Patent Application
20250219740
Optical on/off-keying (OOK) utilizes a signal that have two states that are designated as on or off. Data is encoded using OOK to modulate an optical signal and then decoded by demodulating the signal when received. Each state represents a binary value of zero or one. Demodulation traditionally utilizes a direct detection receiver architecture due to its simplicity. Pulse-position modulation (PPM) can be considered an extension of OOK wherein the encoding selects a particular timeslot in a range of possible timeslots where a transmitted signal is turned on versus turned off. Furthermore, OOK modulation is only one protocol type, and newer optical communications transceivers tend to utilize phase-shift keying (PSK) modulation types due to greater spectral efficiency. With PSK modulation types, demodulators must be able to detect and recover optical signal phase measurements to continuously track and recover an optical carrier phase (termed “coherent”). With such PSK modulation, direct detection receiver architectures are not sufficient.
In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.
Prior optical receiver front-end architectures did not allow for coherent phase recovery of received optical on/off-keying (OOK) signals. Prior research has indicated that carrier phase recovery may be possible if performed in the optical analog domain via beat detection to phase lock a receiver laser to a pulsed transmitter laser. However, such work does not shed light on how to perform phase recovery when the local mixing laser is not physically locked input signal carrier frequency in the analog domain.
For OOK receivers, there is an opportunity to realize a communications performance improvement as long as both symbol timing and carrier frequency/phase can be simultaneously resolved and recovered versus the straightforward approach of ignoring phase information altogether for the OOK modes and only performing magnitude-based timing recovery and demodulation.
An improved receiver system effectively and practically recovers both symbol timing and carrier frequency/phase using customized digital signal processing (DSP) techniques and algorithms, resulting in an example improvement of approximately 2 dB over straightforward approaches. In one example, an all-digital approach may use a coherent optical receiver front end with analog signal processing approaches.
The improved receiver system may be used for optical intersatellite nodes, fiberoptic communications using OOK, or PPM (pulse position modulation) modulation, and/or even simultaneous laser ranging and communication (LiDAR (light detection and ranging)+Comm).
The improved receiver system may also be used in the wireless RF (radio frequency) and microwave spaces, OOK and PPM are used mainly in low cost, power efficient applications, such as remote controls, garage door openers, wireless doorbells, keyfobs, etc in the ISM band, but it could also be useful in low power sensor networks where the receiving base station can tolerate higher complexity in cost to enable better performance, but the high volume of transmitter modules would benefit greatly from lower cost.
In one example, the improved receiver system performs a coherent detection for what would be considered an intensity modulated waveform. The improved receiver includes a coherent optical front-end receiver that utilizes a reference laser LO (locally mixed) and quadrature detection of either a polarization-invariant or dual polarization signal using either single or double-balanced photodetectors/outputs to process a received intensity modulated waveform, such as an OOK optical waveform. A symbol/slot timing recovery circuit includes a timing error detector, a loop filter, and a steered sampling oscillator (numerical or otherwise). A carrier frequency/phase recovery circuit, either feed-forward or feedback-based is used to recover the frequency and phase (e.g. PLL). Optionally, a dual polarization path combining circuit may be used where dual polarization is performed. A soft or hard decision demodulation circuit is used to provide a final digital output of the received intensity modulated waveform.
The improved receiver system is highly beneficial for free space optical communications such as OISL because OOK modulations are very common. Quantitatively, a 2 dB communication performance improvement can mean either ˜60% better transmitter power efficiency, or 2.5× further communication range, or a ˜2× better bit error rate, or a combination of improvement in any or all of these aspects. These improvements create a positive net impact to network performance and uptime and can therefore considerably reduce costs by permitting the use of fewer nodes in an infrastructure, especially impactful for expensive communication network infrastructure.
Receiver font end 100 includes a reference laser 127 that generates a reference ELO signal 130 that is selected to match the carrier frequency. However, due to design tolerances and changes in the received optical signal 110, the reference ELO signal 130 may not exactly match that received optical signal 110 in phase and/or frequency.
The signals are coupled at a coupler 135 to perform mixing and to generate coherent signals on line 140 ((1/(2−2)) (ES+EASE1+jELO)) and 150 ((1/(2−2)) (jES+jEASE2+ELO)), which are detected via balanced photo detectors 155 and 160 to generate a electrical output 170. Receiver front end 100 ensures the reference ELO signal and received optical signal 110 are coherent for purposes of generating output 170 via photo detectors 155 and 160. The output 170 may then be further processed to produce a digital output for decoding. In other examples described in further detail below, the received optical signal and reference ELO signals may be split into polarized signals, which are then detected by four balanced photo detectors prior to producing the output 170 in the form of an electrical signal representative of the received optical signal 110 and the reference ELO signal 130. When polarized in that manner, polarization combining may be performed prior to demodulation.
