Radio-over-fibre transmission in communications networks

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 optical carrier signal portion of the optical carrier signal portions and a respective modulated radio frequency subcarrier signal of a plurality of modulated radio frequency subcarrier signals, and each of the plurality of electro-optic modulation apparatus arranged to modulate the respective optical carrier signal portions with the respective modulated radio frequency subcarrier signal and arranged to suppress onward transmission of the respective optical carrier signal portions, to form a respective carrier suppressed optical subcarrier signal at a respective optical frequency, different from the carrier optical frequency; an optical combiner arranged to receive the carrier suppressed optical subcarrier signals and an optical carrier signal portion which is not suppressed for onward transmission and arranged to combine said carrier suppressed optical subcarrier signals and the optical carrier signal portion which is not suppressed for onward transmission to form a subcarrier multiplexed optical signal; and polarization apparatus arranged to ensure that the carrier suppressed optical subcarrier signals and the optical carrier signal portion which is not suppressed for onward transmission each have a same polarization state at the optical combiner.

2. The 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 optical carrier signal portion which is not suppressed for onward transmission.

3. The 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 plurality of 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 plurality of electro-optic modulation apparatus and to couple the plurality of electro-optic modulation apparatus to the optical combiner.

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

5. The 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 plurality of electro-optic modulation apparatus by a respective second optical path length and each of the plurality of electro-optic modulation apparatus is separated from the optical combiner by a third optical path length, wherein the optical carrier signal portion which is not suppressed for onward transmission 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, the second and the 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. The 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: the 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 the second base station nodes further comprises: the 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. The communications network base station system as claimed in claim 8, wherein the radio-over-fibre transmitter of the second base station node comprises: a plurality of electrical modulators each arranged to receive a respective radio frequency communications signal of a plurality of radio frequency communications signals and a respective radio frequency subcarrier signal of a plurality of radio frequency subcarrier signals, each of the plurality of radio frequency subcarrier signals having a different frequency, and each of the plurality of electrical modulators arranged to modulate the respective radio frequency subcarrier signal with the respective radio frequency communications signal to form the respective modulated radio frequency subcarrier signal.

10. A communications network comprising: a first base station node and a second base station node, wherein each of the first and the second base station nodes further comprises the 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 further comprises: a plurality of electrical modulators each arranged to receive the respective radio frequency communications signal of a plurality of radio frequency communications signals and a respective radio frequency subcarrier signal of the plurality of radio frequency subcarrier signals, each radio frequency subcarrier signal having a different frequency, and each of the plurality of electrical modulators arranged to modulate the respective radio frequency subcarrier signal with the respective radio frequency communications signal to form the respective modulated radio frequency subcarrier signal, and wherein each of the plurality of electrical modulators is arranged to receive the respective radio frequency communications signal 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 one of other optical carrier signal portions with a respective modulated radio frequency subcarrier signal of the plurality of modulated radio frequency subcarrier signals and suppressing onward transmission of each of said other optical carrier signal portions, to form a plurality of carrier suppressed optical subcarrier signals each having a respective optical frequency, different from 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 controlling the carrier suppressed optical subcarrier signals and the selected optical carrier signal portion to each have a same polarization state when they are combined.

12. The 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. The method as claimed in claim 11 , wherein the optical carrier signal has a polarization state and the method further 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. The method as claimed in claim 11, wherein the method further 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 of the plurality of radio frequency subcarrier signals with a respective radio frequency communications signal of the plurality of radio frequency communications signals to form the plurality of modulated radio frequency subcarrier signals.

15. A nontransitory computer readable storage medium having computer readable instructions embodied therein, the computer readable instructions being used 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 one of other optical carrier signal portions with a respective modulated radio frequency subcarrier signal of the plurality of modulated radio frequency subcarrier signals and suppressing onward transmission of each of said other optical carrier signal portions, to form a plurality of carrier suppressed optical subcarrier signals each having a respective optical frequency, different from 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 controlling the carrier suppressed optical subcarrier signals and the selected optical carrier signal portion to each have a same polarization state when they are combined.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of a radio-over-fibre transmitter according to a first embodiment of the invention;

(2) FIG. 2 is a schematic representation of a radio-over-fibre transmitter according to a second embodiment of the invention;

(3) FIG. 3 is a schematic representation of a radio-over-fibre transmitter according to a third embodiment of the invention;

(4) FIG. 4 is a schematic representation of a radio-over-fibre transmitter according to a fourth embodiment of the invention;

(5) FIG. 5 is a schematic representation of a radio-over-fibre transmitter according to a fifth embodiment of the invention;

