Highlighting the requirements of 5G technologies from a PIC perspective

Lê Nguyên Bình, technical director at Huawei's European Research Institute in Germany, examines the role of integrated photonics in addressing the convergence of wireless and optical transport networks.

Wireless mobile networks have evolved from moderately to extremely dense capacities with 4G and now the emerging 5G technologies coupled with cloud Radio Access Networks (cloud-RAN). The delivery of data to and from such mobile access users is critical in reaching the required level of quality in these services. Furthermore, where you have large communities of users, it is no longer possible for the microwave backhaul to transport such high-rate access.

In parallel to the dramatic growth in demand for mobile data transport, optical transport networks have evolved to ultra-high capacity with a basic bit rate per wavelength channel of 100 Gb/s, followed by 400 G and then Tbps in dense wavelength division networks of 50 GHz spacing. Because the uniform spectral grid distribution of optical data channels will no longer meet the demand of pay-on-demand services in the future, different bit rates occupying variable bandwidth will be employed to transport these flexible channels, and flexible optical transport networks (Flex-OTN). The backhaul and front haul of wireless networks must be transported via the optical networks to the points very close to the radiating antenna.

Wireless and optical convergence

The service rate per mobile user is expected to reach 1 Gb/s. The frequency bands for 5G wireless networks are recommended in the range 2-6 GHz, and then in the millimeter wave regions of 28.6 GHz, 58.6 GHz and 90 GHz with a 7 GHz free-license band. The capacity to be delivered to the 5G front haul access would reach several Tbps.

It is obvious that the only possible medium to deliver such ultra-high capacity is the optical domain. Thus the convergence of wireless and optical networking is evolving. Optical networks must be the backhaul for such a 5G wireless delivery proposition. Figure 1 shows a vision of the backhauling and front hauling of wireless and optical networks. It's beyond doubt that the wireless up- and down links will be active and feature steerable beam focusing to users or communities of users of at least Gb/s rate per user. Hence, multiple-input and multiple-output antenna arrays are essential for such wireless deployment. It is expected that the cloud RAN would act as distributed DC, which acts as dynamic data servers for wireless front haul access.

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Figure 1: Vision of (a) future 5G wireless front haul access and optical backhauling networks; (b) some examples of delivery and access scenarios (Image courtesy of Huawei Technologies)

Integrated photonic technology will offer the significant transfer of optical channels to wireless in the mmW region. The mmW antenna must be structured to support multiple-input multiple-output (MIMO) so that wireless beam steering can be done to focus the delivery to users or communities of users as desired. Figure 2 illustrates the front haul delivery of data from a branch of a Cloud Network to the MIMO Antenna wireless front end.

Figure 2: Generic MIMO systems employing photonic-RF technology. Legend: RAN = radio access networks, MIMO = multiple input multiple output. mmW = millimeter wave (frequency range 18-140GHz) (Image courtesy of Huawei Technology)

The total capacity to be delivered to the 5G wireless region would reach several Tbps per MIMO antenna array. Therefore, the main issue is how to delivered such ultra-high capacity information channels from the data centres to the cloud RAN and then onto the antenna radiation sites. The transmission medium must be optical guided fibre; the channel carriers occupy mainly the C-band under both coherent and direct detection reception techniques. Coherent schemes for the system in the long haul and direct detection for front haul in order to economize the access cost.

These receptions determine the technology of the optical to wireless conversion to transmit information to mobile users. At such high frequency millimetre RF carriers it is inevitable that integrated electronics and optical signals must be integrated either fully or partially so as to avoid the high frequency transmission problems.

Integrated photonic circuits are required for the following -

Firstly, the optical information channels must be detected and converted back in the electronic domain after a long haul transmission distance to the antenna site from the cloud RAN (See Figure 2); the RF channels can be generated in the base-band and via the optical modulators, thus very simple with the 7 GHz band to be embedded in the RF carriers.

These data base-bands channels are then mixing with a referenced laser carrier (or comb laser of multiple lightwave carriers) whose frequencies are spaced with respect to that of the optical carriers carrying the baseband channels by an amount equal to the RF frequency to be fed to the MIMI antenna.

Si-photonic circuitry will play the major part in the convergence of wireless and optical technologies for economic reasons provided that the driving voltage for Si-photonic modulators is lowered to the level compatible with that of the outputs of digital to analogue converters so that they can be driven without resorting to any mmW power amplifier.

Furthermore, the availability of laser sources integrated in Si-PIC is essential. We expect flip-chip bonded group II-V compound semiconductor lasers to be playing a major part in the hybrid integrated PIC. The lasers to be employed in outside environment must be stable with temperature fluctuation. Furthermore, comb lasers may also be usefully employed in the case that multiple optical channels are required in such a PIC.

Key devices include: opto-electronic (O/E) and photonic transponders; Si-based PC and hybrid integrated lasers in such PIC and integrated photonic signal processors.

Within these, the various PIC functional elements are splitters and combiner, hybrid couplers, phase modulators, wideband Mach-Zehnder interferometric modulator, resonant RF-optical modulator, Ge-on Si photodetector, flip-chip bonded semiconductor lasers; mode converters and grating couplers, comb lasers, balanced photodetectors; micro-ring modulators/filters, de-multiplexers or multiplexers, mode converters so as to mode matching with those from and to optical fibres etc.

Alternatively, one can approach the generation of the wireless data channels via the electrical mmW integrated circuitry using RF signal generation by frequency multiplying from microwave to mmW regions. These functions are implemented in the ultra-high frequency regions and thus highly integrated circuits be used. However, it is preferably that the RF carriers be embedded with the data channels via the optical mixing as briefly described above.

Traditionally radio access units (RAU) are used to transport data channels through the optical fibres and then recovered in the electrical domain and then transported to antenna site when the RF carriers are in the lower (<2 GHz) frequency range. In 5G, the frequency is expanded to upper mmW region and hence the tremendous convergence of optical and wireless techniques are well expected to be challenging designers and implementers both in terms of PIC and ultra-high frequency micro - and millimetre wave disciplines.

Figure 3: Flex grid in advanced Optical Transport Networks (Flex OTN) (a) super channels forspectral flexible grid; (b) Frequency spectra of optical channels of different bandwidths (image courtesy of Huawei Technologies)

It is also highly expected that flexible optical transport networks (Flex OTN) will emerge in the next generation networks in which variable rates (100 G, 200 G, 400 G or 1 Tbps, 2 Tbps, 3Tbps) and hence bandwidth optical channels are employed to supply on-demand networks (see Figure 3). The technological developments for the optical-wireless-optical convergence under the role of PIC, will be even much more important.