The symbol timing recovery circuit is used to coherently recover phase and demodulate a received OOK signal. Circuits 330 and 335 have been added to the symbol timing recovery loop 325 to obtain complex-valued samples and magnitude values from the OOK sampled signals. The magnitude computation and bias removal are applied only on input to the timing error detector within the symbol timing recovery loop, but the complex-valued samples are passed out to the carrier phase recover circuitry 220 for phase recovery processing.
The CPR circuit 215 can take a variety of forms. One form could be that of the Viterbi & Viterbi CPR algorithm modified to meet the input for OOK receivers in any of its transmitted optical variants. Another would be to use a phase-locked loop (PLL) circuit in conjunction with the input. In this case, there is a novelty in gating the output of the phase error detector that feeds in to the PLL loop filter to improve performance specifically for higher order PPM modes, which has been modeled and simulated to show its improved performance versus non-gating techniques. There are also decision-directed and blind phase search algorithms that can be used for CPR.
Finally, for certain CPR algorithms that feed phase estimates back it is possible to apply these estimates prior to the symbol timing recovery circuit 215, effectively becoming a joint timing and phase recovery loop that allows both tracking loops to converge in cooperation and possibly lead to even better performance. There are generally known practical and other performance benefits to joint tracking loop operation, though the desired form would adaptively disable the complex magnitude operation prior to timing error detector 340 after locking the carrier phase digitally to improve the symbol timing recovery error estimates and would be particularly useful for common pointing-induced fading scenarios experienced in free-space optical communication links.
With dual-polarization front end input there is more than one input that is processed compared to previous OOK based receivers.
The improved coherent OOK receiver may be used in microwave/radio communications applications in addition to optical communications such as for use in low power sensor networks here if those transmitters can be simplified or even use PPM modes more heavily to become even more power efficient, with minimal effect on the receiver network complexity, but greatly increased relative range.
Traditionally, optical OOK demodulation utilizes a direct detection receiver architecture due to its simplicity. When the more complex coherent receiver architecture is used instead such as is done in Space BACN since other coherent modes are already supported, there is an opportunity to realize a communications performance improvement in the OOK modes as long as both symbol timing and carrier frequency/phase can be simultaneously resolved and recovered since any received symbol with large negative component would no longer be flipped to cross the decision threshold.
Operation 720 performs symbol/slot timing recovery on the electrical signal using a timing error detector. The timing error detector gates an output on a per symbol basis by utilizing symbol timing alignment information fed back from a downstream symbol synchronizer of the demodulation circuit. Carrier frequency and phase recovery is performed at operation 730 on a symbol/slot signal to generate a frequency and phase recovered signal. Carrier frequency and phase recovery may be performed continuously tracking and removing carrier and phase offset from the time aligned signal. Symbol/slot timing recovery in one example is based on the quadrature detection of the optical signal using a timing error detector to provide a time aligned signal. In one example phase correction is applied prior to symbol timing error detection.
Operation 740 demodulates the frequency and phase recovered signal via a demodulation circuit to provide a digital output representative of the received on-off keying optical signal.
Operation 720 may be performed by calculating a complex magnitude and removes bias prior to performing timing error detection. In one example, bias is not removed for performing carrier frequency and phase recovery.
Removing bias in operation 720 may be performed by calculating a bias estimate as a mean, median, or highest m-quantile of the complex magnitude, where m is an on-off keyed modulation order.
In one example, a locally mixed laser LO may be tuned to a selected carrier frequency.
Example 1 is a method that includes receiving an on-off keyed optical signal via a coherent optical front-end receiver utilizing a locally mixed laser LO for a reference signal to perform quadrature detection of the optical signal to generate an electrical signal. Symbol/slot timing recovery are performed on the electrical signal using a timing error detector. Carrier frequency and phase recovery are performed on a symbol/slot signal to generate a frequency and phase recovered signal. The frequency and phase recovered signal are demodulated via a demodulation circuit to provide a digital output representative of the received on-off keyed optical signal.
Example 2 includes the method of example 1 wherein performing symbol/slot timing recovery is based on the quadrature detection of the optical signal using a timing error detector to provide a time aligned signal.
Example 3 includes the method of example 2 wherein performing carrier frequency and phase recovery comprises continuously tracking and removing carrier and phase offset from the time aligned signal.
Example 4 includes the method of any of examples 2-3 and further comprising calculating a complex magnitude and removing bias prior to performing timing error detection.
Example 5 includes the method of example 4 wherein magnitude calculation and bias removal are avoided to enable performing carrier frequency and phase recovery.
Example 6 includes the method of example 5 wherein removing bias is performed by calculating a bias estimate as a mean, median, or highest m-quantile of the complex magnitude, where m is an on-off keyed modulation order.
Example 7 includes the method of any of examples 2-6 wherein the timing error detector gates an output on a per symbol basis by utilizing symbol timing alignment information fed back from a downstream symbol synchronizer of the demodulation circuit.
