> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 All-Analogue Real-Time Broadband Filter Bank Multicarrier Optical Communications System Fernando A. Gutiérrez, Philip Perry, Member, IEEE, Eamonn P. Martin, Andrew D. Ellis, Member, IEEE, Frank Smyth, Member, IEEE, and Liam P. Barry, Senior Member, IEEE. their optical counterparts. Consequently, wavelength division Abstract— This paper studies the key aspects of an optical link multiplexing (WDM) schemes have been implemented which transmits a broadband microwave filter bank multicarrier employing electrical subsystems in the transmitter and/or the (FBMC) signal. The work is presented in the context of creating receiver, as in coherent WDM [3] or Nyquist WDM [4]. an all-analogue real-time multi-gigabit orthogonal frequency However, to decrease the number of total optical wavelengths, division multiplexing (OFDM) electro-optical transceiver for short range and high capacity data center networks. Passive these systems require broadband baseband signals of tens of microwave filters are used to perform the pulse shaping of the bit Gbaud, which limits its performance and increases the cost streams, allowing an orthogonal transmission without the and difficulty of any practical real-time implementation. necessity of digital signal processing (DSP). Accordingly, a cyclic Subcarrier multiplexing (SCM) is a different alternative that prefix that would cause a reduction in the net data rate is not combines several RF subchannels into one signal, which is required. An experiment consisting of three orthogonally spaced then modulated onto an optical wavelength before 2.7 Gbaud quadrature phase shift keyed (QPSK) subchannels demonstrates that the spectral efficiency of traditional DSP-less transmission over fiber. Thus, WDM/SCM systems present the subcarrier multiplexed (SCM) links can be potentially doubled. A advantage of employing microwave components with sensitivity of -29.5 dBm is achieved in a 1 kilometer link. narrower baseband signals [5]. SCM has been employed in many implementations. Early Index Terms— Electro optical transceiver, Filter Bank SCM systems were used to transmit a number of low Multicarrier (FBMC), Orthogonal Frequency Division bandwidth video signals [6]. Later on, and in line with present Multiplexing (OFDM), Subcarrier Multiplexing (SCM). requirements, frequency plans with a smaller number of broadband subchannels were reported [7, 8]. Currently, the I. INTRODUCTION advances in high-speed analog-to-digital and digital-to-analog HE increasing demand for capacity in optical links has converters (ADC, DAC) have motivated the implementation T motivated the development of more spectrally efficient of SCM schemes with hybrid analogue-digital processing [9, transmission schemes during the last few decades. More 10]. Most importantly, spectral efficiency can be maximized recently, the proliferation of data centers has stimulated a by transmitting multiple orthogonally overlapping growing research effort in optical interconnects and its subchannels, like in orthogonal frequency division associated technologies [1]. Digital multicarrier techniques multiplexing (OFDM) [11]. However, the implementation of have been thoroughly explored as they can divide the available real-time broadband OFDM schemes requires demanding and transmission bandwidth into narrower-band radio frequency complicated digital signal processing (DSP) techniques [12]. (RF) and optical subsystems that can be processed In intensive DSP based systems, the main disadvantages are independently. Although all-optical multicarrier schemes have the associated high power consumptions [13] and often proven to be valid for real-time implementations [2], they are latencies. Depending on their size and their targeted market, still complex and require stable coherent optical frequency different subsystems inside optical networks can present combs. For more practical and industrial applications, totally dissimilar requirements of spectral efficiency, power electrical and microwave components are preferred because of consumption, and latency. A purely analogue orthogonal SCM the maturity and stability that they present, in contrast with (OSCM) system potentially offers the best trade-off between those key parameters in certain real-time applications. Manuscript submitted on July 14, 2015, revised October 9, 2015. This It can be concluded that traditional broadband SCM is a work was supported in part by the EI CFTD grant CF/2011/1627, SFI grants 09/IN.1/12653 and 10/CE/I1853 and EPSRC grant EP/L000091/1. reliable technique, whose main weakness in spectral efficiency F. A. Gutierrez, P. Perry, E. P. Martin, F. Smyth and L. P. Barry are with can be eliminated