WO2016181197A1

WO2016181197A1 - High-accuracy round trip time (rtt) ranging

Info

Publication number: WO2016181197A1
Application number: PCT/IB2015/055448
Authority: WO
Inventors: Garry Irvine; Magnus Sandgren; Peter Jakobsson; Michael Breschel; Chunhui Zhang; Mikael OLOFSSON
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-05-14
Filing date: 2015-07-17
Publication date: 2016-11-17

Abstract

A method and system for determining a distance between a first base station and a second base station. One method includes receiving from the first base station a first radio signal transmitted from the first base station at a first timestamp, T1. A second timestamp, T2, indicating a time of receipt of the first radio signal at the second base station is determined. A third timestamp, T3, indicating a time of transmission of a second radio signal from the second base station is determined. The second radio signal is transmitted to the first base station. A value of a fourth timestamp, T4, is sent to the second base station, the fourth timestamp being a time of receipt at the first base station of the second radio signal. A distance between the first base station and the second base station is computed based on T1, T2, T3 and T4.

Description

HIGH-ACCURACY ROUND TRIP TIME (RTT) RANGING

TECHNICAL FIELD

A method and system for determining distance between base stations using round trip delay. BACKGROUND

Third generation partnership project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations, e.g., eNodeBs, to wireless devices such as mobile stations (also referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub- carriers in the frequency domain. The basic unit of transmission in LTE is a resource block (RB) which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot). As shown in the LTE physical resource diagram of FIG. 1 , a unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE). Thus, an RB consists of 84 REs. An LTE radio subframe is composed of two slots in time and multiple resource blocks in frequency with the number of RBs determining the bandwidth of the system (see FIG. 2). Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75 and 100 RB pairs. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe = 1 ms.

The signal transmitted by the eNB in a downlink, which is the link carrying transmissions from the eNB to the UE, may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols. In Rel-11 of the LTE standards and in prior releases of the LTE standards, there are multiple types of reference symbols. The common reference symbols are used for channel estimation during demodulation of control and data messages in addition to synchronization. The common reference symbols occur once every subframe. These reference symbols are shown in FIG. 2.

Heterogeneous networks, where the macro cells and the small cells have vastly different transmit powers, may be deployed in two main ways. In the first deployment type, the small cell layer and the macro cell layer share the same carrier frequencies which creates interference between the two layers. In the second deployment type, the small cell layer and macro cell layer are on separate frequencies.

Referring to FIG. 3, the network architecture for LTE allows messages to be sent between eNBs 11 via an X2 interface. An eNB 11 also can communicate with other nodes in the network, e.g., to the Mobility Management Entity (MME) 14 via the SI interface. In current LTE specifications, methods are specified that allow some self-organizing network (SON) functionality where an eNB 11 can request information regarding another eNB, e.g., eNB 8 via the MME 14.

High-accuracy ranging, i.e., distance determination, capability between base station antennas has various applications. For PICO infrastructures (associated with small cells), ranging capability can be used to establish relative position information between a network of PICOs in a local area such as a floor of a building. This is useful to avoid the need to manually survey every PICO station. Other applications of high-accuracy ranging are to automatically survey planned incremental modifications to the PICO network (additions/deletions/changed- location); and using a periodic survey monitor for unexpected changes in the network.

High-accuracy ranging can generally be viewed as a foundation capability to reduce the complexity of PICO station deployment and maintenance. Due to the relatively small dimensions associated with indoor cells and hence inter-PICO station distances, high accuracy is important (ideally on the order of a few meters). An inter- PICO positioning survey can be scheduled as an initial deployment activity and/or as a periodic activity to ensure network integrity. Periodic surveying may lead to undesired interruptions in service.

There currently is inadequate ranging functionality between PICO stations supported by LTE. The position of PICO stations within an indoor coverage area must be manually entered, which is a tedious process subject to human error. PICO station positioning is needed to support mobile device positioning services dependent on PICO station infrastructure, as well as for specific PICO station maintenance. Incremental changes to the PICO network must be manually surveyed. Currently, there is no ability to monitor the position of PICO stations post-deployment other than through manual inspection.

SUMMARY

The present embodiments advantageously provide a method and system for determining a distance between a first base station and a second base station.

According to one aspect, a method includes receiving from the first base station a first radio signal transmitted from the first base station at a first timestamp, Tl. A second timestamp, T2, indicating a time of receipt of the first radio signal at the second base station is determined. Further, a third timestamp, T3, indicating a time of transmission of a second radio signal from the second base station is determined. The method includes transmitting the second radio signal to the first base station. The method further includes receiving from the first base station a value of a fourth timestamp, T4, the fourth timestamp, T4, being a time of receipt at the first base station of the second radio signal. A distance between the first base station and the second base station is computed as C*((T4-Tl)-(T3-T2))/2, where C is the speed of light.

According to this aspect, in some embodiments, the computing is performed at a ranging server. In some embodiments, the ranging server comprises the second base station. In some embodiments, the first and second radio signals are position reference signals, PRSs. In some embodiments, the timestamps Tl and T3 are determined from a PRS transmission schedule. In some embodiments, an accuracy of determining a timestamp is increased by observing an internal round trip delay within a base station to compensate for processing delays internal to the base station. In some embodiments, observing the internal round trip delay includes coupling the transmitted radio signal to the receiver, time stamping the transmitted radio signal, and simultaneously time stamping the receipt of the coupled transmitted radio signal in the receiver. In some embodiments, the method further includes steering a null to suppress a beam from at least a first direction to increase a signal to noise plus interference ratio of a line of sight beam from a second direction.

