Radio access network using radio over fibre

A radio communication system comprising an optical carrier generator for generating at least a pair of frequency spaced optical carrier signals, a transceiver configured to modulate a first portion of the pair of spaced optical carrier signals with downlink (DL) information to generate a modulated first optical signal, combine an unmodulated second optical signal formed of a remaining unmodulated second portion of the pair of spaced optical carrier signals with the modulated first optical signal to form a combined optical signal for transmission over an optical link, receive an optical uplink (UL) signal from said optical link, said optical UL signal comprising UL information modulated on said unmodulated second portion of the spaced optical carrier signals and down convert said received optical UL signal using a photodetector to output an electrical signal at a baseband frequency.

FIELD

The present matter is directed to wireless communications and, more particularly to radio access network (RAN) architectures using optical links, such as radio-over-fiber (RoF).

BACKGROUND

The RAN defined in the Long term evolution (LTE) standard includes an evolved Node B (eNodeB) architecture as defined in the standard. C-RAN is an acronym for centralised or cloud radio access network, which is a more efficient implementation of the traditional eNodeB. In C-RAN, baseband units (BBU's) are housed on a central office (CO) or super macro site sometimes referred to as a BBU hotel. Connectivity from each BBU to its corresponding eNodeB, which may house a remote radio head (RRH), is typically provided using a common public radio interface (CPRI) interface over an optical fiber link. Communication over the optical fiber is achieved by modulating a light signal, usually provided by a laser, using a radio frequency (RF) signal which is then transmitted over the optical fiber link. This architecture is usually termed Radio over Fiber (RoF). The fiber may carry a digital representation of an RF radio signal which is then converted to the RF signal directly at the RRH. Transmission over the standard fibre introduces low loss in the telecommunications band which allows distances between central stations and wireless end users to be extended, maximizing coverage in for example microcell networks. In the BBU hotel, a router connects all the BBU's and, in turn, connects to the evolved packet core (EPC) usually over an Si interface. For a cloud-based architecture the BBU's are virtualised in software running on a common off-the-shelf computer architecture, such as an x86 based server. Depending on the power of the server hardware, multiple virtual BBUs may be supported along with a virtual router. This provides a less complex, more efficient, and lower cost platform.

RoF implementations are expected to dominate deployments of future 5G (fifth generation) wireless networks, as this allows the transmission of 5G broadband RF signals over low loss fibers.

SUMMARY

In accordance with some aspects of the present matter, radio access network comprises multiple geographically distributed base transceiver stations, a fronthaul network, and a central processor, wherein the base transceiver stations are connected to the central processor via the fronthaul network. The fronthaul network may include optical transport networks, including one or more of fiber optic links, free space optical links, and wireless links for connecting each base transceiver station to the central processor.

In accordance with an aspect of the present matter the fronthaul optical transport network provides for transmission of broadband RF signals by modulating laser carrier signals over the optical transport network. The optical transport network comprising optical fibers having high-bandwidth, low-latency, and low loss.

Furthermore, there is provided bidirectional or full-duplex communication over the optical transport network between the central processor and each base transceiver station. The present architecture also provides benefits in that uplink (UL) and downlink (DL) signals are not transmitted on two different fibers, thereby simplifying hardware and reducing reliance on complex components in setup.

The present architecture further provides benefits of frequency conversion, wherein a multi tone optical carrier may be used in frequency up-conversion and down-conversion of UL and DL signals. In one embodiment the central processor may generate the multi tone optical carrier from a single optical tone.

An aspect of the present matter includes a method implemented in a mobile communications system wherein, the base transceiver stations may operate solely as radio units (e.g., remote radio heads), while the RAN baseband processing is performed at the central processor within the operator's network. The present architecture may be adaptable to C-RAN architecture. In one aspect the architecture is adaptable to the C-RAN architecture by providing bidirectional or full-duplex communication over the optical network between the central processor and each base transceiver station. The central processor may include one or more cloud baseband units (C-BBU).

In accordance with a further aspect of the present matter, the architecture provides for reducing interference and cross talk when frequency band of uplink and the downlink signals are adjacent

In accordance with another aspect of the present matter the architecture provides for use of inexpensive off-the shelf components in implementing the central processor. In one aspect the present matter provides for use of inexpensive lasers obviating complex stabilization circuits. Examples of complex stabilization circuits include polarization independent reflective modulators (PIRM) to provide compensation for instability of optical modulators with temperature change.

