Technical Field

The present disclosure relates to a network node and method for photonic beamforming.

Background

The future generations of communication systems must be able to provide efficient, high-performing solutions for all services, and they will need to address a large number of technological challenges. A massive growth in traffic volume is expected, with an analog growth in the number of connected devices, including communication in robotics, industries, domotics and security applications. These heterogeneous communications will define a wide range of requirements and characteristics in terms of data rates, latency, reliability, device energy consumption and cost. Mobile-broadband services such as video streaming, data sharing and cloud services will remain, and will continue to drive a demand for higher consumer data rates. Reliably achievable multi-Giga bytes per second (Gbps) data rates should be available in special scenarios, such as office spaces or dense urban outdoor environments, in order to support applications such as the synchronization of local storage devices to cloud drives, streaming of ultra-high-resolution video, and virtual and augmented reality. Even more important, consumer data rates of hundreds of Mbps should be generally available as a step toward realizing the vision of unlimited access to information. The end-to-end latencies will need to be reduced to a few milliseconds to support the multi-Gbps data rates and enhance virtual- and augmented-reality applications.

The cost of deploying, operating and maintaining a network, as well as the cost of the devices, should also be at a level that enables popular services to be provided at an attractive price for users, while maintaining attractive business cases for network operators. Energy efficiency should also be an important focus area, in order to achieve and retain a low network-operation cost even with the expected massive increase in traffic. Smart antennas including a very large number of steerable antenna elements, larger available spectrum and an increased coordination between base stations will help to provide such very high service levels. The mobile-broadband technologies will also expand into new deployment scenarios, such as dense small-cell deployments, and new use cases, such as different kinds of machine-type communication. To address the challenge of being able to provide extremely high traffic capacity and multi- Gbps data rates in specific scenarios, the introduction of ultra-dense network deployments is foreseen, with nodes operating with very wide transmission bandwidths in higher-frequency bands relying on new radio-access technologies. Ultra-dense networks will consist of low-power access nodes that will be deployed with much higher density than existing networks. In extreme cases, one can foresee indoor deployments with access nodes in every room and outdoor deployments with access nodes at lamppost distances apart. To reliably support multi-Gbps data rates, ultra-dense networks should support minimum transmission bandwidths of several hundreds of MHz with the possibility of an extension up to a few GHz of bandwidth. Ultra-dense networks will primarily operate in the 10-100GHz range as higher frequency bands enable the very wide transmission bandwidth needed to reliably support multi-Gbps data rates. Although these ultra-dense networks will operate in different spectrum regions and will most likely be based on new radio-access technologies, they should be well integrated with the overlaid cellular networks, providing a seamless user experience as devices move in and out of ultra-dense network coverage.

To reduce the interference between densely deployed cells, arrayed antennas using beamforming techniques should be introduced, for example in order to steer the beam in both the vertical and horizontal planes. The use of arrayed antennas could imply an increase of power consumption, complexity and cost, and given the huge number of base stations in micro- and pico-cells, reduction of costs and power consumption by system integration is a necessary step for making this solution feasible. As a rule of thumb, the power consumption should be kept at the current level in front of a traffic volume increase by a factor of 10 ³ .

In future mobile systems, both LTE-A and 5G, the aggressive scenario of a frequency reuse of one is going to be adopted, where all the cells use the same frequency. A reuse of one provides the highest network efficiency and enables high data rates close to the base station. The challenge with a reuse of one is the high inter-cell interference when the terminal (User Equipment, UE) is located between two cells.

Beamforming (BF) will be introduced in 5G to have the capability of pointing the radio beam to cover a precise area, tuning its orientation in order to reduce interference with closed radio sectors. In such a way, it is possible to cover several small crowded areas with radio beams working at microwave frequencies. In perspective, it is also envisaged that beamforming will introduce the capability of following users with the radio beams, for example having multiple-beams irradiated by the same antenna. However, to cover the same area or close area with radio beams, the latter has to work using different frequencies to avoid interference, so several licensed frequencies must be adopted. With the likely increase in the number of antenna elements being used in antenna arrays, together with an increase in the number of antennas, existing beamforming solutions have many limitations.

