Fast target acquisition for steerable beams in optical wireless communication systems

Abstract

The present invention relates to a target acquisition method and system for a steerable light beam in an optical wireless communication or free space optical communication system, in which the duration during which the steerable light beam of a transmitter must illuminate a light detector at a receiver for acquisition can be minimized, and may allow a slow feedback channel from a receiver to a transmitter. This can be accomplished by transmitting a pseudorandom sequence having the attribute that knowledge of a small number of bits uniquely allows determination of the beam direction in which the steerable beam of the transmitter is received by the receiver. In one example, a linear feedback shift register sequence is a suitable and very efficient choice.

Description

Technical Field

[0001] The present invention relates to the field of communication in optical wireless networks, such as but not limited to optical wireless communication (OWC) or free space optics (FSO) communication, for use in various different applications indoors, e.g. for homes, offices, retail, hotels and industry, or outdoors, e.g. in car-to-X (to infrastructure, to other cars, ...) or in military applications. The present invention can be used for communication between infrastructure and end nodes, and can be used as part of a communication link in the infrastructure, such as between satellites, aircraft and ground stations. Background Art

[0002] FSO is an optical communication technology that uses the propagation of visible or invisible radiation in free space to transmit data wirelessly for telecommunications or computer networks. "Free space" means air, outer space, a vacuum, or something similar. Free-space point-to-point optical links can be implemented using infrared laser light, although low data rate communications over short distances using LEDs may also be possible. Infrared Data Association (IrDA) technology is a very simple form of FSO communication. In terms of communications, FSO technology is considered part of OWC applications.

[0003] OWC networks such as Li-Fi networks (named like Wi-Fi networks) enable mobile user devices (hereinafter referred to as endpoints (EP)) such as laptops, tablets, smart phones, etc. to wirelessly connect to the Internet or other data communication networks. Wi-Fi uses radio frequencies to achieve this, but Li-Fi uses visible and invisible light spectrums (which include ultraviolet (UV) and infrared (IR) light) to achieve this, which can achieve unprecedented data transfer speeds and bandwidths. In addition, it can be used in areas susceptible to electromagnetic interference.

[0004] Based on the modulation, any suitable light sensor can be used to detect the information embedded in the coded light. This can be a dedicated photocell (point detector), an array of photocells with a diffuser that may be a lens, a reflector, a phosphor converter, or a camera including an array of photocells (pixels) and a lens for forming an image on the array. For example, the light sensor can be a dedicated photocell included in a dongle inserted into the EP, or the sensor can be a general-purpose (visible light or infrared light) camera of the EP or an infrared detector originally designed for, for example, 3D facial recognition. Either way, this can enable applications running on the EP to receive data via light.

[0005] LiFi devices can be integrated into the luminaires of a lighting system to provide high-speed wireless connectivity over large spaces, such as conference rooms and office floors. There is seamless switching between each LiFi-enabled luminaire, enabling users to roam around. As an example, a USB access key plugged into a laptop can be used to receive LiFi signals and act as a transmitter to send data back to the luminaire.

[0006] Additionally, fixed point-to-point systems can be provided that act like wireless cables for connecting devices. Potential applications include connecting robots or machines in radio frequency (RF) harsh environments such as factories or hospitals, where RF communications may not be allowed, or where large data files need to be sent and received securely and quickly.

[0007] Another broad area of ​​FSO is outdoor communication links. For example, in smart mobile applications, a communication link with a moving car is needed. In military operations, communication without radio signals has advantages, because a directional laser beam can avoid detection and can limit the area where the signal is intercepted. For communication between satellites, aircraft and ground stations, laser beams can be used to cover large distances and nevertheless allow very high bit rates. In the example of a satellite or aircraft, the position it has is initially unknown for communication, so that there is uncertainty within a range of several kilometers, so that finding the correct direction of the laser beam within a range of a few centimeters may be (too) time-consuming.

[0008] In optical wireless communications using steerable light beams (e.g., laser beams), ensuring that the beam is sent in the correct direction to reach the corresponding station is a challenge. Therefore, there is a distinction between acquisition (i.e., the process of finding the corresponding station with the beam from an initial situation where the relative position and direction are unknown) and tracking (i.e., the process of maintaining the accurate direction of the beam despite vibrations or noise, starting from a situation where the correct direction is known).

[0009] A commonly encountered situation is that the transmitter system transmits a narrow beam in scanning (acquisition) mode. This beam may have a high bit rate, but only covers a narrow area. The return channel in acquisition mode is not (yet) established and / or has a much lower bandwidth and / or has a very asymmetric delay and / or has a limited timing resolution. This may hinder the link establishment speed if the transmitter system needs to wait for feedback messages (or wait for a timeout when the forward beam does not hit the corresponding station), which delays the process of finding a direction lock.

[0010] International patent application WO 02 / 086555A2 discloses a method for aligning an optical wireless link using feedback information transmitted via an aligned beam. Each link performs acquisition routing, in which its beam sweeps a predefined pattern while transmitting its beam alignment information. The alignment information may be a control packet containing the position of the transmitted beam, i.e., x and y coordinates relative to some reference point. When a link receives beam alignment information from a remote link, it updates its transmission to include the alignment information received from the remote link. The result is that at some point the remote link will receive the alignment information "echoed" back to itself, and will align its beam accordingly. Summary of the invention

[0011] An object of the present invention is to provide an accelerated acquisition solution for OWC or FSO networks.

[0012] This object is achieved by a source device as claimed in claim 1, a target device as claimed in claim 8, an optical wireless communication apparatus as claimed in claim 10, an optical wireless communication system as claimed in claim 12, and a method as claimed in claims 14 and 15.

[0013] According to a first aspect, there is provided a source device for controlling acquisition of a target receiver by a steered light beam for optical wireless communication, wherein the source device is configured to:

[0014] adding address information to the steered light beam indicating a current direction or position of the steered light beam;

[0015] Scanning a scanning area along a trajectory by the steered light beam; and

[0016] The location of the target receiver is derived based on address information received from the target receiver in a hit report via a feedback channel.

[0017] According to a second aspect, there is provided a target device for arranging communication with a source transmitter via a steered light beam for optical wireless communication, wherein the target device is configured to:

[0018] determining detection of the steered light beam by the photodetector during an acquisition scan phase;

[0019] deriving from the detection address information embedded in the steered light beam and indicative of a current direction or position of the steered light beam; and

[0020] The derived address information is transmitted to the source transmitter in a hit report via a feedback channel.

[0021] According to a third aspect, there is provided an optical wireless communication device comprising at least one of the source device of the first aspect and the target device of the second aspect. Such a device will be able to transmit via a steered light beam with address information and receive a receipt confirmation from another device via a feedback channel. Simultaneously (or sequentially), when the device detects a steered light beam with address information from another device, the device may also provide a receipt confirmation via a feedback channel. Thus, two devices according to the third aspect may be co-aligned and set up a high-speed bidirectional communication link when they are in line of sight with each other.

[0022] According to a fourth aspect, there is provided an optical wireless communication system comprising at least one optical wireless communication device having a source apparatus of the first aspect and at least one optical wireless communication device having a target apparatus of the second aspect.

[0023] According to a fifth aspect, there is provided a method of controlling acquisition of a target receiver by a steered light beam for optical wireless communication, wherein the method comprises:

[0024] adding address information to the steered light beam indicating a current direction or position of the steered light beam;

[0025] scanning a scanning area along the trajectory by the steered light beam; and

[0026] The location of the target receiver is derived based on address information received from the target receiver in a hit report via a feedback channel.

[0027] According to a sixth aspect, there is provided a method of communicating with a source transmitter via a steered light beam arrangement for optical wireless communication, wherein the method comprises:

[0028] determining detection of the steered light beam by the photodetector during an acquisition scan phase;

[0029] deriving from the detection address information embedded in the steered light beam and indicative of a current direction or position of the steered light beam; and

[0030] The derived address information is transmitted to the source transmitter in a hit report via a feedback channel.

[0031] Thus, a very effective and efficient acquisition solution can be provided, which enables a fast way to establish a directional hit, without being hampered by delays in the return channel. The address information associated with the directional hit may indicate a position in the coverage area or a direction towards the hit detector. The scanning trajectory may be determined based on a predetermined scanning pattern or even "randomly", as long as the trajectory is continuous and a known relationship between the trajectory and the associated direction and address information is ensured.