Furthermore, embodiments of the present matter may be used in transmitting broadband 5G signals at wideband frequency carriers in a range down to millimeter wavelengths.

Furthermore, embodiments of the present matter provide for less complex and expensive transceiver units. The present architecture provides some benefits over typical receiver where previously a feedback path was used to mitigate crosstalk of the received optical signal due to the change in the state of polarization (SOP).

The present matter provides a system and method for a C-RAN architecture having full-duplex data communication using radio-over-fiber or free space optics.

In one aspect the present matter provides a system and method in a communication network architecture for simultaneous two-way communications between a central baseband point and distant radio access units. The network can support broadband 5G signals and mixer-less frequency conversions. In one aspect, the architecture includes a central station connected in a star architecture to multiple remote units through fiber optics cables. The central station generates two-tone optical carrier signal by means of modulating a single optical laser tone. In addition to being used as a carrier signal, the two-tone signal may be used as frequency up and down conversion for the information signals. Two linear orthogonal polarization states are used to carry the data in the downlink (DL) and uplink (UL) directions. This advantageously provides for one standard single mode (SM) fiber optic cable to establish full-duplex communication between central station and each remote unit. This architecture has no restrictions on the frequencies assigned for both DL and UL signals.

Furthermore, the present architecture provides benefits of unrestricted assignment of the frequencies for both DL and UL signals. And further provides for real time compensation using a calibrated observation path (COP). In an embodiment of the present matter the COP utilises a copy of the output RF signal from the remote units at the central station.

In some aspects of the present matter there is provided a non-transitory computer-readable medium with instructions stored thereon.

DETAILED DESCRIPTION

In the present description similar components in the figures are represented by like numerals. Furthermore in the description, it is conventionally understood that DL and UL are defined in terms of the separate air interfaces used in the context of LTE, namely DL refers to communication from tower to device, and UL refers to communication from device to tower. However this terminology is used for convenience of only, embodiments of the RoF architecture described herein may be integrated into a large variety of applications, including military radar, radio astronomy and spectroscopy, secure sensing, photonic signal generation, data up-conversion techniques, and massive multiple-input-multiple output (MIMO) systems.

Referring toFIG. 1there is shown a general block diagram of a single link RAN system100according to an embodiment of the present matter. The RAN or C-RAN system100includes a central processor termed a centralized base band Unit (C-BBU)102, a base transceiver unit104termed a remote radio head (RRH) and a fronthaul link106connecting the C-BBU to the RRH. The fronthaul link106includes, in an embodiment, at least a fiber optic link107. In further embodiments free air optic links may also be employed.

In the illustrated embodiment, the C-BBU102includes a DSP block110, and an optical carrier generation and polarization block112configured to generate at least a pair of frequency spaced optical carriers, and further configured to generate orthogonally polarised optical signals X, Y based on the generated spaced optical carriers. The C-BBU102further includes a BBU transceiver114coupled between the DSP110and the link106, and is driven by optical signals from the optical carrier generation and polarization block112and electrical signals from the DSP110. The RRH104includes an RRH transceiver116and an RF front-end module (FEM)118which connects to one or more transmit and receive antennas120. The RRH transceiver116is in turn coupled between the fronthaul link106and the FEM118. The DSP110includes a signal generator109for providing the baseband, IF or RF modulated electrical signals (Signal_OUT) for the DL signal, and an UL signal processor111for processing the received electrical signals (Signal_IN), from the BBU transceiver114.

The BBU transceiver block114includes in a DL signal path an optional intermediate frequency (IF) mixer124, and a Mach-Zehnder modulator (MZM)126a polarization beam (PBC) combiner128. The mixer124receives the signals (signal_OUT) for the DL from the DSP110, and drives the MZM126to modulate one of the polarised optical signals X input from the optical carrier generation and polarization block112, which is outputs a modulated optical signal Xmodto the PBC128. The PBC128combines the modulated optical signal Xmodwith the other (unmodulated) optical signal Y from the polarization block112to form a DL signal. The DL signal is coupled to a first port of an optical circulator130which has a second port coupled to the link106conveying the signal to the RRH.

