one clock offset variable; and determining an estimated maximum error of the determined clock offset variable based on a maximum error of the phase difference to determine whether the determined clock offset variable enables unambiguous determination of a clock offset value.

Description

[Technical Field]

[0001] The present invention relates generally to wireless communications and localization. More particularly, the present invention relates to determining a clock offset between the local clocks of at least a first wireless unit and a second wireless unit by utilizing at least a measurement of the phase of a received signal relative to a local oscillator of the wireless unit.

[0002] Accurate time and phase synchronization of terrestrial wireless nodes is important for many applications, such as positioning and advanced wireless communication applications.

[0003] Information about the phase and time differences between the local oscillators of multiple wireless nodes, such as radio units, can be used directly in positioning algorithms or to maintain a fixed time / phase relationship in the system (see RTK GNSS).

[0004] Information about the phase and time differences between the local oscillators of each of multiple wireless nodes can also be used in cooperative multi-point communication.

[0005] Radio systems often use the system's backhaul to synchronize the local oscillators of the radio units. In such systems, a fixed, separate reference is always required for synchronization. However, the backhaul is typically based on fiber optic technology, and the time synchronization accuracy is limited to about one nanosecond.

[0006] For example, in cellular communication systems, the required accuracy is at the picosecond level. Time synchronization accuracy on the nanosecond time scale is not suitable for phase-coherent transmission, which requires accuracy at the picosecond level. Furthermore, many prior art methods require line-of-sight (LoS) between wireless units to function properly. Factors such as weather conditions can also affect the accuracy of systems that utilize known methods for determining the phase difference between the local oscillators of wireless units. Summary of the Invention

[0007] It is an object of the present invention to alleviate at least some of the problems in the prior art. According to a first aspect of the present invention, there is provided a method for determining a clock offset between local clocks of at least one pair of radio units including at least a first radio unit and a second radio unit, the method comprising: a) performing a first bidirectional transmission between at least one pair of radio units using a first signal including a selected first frequency, the transmission being sent as a broadcast and received by at least one non-transmitting radio unit to obtain the at least one pair of radio units; b) determining first phase information for the first signal received at the wireless unit; c) determining, for each pair of wireless units, a first phase difference as a difference between the first phase information determined for each of the wireless units in the pair; d) performing second or subsequent bidirectional transmissions between the at least one pair of radio units using second or subsequent signals including the selected second or subsequent frequencies; e) determining second and subsequent phase information for the second and subsequent signals received at the wireless unit; f) determining, for each pair of wireless units, a second or subsequent phase difference as a difference between the second or subsequent phase information determined for each of the wireless units in the pair; g) determining the difference between the first phase difference and the second or subsequent phase difference, or the difference between the phase difference determined at the highest or lowest signal frequency and a subsequent phase difference; h) for each of the radio unit pairs, determining at least one clock offset variable indicative of a clock offset estimate between the radio units in the radio unit pair based on the difference determined in step g; i) determining an estimated maximum error of the determined clock offset variable based at least on the maximum error of the first phase difference and the maximum errors of the second and subsequent phase differences; j) determining whether a maximum error of the clock offset variable allows a clock offset to be determined unambiguously by determining a set of candidate clock offset values ​​obtained by varying the clock offset corresponding to an integer number of half-cycle period variations at the first frequency or the subsequent frequency, the set of clock offset values ​​being limited by the estimated maximum error of the determined clock offset variable; k) if it is determined that the clock offset cannot be uniquely determined, repeating step dj using a selected next frequency that differs from the first frequency by an amount greater than the difference between the first frequency and the second frequency or a previously used frequency; Includes:

[0008] The invention also relates to a computer program product according to claim 21 and to an arrangement according to claim 20.

[0009] The present invention describes a system or configuration that can continuously measure mutual time synchronization and time drift (advance or delay) with sub-picosecond level accuracy without relying on backhaul technology.

[0010] According to the present invention, the clock offset and phase difference of the local oscillators of the wireless units between two or more wireless nodes can be determined without backhaul.

[0011] Because the error in the determined clock offset is small, the clock offset determination may remain essentially unaffected by slow movement of the node, such as the swaying of a light pole. In some embodiments, movement of one or more wireless units may be taken into account and compensated for through models and / or measurements to normalize or equalize the frame of reference for the clock offset determination measurements.

[0012] Determining clock offsets according to embodiments of the present invention in which multiple radio units are used may not be affected by whether there is line of sight between nodes.

[0013] In some embodiments, the above clock offset determination may also be applicable when there is no radio link between all the nodes / radio units (eg, when the furthest nodes are too far apart from each other).

[0014] Thus, the present invention may provide a method for determining clock offsets between two or more transceivers (radio units) without requiring additional transmitters or receivers in determining the clock offsets between those transceivers (radio units).

[0015] The invention can be used in terrestrial or space based systems, and can also be used indoors, such as in indoor communication or positioning systems.

[0016] Knowledge of accurate clock offsets enables coherent processing of radio signals across multiple distributed radio units, as required for phase-based positioning techniques and, for example, coordinated multipoint communication systems. Cooperative multipoint (CoMP) communication refers to a wireless communication system in which multiple communication nodes transmit (or receive) to (from) a mobile node with phase coherence. This configuration can improve the capacity, communication range, and reliability of the wireless communication system. This is also known as multipoint MIMO. CoMP may be integrated with the present invention, and the same radio components and antennas used for the clock offset determination presented herein may also be used for the communication service. This communication service may utilize frequencies slightly different from or adjacent to the frequencies used for clock offset determination, thereby avoiding interference. However, these frequencies may be sufficiently close to each other that the cable phase length and clock offset information obtained via the present invention is accurate enough for coherent CoMP transmission and reception.

[0017] The present invention may enable the determination or estimation of clock offsets between radio units where the instantaneous bandwidth of the frequency used for the transmitted signal is narrow (e.g., bandwidths as low as 40 MHz or 10 kHz). The present invention provides a method and arrangement that can be implemented inexpensively, allowing the use of inexpensive narrowband receivers.

[0018] Due to the narrow operating bandwidth of the present invention, the system may operate in frequency bands / ranges that allow for high transmit power. This allows for better range and accuracy than, for example, UWB-based time synchronization systems, which must operate at much lower transmit power. Bands that can be used with the present invention may be, for example, 5 GHz RLAN (which allows for transmit powers of 100 mW or 1 W) or the WIA band (which allows for transmit powers of 400 mW). Therefore, the power used to transmit one or more signals (e.g., primary and / or auxiliary signals) may be tens of mW or more, such as 20 mW or more, 50 mW or more, or 80 mW or more.

[0019] The present invention may also facilitate adapting the narrowband utilized between, for example, Wi-Fi network channels.

[0020] In some embodiments, the signals are transmitted by at least some of the radio units sequentially in a predetermined order, with each sequentially transmitting radio unit transmitting its respective signal in its own predetermined time slot.

[0021] In some embodiments, the set of candidate clock offset values ​​may be based on a variation in clock offset corresponding to an integer number of half-period variations in the highest frequency used.

[0022] The clock offset may be determined based on at least one of the determined phase differences, and in some cases may be determined based on multiple or all of the determined phase differences.

[0023] In some embodiments, the second (or any subsequent) frequency range may be selected based on an estimated maximum error of the determined clock offset variable, with the difference between the first and second frequency ranges being selected to ensure that unaccounted phase rotation is avoided. In some embodiments, the possible range of the clock offset variable is determined based on the maximum error of that clock offset variable, and the second frequency range is determined such that the expected minimum and maximum values ​​of the first auxiliary phase difference corresponding to the minimum and maximum values ​​of the first clock offset variable do not differ by more than a threshold value, such as 2π. Using a threshold value of 2π or smaller prevents phase ambiguity when the first auxiliary phase difference (or any subsequent phase difference) is used to further restrict the set of candidate clock offset values.

[0024] One of the radio units, for example the first radio unit, may be selected as the reference unit, in which case the phase of the local oscillator of the reference unit may be set as zero.

[0025] In some embodiments, the method may include unambiguously determining a clock offset at least once in an integer ambiguity mode, and thereafter repeatedly transmitting subsequent signals in a tracking mode at selected time intervals and optionally in selected frequency ranges to determine subsequent phase differences and repeatedly determining clock offset information indicative of changes in clock offset between the first and second wireless units over the selected time intervals.

[0026] Thus, embodiments of the present invention may provide structures and methods for continuous clock offset tracking or location information provision, where integer ambiguities (IA) may be resolved / determined, for example, once or at predetermined intervals, while otherwise operating in a tracking mode and utilizing signals consisting of only one narrow frequency band, e.g., only the first frequency band and primary signal, as described herein. Clock offsets between wireless units may be tracked without redetermining integer ambiguities.

[0027] In continuous clock offset tracking or tracking mode, subsequent signals may be transmitted at predetermined sufficiently short intervals that it can be assumed that the integer ambiguity problem will not recur, i.e., that the uncertainty in the clock offset between radio units during the time interval between transmissions of subsequent signals will increase by less than the amount that would result in an unaccounted cycle slip.

[0028] In some embodiments, an estimator may be used to track / estimate the clock offset. The clock offset may be tracked using, for example, a simple interpolator, a Kalman filter, an extended Kalman filter, or a particle filter. Using such an estimator may allow the phase difference (e.g., the determined primary phase difference) to be measured at a lower repetition rate (i.e., using fewer and / or less frequent primary signals, etc.) without the risk of uncounted 2π phase slips in the phase difference.

[0029] This configuration can be used for clock offset tracking, and in most cases the transmitted signal only needs to be in one narrow frequency band (e.g., the first frequency band).

[0030] An embodiment of the method may include obtaining or determining an interim clock offset variable as a first approximation of the clock offset, preferably before performing the first bidirectional transmission, to determine the maximum possible value of the clock offset.

[0031] An embodiment of the method may include resolving integer ambiguities by at least performing bidirectional transmissions in at least two frequency ranges to determine a set of clock offset values, and determining a clock offset, wherein determining the clock offset includes: transmitting a primary signal having a frequency in a first frequency range, and selecting at least one of the set of one or more candidate clock offset values; performing bidirectional communication using at least a first primary frequency and a second primary frequency; determining at least first and second primary phase information; determining at least a first primary phase difference and a second primary phase difference; determining a first clock offset variable and an estimated maximum error thereof, optionally based on the first primary phase difference and the second primary phase difference and a maximum error thereof; determining a set of candidate clock offset values ​​based on the first clock offset variable and its estimated maximum error; and determining through; transmitting one or more auxiliary signals including at least one frequency in a second frequency range, and adjusting the clock offset to: performing two-way communication using at least a first auxiliary frequency; determining at least first auxiliary phase information; determining at least a first auxiliary phase difference; determining a second clock offset variable and its estimated maximum error based on the first primary phase difference and the first auxiliary phase difference and their maximum errors; determining the clock offset based on a most likely clock offset value selected from the set of candidate clock offset values, the most likely clock offset value being selected to fit an error margin in the second clock offset variable; and determining through; Including, The method further includes determining whether the selected most likely clock offset value can be uniquely selected from the set of candidate clock offset values, and if not, transmitting one or more second or subsequent auxiliary signals including frequencies in third or subsequent frequency ranges.

[0032] In some embodiments, the method may include transmitting a plurality of primary signals. The method may further include transmitting a plurality of auxiliary signals.