[0032] According to a first option of any one of the first to seventh aspects, scanning of the steered light beam may continue without waiting for receipt of a hit report or its absence. Since the address information provides the information required to determine the direction from which the signal was received, it is no longer necessary to wait for a hit report. Thereby, an accelerated scanning phase may be achieved and after receiving a hit report(s), the direction(s) of(the)(one or more) hit detector(s) may be locked.

[0033] According to a second option of any one of the first to seventh aspects that can be combined with the first option, the output of the pseudo-random sequence can be used as address information, or the address information can be embedded in a data packet with a predetermined number of bits for each axis of a header, a synchronization word, and a scan area. Therefore, an alternative option with corresponding advantages can be provided for transmission direction related address information. Due to the reduction in the number of bits required for signaling the pseudo-random sequence, the first option of directly embedding the output of the pseudo-random sequence achieves fast scanning and higher directional accuracy. Due to additional synchronization and header information, the second option of embedding address information in the data packet achieves higher transmission reliability.

[0034] According to the third option of any one of the first to seventh aspects which can be combined with the first or second option, having a length L and a sequence length 2 L A hybrid or linear feedback shift register of -1 can be used to generate a pseudo-random sequence, where each bit sequence of length L uniquely specifies a different beam position. Therefore, the easily generated pseudo-random sequence can be used as an address identification source that is successively added to the continuous run of the steered beam. This provides an efficient and effective direction-dependent addressing option. The advantage of this approach is that since there is no clear "message start" and / or "message end", detection can start at any point within the pseudo-random sequence. As a result, the pseudo-random sequence can be transmitted as a continuous bit signal during continuous scanning, and there is no need to transmit discrete position address information packets at discrete locations.

[0035] According to a fourth option of any one of the first to seventh aspects which can be combined with any one of the first to third options, the receiving end can be configured to derive the address information of the detected steered light beam by reading a predetermined number of adjacent bits, wherein the predetermined number is at least the logarithm of the length of the pseudo-random sequence to the base 2. This ensures that a sufficient number of bits are read to derive and feed back a clear direction of the steered light beam.

[0036] According to the fifth option of any one of the first to seventh aspects which can be combined with any one of the first to fourth options, since the capture area of ​​the photodetector is larger than the minimum capture area, more than a predetermined number of bits can be captured at the receiving end, so that the available excess bits can be used for error correction or for detecting a more specific position of the photodetector within the beam coverage area of ​​the steered light beam. Thereby, more reliable and more accurate feedback of the direction-related address information can be achieved.

[0037] According to a sixth option of any one of the first to seventh aspects which can be combined with any one of the first to fifth options, a steered beam can be generated having a beam coverage area wider than the beam coverage area used for communication with a target receiver after acquisition. This measure allows a faster acquisition process because the scan area can be covered with a shorter trajectory.

[0038] According to a seventh option of any one of the first to seventh aspects which can be combined with any one of the first to sixth options, the number of bits of the address information can be set to at least log 2 (A C / A B ), where A C represents the size of the scan area, and A B Represents the size of the coverage area of ​​the steered light beam. Thus, the length of the address information can be optimized according to the scanning parameters to obtain an efficient acquisition process. In one example, when the target device receives the signal, but is unable to successfully extract the address information (e.g., no bits, unable to correctly verify the checksum (if present)), it can indicate this through the feedback channel (e.g., "I saw something, but could not recover the address information"). This can then allow the source device to perform a slower transfer along the same path.

[0039] According to an eighth option of any one of the first to seventh aspects which can be combined with any one of the first to seventh options, the scanning speed of the steered light beam can be set to 0.7 beam radius of the steered light beam per address information duration. This measure ensures that the scanning speed of the steered light beam can be optimized according to the scanning parameters to obtain an efficient acquisition process.

[0040] According to a ninth option of any of the first to seventh aspects which may be combined with any of the first to eighth options, the trajectory may be dynamically selected based on a priori address information stored in the memory. Thus, a priori knowledge of the target receiver position may be used to shorten acquisition time.

[0041] According to the tenth option of any one of the first to seventh aspects which can be combined with any one of the first to ninth options, the source transmitter may include a position actuator element having at least one galvanometer mirror for steering the steered light beam. Thus, an efficient beam steering process with an integrated direction sensing option can be provided.

[0042] Note that the above-mentioned source device and target device can be implemented based on an arrangement of discrete hardware circuits, integrated chips, or chip modules with discrete hardware components, or based on a signal processing device or chip controlled by a software routine or program stored in a memory, written on a computer-readable medium, or downloaded from a network (such as the Internet).

[0043] It should be understood that the source device of claim 1, the target device of claim 8, the optical wireless communication apparatus of claim 10, the optical wireless communication system of claim 12, the method of claim 13 or 14 may have similar and / or identical preferred embodiments, in particular as defined in the dependent claims.

[0044] It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or the above-described embodiments with the respective independent claim.

[0045] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In the following drawings:

[0047] Figure 1 schematically illustrates a communication link of an optical communication system prior to acquisition;

[0048] Figure 2 Schematically illustrates the architecture of a wireless optical system with a beam scanning area, a steerable beam width, and a capture area;

[0049] Figure 3 A pseudo-random sequence with a detector window and a minimum window for fast acquisition is shown;

[0050] Figure 4 Schematically shows a block diagram of an optical wireless communication system with a fast acquisition function according to an embodiment;

[0051] Figure 5 schematically shows a sequence of beam coverage areas with address IDs along a beam scanning trajectory;

[0052] Figure 6 schematically showing a sequence of beam coverage areas and associated sub-areas along a beam scanning trajectory;

[0053] Figure 7 schematically showing a sequence of beam coverage areas and associated sub-areas along a beam scanning trajectory and the distances required for determining an optimal scanning speed;

[0054] Figure 8 schematically illustrates a block diagram of an exemplary linear feedback shift register;

[0055] Fig. 9 Shows Figure 8 A table of output values ​​of a linear feedback shift register;

[0056] Fig.10 schematically shows the beam coverage area and the sequence of associated symbols along the beam scanning trajectory;

[0057] Fig.11 A flow chart showing a fast acquisition process at a transmitter according to one embodiment; and

[0058] Fig.12 A flow chart of a fast acquisition process at a receiver according to one embodiment is shown. DETAILED DESCRIPTION

[0059] Various embodiments of the present invention are now described based on an OWC system with an OWC, FSO or LiFi communication device.

[0060] Throughout the following, LiFi, FSO, and OWC are used interchangeably, and a light source or optical transmitter may be understood as a radiation source that generates visible or invisible light (i.e., including infrared (IR) or ultraviolet (UV)) for communication or other purposes (e.g., safety / security light barriers). Light sources suitable for OWC applications may vary from application to application and may be selected based on application requirements (such as bandwidth, cost, or power budget). Light sources often used in OWC systems are, for example, light emitting diodes (LEDs), vertical cavity surface emitting lasers (VCSELs), or other types of laser sources; optionally equipped with suitable optics to achieve the desired emission field of view. The light source may be included in an illuminator, such as an embedded or surface mounted incandescent lamp, fluorescent lamp, or other discharge illuminator.

[0061] The basic idea of ​​LiFi communication is to provide line of sight between LiFi devices. As a result, the LiFi infrastructure needs to be well positioned or aligned to provide optical wireless communication which requires line of sight.

[0062] Figure 1 Schematically illustrated is a communication link between a first LiFi device 10 (eg, an AP) having a steerable light beam 20 and a second LiFi device 30 having an optical detector and a detection area 32 prior to acquisition.

[0063] Note that throughout this disclosure, the structure and / or function of a block having the same reference numeral as previously described will not be described again unless additional specific functions are involved. In addition, only those structural elements and functions that are helpful for understanding the embodiments are shown. For the sake of brevity, other structural elements and functions are omitted.

[0064] Figure 1 An optical communication link will be installed or set between a first LiFi device 10 having a steerable light beam 20 (movably) mounted at a first rack 100 or another installation facility and a second LiFi device 30 (movably mounted at a second rack 110 or another installation facility. The second LiFi device 30 includes an optical detector having a detection area 32.

[0065] Alternatively, at least one of the first and second LiFi devices 10 , 30 may be a mobile device that may be carried and / or operated by a user.

[0066] As a further option, the first and second LiFi devices may belong to a LiFi network comprising a plurality of APs (e.g. luminaires of a lighting system) connected to a backbone network (e.g. Ethernet or similar) e.g. via switches (e.g. Ethernet switches), whereby each AP controls one or more transceivers (i.e. combined transmitters (optical transmitters) and receivers (light sensors)) for optical communication towards an EP (e.g. a mobile user device). The respective downlink light beams generated by the transceivers of the APs and defining a coverage area on the plane(s) of the EP may be steerable.

