## 7 Towards a Simplified Optical Transceiver
### SCOH System Classification
#### 7.3.2 Direct-Detection-Based Balanced Receiver  
The adoption of field modulation in the SCOH schemes facilitates phase diversity realization by the direct detection receivers. Although at the system expense, balanced receiver-based direct detection offers benefits such as the feasibility of recovering phase diverse signals and instinctive elimination of SSBN by means of the BPD. This helps address the associated high computational complexity of the iterative SSBN cancellation algorithm. Conceptually, the traditional SCOH scheme that supports coherent-like detection without an LO copropagates the carrier in conjunction with the signal at the transmitter. Then, at the receiver, the signal and the carrier are separated again. The resulting signal is separated into two paths used to drive the two inputs of the standard Co-Rx [9]. For instance, as illustrated in Figure 17a, the polarization division multiplexing-direct detection (PDM-DD) employs a narrow bandwidth low-pass filter to separate the signal and the carrier in the frequency domain [9; 138]. In this implementation, a frequency gap is highly desirable between the signal and the carrier. Primarily, the frequency gap is normally reserved to ensure that the signal-signal beating being instigated by the photodetection is less than the subcarriers' frequency band. Therefore, the gap is intended to facilitate carrier and subcarrier bands' separation by means of optical filters. It is worth noting that this process limits the SE of direct-detection-based systems. Furthermore, the performance of the optical filter is another limitation that deserves considerable attention [138].  
In addition, it should be noted that the laser frequency drift can be about 10 GHz, and the realization of the low-pass bandwidth in this range with the existing commercial filter is very expensive. Consequently, the PDM-DD scheme not only exhibits a high system cost, but also wastes the SE. To address the associated issue of the PDM-DD scheme, a signal carrier interleaved-direct detection (SCI-DD) has been presented. The SCI-DD scheme separates the signal and the carrier in the time domain by assigning the main carrier and the signals to different time slots. As illustrated in Figure 17b, one carrier block is appended to two consecutive signal blocks at the transmitter. At the receiver, the stream is separated into two different paths, and the lower path is delayed by one block length. This helps ensure that the signal and the carrier from different paths beat effectively with each other at the receiver [9; 40]. Furthermore, an optical hybrid, as well as differential detection are employed in the recovery of the DSB signals. Therefore, provided that the balanced receiver offers a common-mode rejection ratio that is high enough, the DSP implementation for the SSBN cancellation is not required [40]. However, one-third of the SE is sacrificed by the SCI-DD [9].  
In an effort to further enhance the system AIR per wavelength, an SV-DD scheme has been presented. This scheme is capable of realizing a 100% SE with regard to the COH detection of single-polarization modulation [9]. As depicted in Figure 17c, in the SV-DD scheme, the transmitter locates the signal and the carrier at two orthogonal polarizations using a polarization beam combiner (PBC). Furthermore, at the Stokes vector receiver, a 3D detection is conducted for the signal representation recovery in the Stokes space. With the help of a DSP-based 3 \(\times\) 3 Stokes vector rotation, the desired signal can be well retrieved with the full phase diversity [9; 23]. Moreover, the associated concepts and implementations of various prospective transceiver structures for the Stokes system are presented in Section 8.