Patent Description:
Generally, for network services that use time division multiplexing (TDM) technologies such as synchronous optical network (SONET), synchronous digital hierarchy (SDH) or asynchronous transfer mode (ATM), it is necessary to match clock frequencies between network systems, in other words, to synchronize networks, so that data can be transmitted and received. Synchronization technologies are fundamental network service technologies that are widely used by telecommunications carriers around the world. The standardization of these technologies has been ongoing for many years in international organizations. For <NUM>-NR, the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) defined <NUM> microseconds (+/- <NUM> microseconds end-to-end) as the synchronization accuracy requirement for the next generation Node B (gNB).

In the case of time difference of arrival (TDOA) based positioning system, the network synchronization error can be a critical issue since the error is added to the TDOA measurement and further translates into high position error. For instance, having a +/- <NUM> microseconds end-to-end requirement, the error translates to <NUM> meters of error. As commercial <NUM> positioning use cases that have a horizontal positioning requirement of <NUM> meters accuracy and the US Federal Communications Commission (FCC) regulatory requirements mandate <NUM> meters horizontal accuracy, the TDOA based positioning is severely affected by the listed synchronization error.

The synchronization error of the network could be reduced, for instance by installing high precision oscillators and synchronization devices which leads to a cost-intensive setup. A Round Trip Time (RTT) measurement method or Global Navigation Satellite System (GNSS) can also be introduced in order to minimize the synchronization error at the cost of additional network resources.

For example, the document <CIT> shows a position determination method of wireless communication equipment for correcting the influence of the distance error. A hybrid of satellite based Global Positioning System (GPS) is utilized on a mobile communication network in order to minimize the delay influence. However, GPS-based positioning is not suitable to establish indoor locations, since microwaves will be considerably attenuated and scattered by roofs, walls and other objects. In addition, the implementation of GPS-based positioning on the mobile network requires additional network resources.

Other examples of prior art include, document <CIT> which discloses base station synchronization, document <CIT> which discloses relative timing measurements for supporting positioning and document <CIT> which discloses joint processing between base stations using a synchronization parameter.

Accordingly, the object of the invention is to provide a system and a method for network synchronization correction in a cost-effective manner without consuming additional network resources, especially to address both regulatory and commercial positioning requirements in <NUM>-NR.

The object is solved by the features of the first independent claim for the system and by the features of the second independent claim for the method. The dependent claims contain further developments.

According to a first aspect of the invention, a system for network synchronization correction is provided. The system comprises at least two base stations and a user equipment. In this context, the base stations are adapted to transmit a reference signal. Furthermore, at least one of the base stations is adapted to receive the reference signal of the other base station and is adapted to calculate a time of arrival of the reference signal. Therefore, a base station, for instance a gNB in <NUM>-NR network, transmits a reference signal as well as listens to the reference signals from neighbor base stations. Since the base station precisely knows at which time instant the neighbor base station has transmitted the signal, it can thereby calculate the time of arrival (TOA) of the signal from the neighbor.

The reference signal can be any of the conventional transmitted signals from the base station, for instance Positioning Reference Signals (PRS), Primary Synchronization Signals (PSS), Secondary Synchronization signals (SSS) and the like. Alternatively, all of these signals may be used for more flexibility since the user equipment knows at which point in time and on which carrier the signal may come.

According to a first implementation form of said first aspect of the invention, the distance between the base stations are known. The position of the base stations can be defined according to a set of parameters standardized by the World Geodetic System (WGS), for instance WGS <NUM> in order to effectively identify the coordinates and derived constants. Advantageously, a pre-determined distance between two base stations can be achieved in the case of millimeter wave usage for <NUM> technology.

According to a second implementation form of said first aspect of the invention, the at least one of the base stations is further adapted to calculate a synchronization error between the two base stations based on the known distance and the calculated time of arrival of the reference signal. In this context, the base station calculates a relative distance based on the known distance from the neighbor base station and the TOA calculations. With the difference between the known distance and the relative distance, the base station further calculates the synchronization error in the form of, for instance a time offset with respect to the neighbor base station.