[0067] A central global controller entity or function (e.g. provided in one of the (collocated or integrated) APs or in a LiFi controller) may be connected to the backbone network and configured to manage the LiFi network, which may include interference handling coordination. Interference handling may be achieved by providing time division multiple access (TDMA), where the medium access control (MAC) cycles of the APs are aligned and divided into time slots. Furthermore, the global controller entity may be configured to control switching when one of the EPs moves in and out of an overlapping coverage area of ​​the AP. The global controller entity may be connected to the APs via switches of the backbone network.

[0068] According to various embodiments, systems and methods are provided in which a beam direction is determined (acquired) for a steerable optical emitter to hit a target detector (i.e., covered by a coverage area of ​​the steerable beam 20). Figure 1 The detector may have a feedback mechanism to signal that it has been covered (hit) by the search beam.

[0069] However, for such hit detection, a minimum amount of signal strength is required to ensure high reliability. Furthermore, the bandwidth of the target detector may be limited. In fact, a photodetector that must be sensitive to capture photons from a large area and needs to do so for photons entering through a large range of angles needs to have a large surface area. This is a fundamental limitation imposed by the so-called "etendu" limitation.

[0070] Etendue is a property of light in an optical system that characterizes how much the light is "spread" over area and angle. Thus, etendue can be thought of as a volume in phase space. It corresponds to the beam parameter product (BPP) in Gaussian beam optics. Other names for etendue include acceptance, throughput, light grabbing, focusing power, optical range, and AΩ product. Throughput and AΩ product are particularly useful in radiation measurement and radiation delivery where they are related to view factor (or form factor).

[0071] A perfect optical system produces an image with the same etendue as the source. Etendue is related to Lagrangian and optical invariants, which share constant properties in ideal optical systems. The radiance of an optical system is equal to the derivative of the radiant flux with respect to the etendue. For the system proposed here, the etendue law requires that the surface of the photodiode cannot be small if we expect to capture a strong signal by using a large detector area (e.g., a large lens) and if we simultaneously want to detect signals from a wide variety of possible angles of incidence.

[0072] Therefore, optical photodiodes with large surface area have high capacitance and are slow.

[0073] The trade-offs for appropriate selection of beam width may involve (at least) three challenges: the etendue challenge of making an optical detector with a detection area 32 fast and sensitive to wide angles within the steered beam 20 (as described above); second, the desire to keep the exit beam width at the transmitter of the first LiFi device 10 small in the forward system to ensure a strong signal that can carry a high bit rate; and third, the challenge of transmitter design in the return or feedback channel.

[0074] Regarding the third challenge, the feedback signal must be transmitted wirelessly in the opposite direction through a link that may not yet know the direction the incident beam arrived, so the feedback signal needs to be sent out through a wide beam. Since this results in a higher loss in signal strength, the feedback channel can be much slower than the forward channel.

[0075] The following embodiments propose solutions to these challenges, especially the asymmetry of the forward and feedback channels.

[0076] Figure 2 Schematically shows a beam scanning area A C , the controllable beam covers area A B 22 and capture area A D 32 Architecture of a wireless optical system.

[0077] From radar and radio communications using radio waves, it is known that if the system is limited by additive white Gaussian noise (AWGN) and if a matched filter detector is used, the reliability of detecting a pulse depends on the energy in the pulse divided by the noise spectral density. Energy is the integral of the square of the pulse amplitude over time. Matched filter theory states that it does not matter how the pulse shape is structured over time and over frequency.

[0078] For simplicity (i.e., uniform power density inside the beam and zero power density outside the beam), it can be assumed that the path loss is proportional to the beam coverage area A. B 22 and is inversely proportional to the beam width A B The received signal power is related to the receiving antenna aperture (i.e., capture area) A D Directly proportional.

[0079] These considerations can be applied to a system with a width (i.e., coverage area) A B 22, the light beam 20 scans the entire scanning area A C to hit the capture area 32 of the photodetector of the target receiver 30. In optical communications, the detector size (i.e., capture area A) is D ) can be used as a measure of sensitivity (which corresponds to the antenna gain in a radio system). In order to deliver the minimum required energy E, with a transmission power P T The beam needs to illuminate the target for at least T 1 Second:

[0080]

[0081] Here, the minimum required energy E depends on the noise level and on the reliability with which the beam signal needs to be detected. If the spectral noise density is N 0 (in W / Hz), an E / N greater than, for example, 10 may be required 0 .

[0082] Minimum irradiation time T 1 can be interpreted as the dwell time required at each possible position, thus searching the entire scan area A C Requires A C / A B times T 1 Therefore, T 1can be interpreted as the minimum duration for which the signal needs to be retransmitted (or received) to reliably determine the detected beam. However, this requires an immediate feedback channel that immediately informs the transmitter that the current direction is the correct or appropriate direction, i.e. Figure 2 The direction indicated in d prop . Typically, the feedback link is several orders of magnitude slower.

[0083] The current direction of the steered beam 20 can be indicated by adding a T This asymmetry in bit rate can be handled by using direction-dependent address information (e.g., direction address identification (ID) or direction count, etc.) that can be reported back via a feedback channel. However, this requires a minimum illumination time T 1 Large enough to carry complete direction-related information (e.g., address ID). Scanning speed and T 1 It may be mainly limited by the bandwidth of the feedback channel.

[0084] Considering a typical form of an optical detector and its electronic amplifier for LiFi, the first detector property is the signal-to-noise ratio (SNR) versus the received optical power P opt In fact, after the transimpedance amplifier used for detection, the received electric power P RX is the optical power P received by the photodiode opt squared. This can be expressed as follows:

[0085]

[0086] The constant c 1 Includes aspects such as the responsivity of the photodiode.

[0087] The above expression requires that the beam covers area A B Larger than the capture area A of the detector D If A D >A B And the beam covers area A B Falling into the capture area A of the detector D The received electric power is P RX =c 1 P opt 2 , because the photodiode captures all photons anyway.

[0088] Therefore, in order to reduce the search time, the beam size (beam coverage area A B) should be small. This results in a strong transient signal, and its effect is quadratic in the detector system. In fact, the proposed embodiment allows the system to scan quickly with a very narrow beam width, which may not be much larger than the detector size, or even smaller than the detector size.

[0089] Furthermore, a second property of optical detectors is limited bandwidth. For very small beam widths, it is not the received power that limits the dwell time, but the bandwidth of the detector. In this case, one may decide to increase the number of bits embedded in each symbol at the transmitter. Thus, the directional address ID can be signaled faster within the same bandwidth. However, to carry m bits, the number of signal levels is exponential: M = 2 m , which increases rapidly with m. Since the required power is proportional to M, the average power increases rapidly with m. To exploit a particular SNR margin, the system can either increase m (which consumes the margin exponentially) or it can expand the beam width (which consumes the margin quadratically).

[0090] In fact, bandwidth limitations may be particularly relevant to acquisition (scanning) modes, where the receiver needs to be able to capture signals from unknown directions. This typically means that the detector has a relatively large surface area and may be more limited in bandwidth than a detector that may be designed and optimized for reception from known directions. The imaging system (e.g., a lens in front of the detector) may ensure that signals from a large capture area A are captured. D All light from the detector is mapped to a focused spot on the detection photodiode. However, if the angle of arrival θ is not known in advance R , the position of the spot is unknown, so a larger detector may be needed. In fact, the etendue law states that if a large capture area A is desired D and a large arrival angle spread, then basically the photodiode area cannot be reduced.

[0091] The following embodiments focus on the problem of how, with a given beam width, a given bandwidth, and a given power, a system can efficiently utilize fast beam operation and avoid the need for a high bandwidth return channel, at least during the initial acquisition phase of setting up a link. Furthermore, encoding address information allows the forward beam to be scanned in a very efficient manner while mitigating the problems caused by limited signal power and / or limited bandwidth.

[0092] One way to speed up the acquisition process is to allow the scanning beam to continue moving while the receiver is still in the process of preparing confirmation that the beam has successfully made contact (i.e., sufficient illumination time has been reached). However, the feedback must then include when or in which direction the beam has successfully made contact, because in the meantime the beam may have visited hundreds of other locations, all of which may not be the correct location except one.