According to another aspect, some embodiments include a central node configured to facilitate computation of a distance between a first base station and a second base station. The central node includes a receiver. The receiver is configured to: receive a first timestamp, Tl, indicative of a time at which a first radio signal is transmitted from the second base station; receive a second timestamp, T2, indicating a time of receipt of the first radio signal at the first base station; receive a third timestamp, T3, indicating a time of transmission of a second radio signal from the first base station; and receive a fourth timestamp, T4, the fourth timestamp, T4, being a time of receipt at the second base station of the second radio signal

According to this aspect, in some embodiments, the central node includes a processor to compute a position of at least one of the first base station and the second base station. In some embodiments, the central node is located at one of the first base station and the second base station. In some embodiments, the first and second radio signals are position reference signals, PRSs. In some embodiments, the timestamps Tl and T3 are determined from a PRS transmission schedule. In some embodiments, the time stamps are referenced to an antenna reference point. In some embodiments, at least one timestamp is calibrated to compensate for an internal processing delay of a base station.

According to yet another aspect, some embodiments include a central node configured to facilitate computation of a distance between a first base station and a second base station. The central node includes a receiver module configured to: receive a first timestamp Tl indicative of a time at which a first radio signal is transmitted from the second base station; receive a second timestamp, T2, indicating a time of receipt of the first radio signal at the first base station; receive a third timestamp, T3, indicating a time of transmission of a second radio signal from the first base station; and receive a fourth timestamp, T4, the fourth timestamp, T4, being a time of receipt at the second base station of a second radio signal received from the first base station. The central node also includes a calculator module configured to calculate a distance between the first base station and the second base station. According to this aspect, in some embodiments, the calculator module is further configured to calculate a position of at least one of the first base station and the second base station. In some embodiments, the central node is located at one of the first and second base stations. In some embodiments, the first and second radio signals are position reference signals, PRSs. In some embodiments, the timestamps Tl and T3 are determined from a PRS transmission schedule. In some embodiments, the time stamps are referenced to an antenna reference point. In some embodiments, at least two timestamps are calibrated to compensate for an internal processing delay of a base station.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagram of a unit of one subcarrier and an OFDM symbol;

FIG. 2 is diagram of a subframe having reference symbols;

FIG. 3 is a diagram of a communications network;

FIG. 4A is a diagram of distances between base stations;

FIG. 4B is a diagram using part of FIG. 4A to show triangulating to determine relative positions of base stations;

FIG. 5 is a block diagram of two base stations exchanging position reference signals;

FIG. 6 is a signaling diagram for exchanging reference signals between base stations;

FIG. 7, comprised of FIGS. 7A and 7B, is a block diagram of calibration of base stations in an operational mode;

FIG. 8 is a block diagram of off-line calibration of a base station;

FIG. 9 is a block diagram showing internal timing measurements;

FIG. 10 is a graph showing relative internal timing measurements;

FIG. 11 is a block diagram of an embodiment of a base station constructed as described herein; FIG. 12 is a block diagram of an alternative embodiment of a base station constructed as described herein;

FIG. 13 is a block diagram of a ranging system including a central node; and FIG. 14 is a flowchart of an exemplary process for exchanging time stamp information to determine a range between base stations.

DETAILED DESCRIPTION

Before describing in detail exemplary embodiments that are in accordance with the present disclosure, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to determining ranges between base stations in a wireless communication system. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

As used herein, relational terms, such as "first" and "second," "top" and "bottom," and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements.

Although terminology from 3GPP LTE has been used in this disclosure to describe some embodiments, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. Other wireless systems, including WCDMA, WiMax, UMB and GSM, may also benefit from exploiting the subject matter covered within this disclosure.

Also note that terminology such as base station (or eNodeB) and mobile device (or UE) should be considering non-limiting and does not imply a certain hierarchical relation between elements; in general "base station" (or eNodeB) could be considered as device 1 and "mobile device" (or UE) device 2, and these two devices may communicate with each other over some radio channel. Similarly, when talking about signaling over an X2 or an SI interface, the solutions are not limited to communication between base stations, e.g., eNBs, or between a base station, e.g., eNB, and a Core Network (CN) but the communicating nodes can be any node terminating the interface over which the information described is transmitted.

Note that, in some embodiments, the procedures described herein may be employed to determine range between two base stations, between two mobile devices, e.g., UEs, or between a base station and a mobile device, e.g., UE. Thus, generally, ranging can be achieved between two nodes, where a node can be a base station or a mobile device, e.g., UE. For example, the mobile device, e.g., UE, may be employed as a relay node between two base stations, between two mobile devices, e.g., UEs, or between a base station and a mobile device, e.g., UE.

Although references to PICOs or PICO stations are made herein, the embodiments and methods disclose herein are applicable to base stations, generally. Thus, term "base station" as used herein encompasses PICO stations.

According to some embodiments, ranging between a first node and a second node is achieved by sending radio signals over the air between the nodes. As described herein, in some embodiments, the first node may be a source node which may provide the timing base to which the second node, which may be referred to as the target node, is to be synchronized. Thus, although reference is made between ranging between a source node and a target node which are time-synchronized, it will be understood that the methods for ranging described herein relate to first and second nodes, generally, which may or may not be time-synchronized. Further, although reference may be made to first and fourth time stamps associated with a source node and second and third time stamps associated with a target node, in some

embodiments, the first and fourth time stamps may be associated with the target node and the second and third time stamps may be associated with the source node.

Under this arrangement, the nodes, e.g., base stations, include receivers that monitor the downlink transmissions, e.g., a downlink reference signal, from other nodes, e.g., base stations, to extract the signal used for ranging. Such an arrangement allows bidirectional measurement of signals to allow for ranging. According to principles of the present disclosure, a target node clock is synchronized with a source node clock with a clock offset that compensates or takes into account the propagation delay from the source node to the target node. In some embodiments, the clock offset is determined based on the estimated round-trip propagation delay (also referred to as a Round-Trip Time or RTT) between the source and the target nodes. For example, the clock offset may be determined based on the transmission and reception times of a first radio signal transmitted from the source node to the second node as well as a second radio signal transmitted from the target node to the source node. In other embodiments, the clock offset is determined based on a "one-way" propagation delay (e.g. delay of the radio signal transmissions) which can be estimated by taking half of the estimated RTT between the source and the target nodes. With an estimated propagation delay, the clock offset can then be determined based the transmission/reception times associated with a given radio or reference signal between the source node and the target node, assuming substantially equal propagation time for both forward and return paths.