Summary

According to a first aspect, there is provided a network node for use in photonic beamforming. The network node comprises an input for receiving a mode locked laser (MLL) signal. The network node comprises an optical modulator coupled to receive the MLL signal and a clock signal. The network node comprises a clock derivation module for deriving the clock signal from the MLL signal.

The optical modulator may comprise a first transmitting optical modulator coupled to receive the MLL signal, the first transmitting optical modulator being configured to generate a comb signal of continuous waves. The first transmitting optical modulator may be configured to be driven by the clock signal. It may also be configured to output a comb signal of continuous waves in the form of a single sideband modulation with carrier suppression.

In some embodiments the network node further comprises a second

transmitting optical modulator coupled to receive the MLL signal, wherein the second transmitting optical modulator is configured to be driven by an intermediate frequency signal.

The second transmitting optical modulator may be configured to output a data sideband signal for each respective laser line of the comb signal, at a frequency distance of the intermediate frequency from each respective laser line. Each data sideband signal may be a single sideband modulation with carrier suppressions. In some embodiments the network node further comprises a first selective phase shifter coupled to receive the output of the first transmitting optical modulator, and configured to selectively control the phase of each continuous wave. The first selective phase shifter may be further coupled to receive the output of the second transmitting optical modulator. The output of the first selective phase shifter may be coupled to a first demultiplexer.

In some embodiments, the second transmitting optical modulator is coupled to the first demultiplexer.

The clock signal may be configured to be derived by dividing a frequency of a reference clock provided by the MLL signal by an integer number. A frequency of the clock signal may be a frequency of a reference clock of the MLL signal. The optical modulator may be coupled to receive the MLL signal via a de- interleaving module.

The optical modulator may comprise a receiving optical modulator coupled to receive the MLL signal, and which is configured to generate a comb signal of continuous waves.

The receiving optical modulator may be configured to be driven by the clock signal. The receiving optical modulator may be configured to output a comb signal of continuous waves in the form of a single sideband modulation with carrier suppression.

The MLL signal may be further coupled to a second demultiplexer. The second demultiplexer may be configured to separate the modes of the MLL signal and send each mode signal to one of a plurality of receiving antenna elements which modulate the mode signals with received signals to provide modulated signals comprising single-sideband modulation with carrier suppression.

The modulated signals from the plurality of receiving antenna elements may be coupled through a multiplexer to form a combined receiver signal. The network node may further comprises a summing module configured to couple the combined receiver signal and the output of the receiving optical modulator. A second selective phase shifter may be coupled between the input and the second demultiplexer, and configured to selectively control the phase of each mode of the MLL signal. A second selective phase shifter may be coupled between the receiving optical modulator and the summing module, and configured to selectively control the phase of each continuous wave.

According to a further aspect, there is provided a method of photonic

beamforming for use in a network node. The method comprises receiving a mode locked laser (MLL) signal, and inputting the MLL signal into an optical modulator. A clock signal is derived from the MLL signal, and the clock signal used to drive the optical modulator. Brief Description of the Drawings

For a better understanding of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 illustrates an example of a network node according to an

embodiment;

Figure 2 illustrates an example of a method carried out by a network node 100 according to an embodiment;

Figure 3 illustrates an example of a network node according to another embodiment;

Figure 4 illustrates an example of a network node according to another embodiment; Figure 5 illustrates an example of a network node according to another embodiment; Figure 6 shows a block diagram of a network node 600 according to an embodiment.

Figure 7 shows a block diagram of a network node 700 according to an embodiment.

Detailed Description

In recent years, photonics-based wireless systems, although still at research level, are moving towards a new generation of multifunctional systems able to manage the wireless communications with several different frequencies and protocols, even simultaneously, while also realizing surveillance operations. Photonics matches the new requirements of flexibility for software-defined architectures due to its ultra-wide bandwidths, and ease of tunability, guaranteeing low footprint and weight as a result of integrated photonic technologies. Moreover, photonics also allows increased resolution and sensitivity by means of the inherently low phase noise of lasers.

