RADIO-OVER-FIBRE TRANSMISSION IN COMMUNICATIONS NETWORKS

• US 20170033885A1
• Filed: 04/10/2014
• Published: 02/02/2017
• Est. Priority Date: 04/10/2014
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: PONZINI, Filippo, CAVALIERE, Fabio, GIORGI, Luca
• Granted Patent: US 10,009,138 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: H04B 10/2572 due to forms of polarisation-dependent distortion other than PMD
• CPC Class Codes: H04B 10/2575 Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
• CPC Class Codes: H04B 10/25759 Details of the reception of RF signal or the optical conversion before the optical fibre
• CPC Class Codes: H04B 10/588 in external modulation systems
• CPC Class Codes: H04J 14/06 Polarisation multiplex systems

Abstract

A radio-over-fibre transmitter comprising: an optical splitter arranged to receive an optical carrier signal having a carrier optical frequency, and split it into a plurality of portions; electro-optic modulation apparatus each arranged to receive a respective optical carrier signal portion and a respective modulated radio frequency subcarrier signal, and arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal and arranged to suppress onward transmission of the respective optical carrier signal portion, to form a respective carrier suppressed optical subcarrier signal; an optical combiner arranged to receive the carrier suppressed optical subcarrier signals and one of the optical carrier signal portions and arranged to combine them to form a subcarrier multiplexed optical signal; and polarisation apparatus arranged to ensure that the carrier suppressed optical subcarrier signals and said optical carrier signal portion each have the same polarisation state at the optical combiner.

Claims

1. A radio-over-fibre transmitter comprising:

an optical splitter arranged to receive an optical carrier signal having a carrier optical frequency and arranged to split the optical carrier signal into a plurality of optical carrier signal portions;

a plurality of electro-optic modulation apparatus each arranged to receive a respective one of the optical carrier signal portions and a respective one of a plurality of modulated radio frequency subcarrier signals, and each electro-optic modulation apparatus arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal and arranged to suppress onward transmission of the respective optical carrier signal portion, to thereby form a respective carrier suppressed optical subcarrier signal at a respective optical frequency, different to the carrier optical frequency;

an optical combiner arranged to receive the carrier suppressed optical subcarrier signals and one of the optical carrier signal portions and arranged to combine said carrier suppressed optical subcarrier signals and the said one of the optical carrier signal portions to form a subcarrier multiplexed optical signal; and

polarization apparatus arranged to ensure that the carrier suppressed optical subcarrier signals and the said one of the optical carrier signal portions each have the same polarization state at the optical combiner.

2. A radio-over-fibre transmitter as claimed in claim 1, further comprising an optical attenuator provided between the optical splitter and the optical combiner, the optical attenuator arranged to apply an attenuation to the said one of the optical carrier signal portions.

3. A radio-over-fibre transmitter as claimed in claim 1, wherein the polarization apparatus comprises the optical splitter, which is a polarization maintaining optical splitter, the electro-optic modulation apparatus, each of which is polarization maintaining, and a plurality of polarization maintaining optical waveguides arranged to couple the optical splitter to the electro-optic modulation apparatus and to couple the electro-optic modulation apparatus to the optical combiner.

4. A radio-over-fibre transmitter as claimed in claim 1, further comprising a plurality of electrical modulators each arranged to receive a respective one of a plurality of radio frequency communications signals and a respective radio frequency subcarrier signal, each radio frequency subcarrier signal having a different frequency, and each electrical modulator arranged to modulate the respective radio frequency subcarrier signal with the respective radio frequency communications signal to form a respective one of the modulated radio frequency subcarrier signals.

5. A radio-over-fibre transmitter as claimed in claim 1, wherein the optical splitter is separated from the optical combiner by a first optical path length, the optical splitter is separated from each of the electro-optic modulation apparatus by a respective second optical path length and each of the electro-optic modulation apparatus is separated from the optical combiner by a third optical path length, wherein the said one of the optical carrier signal portions is transmitted across a first total optical path length from the optical splitter to the optical combiner and each of the carrier suppressed optical subcarrier signals is transmitted across a respective second total optical path length from the optical splitter to the optical combiner, and the first, second and third optical path lengths are selected such that a difference between the first total optical path length and the respective second total optical path lengths is less than a preselected maximum path length difference.

6. A radio-over-fibre transmitter as claimed in claim 1, wherein the radio-over-fibre transmitter is a photonic integrated structure.