In some embodiments, the radio signals used for estimating the one-way propagation delay (or RTT) include periodic reference signals transmitted using common time and frequency resources. In that scenario, the reference signals may be configured with a muting pattern such that each of the source and target node can mute at least one of their reference signal transmissions to properly receive a reference signal transmission from the correspondent node with which

synchronization is performed. Examples of suitable reference signals includes a new or existing reference signal such as a primary synchronization signal (PSS), a secondary synchronization signal (SSS) or a position reference signal (PRS), thereby providing an efficient means, both in terms of wireless and processing resources, to facilitate synchronization and ranging. Because signals such as the PSS, SSS and PRS are already accounted for and exist in the general sense in for example, the current LTE air interface, using these signals for synchronization as described herein does not add a further burden to the air interface. That said, the disclosure is not limited solely to the use of the PRS, PSS or SSS. The embodiments can be implemented using any suitable reference signal. For clarity, the embodiments described herein

In some embodiments, the reference signals exchanged between source/target node pairs use the same frequency band in order to avoid asymmetry in propagation delay which contributes to propagation delay estimation error. In embodiments where the same periodic reference signals are used at the source and target nodes (e.g. PRS, PSS or SSS signals) it is desirable to avoid simultaneous transmission of both reference signals used to determine the propagation delay or RTT . This is because the nodes would have difficulty receiving one of the reference signal transmissions used while actively transmitting the other reference signal transmission. Hence it is beneficial to schedule the reference signal transmissions such that they are mutually orthogonal in the time domain between source/target node pairs (e.g., source/target transmits a reference signal during interval Tn and receives a reference signal from a target/source during interval Tm, where Tn and Tm are different or do not overlap).

In some embodiments where PRS signals are used, scheduling flexibility of the PRS is particularly advantageous for this purpose since a muting pattern can be configured at both the source and target nodes or at all nodes in a particular area or neighborhood such that time domain orthogonality is universally achieved for all source/target node pairs in the neighborhood. Without muting configured, a node supporting PRS transmits its PRS on every PRS occasion which is configured to have periodicity of 160, 320, 640 or 1280ms. With PRS muting configured, a node transmits only during a subset of the configured PRS occasions according to the PRS muting information bits which indicate the active PRS occasions in a 2, 4, 8 or 16 bit cycle (i.e. bit equal to 0 when PRS is muted). In one example, each of the source and target nodes is configured to mute at least one PRS occasion during which it can receive a reference signal transmission used for synchronization. It is understood that in implementations where it is desirable to average the propagation delay over multiple measurements (i.e., using multiple sets of PRS transmissions), a

corresponding number of PRS occasions would need to be muted. Generally, the number of muted PRS occasions at each node is a function of the number of propagation measurements required. By achieving a coarse degree of time- synchronization prior to becoming active transmitters, nodes involved in a synchronization or ranging procedure as described herein are able to achieve time domain orthogonality of their respective PRS transmissions using the appropriate PRS muting configuration. In implementations where muting is used, there is no need to trigger or coordinate individual PRS transmissions since these would occur autonomously as dictated by the PRS configuration used at each node. It is to be understood that the present disclosure is not limited to PRS signals and generally, any signal that are configured to achieve mutual orthogonality in the time domain (e.g., with periodicity and mutability) can be used.

In some embodiments, a reference signal is utilized to achieve more accurate ranging between base stations. For example, in some embodiments, the position reference signal (PRS), which is already in use for over the air time difference of arrival (OTDOA) determination of the location of mobile devices, is used for ranging between base stations to determine base station locations. In traditional use of PRSs, the PRS transmissions of many cells are observed in sub frames by the mobile device. The mobile device then reports relative arrival times between the received PRSs from the different base stations to a serving base station that determines mobile device position from the reported arrival times. As a practical matter, any two base stations involved in bilateral ranging observation should transmit their respective PRS in separate sub frames. Thus, muting may be employed such that only one base station transmits its PRS at a time. Note that in some embodiments, ranging is done only occasionally, whereas synchronization of base stations may be done periodically, i.e., more frequently.

Other reference signals that may be used for ranging between base stations to determine base station positioning may be downlink (from base station to mobile device) signals such as the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). Reuse of 3GPP defined signaling such as the Position- Reference-Signal (PRS) introduced in LTE Release 9 is suited to the task of ranging for several reasons: hearability, scheduling orthogonality, and timing-resolution of the autocorrelation leading to superior Range-Mean-Squared-Error (RMSE). Aside from coordination with supported traffic, another advantage is that (already verified) base station functionality, including baseband processing, is reused.

Relative frequency offset between two base stations involved in a ranging measurement can lead to significant error in the RTT. This impairment can be minimized by coordinating the PRS occasions used to calculate RTT to be as close as possible. PRS transmit occasions can be scheduled to be at 160, 320, 640 or 1280 ms apart, and an optional muting pattern adds further scheduling flexibility. A set of base stations in a coverage area are configured to transmit a reference signal, such as a PRS, for the purpose of ranging between pairs of base stations as shown in FIGS. 4A and 4B. FIG. 4A shows eight exemplary base stations that may be distributed throughout a floor of a building. Of course, the disclosure is not limited to eight base stations, and more or fewer may be implemented, as needed. The distance (Djk) between pairs of the base stations may be established by the methods provided herein. For example, D23 is the distance between base stations 12a (BS 2) and 12b (BS 3). A unique relative position map can be established knowing a significant number of the Djk. Finally, by surveying the position of some of the base stations relative to the coverage area features manually or by other means, the relative position map of all eight base stations can be associated with the coverage area. Referring to FIG. 4B, it can be shown that for any three base stations where the ranges of all three pairs are available, D12, D13, and D23, the spatial orientation of the three base stations in a triangular shape can be established. This process can be extended incrementally for other base station triplets. Then the triangles can be fit together piece-wise, to form the spatial orientation of all base stations in the neighborhood area. This relative position map derived from ranging measurements can also be overlaid on a building plan as a centralized function in order to further optimize the estimated actual position of the base stations.