[0033] When multiple primary or auxiliary signals are transmitted, the difference in frequency separating at least consecutive primary signals and / or consecutive auxiliary signals is preferably less than 20 MHz, more preferably less than 10 MHz, e.g., less than 5 MHz, in some embodiments.

[0034] Advantageously, the difference between the first frequency range and the second and subsequent frequency ranges is at least 150 MHz, preferably at least 200 MHz, and most preferably at least 500 MHz. These frequency ranges may be entirely different radio bands. For example, the higher frequency band may be the 5 GHz RLAN band or the new 6 GHz unlicensed band, and the lower frequency band may be the 2.4 GHz ISM band. That is, a frequency difference of more than 3 GHz is possible. Thus, the frequency difference separating the first and second frequency ranges may be, for example, 500 MHz to 5 GHz.

[0035] The first frequency range and / or the second frequency range may encompass a maximum bandwidth of 100Hz-100kHz, preferably 10-100kHz, if only one signal is transmitted in the range, or 5-100MHz, preferably 50MHz, if multiple signals are transmitted in said range.

[0036] In embodiments where only one first primary signal is used, for example, the bandwidth of the first frequency range may be considered to consist essentially of only one frequency, allowing for an inexpensive design and even coin cell battery operation, and the same applies to the second frequency range when only one first auxiliary signal is used.

[0037] For simultaneous transmission, two or more signals to be transmitted by one radio unit are transmitted at the same time, but different radio units may each transmit in their own time slots.

[0038] In some embodiments of the present invention, the first and / or second frequency ranges may have set bandwidths, but one or more signals within these frequency ranges may be present at specific frequencies within the frequency ranges, rather than necessarily spanning the entire bandwidth.

[0039] All of the primary and / or auxiliary signals to be transmitted by the radio units may be transmitted simultaneously, although in some embodiments of the invention, all signals may be transmitted sequentially by at least one or all of the radio units, allowing the system to utilize simpler and / or cheaper radio units that can transmit on only one frequency at a given time, e.g., that can be powered by a coin battery.

[0040] It is also possible to use any number of auxiliary signals in various different frequency ranges (e.g., a third frequency range, a fourth frequency range, etc.) For simplicity, the following detailed description will focus on using only one frequency range (the second frequency range) for the auxiliary signals.

[0041] In some embodiments of the present invention, at least a second primary signal may be transmitted to determine respective phase information (second primary phase information) and a second primary phase difference. A clock offset variable may be determined by comparing at least the first primary phase difference and the second primary phase difference. In some embodiments, the clock offset variable may be determined based on the difference between the first primary phase difference and the second primary phase difference. The first and subsequent primary signals may be transmitted to determine respective phase information to obtain multiple phase differences, while the difference between the phase differences (e.g., the difference between the first phase difference and each subsequent phase difference) may be used to determine a clock offset variable indicating an approximate clock offset between the first wireless unit and the second wireless unit.

[0042] In some embodiments, the maximum error of the clock offset variable may be determined based on other information, for example, obtained as a previously determined parameter.

[0043] In some embodiments, the set of candidate clock offset values ​​may be determined based on at least a first primary phase difference or phase differences measured at one of the frequencies used and a clock offset variation corresponding to an integer number of half periods at at least one of the frequencies used.

[0044] However, if the maximum error of the clock offset variable is known, the candidate clock offset values ​​may be restricted to those that are within the maximum error value of the clock offset variable. This can then be used to determine a set of candidate clock offset values, thereby providing candidate clock offsets between radio units in terms of clock offsets that differ from each other by half cycle period times integer ambiguity (IA).

[0045] The second clock offset variable may correspond to a second approximate clock offset measurement between the first radio unit and the second radio unit. Determining the second clock offset variable is based on comparing at least the first primary phase difference and the first auxiliary phase difference, and in some embodiments, based on the difference between the first primary phase difference and the first auxiliary phase difference divided by the frequency difference between the first primary signal and the first auxiliary signal.

[0046] The maximum error of the second clock offset variable may limit the candidate clock offset values. When determining the second clock offset variable based on the first primary phase difference and the first auxiliary phase difference, advantageously, the maximum error of the second clock offset variable leaves only one candidate clock offset value. From the set of candidate clock offset values, the most likely clock offset value that meets the error margin of the second clock offset variable may be selected as the clock offset value. This error margin is determined by the estimated maximum error of the first primary phase difference and / or the estimated maximum error of the first auxiliary phase difference.

[0047] The most likely clock offset value, or the specifically determined clock offset value, may correspond to, essentially correspond to, or indicate the actual clock offset between the first radio unit and the second radio unit.

[0048] The method may include performing bidirectional transmissions between a plurality of radio units and determining a plurality of clock offsets between pairs of the radio units.

[0049] When at least three wireless units are employed and at least two clock offsets are determined, the clock offsets that can be determined directly by phase measurements can also be used to determine clock offsets between wireless nodes that are not transmitting or receiving signals from each other, thereby making it possible to determine clock offsets between wireless units that are not or cannot communicate with each other.

[0050] Furthermore, when multiple radio units are used to determine multiple clock offsets in accordance with embodiments of the present invention, time and / or resources may be saved. In conventional systems with multiple radio nodes, measurements are performed in connection with each radio link, i.e., each pair of radio units transmits a signal individually to each of the remaining radio units. For example, in a system or configuration with 10 radio units, 45 bidirectional signals should be utilized, resulting in at least 90 total transmissions. However, in the present invention, the clock offset between each radio unit can be determined with as few as 10 transmissions, significantly reducing the resources and time required for measurements and / or transmissions.

[0051] In some embodiments, at least some of the transmitting wireless units may transmit at least one signal in a predetermined time slot and in a predetermined sequence. These transmissions may occur in consecutive time slots such that no empty time slots are left between transmissions. The transmissions and time slots may also be proportioned such that there is a time interval between the end of a transmission and the start of a subsequent time slot in which a subsequent wireless unit begins transmission that is less than a selected "empty" time interval. The time interval between the end of a transmission and the start of a subsequent transmission may be less than 16 μs.

[0052] In embodiments of the present invention in which multiple transmitting radio units each transmit at least one signal in a predetermined order within a predetermined time slot, the subsequent provision of compact transmission signals may be advantageously used in combination with, for example, a Wi-Fi network, whereby a radio channel for transmission needs to be reserved only once per measurement cycle. This feature may enable compatibility with networks such as Wi-Fi described above.

[0053] If the transmissions are not performed in a predetermined time slot and in a predetermined order, the measurement cycle may take a longer and unknown time to complete. This is because a radio channel, which requires the channel to be contended only once, as defined, for example, in ETSI EN 301 893 (the standard specification regulating 5 GHz Wi-Fi transmissions), does not effectively execute one measurement cycle as one transmission. Because each transmitting antenna unit must contend for the channel separately during transmission, the measurement sequence may become significantly longer if the channel is occupied by other users between transmissions.

[0054] Delays in the measurement sequence, due to the transmissions not being effectively transmitted as a single transmission, can easily lead to situations where the channel changes by more than a wavelength between sequences (causing an N*π ambiguity in the phase difference), potentially resulting in wasted measurements. Delays also lead to the possibility that the distance between radio units changes between sequences, making it unclear. This also means that the quality of the local oscillators in the radio units must be very high to maintain phase coherence between different radio units over the long and uncertain measurement intervals, even if the distance changes slowly. However, our solution allows for the use of lower quality oscillators in this regard, allowing for a low-cost implementation.

[0055] In some embodiments, the first radio unit may be a master unit and the remaining radio units may be slave units. The master unit may be configured to transmit the first signal in a measurement cycle. The master unit may be configured to check whether the radio channel is free for transmission before transmitting the first signal in each measurement cycle. If the radio channel is free, at least the first signal (e.g., the first primary signal) in the measurement cycle is transmitted. If the radio channel is not free, no transmission is performed.

[0056] The configuration may advantageously utilize a radio band / channel requiring a Listen Before Talk (LBT) function, and the master unit may check whether the radio channel is free before transmitting the first signal, and if yes, may continue with the measurement cycle, and the radio channel may be reserved by the configuration for at least one measurement cycle. If it is determined that the radio channel is not free, the first signal is not transmitted, and the measurement cycle may be aborted or canceled without transmitting a signal. The master unit or first radio unit may then wait a predetermined time between measurement cycles, and in the next measurement cycle, may again check whether the radio band is free, and if the radio band is free, may continue with transmitting the first signal to start the measurement cycle.

[0057] In some embodiments having a master radio unit and one or more slave radio units, a slave unit may be configured to, before transmitting a signal in a measurement cycle, determine whether a previous radio unit in a predetermined order of radio units transmitted a signal in that measurement cycle, and if so, transmit the signal. On the other hand, if it determines that the previous radio unit did not transmit a signal, i.e., if a valid measurement signal was not received, then no signal is transmitted (waiting for a complete measurement cycle). The determination of whether a previous radio unit transmitted a signal can be based, for example, on other radio units having knowledge of the exact signal characteristics and being able to detect the previous transmission based on well-known correlation techniques.

[0058] In some embodiments, one of the radio units may be set as a reference radio unit by setting the phase of at least one of the received signals relative to the local oscillator of the reference radio unit as a reference phase.

[0059] In some embodiments, a clock rate difference between at least the first and second radio units may be determined, and this clock rate difference may be taken into account in determining the clock offset. The clock rate difference may be determined by repeatedly transmitting at least a (primary) signal and repeatedly determining at least a (primary) phase difference, i.e., performing at least two bidirectional transmissions using the same frequency. The repeated transmissions, e.g., of the primary signal, may be spaced apart, e.g., 100 μs to 1 ms, but using the same frequency.

[0060] In some embodiments, a Doppler frequency between at least a first radio unit and a second radio unit may be determined as a result of relative motion between these radio units. This Doppler frequency may be taken into account in determining a clock offset to compensate for the relative motion between the radio units. The Doppler frequency may be determined by repeatedly transmitting at least a (primary) signal and repeatedly determining at least a (primary) phase difference, i.e., performing at least two bidirectional transmissions using the same frequency. This may be estimated and taken into account independently of the aforementioned clock rate difference. The same set of measurements may be used to determine both the clock rate difference and the Doppler frequency.

[0061] In some embodiments, for example, at least the first primary signal and the first auxiliary signal (or any one of the first, second, and / or subsequent signals) may be transmitted sequentially (transmitted by the same single radio unit).

[0062] In some embodiments, at least a first, e.g., primary signal and a first, auxiliary signal (or any of the signals transmitted by the same radio unit) may be transmitted at least partially simultaneously, and multiple, e.g., primary signals and / or multiple auxiliary signals may be transmitted simultaneously.

[0063] The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. However, the invention itself, both as to organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of certain illustrative embodiments read in conjunction with the accompanying drawings.