7. A communications network base station node comprising a radio-over-fibre transmitter as claimed in claim 1.

8. A communications network base station system comprising:

a first base station node and a second base station node, wherein each of the first and second base station nodes comprises a radio-over-fibre transmitter as claimed in claim 1; and

wherein the communications network further comprises an optical fibre coupled between the first base station node and the second base station node.

9. A communications network base station system as claimed in claim 8, wherein the radio-over-fibre transmitter of the second base station comprises a plurality of electrical modulators each arranged to receive a respective one of a plurality of radio frequency communications signals and a respective radio frequency subcarrier signal, each radio frequency subcarrier signal having a different frequency, and each electrical modulator arranged to modulate the respective radio frequency subcarrier signal with the respective radio frequency communications signal to form a respective one of the modulated radio frequency subcarrier signals,

wherein each electrical modulator is arranged to receive a respective one of the plurality of radio frequency communications signals.

10. A communications network comprising:

a first base station node and a second base station node, wherein each of the first and second base station nodes comprises a radio-over-fibre transmitter as claimed in claim 1; and

a plurality of radio antennas each configured to transmit a respective radio frequency communications signal;

wherein the radio-over-fibre transmitter of the second base station node comprises a plurality of electrical modulators each arranged to receive a respective one of a plurality of radio frequency communications signals and a respective radio frequency subcarrier signal, each radio frequency subcarrier signal having a different frequency, and each electrical modulator arranged to modulate the respective radio frequency subcarrier signal with the respective radio frequency communications signal to form a respective one of the modulated radio frequency subcarrier signals, and

wherein each electrical modulator is arranged to receive a respective one of the radio frequency communications signals transmitted by the radio antennas; and

wherein the communications network further comprises an optical fibre coupled between the first base station node and the second base station node.

11. A method of transmitting radio frequency communications signals over an optical fibre, the method comprising:

receiving a plurality of modulated radio frequency subcarrier signals;

providing an optical carrier signal having a carrier optical frequency and splitting the optical carrier signal into a plurality of optical carrier signal portions;

selecting one of the optical carrier signal portions for onward transmission;

modulating each other optical carrier signal portion with a respective one of the modulated radio frequency subcarrier signals and suppressing onward transmission of each said other optical carrier signal portion, to thereby form a plurality of carrier suppressed optical subcarrier signals each having a respective optical frequency, different to the carrier optical frequency;

combining the selected optical carrier signal portion and the carrier suppressed optical subcarrier signals to form a subcarrier multiplexed optical signal; and

ensuring that the carrier suppressed optical subcarrier signals and the selected optical carrier signal portion each have the same polarization state when they are combined.

12. A method as claimed in claim 11, further comprising attenuating the selected optical carrier signal portion before combining it with the carrier suppressed optical subcarrier signals.

13. A method as claimed in claim 11, wherein the optical carrier signal has a polarization state and the method comprises maintaining the polarization state during the splitting of the optical carrier signal into the optical carrier signal portions and maintaining the polarization state during the modulation of the optical carrier signal portions, such that the selected optical carrier signal portion and the carrier suppressed optical carrier signals each have the same polarization state when they are combined.

14. A method as claimed in claim 11, wherein the method additionally comprises:

receiving a plurality of radio frequency communications signals;

providing a plurality of radio frequency subcarrier signals, each radio frequency subcarrier signal having a different frequency; and

modulating each radio frequency subcarrier signal with a respective one of the radio frequency communications signals to form a respective one of the modulated radio frequency subcarrier signals.

15. A nontransitory computer readable storage medium having computer readable instructions embodied therein, the computer readable instructions being for providing access to resources available on a processor and the computer readable instructions comprising instructions to cause the processor to perform a method of transmitting radio frequency communications signals over an optical fibre, wherein the method comprises:

receiving a plurality of modulated radio frequency subcarrier signals;

providing an optical carrier signal having a carrier optical frequency and splitting the optical carrier signal into a plurality of optical carrier signal portions;

selecting one of the optical carrier signal portions for onward transmission;

modulating each other optical carrier signal portion with a respective one of the modulated radio frequency subcarrier signals and suppressing onward transmission of each said other optical carrier signal portion, to thereby form a plurality of carrier suppressed optical subcarrier signals each having a respective optical frequency, different to the carrier optical frequency;

combining the selected optical carrier signal portion and the carrier suppressed optical subcarrier signals to form a subcarrier multiplexed optical signal; and

ensuring that the carrier suppressed optical subcarrier signals and the selected optical carrier signal portion each have the same polarization state when they are combined.