It is understood that the two-dimensional problem identified in FIGS. 4A and

4B can also be generalized to three dimensions. Certain impediments to 3- dimensional positioning exist; for example, a line of sight (LOS) path between base stations on different floors of a building is likely to be highly attenuated and may be significantly higher in path loss compared to another reflected path.

FIG. 5 illustrates the observations associated with bilateral ranging, D12, between two base stations, 12a and 12b (referred to collectively herein as base stations 12). As a practical matter, two base stations 12a and 12b involved in a bilateral ranging observation must transmit their respective PRS in separate sub frames. Because the PRSs share the same downlink (DL) allocation, PRS

transmissions during a given sub frame will desensitize PRS reception in the same base station over the same sub frame. This prevents the transmission of PRS 1 and reception of PRS 2 in the same sub frame, which can be avoided by configuring appropriate muting patterns. In FIG. 5, the measured timestamps Tl, T2, T3 and T4 may be sent to a ranging server 16 which computes the distance between the base stations 12a and 12b using C*((T4-Tl)-(T3-T2))/2, where C is the speed of light. Tl is the time at which the BS 1, e.g., base station 12a, determines that a first over the air (OTA) message has been sent; T2 is the time at which the BS 2, e.g., base station 12b, determines that the OTA message from the BS 1 has been received; T3 is the time at which the BS 2 determines that a second OTA message has been sent; and T4 is the time at which the BS 1 determines that the second OTA message from the BS 2 has been received. The ranging server 16 of FIG. 5 is shown separate from the base stations 12a (BS 1) and 12b (BS 2), but in some embodiments, the ranging server 16 can be included at a base station 12.

In some embodiments, a target node (e.g., a first base station) synchronizes with a source node (a second base station) by calculating a clock offset based on the transmission and reception times of two reference signals. In one example, a first message (e.g. a first reference signal) is generated and sent over the air from the source node to the target node. At the time of sending such message a time stamp indicative of a time of transmission of the first message is generated by the source node and sent to the target node. Such timestamp is referred to herein as a first timestamp, Tl. Alternatively, the time, Tl, of transmission of the first message can be ascertained independently by the target node based on a known or predetermined transmission schedule for the messages used. At the time of reception of the first message at the target node, a new timestamp is generated by the target node, referred to herein as a second timestamp, T2. Similarly, the target node generates a second message (e.g. a second reference signal) over the air interface towards the source node.

At the time this second message is sent, a third timestamp is generated by the target node, referred to herein as the third timestamp, T3. Alternatively, and similarly to the first timestamp, Tl, the time, T3, of transmission of the second message can be ascertained by the target node independently from a known or predetermined transmission schedule for the messages used. Finally, at the time the second message is received by the source node, a fourth time stamp will be generated by the source node, referred to herein as the fourth timestamp, T4. The naming of Tl, T2, T3 and T4 and source and target node is shown in FIG. 6. It can be appreciated that the node initiating the synchronization procedure described herein may be the source node or the target node. In the example embodiment of FIG. 6, the source node (BS1) has been chosen to initiate the procedure.

It is important to note that the order in which the first and second messages are transmitted can vary. In some embodiments such as shown in FIG. 6, the first message is transmitted before the second message. In other embodiments, the first message is transmitted after the second message. In yet other embodiments, the messages are transmitted concurrently but are non-overlapping in time at each node. Generally, to the extent each of the source and target nodes can properly determine the transmission and reception times Tl, T2, T3, T4 (e.g. with appropriate muting patterns and/or transmission times configured such that each nodes does not simultaneously transmit and receive at the same time or such that the first and second messages are scheduled as non-overlapping transmissions at each node), the order in which the first and second messages are transmitted can vary.

In some embodiments, the node that generated Tl and T4 (in this example the source node) signals such timestamps to the node that generated T2 and T3 (in this example the target node) via a direct or indirect interface. In some examples, Tl and T4 are sent over the air, e.g., encoded in a radio or reference signal transmission to the target node. In other examples, Tl and T4 are sent via a direct source node-to-target node interface (e.g. an X2 interface) or indirectly via an intermediate node (e.g. an MME node) and/or interface (e.g. an SI interface). With the Tl and T4 information, the target node generating T2 and T3 can therefore calculate a clock offset (defined as the difference between the target node clock and the source node clock) as a function of the times Tl, T2, T3, T4. Note also, that the same time stamps used for synchronization can be used for ranging, and vice versa.

In FIG. 6, to ascertain a range between a first base station, BS 1, 12a and a second base station, BS 2, 12b the round trip time (RTT) of propagation of a signal between BS 1 and BS 2 is determined. This RTT is divided by 2 and multiplied by the speed of light traveling between the base stations BS 1 and BS 2. The RTT is determined based on times of transmission and reception of radio signals traveling between the BS 1 12a and BS 2 12b. The round trip time, RTT12 is equal to (T4-T1)-(T3-T2). Note that Tl and T4 are observed in the clock-domain of BS 1 12a while T2 and T3 are observed in the clock-domain of BS 2 12b. D12, is then derived as ½ (RTT12) * C, where C is the speed of light. The sources of error of D12, include path propagation asymmetry, deviation of path propagation from line-of-sight (LOS), and errors in the observation of Tl, T2, T3 and T4.

A local cluster of base stations may be assigned to a ranging server which gathers the ranging data and may coordinate with a positioning server in order to configure reference signal transmissions. Note that in principle, Tl and T3 may be known a priori as scheduled transmissions in a given LTE frame number, while T2 and T4 are special-purpose observations dependent on the actual propagation delay associated with D12.

