Orthogonal Chirp-Division Multiplexing for Future Converged Optical / Millimeter-Wave Radio Access Networks

Selection of constellation signals received over the full optical and wireless transmission link with a ROP of 0 dBm; (a) wideband 16-QAM OCDM, (b) wideband 16-QAM OFDM with FDE, (c) mobile 256-QAM OCDM and (d) mobile 128-QAM OCDM.

Envisaged network scaling in the beyond 5G and 6G era makes the optical transport of high bandwidth radio signals a critical aspect for future radio access networks (RANs), while the move toward wireless transmission in millimeter-wave (mm-wave) and terahertz (THz) environments is pushing a departure from the currently deployed orthogonal frequency division multiplexing (OFDM) modulation scheme. In this work, the orthogonal chirp-division multiplexing (OCDM) waveform is experimentally deployed in a converged optical/mm-wave transmission system comprising 10 km analog radio-over-fiber (A-RoF) transmission, remote mm-wave generation and 2 m wireless transmission at 60 GHz. System performance is evaluated in terms of both bit error ratio (BER) and error vector magnitude (EVM) for a wideband 4 GHz 16 Gb/s signal and 128/256-Quadrature Amplitude Modulation (QAM) mobile signals compatible with 5G new radio numerology. OCDM is shown to outperform OFDM by offering enhanced robustness to channel frequency selectivity, enabling performances below the forward error correction (FEC) limit in all cases and exhibiting an EVM as low as 3.4% in the case of the mobile signal transmission.

The delivery of 5G mobile communications is well underway and is driving network evolution today. From the current standpoint it is difficult to say exactly what will constitute the 6G era of mobile communications but it is clear that, as well as the key drivers of increased bandwidth, lower latency and enhanced connectivity, future wireless communications must facilitate scaling in a way that supports a vast array of user types (human and otherwise) in potentially challenging environments. This points to the ‘super-convergence’ of emerging flexible radio and optical technologies and network types which are capable of providing a variety of extremely high-throughput services in diverse environments. 5G’s frequency range 2 (FR2) has standardized the use of mm-wave frequencies up to 48 GHz in order to deliver enhanced mobile channel bandwidths of up to 400 MHz. This trend looks set to continue with higher frequency mm-wave ranges (around 60 GHz and 90 GHz) and THz frequencies (0.1 - 1 THz) being identified as key platforms for developing high bandwidth and multi-Gb/s wireless technologies for ‘beyond’ 5G and 6G. Indeed, outside of current mobile standards, WiGig protocols, including the recently released 802.11ay standard, specify multi-GHz mm-wave channel bandwidths for the delivery of 10’s Gb/s per channel in the 60 GHz unlicensed band.

The key optical infrastructures charged with the delivery of broadband and mobile services are the passive optical network (PON) and the RAN, respectively. A central unit (CU), distributed unit (DU) and remote unit (RU) constitute the three main functional elements of a RAN. Various RAN topologies can be implemented through the relative distribution of these elements along the path from the RAN’s central office (CO) to the antenna site, and these are well summarized in. A more centralized RAN (C-RAN) design, allowing the CU and DU functionalities to be co-located at the CO, is preferable in cases where the footprint and cost of the antenna site are of primary concern. Considering the vast proliferation of antenna sites envisaged beyond 5G, and the antenna site complexity synonymous with the transmission of very high frequency carriers, it is likely that a highly centralized RAN architecture will be required to facilitate scaling for wireless networks incorporating THz and mm-wave functionality. Increased centralization of RAN resources at the CO emphasises the role of RoF transport of mobile data between the CO and the RU (i.e. fronthaul) which can span 10–20 km’s. As the number of users and antenna sites grow, the associated increase in fronthaul capacity demands has shifted focus away from simplistic point-to-point fronthaul links, to those harnessing optical access networking technologies such as wavelength division multiplexing (WDM). Unsurprisingly, these trends have led to proposals for the co-operation of PON and C-RAN infrastructures whereby RAN traffic can be provisioned over a PON topology — and indeed such operation is included in the ITU G-series recommendations.

The recent deployment of enhanced-common public radio interface (eCPRI) technology has provided some improvement in spectral efficiency for current fronthaul transmission, but this is ultimately limited by the binary modulation scheme currently employed. An alternative approach is to use Analog (i.e. multi-carrier) RoF (A-RoF), whereby the mobile signal is transmitted over the fronthaul link in its native multi-carrier format. At the cost of decreased robustness to system nonlinearity, this technique provides excellent spectral efficiency and results in the simplification of the RU site compared to a D-RoF approach — ultimately facilitating network antenna ‘densification’.

The authors’ prior works have examined how remote mm-wave generation through optical means can be incorporated into a C-RAN architecture. This type of optical-wireless converged system design makes use of the optical heterodyne operation whereby two optical carriers, centrally distributed over a fronthaul link, undergo heterodyne detection at a photo-diode (PD) stage at the antenna site; producing a mm-wave signal for wireless propagation. This approach avoids the difficulty/expense of mm-wave carrier generation in the electronic domain, is capable of providing a wide mm-wave carrier tuning range and offers relative ease of convergence with the hardware centralized implementation of C-RAN. Optical heterodyning in this way can introduce significant amounts of optical phase noise and frequency offset which are performance limiting factors for A-RoF systems. Our prior work has proposed several hardware and software based techniques to mitigate these effects, thereby alleviating strict optical source linewidth and stability requirements and making remote optical heterodyne operation a viable option for future mm-wave A-RoF fronthaul.

Nevertheless, other challenges associated with operation in the mm-wave and THz regime, such as frequency selective fading, severe multi-path interference and enhanced amplitude noise effects remain highly problematic. To tackle these, a departure from the 4G/5G waveform of choice — OFDM — is expected. Recently, we demonstrated the use of the novel waveform orthogonal chirp division multiplexing (OCDM) in a converged optical / mm-wave system; OCDM was shown to outperform equivalent OFDM signals in the system owing to its robustness to channel fading (assigned by its chirp spread-spectrum form and its capability of providing enhanced channel estimation. The authors of have previously demonstrated the application of OCDM in a RAN implementation incorporating optical heterodyne operation and mm-wave wireless transmission — with results aimed at highlighting OCDM’s tolerance to interference from adjacent wireless services. In this work, we augment our previous OCDM converged system analysis by demonstrating an end-to-end C-RAN transmission path including 10 km A-RoF fronthaul and 2 m wireless transmission at 60 GHz. In addition to performance evaluation using equivalent 5G new radio (NR) OCDM/OFDM compatible signals with up to 256-QAM, a 4 GHz bandwidth OCDM signal delivering 16 Gb/s is successfully transmitted over the link, exhibiting near-uniform performance across the frequency range; highlighting OCDM’s potential to provide extremely high throughput future mm-wave wireless communications enabled through converged optical networking.

Orthogonal Chirp-Division Multiplexing for Future Converged Optical/Millimeter-Wave Radio Access Networks | IEEE Journals & Magazine | IEEE Xplore

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