**2.3 Receiver side**

After propagating over the fiber, an SDM-DE-MUX which tend to disengage propagating modes (sharing the same MCF or FMF) and oriented them to particular SMFs. In principle, SDM-DE-MUX devices or techniques are the same as SDM-MUX but in the inverse sense (known also as fan-out devices).

**Figure 3.** *Several kinds of fibers used for SDM communication system.*

*Multiplexing, Transmission and De-Multiplexing of OAM Modes through Specialty Fibers DOI: http://dx.doi.org/10.5772/intechopen.101340*

After retrieving the optical signal (DE-MUX), optical photodectors are employed at the end of each SMFi, aiming to detect each particular mode (data carrier from each SMFi) and convert the modulated optical signal into an electrical signal. The most commonly used photodectors are semiconductor photodiodes. Semiconductor based PIN photodiode and the Avalanche photodiode (APDs) are examples of such photodectors. In principle, the selection of these devices is based on the following requirement: high responsivity, bandwidth, noise characteristics, low cost, and so on [34]. Thereafter, the obtained electrical signals are converted to digital ones using electrical-to-digital converters (ADCs). At the end, a MIMO DSP block is used to mitigate the crosstalk effects on different mode channels. The digital signal undergos a normalizing/resampling and symbol synchronizing operations. Then, the obtained signals are equalized using adaptive time-domain equalization (TDE) or frequency-domain equalization (FDE). MIMO DSP are composed of equalizers (i.e. FIR filters) of coefficients hij. The number of these equalizers is related to the number of the square of the transmitted modes (N × N), the length of the transmission link, and the difference between modes delays [35, 36].