Existing wireless communication systems need specific hardware working at specific radio frequencies, with characteristic signal waveforms and bandwidths, requiring dedicated apparatus for each single application. Moreover, personal communications will soon need to exploit new frequency bands such as the unlicensed millimeter waveband around 60GHz. However, current electronic devices present increasing limitations and worsening performance as the frequency gets higher. Similarly, surveillance systems will soon require a higher carrier frequency for smaller antennas, broadened bandwidth for increased resolution, and software-defined signal generation and detection (the so-called software-defined radio approach, SDR) for flexibility in variable environments. Therefore, in most of the future transceivers for wireless applications (either communications or surveillance) it will be necessary to generate and receive very stable high frequencies and wideband radio-frequency signals by means of reliable transmitters/receivers that also respect constrains in size, weight and power consumption (SWaP). Besides this, flexibility and re-configurability will be another fundamental requirement for future systems.

Existing microwave components suffer limited bandwidth, poor flexibility, and high noise at increasing carrier frequencies. In conventional electronic radio- frequency transceivers, the major limitations come from the reduced dynamic range due to multiple-stage down-converting mixers, the limited port-to-port isolation at the mixer, and the excessive size, weight, and power requirements of the front-ends. Such front-ends are known to be noisy and suffer from electromagnetic interferences, causing degradation of the sensitivity and of the dynamic range of the final system. These deficiencies are magnified as wider bandwidth systems are developed. An analog-digital converter accepting signal frequencies above 10GHz with a decent signal-to-noise ratio (for example greater than 50dB) need to show extremely low sampling jitter, and very fast switching devices that are difficult to achieve with the current electronic technologies.

On the contrary, photonics has proved to have high precision and ultra-wide bandwidth, allowing the generation of extremely stable multiple radio-frequency (RF) signals with arbitrary waveforms up to the millimeter waves, while also allowing their detection and precise direct digitization (i.e., without noisy RF down-conversions). The photonics-based generation and detection of RF signals are usually studied only separately. Only recently a fully photonics- based RF transceiver has been developed and characterized, and is now being tested in a radar application. The exploited architecture makes use of a single pulsed laser for both generating and detecting the tunable RF signals, avoiding RF up-/down-conversions and guaranteeing a software-defined approach, multiple functionalities, and high resolution, with performance exceeding the state-of-the-art electronics at carrier frequencies above 10GHz. In the past few years, simple architectures for the photonic generation of RF signals have been envisioned based on the heterodyning of two independent lasers, but these implementations do not allow for a stable RF generation, making the obtained signals not useful for the requirements of future systems. In order to improve RF stability, phase locking of the beating lasers is necessary, and this usually requires more complex and cumbersome set-ups. One technique for generating phase locked laser lines is the mode locking of lasers: the intrinsic phase-locking condition of the mode-locked laser (MLL) ensures an extremely low phase noise of the generated RF signal. Moreover, the possibility of selecting laser modes with variable wavelength detuning allows the flexible production of RF carriers with tunable frequency, potentially generating any multiple frequency of the MLL repetition rate. Moreover, the phase noise of the obtained RF carriers have been measured and analyzed, demonstrating that they can be significantly less noisy than the signals generated by the state-of-the-art RF synthesizer.

To implement advanced communication functionalities, the photonics-generated carriers must be modulated into amplitude- and/or phase-coded signals via electronics methods, which would require frequency-specific RF components that become more expensive at increasing frequency. An alternative modulation and coding approach based on photonics could instead allow broad RF bandwidth without restrictions on the carrier frequency selection.

The present embodiments exploit the same approach as for the carrier generation, based on the use of a MLL. In these schemes, the modulated RF signal is generated by heterodyning two modes from a MLL, one of which is modulated by the low-pass modulation signal. In this approach, the modulation signal can be generated by a digital synthesizer with narrow analog bandwidth, and directly up-converted by photonic techniques through the heterodyning. Typical Wi-Fi OFDM (orthogonal frequency-division multiplexing) signals and compressed radar pulses have been generated with these techniques, with carrier frequencies up to 40GHz. The schemes allow the photonic generation of arbitrary phase-modulated RF pulses with flexible carrier frequency, and phase stability suitable for coherent radar systems.