Although an LTE base station has inherent capability to precisely know the timing of transmitted and received signals at the antenna reference plane relative to its clock-domain, uncertainties in timing are likely intolerably high for the purposes of ranging. Uncertainties related to observing Tl, T2, T3 andT4 can be improved by calibration. Calibration can improve observation uncertainties with varying effectiveness depending on need. In order of effectiveness, various calibration methods include: characterization by measuring samples; per-unit factory calibration; intermediate transmit/receive loopback operational calibration; RF loopback operational calibration including any components with significant group-delay.

FIG. 7, which includes FIGS. 7A and 7B, is a block diagram of an operational calibration configuration. In the left of FIG. 7, block A, an operational loopback configuration is illustrated for observing an internal round trip delay. This loopback can be physically implemented external to the base station by coupling the antenna ports 18 and 19 together. The observation of the internal round trip delay is achieved by coupling a transmitted radio signal from an antenna port 18 of the transmitter of the base station 12 directly to the receiver antenna port 19 of the base station 12, time stamping the transmitted radio signal and simultaneously time stamping the receipt of the coupled transmitted radio signal. In another embodiment, the observation of the internal round trip delay may be achieved by coupling a transmitted radio signal radiated from an antenna of the transmitter of the base station directly to the receiver antenna of the base station 12, time stamping the transmitted radio signal and simultaneously time stamping the receipt of the coupled transmitted radio signal. The internal round trip delay may be applied to compensate for processing delays internal to the base station 12. In the right block of FIG. 7, shown as FIG. 7B, the transmit signal is internally coupled directly from the transmitter 23 to the receiver 24, bypassing the antennas and antenna ports. It is understood that base station 12 includes other elements and places where internal loopback can be FIG. 8 shows a calibration method employed when the base station is not operating, i.e., offline, using an external test equipment 26 which emulates a calibrated base station. Further, the internal round trip delay can be observed for regular PRS transmissions in a way that does not interfere with an intended radiated emission of the transmitted PRS. Also, the round trip delay may be observed periodically for the purpose of monitoring changes to the base stations.

To improve synchronization accuracy, the clock offset should also compensate for internal delays and variations in the transmit and receive paths of the source and target nodes. Since internal delays are properties which are dependent on the node design, the source and target nodes keep track of, and compensate for, internal delays and variations. Depending on the node design and the synchronization accuracy required, different compensation methods could be used.

Returning to FIG. 6, the time stamps for Tl, T2, T3 and T4 are defined or determined at an Antenna Reference Point (ARP). This approach is particularly well suited for LTE which requires deterministic handling of the timing of signals at the ARP, and hence careful handling of timing uncertainty of the data path from baseband to ARP for transmit signals and from ARP to baseband for receive signals. By defining the times Tl, T2, T3 and T4 in relation to the nodes' ARPs instead of a different point within the nodes (for example, baseband defined Τ , Τ2', Τ3', T4' - described further below in relation to FIGS. 9 and 10), the propagation delay (or RTT) is effectively calibrated for possible variations in and between the transmit and receive paths. Another way could be that compensation data indicative of the source node delay is sent to the target node (together with the time stamps Tl, T4 or in separate transmissions) so that the target node calculates its clock offset also as a function of its own compensation data and the compensation data received. But other possibilities exist as internal delays and variations are dependent on the node design chosen. As such, the compensation method used may vary. The compensation can e.g. be based on characterization or calibration of internal delays close to or during actual round trip time measurement, to avoid or reduce variations over time. Thus, the time stamps may be referred to an antenna reference point (ARP). Note also, that the calibration applied at the source node may be similarly applied at the target node.

As noted above, in some embodiments, the time message to stamp is the radio frame which is a base band IQ frame containing a reference signal such as a primary synchronization signal (PSS), a secondary synchronization signal (SSS) or a position reference signal (PRS). The time stamp may be applied to the reference signal before the up conversion to the radio frequency for the downlink and after the down conversion from radio frequency to base band in the uplink. The base band processor of each node may derive the position of the reference signal relative to the start of the time stamped radio frame. Note that in some embodiments, a radio signal transmitted between two nodes to achieve timing synchronization can be a reference signal, e.g., PSS/SSS/PRS.

For example, referring to FIGS. 9 and 10, for the time between source node 12a, e.g., base station 12a, and target node 12b, e.g., base station 12b, the time stamp T21 is performed by time measurement at two places, one for the IQ stream at the DDC 32b (digital down conversion) of the target node 12b which is responsible to convert the RF signal to baseband signal. The other is ΔΤ2 at the base band processer 20b which processes the IQ data and identifies the start position of the reference signal (PRS/PSS/SSS). Finally, T2 is derived by adding T21 and ΔΤ2. Similarly from target node 12b to source node 12a, T4 is derived by adding T41 and ΔΤ4, where T41 is measured at the DDC 32a of the source node 12a and ΔΤ4 is measured at the base band processor 20a. Note that the nodes 12a and 12b also include digital up converters 22a and 22b, respectively.

In some embodiments, the message containing time stamp information can be transferred over the SI interface. In some embodiments, the message containing time stamp information can be transferred over the X2 interface. In some embodiments, the message containing time stamp information and configuration can be transferred over the OSS interface. Multipath propagation introduces a potential impairment to ranging whereby the transmitted reference signal from one base station arrives at another base station via reflections off of surfaces in the coverage area obscuring the aggregate received signal. This impairment can be mitigated by maximizing the operational bandwidth and hence minimizing the resolution of the receiver. Hence, the earliest arriving signal, which is more likely to travel a line-of-sight (LOS) path between the base stations, can be differentiated from reflections that are non-LOS. Other strategies involve forming an antenna null and steering that null to reduce coupling toward a high level but non-LOS path and hence allowing reception of a lower level but LOS path that may be attenuated by an obstruction. In particular, a null may be steered to suppress a beam from at least a first direction to increase a signal to noise plus interference ratio of a beam from a second direction to determine a minimum round trip delay time from among signals from a plurality of candidate source nodes. Null steering provides at least two benefits. First, null steering may attenuate a low-loss non-LOS path. Second, null steering can attenuate a nearby interfering signal from another base station. In some embodiments the source node is selected having the minimum round trip delay time from the plurality of candidate source nodes. Thus, in some embodiments, the selection of a signal to be used in determining round trip delay may be aided by beam, i.e., signal, steering. In these embodiments, the base station steers a null to suppress a beam from at least a first direction to increase a signal to noise plus interference ratio of a beam from a second direction (or other directions) to determine a minimum round trip delay time from among signals from at least one other base station. The base station steering the null can select the signal having the minimum round trip delay time from the plurality of candidate signals. Exemplary arrangements and processes for steering a null to suppress a beam a direction to increase a signal to noise plus interference ratio of a beam from a second direction can be found in Patent Cooperation Treaty Application No.