[0064] As will be appreciated by those skilled in the art, the discussions presented above with respect to various method embodiments can be flexibly applied mutatis mutandis to apparatus embodiments, and vice versa. [Brief explanation of the drawings]

[0065] The invention will now be described in more detail with reference to exemplary embodiments according to the accompanying drawings. [Figure 1] 1 illustrates one exemplary configuration according to an embodiment of the present invention. [Figure 2] 10 illustrates a further exemplary configuration in accordance with an embodiment of the present invention. [Figure 3] 1 illustrates exemplary first and second antenna units and first and second radio units that may be used in one configuration. [Figure 4] 10 illustrates other exemplary first and second antenna units and first and second radio units that may be used in a configuration. [Figure 5] 1 shows a graph of determined phase differences as a function of transmitted signal frequency, with lines corresponding to sets of primary phase differences, auxiliary phase differences, and determined clock offset values ​​that may be determined in one use case scenario in accordance with one embodiment of the present invention. [Figure 6] 1 shows radio units that may be used in a configuration. [Figure 7] 1 shows the allocation of time slots in a measurement cycle. [Figure 8] 1 illustrates a flowchart of a method according to an embodiment of the present invention. [Figure 9] 3 shows a flowchart of a method for selecting a frequency range for use in an embodiment of the present invention. [Figure 10] 1 shows a flowchart of a method according to an alternative embodiment of the present invention. [Figure 11] 1 illustrates, in terms of time and frequency, how signals are transmitted in an embodiment of the present invention. Detailed explanation

[0066] 1 illustrates an arrangement 100 according to one embodiment of the present invention. The arrangement includes at least a first antenna unit (AU) 104 and a second antenna unit 106, which are associated with a first radio unit 108 and a second radio unit 110, respectively. The antenna units 104, 106 may be located within the radio units 108, 110, or may be coupled to the radio units via, for example, cables. The arrangement 100 may include other numbers of antenna units or radio units, such as a third radio unit or a fourth radio unit. Any two radio units (or antenna units) may be considered to be separated by a baseline or distance D and to transmit and receive one or more signals between them.

[0067] The radio units 108, 110 are coupled to at least one processing unit 102. The processing unit 102 may be a controller unit external to the radio units 108, 110, may be implemented as a microprocessor unit, or may be provided as part of a larger computing device such as a personal computer. However, in some embodiments, the processing unit 102 may be located within the radio units 108, 110 or may be considered part of the radio units 108, 110.

[0068] The processing unit 102 may be configured to control the radio units and / or antenna units comprised in the arrangement 100. The processing unit 102 may further receive data from the antenna units 104, 106 or the radio units 108, 110.

[0069] Additionally or alternatively, the processing unit 102 may be configured to receive data from antenna units and / or radio units included in the arrangement 100 in a wired (e.g., Ethernet) or wireless (e.g., WLAN) manner. Figure 2 shows an embodiment of the arrangement 100 in which the processing unit 102 is wirelessly coupled to the radio units 108, 110. The processing unit 102 may be associated with a processor-antenna unit 112.

[0070] The processing unit 102 and wireless units 108, 110 may be powered using, for example, Power-over-Ethernet (PoE), direct mains power, batteries, solar panels, or mechanical generators (such as wind turbine blades).

[0071] In some embodiments, configuration 100 may utilize a remotely located processing device in addition to processing device 102, which may be, for example, a locally provided processing device. Alternatively, processing device 102 may be implemented as a remote processing device that does not require a local processing device. The remote processing device may receive any of the obtained data and, for example, perform at least some of the data determinations performed by configuration 100. A remote processing device may refer to a processing device that may be accessed via cloud computing. Alternatively, a remote processing device may refer to, for example, a virtual processor configured at multiple locations that may be configured to perform the processing presented herein through parallel processing means.

[0072] In the following example, the configuration 100 and its functionality are described in relation to a first radio unit 108 and a second radio unit 110, where at least a first primary signal and a first auxiliary signal are transmitted by both radio units. As will be appreciated by those skilled in the art, similar considerations may apply to any transmission that may be transmitted in the present method.

[0073] The first radio unit 108 is configured to transmit at least a first primary signal through the first antenna unit 104. This primary signal may be a radio frequency (RF) signal and have a first primary frequency. This primary signal is preferably a sine wave, but may be any signal with known modulation. The transmitted signal may also be a sine wave including a scrambling code. The first radio unit 108 may also transmit subsequent primary signals, as described below.

[0074] The first primary frequency (and possibly subsequent primary signals) may be included in a first frequency range, which may for example encompass a maximum bandwidth of 100 Hz-100 kHz, preferably 10-100 kHz, if only one signal is transmitted in that range, or 5-100 MHz, preferably 10-50 MHz, for example 40 MHz, if multiple signals are transmitted in said range.

[0075] The duration of the first primary signal (and subsequent signals transmitted by any of the radio or antenna units of the arrangement 100) may be between 10-10000 μs, depending, for example, on the distance between the antenna or radio units, the time interval between measurement cycles, and / or the quality of the local oscillators provided by the radio units 108, 110. The duration of the signal may be, for example, around 100 μs.

[0076] The first primary signal is then received by the second radio unit 110 via the second antenna unit 106. Based on the received first primary signal, at least first primary phase information associated with the first primary signal is determined, the first primary phase information indicating the phase of the received first primary signal relative to a local oscillator of the second radio unit 110.

[0077] To be precise, the signal frequency is usually higher than the local oscillator frequency, and the phase measurement is often done in digital baseband using a fast Fourier transform or similar, which is essentially equivalent to measuring the phase with respect to a local oscillator, which for simplicity is understood to operate at the signal frequency.

[0078] If configuration 100 includes additional radio units, such as a third radio unit, the (first) primary signal may also be received by the third radio unit, and the (first) primary phase information may also be determined by the third (and other) radio units. In general, the signal may be transmitted as a broadcast so that it may also be received by at least some of the remaining non-transmitting radio units in configuration 100. The pairs of radio units may include all possible pairs of radio units possible based on the radio units included in configuration 100, or may include only some pairs. For example, a link failure between a possible pair of radio units may prevent the broadcast signal from reaching the other radio unit.

[0079] The second radio unit 110 is configured to transmit at least a first primary signal through the second antenna unit 106. The first primary signal may be identical to the first primary signal transmitted by the first radio unit and essentially correspond to the first primary signal at least in frequency. The second radio unit 106 may be configured to transmit subsequent primary signals. The subsequent primary signals transmitted by the second antenna unit may essentially correspond to subsequent primary signals transmitted by the first radio unit, etc.

[0080] The first primary signal transmitted by the second radio unit 110 is received by the first radio unit 108 via the first antenna unit 104. Thus, transmission between the radio units in a pair of radio units is bidirectional, i.e., the paired radio units transmit similar signals to each other.

[0081] At least first primary phase information is determined based on the received first primary signal, the first primary phase information indicating the phase of the received first primary signal relative to a local oscillator of the first radio unit 108.

[0082] If the configuration 100 includes multiple radio units 108, 110, each of the radio units of the configuration 100 may be configured to transmit, for example, a first primary signal. This signal may be transmitted as a broadcast after a preceding radio unit in the sequence, or at least the first radio unit 108, has transmitted its first primary signal. This signal may also be received by at least some of the other radio units of the configuration 100. For each received signal, corresponding phase information may be determined. Thus, bidirectional phase information may be determined for each pair of radio units and each bidirectional transmission.

[0083] The first primary phase information is used (by the processing unit 102) to determine at least a first primary phase difference, which indicates the difference in first primary phase information for the first primary signal received at the second radio unit 110 and the first primary signal received at the first radio unit 108.

[0084] The first radio unit 108 and the second radio unit 100 may be configured to transmit subsequent primary signals, e.g., at least a second primary signal that is at a different frequency than the first primary signal, but preferably the first primary signal and the subsequent primary signal are within a first frequency range and may be transmitted simultaneously or sequentially.

[0085] The subsequent primary signal may be received by a radio band that is not transmitting from a radio unit included in the configuration 100, and subsequent primary phase information (e.g., at least a second primary phase information) may be determined.

[0086] From the subsequent primary phase information, a subsequent primary phase difference, for example at least a second primary phase difference, may be determined.

[0087] If the configuration 100 includes two or more radio units 108, 110, any one of them may transmit and receive the above-mentioned signals, each transmitting one signal in a pre-assigned slot, and the clock offset between any two radio units that have transmitted and received at least one signal from each other may be evaluated. If a pair of radio units that have transmitted bidirectionally is obtained, bidirectional phase information may be determined. For a pair of radio units that do not transmit signals to each other, it may also be possible to determine the clock offset between the pair of radio units from the clock offset determined based on the phase measurement if the two radio units transmit bidirectional signals to one or more third radio units common to both. In this case, the clock offset may be determined as the sum of the individual clock offsets on the links connecting the two radio units.

[0088] A set of candidate clock offset values ​​is then determined based at least on the first primary phase difference, the determined first clock offset variable indicating a first clock offset estimate between the antenna units, and an estimated maximum error of the determined first clock offset variable. In some embodiments, the clock offset variable can be based on an approximate estimate of the clock offset between the antenna units. Possible methods for determining the set of candidate clock offset values ​​are described in detail below. A set of candidate clock offset values ​​may be determined for each pair of wireless units.

[0089] The first radio unit 108 is also configured to transmit at least a first auxiliary signal having an auxiliary frequency. The first auxiliary signal may essentially correspond to the first primary signal except for frequency. The first auxiliary frequency may represent a frequency included in a second frequency range. The second frequency range may encompass a maximum bandwidth of 100 Hz-100 kHz, preferably 10-100 kHz, if only one signal is transmitted in the range, or a maximum bandwidth of 5-100 MHz, preferably 10-50 MHz, e.g., 40 MHz, if multiple signals are transmitted in the range.

[0090] The difference between the first frequency range (the frequency range of the first primary signal and possibly subsequent primary signals) and the second frequency range may be at least 150 MHz, preferably at least 200 MHz, and most preferably at least 500 MHz. For example, if the two frequency bands are in completely different radio bands, such as 2.4 GHz ISM, 5 GHz RLAN / ISM, and 6 GHz unlicensed bands, the difference between the first frequency range and the second frequency range may even be 3 GHz or more.

[0091] The frequency values ​​of the first frequency range, the second frequency range, and / or any subsequent frequency range may include essentially any frequency values. More important than the frequency values ​​included in the frequency ranges may be the frequency separation or distance or difference between two frequency ranges that are separated from each other, or at least between the frequency of the first primary signal and the frequency of the first auxiliary signal.

[0092] The second antenna unit 106 may receive the first auxiliary signal, and first auxiliary phase information may be determined for the second radio unit 110, where the first auxiliary phase information indicates the phase of the received first auxiliary signal relative to a local oscillator of the second radio unit 110.

[0093] The second radio unit 110 then transmits a first auxiliary signal that substantially matches the first auxiliary signal transmitted by the first radio unit 108 .

[0094] The first auxiliary signal transmitted by the second antenna unit 110 is received at the first radio unit 108, and corresponding first auxiliary phase information is determined, where the first auxiliary phase information indicates the phase of the received first auxiliary signal relative to a local oscillator of the first radio unit 108.

[0095] This first auxiliary phase information is used to determine at least a first auxiliary phase difference indicating the difference between the first auxiliary phase information for the first auxiliary signal received at the second radio unit 110 and the first auxiliary phase information for the first auxiliary signal received at the first radio unit 108.

[0096] If the configuration 100 includes multiple radio units 108, 110, each radio unit may be configured to transmit, preferably consecutively, a signal corresponding to the first auxiliary signal in its respective time slot. Each signal may be received by (at least some of) the remaining radio units of the configuration 100 that are not transmitting, and corresponding first auxiliary phase information may be determined. A first auxiliary phase difference may be determined for each pair of radio units that transmitted a bidirectional signal corresponding to the first auxiliary signal.