(6) FIG. 6 is a schematic representation of a radio-over-fibre transmitter according to a sixth embodiment of the invention;

(7) FIG. 7 is a schematic representation of a communications network base station node according to a seventh embodiment of the invention;

(8) FIG. 8 is a schematic representation of a communications network base station node according to an eighth embodiment of the invention;

(9) FIG. 9 is a schematic representation of a communications network base station system according to a ninth embodiment of the invention;

(10) FIG. 10 is a schematic representation of a communications network base station system according to a tenth embodiment of the invention;

(11) FIG. 11 is a schematic representation of a communications network according to an eleventh embodiment of the invention;

(12) FIG. 12 shows the steps of a method according to a twelfth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre;

(13) FIG. 13 shows the steps of a method according to a thirteenth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre;

(14) FIG. 14 shows the steps of a method according to a fourteenth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre; and

(15) FIG. 15 shows the steps of a method according to a fifteenth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre.

DETAILED DESCRIPTION

(16) Referring to FIG. 1, a first embodiment of the invention provides a radio-over-fibre, RoF, transmitter 10 comprising an optical splitter 12, a plurality of electro-optic modulation apparatus 24, an optical combiner 28 and polarisation apparatus.

(17) The optical splitter 12 is arranged to receive an optical carrier signal 20 having a carrier optical frequency. The optical splitter is arranged split the optical carrier signal into a plurality of optical carrier signal portions 22.sub.1-22.sub.N and 22.sub.C.

(18) Each of the electro-optic modulation apparatus 24 is 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 14. Each electro-optic modulation apparatus is arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal. In addition, each electro-optic modulation apparatus is arranged to suppress onward transmission of the respective optical carrier signal portion, to thereby form a respective carrier suppressed optical subcarrier signal 26.sub.1-26.sub.N at a respective optical frequency. The optical frequency of each carrier suppressed optical subcarrier signal is different to the carrier optical frequency.

(19) The optical combiner 28 is arranged to receive the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining one of the optical carrier signal portions 22.sub.C The optical combiner is arranged to combine the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining optical carrier signal portion 22.sub.C to form a subcarrier multiplexed optical signal 30.

(20) The polarisation apparatus is this embodiment comprises the optical splitter and the electro-optic modulation apparatus, each of which are arranged to ensure that the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining optical carrier signal portion 22.sub.C each have the same polarisation state at the optical combiner. However it will be appreciated that any arrangement of optical apparatus can be used which ensures that the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining optical carrier signal portion 22.sub.C each have the same polarisation state at the optical combiner.

(21) It will be appreciated that radio frequency is used here to mean any radio wave signal within the electromagnetic spectrum, including RF, microwave and millimetre wave signals.

(22) A second embodiment of the invention provides an RoF transmitter 40 as shown in FIG. 2. The RoF transmitter 40 of this embodiment is similar to the RoF transmitter 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(23) In this embodiment, an optical attenuator 42 is provided between the optical splitter 12 and the optical combiner 28 in the path of the remaining optical carrier signal portion 22.sub.C. The optical attenuator is arranged to apply an attenuation to the remaining optical carrier signal portion 22.sub.C.

(24) A third embodiment of the invention provides an RoF transmitter 50 as shown in FIG. 3. The RoF transmitter 50 of this embodiment is similar to the RoF transmitter 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(25) In this embodiment, the RoF transmitter additionally comprises a plurality of polarisation maintaining optical waveguides 56 provided between the optical splitter 52 and the electro-optic modulation apparatus 54 and between the electro-optic modulation apparatus 54 and the optical combiner 28. The optical splitter 52 is polarisation maintaining and the electro-optic modulation apparatus 54 are each polarisation maintaining. The optical splitter 52, electro-optic modulation apparatus 54 and the optical waveguides 56 together form the polarisation apparatus which ensures that the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining optical carrier signal portion 22.sub.C each have the same polarisation state at the optical combiner.

(26) A fourth embodiment of the invention provides an RoF transmitter 60 as shown in FIG. 4. The RoF transmitter 60 of this embodiment is similar to the RoF transmitter 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(27) In this embodiment, the RoF transmitter 60 additionally comprises a plurality of electrical modulators 62. Each electrical modulator is arranged to receive a respective one of a plurality of radio frequency communications signals 64 and a respective radio frequency subcarrier signal 66. Each radio frequency subcarrier signal has a different frequency. Each electrical modulator is 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 14.