PCT/IB2014/060272, entitled OBSERVED TIME DIFFERENCE OF ARRIVAL ANGLE OF ARRIVAL DISCRIMINATOR, the entire contents of which is incorporated herein by reference.

Another mitigating strategy is to use multiple frequency bands supported by the base station including unlicensed bands, which may exhibit less multipath effects between certain base stations or have larger operational bandwidth, hence, minimizing the resolution of the receiver.

Range mean square error (RMSE) is a measure of the expected accuracy of the range estimate. RMSE is, in general, related to signal to noise ratio (SNR), signal bandwidth and a duration of the signal that is correlated at the receiver. The LTE PRS signal is well suited to the purpose of ranging because its structure and scheduling flexibility allows limiting of co-channel interference. The PRS structure allows 6 mutually orthogonal PRSs in the LTE frequency-time resource-grid during a given PRS occasion. PRS-muting, which mutes the PRS during some PRS-occasions increases orthogonality even further. In addition the PRS allocation can include a number of consecutive sub frames assigned to the reference signal, which increases correlation duration. The PRS bandwidth can be configured to be as large as the LTE channel bandwidth which is dependent on the band provided by the operator. Finally, the PRS has good auto-correlation and cross-correlation properties which make the correlation receiver robust to undesirable false -peaks from the PRRS of interest as well as other PRSs, respectively. The SNR dependency implies that closer base station pairs will have more accurate range data, provided that their propagation paths are not obstructed.

Optionally, ranging observations can be coordinated with received signal strength indicator (RSSI) measurements in order to jointly derive position of the base stations. Although RSSI ranging error degrades significantly versus distance from the base station and is likely to be significantly inferior to RTT discussed above, it can be useful in certain cases.

FIG. 11 is a block diagram of a first base station 12 configured to facilitate computation of a distance to a second base station. The first base station 12 includes a processor 28, a memory 30, a transmitter 32 and a receiver 34. The memory 30 includes computer instructions 36 that, when executed by the processor 28, configure the processor 28 to perform timing functions via a timer 38. A calculator 40 calculates a distance between the first and second base stations based on the timing from the timer 38. A null steering unit 42 instructs the processor to determine a null steering direction and to steer a null in a direction to suppress a non-LOS signal to increase sensitivity of the receiver to a LOS signal. In one embodiment, the transmitter 32 of, for example, a first base station 12a is configured to transmit radio signals to a second base station, e.g., base station 12b. The transmitter 32 may transmit a first radio signal to a second base station 12b at a timestamp Tl determined by the timer 38. For example, the first radio signal may be a first PRS and Tl may be determined from a PRS transmission schedule. The receiver 34 may receive a second radio signal from the second base station 12b. The second radio signal may be transmitted from the second base station 12b at a timestamp T3 which occurs after the first radio signal is received at the second base station 12b at a timestamp T2. The second radio signal may be, for example, a second PRS and T3 may be determined from a PRS transmission schedule. The timer 38 may compute a timestamp T4 at which the second radio signal is received at the first base station 12a. In one embodiment, the transmitter 32 may transmit the timestamp T4 to the second base station 12b which knows Tl, T2 and T3 and computes the distance between the first and second base station as C*((T4-T1)-(T3-T2)), where C is the speed of light. In some embodiments, the first base station 12a may send Tl and T4 to a ranging server and the second base station 12b may send T2 and T3 to the ranging server. In these embodiments, the ranging server computes the distance between the two base stations. In some embodiments, the ranging server is located at a base station. In other embodiments, the ranging server is at a centralized node.

The modules shown in FIG. 11 may be implemented at least partially in the memory 30 in the form of software or computer-implemented instructions executed by the processor 28 within the node 12 or distributed across two or more nodes, e.g., the node 12 and another node. In another example, the processor 28 may include one or more hardware components such as application specific integrated circuits (ASICs) that provide some or all of the functionality described above. In another embodiment, the processor 28 may include one or more hardware components, e.g., Central Processing Units (CPUs), and some or all of the functionality described above is implemented in software stored in, e.g., the memory 30 and executed by the processor 28. In yet another embodiment, the processor 28 and memory 30 form processing means (not shown) configured to perform the functionality described above.

FIG. 12 is a block diagram of an alternative configuration of a base station 12 which consists of modules for performing the various functions described with reference to FIG. 11. One or more of the modules can be implemented as software modules executed by a processor. The base station 12 has a timing module 44 configured to determine one or more of the timestamps Tl, T2, T3 and T4. A calculator module 46 calculates the distance between two base stations exchanging radio signals at times defined by Tl, T2, T3 and T4. The null steering module 48 determines a null direction and steers a null to a direction to suppress a non-LOS beam in favor of a LOS beam. The transmitter module 50 is configured to transmit radio signals at times defined by Tl or T3. The receive module 52 is configured to receive radio signals at times defined by T2 or T4.

Note that the roles of the first and second base station may be reversed. Thus, the first base station 12a may receive at timestamp T2 the first radio signal transmitted from the second base station 12b at a prior timestamp Tl, and may transmit the second radio signal at timestamp T3. The second base station 12b may compute the timestamp T4. This timing information may be used by the first base station 12a to compute the distance between the base stations. Further note that the order in which the first and second radio signals are transmitted may be reversed.

