Multiple access techniques, on their part, are implemented at the RF band level. A multiple access scheme defines the access method that is employed to allow multiple telecommunication users –through the use of an appropriate multiplexing technique – to access and share a common communications channel typically in a wireless telecommunication network. Multiple access schemes are typically applicable to multiple channel per carrier (MCPC) access scenarios as opposed to single channel per carrier (SCPC) applications. Although a huge number of sharing methods have been developed and are also been actively explored, the multiple access techniques that have been garnering the highest usage across telecommunication networks are Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), which are defined by access schemes that are based on the frequency domain and the time domain respectively. There is also the Demand-Assigned Multiple Access (DAMA) scheme, which is unique to satellite communications.

Code Division Multiple Access (CDMA) – a generic access method like FDMA and TDMA – and Multi Frequency-Time Division Multiple Access (MF-TDMA) are also finding increasing applications in the telecommunications industry. For some applications, the Orthogonal Frequency Division Multiple Access (OFDMA) and the random multiple access Carrier Sense Multiple Access (CSMA) – which has been finding increasing applications in LANs – are also exhibiting their capabilities, while an OFDMA variant, the Non-Orthogonal Multiple Access (NOMA), continues to be actively explored, particularly for 6G technology applications. There are also less common multiple access techniques such as the Polarization Division Multiple Access (PDMA) and the Space (or Spatial) Division Multiple Access (SDMA), which is still under development. These access schemes make use of resources such as frequency, time, and space. In many telecommunications applications, a combination of the resources is usually featured in order to increase capacity as well as enhance flexibility and efficiency. The implication, therefore, is that meeting the ever-changing demands of digital telecommunications will require a combination of multiple access techniques rather than the use of a single access technique.

ACCESS BASED ON FREQUENCY AND TIME DOMAINS

The FDMA, which is traditionally an analogue system, is typically used in satellite communications and 5G cellular technologies. Historically, FDMA is the multiple access method for cellular systems and it is usually implemented in narrowband systems. It is somewhat related to Wavelength Division Multiple Access (WDMA) – widely used in optical communications systems – where wavelengths are used for propagation within a single optical fiber.

As a frequency domain assigned access technique, FDMA allows multiple users to share an allocated spectrum at the same time by dividing the spectrum into separate frequency bands and assigning each user a dedicated frequency slot for communication. Access is generally on a single-channel-per-carrier (SCPC) basis or multiple-channel-per-carrier (MCPC) basis. All users are, thus, able to communicate simultaneous and continuously on their assigned channels. This fact presupposes the importance of filtering (using bandpass filters) with respect to the frequency domain in order to ensure interference-free signal separation, a requirement that is unnecessary in TDMA systems. One problem, though, remains that of spectral resource idleness as an assigned FDMA channel cannot be used by other users in the network when that particular channel is not in use.

The Nigerian Aeronautical Satellite Telecommunication Network, which is an integral part of the Satellite Telecommunication Network for Central and Western Africa (now known as the AFISNET network) operating at the C-Band (6/4 GHz) based on the INTELSAT Standard B/ Standard F1 configurations at 64 kb/s using ¾ Forward-Error-Correction techniques, for example, is anchored on the FDMA scheme. Operating on a transponder aboard Intelsat 10.02 satellite, the network’s usable bandwidth of 36 MHz is subdivided into smaller bandwidths that are assigned to users. The 36 MHz bandwidth occupies one of the 12 segments (or transponders) of the total satellite bandwidth, which is typically 500 MHz for a C-band, Ku-band or Ka-band satellite system. With an IF of 70 MHz, a total of 1600 channels are made available for a terminal. Given a bandwidth of 36 MHz and an IF of 70 MHz±18 MHz,

1 channel = 36 MHz/1600

                  = 22.5 kHz.

The step or spacing of each channel within the dedicated 36 MHz is, therefore, 22.5 kHz. For a typical GSM cellular use case, however, the useable frequency of 25 MHz is divided into 124 carrier frequencies that are spaced at 200 kHz to yield a total of 125 channels.

Frequency management is an important requirement in an FDMA operational terrain. The FDMA, in spite of its wide application, has limitations in terms of channel capacity and operational flexibility with a fixed bit-rate per channel. However, capacity can be increased considerably in FDMA systems by implementing a robust digital coding scheme as well as lowering information bit rate. Given the limitation of frequency spectrum, there is the problem of frequency slot saturation where the population of potential users outstrips the number of available frequency slots. In terms of system complexity, FDMA requires the implementation of dedicated MODEM at the baseband level for each of the communicators thus creating difficulty in terms of cost-effectiveness. However, FDMA systems are advantageous when it comes to hardware simplicity and the simplicity of channel assignment as they are not as complex as the time-domain assigned TDMA. In satellite applications, there are also disadvantages regarding intermodulation products problem and the inability to maximize the use of the power of the satellite transponder. Thus, the need for a close coordination of uplink power levels becomes constantly imperative. In other words, a transponder cannot be operated at maximum power without facing the need to back off the input, thus downgrading capacity and efficiency.

The industry, however, continues to devise techniques for mitigating the disadvantages of FDMA. One is the effective reuse of frequency slots in order to accommodate as many users as possible. In cellular applications, multiple users are located in separate cells within a frequency slot and can simultaneously make use of the frequency segment provided the users are sufficiently geographically dispersed in an attempt to preclude the signal of one cell from affecting the signal of another cell using the same frequency segment.