[0093] To cope with this asymmetry between the forward channel and the feedback channel, it is proposed that the transmitter adds or embeds address information to the scanning beam 20 (e.g., as a payload), and the receiver can signal the address information back via the slower feedback channel. In one example, each beam position can carry different address information. Or, as another example, ensure that the same position address is distributed in a way that it can be unambiguously linked to a single direction (e.g., based on an address sequence).

[0094] Thus, a very effective and efficient acquisition solution can be provided, which enables a directional hit to be established quickly without being hampered by delays in the return channel. This idea can also be used in satellites that rotate internal mirrors, or that rotate on their own axis and thereby sweep a narrow beam. In such long-distance links, not only the bandwidth of the return channel can be a problem, but also the delay caused by the time it takes for light to travel over several kilometers.

[0095] In one embodiment, the proposed solution can be implemented in an optical positioning system using the output of a pseudo-random sequence, where each bit in the sequence is projected at a different (possibly partially overlapping) position and where a detector reads a number of adjacent bits, where this number is at least the base-2 logarithm of the sequence length.

[0096] Figure 3 A pseudo-random sequence with a detector window and a minimum window for fast acquisition according to the above-described embodiments is shown.

[0097] like Figure 3 As indicated in , the detector captures a portion of the pseudo-random sequence in its capture area 32. A minimum number of bits, indicated by the minimum capture area 34, is required to uniquely or unambiguously determine the position of the received light beam based on the sequence. Excess bits captured because the actual capture area 32 is larger than the minimum capture area 34 can optionally be used, for example, for error correction or for detecting a more specific position of the detector within the light beam coverage area based on a longer sequence of bits captured.

[0098] In one example, the pseudo-random sequence may be generated by a hybrid feedback shift register (MFSR) of length L and may have a sequence length of 2. L-1 (two to the power of L minus one). Each bit sequence of length L can uniquely identify a different beam position in the code. If more than L bits are received, these extra bits can be used to correct errors because in fact the L bits uniquely define the entire remainder of the sequence. Other pseudo-random sequences can be used as long as any L bits uniquely identify a position in a sequence of length K, where L is small, preferably much smaller than K.

[0099] Because a set of L consecutive bits defines the corresponding light beam position (essentially the direction of the transmitter, the position of the receiver when received by the receiver), the average detection speed is improved compared to a system that transmits coordinate packets. For a packet receiver to receive a complete coordinate packet, the packet receiver will need to wait for the start of the coordinate packet and then collect all the bits from the next coordinate packet. Although it is possible that the light beam may accidentally enter the field of view of the packet receiver for the first time at the beginning of a coordinate packet, this will not usually be the case. The larger the coordinate packet size, the lower the probability that this will be the case. Assuming that the light beam advances uniformly linearly along the trajectory, it is expected that on average, the packet receiver will need to wait for a duration of half the transmission time of the coordinate packet size.

[0100] Furthermore, the coordinate packet size also imposes limitations on prior art packet transmitters, particularly with respect to the speed at which the light beam advances along the trajectory. In a worst case scenario, the light beam may first impinge on a packet receiver at the moment when the second bit (or symbol) of the coordinate packet size has just been sent out. In this case, the packet receiver will need to wait for the transmission duration of almost one full coordinate packet size until the next coordinate packet begins so that it can then detect the full coordinate packet. This means that the speed at which the light beam sweeps along the acquisition pattern will need to be adjusted so that the time "spent" in the field of view of the receiver is at least twice the full coordinate packet transmission duration.

[0101] By outputting a continuous stream of bits of a pseudo-random sequence, the receiving device does not need to wait until it receives the start of a packet header / coordinate packet, but can instead collect L consecutive bits from the input data stream in a "random access" manner, compared to using the present invention. There is no need to wait for the start of a coordinate packet, as any bit received can be used. Furthermore, at the transmitter, the sweep speed only needs to accommodate the reception of L bits (or symbols) (rather than 2L as described above).

[0102] Therefore, it is valuable to use a source device to control the acquisition of a steered light beam for optical wireless communication to a target receiver, wherein the source device is configured to: add address information indicating the current direction or position of the steered light beam to the steered light beam; control a sequence having a length L and a sequence length 2 L-1 hybrid or linear feedback shift register to generate a pseudo-random sequence; the steered light beam scans the scanning area along the trajectory while outputting a continuous bit stream of the pseudo-random sequence; and the position of the target receiver is derived based on the address information received from the target receiver in the hit report via the feedback channel, and each bit sequence of length L uniquely specifies a different light beam position along the trajectory.

[0103] The bit sequence of length L can be used to determine at the transmitter side the direction in which the light beam was pointing when the L bit sequence was transmitted. This can be established by "time stamping" certain parts of the trajectory scan a priori based on the known operation of the source device, and then determining when one of the interpolated bits in the L bit sequence was transmitted. Based on this, it can then be determined where the light beam was pointing at that time.

[0104] Alternatively, if the specific timing of the shift register bit outputs and the progression along the trajectory is known in advance, this can similarly be accomplished by calculation. Here, for example, the time at which the middle bit(s) in the L-bit sequence is transmitted can be first determined, and then that time used to determine the relative direction the beam is pointing at at that time, and from there infer where the target device is located relative to the source device.

[0105] Those skilled in the art will appreciate that this determination does not determine the exact location of the target device, nor does it determine the absolute orientation. However, it does enable the source device to steer the light beam in the same direction as the target device.

[0106] Similarly, it is valuable to use a target device to set up communication with a source device via a steered beam for optical wireless communication, wherein the target device is configured to: determine detection of the steered beam by a photodetector during an acquisition scan phase; derive address information embedded in the steered beam and indicating the current direction or position of the steered beam from the detection; and transmit the derived address information to the source transmitter in a hit report via a feedback channel; wherein the address information is derived by reading a predetermined number of adjacent bits from the detection, the predetermined number of adjacent bits representing a pseudo-random sequence indicating the position of the beam along a trajectory embedded by the source device, and wherein the predetermined number is at least the base-2 logarithm of the length of the pseudo-random sequence.

[0107] In an embodiment, the scanning speed of the target LiFi device 30 with the target detector and also the source LiFi device 10 with the steerable light beam 20 can be configured to allow the detection of address information by reading a predetermined number of adjacent bits at the target LiFi device 30, where the predetermined number can be at least a number L of adjacent bits to uniquely determine the position in the code sequence. For any pseudo-random sequence, L can be at least the base 2 logarithm of the length of the pseudo-random sequence. The maximum length LFSR sequence achieves this to a very good approximation because its sequence length is 2 L -1, so the L bit can be enabled for 2 L It is missing only one address in the possible space of addresses.

[0108] However, there may be at least two reasons for reading more bits: First, depending on the angular scanning speed of the source LiFi device 10, the distance and orientation of the target LiFi device 30, more consecutive bits may be read by the target LiFi device 30 anyway. In this case, taking a longer sequence or reading the middle of the bits allows a more accurate determination of the best directional address in the total pseudo-random sequence. Second, during system design, a deliberate choice may be made to allow more consecutive bits to be read to facilitate error correction. This may even mean that the scanning speed is set slightly lower, or the modulation is deliberately set faster, exceeding the speed at which the reliability of the detection begins to degrade. In one example, a good choice may be to design at least a few (e.g., 8) more bits than the lowest bit required to determine the position in the pseudo-random sequence. Having a few extra bits (e.g., L+8 in total) already allows a good degree of error correction, but has no significant effect on the scanning speed. If the system is optimized for scanning speed, using a few extra bits for error correction means that the system can work when the bit reading is less reliable, so that the energy of each received bit can be reduced. This can allow higher modulation speeds and faster scanning. However, an excessive number of bits used for error correction may be counterproductive. For example, if more than 2xL bits are used, error correction coding may become less effective than at a lower rate modulation.

[0109] In another example, the sequence can be combined with Manchester encoding to allow self-synchronization and immediate estimation of 0 and 1 levels, which is possible even when the beam signal received at the detector has varying intensity while the beam is steered to sweep across the target.

[0110] Figure 4 A block diagram of an optical wireless communication system with a fast acquisition function according to an embodiment is schematically shown.

[0111] In this embodiment, a transmitter 10 (e.g., an AP or other LiFi device) uses a light source (LS) 48 (e.g., an LED or laser) to direct a steerable light beam 20 to a receiver 30 in a communication system. The communication system may be switched by an electronic or mechanical or software-based switching element (SW) 49 to add or combine or embed a user data stream generated by a data source (DS) 43 for communication, or a pseudo-random sequence or other direction-related address information generated by a code generator (CG) 44 for acquisition during the acquisition phase.