Note further that the times Tl, T2, T3 and T4 may be sent to a central node that calculates the round-trip propagation time as (T4-T1)-(T3-T2). In some embodiments, assuming that the central node can uniquely associate the transmission / reception times Tl, T2, T3 and T4 with the appropriate reference signals used for the RTT estimation, instead of conveying T2 and T3 as individual values to the central node, the target node can convey the time information determined in the form of a time interval between reception of the first reference signal and transmission of the second reference signal (e.g. expressed as T3-T2). Similarly, the source node can convey the time information determined at its end in the form of time interval between the transmission of the first reference signal and the reception of the second reference signal (e.g. expressed as T4-T1). A benefit of this approach is that the same resolution can be conveyed with fewer information bits. Also note that in some embodiments, the times Tl, and/or T3 may be ascertained from a schedule of transmissions for the reference signals and thus need not be sent to the central node. FIG. 13 is a block diagram of an embodiment of a network with a centralized server 70 in communication with a source node 12a and at least one target node 12b. The centralized server 70 includes a processor 72, a memory 74, a transmitter 76 and a receiver 78. The processor 72 is configured to execute computer instructions stored in the memory 74. The memory 74 may be organized into modules that include storage of time measurements 80 and a calculator module 82 for instructing the processor to compute and/or store the time measurements 80.

The time measurements may be received from a source node and a target node via the receiver 78 automatically and periodically or in response to a request sent to the target and/or source nodes via the transmitter 76. The time measurements can be the values, T1-T4, or alternatively, they may be time intervals T4-T3, T2-T1, T4-T1 and/or T3-T2. In operation, the centralized server 70, the source node 12a or one or more of the target nodes 12b may initiate a synchronization procedure based on RTT estimation) by sending a signal to another node such as the centralized server 70.

In one embodiment, upon receipt of a request from a node 12, or automatically and periodically, the centralized server 70 may initiate a RTT measurement process by causing the source node 12a to send a first radio message to one or more of the target nodes 12b. The time Tl at which the first radio message is sent to a target node 12b may be known a priori by the centralized server 70 or may be received from the source node 12a. The time, T2, at which the first radio message is received at a target node 12b may be sent to the centralized server 70. The time T3 at which a target node 12b transmits a second radio signal, is either sent to the centralized server 70 or predetermined at the centralized server 70 and corresponding scheduling information is sent to the target node 12b in time for the transmission of the second radio signal at T3. After the second radio signal transmitted at time T3 is received by the source node 12a at time T4, the time T4 is transmitted to the centralized server 70 from the source node 12a. Note that an alternative embodiment of the centralized server 70 can be implemented as software modules that when executed by the processor perform the functions described herein.

Each of the node 12 and the centralized server 70 includes a transmitter and receiver and circuitry containing computer-implemented instructions which when executed by one or more processors cause their respective node 12 and centralized server 70 to perform some or all of the functionality described above. In yet another variant, the circuitry includes the respective memories and processor(s) which may be implemented in many different ways. In one example, the memories contain instructions which, when executed, cause the respective node 12 and centralized server 70 to perform some or all of the functionality described above. Other implementations are possible.

Further, although embodiments are described herein as using a base station 12 as the mechanism for exchanging radio signals to determine a round trip time between base stations, the disclosure is not limited solely to LTE or 3GPP base stations. It is contemplated that other wireless transmission devices that transmit a signal and can monitor wireless transmissions can implement the functionality described herein. Further, although FIG. 11 shows a processor 28 that may be used to calculate the distance between the base stations, the disclosure is not limited to this arrangement. As described herein, the timing data Tl, T2, T3 and T4 can be transmitted, using transmitter 32, for example, or some other wired or wireless transmitter to the ranging server which can determine the range between base stations. An exemplary ranging server can include memory, a processor, a transmitter and receiver that interoperate to perform the ranging calculation functions described herein.

FIG. 14 is a flowchart of an exemplary process for computing a distance between a first base station and a second base station. A first radio signal is received from a first base station 12a, the first radio signal transmitted at a timestamp, Tl,

(block SI 00). A second timestamp T2 at which the first radio signal is received at a second base station 12b is determined (block S102). A third time stamp, T3, indicating a time of transmission of a second radio signal from the second base station 12b to the first base station 12a is determined (block SI 04). The second radio signal is transmitted to the first base station 12a (block S106). A fourth time stamp, T4, indicating a time of receipt of the second reference signal at the first base station 12a is received at the second base station 12b (block S108). A distance between the first base station 12a and the second base station 12b is computed as C*((T4-T1)-(T3- T2))/2, where C is a speed of light (block SI 10).

It is contemplated that the distance between the first base station and the second base station can be determined based on the difference between the round trip time interval, i.e., T4 minus Tl, and the processing delay within the second base station, i.e., T3 minus T2. In this case, a first time interval between a first timestamp, Tl, indicating a time of transmission of a first reference signal from the first base station to the second base station and a fourth timestamp, T4, indicating a time of receipt of a second reference signal from the second base station to the first base station is determined. A second time interval between a second timestamp, T2, indicating a time of receipt of the first reference signal from the first base station and a third timestamp, T3, indicating a time of transmission of the second reference signal from the second base station to the first base station is determined. A distance between the first base station and the second base station can be computed as C*(the first time interval - the second time interval)/2, where C is the speed of light. In accordance with this arrangement, in one embodiment, the first time interval can be received by the ranging server from the first base station and the second time interval can be received by the ranging server from the second base station. As another alternative, the time intervals can be determined by a device such as the ranging server by receiving the first timestamp Tl and the fourth timestamp T4 from the first base station, and receiving the second timestamp, T2, and the third timestamp T3 from the second base station.