The receivers for Software Defined Radios (SDR) would need high speed analog-digital convertors with a huge analog input bandwidth spanning over several tens of GHz, and with high spurious-free dynamic range (SFDR) as well as signal to noise ratio. However, precise electronic analog-digital convertors show limited analog input bandwidth, since at high input frequency the aperture jitter of the sampling clock affects the accuracy of the digitized signal. Existing electronic analog-digital convertors show an aperture jitter of hundreds of femtoseconds with only a few GHz of analog bandwidth. Optical sampling can overcome the limitations faced by electronic analog-digital convertors, and in the last decade several photonics-assisted analog-digital convertors have been proposed, based on the electrical detection of modulated optical pulse trains with subsequent sample parallelization schemes. Most of these converters resort to the concept of under-sampling to acquire radio frequency signals with bandwidth up to a few GHz but a carrier frequency up to several tens of GHz. The use of narrow-pulse MLLs with very low temporal jitter guarantees a precise sampling time and a digitized signal with a low jitter-limited noise floor. The high electro-optical bandwidth of the optical modulators can broaden the analog input bandwidth of photonic-assisted ADCs up to tens of GHz. Sample parallelization by time- or wavelength-interleaving schemes have been proposed to enlarge the instantaneous bandwidth (i.e., the maximum signal bandwidth) of the photonic ADCs, by exploiting a MLL with high repetition rate and a set of parallel low-speed high-precision electronic converters. However, the data interleaving can also produce spurious peaks due to the inequalities of the data arrays in the parallel channels, and to the non-idealities of the parallelizing method as time skew and crosstalk.

While wavelength-interleaving is most sensible to the time skew, time- interleaving suffers the inter-channels crosstalk due to the limited extinction ratio of the optical switching matrix. Digital post-processing techniques are usually applied to minimize the effect of such spurious components and to maximize the precision of the photonic ADC. The present Applicant has proposed the exploitation of the time-interleaving approach to avoid the time skew issues, and have presented a photonic ADC based on a 4-fold time- interleaving with an extremely low sampling jitter where the limited extinction ratio of the optical switching matrix is compensated for by a real-time digital post-processing, for reducing the spurious tones. The realized ADC has shown a state-of-the-art precision above 7 effective bits up to 40GHz with an instantaneous bandwidth of 200MHz. The scheme demonstrates to approach the theoretical limit imposed by the sampling jitter, and to be easily scalable to larger signal bandwidth with the current photonic technologies.

Nowadays, electronic solutions for the implementation of a beamforming network make use of phase shifters at each antenna element. Such solutions show significant insertion loss and power consumption, with a non-negligible phase error.

In the future, an increase in the number of antenna elements in the array is to be expected, together with an increase in the number of antennas. Therefore, the limitations reported above will progressively affect the running cost of the beamforming network.

Beamforming networks based on photonics have been proposed, either exploiting true-time delay or phase shift. The solutions based on true-time delay have the advantage of avoiding the squint of the beam, even for broadband signals. Photonics has been proposed for this application, thanks to the broad bandwidth it assures. The true-time delay has been proposed by means of chirped fiber Bragg gratings and tunable laser sources. This solution requires one tunable laser per antenna element, therefore it becomes unpractical if the antenna array has several elements. Another technical implementation of true-time delay consists in realizing a set of different delays which propagate the optical signal. This approach lacks the continuity of the delay that would be necessary for a precise beam control.

Optical beam-forming based on phase shift is realized by shifting the carrier with respect to the sidebands. This can be done, for example, by exploiting 2- D arrays of pixels based on liquid crystal on silicon and multi-wavelength optical sources. However, such a solution requires that the carrier and its sideband are separated more that the resolution of the pixels.

All the examples above consider the modulation of laser sources by means of a signal at radio frequency. The stability of the signal therefore relies on electronic oscillators.

Recently the simultaneous radio frequency signal generation and steering has been proposed exploiting the true-time delay induced by chromatic dispersion to the different modes of a mode-locked laser. This solution also needs radio frequency filters at the antenna elements to select the correct beating after the photodiode. Besides this representing additional complexity, it also implies a reduced efficiency in the radio frequency signal generation process.