[0112] The receiver 30 at the receiving end (e.g., an EP or other LiFi device or mobile user equipment) has a transmitter (TX) 412 for providing transmission via a feedback channel 40. The feedback channel 40 may have a lower transmission / data rate (bandwidth), or may have other causes of latency and latency variability, and may be used to provide feedback over a separate channel, such as an omnidirectional optical transmission, or an omnidirectional RF transmission (such as Bluetooth Low Energy (BLE), Wi-Fi, etc.).

[0113] In addition, the receiver 30 includes a photodetector (PD) 418 (e.g., a semiconductor, PIN or avalanche photodiode, a phototransistor or other optical detector chip, etc.) for detecting hits by the light beam 20 when the light beam 20 is steered along a predetermined or random trajectory 200 by a position actuator (PA) 47 of the transmitter 10. In addition, the receiver 30 includes a data receiver (DRX) 416 for receiving a data stream transmitted by the transmitter 12 during a communication phase following the acquisition phase and amplified by an amplifier 417 of the receiver 30.

[0114] During the acquisition phase, a code detector (CD) 414 (e.g., a matched filter or other filter or decoder) is used to decrypt the received pattern (i.e., a pseudo-random sequence or other direction-related information) added to, combined with, or embedded in the received beam 20 from a given code into its original form. The derived pseudo-random sequence or other direction-related information is then transmitted back to the transmitter 10 via the feedback channel 40.

[0115] The fed-back pseudo-random sequence or other direction-related information is received by a feedback receiver (RX) 46 of the transmitter 10 for receiving omnidirectional transmissions or RF transmissions (such as Bluetooth Low Energy (BLE), Wi-Fi, etc.) from a receiving end.

[0116] The received pseudo-random sequence or other direction-dependent address information is forwarded to a microprocessor (MP) 42 or other processor or controller of the transmitter 10 and may be stored by the microprocessor 42 in a memory (MEM) 41 for later retrieval.

[0117] In addition, the microprocessor 42 uses the received pseudo-random sequence or other direction-related information to derive the position of a receiver (e.g., receiver 30) that has been hit by the steerable beam 20 and controls the phase actuator 47 to steer the beam 20 of the light source 48 to point to the derived receiver position, for example based on the current beam position signaled by the position sensor (PS) 45 and stored in the memory 41.

[0118] During the initial acquisition phase, the light source 48 may be controlled (e.g., by the microprocessor 42) to generate a steered light beam 20 having a wider beam angle than that used for communication to sweep across the desired scanning area (e.g., a room). Since the feedback channel 40 may be far from instantaneous, reports of hits may arrive with significant and potentially varying delays. Such delays and temporal resolution of the feedback channel 40 may be so coarse that the transmitter control system (e.g., the microprocessor 42) may not be able to adequately return to (re-determine) the specific beam settings of the light source 48 that hit the target detector (e.g., the photodetector 418).

[0119] According to this embodiment, the transmitter 10 can therefore be controlled (e.g., by a microprocessor 42) to add, combine, or embed a code data stream (i.e., a pseudo-random sequence or other direction-related information) obtained from the code generator 44 into / into the light beam 20 and traverse a search area (i.e., a desired scanning area) at a certain speed according to the trajectory 200, so that a receiving device (e.g., a receiver 30) in the coverage area of ​​the acquisition search will receive sufficient portions of the code data stream to derive the encoded current direction of the steered light beam 20.

[0120] After acquiring the direction of the target detector, a directional lock toward the target detector can be established and the system can switch to data communication (e.g., using beam tracking). The locked direction can also be recalibrated intermittently (especially when the resolution is (much) smaller than the beam width) so that the beam 20 can be kept centered at the receiver 30.

[0121] In one example, during a scanning operation based on the trajectory 200 in the initial acquisition phase, the transmitter 10 embeds direction-related address information by modulating the steered light beam 20. The address information (e.g., address identification (ID)) can be a unique identifier of the location and / or direction of the target detector. In order to identify the entire coverage area A C Medium spot size A B All possible positions of (beam coverage area) require at least n addr =log 2 (A C / A B ) bits.

[0122] One option for adding address information to the steered beam 20 is to use an address embedded in a data packet with a header and synchronization words. For example, 8 bits per axis may be used, so that 16 bits are required for a two-dimensional scan area with axes x and y, plus a unique header word for frame synchronization (typically 8 or 16 bits), plus an 8-bit cyclic redundancy check (CRC) code. Thus, a total of 32 bits of address packets would be required, where the header bits should not coincide with any data values.

[0123] During the search in the acquisition phase, the light beam 20 may have a non-zero velocity and may move over the target detector while transmitting its address ID. Since the photodetector may only partially overlap the area where the complete address ID can be recovered, more overlapping beam directions may have to be searched and identified, which may require potentially more bits of the address ID. In one example, to ensure that a complete sequence can be detected, the receiving system at the receiver 30 may be configured to receive 64 bits.

[0124] As another example, 16 bits per axis (i.e., 32 bits for the x-axis and y-axis), a unique header word for 16-bit frame synchronization, and an 8-bit CRC check may be used, resulting in an addressing identifier packet of 64 bits in length. To ensure that a complete sequence is seen, the photodetector may now need to receive 128 bits.

[0125] Figure 5 A sequence of beam coverage areas with address IDs along a beam scanning trajectory is schematically shown as an illustrative example of the proposed concept and its advantages.

[0126] exist Figure 5 In the example of , the address ID of the beam is selected to encode seven positions, and three bits ("000" to "111") are used to identify the beam direction. Additionally, a header "HEAD" of four symbols is used as a preamble of the address ID.

[0127] The light beam can be moved continuously along the trajectory, because the actuation of the steering can be several orders of magnitude slower than the signaling of the data. The circle indicates the area where a packet with the address ID can be received in full at the beginning of packet transmission. The circle of the next packet also shows the area where the previous packet can be received at the moment of the end of the packet. Therefore, the area where a complete packet can be received is the common area of ​​the lens shape of the two consecutive circles. One of these areas is Figure 5 The area of ​​the lens shape is indicated as a shaded lens in . The area of ​​the lens shape is much smaller than the beam size because the target detector must be in the beam at the beginning and at the end of the transmission.

[0128] In one embodiment, the transmitter 10 may be an AP of an OWC network, and the receiver 30 may be an EP of the OWC network. In this case, the downlink (AP to EP) transmission may be a high-speed transmission, and the feedback transmitter 412 of the (one or more) EPs may be configured as an omnidirectional transmitter, and the photodetector 418 of the EP may be configured as an angle diversity receiver.

[0129] In an alternative embodiment, the proposed fast acquisition scheme can be used in a peer-to-peer link between two FSO devices, where each FSO device is capable of beam steering. That is, each FSO device at the two transmitting ends includes Figure 4 The transmitter 10 and the receiver 30 of the FSO device are shown in Figure 1. Thus, each FSO device can be configured to search for another FSO device using the onboard transmit and receive functions, so that two direction determinations can be initiated in parallel or sequentially. In the latter case, when an angle diversity receiver is used, the second determination can utilize a narrower search area.

[0130] In one embodiment, Figure 4 The transmitter 10 can be configured to always scan the entire scanning area at a fixed clock speed and fixed trajectory 200 of beam directions. Then, any specific received direction-related address ID can correspond to a specific direction that can be calculated back. The transmitter 10 can also be configured to record an exhaustive list of all addresses and corresponding beam directions in the memory 41, but as mentioned above, this may require 2 times the sequence length for L bits. L -1 The memory at the address location.

[0131] In another embodiment, the transmitter 10 can be configured to store only a few address points along the trace 200. A search can then be performed by clocking the code generator 44 (e.g., a linear feedback shift register (LFSR)) backwards starting from the received address until the stored location is reached. In fact, since the output stream is reversible, a code generator (e.g., an LFSR) with mirrored taps can cycle through the output sequence in reverse order.

[0132] In other embodiments, the pseudo noise sequence (eg, address ID) and the bit clock may remain fixed regardless of the beam sweep speed. When the beam moves slower, the photodetector will see a longer set of bits, so the receiver 30 can better correct errors.