It is also noted that the first radio signal and second radio signal can be scheduled to be transmitted at different PRS occasions and thus may be orthogonal in time. In this case, orthogonality can be achieved using different PRS muting applied to the first and second base stations. In other words, distance can be determined based on times Tl to T4 reported to a node such as a ranging server that will compute the distance based on time intervals, for example time intervals associated with round trip time and processing delay time. Of course, it is understood that one of the base stations can also determine the distance and report the distance to another network entity. Also, in some embodiments, because Tl and T3 can be determined from a PRS schedule, it may not be reported to the ranging server or node that is computing the distance between the first and second base stations.

Ranging accurate enough to establish positions of PICO stations in a coverage area with adequate certainty while minimizing (if not eliminating) manual site-survey activity may be achieved by embodiments described above. Embodiments enable automatic re-survey of the positions of the PICO network to accommodate planned changes: additions/deletions/changed-location of PICOs. Some embodiments enable monitoring unplanned changes to a PICO network by monitoring changes to ranging observation data. The integrity of transmit/receive functionality, synchronization, and position of PICO stations in a coverage area can thus be assessed and confirmed. Further, complexity of deploying temporary wireless infrastructure for a special event, e.g., a concert, is reduced. Embodiments, may further be applied to enable dynamic, i.e., self-relocating, infrastructure, and could form an input to a control mechanism where the self -relocating infrastructure is positioned.

Embodiments can be realized in hardware, or a combination of hardware and software. Any kind of computing system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein. A typical combination of hardware and software could be a specialized computer system, having one or more processing elements and a computer program stored on a storage medium that, when loaded and executed, controls the computer system such that it carries out the methods described herein. The arrangements described herein can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which, when loaded in a computing system is able to carry out these methods. Storage medium refers to any volatile or non-volatile storage device.

Computer program or application in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following a) conversion to another language, code or notation; b) reproduction in a different material form.

It will be appreciated by persons skilled in the art that the present

embodiments are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings.

Claims

What is claimed is:

1. A method of determining a distance between a first base station (12a) and a second base station (12b), the method comprising:

receiving from the first base station (12a) a first radio signal transmitted from the first base station (12a) at a first timestamp, Tl (SI 00);

determining a second timestamp, T2, indicating a time of receipt of the first radio signal at the second base station (12b) (SI 02);

determining a third timestamp, T3, indicating a time of transmission of a second radio signal from the second base station (12b) (SI 04);

transmitting the second radio signal to the first base station (12a) (SI 06); receiving from the first base station a value of a fourth timestamp, T4, the fourth timestamp, T4, being a time of receipt at the first base station (12a) of the second radio signal (SI 08); and

computing a distance between the first base station (12a) and the second base station (12b) as C*((T4-Tl)-(T3-T2))/2, where C is the speed of light (SI 10).

2. The method of Claim 1 , wherein the computing is performed at a ranging server (16).

3. The method of Claim 2, wherein the ranging server (16) comprises the second base station (12b).

4. The method of Claim 1 , wherein the first and second radio signals are position reference signals, PRSs.

5. The method of Claim 4, wherein the timestamps Tl and T3 are determined from a PRS transmission schedule.

6. The method of Claim 1 , wherein an accuracy of determining a timestamp is increased by observing an internal round trip delay within a base station (12) to compensate for processing delays internal to the base station (12).

7. The method of Claim 6, wherein observing the internal round trip delay includes coupling the transmitted radio signal to a receiver (24), time stamping the transmitted radio signal, and simultaneously time stamping the receipt of the coupled transmitted radio signal in the receiver (24).

8. The method of Claim 1 , further comprising:

steering a null to suppress a beam from at least a first direction to increase a signal to noise plus interference ratio of a line of sight beam from a second direction. .

9. A central node configured to facilitate computation of a distance between a first base station (12a) and a second base station (12b), the central node comprising: a receiver (70) configured to:

receive a first timestamp, Tl, indicative of a time at which a first radio signal is transmitted from the second base station (12b);

receive a second timestamp, T2, indicating a time of receipt of the first radio signal at the first base station (12a);

receive a third timestamp, T3, indicating a time of transmission of a second radio signal from the first base station (12a); and

receive a fourth timestamp, T4, the fourth timestamp, T4, being a time of receipt at the second base station (12b) of the second radio signal.

10. The central node of Claim 9, further comprising a processor (72) to compute a position of at least one of the first base station (12a) and the second base station (12b).

11. The central node of Claim 9, wherein the central node is located at one of the first base station (12a) and the second base station (12b).

12. The central node of Claim 9, wherein the first and second radio signals position reference signals, PRSs.

13. The central node of Claim 12, wherein the timestamps Tl and T3 are determined from a PRS transmission schedule.

14. The central node of Claim 9, wherein the timestamps are in reference to an antenna reference point.

15. The central node of Claim 9, wherein at least one timestamp is calibrated to compensate for an internal processing delay of a base station.

16. A central node configured to facilitate computation of a distance between a first base station (12a) and a second base station (12b), the central node comprising: a receiver module (78) configured to:

receive a first timestamp Tl indicative of a time at which a first radio signal is transmitted from the second base station (12b);

receive a fourth timestamp, T4, the fourth timestamp, T4, being a time of receipt at the second base station (12b) of a second radio signal received from the first base station (12a); and

a calculator module (82) configured to calculate a distance between the first base station (12a) and the second base station (12b).

17. The central node of Claim 16, wherein the calculator module is further configured to calculate a position of at least one of the first base station (12a) and the second base station (12b).

18. The central node of Claim 16, wherein the central node is located at one of the first (12a) and second (12b) base stations.

19. The central node of Claim 16, wherein the first and second radio signals are position reference signals, PRSs.

20. The central node of Claim 19, wherein the timestamps Tl and T3 are determined from a PRS transmission schedule.

21. The central node of Claim 16, wherein the timestamps are in reference to an antenna reference point.

22. The central node of Claim 16, wherein at least two timestamps are calibrated to compensate for an internal processing delay of a base station.