Most of the beamforming schemes proposed so far only consider the steering of the transmitted signal, neglecting the control of the received signal. The embodiments described herein exploit a mode-locked laser as a laser source, which works as both a laser comb and a system clock. Each component of the comb may be modulated in carrier suppression to separately generate the phase controlled carrier and the data sideband.

As will be described in greater detail below, this approach allows the phase of the carriers to be easily controlled, without limitation on the frequency of the RF signal. Embodiments described herein also consider the beamforming in both transmission and reception, and examples can compensate separately the phase fluctuations in the two directions.

The exploitation of photonics and the described embodiments can allow an antenna array to be remotely controlled, with reduced complexity and power requirement at the antenna site.

Figure 1 illustrates a network node 100 according to an example of an embodiment. The network node 100, which may be a basestation or other form of network node, is used for photonic beamforming.

The network node may be a basestation, a receiving or transmitting radio module or antenna, or a combination of a basestation and receiving and/or transmitting radio module (s) or antenna(s).

The network node 100 comprises an input 101 for receiving a mode locked laser (MLL) signal. In some examples, the MLL may be provided in the network node 100. In other examples, the MLL may be remote from the network node 100. The network node 100 also comprises an optical modulator 103 which is coupled to receive the MLL signal and a clock signal 105. A clock derivation module 106 is provided for deriving the clock signal 105 from the MLL signal. Such an embodiment has an advantage in that, in addition to using an MLL signal by an optical modulator for photonic beamforming (for example whereby the MLL signal is used as a laser comb), the MLL signal is also used by a clock derivation module for providing a clock signal to the optical modulator. This has an advantage of providing a more stable and accurate optical modulator for photonic beamforming.

In one example, the MLL is at a frequency mF. The signal from the MLL may be split to feed an optical modulator in the transmitting and/or the receiving parts.

The clock derivation module may comprise circuitry. The term circuitry includes hardware only circuit implementations, such as implementations in only analogue and or digital circuitry. The term circuitry also includes implementations including a combination of circuits, including for example a processor, and software.

In some cases the clock signal 105 can be derived by the clock derivation module 106 which in this example would comprise dividing circuitry for dividing a frequency of a reference clock of the MLL signal by an integer number. In other embodiments, the clock derivation module comprises circuitry for obtaining the frequency of the reference clock of the MLL signal and the reference clock frequency may be used for the clock signal .

The reference clock is a radio-frequency signal equal to the repetition rate of the optical pulses generated by the MLL, or, equivalently, equal to the frequency detuning between the laser lines constituting the comb generated by the MLL. It is usually provided by the MLL, or it can be derived by detecting the optical signal of the MLL with a photodiode. Figure 2 illustrates a method carried out by a network node 100 according to example embodiments.

In step 201 the network node 100 receives a mode locked laser (MLL) signal. The MLL signal is input into an optical modulator in step 203.

In step 205 a clock signal is derived from the MLL signal, and in step 207, the clock signal is used to drive the optical modulator. Figure 3 illustrates an example of a network node according to another embodiment.

In this figure, the optical modulator 103 comprises a first transmitting optical modulator 107, which is coupled to receive the MLL signal from the input 101 . This first transmitting optical modulator 107 is configured to generate a comb signal of continuous waves as shown in the graph 109.

The first transmitting optical modulator 107, which configured to be driven by the clock signal 105, may in some examples produce a comb signal of continuous waves in the form of a single sideband modulation.

In some examples the network node 100 further comprises a second

transmitting optical modulator 1 1 1 which is also coupled to receive the MLL signal. In this example the second transmitting optical modulator is driven by an intermediate frequency signal 1 13.

The second transmitting optical modulator 1 1 1 may be configured to output a data sideband signal 1 15 for each respective laser line of the comb signal, at a frequency distance of the intermediate frequency from each respective laser line, as shown in the graph 1 15. In some embodiments each data sideband signal may be a single sideband modulation with carrier suppressions (SSB-CS).