[0097] The processing of information may occur in a different order than that suggested here, for example, the determination of the set of candidate clock offset values ​​described above may occur after transmitting (and receiving) the auxiliary signal, or the auxiliary signal may be transmitted simultaneously with the primary signal.

[0098] A subsequent auxiliary signal may be transmitted to determine subsequent auxiliary phase information and subsequent auxiliary phase differences.

[0099] The subsequent auxiliary signal may include frequencies within a second frequency range.

[0100] When a plurality of auxiliary signals are transmitted, they may be transmitted simultaneously or sequentially.

[0101] A most likely clock offset value is determined or selected from a set of candidate clock offset values ​​based at least on the determined first primary phase difference and first auxiliary phase difference (assuming the difference between the first frequency range and the second frequency range is sufficient to make an unambiguous selection). The selection of the most likely clock offset value is described in more detail below.

[0102] Figure 3 shows exemplary first and second antenna units 104 and 106, as well as radio units 108 and 110, that may be used in the configuration. In the example of Figure 3, the antenna units 104 and 106 are provided separately from the radio units 108 and 110. Figure 3 also shows schematically how, in a pair of antenna units that transmit and receive at least one signal to and from each other, phase information associated with signals transmitted and received between the two antenna units 104 and 106 may be used to estimate a clock offset therebetween. Corresponding considerations apply to other pairs of radio units that may be obtained in various embodiments of the configuration.

[0103] Assuming that the transmitting first radio unit 108 transmits at least one first primary signal at zero phase relative to its local clock / oscillator (LO), the measured / determined phase φ of the primary signal received at the second radio unit 110 is 12 (or the first primary phase information) can be determined (as can be seen from FIG. 3) by the following formula:

[0104] φ 12 (t1) = θ C,1 (t1) - θ T,1 -θ A,1 - Φ 12 (t1) - θ A,2 -θ R,2 -θ C,2 (t1) (1)

[0105] where θ C,1 (t1) and θ C,2(t1) is the phase of the local oscillator of the first and second radio units 108, 110, respectively (where t1 is the time when the first radio unit 108 transmits). This essentially translates to an indication of the quantity of interest, namely the clock offset. Φ 12 (t1) is the geometric phase corresponding to the distance or baseline or connecting geometric line, D, between the first antenna unit 104 and the second antenna unit 106. θ T,1 is the phase length of the transmitting branch (transmitting section) corresponding to the first antenna unit 104, and θ R,2 is the phase length of the receiving branch (receiving section) corresponding to the second antenna unit 106. (Here, phase length refers to the phase shift that occurs in a signal traversing a certain distance.) θ A,1 and θ A,2 are the phase lengths of the antenna feed cables of the first antenna unit 104 and the second antenna unit 106, respectively. The phase lengths of the branches and cables can be assumed to be constant, so there is no time dependency.

[0106] The phase length of the transmitting and receiving branches, e.g., θ T,1 , θ R,2 constitutes the phase length due to the physical length of the transmit and receive branches of the associated radio unit, e.g., amplifiers within the radio unit and possibly cables. For example, the phase length θ T,1 corresponds to the length of the transmit branch from the digital-to-analog converter (DAC) to the antenna port of the first radio unit 108.

[0107] The second radio unit 110 may then also transmit at least one primary signal, which may occur in a pre-assigned time slot after determining that a corresponding signal from the first radio unit 108 has been transmitted. The transmitted first primary signal is received by at least the first radio unit 108.

[0108] Now, assuming that the second radio unit 110 transmits the first primary signal at zero phase relative to its local clock / oscillator (LO), the measured / determined phase φ of the primary signal received at the first radio unit 108 is: 21 (or the first primary phase information) may be determined by the following formula:

[0109] φ 21 (t2) = θ C,2 (t2) - θ T,2 -θ A,1 - Φ 21 (t2) - θ A,2 -θ R,1 -θ C,1 (t2) (2)

[0110] where θ C,2 (t2) and θ C,1 (t2) is the phase of the local oscillator of the second and first wireless units 110 and 108, respectively (where the transmission time of the second wireless unit 110 is assumed to be t2). 21 (t2) is the geometric phase corresponding to the distance or the baseline or the connecting geometric line between the second antenna unit 106 and the first antenna unit 104. θ T,2 and θ R,1 are the transmit branch phase lengths and receive branch phase lengths corresponding to the second antenna unit 106 and the first antenna unit 104, respectively.

[0111] The radio unit may transmit the determined phase information (and possibly other information, such as amplitude information) to the processing unit 102 (although the processing unit 102 may be incorporated into any of the radio units). The processing unit 102 may then determine at least a first primary phase difference.

[0112] The phase difference may be determined as the difference in phase information associated with a signal transmitted by one radio unit and received at one other radio unit and a corresponding signal transmitted by the other radio unit (e.g., the second radio unit 110) and received at the one radio unit (e.g., the first radio unit 108). In the example of Figure 3 with the first radio unit 108 and the second radio unit 110, the phase difference (e.g., the first first order phase difference) Φ d may be determined as follows:

[0113] Φ d = φ 12 - φ 21 = θ C,1 (t1) - θ T,1 -θ A,1 - Φ 12 (t1) - θ A,2 -θ R,2 -θ C,2 (t1) - [θ C,2 (t2) - θ T,2 -θ A,1 - Φ 21 (t2) - θ A,2 -θ R,1 -θ C,1 (t2)] = [θ C,1 (t1)-θ C,2 (t1)]+ [θ C,1 (t2)-θ C,2 (t2)] + [Φ 21 (t2)-Φ 12 (t1)] + (θ T,2 + θ A,1 + θ A,2 + θ R,1 ) -(θ T,1 + θ A,1 + θ A,2 + θ R,2 ) (3) However, for simplicity, the time dependence of φ is omitted. (θ T,2 + θ A,1 + θ A,2 + θ R,1 )-(θ T,1+ θ A,1 + θ A,2 + θ R,2 ) term can be considered to be cancelled (if the wireless node and antenna are identical to each other) or can be taken into account by well-known transmit / receive RF chain calibration methods.

[0114] In this assumption TIFF0007818016000001.tif16170 where, TIFF0007818016000002.tif1026 is the average phase difference of the local oscillator between t1 and t2.

[0115] For a high quality oscillator, the drift rate dθ C / dt, i.e., the frequency offset between the local oscillator of the first radio unit 108 and the local oscillator of the second radio unit 110 is substantially constant and can be written as,

[0116] Φ d = 2Δθ C (t1) +(dθ C ) / dt (t2-t1) (5)

[0117] Here, ΔθC(t1)≡θ( C,1 )(t1)-θ( C,2 )(t1), and Φ 12 (t) = Φ 21 Let (t) = Φ(t). If the wireless channel is constant (i.e., the distance between the first and second wireless units does not change and the objects contributing to the reflection do not move relative to the wireless units), the Doppler frequency dΦ / dt is zero. This is a reasonable assumption in most cases, such as for fixed wireless base stations and positioning nodes. However, if this assumption cannot be made, dΦ / dt can be determined as described below.

[0118] From equation (5), the LO phase difference at time t1 is calculated as follows:

[0119] Δθ C(t1) = 1 / 2 [Φ d -(dθ C ) / dt (t2-t1)] (6)

[0120] Frequency offset dθ C Even if / dt is unknown, the LO phase difference at the exact midpoint between t1 and t2 can be found as follows:

[0121] Δθ C (t1+ (t2-t1) / 2) = 1 / 2 Φ d (7)

[0122] A candidate frequency offset between the local oscillators of the radio units 108 and 110 can be determined as described below, which allows the clock offset epoch to be converted to any time in the vicinity of t1 for which a linear phase drift holds. C,re One of the wireless nodes (e.g., the first wireless unit) can be selected as the reference clock (or reference station or unit) by setting f = 0. Then, a system equation can be constructed to determine the remaining unknown local oscillator phase offset θc from the difference Δθ.

[0123] LO phase offset θ C and the clock offset τ must obey the simple relationship:

[0124] θ C = 2π τ f + θ inst (f) (8)

[0125] This relationship allows the instrumental term θ inst If (f) is known, the LO phase offset can be calculated from the clock offset τ. In the following analysis, θ inst(f) is assumed to be zero to simplify the equation. In practice, this value can be reasonably assumed to be constant at a given frequency f and can be measured with appropriate measuring equipment. From (7) and (8), and the measured phase difference Φ d Considering that only values ​​within the range [π, π] can be obtained, the following formula is obtained:

[0126] TIFF0007818016000003.tif22170

[0127] 4 illustrates exemplary first and second antenna units 104, 106, and radio units 108, 110 that may be used in configuration 100. In this embodiment, antenna units 104, 106 are mounted within radio units 108, 110 or are connected by only a single antenna cable. In the examples presented, the radio units should be considered to include antenna units, regardless of whether the antenna units are implemented as part of the radio unit itself.

[0128] Figure 5 illustrates a graph of phase difference determined as a function of transmit signal frequency, with lines corresponding to a set of possible primary phase difference, auxiliary phase difference, and clock offset values ​​that may be determined in one use case scenario in accordance with an embodiment of the present invention. The numbers, lines, calculated values, etc. in Figure 5 are merely exemplary and intended as a visual aid in explaining the present invention. Accurate representations may be possible, for example, when the clock difference is only about 200 picoseconds. However, the principles are the same for any clock difference.

[0129] The depicted points 302 and 304 may correspond to a first and a second primary phase difference, respectively. In this example, first and second primary signals (having frequencies f1 and f2, respectively) are transmitted. The primary signals are transmitted in a first frequency range f a An example of the first frequency range f aspans a frequency range of about 40 MHz. The number of signals transmitted and the frequency range f a The frequencies included in the first band (frequency range), the width of the first band (frequency range), etc. may of course vary depending on the use case.

[0130] The primary signal may be transmitted in a single transmission (one radio unit transmitting in each time slot), or multiple signals, e.g., sinusoidal waves, may be transmitted simultaneously. The frequency difference between successive signals may be, e.g., 1-40 MHz, 5-20 MHz, e.g., about 10 MHz.

[0131] Points 308 and 310 may correspond to a first and second auxiliary phase difference, respectively. In this example, first and second auxiliary signals (at frequencies f3 and f4, respectively) are transmitted. The auxiliary signals are in a second frequency range f b An example second frequency range f b spans a frequency range of about 40 MHz. Again, the number of signals transmitted and the frequency of the second frequency range f b The frequencies included in the second band (which may be only one frequency, for example) may be different, as well as the width of the second band.

[0132] First and second frequency ranges f a , f b The first and second frequency bands may be the same or different in bandwidth, but advantageously both are narrow enough to allow the use of narrowband receivers (see Wi-Fi receivers) or Internet of Things (IoT) receivers powered by coin cell batteries.

[0133] First frequency range f a and the second frequency range f b The difference Δf between the primary signal and the auxiliary signal is approximately 550 MHz in the example of Figure 5. The frequency of the primary signal may be greater or less than the frequency of the auxiliary signal, but it is desirable that there be a frequency difference or frequency range difference Δf of sufficient magnitude to determine a clock offset value that is considered appropriate.

[0134] In some embodiments of the present invention, the signal is in a first band or first frequency range f a and a second band or second frequency range f b In addition, it may also be transmitted in a third and possibly fourth and subsequent narrowbands.