Example 8 includes the method of any of examples 1-7 wherein phase correction is applied prior to symbol timing error detection.
Example 9 includes the method of any of examples 1-8 and further comprising tuning the locally mixed laser LO to a selected carrier frequency.
Example 10 includes the method of any of examples 1-9 wherein quadrature detection is performed once for the received optical signal without any polarization splitting.
Example 11 includes the method of any of examples 1-10 wherein quadrature detection is performed separately for dual orthogonal polarizations of the of the received optical signal.
Example 12 is an optical receiver system that includes a coherent optical front-end receiver utilizing a locally mixed laser LO for a reference signal to perform quadrature detection of an on-off keyed received optical signal to generate an electrical signal. A timing error detector circuit is configured to receive the electrical signal and perform symbol/slot timing recovery. A carrier phase recovery circuit is configured to perform carrier frequency and phase recovery on a symbol/slot aligned signal to generate a frequency and phase recovered signal. A demodulation circuit is configured to receive and demodulate the frequency and phase recovered signal to provide a digital output representative of the received on-off keyed optical signal.
Example 13 includes the system of example 12 wherein the timing error detector is configured to perform symbol/slot timing recovery based on the quadrature detection of the optical signal and subsequently provide a time aligned signal.
Example 14 includes the system of example 13 wherein carrier phase recovery circuit is further configured to perform carrier frequency and phase recovery by continuously tracking and removing carrier and phase offset from the time aligned signal.
Example 15 includes the system of any of examples 12-14 wherein the timing error detector circuit is further configured to calculate a complex magnitude and remove bias prior to performing timing error detection.
Example 16 includes the system of any of examples 12-15 wherein magnitude calculation and bias removal are avoided to enable performing carrier frequency and phase recovery.
Example 17 includes the system of example 16 wherein the timing error detector circuit is further configured to remove bias from the complex magnitude by a bias estimate as the mean, median, or highest m-quantile of the complex magnitude, where m is an on-off keyed modulation order.
Example 18 includes the system of any of examples 16-17 wherein the timing error detector circuit gates an output on a per symbol basis by utilizing symbol timing alignment information fed back from a downstream symbol synchronizer of the demodulation circuit.
Example 19 includes the system of any of examples 12-18 wherein the carrier phase recovery circuit is further configured to perform phase error correction prior to symbol timing error detection.
Example 20 includes the system of any of examples 12-19 wherein quadrature detection is performed separately for dual orthogonal polarizations of the on-off keyed received optical signal.
The functions or algorithms described herein may be implemented in software in one embodiment. The software may consist of computer executable instructions stored on computer readable media or computer readable storage device such as one or more non-transitory memories or other type of hardware-based storage devices, either local or networked. Further, such functions correspond to modules, which may be software, hardware, firmware or any combination thereof. Multiple functions may be performed in one or more modules as desired, and the embodiments described are merely examples. The software may be executed on a digital signal processor, ASIC, microprocessor, or other type of processor operating on a computer system, such as a personal computer, server or other computer system, turning such computer system into a specifically programmed machine.
The functionality can be configured to perform an operation using, for instance, software, hardware, firmware, or the like. For example, the phrase “configured to” can refer to a logic circuit structure of a hardware element that is to implement the associated functionality. The phrase “configured to” can also refer to a logic circuit structure of a hardware element that is to implement the coding design of associated functionality of firmware or software. The term “module” refers to a structural element that can be implemented using any suitable hardware (e.g., a processor, among others), software (e.g., an application, among others), firmware, or any combination of hardware, software, and firmware. The term, “logic” encompasses any functionality for performing a task. For instance, each operation illustrated in the flowcharts corresponds to logic for performing that operation. An operation can be performed using, software, hardware, firmware, or the like. The terms, “component,” “system,” and the like may refer to computer-related entities, hardware, and software in execution, firmware, or combination thereof. A component may be a process running on a processor, an object, an executable, a program, a function, a subroutine, a computer, or a combination of software and hardware. The term, “processor,” may refer to a hardware component, such as a processing unit of a computer system.
Furthermore, the claimed subject matter may be implemented as a method, apparatus, or article of manufacture using standard programming and engineering techniques to produce software, firmware, hardware, or any combination thereof to control a computing device to implement the disclosed subject matter. The term, “article of manufacture,” as used herein is intended to encompass a computer program accessible from any computer-readable storage device or media. Computer-readable storage media can include, but are not limited to, magnetic storage devices, e.g., hard disk, floppy disk, magnetic strips, optical disk, compact disk (CD), digital versatile disk (DVD), smart cards, flash memory devices, among others. In contrast, computer-readable media, i.e., not storage media, may additionally include communication media such as transmission media for wireless signals and the like.
Although a few embodiments have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Other embodiments may be within the scope of the following claims.