According to a further implementation form of said first aspect of the invention, the user equipment is adapted to receive the respective reference signals from the base stations and is further adapted to measure a time difference of arrival between the reference signals. Thus, the user equipment measures the time interval that is observed between the reception of downlink signals from the two base stations.

According to a further implementation form of said first aspect of the invention, the at least one of the base stations is further adapted to transmit the synchronization error to the user equipment and whereby the user equipment is further adapted to optimize the measured time difference of arrival between the reference signals based on the synchronization error. Advantageously, the base station sends the synchronization error to the user equipment, for instance in the form of an assistance data, and the user equipment corrects the TDOA measurement based on the synchronization error.

According to a further implementation form of said first aspect of the invention, the user equipment is further adapted to transmit the measured time difference of arrival between the reference signals to at least one of the base stations and whereby the base station is further adapted to optimize the synchronization error based on the measured time difference of arrival between the reference signals. In addition to the user equipment correcting the TDOA measurements, the network is advantageously able to correct the TDOA measurements on its own. Here, the user equipment sends the TDOA measurements to the base station where the base station corrects the synchronization error.

According to a further implementation form of said first aspect of the invention, the user equipment is further adapted to perform at least three time difference of arrival measurements between the reference signals of the base stations. Advantageously, a three-dimensional (3D) position calculation is performed.

According to a further implementation form of said first aspect of the invention, the system further comprises a synchronization error database and wherein the synchronization error is stored within the database. The database can be a physical or logical entity, for instance a Location Server (LCS), which contains and manages the synchronization error data.

According to a further implementation form of said first aspect of the invention, the synchronization error database is updated periodically. Advantageously, the data update rate is effectively matched with respect to the gradual fluctuation rate of synchronization error due to, for instance the drift of the oscillator clock of the base station.

According to a second aspect of the invention, a method for network synchronization correction comprising at least two base stations and a user equipment is provided. The method comprises the steps of transmitting a reference signal from the base stations, receiving the reference signal by at least one of the base stations and calculating a time of arrival of the reference signal. Therefore, a base station, for instance a gNB in <NUM>-NR network, transmits a reference signal as well as listen to the reference signals from neighbor base stations. Since the base station precisely knows at which time instant the neighbor base station has transmitted the signal, it can thereby calculate the time of arrival (TOA) of the signal from the neighbor.

According to a first implementation form of said second aspect of the invention, the distances between the base stations are known. Advantageously, a pre-determined distance between two base stations can be achieved in the case of millimeter wave usage for <NUM> technology.

According to a second implementation form of said second aspect of the invention, the method further comprises the step of calculating a synchronization error between the two base stations based on the known distance and the calculated time of arrival of the reference signal. Hence, the base station calculates a relative distance based on the known distance and the TOA calculations and thereby calculates the synchronization error in the form of, for instance a time offset with respect to the neighbor base station.

According to a further implementation form of said second aspect of the invention, the method further comprises the steps of receiving the respective reference signals by the user equipment from the base stations and measuring a time difference of arrival between the reference signals. Thus, the user equipment measures the time interval that is observed between the reception of downlink signals from the two base stations.

According to a further implementation form of said second aspect of the invention, the method further comprises the steps of transmitting the synchronization error to the user equipment and optimizing the measured time difference of arrival between the reference signals based on the synchronization error by the user equipment. Advantageously, the base station sends the synchronization error to the user equipment, for instance in the form of an assistance data, and the user equipment corrects the TDOA measurement based on the synchronization error.

According to a further implementation form of said second aspect of the invention, the method further comprises the steps of transmitting the measured time difference of arrival between the reference signals to at least one of the base stations and optimizing the synchronization error based on the measured time difference of arrival between the reference signals by the base station. In addition to the user equipment correcting the TDOA measurements, the network is advantageously able to correct the TDOA measurements on its own.

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. However, the following embodiments of the present invention may be variously modified and the range of the present invention is not limited by the following embodiments.