The example shown in Figure 3 also comprises a first selective phase shifter 1 17 which is coupled to receive the output of the first transmitting optical modulator 107. This first selective phase shifter 1 17 may be configured to selectively control the phase of each continuous wave.

In the embodiment shown in Figure 3, the output of the first selective phase shifter 1 17 is coupled to a first demultiplexer 1 19. The first demultiplexer 1 19 may be situated within a transmitter, as shown in Figure 3, or within the network node 100 itself, for example within a basestation.

The output from the second transmitting optical modulator 1 1 1 is coupled with the output of the first selective phase shifter 1 17 and is also input into the first demultiplexer 1 19. In some embodiments, the coupled signal may first be amplified before being input into the first demultiplexer 1 19. For example, an Erbium Doped Fibre Amplifier (EDFA), as shown in Figure 3, may be provided for amplifying the outputs of the first selective phase shifter 1 17 and second transmitting optical modulator 1 1 1 . The EDFA can be placed either at the network node 100 (e.g. basestation), or at the transmitter.

The first demultiplexer 1 19 then separates the pairs of optical signals: each signal pair, composed of the data sideband and the phase-controlled CW generated from the same MLL mode, is selected and sent to an output port of the first demultiplexer 1 19.

At each output port, the signal pair is sent to a photodiode (PD) which generates the beating between the signals, producing the RF signal to be transmitted by the antenna element, with a controlled phase, at frequency F+IF. In this example of Figure 3, the clock signal is derived from the MLL reference clock frequency by dividing the frequency of reference clock by an integer number 'm'. However, it will be appreciated that alternative methods could be used for deriving the clock signal from the MLL signal.

In the receiving section of this example of the embodiment of Figure 3, the optical modulator 103 also comprises a receiving optical modulator 121 which is coupled to receive the MLL signal. Again, this receiving optical modulator 121 generates a comb signal of continuous waves.

This receiving optical modulator 121 is also driven by the clock signal 105 which is derived from the MLL signal. The output of the receiving optical modulator 121 may be in the form of a single sideband modulation with carrier

suppression.

In this example, the MLL signal is also coupled to a second demultiplexer 123. Again, this second demultiplexer may be located within the network node 100 (e.g. the basestation) or in a receiver as shown in Figure 3.

The second demultiplexer 123 is configured to separate the modes of the MLL signal and send each mode signal to one of a plurality of receiving antenna elements 125. These receiving antenna elements 125 (for example optical modulators for providing single sideband modulation with carrier suppression, SSB-CS) modulate the mode signals with the signals which they receive. This provides modulated signals as shown in the graphs 127. As such, these modulated signals may comprise single sideband modulation with carrier suppression. The modulated signals are then coupled, for example through a multiplexer 129. This multiplexer 129 may be located within either the network node 100 (e.g. basestation) or the receiver. The multiplexer 129 outputs a combined receiver signal. This combined receiver signal is input into a summing block 131 . The summing block 131 may be configured to couple the combined receiver signal with the output of a second selective phase shifter 133. The second selective phase shifter 133 may be coupled between the receiving optical modulator 121 and the summing block 131 (and which may be configured to selectively control the phase of each continuous wave). In some examples, the summing block 131 can control the reciprocal polarization and phase fluctuation of the two added signals.

This control can be implemented, for example, by means of a common PLL. Once the phase-controlled modes are coupled with the modulated signals, they are all sent to a photo diode PD, which generates all the beatings at a frequency IF from each receiving antenna element, and sums them together generating the beam-formed signal.

The embodiment of Figure 3 has an advantage in that an MLL signal is used not only as a laser comb, but also to derive a clock signal for controlling an optical modulator.

Figure 4 illustrates another example of a network node according to another embodiment.

The elements Figure 4 which are similar to those of Figure 3 having been given the same reference numerals.

Figure 4 differs from Figure 3 in that the clock signal is taken or derived directly from the frequency of the input MLL signal. Therefore the optical modulator 103 (comprising the first transmitting optical modulator 107 and receiving optical modulator 121 in this example) receives the MLL signal via a de-interleaving module 201 . The second transmitting optical modulator 1 1 1 and the second demultiplexer 123 are also coupled to receive the clock signal derived directly from the MLL signal via the de-interleaving module 201 .

