## 6 Transceiver Architectures' Classification and Basic Concepts
### 1D-Based Architectures  
The 1D-based architectures exploit just one DoF and are presented as follows:  
IM-DD  
The IM-DD is a 1D scheme that offers the simplest configuration. Its transmitter is based on the IM, and the receiver is based on direct detection, as depicted in Figure 10a. The radio frequency (RF) signal modulates the light wave intensity from the CW laser at the transmitter. In regard to the CW laser frequency, the obtained optical spectrum after modulation is a double-sideband (DSB) signal in which a strong CW tone is located precisely in the middle, as well as locked \"in-phase\" with the information-bearing signal [17]. At the receiver, the intensity of the received optical signal is detected by a photodiode (PD). Therefore, the transmitter and receiver arrangement is usually referred to as the IM-DD scheme. Furthermore, depending on the adopted technique, a 1D IM-DD scheme can be realized without or with a digital-to-analog converter (DAC) or analog-to-digital converter (ADC) and DSP at the receiver, as depicted in Figure 10a(i) and Figure 10a(ii), respectively [10,14]. Moreover, regarding the achievable optical spectrum efficiency, the IM-DD scheme can realize 25% optical spectrum efficiency with reference to a DP-COH system. Digital chromatic dispersion compensation is not supported due to the undetected phase information of the received field. Furthermore, the optical spectrum of an IM-DD signal is conjugate symmetric around the optical carrier. This is in view of the fact that the baseband RF signal is real. Similarly, for effective detection of an IM signal with an optical bandwidth, \(R\), the minimum required electrical bandwidth of the receiver is \(R/2\)[14]. Figure 9: Transceiver classification by degrees of freedom (DoFs). \(x-D\): \(x\)-dimensional, with \(x=1,\dots,4\); C: carrier; IM: intensity modulation; DD: direct detection; DP: dual-polarization; SVR: Stokes vector receiver; PolMux: polarization multiplexing; COH: coherent; \(I\): in-phase; Q: quadrature; IPM: intensity-and-phase modulation; KK: Kramers–Kronig; IC: interference cancellation; RLOD: remote local oscillator delivery. It is worth noting that the classification of transceivers regarding the DoFs is not consensual. For instance, \(IQ+\)offset \(C/\)DD and DD-KK or DD-IC were considered both as 2D schemes [14] and as 1D schemes [9,12,13,24,38]. In this work, we group them as 1D schemes. This is owed to the fact that we employed a 1D subsystem with the IM-DD as the baseline. In this context, the IM-DD can recover a 2B-baud IM signal with a B-Hz receiver bandwidth, in accordance with the Nyquist theorem. It is noteworthy that based on the AIR, the 2B-baud IM signal is equivalent to a B-baud complex signal [13]. Although they are indeed full-field schemes, the associated phase and amplitude are not independent. Consequently, their DoF is still just one since only one dimension, or just an amplitude, is detected. \(IQ+\)offset \(C\)/DD  
The \(IQ+\)shifted (offset) \(C\) scheme is a variety of the SCOH systems that frequency multiplexes a frequency shifted copy of the transmitter laser with the modulated signal. The scheme is depicted in Figure 10b. At its transmitter, a shifted copy of the CW laser is inserted into the complex modulated signal, while a frequency gap that is equal to \(R\) is maintained between the signal and tone. At the receiver, owing to the adequate frequency gap, a single PD with electrical bandwidth \(\geq 2R\) offers the bandpass carrier-signal beating where the unwanted SSBN can be removed by means of filtering in DSP. Therefore, the DSP stack of the \(IQ+\)\(C\) SCOH scheme demands the SSBN cancellation stage. Likewise, the reconstructed field can be chromatic dispersion compensated after SSBN cancellation [14]. \(IQ+C\)/DD-KK or DD-IC  
The \(IQ+C\) scheme is illustrated in Figure 10c. It is similar to the \(IQ+\)offset \(C\) system, but it doubles the SE.