The BBU transceiver block114further includes in an UL signal path, an erbium doped fiber amplifier (EDFA)132and, a photodetector134. A received UL signal on the link106is coupled from a third port of the circulator130to the input of the EDFA132to drive the photodetector134which outputs an electrical signal (Signal_IN) for the UL signal to the DSP110.

The RRH transceiver116includes in a DL signal path, an optical circulator129, an optical splitter133, and a PD139. The optical circulator129has a first port connected to the link106, a second port connected to the optical splitter133for recovering second orthogonally polarised signals X′, Y′ from the previously combined X, Y signals in the DL signal, wherein the one of the orthogonal signals X′ is input to the PD139.

The RRH transceiver116further includes in an UL signal path an MZM135. The other of the orthogonal signals Y′ recovered by the splitter133is output to an optical input of the MZM135. An optical output of the MZM135is coupled to a third input port of the circulator129.

In the DL signal path, the PD139outputs an electrical signal to the FEM118. In the UL signal path, the MZM135receives, and is driven by an electrical signal output from the FEM118.

The FEM118includes a power amplifier (PA)140for amplifying input DL information signals before transmission, a low noise amplifier (LNA)142for amplifying received UL information signals, and a diplexer144connected to the LNA142and PA140and the antenna120for transmitting and receiving, respectively, the DL and UL information signals to and from the antenna120.

As may be seen from the illustrated architecture100, the UL signal path does not include local oscillators to extract the baseband signal, but instead by having frequency spaced optical carriers, a baseband signal may be automatically provided at the output of the PD134to the DSP110. This may be better understood by referring to the description below.

Referring toFIG. 2, there is shown a detailed description of the optical components200for the C-RAN system100according to an embodiment of the present matter. For brevity, similar components described in the figures will be referenced with the same numerals. In the illustrated embodiment, the optical carrier generation and polarization block112is comprised of an off-the-shelf distributed feedback laser (DFBL) source201, a single-drive (one input/output optical port) Mach-Zehnder optical interferometer (MZM)203, a local oscillator (LO)202which may have a tunable frequency, an Erbium doped fiber amplifier (EDFA)204, polarization controller205and a polarization beam splitter (PBS)206. The DFBL201emits light at an optical frequency fop. It may be used as a sole optical carrier source to operate the entire network, as will be explained later. The LO202may be implemented as any known electrical signal generator to output a sinusoidal wave with a tunable frequency fLO. A first electro-optic modulation process occurs at the MZM optical interferometer203. It is to be noted that the optical modulator as used herein may be a single-drive modulator with one RF port, and one DC port. The laser signal output from DFBL201is electrically modulated at the MZM optical interferometer203by the sinusoidal signal that is generated from the LO202. The modulator203together with the LO202, are configured to generate the spaced optical carriers. For example, with a fixed LO frequency fLOthe generated optical carriers are spaced at 2fLO. The optical signal may then be amplified by the EDFA204and have a random SOP adjusted by the polarization controller205before it is orthogonally split by the polarization beam splitter (PBS)206to output two linear orthogonal SOP signals from the PBS, which are referenced inFIG. 1as the X and Y signals and which is input to the C-BBU transceiver block114.

Turning now to the optical splitter133included in the RRH transceiver116. The optical splitter133includes a polarization controller207and a PBS208. The polarization controller receives the DL optical signal from the circulator129and adjusts the random SOP in the received DL signal for polarization splitting at the PBS208, which in turn recovers the orthogonally polarised optical beams X′ and Y′. The PD139receives one of the polarised optical DL information signals (X′ or Y′) from the C-BBU transceiver block114, in the illustrated embodiment shown as signal X′, and converts it into an electrical signal. The electrical DL signal is input to the PA140which boosts the DL signal, at the desired frequency and outputs this amplified DL signal to the diplexer144for broadcasting to the air by the antenna120.

Furthermore for an UL signal received at the antenna120, the antenna passes the signal to the diplexer144, which in turn passes the signal to the LNA142, where the UL signal is filtered at a chosen frequency and amplified by the LNA142. The LNA142outputs the signal to optical modulator135wherein the other of the orthogonally polarised signals (Y′) is modulated by the input UL signal from the LNA142. The optical modulator135outputs an optical carrier modulated by the UL information signal. This signal is directed by the circulator129through the fiber link107, to the circulator130and finally to the BBU transceiver block114, wherein the EDFA132optically boosts the modulated UL signal before outputting the signal to the PD134.