In <FIG>, a first exemplary embodiment of the system <NUM> according to the first aspect of the invention is illustrated. The system <NUM> comprises two base stations <NUM>, <NUM> and a user equipment <NUM>. Each base station transmits a reference signal <NUM>, for instance PRS, PSS, SSS and so on. The base stations <NUM>, <NUM> are separated by a pre-defined distance D. Typically, the reference signal <NUM> transmitted by each base station <NUM>, <NUM> is received by the user equipment <NUM> through downlink transmission paths <NUM>, <NUM>. The user equipment <NUM> then performs the TDOA measurements based on the reference signals <NUM> received from each of the base stations <NUM>, <NUM>.

In addition, each base station <NUM>, <NUM> further listens for the neighboring reference signals <NUM> and receives the reference signal <NUM> from its neighbor, for instance through signal path <NUM>. The base station <NUM>, <NUM> then calculates the TOA of the reference signal <NUM> from its neighbor. With the help of TOA measurements, the base station <NUM>, <NUM> calculates a relative distance D̂. With the difference between the known distance D and the relative distance D̂, the base station <NUM>, <NUM> calculates the time offset as the synchronization error with respect to its neighbor.

Advantageously, the system <NUM> allows two possibilities to correct the network synchronization, one is handled by the user equipment <NUM> and the other is performed by the network itself. In the former case, the base station <NUM>, <NUM> transmits the time offset to the user equipment <NUM> as an assistance data and the user equipment <NUM> corrects the TDOA measurement. In the case where the network corrects the synchronization error, the user equipment <NUM> transmits the TDOA measurement, for instance through the uplink path <NUM> to the base stations <NUM> and the base station <NUM> corrects the synchronization error based on the TDOA measurement.

The system <NUM> further comprises a synchronization error database <NUM>, connected to the base stations <NUM>, <NUM>, where the error data are stored and are periodically updated. The system <NUM> effectively implements TDOA based positioning, which does not consume additional network resources. Hence, the system <NUM> provides a cost-effective solution for network synchronization in order to reduce network synchronization error.

It is to be noted, the user equipment <NUM> denotes a mobile or fixed type user terminal that transmit and receive user data and control information to and from a base station. The user equipment <NUM> may be referred to as a Terminal Equipment (TE), a Mobile Station (MS), a Mobile Terminal (MT), a User Terminal (UT), a Subscriber Station (SS), a wireless device, a Personal Digital Assistant (PDA), a wireless modem, or a handheld device. Furthermore, the base station <NUM>, <NUM> denotes a fixed station that performs communication with a user equipment <NUM> and/or another base stations <NUM>, <NUM>, and exchanges various kinds of data and control information with the user equipment <NUM> and another base station <NUM>, <NUM>. The base station may be referred to as another terminology such as an Access Point (AP) or a next generation Node B (gNB).

In <FIG>, exemplary block diagrams of the user equipment and base station according to the first aspect of the invention are illustrated. The user equipment <NUM> serves as a transmitter on the uplink and as a receiver on the downlink. In contrast, the base station <NUM> may serve as a receiver on the uplink and as a transmitter on the downlink. Although only one base station <NUM> is illustrated in <FIG>, the features and functionality as described herein hold true to the other base station <NUM>.

The user equipment <NUM> and the base station <NUM> comprise antennas <NUM>, <NUM> for transmitting and/or receiving data and control signals. In particular, the base station antenna <NUM> transmits reference signals to the user equipment <NUM> and further receives reference signals from any neighbor base station. Both antennas <NUM>, <NUM> are connected to transmit/receive circuitry <NUM>, <NUM> followed by processing units <NUM>, <NUM>. The processing unit <NUM> of the base station <NUM> calculates time offsets from neighboring reference signals and transmits the time offset to the user equipment <NUM>. In addition to the synchronization error database illustrated in <FIG>, the base station <NUM> may further comprise memory <NUM> in order to queue the processed calculations from the processing unit <NUM>. The base station <NUM> further stores the TDOA measurements received from the user equipment <NUM> if the network is performing the synchronization correction on its own.

The processing unit <NUM> of the user equipment <NUM> performs TDOA measurements based on the received reference signals and stores these results in a memory <NUM>. The user equipment <NUM> further stores the time offset measurements as received from the base station <NUM> if the user equipment <NUM> is performing the network synchronization correction. The processing units <NUM>, <NUM> may also be referred to as microcontrollers, microprocessors, microcomputers, etc. The processing units <NUM>, <NUM> may be configured in hardware, firmware, software, or their combination.

