Mitigating noise and OBI in RFoG networks
View Patent ↗A system for communicating data signals over an optical transmission path combines forward signals onto a fiber that also carriers return signals. The system includes a return receiver for detecting an optical beat interference (OBI) event. Return signals are received by return receivers, one for each group of users. The fiber is provided to a splitter that distributes the forward optical receivers to the end users (RTx), which each comprise a receiver for the forward wavelength and a transmitter for the return wavelength. The transmitters provide return signals. When multiple transmitters are on at the same time, OBI events can take place when transmitter wavelengths coincide. When the return receiver detects an OBI event, it may signal the end user devices via a forward communication. The end user devices are responsive to the OBI detection signal to adjust a return wavelength to reduce or eliminate OBI.
1. A system in at least one of a headend or a node for detecting an optical beat interference (OBI) event in a two way optical communication path, the system comprising:
at least one forward transmitter for transmitting forward signals for distribution to one or more groups of end user devices, wherein each of the one or more groups includes a plurality of end user devices;
at least one return receiver for receiving return signals transmitted from the one or more groups of end user devices, the return receiver including a photodetector coupled to the two way optical communication path for detecting return signals from the plurality of end user devices;
a processor; and
a computer readable memory including computer readable instructions which, when executed by the processor, cause the processor to:
monitor a spectrum of the return signals from each group of end users, and
determine from the monitored spectrum of return signals if an OBI event has occurred for each respective group of end user devices,
identify an end user device associated with the OBI event,
generate an OBI detection signal indicative of the OBI event and identifying the end user device associated with the OBI event, and
cause at least one forward transmitter to transmit the OBI detection signal in one of more of the forward signals.
2. The system of claim 1 , wherein at least one return transmitter embeds information in to a signal returned to the headend or node, the information including at least one of a status of the return transmitter, a history of wavelengths of the return transmitter, an amount of wavelength shift applied, or an actual operating temperature.
3. The system of claim 1 , wherein determining whether an OBI event has occurred includes monitoring power outside of a predetermined signal frequency band.
4. The system of claim 3 , wherein the processor determines that an OBI event has occurred if the power outside of the predetermined signal frequency band exceeds a predetermined threshold.
5. The system of claim 1 , wherein at least one return receiver includes a return receiver for each group of end user devices, the return signals from each group of end user devices received at a respective return receiver.
6. The system of claim 1 , wherein the processor monitors the return signals by monitoring an average radio frequency (RF) power and an optical power of the return signals, and detects an OBI event based on an average RF power exceeding the optical power level.
7. The system of claim 1 , wherein an end user device receiving the OBI detection signal is responsive to the OBI detection signal to adjust an operating wavelength for transmitting return signals to the at least one of the headend or the node.
8. The system of claim 1 , wherein the OBI detection signal indicates a group ID associated with the OBI event.
9. The system of claim 1 , wherein the OBI detection signal is provided to a plurality of return receivers in communication with the at least one forward transmitter.
10. The system of claim 1 , wherein the OBI detection signal is a radio frequency (RF) signal modulated onto a forward laser and combined with at least one of the forward signals output from the system for distribution to end users.
11. The system of claim 1 , wherein the processor further collects return signals to identify OBI events for each of the groups of end user devices and accordingly generates forward signals for transmission to the end user devices to prevent OBI events.
12. The system of claim 1 , wherein at least one end user with an active transmitter in a time interval corresponding to the OBI event is responsive to the OBI detection signal by shifting their respective transmit wavelengths.
13. The system of claim 12 , wherein the OBI detection signal causes an automatic re-distribution of wavelengths among the plurality of end user device transmitters that could have contributed to the OBI event during a time interval of the OBI event.
14. The system of claim 1 , further comprising an end user device responsive to a downstream signal for adjusting a wavelength, the end user device comprising:
an input for receiving a downstream signal identifying an OBI detect event;
at least one return transmitter for transmitting return signals at a return wavelengths; and a processor; and
a memory including computer readable instructions which, when executed by the processor, cause the processor to generate an end user device wavelength, wherein the return wavelength is adjustable responsive to the downstream signal.
15. The end user device of claim 14 , wherein the end user device responsive to the downstream signal can adjust its wavelength to avoid wavelengths taken by other end user devices.
16. The end user device of claim 14 , wherein the end user device responsive to the downstream signal may signal its current wavelength and identification code upstream to a headed or node.
17. The end user device of claim 14 , wherein responsive to identifying a wavelength conflict with another end user device at the same port, the end user device wavelength is adjustable by the ONU.
18. The end user device of claim 14 , wherein the downstream signal identifies wavelength labels for other end user device connected to the splitter for selecting a non-conflicting wavelength.
19. The end user device of claim 14 , wherein the end user device responsive to a downstream signal detects a radio frequency (RF) signal identifying the OBI event and, responsive to the RF signal, controls a wavelength shifter for its return lasers.
20. The end user device of claim 14 , wherein responsive to the detection of an OBI event, instructs a return transmitter to change its wavelength to a particular value.
21. The end user device of claim 14 , wherein the return transmitters that receive an OBI event message respond by shifting their wavelength, or they do not respond to the OBI event.
22. The end user device of claim 14 , wherein return transmitters may also request confirmation signals to be embedded in the forward path to determine what return receiver group to which they belong.
23. The end user device of claim 14 , wherein thermal control of the transmit lasers may be applied to offset the laser wavelength in a limited range.
24. The end user device of claim 14 , wherein the end user device is responsive to the downstream signal to initiate operation in an absence of optical beat interference (OBI).