**2.3 Optical space-division multiplexing for MIMO systems**

Space-Division Multiplexing (SDM) is a multiplexing technique for optical data transmission where multiple spatial channels are utilized. **Figure 6** shows a generic optical MIMO-SDM system. At the transmitter, the user data signals are encoded, modulated, E/O converted and then multiplexed onto different wavelengths (λ1, λ<sup>2</sup> ... λ*N*) in a WDM multiplexer [17].

At the receiver, the transmitted signals are recovered using MIMO digital signal processing consisting of an N N array of equalizers by DSP (digital signal processor). First, the N channels signal is demultiplexed by an SDM demultiplexer. Then

**Figure 6.** *Space-division multiplexing for optical communications and application to 5G systems.*

the separate signals *r*<sup>1</sup> … *rN* are fed into the N N MIMO DSP block that is capable of eliminating all linear impairments of the transmission system and giving the reconstructed signals as output. Optical fibers are utilized at fronthaul, midhauld and backhaul of 5G systems. Data from RUs (at smallcells, for example) are multiplexed and transmitted to DU via fronthaul by optical fiber [18]. At present, a dense optical wavelength-multiplexing system has a transmission capacity of more than 1 Tera bit/s (1000 Gbit/s) per wavelength [19].