Referring toFIG. 3there is shown signal frequency spectra300of both the optical and electrical signals in the UL signal and the DL signal directions in the C-RAN system100according to an embodiment of the present matter. Each of the spectra is referenced to a location in the C-RAN system100as shown inFIG. 2. Accordingly, reference is made to bothFIG. 2andFIG. 3. The dashed lines in the figures refer to the spectra of the optical signals and the solid lines refer to spectra of the electrical signal. The DFB laser201generates an optical spectrum301at an optical frequency fop. Diagram302shows an electrical sinusoidal signal spectrum generated from the LO202at the frequency fLO. Diagram303shows the spectrum of a pair of frequency spaced optical carriers generated by the modulator203. These signals303may be linearly polarized in an arbitrary direction. The spacing or frequency difference between the optical carrier is 2fLOwhich in the illustrated embodiment provides a two-tone optical carrier used in the DL and UL signals. Diagram304shows a spectrum of the X polarised signal at one of the outputs of the PBS206, and diagram305shows a spectrum of the Y polarised signal at the other output the PBS206stage. As may be seen the spectra304and305are mutually orthogonal and spaced respectively at 2fLO. Diagram306shows a spectrum in the electrical domain of the DL information signal at a frequency ft1. Diagram307shows a spectrum of the modulated X two-tone optical carrier signal output from the modulator126, wherein the solid lines represent the double side bands of the information signal spaced by ft1around each carrier, represented by the dashed lines. Diagram308shows a spectrum output from the PBS128where unmodulated Y polarised signal is combined orthogonally with the modulated X polarised signal. Diagram309shows a spectrum after passing through the first circulator130, the fiber cable107, and the second circulator132, for input to the polarization controller207. The random polarization is adjusted at207so that the X′ and Y′ mutually orthogonal polarised signals are produced. Diagram310shows a spectrum of the modulated X′ polarised signal after PBS in208. Diagram311shows a spectrum of the modulated Y′ polarised signal after PBS in208. Diagram312shows an electrical domain spectrum of the extracted DL information signal at a chosen transmit frequency ftx1=2fLO+ft1. This signal is produced at the output of the PA140after passing through the PD139and the PA140stages. Diagram313shows an electrical spectrum of the UL signal received by the antenna120, which is filtered and amplified at a receive frequency frx1, by the LNA142. Diagrams314and315show an optical domain of the spectrum of the Y′ signal after being modulated by the received UL information signal at the modulator135before being optically amplified by the EDFA132. Diagram316shows an electrical domain spectrum of the UL information signal at the chosen frequency fr1after being received at the C-BBU102and extracted at the PD134output. The transmitter frequency ftx and the receiver frequency frx are obtained from the beating of the optical tones with the information bands in the PD and they can be given as the formulas on the figure. In other words, for example by having the two-tone optical carrier there is a choice to receive the DL signal either at fDL=ftor, fDL=ft−2fLO, or at fDL=ft+2fLO. Furthermore, the two-tone optical carrier can automatically convert the UL frequency to different bands fUL=fr, or fUL=fr−2fLO, or fUL=fr+2fLO.

Referring toFIG. 4there is shown a flow chart400for a method of operation of the C-RAN network100according to an embodiment of the present matter. At step402an optical carrier with a spectrum301, may be generated by, for example, an inexpensive off-the-shelf DFBL. Such DFBL's may typically emit light at a wavelength of 1548.51 nm (Fop), with a power in a range of 30 mW or so, and a maximum spectral width of 5 MHz. A temperature stability circuit for this laser may not be necessary as it may be seen from the architecture of the transceivers and the spectra, that a beat frequency of the carriers and information signals is independent of the temperature drift.

At step404the two-tone optical carrier is generated, the laser signal from step402is modulated by for example the LO202signal that is fed to the RF port of the modulator203. The frequency of the LO is based on a chosen frequency up/down conversion. The resulting two-tone optical carrier is obtained by adjusting a dc bias of the modulator203, for example, to minimum transmission point (MTP). At this point the main laser tone at fopis suppressed while the double side bands are left at their maxima, as shown in303or304inFIG. 3. The optical signal may be amplified at406stage by for example using the amplifier204in order to overcome signal losses in the transceiver signal path. This amplification produces a random SOP that is adjusted at step408immediately by the polarization controller205.