[0135] The number of frequency ranges that are desirable to use to be able to determine or select the most suitable clock offset value from the determined set of candidate clock offset values ​​will vary depending on the environment, use case, or embodiment.

[0136] When at least two signals are utilized, e.g., a first primary signal and a second primary signal having carrier frequencies f1 and f2, a clock offset variable indicating the clock offset between the first radio unit and the second radio unit can be expressed as the difference between the respective phase differences (the difference between the first (primary) phase difference and the second (primary) phase difference).

[0137] Δτ1= (Φ d,f1 -Φ d,f2 +N·4π) / [4π(f1-f2)] (10)

[0138] This can be seen from the relationship between the clock offset and the measured phase difference given in equation (9): The maximum error in the clock offset variable is:

[0139] Δτ max = 2ΔΦ d,max / [4π(f1-f2)] (11)

[0140] where ΔΦ d,max is the maximum error in a single phase difference measurement. We also assume that N is known. In other words, f1-f2 are chosen to be appropriately small to avoid phase ambiguity. Such ambiguity is expressed in equation (10) as |Φ d,f1 -Φ d,f2It can be avoided if |<2π, i.e.

[0141] Δf_(max )=|f1- f2| max < 1 / (2Δτ max ) (12)

[0142] However, the clock offset inaccuracy determined by equation (11) is due to the phase difference Φ d,f1 and Φ d,f2 It can be relatively high due to measurement or estimation errors in the measurement of phase difference. Generally, phase measurement errors are due to thermal or phase noise in the radio receiver. In high-quality oscillators, thermal noise is the dominant cause. Because thermal noise has well-known statistical properties, the typical maximum error in phase difference can be easily estimated from known system noise and signal levels.

[0143] Estimated maximum error value ΔΦ d,max From this, it is possible to determine a limit on the value of the error that can be determined using the determined phase information. d,max An estimate of ΔΦ may be sufficient for the procedures described herein to be feasible. d,max is to be understood as the maximum possible phase measurement error.

[0144] Maximum measurement error ΔΦ d,max may be determined for each use case or each configuration 100. The maximum measurement error may be known a priori or may be received by the configuration 100. For example, the maximum measurement error ΔΦ d,max may be determined based on the known phase measurement / determination accuracy of the arrangement 100.

[0145] ΔΦ d,max may be a value chosen such that it is known that the true error in the phase measurement is always likely to be less than this value.

[0146] where ΔΦ d,maxUsing too large a value for ΔΦ will not invalidate the following procedure. However, if the value is too small, the phase rotation between frequency ranges will not be taken into account, which may result in an incorrect clock offset determination. d,max It is desirable to select conservatively.

[0147] For example, there is a 40 MHz difference between f1 and f2, and a measurement error ΔΦ of 10 degrees d,max Considering this, the clock offset variable Δτ is determined max The maximum error is about 700 picoseconds, as can be seen from equation (11).

[0148] In some embodiments, the clock offset estimate determined between at least a pair of radio units, including at least a first radio unit and a second radio unit, may be obtained in a different manner (i.e., different from the phase measurement described above). For example, it may be obtained from prior knowledge or from a measured approximate clock offset (e.g., measured using electrical or optical methods). This clock offset estimate may be used as a provisional clock offset variable that can be used to determine a provisional set of candidate clock offset values. Then, for example, a maximum error of the measured provisional clock offset (variable) may be determined or obtained.

[0149] The tentative clock offset variable may be used as a first approximation of the clock offset and may be determined before any transmission is performed to determine the maximum value of the clock offset candidate.

[0150] If a tentative clock offset variable is obtained, the first signal and the second signal (or the first primary signal and the first auxiliary signal) may be sufficient to unambiguously determine the clock offset value.

[0151] Measurement error ΔΦ d,max In addition to the error due to d,f1There could be an ambiguity of 2π in , which would mean an ambiguity of about 13 ns considering the example scenario above (from equation (10)). In this case, if we had prior information about the clock offset more accurate than 13 ns, we could eliminate the 2π inaccuracy.

[0152] This 2π uncertainty problem can also be reduced by transmitting consecutive primary signals whose frequencies differ by less than a threshold value. The difference in frequency between consecutive primary signals (e.g., f1 and f2) may be, for example, less than 20 MHz, for example, less than 15 MHz, for example, less than 10 MHz, or for example, less than 5 MHz. If the signal frequency difference (e.g., the difference in frequency between f1 and f2) is 5 MHz, the 2π ambiguity in the clock offset determined from Equation (10) would be approximately 100 ns. In this particular case, a priori knowledge of the clock offset with an accuracy better than 100 ns would eliminate the 2π uncertainty. Such accuracy is achievable, for example, with a synchronization sequence utilizing the full 40 MHz instantaneous bandwidth. However, to limit the imprecision in determining the clock offset variable, it is advantageous for the transmitted primary signals to cover a frequency range whose total extent is, for example, at least 40 MHz.

[0153] Considering that the clock offset τ must be the difference between N+IA (the integer ambiguity) half period T1 in the primary frequency and the fractional component (whose magnitude is always less than 1 / 4 period), equation (10) could be used to resolve the integer ambiguity through the following relationship:

[0154] τ = (N + IA) *(T1 / 2) + τ frac = (Φ d,f1 -Φ d,f2 ) / [4π(f1- f2)] (13) where T1 is the period of the first primary signal and Φ d is given in radians.

[0155] (Φ d,f1 -Φd,f2 ) in the measurement, the maximum error (which is 2ΔΦ d,max ), considering this, the candidates for the value of N + IA correspond to the values that satisfy Equation (13). Therefore, ΔΦ d,max defines the range within which the integer ambiguity value IA or N + IA can take values, and gives a set of candidates for the integer ambiguity value. τ frac If τ can be determined, the set of candidates for the integer ambiguity value will either match or define the set of candidates for the clock offset value τ.

[0156] The best estimate N of the number of half - periods between clocks can be derived as the one that best fits Equation (13) by setting IA = 0.

[0157] The possible range of IA (-ΔIA < IA < ΔIA) is restricted by this maximum phase estimation error ΔΦ d,max as follows (and as derived from the previous equation).

[0158] ΔIA = ΔΦ d,max / (π(1 - f2 / f1)) (14)

[0159] Setting f2 in Equation (13) (and the corresponding Φ d,f2 ) to zero, and noting that Φ d,f1 must match the fractional component of τ frac , the phase difference satisfies the following equation.

[0160] Φ d,f1 = 4π·f1·τ+(N + IA)·2π (15)

[0161] However, the instrument phase error that cannot be canceled in the basic measurement, that is, (θ T,2 +θ A,1 +θ A,2 +θ R,1 )-(θ T,1 +θ A,1 +θ A,2 +θ R,2 ), and θ instis assumed to be zero (or is assumed to be determined separately and excluded from the calculation). From this, the clock offset is determined as follows:

[0162] TIFF0007818016000004.tif24170

[0163] From this equation, τ can be determined with greater accuracy than from equation 10, because f1 is much larger than f1-f2.

[0164] The problem with equation (16) is an integer ambiguity. (The value of N+IA is unknown, or if N is determined from equation (13), the value of IA is unknown.) For example, if the signal frequency f1 is 6 GHz, the ambiguity in τ is IA*83 picoseconds. However, equation (16) may be used to determine a set of candidate clock offset values, and the most likely clock offset value may be determined based on the obtained phase information, the approximate clock offset, and the estimated maximum error.

[0165] As shown above, f1 and f2 are 40 MHz apart, and the maximum measurement error of the phase difference is ΔΦ d,max is 10 degrees, the phase difference Φ d,f1 -Φ, d,f2 By determining the ambiguity, the clock offset can be determined to an accuracy of approximately ±700 picoseconds. When f1=5.8GHz and the half-period is 86 picoseconds, the integer ambiguity is limited to approximately 16 candidate values. (This is sometimes referred to as the "set of candidate integer ambiguity values.")

[0166] When considering the phase difference as a function of the transmit frequency, the set of candidate integer ambiguity values ​​can be understood to correspond to integer ambiguity lines on a graph, with a slope determined by the clock offset (and a scaling factor of 4π) given by equation (16). This is shown in Figure 5. The set of candidate IA values, or candidate clock offset values, is shown as the integer ambiguity lines intersecting the first primary phase difference 302. The line corresponding to the slope determined by equation (13) for IA = 0 (best tentative match), which also determines the value of N, is shown as 316. The neighboring candidates are IA = +1 (318) and IA = -1 (314), corresponding to clock offset differences that are larger and smaller by a half-cycle period, respectively. These all fall within an error bound of 2ΔΦ around the measured primary phase difference, indicated by the error bars. d,max and is therefore part of the set of candidate IA or clock offset values.

[0167] When considering the first and second primary signals, the error limit considered when determining a set of candidate clock offset values ​​for the determined first clock offset variable may be limited by the maximum error in the phase difference and frequency difference between the first and second primary signals, as can be seen from equation (10). Thus, the maximum error in the first clock offset variable is 2ΔΦ d,max / [4π(f1-f2)], which may be used to provide an error bound on the first clock offset variable that restricts the set of candidate clock offset values.

[0168] In embodiments where subsequent primary or auxiliary phase differences are determined, the integer ambiguity line IA=0 may be determined as the line passing through two of the determined primary phase differences or as the line with the best least squares fit to multiple primary phase difference points.

[0169] When three or more primary signals are utilized, the first clock offset variable may be determined as a least-squares fit, and bounds for the first clock offset variable for a given probability value may be derived using, for example, statistical estimation methods to determine the maximum error of the first clock offset variable. When three or more primary signals are utilized, the first and second primary signals discussed should be understood to refer to the two primary signals that are furthest apart in frequency, with the third primary signal and subsequent primary signals (if present) having frequencies between the first and second primary signals.

[0170] When transmitting at least one auxiliary signal, preferably when the auxiliary signal frequency f3 differs from f1 or f2 by at least, for example, 400 MHz, at least a first auxiliary phase difference Φ d,f3 The determined second phase difference Φ d,f1 -Φ d,f3 , f1-f3, equation (10) may be used to determine a second, better estimate of the clock offset variable Δτ2, with accuracy in this case Φ d,f1 -Φ d,f2 For example, it may be ±70 ps, ​​rather than ±700 ps as in the first estimate obtained from equation (10) using f1-f2 and f1-f2. Note that this value is estimated for the frequency considered and may vary depending on the use case. Numerical values ​​are provided here to illustrate the difference in the magnitude of the error in the estimate of the determined clock offset variable.

[0171] Using equation (10), but with a second phase difference Φ d,f1 -Φ d,f3 Using the first primary phase difference and the first auxiliary phase difference to determine a second clock offset variable Δτ2 indicative of a second clock offset estimate, the second clock offset variable Δτ2 may be determined as follows:

[0172] Δτ2= (Φ d,f1 - Φ d,f2 ) / [4π(f1-f2)] (17)

[0173] Estimated maximum error ΔΦ of the first primary phase difference d,max and / or the estimated maximum error in the first auxiliary phase difference (which is also e.g., ΔΦ d,max , can be used to d,f1 -Φ d,f3 (This gives a total of 2ΔΦ d,max ) This may also give the maximum error of the second clock offset variable.

