# Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing
## 3 Receiver Comb Technologies
### Exploiting Subchannel Phase Coherence  
The frequency-locked and phase coherent properties of subchannels can be exploited to either completely replace or increase the efficiency of the DSP carrier recovery in the receiver. In a phase noise limited system, this can be utilized to increase performance. Additionally, even if performance is not limited by phase noise, eliminating or simplifying the carrier recovery leads to a reduction in DSP complexity and power consumption. This is particularly interesting for short-reach links using higher-order modulation formats, whose higher phase-noise sensitivity increases the complexity of the carrier recovery. In addition, shorter links need to spend less power on the dispersion compensation, making the carrier recovery a proportionally bigger part of the DSP power consumption.  
In this context, connections between comb-based systems and systems employing space-division multiplexing (SDM) can be made. For SDM, using the same laser for several channels transmitted over different cores/modes in an SDM fiber leads to these spatial channels sharing the same frequency and the same phase noise. Similarly, all spectral channels originating from an optical frequency comb mostly share the same phase noise. However, while the scenarios are very similar, the two are not identical. As discussed in Section 1, frequency combs are inherently characterized by two frequencies, the central frequency and the line spacing, and in general, there will be phase noise differences between the lines.  
The methods for exploiting subchannel coherence can be roughly divided into two categories, pilot tone based or methods based on joint carrier recovery in DSP. The pilot tone-based methods work by multiplexing one or several pilot tones extracted directly from the carrier light source with the superchannel. The carrier phase can then be extracted from the pilot tones in the receiver. In contrast, joint carrier-recovery methods extract the carrier phase information using DSP on the data channels, further discussed in Section 4.  
The phase information carried in pilot tones can be extracted or detected in the receiver using several different methods, which can be divided into optical and electrical/digital methods. In the optical approach, the pilot tones are used (directly or with additional analog processing) to create LOs that are phase-locked to the carriers, causing the downconverted signals to be (ideally) without carrier offset. These methods are commonly known as self-homodyne methods [84]. Self-homodyne methods can enable coherent transmission using high-linewidth light sources [85], as well as savings in DSP complexity [86].  
The amount of processing of the pilot tones needed for self-homodyne detection differs depending on the system type. In SDM systems, processing is usually limited to amplification and filtering of the pilot tones, while comb-based superchannels need to regenerate a full comb from co-transmitted pilot tones. Such comb regeneration can be done in several ways and will be discussed below in Sections 3.2-3.4.  
Figure 4: Schematic of two different comb-based receiver principles. (**a**) Comb regeneration based on two pilot tones. Since the local-oscillator (LO) comb is phase-locked to the transmitter comb, the receiver digital signal processing (DSP) can be simplified. (**b**) Unsynchronized local oscillator comb and joint DSP. RX: Receiver.  
For the electrical/digital approach, the pilot tones are detected jointly with the signals using a separate (free-running) LO, and then, the relative phase between pilot and signal is extracted in DSP. This is known as digital self-homodyne detection [87] or shared-carrier reception [88].