At step410orthogonal polarization, as one example, is used to split the adjusted optical signals in order to achieve full-duplex link operation, using for example the PBS206to produce the two-tone signals, i.e. a x-polarised signal X for the DL and a y-polarised signal Y for the UL signal processing.

At step414data communication between the central station102and remote unit104is started. The x-polarised DL optical carrier X is modulated, step412by for example at the optical modulator126by the information DL signals from the signal generator109and the optional IF mixer124while the y-polarised signal Y is sent without modulation, step416, across the optical link107. At step418, the modulated x-polarised signal and the unmodulated y-polarised signals are combined by for example the PBS128to form as the combined signal, the DL signals.

At step422signals are either sent across the link107as the DL signal or received from the link107as the UL signal.

In the case of an UL signal received at step422at the RRH102, the UL signal is amplified at step424and converted at step426to an electrical signal and demodulated at step428.

However, in the case of a DL signal at step422the DL signal is sent, step420across the link107to the RRH104. At step430the SOP is adjusted in the RRH104and split into the orthogonal, x-polarised and y-polarised signals X′, Y′ at step434.

At step442, the polarised X′ signal and Y′ signal, generated at step434, are processed differently. At step436the X′ signal is converted to the electrical domain, amplified at step438and broadcast at step440.

The Y′ signal is used in the optical domain at step446to modulate the received UL signal, converted at step446from the electrical domain to the optical domain. For the UL path, the received UL information signal is captured by the Rx antenna at step450and filtered and amplified by the LNA142at step448, which may then be converted to the optical domain in the step446. The modulated UL signal is then passed to the C-BBU102where it is processed in step422as described above.

It may be seen that the y-polarised optical carrier is modulated by the UL information signal at modulator135. Both circulators,129at the RRH and130at the C-C-BBU, work together with the optical link107to deliver the UL information signal back to the C-BBU transceiver114. The incoming UL signal is amplified optically by the EDFA132. The optical spectrum of the modulated UL signal is shown in the spectrum diagram314ofFIG. 3. The PD134extracts the desired UL signal as shown in the spectrum diagram315ofFIG. 3. The second circulator129forwards the DL signal to the polarization controller207. The controller207adjusts the random polarization to linear polarization and aligned equally to the PBS208so that the x-polarized signal goes to the PD139and the y-polarised signal goes to the UL modulator135. The DL information signal is converted to the electrical domain, filtered and amplified by the PA140and broadcasted to air by the DL antenna, as shown in the spectrum plot312ofFIG. 3.

Referring toFIG. 5there is shown a block diagram of a linearized C-RAN system500according to a further embodiment of the present matter. Similar to the C-RAN system100, the system500includes a C-BBU502, a RRH504, and a fronthaul optic link507connecting the C-BBU502to the RRH504.

The BBU502includes an optical generation block512, a DSP block510and a BBU transceiver514.

The optical generation block512includes a DFBL source501, a first EDFA503, a polarization controller505, and a PBS506.

The DSP block510includes a DL signal generator509, an UL signal processor511, and a DPD block513.

The BBU transceiver514includes a transmit path having an optional IF mixer524, an MZM526, a PBS combiner528, an optical circulator530, and a receive path having a second EDFA532, photodetector534, a power divider538, and first band pass filter (BPF)536and a second BPF537.

Operation of the BBU transceiver514is similar to that as previously described embodiments. However, this embodiment includes a DPD function block for linearization of the signals to compensate for any nonlinearity and hardware impairment generated in the optical and or electrical paths. As previously described, the optical generation block512generates the mutually orthogonal polarised optical signals X and Y. The IF mixer524receives an electrical signal (signal_OUT) from the DL signal generator509in the DSP510, and drives the MZM526with an output signal to modulate one of the optical signals X input from the optical carrier generation and polarization block512. The MZM526outputs a modulated optical signal to the PBS combiner528. The PBS528combines the modulated optical signal Xmodwith the other (unmodulated) optical signal Y from the polarization block512. The combined signals are output to a first port of the optical circulator530which has a second port coupled to the fiber optic cable507. A third port of the circulator530is coupled in the receive path, to an input of the second EDFA532which couples an UL signal from the link507to drive the photodetector534which in turn outputs an electrical signal to the power divider538. The power divider feeds the respective split signals in parallel to the respective first and second band pass filters BPF536and537. The first BPF536has an output coupled to feed a signal to the DPD block513, and the second BPF537has an output coupled to feed a signal to the UL block511.