[0133] In yet another embodiment, the mapping of address ID to beam position / direction may not be fixed. That is, in another scan, another direction may correspond to a specific address ID. The transmitter 10 can be configured to continue to maintain only the pseudo-noise sequence of the address ID and record the outgoing beam position or beam direction for each clock cycle or at a certain sub-sampling rate (e.g., once every 64 clock cycles). The beam position / direction at these intervals can be interpolated to obtain the final position / direction. In this setting, if a galvanometer mirror is used as / in (one or more) position actuator elements 47 to manipulate the light beam 20, the beam direction can be monitored by obtaining the beam angle from feedback from a galvanometer angle sensor (e.g., provided as a position sensor 45).

[0134] Further details of suitable galvanometer mirrors are described in "High speed laser scanning microscopy by iterative learning control of a galvanometer scanner", Control Engineering Practice, volume 50, 2016, pages 12-21, ISSN 0967-0661 by Han Woong Yoo et al., where iterative learning control (ILC) of a galvanometer scanner is proposed to achieve high-speed, linear and accurate bidirectional scanning of a scanning laser microscope. The galvanometer scanner, which is a low-stiffness actuator, is first stabilized with feedback control that compensates for disturbances and nonlinearities at low frequencies, and the ILC is applied to the control of fast scanning motions. For stable inversion of non-minimum phase zeros, time delay approximation and zero phase approximation are used for the design of ILC, and their achievable bandwidths are analyzed. The experimental results can be used to verify the advantages of ILC: it has a wide control bandwidth, thereby achieving faster, more linear, and more accurate scanning without phase lag and gain mismatch. At a scan rate of 4112 lines per second, the ILC can achieve a 73-fold reduction in root mean square (RMS) error compared to a commercial system's feedback-controlled galvanometer scanner.

[0135] In one example, for the galvanometer motor and mirror assembly of the position actuator element 47, a small beam diameter scanning galvanometer mirror system may be used that is operated by a 100 Hz square wave, a 175 Hz triangle wave or sawtooth wave, or a 250 Hz sine wave.

[0136] In fact, the trajectory 200 to be scanned may be dynamically selected based on a priori information that the transmitter 30 may have stored in the memory 41. Thus, the user movement may be tracked, and if the communication link is lost, the system may scan over a path that is first searched at the most likely location given the previously observed and stored movement trajectory.

[0137] If the transmitter 10 is on a mobile device, it can use its own gyroscope and acceleration sensors to bias the search to check possible new directions first. A Kalman filter and / or a particle filter can be used to track the most likely new positions, thereby calculating the best or preferred search direction.

[0138] The solution proposed according to various embodiments described herein allows for the detection of mechanical delays in the loop. In particular, if the scan is done quickly and the beam dwell time is a few nanoseconds, the mechanical beam direction may be out of sync with the beam modulation process. This may occur, for example, in a feed-forward control loop, where the transmitter 10 is controlled (e.g., by the microprocessor 42) to set the beam 20 to a position near the hit address reported via the feedback channel 40, without correcting the error between the actual beam direction and the beam direction assumed by the transmitter 10. It is then controlled to search slowly around this position at a speed that minimizes overshoot and allows the system to find a locked position.

[0139] In one example, the desynchronization may be estimated based on, for example, known mechanical properties of the beam steering system, and / or the scans may be done in reverse order so that the desynchronization error may now be opposite to the first scan.

[0140] In another example, a second scan may be done according to another grid, for example rotated 90 degrees. The location of the target detector may then be at the intersection of the first and second searches.

[0141] In a further embodiment, a closed loop mirror positioning system may be used in which the angular orientation (position) of the mirror of the position actuator element 47 is optically encoded using a photocell array and a light source, both of which are integrated into the interior of the galvanometer housing. Each mirror orientation corresponds to a unique ratio of the signal from the photodiode, which allows closed loop operation of the galvanometer mirror system.

[0142] In one example, the galvanometer systems can be driven to scan their full mechanical range of ±12.5° at a frequency of 100 Hz when a square wave control input voltage is used, or at a frequency of 250 Hz when a sine wave is used. For a single small angular step of 0.2°, the mirrors require 300 μs to stop at the commanded position. The scanning frequency range can be DC to 1 kHz, and the angular resolution can be 0.0008° (15 μrad).

[0143] In another example, a galvanometer system can consist of a galvanometer-based scanning motor with an optical mirror mounted on the motor shaft and a detector that provides position feedback to a control board (e.g., microprocessor 42). To provide fast response time and high system resonant frequency, a moving magnet design for the galvanometer motor can be selected rather than a fixed magnet and rotating coil design. The position of the mirror can be encoded using an optical sensing system located inside the motor housing.

[0144] Due to the large angular accelerations of the rotating axis, the size, shape, and inertia of the mirror can become a significant factor in the design of a high-performance galvanometer system. In addition, the mirror may need to remain rigid (flat) even when subjected to large accelerations. All of these factors need to be balanced to match the characteristics of the galvanometer motor and maximize the performance of the system.

[0145] Galvanometer mirrors can be coated with silver or gold, or can have broadband or high power dual band coatings.

[0146] In one example, if there is no known communication partner, the search pattern of trajectory 200 may spiral outward from the historical position to mitigate the loss of the scanning mechanism as compared to a grid-type line scan. Otherwise, if the communication partner is previously known, the scanning process may begin from the last known position of the communication partner before spiraling outward.

[0147] Although the above embodiments have been described in connection with indoor (LiFi) applications, the proposed solution can also be advantageously applied to long distance communications where the beam divergence can be of the order of millirads. For example, if a satellite at a distance of tens, hundreds or thousands of kilometers has a position with an uncertainty of several kilometers before acquisition, even with a priori knowledge of the trajectory. The search time for a beam with a width of a fraction of a meter can be very long, because the number of possible positions is very large and the propagation delay in the link is also very long.

[0148] Figure 6 The sequence of beam coverage areas and associated sub-areas along a scanning trajectory of a beam with an address ID is schematically shown.

[0149] exist Figure 6 In , circles C1, C2, C3 represent the coverage area of ​​the light beam at the instants t1, t2 and t3 at the start of packets 1, 2 and 3, respectively. The instants s1, s2, s3 indicate the corresponding detection ends of the coverage area at the central observation point. Therefore, the position of the coverage area of ​​the light beam as the light beam evolves over time according to the linear beam motion (BM) is shown. When the light beam covers the circle C1 including the sub-areas A1, A2 and A3, the first address ID starts to be transmitted sequentially. When the light beam occupies the positions of circles C2 and C3, respectively, the transmission of the second and third address IDs starts. In order to correctly receive the complete first address ID, the receiver must see the light beam at the start instant t1, at the end instant s1, and at any intermediate instants. A sufficient condition is that it sees the light beam at the start instant t1 and at the end instant s1, because this means that it sees the light beam during this duration (in Figure 6 The beam is also seen at any intermediate moment (indicated as Dp in ). This applies to a receiver located in the intersection area of ​​C1 and C2 (and thus the combined area of ​​A2 and A3). Similarly, it can be considered that the second address ID can be received in the intersection area of ​​C2 and C3 (and thus the combined area of ​​A3 and A4). In area A3, both the first and second address IDs can be detected. This seems to indicate that sub-area A3 gives redundant address ID information and that the sub-areas of A3 can be reduced to zero to rely entirely on sub-areas A2 and A4 only. However, this is not advantageous because the width of the guaranteed beam delivering at least one address ID in this area is reduced to zero. Figure 6 In FIG. 5 , the scan width SW corresponds to the vertical size of the sub-area A3.

[0150] Figure 7 The beam coverage area along the beam scanning trajectory, the sequence of associated sub-areas A1 to A5, and the distances and angles Φ between characteristic points Q1 to Q4 required for determining the optimal scanning speed are schematically shown.

[0151] Assuming that the bandwidth of the photodetector is a limiting factor and that the time taken to communicate a single address ID is limited to a maximum duration, the optimal scan speed can be calculated. The trade-off is to optimize beam width and search speed (scan speed). One extreme is to move the beam as fast as possible to ensure that the horizontal displacement is maximum and the scan width becomes zero, so that in the case of a grid pattern, the area needs to be scanned along many parallel very closely spaced beam paths. The other extreme is to move the beam slowly to ensure a large beam scan width, perhaps even close to the beam width, but the forward progress is small.

[0152] Without loss of generality, the beam coverage area (intersection area C i) can be set to one. Then, the area A effectively scanned during each address ID transmission within the scan width (SW) is equal to:

[0153] A=2d(Q3,Q4)d(Q1,Q3),

[0154] where d(Qx,Qy) represents the distance between points Qx and Qy, d(Q3,Q4) = sinΦ, and applying Pythagoras, d 2 (Q1,Q3)=1-sin 2 Φ=cos 2 Φ.