Figure 5 illustrates another example of a network node according to another embodiment. Again, the elements of this figure which are similar to those of Figure 3 have been given the same reference numerals.

In this embodiment the first selective phase shifter 1 17 is not only coupled to the first transmitting optical modulator 107 to selectively control the phase of each continuous wave, but it is also coupled to receive the output of the second transmitting optical modulator 1 1 1 . In particular, in this example, the first selective phase shifter 1 17 receives a signal which is the coupling of the signals output from the first and second transmitting optical modulators 107, 1 1 1 . In this example, therefore, the first demultiplexer 1 19 does not receive the output of the second transmitting optical modulator 1 1 1 , but receives the output of the first selective phase shifter 1 17. In this example, the first demultiplexer 1 19 is shown located within the network node 100 (e.g. basestation). It will be appreciated that the first demultiplexer 1 19 may be located within the

transmitter, similarly to that shown in Figure 3.

Furthermore, in this example of Figure 5 the second selective phase shifter 133 is instead coupled between the input 101 and the second demultiplexer 123. Again, the second demultiplexer 123 is shown as being located within the network node 100 (e.g. basestation), but it will be appreciated that the second demultiplexer 123 may be located within the receiver as shown in Figure 3. Therefore, in this example, the second selective phase shifter 133 is configured to selectively control the phase of each mode of the MLL signal. From the embodiments described above it can be seen that the MLL signal therefore acts as a very precise clock, and as laser comb. Each mode of the MLL feeds an antenna element and the association between the MLL mode and the antenna element may be determined by the wavelength of the mode, through the wavelength demultiplexers.

From each MLL mode, two signals are generated. One holds the information to be transmitted (in the transmitting part) or received (in the receiving part), the other acts as a local oscillator (LO) realizing, respectively, an up- conversion or down-conversion.

The LOs are very stable, since they are generated exploiting an electrical clock signal derived by the MLL.

In examples described herein each photodiode in the transmitter side receives only two optical signals. As such, the photodiode generates only the desired radio frequency signal to be transmitted. This has an advantage of not requiring radio frequency filtering after the PD, and this increases the efficiency of the scheme. The demultiplexing and multiplexing based on signal wavelength minimize the insertion losses, compared for example with star couplers or splitters, especially for high numbers of antenna elements.

The use of a MLL at multiple frequencies allows for an easy separation of the pairs of optical signals with wavelength demultiplexers.

The phase control is separated for transmission and reception. This ensures the optimal setting of the beamforming in both directions.

Although the described embodiments have depicted certain features as being located within the basestation and certain features as being located within the transmitting and receiving radio modules or antennas, it should be appreciated that any distribution of the features within these structures is within the scope of the present embodiments. The positioning of each feature is a matter of choice and implementation.

For example, the optical modulators and/or the phase shifter may be located within the basestation or within the radio modules or antennas.

It should also be appreciated that the MLL may also itself be located within the basestation or indeed within the radio modules or antennas. It is further noted that one MLL can be used to serve multiple radio modules or antennas, for example if the MLL is located within the basestation. Alternatively, an MLL could be used to serve a single radio module or antenna, and the MLL could then be located within that particular radio module or antenna.

Figure 6 shows a block diagram of a network node 600.

The network node 600 is configured for use in photonic beamforming and comprises, a processor 601 and memory 603, the memory 603 containing instructions executable by the processor 601 . The network node 600 is operative to receive a mode locked laser (MLL) signal, input the MLL signal into an optical modulator, derive a clock signal from the MLL signal, and use the clock signal to drive the optical modulator. Figure 7 illustrates a network node 700 according to another example, for use in photonic beamforming. The network node 700 comprises a first module 701 configured to receive a mode locked laser (MLL) signal. A second module 703 is configured to input the MLL signal into an optical modulator. A third module 705 is configured to derive a clock signal from the MLL signal, wherein the clock signal is used to drive the optical modulator. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.