Turning now to the RRH504, the RRH includes an RRH transceiver516, and a FEM518. The RRH transceiver516includes a second optical circulator529, an optical splitter533, and an MZM535, and a PD539. The optical circulator529has a first port connected to the fiber optic cable507, a second port connected to the optical splitter533for generating second orthogonally polarised signals X′, Y′, wherein the one of the orthogonal signals X′ is input to the PD539and the other orthogonal signal Y′ is output to an optical input of the MZM535. An optical output of the MZM535is coupled to a third input port of the circulator529. The PD539outputs an electrical signal to the FEM518and the MZM535receives an electrical signal output from the FEM518.

The FEM518includes the PD539, a PA540, a diplexer544, an attenuator546, an LNA542, and a power combiner548. The FEM518, in this embodiment, includes a feed back path providing a signal for use in linearization. The DL signal is fed to the PA540where it is amplified and output to the diplexer544, and signal is also fed back through the attenuator546to one input of the power combiner548. The power combiner548has a second input for receiving an UL signal output from the LNA542. The power combiner548combines both the attenuated DL feed back signal from 546 and the received UL signal and outputs a combined signal to the single MZM535which uses the signal to modulate the Y′ signal for output to the circulator529for transmission back to the BBU502across link507.

The system500illustrates a linearized RoF transmitter using a DPD in the architecture of the RoF transceivers (BBU transceiver and RRH transceiver) augmented with a feedback path for digital predistortion (DPD) using a single MZM at the RRH to convert a combined RF feedback/DL signal and a RF received/UL signal from electrical signals to optical signals. In other words, the embodiment provides for modification of the RoF Transceiver architecture to include a feedback/observation path to sample the DL signal at the output of power amplifier before transmission over the air by the antenna. This down-converted feedback signal at the BBU along with the input baseband signal is used to construct a digital base band predistortion model to predistort the DL signal, by the post-processing block in BBU. This may compensate for any nonlinearity and hardware impairment generated in RoF transmitter by both the electrical and optical components.

Referring now toFIG. 6there is shown a block diagram of a linearized C-RAN system600according to a still further embodiment of the present matter. Similar to the linearized C-RAN system500, the system600includes a C-BBU602, a RRH604, and a fronthaul optic link607connecting the C-BBU602to the RRH604. The linearized RoF transmitter in this embodiment600is augmented with a feedback path for digital predistortion (DPD) linearization using feedback signals in the optical domain. In other words the transceivers are configured to use two MZMs at the RRH604to respectively and separately use a RF feedback/DL signal and a RF received/UL signal to modulate optical signals, which are then combined to be transmitted across the fiber to the BBU.

The BBU602includes an optical generation block612, a DSP block610and a BBU transceiver614.

The optical generation block612includes a DFBL source601, a first EDFA603, a polarization controller605, and a PBS606.

The DSP block610includes a DL signal generator609, an UL signal processor611, and a DPD block613.

The BBU transceiver614includes a transmit path having an optional IF mixer624, an MZM626, a PBS combiner628, an optical circulator630, and a receive path having a second EDFA632, an optical polarization splitter638, a first photodetector634-1, a second photodetector634-2.

Operation of the BBU transceiver614is similar to that as previously described embodiments. However, this embodiment includes a DPD function block613for linearization of the signals to compensate for any nonlinearity and hardware impairment generated in the optical and or electrical paths. As previously described, the optical generation block612generates the mutually orthogonal polarised optical signals X and Y. The IF mixer624receives an electrical signal (signal_OUT) from the DL signal generator609in the DSP610, and drives the MZM626with an output signal to modulate one of the optical signals X input from the optical carrier generation and polarization block612. The MZM626outputs a modulated optical signal to the PBS combiner628. The PBS628combines the modulated optical signal Xmodwith the other (unmodulated) optical signal Y from the polarization block612. The combined signals are output to a first port of the optical circulator630which has a second port coupled to the fiber optic cable607. A third port of the circulator630is coupled in the receive path, to an input of the second EDFA632which couples an UL signal from the link607to the optical polarization splitter638. The splitter outputs two signal YY′ and YX′ coupled to the respective photodetectors634-1and634-2. The first photodetector (converter)634-1has an output coupled to feed a signal to the DPD block613, and the second photodetector (converter)634-2has an output coupled to feed a signal to the UL block611.