(28) A fifth embodiment of the invention provides an RoF transmitter 70 as shown in FIG. 5. The RoF transmitter 70 of this embodiment is similar to the RoF transmitter 60 of the previous embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(29) The RoF transmitter 70 of this embodiment is provided as a photonic integrated structure comprising an optical splitter, PS, 74, a plurality of electro-optic modulation apparatus 78, a plurality of electrical modulators 82, a plurality of polarisation maintaining waveguides 76, an optical attenuator 72 and an optical combiner 80. A laser 84 is also provided to generate the optical carrier signal 20. The laser 84 may be incorporated in the photonic integrated structure or may be provided separately to it.

(30) Each electrical modulator is an electrical mixer 82 arranged to receive a respective radio frequency communications signal 64 and a respective radio frequency subcarrier signal 66. Each electrical mixer is arranged to modulate the respective radio frequency subcarrier signal with the respective radio frequency communications signal to form a respective modulated radio frequency subcarrier signal 14.

(31) The optical splitter 74 is arranged to receive the optical carrier signal 20 from the laser 84 and split the optical carrier signal into a plurality of optical carrier signal portions 22.sub.1-22.sub.N and 22.sub.C. The optical splitter 74 is polarisation maintaining.

(32) In this embodiment, each of the electro-optic modulation apparatus is a polarisation maintaining electro-optic modulator 78 having a respective transfer function having a minimum transmission point. Each electro-optic modulator is arranged to receive a respective one of the optical carrier signal portions 22.sub.1-22.sub.N and a respective one of the modulated radio frequency subcarrier signals 14. Each electro-optic modulator is arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal. Each electro-optic modulator is biased so that its minimum transmission point is at the optical frequency of the optical carrier signal, so each electro-optic modulator suppresses onward transmission of the respective optical carrier signal portion. Each electro-optic modulator therefore forms a respective carrier suppressed optical subcarrier signal 26.sub.1-26.sub.N at a respective optical frequency. The optical frequency of each carrier suppressed optical subcarrier signal is different to the carrier optical frequency.

(33) The optical combiner in this embodiment is a beam collimator, BC, 80 which is arranged to receive the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining one of the optical carrier signal portions 22.sub.C The beam collimator is arranged to combine the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining optical carrier signal portion 22.sub.C to form a subcarrier multiplexed optical signal 30.

(34) The optical attenuator 72 is provided between the optical splitter 74 and the beam collimator 80 in the path of the remaining optical carrier signal portion 22.sub.C. The optical attenuator is arranged to apply an attenuation to the remaining optical carrier signal portion 22.sub.C.

(35) Polarisation maintaining optical waveguides 76 are provided between the optical splitter 74 and the electro-optic modulators 78, between the electro-optic modulators and the beam collimator 80, between the optical splitter 74 and the optical attenuator 72, and between the optical attenuator and the beam collimator.

(36) The length of each optical waveguide 76 is selected such that any difference in the optical path length from the optical splitter 74 to the beam collimator 80, along any of the optical paths via an electro-optic modulator or via the optical attenuator, is less than a maximum optical path length difference. This is the optical path length difference that equates to the maximum noise generated by the resulting phase mismatch between the remaining optical carrier signal portion 22.sub.C and the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N which can be compensated for at a receiver apparatus using electronic digital signal processing, DSP. Conventional direct detection receiver apparatus and DSP techniques may be used.

(37) Referring to FIG. 6, a sixth embodiment of the invention provides an RoF transmitter 90. The RoF transmitter 90 of this embodiment is similar to the RoF transmitter 10 of the first embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(38) In this embodiment, the RoF transmitter additionally comprises a plurality of polarisation controllers, PC, 92. A respective polarisation controller is provided between each electro-optic modulation apparatus 24, in the paths of the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N. A further polarisation controller is provided between the optical splitter 12 and the optical combiner, in the path of the remaining optical carrier signal portion 22.sub.C. The polarisation controllers 92 are each arranged to ensure that the carrier suppressed optical subcarrier signals 26.sub.1-26.sub.N and the remaining optical carrier signal portion 22.sub.C each have the same polarisation state at the optical combiner.

(39) Each polarisation controller may be an active polarisation controller or a passive polarisation controller.

(40) Referring to FIG. 7, a seventh embodiment of the invention provides a communications network base station node 100 comprising an RoF transmitter 10, as shown in FIG. 1.

(41) Referring to FIG. 8, an eighth embodiment of the invention provides a communications network base station node 110 comprising an RoF transmitter 60, as shown in FIG. 4. It will be appreciated that any of the RoF transmitters 40, 50, 70 shown in FIGS. 2, 3, 5 and 6 may alternatively be used.

(42) FIG. 9 shows a communications network base station system 200 according to a ninth embodiment of the invention.

(43) The communications network base station system 200 comprises first and second base station nodes 100a, 100b, according to the seventh embodiment of the invention shown in FIG. 7, and an optical fibre 202 coupled between the first base station node and the second base station node.