[0174] The second clock offset variable Δτ2 can be used to determine candidate clock offset values ​​for the clock offset that correspond to an integer number of half-period clock offset variations in one of the frequencies used, such as the first primary frequency. The maximum error of the second clock offset variable may provide an error bound within which the most likely clock offset value should fall.

[0175] As a result of the above, there is only one candidate for the clock offset value, in other words, there is only one candidate for the IA value, and the most likely appropriate IA value or clock offset value is determined, thereby resolving the integer ambiguity.

[0176] Using the determined most likely integer ambiguity value I A, the clock offset τ is calculated / determined using equation (16), and the clock offset between the first wireless unit 108 and the second wireless unit 110 can be determined with an accuracy of, for example, a few picoseconds or less.

[0177] If the set of candidate clock offset values ​​is not expected to be limited to one, a third frequency range may be selected that differs from the second frequency range by the selected frequency difference, and an auxiliary signal may be transmitted in the third frequency range to further limit the set of candidate clock offset values.

[0178] In Figure 5, the most likely integer ambiguity value is the measurement error ΔΦ d,maxIn this example, IA=0 corresponds to the line 316.

[0179] If multiple primary and / or auxiliary phase differences are determined, an integer ambiguity line may be determined, for example, using least squares fitting or other fitting techniques. The integer ambiguity line is fitted taking into account preferably all of the measured phase differences. Furthermore, a candidate clock offset value may be determined from the slope of the integer ambiguity line.

[0180] In some embodiments, the arrangement 100 may be configured to perform the above-described integer ambiguity resolution protocol at least once and then operate in a tracking mode. In the tracking mode, the arrangement 100 may be configured to track the clock offset between the first wireless unit 108 and the second wireless unit 110 by repeatedly transmitting subsequent primary signals, determining subsequent primary phases, determining primary phase differences, and repeatedly determining clock offset information indicative of changes in the clock offset between the first wireless unit 108 and the second wireless unit 110. The subsequent primary signals may be transmitted, for example, over a narrow frequency band f a By summing up such clock offset changes, this mode allows for continuous tracking of the true clock offset.

[0181] After the integer ambiguity has been resolved at least once, it may be assumed that the integer ambiguity does not change between subsequent measurements in tracking mode. This may be based, for example, on known or estimated changes in the clock offset between the first wireless unit 108 and the second wireless unit 110. The integer ambiguity may be resolved, for example, at predetermined time intervals, to ensure that the previously resolved integer ambiguity value is still valid, i.e., no phase slip has occurred.

[0182] During normal operation, signals are transmitted in one narrow frequency band (e.g., a first frequency band f containing the primary signal). a The tracking mode is convenient to use for tracking clock offsets, since only the clock offset may need to be tracked in another (narrower) frequency band (e.g., a second frequency band f b ) may be required only once before transitioning to tracking mode, or may need to be transmitted much less frequently compared to the signals transmitted in tracking mode. For example, phase tracking (through transmission of a (primary) signal in the first narrowband) may be repeated at a time interval ranging, for example, from 0.1 to 50 ms or from 1 to 20 ms, for example, every 10 ms. A new IA determination (through additional transmission of at least one (auxiliary) signal) in the second narrowband may be performed only once per second, for example, during a time interval ranging, for example, from 0.1 to 10 seconds, or from 0.5 to 5 seconds.

[0183] The procedure to be followed in embodiments of the present invention may be described without indicating frequency ranges as presented above. The transmitted signals may be considered to be at least a first signal having a first frequency, a second signal having a second frequency, followed by a subsequent signal having a subsequent frequency, e.g., a third signal having a third frequency.

[0184] Based on the phase information determined to be bidirectional transmission, the first phase difference and the second phase difference may be determined, and the first phase difference and the second and subsequent phase differences may also be determined.

[0185] At least one clock offset variable may then be determined based on the difference between the first phase difference and the second (or subsequent) phase difference, and a maximum error of the determined clock offset variable may be determined based on the maximum error of at least the first phase difference and the maximum error of the second or subsequent phase differences.

[0186] At least one of a set of candidate clock offset values ​​may be determined through a variation of the clock offset corresponding to an integer number of half-period variations in at least one of the used frequencies, such as the first primary frequency, wherein the set of candidate clock offset values ​​is limited by an estimated maximum error of the determined clock offset variable.

[0187] The clock offset may be varied, for example, by varying an integer number of half periods at the highest frequency used. In this case, it is known that if the clock offset can be uniquely determined at the highest frequency, the clock offset can also be uniquely determined at a lower frequency.

[0188] Subsequent two-way transmissions on the selected frequency may be repeated to determine the maximum error associated with the clock offset variable and a set of candidate clock offset values, and may be repeated until only one candidate clock offset value remains and the clock offset can be unambiguously determined.

[0189] Subsequent frequencies may be selected such that the subsequent frequency differs from the first frequency by more than the difference between the first frequency and the second or previously used frequency.

[0190] A final, unambiguous clock offset value may be determined via Equation (16) using any of the determined phase differences. In some embodiments, the final clock offset value may be determined using one or more phase differences or all of the phase differences. One other possible way to determine the final clock offset value may be to determine the clock offset value individually (using Equation (16)) for several or each of the determined phase differences and then determine the final clock offset value as their (weighted, in some embodiments) average. Another option is to find the value of the clock offset that provides the best least-squares fit to the phase differences determined using several different frequencies. Such a least-squares fit calculation can also be performed in the complex domain instead of using real phase values.

[0191] Of course, the above procedure can also be considered to have frequencies selected in at least a first and a second frequency range. In the embodiments of the invention described herein, the frequency ranges do not necessarily need to be pre-selected, since the clock offset value may be determined by selecting second and subsequent frequencies to ultimately cover the first and second frequency ranges, or the entire frequency range encompassing at least the primary and auxiliary frequencies considered.

[0192] The clock offset variable determined in connection with any of the second or subsequent (or further primary or auxiliary) transmissions performed (or at least optionally after obtaining an interim clock offset variable) can be found using equation (10), where frequency f2 may be replaced by the transmission frequency used in each step or cycle, and f1 may be replaced by the highest or lowest signal frequency used.

[0193] In yet another embodiment of the present invention, in determining the clock offset, a frequency offset between local oscillators and / or a possible Doppler frequency between antenna units can be determined and compensated for. After transmitting at least a first primary signal and determining first primary phase information, transmitting at least a repeated primary signal comprising the first primary frequency and determining repeated primary phase information for the signal received at the second radio unit.

[0194] φ 12 (t3) = θ C,1 (t3) - θ T,1 -θ A,1 - Φ 12 (t3) - θ A,2 -θ R,2 -θ C,2 (t3) (18)

[0195] Also, it determines recursive primary phase information for the signal received at the first radio unit.

[0196] φ 21 (t4) = θ C,2 (t4) - θ T,2 -θ A,1 - Φ 21 (t4) - θ A,2 -θ R,1 -θ C,1 (t4) (19)

[0197] If the first wireless node is the reference station, then θ C,1 (t) = 0. Assume that the time between the signals transmitted by the first and second radio units is kept constant. That is, t3 - t1 = t4 - t2 = Δt. We can also assume that the instrumental terms are constant. With these assumptions, and Φ 12 (t)=Φ 21 Noting that (t)=Φ(t), the iterative first order phase difference for the signal received at the second radio unit is given by:

[0198] φ 12 (t3) - φ12 (t1) = -dΦ / dt Δt - (dθ C,2 ) / dt Δt (20)

[0199] And the iterative first order phase difference for the signal received at the first radio unit is:

[0200] φ 21 (t4) - φ 21 (t2) = -dΦ / dt Δt + (dθ C,2 ) / dt Δt (21)

[0201] Doppler dΦ / dt and LO frequency offset dθ C,2 / dt can be solved from these two equations and corrected in equation (5). Note that this solution can be used even if the signal spacing changes, as long as the spacing is known.

[0202] The additional phase measurements from the repeated primary signal may be performed every complete measurement cycle or intermittently. The signal used to determine the frequency offset and / or Doppler frequency may be another signal, such as a repeated auxiliary signal, or may be the same signal used in determining the set of candidate clock offset values, for example, as described above with reference to FIG. 5.

[0203] In some embodiments, the described phase difference measurements may be used as inputs to a Kalman estimator to track, for example, Doppler and LO frequency offset, removing the requirement that these be constant.

[0204] The Doppler and LO frequency offset considerations also apply to first, second, and subsequent signals (and associated signals that have been called primary and auxiliary signals) transmitted without explicit selection of the first and second frequency ranges.

[0205] Figure 6 shows one possible embodiment of radio units 108, 110 that may be used in the configuration 100. The antenna units 104, 106 are provided within the radio units 108, 110. The radio units 108, 110 of Figure 6 each include two receivers and two transmitters, the frequencies of which can be set separately.

[0206] Utilizing radio units 108, 110 with multiple receivers may enable simultaneous measurement of multiple bands, such as a first band including the primary frequency and at least a second band including the auxiliary frequency. At least a portion of the primary signal to be transmitted and at least a portion of the auxiliary signal to be transmitted may be transmitted at least partially simultaneously.

[0207] In some embodiments, the radio units 108, 110 may include two or more receivers, and two or more primary or auxiliary signals may be transmitted (and received) simultaneously. In addition to a first band containing the primary frequency, a second band containing auxiliary frequencies, e.g., a third band containing further auxiliary frequencies, may be transmitted and received at least partially simultaneously.

[0208] In yet another embodiment, primary and auxiliary signals in multiple frequency bands can be transmitted sequentially (only one signal at a time). Such a system can operate over a very narrow bandwidth (e.g., 100 Hz to 100 kHz) and can use very low-cost hardware and small batteries.

[0209] 7A and 7B illustrate how time slots may be allocated in a measurement cycle for transmitting and receiving signals, and possibly communicating data, in configuration 100. A measurement cycle may represent a set of signals to be transmitted, or may represent a time duration during which signals are transmitted one after the other such that the time between transmissions is less than a threshold. For example, a first measurement cycle may include transmitting (and receiving) a primary signal and an auxiliary signal. In some embodiments, a second measurement cycle may be implemented. The second measurement cycle may be identical to the first measurement cycle, for example, or the second measurement cycle may include only transmitting (and receiving) a primary signal, for example, in embodiments where a tracking mode is utilized.

[0210] One measurement cycle may include at least one measurement frame (having N measurement slots). During the measurement frame, at least the first radio unit 108 and the second radio unit 110 may transmit their respective signals separately in their assigned time slots. One measurement frame may include transmission of a signal having one frequency. For example, a primary signal may be transmitted in the first measurement frame and an auxiliary signal may be transmitted in the second measurement frame. The measurement cycle of FIG. 7A is applicable to a configuration 100 having N radio units, and a clock offset may be evaluated between each radio unit. Each radio unit may transmit its respective signal in its respective time slot.

[0211] In embodiments in which at least the primary signal is repeatedly transmitted, a measurement cycle may consist of at least three measurement frames, where the primary signal is transmitted in a first measurement frame, the primary signal is repeatedly transmitted in a second measurement frame, and an auxiliary signal is transmitted simultaneously with the primary signal in the first and second measurement frames, where one or more LO frequency differences and / or one or more Doppler frequencies may also be determined. Also, depending on hardware capabilities (i.e., if simultaneous transmission of the primary and auxiliary signals is not possible), a third measurement frame may be used to transmit an auxiliary signal.