[0155] therefore:

[0156] A=2sinΦcosΦ=sin2Φ,

[0157] It has a maximum value at Φ=45°.

[0158] Therefore, the optimal scanning speed is sinΦ=Sqrt2 / 2=0.7 beam radius per address ID duration.

[0159] For N p The address ID grouping of symbols and the maximum data rate r m (in symbols per second), the system p / r m Scan 0.7 times the beam radius R beam , and the area scanned per second is as follows:

[0160]

[0161] In the following examples, (for example, by Figure 4 The code generator 44 of the receiver 30 uses a long pseudo-random sequence to signal the beam direction / position without the need for a header structure. Therefore, the time the beam needs to spend above the target photodetector 418 of the receiver 30 can be reduced. In addition, significantly higher resolution of the position within the beam can be achieved.

[0162] In one example, the address ID may be generated from the LFSR sequence by running a linear feedback shift register continuously and uninterruptedly, and the output sequence may be used as the address ID in the steered light beam 20 .

[0163] As combined Figure 3As already mentioned, any subsequence of L bits uniquely identifies the relevant position in the long LFSR sequence. In the event of any error in the received sequence, adjacent bits can be used for error correction, since the generating polynomial is known. Thus, adjacent bits are completely determined by any L consecutive bits. If the received longer bit sequence deviates from the generator rule, an error is detected. The most likely correct bit sequence is the one with the lowest Hamming distance to the received sequence.

[0164] The photodetector 418 detects (sees) a limited portion of the LFSR sequence and is therefore configured so that it detects at least L symbols in any position in the scan area, where L is the length of the code for the address ID. Any such L symbols (e.g., bits) uniquely determine the position in the entire code sequence.

[0165] If more than L symbols are received, these additional symbols increase redundancy and allow error correction or allow improvement in resolution in the position estimate.

[0166] Figure 8 A block diagram of an exemplary LFSR is schematically shown. Figure 4 An example of a code generator 44 having a length of three bits.

[0167] A shift register is a digital circuit that uses cascaded flip-flops D1 to D3 (storage locations for one bit) where the output of one flip-flop is connected to the input of the next flip-flop. They share a single clock signal, which causes the data stored in the system to shift from one location to the next. By connecting the last flip-flop back to the first flip-flop, data can be circulated within the shifter for an extended period of time.

[0168] In the LFSR of this example, the output of the second flip-flop D2 is logically added to the output of the third flip-flop D1, and the result is input to the first flip-flop D3, whereby the desired LFSR sequence can be obtained at the output of the third flip-flop D1.

[0169] In one example, the three flip-flops D1 to D3 of the LFSR are set to the seed pattern “001”. Then the output of the third flip-flop D1 will be a continuously repeating 7-bit sequence “1001011”.

[0170] Fig. 9 Shows Figure 8A table of output values ​​of the LFSR of the LSFR is shown. The table shows the timing clock pulses CP (0 to 7) and the associated LSFR outputs at the flip-flops D3 to D1 for each specific clock pulse. The output values ​​are stored in the flip-flops as buffer contents that can be used as outputs of the LFSR during a later moment. These buffer contents can then be assigned to locations where the receiver may be located. Therefore, the output sequence of the LSFR can be used as an address that uniquely identifies a moment in time, and thus identifies the current position of the steered light beam 20.

[0171] The output values ​​of flip-flops D3, D2, and D1 can be interpreted as an address in the LFSR sequence. Such addresses have an interesting property: each address has L bits (here L=3), where the L-1 bits are cyclically shifted bits from the adjacent address. In the proposed embodiment, this property can be used in a new context. As the beam 20 advances along the trajectory 200, these common bits are used in multiple adjacent addresses.

[0172] Fig.10 The beam coverage area and the sequence of associated symbols along the beam scanning trajectory 200 are schematically shown.

[0173] The light beam 20 travels along the X axis (position). Figures 5 to 7 , the circles indicate the area of ​​the beam where the corresponding symbol written in the center of each circle begins. The photodetectors located in the vertically shaded area receive the symbol "0101". Both the sequences "010" and "101" are unique addresses in the longer overall sequence. The photodetectors in the horizontally shaded area receive the symbol "010", which still gives a unique position in the sequence.

[0174] Performance criteria for the proposed acquisition system may be how many square meters can be scanned per second, and the accuracy with which the direction or target position can be estimated from such scans.

[0175] The proposed LFSR code embedding for signaling beam direction / position is significantly faster than using an address ID separated by frame synchronization and error correction codes because there is no overhead from the synchronization code and other header information. In addition, the resolution of the LFSR approach is higher by a factor approximately equal to the length of the LFSR sequence.

[0176] While the address method with synchronization and header can be used to report the detector position with an accuracy of approximately one beam width, the LFSR method allows the detector position within the beam to be measured with a longitudinal accuracy of 1 / N times the beam width (where N is the number of symbols “across the beam width”) and a lateral accuracy of approximately Sqrt2 times the beam width.

[0177] In one example, the LFSR address sequence can be used for error correction by taking advantage of the fact that the code for the address ID is generated by a LFSR of length L, based on a feedback polynomial known at the receiver 30. Thus, during each clock cycle, parity bits can be generated on a subset of the previous L bits (i.e., those bits that appear in the feedback polynomial), which are XORed to generate the next input.

[0178] The algorithm to find the position / direction of the receiver being hit by the moving beam is to feed the first received bit of the address ID into the LFSR at the receiver 30 and compare the predicted next value with the received value. If a mismatch occurs, then either the newly received bit is erroneous or one of the already received bits participated in the generation of the newly received bit. This gives a number of bits that are suspected to be erroneous. By continuing to receive more bits, with each new bit, yet another "XOR equation" becomes available. Depending on the number of errors occurring simultaneously and the number of additional bits received, it may even be possible to repair the incorrectly received bits.

[0179] In a further embodiment, the code generator 44 of the transmitter 10 may use a (differential) Manchester code to obtain the address ID.

[0180] Differential Manchester (DM) code is a line code in digital frequency modulation, in which data and clock signals are combined to form a single two-level self-synchronous data stream. In various specific applications, this method is also called by various other names, including biphase mark code (CC), F2F (Frequency / Frequency Multiplication), Aiken biphase and conditional biphase.

[0181] This specific type of code avoids missing zeros at the beginning or end of a hit and balances the DC component in short run lengths. If on-off keying (OOK) is used for code transmission, the photodetector 44 may not be able to distinguish the first symbol initially received (if these symbols are set to zero). In fact, there is no difference in the received signal between a beam that does not hit the detector and a logical zero in the beam that does hit the detector. Similarly, at the end of a hit, a zero will be indistinguishable from a beam that is already outside the detector area. This can be avoided using a line code. Manchester code is a very efficient line code that associates the power-on signal with both a logical zero and a logical one.

[0182] Thus, even in the case where the address IDs have short sequences, improved synchronization can be achieved.

[0183] Furthermore, using bi-phase coding such as (differential) Manchester coding, the receiver 30 receives reference zero and one values ​​during each symbol period to be able to track variations in received signal strength, for example between the centre and the periphery of the received beam.

[0184] Fig.11 FIG. 1 shows a generalized flow chart of a fast acquisition process at a transmitter according to one embodiment, which process may be performed by, for example, Figure 4 The microprocessor 42 is based on the data stored in Figure 4 The instructions in the memory 41 are executed.

[0185] In an initial step S111 , a pseudo-random sequence is embedded as a direction-dependent address ID into a light beam generated by a light source, and the light beam is scanned along a predetermined or random trajectory for acquisition.

[0186] Then, in step S112, it is checked whether a hit with a direction-dependent code (address ID) has been reported from another transmission via the feedback channel.

[0187] If not, the process jumps back to step S111 and continues the scanning process.

[0188] Otherwise, if it is determined in step S112 that a hit has been reported via the feedback channel, the process proceeds to step S113, where the position of the reported hit detector is derived based on the embedded code signaled in the report. This can be achieved, for example, by a memory lookup process and can include an error correction process.

[0189] In the following step S114, the beam direction is fixedly set (locked) to the lead-out position hitting the detector. Optionally, a fine adjustment (loop) can be activated to optimize the retrieved direction.

[0190] Finally, in step S115, the transmitter switches to data communication and a tracking mechanism for tracking any movement of the hit detector. That is, once the beam has found a hit and has converged to a locked communication link, tracking can be achieved by other schemes or mechanisms (such as using a four-quadrant detector).

