Multiplexing is the process of combining multiple signals into one over a shared communication medium. Communication could not have been possible if not for the introduction of multiple access techniques implemented in various baseband units. Just imagine if all the FM radio stations within a locality were broadcasting on the same frequency. It is clear that nothing would be understood, thanks to the technology.

There are various types of multiplexing access techniques, namely: frequency division multiplexing (FDM), wavelength-division multiplexing (WDM), time-division multiplexing (TDM), code-division multiplexing (CDM), space-division multiplexing (SDM), and polarization-division multiplexing (PDM). These techniques are commonly deployed depending on their application needs. Therefore, diverse kinds of analogue and digital information such as images, voice, letters, pictures, video, maps, and charts can communicate with one another over telecommunication media seamlessly without interference if the network is carefully designed with appropriate modulation and multiplexing techniques.

To maintain communication on multiple simultaneous channels, two physical limits must be overcome – the bandwidth limit and the power limit – either of which may act as a constraint on speed and capacity. To overcome the bandwidth limit, frequency division multiple access (FDMA), time-division multiple access (TDMA), or code-division multiple access (CDMA) are typically implemented. For the power limit, higher transmitter power and antennas with high gain are used as well as more robust coding schemes. In satellite communication systems, the most commonly used techniques are Frequency Division Multiple Access (FDMA) and Multi Frequency-Time Division Multiple Access (MF-TDMA).

Typically and talking about modulation, analogue modulation is in the form of frequency modulation (FM) and amplitude modulation (AM) while digital modulation is in the form of PSK, QPSK, 8PSK, 16QAM, and 64QAM used in various communications technology. Digital modulation systems can be divided into several levels based on their symbols such as Amplitude Shift keying (ASK), Frequency Shift keying (FSK), and Phase Shift keying (PSK). According to Wikipedia; modulation is the process of varying one or more properties of a high-frequency periodic waveform called the carrier signal which typically contains information to be transmitted. To improve performance and avoid latency/errors during transmission, the information must be coded. In channel coding, forward error correction (FEC) is usually given with the modulation scheme. The FEC consists of additional bits of useful information to allow detection and correction caused by the transmission channel.

The FDMA is a method consisting of the continual use of a channel in a frequency band without any time limit by dividing the frequency bandwidth. In this method, there is a Single Channel per Carrier (SCPC) which is suitable for a satellite communication network with low traffic and numerous earth stations The Multiple Channel per Carrier (MCPC) transmits many channels with only one carrier wave.

The MF-TDMA, on its part, is a method enabling several earth stations to share one satellite repeater by inserting the same frequency into each different time slot being assigned to each earth station to avoid overlapping several earth station frequencies on the satellite repeater. The hybrid method of assigning channels to multi-operators is by applying the SCPC to the MF-TDMA to support a network structure where terminals having various frequency bandwidth lengths are mixed by using the resource assignment technique. The MF-TDMA is widely applied to the up-link of communication satellites because of its high efficiency and flexibility. This transmission method, however, has disadvantages and it needs to be synchronized to avoid any interference during complex signal processing. In other words, FDMA is superior not only in speed and damages resulting from jamming but also in security and reliability based on its users.

            In Nigeria, the frequency band for a specific application is assigned by the National Communication Commission (NCC). The assignment of the C-Band in the SCPC satellite communication system is based on the 36MHz space segment capacity depending on the intermediate frequency (IF) with the frequency channel separation. In the space segment, bandwidth utilization is managed and controlled by the satellite system operator. Thus, a 36MHz transponder will have a maximum channel capacity of 1600 if the intermediate frequency (IF) of 70MHz and channel spacing of 25 KHz is applied.

The use of IF of 140MHz with the same channel space will reduce the channel capacity by half. Either the channel or frequency can be obtained by using the expression IF = 70 + (N-800) x 0.0225 MHz, where N is the channel number. It should be noted that this process only takes place in the MODEM unit and it is automatically replicated during the conversion from IF to the radio frequency (RF), provided an appropriate centre frequency (C_f) is selected in the up/down converter module. For example, to calculate the operating frequency of an SCPC carrier with an assigned channel 313 will generate an intermediate frequency of 59.0425MHz. In the case of SCPC network equipped with L-Band MODEM, conversion of IF to L-Band Frequency is very necessary. You need to have the intermediate frequency of the particular assigned channel and use the following formula for the conversion:

TX_{IFMHz =} [TX_Sat.freq] _– 5890 ₊ 70MHz

RX_{IFMHz =} [RX_Sat.freq] _– 3665 ₊ 70MHz

TX_{L-Band =} [TX_Sat.freq] _– 4900MHz

RX_{L-Band =} 5150 _– [TX_Sat.freq] MHz

The L-Band frequency can be implemented over the MF-TDMA network in the frequency band of 950-1750MHz and 950-2150MHz. A band of frequency from 950 to 1750 MHz can support 510 while the band 950-2150MHz supports 250 based on 8 and 16 channels capacity respectively. In SkyWAN network, the 16 channels MF-TDMA network use the following air interface parameters for effective operation of the network: Symbol rate between 200-12Mbps, User data per channel 64Kps-20Mbps, Channel Spacing / Roll-off factor 1.2, 1.4 / 0.2, 0.4, maximum numbers of Time Slot/TDMA and channel 256, Data content of the time slot between 100-3000bytes, TDMA frame time between 40-400milliseconds with 250 maximum stations.