## 1 Introduction  
Digital subcarrier multiplexing (SCM) consists of the generation of a multicarrier electrical signal that is then modulated onto an optical carrier and transmitted over fiber [1, 2]. The (de)modulation of the electrical digital subchannels can be accomplished by digital signal processing (DSP) [1] and analogue signal processing (ASP) [2]. Broadband DSP-based SCM schemes can potentially achieve high modulation orders with a high spectral efficiency, but they require expensive analogue-to-digital and digital-to-analogue converters (ADC and DAC) and are also penalised by a high power consumption and latency [3]. In contrast, ASP-based SCM systems rely on the maturity of microwave components, leveraging the excellent stability and frequency selectivity of microwave devices and the low phase noise of microwave oscillators, to potentially achieve broadband communications systems with low power consumption and low latency. Furthermore, all electrical ASP can be implemented with inexpensive monolithic microwave integrated circuit (MMIC) technology [4], benefiting from integration but with a reduced transistor count with respect to DSP. The main weakness of traditional broadband ASP-based SCM systems has been the spectral efficiency. Recently, it has been proven that this disadvantage can be overcome by transmitting orthogonally overlapping electrical subchannels according to filter bank multicarrier (FBMC) theory [5]. For the remainder of the paper, this technique will be referred to as orthogonal subcarrier multiplexing (OSCM). Note that the feasibility of OSCM has been demonstrated in real-time schemes relying largely on off-the-shelf components [5], employing realistic synchronization [6] and in combination with wavelength division multiplexing (WDM) [7].  
The main advantage of SCM with respect to a single channel implementation is the higher tolerance to dispersion [2]. However, standard modulation of an optical carrier produces a double sideband (DSB) spectrum. Due to dispersion, associated subcarriers suffer a different delay in the upper and the lower sidebands, which translates into dispersive fading after photo-detection [8]. Consequently, optical single side band (SSB) modulation is preferred as it eliminates dispersive fading [9] and achieves the higher tolerance to dispersion expected from SCM. SCM/SSB can be directly obtained without employing optical filters with dual-drive Mach Zehnder modulators (DD-MZM) [2] and with optical IQ modulators [10]. The optical modulators exhibit a nonlinear behaviour that produces intermodulation distortion (IMD) between the electrical subchannels. This IMD has been thoroughly analysed in a number of theoretical studies [11; 12], where results are often provided in terms of signal to noise ratio per subchannel in the receiver. However, from a practical perspective, the performance of digital electro-optical transceivers is typically specified in terms of receiver sensitivity, which is the minimum received power at which data can be successfully recovered. Calculations to obtain the best achievable sensitivities in digital broadband SCM links in the presence of optical noise were presented in [2], but did not include the IMD. In contrast, a model that includes IMD was developed in [10] for the case of optical IQ modulators.  
This paper employs the model presented in [10] to estimate the best achievable sensitivities in broadband SCM/SSB and OSCM/SSB links consisting of an optical IQ modulator and a pre-amplified optical receiver as a function of the optical modulation index (OMI). While previous works showed how the mathematical model predicted differential variations in performance [10], this manuscript investigates the accuracy of the model to predict the receiver sensitivity as a function of OMI, which are the key parameters in the design of such an optical link. The model is implemented in MATLAB and compared to simulations performed with the \"VPI Transmission Maker\" simulation platform. A good agreement, especially for the optimum OMI, is obtained.