The memories <NUM>, <NUM> may further store programs required for signal processing and controlling of the processing units <NUM>, <NUM> and temporarily store input and output information. Each of the memories <NUM>, <NUM> may be implemented into a flash memory-type storage medium, a hard disc-type storage medium, a multimedia card micro-type storage medium, a card-type memory, a Random Access Memory (RAM), a Read-Only Memory (ROM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a Programmable Read-Only Memory (PROM), a magnetic memory, a magnetic disc, or an optical disk.

In <FIG>, a second exemplary embodiment of the system <NUM> according to the first aspect of the invention is illustrated. The system <NUM> comprises three base stations, separated by predefined distances D<NUM>,D<NUM>,D<NUM>. Each of the base stations are identical to the base stations illustrated in <FIG> or <FIG> and transmits a reference signal <NUM>. In order to perform position calculation, the user equipment <NUM> is required to perform TDOA measurements for each pair of the base stations with respect to their known distances D<NUM>,D<NUM>,D<NUM>. It is also necessary to obtain at least three TDOA measurements to calculate a 3D position. In this case, the time offsets for the pairs of base stations are required by the user equipment <NUM> to effectively correct the TDOA measurements.

It is to be noted that an absolute synchronization error for the pairs of base stations is not possible to calculate. It is only possible to calculate the relative synchronization error between two base stations.

In <FIG>, a flow chart of an exemplary embodiment of the inventive method according to the second aspect of the invention is illustrated. In a first step <NUM>, a reference signal is transmitted from the base stations. In a second step <NUM>, the reference signal is received by at least one of the base station. In a third step <NUM>, a time of arrival of the reference signal is calculated.

In addition to this, the distance between the base stations are known and the inventive method may further comprise the step of calculating a synchronization error between the two base stations based on the known distance and the calculated time of arrival of the reference signal.

It might be further advantageous if the method further comprises the steps of receiving the respective reference signals by the user equipment from the base stations and measuring a time difference of arrival between the reference signals.

Moreover, the method according to the second aspect of the invention may further comprise the steps of transmitting the synchronization error to the user equipment and optimizing the measured time difference of arrival between the reference signals based on the synchronization error by the user equipment.

In addition to this, the inventive method may further comprise the steps of transmitting the measured time difference of arrival between the reference signals to at least one of the base stations and optimizing the synchronization error based on the measured time difference of arrival between the reference signals by the base station.

Claim 1:
A system (<NUM>) for network synchronization correction comprising:
at least two base stations (<NUM>, <NUM>), and
a user equipment (<NUM>),
wherein the base stations (<NUM>, <NUM>) are adapted to transmit a reference signal (<NUM>),
wherein at least one of the base stations (<NUM>, <NUM>) is adapted to receive the reference signal (<NUM>) of the other base station (<NUM>, <NUM>) and is adapted to calculate a time of arrival of the reference signal (<NUM>), whereby a time instant at which the other base station (<NUM>, <NUM>) transmits the reference signal (<NUM>) is known to the at least one of the base stations (<NUM>, <NUM>),
wherein the at least one of the base stations (<NUM>, <NUM>) is adapted to calculate a relative distance with the help of the calculated time of arrival of the reference signal (<NUM>),
wherein the at least one of the base stations (<NUM>, <NUM>) is further adapted to calculate a time offset as a synchronization error with respect to its neighbor with the difference between a known distance (D) between the base stations (<NUM>, <NUM>) and the relative distance,
wherein the user equipment (<NUM>) is adapted to receive the respective reference signals (<NUM>) from the base stations (<NUM>, <NUM>) and is further adapted to measure a time difference of arrival between the reference signals (<NUM>), and wherein the at least one of the base stations (<NUM>, <NUM>) is further adapted to transmit the synchronization error to the user equipment (<NUM>) as assistance data and whereby the user equipment (<NUM>) is further adapted to correct the measured time difference of arrival between the reference signals (<NUM>) based on the synchronization error.