## 3 Short-Reach Systems Classification and Enabling Technologies
### Optoelectronic Oscillator
#### 3.3.4 Requirements and Challenges  
This part presents a number of system requirements and the specific constraints that make the application of digital conventional COH systems inappropriate for short-reach systems. Consequently, the associated system requirements prompt further research directions on novel transceiver architectures and DSP techniques. This will be discussed in this subsection. Cost  
The short-reach transceiver applications require a low cost, so high-performance devices such as thermoelectric cooling (TEC), LiNbO\({}_{3}\) external modulators, and low-linewidth external cavity lasers (ECL) are unattractive. It is noteworthy that the implementation of LOs and full CO-Rxs is undesirable. On the other hand, most of the short-reach transceivers usually comprise components such as DMLs, low-bandwidth PIN, VCSELs, and EAMs. Direct-detection-based schemes with simplified DSP are the more promising solutions for the short-reach systems compared with the COH schemes and their typical advanced DSP algorithms. Furthermore, since thermoelectric cooling and wavelength locking mechanisms are not required, a so-called coarse-WDM is used where the channel spacing is larger than the standard 50 GHz ITU grids [35]. In general, small form factors are very essential to the majority of optical transceivers. However, the demanded form factor by the short-reach applications such as DCNs is even more stringent. This is mainly due to the fact that the underlying form factors have significant effects on the port number and density, as well as the rack size to support the ports. These parameters determine the size of the DCN as a whole and the required cooling \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Data Rate** & **Form Type** & \begin{tabular}{c} **Transmission** \\ **Distance** \\ \end{tabular} & **Wavelength** & \
\begin{tabular}{c} **Modulation** \\ **Format** \\ \end{tabular} &
\begin{tabular}{c} **Transmitter and** \\ **Receiver** \\ \end{tabular} \\ \hline \hline
**25 G/100 G transceivers for 5G mobile FH networks** & & & & & VCSEL + PIN \\ \hline \multirow{6}{*}{25 Gbit/s} & \multirow{2}{*}{SFP28} & 70\(\sim\)100 m & 850 nm & \multirow{2}{*}{NRZ} & \multirow{2}{*}{FF/DFR + PIN} \\ \cline{3-3} \cline{5-6}  & & 300 m & 1310 nm & & & \\ \cline{3-3} \cline{5-6}  & & 10 km & & & & \\ \cline{2-3} \cline{5-6}  & & & & & & \\ \cline{2-3} \cline{5-6}  & & & & & & \\ \cline{2-3} \cline{5-6}  & & & & & & \\ \hline \hline \multirow{6}{*}{100 Gbit/s} & \multirow{2}{*}{QSFP28 BiDi} & 10/15/20 km & 1270/1330 nm & \multirow{2}{*}{NRZ} & \multirow{2}{*}{DRB + PIN} \\ \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \hline \hline
**25 G/50 G/100 G/200 G/400 G transceivers for 5G midhaul/backhaul networks** & & & & & \\ \hline \hline
25 Gbit/s & \multirow{2}{*}{SFP28} & 40 km & 1310 nm & \multirow{2}{*}{PAM4} & \multirow{2}{*}{EML/DFR + PIN} \\ \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \hline \hline \multirow{6}{*}{100 Gbit/s} & \multirow{2}{*}{QSFP28} & 10 km & CWDM/LWDM & \multirow{2}{*}{NRZ} & \multirow{2}{*}{DRB/EMLs + PINs} \\ \cline{3-3} \cline{5-6}  & & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \cline{1-1} \cline{3-3} \cline{5-6}  & & & & & \\ \hline \hline \end{tabular}
\end{table}