The RRH604includes an RRH transceiver616, and a FEM618. The RRH transceiver616includes a second optical circulator629, first and second optical splitters633-1,633-2, first and second MZM's635-1,635-2, and an optical polarization combiner650.

The optical circulator629has a first port connected to the fiber optic cable607, a second port connected to the first optical splitter633-1for generating a second pair of orthogonally polarised signals X′, Y′, wherein the one of the orthogonal signals X′ is input to the FEM618and the other orthogonal optical signal Y′ is output to the second optical polarization splitter633-2for generating a third pair of orthogonally polarised signals YY, YX, where YY is the y-polarised and YX is the x-polarised signals recovered from the Y′ signal. The optical signal YY is coupled to an optical input of the first MZM635-1and the optical signal YX is coupled to an input of the second MZM635-2. The outputs from the MZM's are coupled to the polarization combiner650where they are combined and forwarded to the link607to the BBU602. Each of the first and second MZM's635-1,635-2are controlled by respective signal output from the FEM618as will be explained below.

The FEM618includes the PD639, a PA640, a diplexer644, an attenuator646, and an LNA642. The FEM618, in this embodiment, includes a feed back path providing a signal for use in linearization. The DL signal from the optical polarization splitter633-1is converted by the PD639and fed to the PA640where it is amplified and output to the diplexer644, the amplifier output is also coupled via the attenuator646to drive the first MZM635-1which modulates the YY signal. An UL signal output from the LNA642is coupled to drive the second MZM635-2which modulates the YX signal. The signals output from the MZM's are combined in the polarization combiner650for output to the circulator629for transmission back to the BBU602across link607.

It may be seen that linearization of the system may be achieved by inclusion of a feedback/observation path to sample the DL signal at the output of power amplifier before transmission in the air by the antenna. The down-converted feedback signal may then be used at the BBU along with the input baseband signal to construct a digital baseband predistortion model to predistort the DL signal. This may be implemented by the post-processing block in the BBU, to compensate for any nonlinearity and hardware impairment generated by both the electrical and optical components in the system.

Referring toFIG. 7there is shown an N-link (N being an integer) C-RAN network architecture700according to an embodiment of the present matter. The C-RAN700includes C-BBU702, a plurality of base transceiver units704-ior RRH's and a fronthaul link706connecting the C-BBU702to the plurality of RRH's704-i. The front haul link706may include one or more fiber optic links707-ior in further embodiments free-air optic links may also be employed or a combination of free-air optics, and fibre optic links.

In this N-link C-RAN architecture700, the C-BBU702includes a dual optical carrier generation and polarization block712for generating the x-polarised and y-polarised optical signals, a DSP block710providing one or more functions of modulation/demodulation, signal encoding, predistortion, and beamforming. The C-BBU702further includes N C-BBU transceivers714-i(i−1 . . . N) each of which is connected via respective ones of N standard single mode (SM) fiber optic cables707-ito one of the respective N RRH's704-i. The transceivers714-iprovide an interface between baseband and modulated information signals.

Furthermore, each RRH's704-iincludes a RRH transceiver716-iand a FEM718-icoupled to a respective antenna Ai120-i.

As will be appreciated the components in the N-link C-RAN network architecture700are configured to perform the same functions as those described with respect to the single link implementation ofFIG. 1above. Therefore, for brevity will not be further described. Furthermore, the linearization and DPD as described in embodiments above may also be implemented in one or more of the N links in the present embodiment700.