(44) Each base station node 100 comprises an RoF transmitter 10 according to the first embodiment of the invention, as shown in FIG. 1. It will be appreciated that any of the RoF transmitters 40, 50, 60, 70, 90 shown in FIGS. 2 to 6 may alternatively be used.

(45) FIG. 10 shows a communications network base station system 220 according to a tenth embodiment of the invention. The communications network base station system 220 of this embodiment is similar to the communications network base station system 200 of the previous embodiment, with the following modifications.

(46) In this embodiment, the second base station node 110 comprises an RoF transmitter 60 according to the fourth embodiment of the invention, shown in FIG. 4. An RoF transmitter 70 according to the fifth embodiment of the invention, shown in FIG. 5, may alternatively be used. As in those embodiments, each electrical modulator 62, 82 is arranged to receive a respective radio frequency communications signal 64.

(47) Referring to FIG. 11, an eleventh embodiment of the invention provides a communications network 300 comprising a first base station node 100, a second base station node 110, a plurality of radio antennas 302 and an optical fibre 306 coupled between the first base station node and the second base station node.

(48) Each radio antenna 302 is configured to transmit a respective radio frequency communications signal 304.

(49) The first base station node 100 is as described in the seventh embodiment, shown in FIG. 7, and the second base station node 110 is as described in the eighth embodiment, shown in FIG. 8. In the second base station node, each electrical modulator 62 of the RoF transmitter 60 is arranged to receive a respective one of the radio frequency communications signals transmitted by the radio antennas.

(50) FIG. 12 shows the steps of a method 400 according to a twelfth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre.

(51) The method 400 comprises receiving a plurality of modulated radio frequency subcarrier signals 402 and providing an optical carrier signal having a carrier optical frequency 404. The optical carrier signal is split into a plurality of optical carrier signal portions 404 and one of the optical carrier signal portions is selected for onward transmission 406.

(52) Each other optical carrier signal portion, that is to say all those not selected for onward transmission, is modulated with a respective one of the modulated radio frequency subcarrier signals 408. Onward transmission of each of these optical carrier signal portions is suppressed. A plurality of carrier suppressed optical subcarrier signals each having a respective optical frequency, different to the carrier optical frequency, is thereby formed 408.

(53) The selected optical carrier signal portion is combined with the carrier suppressed optical subcarrier signals to form a subcarrier multiplexed optical signal 412. The steps of the method are performed ensuring that the polarisations of the carrier suppressed optical subcarrier signals and the polarisation of the selected optical carrier signal portion all have the same polarisation state when they are combined.

(54) FIG. 13 shows the steps of a method 420 according to a thirteenth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre. The method 420 of this embodiment is similar to the method 400 of the previous embodiment, with the following modifications. The same reference numbers are retained for corresponding steps.

(55) In this embodiment, the method 420 comprises the additional step of attenuating the selected optical carrier signal portion 422 before combining it with the carrier suppressed optical subcarrier signals.

(56) FIG. 14 shows the steps of a method 430 according to a fourteenth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre. The method 430 of this embodiment is similar to the method 400 of the twelfth embodiment, with the following modifications. The same reference numbers are retained for corresponding steps.

(57) In this embodiment, the optical carrier signal has a polarisation state 432. The method comprises maintaining the polarisation state during the splitting of the optical carrier signal into the optical carrier signal portions 434 and maintaining the polarisation state during the modulation of the optical carrier signal portions 436. This is to ensure that the selected optical carrier signal portion and the carrier suppressed optical carrier signals each have the same polarisation state when they are combined 438.

(58) FIG. 15 shows the steps of a method 440 according to a fifteenth embodiment of the invention of transmitting radio frequency communications signals over an optical fibre. The method 440 of this embodiment is similar to the method 400 of the twelfth embodiment, with the following modifications. The same reference numbers are retained for corresponding steps.

(59) In this embodiment, the method additionally comprises receiving a plurality of radio frequency communications signals 442 and providing a plurality of radio frequency subcarrier signals, each having a different frequency 444. Each radio frequency subcarrier signal is then modulated with a respective one of the radio frequency communications signals to form a respective modulated radio frequency subcarrier signal 446. These radio frequency subcarrier signals are modulated onto the optical carrier signal portions 408.

(60) A sixteenth embodiment of the invention provides a data carrier having computer readable instructions embodied therein. The computer readable instructions are for providing access to resources available on a processor and comprise instructions to cause the processor to perform any of the steps of the method of transmitting radio frequency communications signals over an optical fibre according to any one of the eleventh to fourteenth embodiments.