The TDMA, on its part, is a time domain-assigned multiple access technique where timing synchronization is a crucial factor. This timing requirement necessarily requires the implementation of a centralized monitoring and control station which not only transmits a periodic TDM-format reference burst in the form of a frame divided into time slots but also ensures that each user in the network transmits at its individual assigned time slot. This synchronization, though, is not a requirement for FDMA operations. The configuration of a TDMA frame – consisting of a number (N) of time slots – is such that each time-slot usually integrates user data bits, bits for synchronization, guard times, control, and so on. Sufficient guard times are inserted between each user’s transmissions to mitigate crosstalk and clock instability effects.

TDMA is also a largely digital access scheme, which typically employs both phase shift keying modulation and time division multiplexing at the baseband level. Its application covers both wireless and wired digital communications terrains. In TDMA, the signals of different multiple users are separated and assigned separate time segments in a sequential manner and on a one-user-per-slot basis, albeit a user can make use of multiple slots. Although all the communicators in a TDMA system operate on a single carrier frequency, different non-overlapping time slots are allocated for transmission and reception using TDD. This effectively makes the implementation of duplexers unnecessary.  Unlike FDMA, TDMA is superior in terms of capacity and flexibility as accesses are quite re-configurable at any time and different numbers of time-slots per frame can be assigned to different communicators. Transmission is also non-continuous in TDMA systems unlike FDMA where transmission takes place continuously and simultaneously.

In satellite communications applications, TDMA allows the transponder to be operated at full power as each user has exclusive and complete use of the frequency band during its assigned time slot, an advantage that FDMA lacks. Additionally, TDMA removes problems associated with overlapping and intermodulation interference among individual carriers. This notwithstanding, TDMA suffers from negligible time delays as communications are stored or queued in a buffer on occasions where the number of requests for time slots to use a communications channel from potential users outstrips the number of available time slots. Two notable disadvantages of TDMA, particularly for cellular system applications, are problems associated with the requirement for guard space (especially when used with FDMA) for users-separation and complex time synchronization. There is also the limitation of a high peak power demand on the uplink which elevates power consumption and shortens battery life. A growing number of mobile networks are, therefore, embracing using TDMA in combination with FDMA or SDMA in order to boost the population of users.

The demand-assigned multiple access (DAMA) is typically applicable to the satellite communication terrain. Because the access method is in the frequency domain, DAMA can be categorized as a subset of FDMA as it allows for bandwidth-sharing such that multiple users are granted access to the sharing of a pool of resources or channels available for assignment on demand. In this manner, a single transponder is used to support several users strictly on call-by-call basis. When a user activates a request, a network control system validates the request and assigns a channel for communication. Upon the completion of a call on the assigned channel, a notification to this effect is received by the control system and the channel is returned to the pool of channels for reassignment on demand.

The MF-TDMA effectively combines the affordances of both the FDMA and the TDMA. The idea is to divide the available frequency band into separate frequency sub-bands with each frequency sub-band further divided into time-slots. Multiple users are then assigned, in a dynamic manner, specific time slots and frequency sub-bands for communications. This assignment is necessarily predicated upon the demands and the bandwidth capabilities of individual users. Although FDMA is noted for permitting simultaneous and continuous transmission, the time domain dependency of MF-TDMA prevents the multiple users on the network from transmitting simultaneously in the same frequency band. This is necessary for precluding interference. The multiple carrier frequencies, though, can be operated with different power levels.

The OFDMA – an upgraded variant of the OFDM technique – is another frequency – and time-domain access scheme that divides the available channel into a number of independently modulating subcarriers and assigns each user a subset of the closely-spaced and orthogonal subcarriers, thus allowing multiple users to communicate simultaneously. This allows for improved network efficiency and increased capacity through low latency and the elimination of network congestion, albeit OFDMA exhibits a great deal of sensitivity to frequency offset. The defining characteristics of OFDMA are its robust spatial diversity, receiver circuit simplicity and intrinsic orthogonality. Additionally, it is possible in OFDMA systems to dynamically change the allocation of subcarriers to users. Another important advantage of OFDMA is the capability of the scheme to combine the use of a multiplicity of low data rate subcarriers (with long symbol duration) with the insertion of cyclic extension or cyclic prefix to address issues surrounding multipath interference.

The OFDMA finds growing applications in cellular standards such as WiMAX and LTE (Long Term Evolution). It is also a crucial feature of Wi-Fi 6/Wi-Fi 7, WMAN (wireless metropolitan area network) and the futuristic air-ground broadband digital datalink system known as L-Band Digital Aeronautical Communications System (LDACS), which utilizes the OFDM modulation technique. The LDAC protocol stack’s physical layer, though, supports the Forward-Reverse transfers of data using a combination of OFDMA and TDMA schemes. On the forward link, an LDAC ground station supports the continuous transmission of a stream of OFDM symbols as well as bi-directional communication links to multiple air traffic, while, on the reverse link, multiple traffic can transmit discontinuously, transmitting a combination of TDMA and OFDMA radio bursts in frequency and time in accordance with the slots assigned to the different traffic on demand by the ground station.