[0212] Transmissions may be performed such that transmissions occur in subsequent time slots such that no empty time slots are left between transmissions. Transmissions and time slots may be balanced such that the time interval between the end of a transmission and the start of the subsequent time slot in which the subsequent antenna unit begins transmitting is less than or equal to a selected maximum time interval. The time interval between the end of a transmission and the start of the subsequent transmission may be less than 50 μs, preferably less than 20 μs, e.g., less than 16 μs.

[0213] The provision of a compact transmission signal can be advantageously used in combination with, for example, a Wi-Fi network: in the present invention, a radio channel for transmission needs to be reserved only once per measurement cycle. This feature may enable compatibility with Wi-Fi-like networks.

[0214] If no transmissions occur in subsequent time slots, the measurement cycle may take a longer, unknown time to complete. This is because a radio channel that requires the channel to be contended only once, as defined, for example, in ETSI EN 301 893 (the standard that regulates 5 GHz Wi-Fi transmissions), cannot effectively execute one measurement cycle as a single transmission. Because each transmitting radio unit must contend for the channel separately during transmission, the measurement sequence may become significantly longer if the channel is occupied by other users between transmissions.

[0215] 7B illustrates how time slots are allocated in a measurement cycle that also employs at least one communication frame (having one or more communication slots). During the communication frame, signals, measurement / determined data, or other data may be transmitted to the processing unit 102. At least one data communication may be transmitted and may be multiplexed in the time or frequency domain with the measurement signals transmitted by the wireless units. The at least one data communication may include at least the determined phase information. Additionally or alternatively, the data communication may include any other information. In this manner, the configuration 100 can simultaneously function as a measurement system and a communication network.

[0216] The required time synchronization accuracy is preferably better than one-quarter of the duration of the possible guard time between subsequent signals to prevent overlapping transmissions. This requirement is much less stringent than the clock offset estimation accuracy, and achieving such coarse synchronization is easily achieved with synchronization sequences, etc.

[0217] 8 illustrates a flowchart of a method according to an embodiment of the present invention. a At least one primary signal having a frequency of is transmitted (802) through the first radio unit 108 and received (804) at the second radio unit 110, and primary phase information is determined through the received primary signal.

[0218] Next, at least one primary signal is transmitted (806) by the second radio unit 110 and received (808) by the first radio unit 108, and respective primary phase information is determined through the received primary signals.

[0219] Based on the determined phase information, at least one primary phase difference is determined 810. An approximate clock offset between the first wireless unit and the second wireless unit and a maximum error in the approximate clock offset may be obtained 812, and a set of candidate clock offset values ​​indicative of the clock offset between the wireless units is determined 814. The set of candidate clock offset values ​​is preferably determined based at least on the approximate clock offset and its error and the first primary phase difference.

[0220] Then, the second frequency range f b At least one auxiliary signal having a frequency of is transmitted (816) through the first radio unit 108, which is received (818) by the second radio unit 110, and auxiliary phase information is determined from the received auxiliary signal.

[0221] Next, at least one auxiliary signal is transmitted (820) by the second radio unit 110, which is received (822) by the first radio unit 108, and auxiliary phase information is determined through the received auxiliary signal.

[0222] Based on the determined auxiliary phase information, at least a first auxiliary phase difference and its maximum error are determined (824).

[0223] A potentially most suitable clock offset value is selected from the set of candidate clock offset values ​​in step 826. Preferably, the potentially most suitable clock offset value is selected based at least on the first primary phase difference, the first auxiliary phase difference, and a maximum error of the first primary phase difference and / or the auxiliary phase difference.

[0224] 9 shows a flowchart of a method for selecting a frequency range, utilized in an embodiment of the present invention. a is selected (902). Then at least a first primary frequency is set. A second primary frequency or a third, fourth, ... primary frequency may also be set.

[0225] The maximum error of the determined phase information may be estimated or obtained in step 904 .

[0226] In step 906, a maximum error in the determined or obtained clock offset estimate between the first wireless unit and the second wireless unit is determined. The maximum error in the clock offset estimate may be based on a maximum error in the phase difference (if at least two primary frequencies are used), where the maximum error in the phase difference may be based on a maximum error in the phase information determined in step 904. Alternatively, any other means for determining the maximum error in the clock offset estimate may be used. For example, the maximum error of another measurement method that may be used to determine the clock offset estimate may be used.

[0227] In step 908, the first frequency range f a and the second frequency range f b The maximum frequency difference Δf between max may be determined so as to avoid unknown phase rotations. max This would be advantageous to most efficiently reduce the set of candidate clock offset values ​​and preferably unambiguously select the most likely suitable clock offset value from the set of candidate clock offset values.

[0228] First frequency range f a is the second frequency range f b It may contain frequencies greater than those contained in the

[0229] Maximum frequency difference Δf maxTo avoid introducing an unknown phase rotation, i.e., a phase slip of 2π, into the determination of Δτ, in some embodiments, the minimum and maximum possible values ​​of at least the first clock offset variable (or clock offset estimate) are used to determine a possible range for at least the first clock offset variable. 1,min ,Δτ 1,max ] (between the minimum and maximum values ​​of Δτ1) and the determined primary phase difference at the first primary frequency f1 is Φ1, then the first auxiliary frequency f3 is in a range bounded by the expected minimum and maximum values ​​of the first auxiliary phase difference, i.e. [Φ 3min ,Φ 3max ] = Φ1+(f1-f3)*phase tilt range = Φ1+(f1-f3)*4π*[Δτ 1,min ,Δτ 1,max ] where Φ3 is the first auxiliary phase difference. The phase slope range refers to the range of possible slopes of the integer ambiguity line, and can also be expressed in terms of the error in the clock offset value. The difference between the expected minimum and maximum values ​​of at least the first auxiliary phase difference |Φ 3max -Φ 3min If | is greater than 2π, the first primary frequency f31 is too far from the first primary frequency f1. In other words, the frequency difference is greater than the maximum frequency difference Δf max (which is the maximum value that satisfies the condition). The difference between the expected minimum and maximum values ​​of at least the first auxiliary phase difference Φ3 may be selected to be less than a threshold value, such as 2π. This may give a range of possibilities for at least one first auxiliary frequency f3, and also for a second frequency range f b You may give a candidate for f b is the first frequency range f a and the maximum is Δf max different.

[0230] Then the second frequency range f bmay be determined, and at least a first auxiliary frequency may also be set (910). After determining at least a first auxiliary phase difference and its error, the size of a set of candidate clock offset values ​​may be determined (912). If only one candidate clock offset value remains in the set, this value may be determined as the most likely clock offset value. However, if it is determined in step 914 that there is ambiguity in the clock offset value, i.e., if the size of the set of candidate clock offset values ​​is greater than one, the process proceeds to step 908, where a second frequency range f b or the first frequency range f a and the maximum frequency difference Δf between the new third frequency band max,2 A third frequency range of signals may be set and subsequent phase information may be determined to determine a new third set of candidate clock offset values.

[0231] The selection of a new auxiliary frequency range may be performed any number of times if it is determined that there is ambiguity in the clock offset value, i.e., if a uniquely suitable clock offset value cannot be selected. If only one candidate clock offset value remains, the process ends at 916, and this becomes the most suitable clock offset value. From this value, the clock offset between the first wireless unit and the second wireless unit can be determined with greater accuracy than the clock offset estimate variable.

[0232] The procedure of Figure 9 may be followed in terms of considering successive signals having selected frequencies without explicitly selecting a frequency range. In this case, the selection of the first frequency in step 902 may be followed by steps (908, 910) of determining and setting the frequencies of the second and subsequent signals. A clock offset variable may be determined in step 906, a maximum error may be determined in step 904, followed by determining the size of a set of candidate clock offset values ​​(912). After determining (914) whether ambiguity remains, if such ambiguity remains, the frequencies of the subsequent signals may be determined (908, 910).

[0233] 10 shows another flowchart of a method according to an embodiment of the present invention. In this embodiment, multiple radio units and at least a first primary signal and a repeated primary signal are employed. The first primary signal (in a first frequency range) may be transmitted (002) through a first radio unit 104. The first radio unit 104 may be a master unit configured to check whether a channel is free for transmission before transmitting a signal.

[0234] The first wireless unit 104 (or other wireless unit in the configuration participating in the transmission) may be selected as the reference unit.

[0235] In step 004, the first primary signal transmitted by 002 may be received by at least some of the remaining non-transmitting radio units of the configuration (radio units not transmitting at 002). Phase information relating to the received signal, relative to the phase of the signal relative to the local oscillator of each receiving radio unit, may then be determined. The phase information may be determined correspondingly as described for the two radio units disclosed above.

[0236] Thereafter, at 006, first primary signals may be transmitted from at least some of the wireless units that have not transmitted the first primary signal at 002. At 006, the first primary signals may be transmitted by each wireless unit in a predetermined time slot, one at a time, sequentially, or in a predetermined order.

[0237] In 008, the first primary signal may be received by the radio units not transmitting in 006, and phase information may be determined for each. Thus, measurements may be performed in which each of multiple radio units transmits at least one similar signal (here, the first primary signal) and each receives multiple signals transmitted by one other radio unit (bidirectional transmission). Pairs of multiple radio units that have transmitted and received at least one signal from each other may be obtained.

[0238] A first primary phase difference may be determined at 010, where the phase difference may be determined essentially as disclosed above, but where at least the first primary phase difference may be determined for each pair of wireless units.

[0239] At least a primary signal may advantageously be repeatedly transmitted from the first radio unit 104 at 012. The repeatedly transmitted primary signal may be received (014) by a plurality of radio units that did not transmit, and phase information may be determined at each of them. At least a plurality of primary signals may be repeatedly transmitted in sequence at 016 by the remaining radio units that did not transmit at 012, preferably in a predetermined order and time slot. The repeatedly transmitted primary signals may be received (018) by a radio unit that did not transmit at 016, and phase information for each of them may be determined. This may result in a plurality of pairs of radio units that transmit and receive signals corresponding to the repeated primary signal to each other.

[0240] At 020, a recursive primary phase difference corresponding to the repeated primary signal may be determined.

[0241] In some embodiments, an LO frequency offset corresponding to a frequency offset between local oscillators in at least one pair of radio units may be determined at 022. The LO frequency offset may be determined for some or all of the pair of radio units.

[0242] In some embodiments, at 024, the Doppler frequency for at least one (or some or all) pairs of wireless units may be determined.

[0243] At 026, an approximate clock offset for each pair of wireless units may be obtained or determined. A maximum error in the approximate clock offset may be obtained or estimated such that a set of candidate clock offset values ​​is determined for each pair of wireless units, thereby determining a plurality of candidate sets of clock offset values ​​(028). The maximum error in the approximate clock offset may be determined based on a maximum error in the determined phase difference. Here, the maximum error in the determined phase difference may be determined based on a maximum error in the phase information or a maximum error in one or more phase measurements.

[0244] At 030, at least a first auxiliary signal (in a second frequency range) may be transmitted by the first radio unit 104. At 032, the transmitted first auxiliary signal may be received by multiple radio units that did not transmit an auxiliary signal at 030, and phase information may be determined at each radio unit.

[0245] At least the first auxiliary signal may be transmitted sequentially from the remaining plurality of wireless units at 034, preferably in a predetermined order and time slot. At 036, the first auxiliary signal may be received by the plurality of wireless units that did not transmit at 034, and phase information may be determined from each of them. This may provide a set of wireless units that have at least mutually transmitted and received signals corresponding to the first auxiliary (response) signal.