Referring toFIG. 8there is shown a block diagram800of an optical splitting or polarization multiplexing arrangement for the N-link C-RAN network architecture700according to an embodiment of the present matter. As previously described inFIG. 7, the dual optical carrier generation and polarization block812generates the x-polarised and y-polarised optical signals from a laser (also as previously described herein). The polarization block812may use single PBS outputting signals to respective two optical splitters863,864. In order to provide carriers for the N RRHs the optical splitters or polarisation maintaining (PM) splitters split the x-polarised and y-polarised signals into multiple N signals to match a desired network size or number of RRHs. This implementation is less costly than using multiple PBSs with one optical splitter. The information signal IF frequencies (f1, f2, . . . fN) for each link may be decided and selected at network design time. Links have the same communication technique protocols but may have different DL and UL data and frequencies.

Referring toFIG. 9there is shown a MU-MIMO architecture900with beamforming using the C-BBU102and RRH104transceivers according to an embodiment of the present matter. The MIMO architecture900with beamforming includes a BBU unit receiving different input signals (I1, Q1) to (Ii, Qi), where i=1 . . . N that are converted to optical signal before transmitting them to the RRHs over N optical fibers as described previously herein. At each respective RRH unit904-1. . .904-i, each respective optical stream is converted to an electrical signal, as previously described herein. However for each RRH the stream is divided into M signals each driving respective ones of M FEMs (e.g. (FEM-11 . . . FEM-1M) . . . (FEM-i1 . . . FEM-iM)) where each FEM in turn is connected to a phased array antenna having M elements. Each FEM includes a PA, an LNA, single-pole double-throw (SPDT) switch along with amplitude and phase controllers which can be configured for applying amplitude and phase control on the signals for beamforming. The frequencies of the signals at the outputs of RRH units are controlled by BBU. In one embodiment of the configuration900all the frequencies may be set to the same value, the system may then be operated in a mode of a full MIMO transceiver with multi-beam capability. In a further embodiment of the configuration900all the frequencies may be set to different values, the system may then be operated in a mode of a multi-user MIMO transceiver with multi-beam capability.

Referring toFIG. 10there is shown a C-RAN architecture1000using the C-BBU102and RRH104transceivers, described previously herein, configured with a free-space-optical (FSO) channel, according to an embodiment of the present matter. In the illustrated embodiment1000the BBU transceivers and the RRH transceivers are configured to use an FSO channel1010instead of the optical fiber. Although in some instance combinations of FSO and optical fiber channels may be used. The BBU transceiver includes a FSO transducer1006which communicates over free space with a corresponding FSO transducer1008at the BBU.

Referring toFIG. 11there is shown a C-RAN architecture1100for satellite (SAT) applications which connects two or more satellites, with ground/mobile stations. For example, as shown in the illustrated embodiment two satellites are shown SAT-11102and SAT-21104and ground/mobile stations1110-1and1110-2. The satellite SAT-11102configured for operation similar to C-BBU102as described previously inFIG. 1. The satellite SAT-11102is linked to the remote SAT-21104which is configured to operate similar the RRH104unit ofFIG. 1. The satellite SAT-11102includes similar components as the C-BBU102except that the DSP unit may be located in the ground/mobile stations1110-1,1110-2. The transceivers1114and1116are configured to route bidirectional optical communications over an FSO channel1010(similar to the fronthaul link107, inFIG. 1), between the satellites SAT-1 and SAT-2. A first optical transducer1006in SAT-1 and a second optical transducer1008in SAT-2 are configured to couple an optical signal from fiber to free space and vice versa. Also, the transceiver1114may route the bidirectional RF signals between SAT-11102and the earth/mobile station1110-1through an FEM118-1. For example, in one embodiment, microwave antennas1120-1in SAT-1 and1122-1at the earth/mobile station may couple the broadcast RF UL signals and DL signals from free space to an RF path.

The SAT-21104includes the transceiver1116configured to operate similar to that described previously for the transceiver116ofFIG. 1, to establish full-duplex optical link between SAT-1 and SAT-2. The transceiver1116routes bidirectional RF signals between SAT-21104and the earth/mobile station1110-2through a FEM118-2in the satellite SAT-2. Microwave antennas1120-2in SAT-2 and1122-2in the earth/mobile station1110-2couple the RF UL signals and DL signals from free space to the RF network.

It may be seen from the above description that embodiments of the present matter provide for base transceiver stations to be physically smaller, less expensive, and easier to deploy. Furthermore, the present architecture reduces reliance on use of a local multipoint distribution system (LDMS) as a carrier which in turn may limit degrees of the freedom of frequency bands as well as overall cost of hardware.