if orthogonally overlapping subchannels are The RINCE Institute, Dublin City University, Glasnevin, Dublin 9, Ireland. employed. The previous discussion motivates the search and (E-mail: [email protected]). F. Smyth is also with Pilot Photonics, Invent Centre, Dublin City the analysis of an all-analogue OSCM implementation, as a University, Glasnevin, Dublin 9, Ireland. means to achieve DSP-less broadband OFDM optical links. A A. D. Ellis is with Aston Institute of Photonic Technologies, Aston similar concept was simulated in [14]. More recent results University, Aston Triangle, Birmingham B4 7ET, UK. Copyright (c) 2015 IEEE. Personal use of this material is permitted. have demonstrated the capabilities of the technique in a WDM However, permission to use this material for any other purposes must be system employing an optical comb based transmitter [15], but obtained from the IEEE by sending a request to [email protected]. > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 Fig. 1 illustrates an optical NIC comparing two different implementations: DSP-based OFDM and OSCM. The interface between a computing system and the NIC is called “peripheral component interconnect express” (PCI express), which currently supports up to 16 digital lines with peak data rates of 4 Gbit/s per line [17]. OFDM, as shown in Fig. 1, requires the computation of the standard and the inverse fast Fourier transforms (FFT and IFFT). Those operations are complicated and integrated circuits need to parallelize the incoming data to reduce the rate per line from Gbit/s to Mbit/s in the transmitter [18]. Equivalently, the opposite multiplexing, from Mbit/s to Gbit/s, is required in the receiver. High-speed DACs and ADCs are necessary to achieve high transmission rates. Finally, a data driver and an amplifier are required in the transmitter and the receiver before the electro-optic and the opto-electronic converters (E/O and O/E). The use of narrowband subchannels Fig. 1. Optical NIC showing two different options for an orthogonal is an advantage to compensate high values of accumulated transmission: FFT-based OFDM and OSCM. fiber dispersion in long-haul optical communications systems a detailed description and analysis of the basic OSCM system [19], but the implementation of the (I)FFT in short reach has not been provided. This manuscript demonstrates and systems might be justified only when spectral efficiency has to thoroughly investigates how broadband SCM schemes can be be maximized regardless of any penalty in resource demand implemented with orthogonally overlapping subchannels and additional complexity [20]. Even for low modulation without requiring DSP. This is achieved by relying on the orders, there are three limitations in these implementations excellent stability and low phase noise of microwave that cannot be avoided. Firstly, due to the (I)FFT based oscillators, the frequency selectivity of microwave filters, and operation, a small amount of dispersion between two the good performance of inexpensive integrated microwave subcarriers affects the timing of consecutive symbols, modulators. To validate the proposed concept, an optical link impairing all the subchannels. As a result, a cyclic prefix (CP) consisting of three orthogonal 2.7 Gbaud quadrature phase that reduces the net rate is required for the transmission in any shift keyed (QPSK) subchannels is employed, with the middle dispersive channel [11, 21]. Secondly, the power consumption subchannel representing the worst case scenario with increases due to the data interfaces and the ADCs and DACs maximum interference. In the experiment, passive microwave [22]. Finally, a high number of subchannels translates into a filters are used to shape the multi-gigabit baseband data very high peak to average power ratio (PAPR), with the streams as Nyquist pulses, which enables the orthogonal corresponding penalty in the dynamic range and the power modulation and demodulation of the subchannels. Moreover, consumption [23]. the rate of 2.7 Gbaud is selected because it is compatible with In contrast, OSCM can potentially enable a simpler interfaces between computers and network interface cards orthogonal implementation with several advantages. Fig. 1 (NIC). shows the typical components of a broadband SCM system The rest of the text is organized as follows. Section II [8]. Baseband data can be directly processed with a low pass compares the integration of OSCM and DSP-based OFDM in filter (LPF) and then used to modulate an RF subcarrier. All optical NICs. Section III explores the key theoretical and the subchannels are combined and multiplexed in a single practical concepts involved in OSCM links, from three signal that is amplified and fed to an