[0246] At 038, at least a first auxiliary phase difference may be determined.

[0247] Steps 030-038 may be repeated for the second, third and subsequent auxiliary signals. Accordingly, steps corresponding to steps 002-010 and / or 012-020 may also be repeated for further (such as the third) primary signals.

[0248] A most likely clock offset value is then selected from the set of candidate clock offset values ​​(040). Preferably, a most likely clock offset value is determined for each pair of radio units. Selection of the most likely clock offset value may be performed for the pair of radio units according to the procedure described above for two radio units.

[0249] If the most likely appropriate clock offset value cannot be determined unambiguously, a third frequency range may be selected for further transmitting an auxiliary signal in the third frequency range, as previously described herein.

[0250] Obtaining the approximate clock offset value, the maximum error in the approximate clock offset value, and the maximum error in the phase difference, determining the set of clock offset values, and selecting the most suitable clock offset value can be performed for embodiments of the present invention utilizing multiple radio units in a manner similar to that disclosed herein for the two radio unit case, as will be understood by those skilled in the art.

[0251] 11 shows a signal transmission method according to an embodiment of the present invention. The horizontal axis represents time, and the vertical axis represents frequency. Here, one or more primary signals and one or more auxiliary signals may be transmitted substantially simultaneously (i.e., transmitted together by one radio unit). Here, one or more primary signals are transmitted in a first frequency range f aand the auxiliary signal is contained in a second frequency range f b Included in f a and f b may be separated by Δf.

[0252] The repeated primary signal (if utilized) and in some embodiments the repeated auxiliary signal(s) may be transmitted at a time that is different from the time that the primary signal(s) and auxiliary signal(s) are transmitted, preferably this time difference being greater than the time difference between the transmission of a signal by one radio unit and the transmission of a corresponding signal by another radio unit.

[0253] Although the present invention has been described with reference to the above embodiments and some advantages of the present invention have been shown, it is not intended that the present invention be limited to these embodiments, but rather to encompass all possible embodiments within the spirit and scope of the inventive concept and the scope of the following claims.

[0254] Unless expressly stated otherwise, the features recited in the dependent claims may be freely combined with one another.

Claims

1. 1. A method for determining a clock offset between local clocks for at least one pair of radio units including at least a first radio unit and a second radio unit, the method comprising: a) performing a first bidirectional transmission between at least one pair of radio units using a first signal including a selected first frequency, the first bidirectional transmission being transmitted as a broadcast and received by at least one radio unit not transmitting the broadcast to obtain the at least one pair of radio units; b) determining first phase information for the received first signal; c) for each pair of radio units, determining a first phase difference as a difference between the first phase information determined for each of the radio units in the pair; d) performing a second or subsequent bidirectional transmission between the at least one pair of radio units using a second or subsequent signal including the selected second or subsequent frequency; e) determining second or subsequent phase information for the second or subsequent signal received in the second or subsequent bidirectional transmission; f) determining, for each pair of wireless units, a second or subsequent phase difference as a difference between the second or subsequent phase information determined for each of the wireless units in the pair; g) determining the difference between the first phase difference and the second or subsequent phase difference, or the difference between the phase difference determined at the highest or lowest signal frequency and a subsequent phase difference; h) for each of the radio unit pairs, determining at least one clock offset variable indicative of a clock offset estimate between the radio units in the radio unit pair based on the difference determined in step g; i) determining an estimated maximum error of the determined clock offset variable based at least on a maximum error of the first phase difference and a maximum error of the second and subsequent phase differences; j) determining whether the maximum error of the clock offset variable allows the clock offset to be determined unambiguously by determining a set of candidate clock offset values ​​obtained by varying the clock offset corresponding to an integer number of half-cycle period variations at the first frequency or the subsequent frequency, the set of clock offset values ​​being limited by the estimated maximum error of the determined clock offset variable; k) if it is determined that the clock offset cannot be uniquely determined, repeating steps dj using a selected next frequency that differs from the first frequency by an amount greater than the difference between the first frequency and the second frequency or a previously used frequency; A method comprising:

2. 2. The method of claim 1, wherein the signals by at least some of the different radio units are transmitted sequentially in a predetermined order, with each sequentially transmitting radio unit transmitting its respective signal in its own predetermined time slot.

3. 2. The method of claim 1, wherein the set of candidate clock offset values ​​is based on a variation in the clock offset corresponding to an integer number of half-period variations in the highest frequency used.

4. 2. The method of claim 1, wherein the clock offset is determined based on at least one of the determined phase differences, and possibly based on a plurality of the determined phase differences, or all of the determined phase differences.

5. 2. The method of claim 1, comprising selecting a frequency of the second or subsequent signal by determining a possible range of the clock offset variable based on a maximum error of the clock offset variable, and selecting the second or subsequent frequencies such that expected minimum and maximum values ​​of the second or subsequent phase differences corresponding to minimum and maximum values ​​of the clock offset variable do not differ by more than a threshold of 2π.

6. 10. The method of claim 1, comprising: performing bidirectional transmissions between a plurality of radio units; and determining a plurality of clock offsets between pairs of radio units.

7. The method of claim 1 , wherein one of the wireless units is selected as a reference unit.

8. 2. The method of claim 1, comprising unambiguously determining a clock offset at least once in an integer ambiguity mode, and then repeatedly transmitting subsequent signals in a tracking mode at selected time intervals and optionally in selected frequency ranges to determine subsequent phase differences, and repeatedly determining clock offset information indicative of changes in clock offset between the first and second wireless units during the selected time intervals.

9. 2. The method of claim 1, comprising obtaining or determining an interim clock offset variable as a first approximation of the clock offset to determine the maximum possible value of the clock offset.

10. and resolving integer ambiguities by performing bidirectional transmissions in at least two frequency ranges to determine the set of clock offset values ​​and determining the clock offsets, wherein determining the clock offsets includes: transmitting a primary signal having a frequency in a first frequency range, and selecting at least one of the set of one or more candidate clock offset values; performing bidirectional communication utilizing at least a first primary frequency and a second primary frequency; determining at least first and second primary phase information; determining at least a first and a second primary phase difference; determining a first clock offset variable and an estimated maximum error thereof, optionally based on the first and second primary phase differences and a maximum error thereof; determining a set of candidate clock offset values ​​based on the first clock offset variable and its estimated maximum error; and to decide through; transmitting one or more auxiliary signals including at least one frequency in a second frequency range, and adjusting the clock offset to: performing two-way communication utilizing at least a first auxiliary frequency; determining at least first auxiliary phase information; determining at least a first auxiliary phase difference; determining a second clock offset variable and its estimated maximum error based on the first primary phase difference and the first auxiliary phase difference and their maximum errors; determining the clock offset based on a most likely clock offset value selected from the set of candidate clock offset values, the most likely clock offset value being selected to fit an error margin in the second clock offset variable; and to decide through; Including, 2. The method of claim 1, further comprising: determining whether the selected most likely clock offset value can be uniquely selected from the set of candidate clock offset values; and, if not, transmitting one or more second or subsequent auxiliary signals including frequencies in third or subsequent frequency ranges.

11. 11. The method of claim 10, comprising transmitting a plurality of primary signals and further comprising transmitting a plurality of auxiliary signals, wherein the frequencies of at least consecutive primary signals and / or consecutive auxiliary signals are separated from each other by less than 20 MHz.

12. 11. The method of claim 10, wherein the difference between the first frequency range and the second frequency range or the third or subsequent frequency range is at least 150 MHz.

13. 11. The method of claim 10, wherein the first frequency range and / or the second frequency range encompass a maximum bandwidth of 100 Hz-100 kHz if only one signal is transmitted in the range, or a maximum bandwidth of 5-100 MHz if multiple signals are transmitted in the range.

14. 10. The method of claim 1, comprising transmitting at least two signals at least partially simultaneously by one radio unit.

15. 10. The method of claim 1, the first radio unit is a master unit, and the remaining radio units including at least the second radio unit are slave units, and the master unit is configured to transmit the first signal; the master unit is configured to check in each measurement cycle whether a radio channel is free for transmission before transmitting the first signal, and if the radio channel is free, at least the first signal is transmitted, and if the radio channel is not free, the transmission is not performed; the slave unit is configured to, before transmitting a signal in a measurement cycle, determine whether a previous wireless unit in a predetermined order of the multiple wireless units has transmitted a signal in the measurement cycle, and, if it is determined that a previous wireless unit has transmitted a signal, transmit the signal; method.

16. 2. The method of claim 1, comprising determining a clock rate difference between at least the first wireless unit and the second wireless unit, and taking the clock rate difference into account when determining a clock offset.

17. 2. The method of claim 1, further comprising determining a Doppler frequency resulting from relative motion between at least the first radio unit and the second radio unit, and taking the Doppler frequency into account in determining a clock offset.

18. 10. The method of claim 1, comprising transmitting signals in one or more time slots within a measurement frame and transmitting data in one or more time slots within a communication frame.

19. The method of claim 1 , wherein the signal comprises a sine wave, and in some embodiments a sine wave with a scrambling code.

20. 1. A system for determining a clock offset between at least a first radio unit and a second radio unit, the system comprising: at least a first radio unit, a second radio unit, and at least one processor; a) performing a first bidirectional transmission between at least one pair of wireless units using a first signal including a selected first frequency, the first bidirectional transmission being transmitted as a broadcast and received by at least one wireless unit not transmitting the broadcast to obtain the at least one pair of wireless units; b) determining first phase information for the received first signal; c) determining, for each pair of wireless units, a first phase difference as a difference between the first phase information determined for each of the wireless units in the pair; d) performing a second or subsequent bidirectional transmission between the at least one pair of radio units using a second or subsequent signal including the selected second or subsequent frequency; and e) determining second or subsequent phase information for the second or subsequent signal received in the second or subsequent bidirectional transmission; f) for each pair of wireless units, determining a second or subsequent phase difference as a difference between the second or subsequent phase information determined for each of the wireless units in the pair; g) determining the difference between the first phase difference and the second or subsequent phase difference, or the difference between the phase difference determined at the highest or lowest signal frequency and a subsequent phase difference; h) for each of the radio unit pairs, determining at least one clock offset variable indicative of a clock offset estimate between the radio units in the radio unit pair based on the difference determined in step g; i) determining an estimated maximum error of the determined clock offset variable based at least on the maximum error of the first phase difference and the maximum errors of the second and subsequent phase differences; j) determining whether a maximum error of the clock offset variable allows a clock offset to be determined unambiguously by determining a set of candidate clock offset values ​​obtained by varying the clock offset corresponding to an integer number of half-cycle period variations at the first frequency or the subsequent frequency, the set of clock offset values ​​being limited by the estimated maximum error of the determined clock offset variable; k) if it is determined that the clock offset cannot be uniquely determined, repeating steps dj using a selected next frequency that differs from the first frequency by an amount greater than the difference between the first frequency and the second frequency or a previously used frequency; A system configured to perform the steps of:

21. A computer program comprising program instructions arranged, when executed by a processing means of a system, to cause said system to perform a method according to any of claims 1 to 19.

22. 20. A system comprising processing means and storage means having stored thereon program instructions which, when executed by said processing means, are configured to cause said system to perform a method according to any one of claims 1 to 19.

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