**2. Short history of multiplexing**

The history of multiplexing goes back to the 1800s, when Samuel Morse developed his telegraph system which enabled long-distance communications [4]. Later in 1874, Thomas Edison invented diplexing to transmit two individual messages over one line at the same time [5]. Later in 1894 and the 1930s, time-division multiplexing (TDM) and frequency division multiplexing (FDM) came into existence [6, 7].

### **3. Application of multiplexing in current technology**

One of the expectations about 5G technologies is to support enormous capacity, approximately 1000 times devices per squared kilometer [8]. In order to satisfy this requirement, several technologies have been suggested and developed, and one of the most attractive approaches is known as massive multiple-input multiple-output (MIMO), and that is where, for example, spatial multiplexing comes to use [9]. From signal processing side, for instance, employing multiple carriers all the way to arranging antennas and network management, multiplexing technique provides accessing resources by dividing and sharing it among users. In the other side, de-multiplexing has to be applied at the receiver side to inverse all the processes and extract the information sent.

One particular example for application of multiplexing is seen in orthogonal frequency division multiplexing (OFDM) systems [10]. The signal is spread between different subcarriers, and the frequency bandwidth is efficiently used [11].