[0191] Fig.12 A general flow chart of a fast acquisition process at the receiver side according to one embodiment is shown, which may be performed by a microprocessor ( Figure 4 Not shown) based on the receiver's memory ( Figure 4 ) to execute instructions stored in the .

[0192] In step S121 , a photodetector at the receiver is activated to allow hit detection.

[0193] Then, in step S122, the receiver checks whether the photodetector has detected a hit of the light beam. If not, the process jumps back to step S121, and the photodetector remains activated and ready to detect.

[0194] Otherwise, if the photodetector has detected a hit, the process proceeds to step S133, where the receiver checks, for example based on a lookup table, whether a valid address ID has been detected during the hit. If not, the process jumps back to step S121, and the photodetector remains activated and ready to detect.

[0195] Otherwise, if the photodetector has detected a valid address ID during the hitting period, the process proceeds to step S124, where the receiver is controlled to transmit the received valid address ID to the transmitter via the feedback channel.

[0196] This enables the transmitter to direct its light beam toward the receiver and begin data communications with the receiver.

[0197] In summary, a target acquisition method and system for a steerable beam in an OWC or FSO system has been described in which the duration during which a steerable beam at a transmitter must illuminate an optical detector at a receiver for acquisition can be minimized and a slow feedback channel from the receiver to the transmitter can be allowed. In one example, this can be achieved by transmitting a pseudo-random sequence having properties that knowledge of a small number of bits uniquely predicts the entire sequence. In another example, a linear feedback shift register sequence is a suitable and very efficient choice.

[0198] As used herein, a trajectory may be described as a scan path and / or alternatively as a series of changes in the direction of the beam used. Using the scan path approach, the trajectory corresponds to the position that is subsequently illuminated by the beam where we expect the communication partner to be. In contrast, using the direction approach, the same trajectory corresponds to a series of changes in the direction of the beam used to find the communication partner.

[0199] Intuitively, a trajectory can be defined, for example, as the scanning path that the light beam follows on the floor when scanning an (empty) room. Since the communication partner will usually not be on the "floor surface", but more likely at tabletop height, the trajectory should preferably scan a height of, for example, about 1 meter of the room, but as indicated, this depends on where we expect the communication partner to be.

[0200] When we take a bird's eye view, the trajectory can describe a path using fast horizontal left-to-right scans and right-to-left scans (which are iterated) combined with slow top-to-bottom scans. The trajectory in this case can be described as a scan path on a surface at, for example, 1 meter high.

[0201] However, this scan path is equivalent to a sequence of beam direction changes using fast horizontal left-to-right scans and right-to-left scans (which are iterated) combined with a slow top-to-bottom scan. Therefore, when describing the trajectory, these two methods can be used to equal effect.

[0202] Although the "zigzag" trajectory highlighted above is a simple pattern, many alternatives are possible. For example, instead of using a "zigzag" pattern, one could also start scanning in the center of the room and spiral outwards, such a trajectory having the advantage that detection can occur earlier if the communication partner is more likely to be present in the center of the room. Many more variations are possible, including patterns where some positions are covered more often than others.

[0203] Although the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description should be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed LiFi-based embodiments, but can be applied to all kinds of optical wireless networks with steerable light beams.

[0204] By studying the drawings, the disclosure and the appended claims, those skilled in the art can understand and implement other variations of the disclosed embodiments when practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "one" or "an" does not exclude multiple. A single processor or other unit can implement the functions of several items listed in the claims. The mere fact that certain measures are listed in mutually different dependent claims does not indicate that the combination of these measures cannot be used advantageously. The foregoing description describes certain embodiments of the present invention in detail. However, it will be appreciated that no matter how detailed the above appears in the text, the present invention can be practiced in a variety of ways and is therefore not limited to the disclosed embodiments. It should be noted that when describing certain features or aspects of the present invention, the use of specific terms should not be understood to imply that the term is redefined here to be limited to include any specific characteristics of the features or aspects of the present invention associated with the term.

[0205] A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0206] Similar to Fig.11 and Fig.12 The described operations of the operations indicated in the can be implemented as program code means of a computer program and / or dedicated hardware of a transmitter device, a receiver device or a transceiver device, respectively. The computer program can be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium, provided together with other hardware or as part of other hardware, but can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

1. A source device for controlling acquisition of a target receiver (30) by a steered light beam (20) for optical wireless communication, wherein the source device is configured to: adding address information indicating a current direction or position of the steered light beam (20) to the steered light beam (20); Control has length L and sequence length 2 L -1 hybrid or linear feedback shift register to generate pseudo-random sequences; Scanning a scanning area along a trajectory (200) by means of a steered light beam (20) while outputting a continuous bit stream of a pseudo-random sequence; and deriving the location of the target receiver based on address information received from the target receiver (30) in a hit report via a feedback channel (40), and Each bit sequence of length L uniquely specifies a different beam position along the trajectory.

2. The source device of claim 1, wherein the source device is configured to continue scanning with the steered light beam (20) without awaiting receipt of a hit report. 3 . The source device of claim 1 , wherein the source device is configured to combine the pseudo-random sequence using Manchester encoding.

4. The source device of claim 1, wherein the source device is configured to generate a steered light beam (20) having a beam coverage area (22) wider than a beam coverage area for communicating with a target receiver (30) after acquisition.

5. The source device according to claim 1, wherein the source device is configured to set the number of bits of the address information to at least log2(A C / A B ), where A C represents the size of the scan area, and A B It represents the size of the coverage area of ​​the steered light beam (20).

6. The source device according to claim 1, wherein the source device is configured to set a scanning speed of the steered light beam (20) to 0.7 beam radius of the steered light beam (20) per duration of the address information.

7. The source device of claim 1, wherein the source device is configured to dynamically select a track (200) based on a priori address information stored in a memory (41).

8. A target device (30) for communicating with a source device (10) via a steered light beam (20) for optical wireless communication, wherein the target device is configured to: determining detection of a steered light beam (20) by a photodetector (418) during an acquisition scan phase; deriving from the detection address information embedded in the steered light beam (20) and indicative of a current direction or position of the steered light beam (20); and transmitting the derived address information in a hit report to a source transmitter (10) via a feedback channel (40); wherein the address information is derived by reading a predetermined number of adjacent bits from the detection, the predetermined number of adjacent bits representing a pseudo-random sequence indicating a position of the light beam along a trajectory embedded by the source device, and wherein the predetermined number is at least the base 2 logarithm of the length of the pseudo-random sequence.

9. A target device according to claim 8, wherein the target device is configured to capture more than a predetermined number of bits due to a capture area (32) of the photodetector (418) being larger than a minimum capture area (34), and use the excess bits for error correction or for detecting a more specific position of the photodetector (418) within the beam coverage area (22) of the steered light beam (20).

10. An optical wireless communication device (10, 30) comprising at least one of the source device according to claim 1 and the target device according to claim 8.

11. The device (10, 30) of claim 10, further comprising a position actuator element (47) having at least one galvanometer mirror for steering the steered light beam (20).

12. An optical wireless communication system comprising at least one optical wireless communication device (10) having a source device according to claim 1 and at least one optical wireless communication device (30) having a target device according to claim 8.

13. A method of controlling acquisition of a steered light beam (20) to a target receiver (30) for optical wireless communication, the method comprising: adding address information indicating a current direction or position of the steered light beam (20) to the steered light beam (20); Control has length L and sequence length 2 L -1 hybrid or linear feedback shift register to generate pseudo-random sequences; The steered light beam (20) scans the scanning area along the trajectory (200), while outputting a continuous bit stream of a pseudo-random sequence; and deriving a location of a target receiver (30) based on address information received from the target receiver (30) in a hit report via a feedback channel (40), and Each bit sequence of length L uniquely specifies a different beam position along the trajectory.

14. A method of arranging communication with a source device (10) via a steered light beam (20) for optical wireless communication, wherein the method comprises: determining detection of a steered light beam (20) by a photodetector (418) during an acquisition scan phase; deriving from the detection address information embedded in the steered light beam (20) and indicating a current direction or position of the steered light beam (20); and transmitting the derived address information in a hit report to the source transmitter (10) via a feedback channel (40), wherein the address information is derived by reading a predetermined number of adjacent bits from the detection, the predetermined number of adjacent bits representing a pseudo-random sequence indicating a position of the light beam along a trajectory embedded by the source device, and wherein the predetermined number is at least the base 2 logarithm of the length of the pseudo-random sequence.

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