MULTI-ACCESS TECHNIQUES
The single communication medium in wireless networks is the radio
spectrum that is shared among multiple users. And Multi-access is a technique
that has been employed to allow multiple users to share the medium
simultaneously without interfering with one another. These techniques are
critical to allow for the maximization of network capacity and efficiency. And
the following are some of the techniques that have been employed:
a) Frequency Division Multiple Access (FDMA)
Frequency Division Multiple Access (FDMA) is one of the oldest
multiple-access techniques that allocates a series of nonoverlapping frequency
slots among multiple earth stations to enable simultaneous communication while
managing issues such as signal interference and power. This is made possible
through sharing of transponder bandwidth.
Figure 1: Frequency Division Multi-Access (FDMA)
https://article.murata.com/en-global/article/multiplexing-and-multiple-access-1
Application: When we tune into a specific frequency to listen to a specific broadcast. In FDMA, each user gets his own radio frequency. An example application is in satellite communication.
Advantages.
- Because FDMA systems use low bit rates (large symbol time) compared to average delay spread, there is low Inter-Symbol Interference (ISI).
- Simple to implement from a hardware standpoint.
- No synchronization is required between users.
- There is hardly any equalization required.
- It is relatively efficient with a small base population, and when traffic is constant.
Disadvantages:
- It is suitable only for analog signals.
- Network planning is cumbersome and time-critical. RF filters may be required to meet the stringent measures for adjacent channel rejections. This means they can be costly.
- The carrying capacity of traffic is relatively low.
- Inefficient use of spectrum because some channels may be idle while others are busy.
- The total number of users is limited by the number of available frequency channels.
b) Time Division Multiple Access
(TDMA)
TDMA divides a single frequency channel into discrete time slots so that each user is assigned a specific time slot to transmit and receive data. Users take turns transmitting in a round-robin fashion.
Figure 2: Time Division Multi-Access (TDMA)
Advantages- TDMA isolates clients with respect to time, thereby ensuring that there is no obstruction through concurrent transmission.
- Relatively more efficient. For example, it is more efficient than FDMA, since a single frequency can be used by multiple users.
- FDMA is well-suited for bursty data traffic.
- It offers an effective use of hierarchical cell structures like Pico.
- It has a relatively high synchronization overhead between the Base Station (BS) and the Mobile Station (MS)
- Signal preparation is also needed to coordinate separation and connection recognition
- Guard bands are needed to prevent interference between time slots.
c) Code Division Multiple Access (CDMA)
CDMA is a wireless transmission technology that allows multiple users to transmit simultaneously over the same frequency. It uses unique pseudo-random codes to spread each user's data over a wide bandwidth. Unique values (pseudo-random codes) are assigned to users, allowing them to broadcast over the same frequencies simultaneously. The receiver uses the same codes to recover the data and reject signals from other
users.
Advantages
- Relatively more efficient. For example, it is more efficient than FDMA, since a single frequency can be used by multiple users.
- FDMA is well-suited for bursty data traffic.
- It offers an effective use of hierarchical cell structures like Pico.
- It has a relatively high synchronization overhead between the Base Station (BS) and the Mobile Station (MS)
- Signal preparation is also needed so as to coordinate separation and connection recognition.
- Guard bands are needed to prevent interference between time slots.
c) Code Division Multiple Access (CDMA)
CDMA is a wireless transmission technology that allows multiple users to transmit simultaneously over the same frequency. It uses unique pseudo-random codes to spread each user's data over a wide bandwidth. Unique values (pseudo-random codes) are assigned to users, allowing them to broadcast over the same frequencies simultaneously. The receiver uses the same codes to recover the data and reject signals from other sources.
Figure 3: Carried Division Multi-Access Technology
CDMA remains a powerful wireless multi-access technology in modern wireless networks because of the following:
- Spread Spectrum Technology: It uses a wideband spectrum technique where each user’s signal is multiplied by a unique code sequence, spreading it across a broad frequency range.
- Unique Code Assignment: Different CDMA networks can occupy the same frequency because each network is assigned a unique code that separates them from one another.
- Signal Interference Management: CDMA minimizes cross-talk and interference so that more users to share the same bandwidth without degrading service or quality. This is carried out through power control and code separation
Application: CDMA protocols have been used in 4G, 5G, and LTE networks.
Advantages:
- Has very low power requirements
- Relatively higher user capacity
- In CDMA, problems like multipath and fading do not occur
- High spectral efficiency and capacity.
- Robust against interference and jamming.
- Provides "soft handoff" which improves call quality.
- It requires a high degree of synchronization between transmitters and receivers. Any little mismatches lead to errors.
- Power variations occur when users are farther from the base station and excess overloads, affecting efficiency and signal quality.
d) Orthogonal Frequency Division Multiple Access (OFDMA)
OFDMA is a variation of the OFDM digital modulation scheme that combines the principles of TDMA and FDMA. It divides the available bandwidth into multiple sub-channels and then allocates these sub-carriers to different users in a time-frequency grid.
Application: It is the foundation of 4G LTE and 5G technologies.
Advantages:
- Flexibility:The possibility of having a sliced spectrum allows the designer to control different parts of the spectrum. As a result, the service provider can customize the type of services to the subscribers. Additionally, it can be easily integrated with adaptive modulation and coding strategies.
- Simpler Equalization: OFDM eliminates the need for a long equalizer in single-carrier systems, requiring only one division operation per subcarrier for equalization. This results in a significant reduction in complexity for high-rate wireless communication systems.
- Mitigates Intercell Signal Interference: Orthogonality helps in preventing Inter-Symbol Interference (ISI) from multiple copies of the same signal. Additionally, OFDM helps avoid interference among transmissions from neighboring cells.
- Mitigates the effects of MIMO: OFDM helps in equalizing the impact of this interference among antennas through the same equalization strategy in an error when Multiple Input Multiple Output (MIMO) systems have become an integral part of infrastructure-based wireless systems. When multiple antennas transmit or receive signals, interference occurs among their transmissions, which OFDM mitigates.
- Simpler Hardware Implementation requirements: In OFDM systems, modulation at the transmit side is performed via an inverse Fast Fourier Transform block, and the demodulation at the receive side is done through a Fast Fourier Transform. In both cases, they are hardware optimized, leading to a simplified implementation.
A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). A Fourier transform converts a signal from its original domain (often time or space) to a representation in the frequency domain and vice versa. -https://en.wikipedia.org/wiki/Fast_Fourier_transform - 10th September 2025
2. FADING IN WIRELESS CHANNELS.
Fading is the variation in the power of a received radio signal. It is a barrier in wireless communication created by the physical environment between the transmitter and receiver. The signal's route is typically not straight; it might be reflected, diffracted, and scattered, resulting in multipath propagation and hence loss in signal strength at the destination. Fading can be large-scale or small-scale.
a) Large-Scale Fading
Large-scale fading is the average signal-power attenuation over large areas. It is largely influenced by terrain configuration between the transmitter and receiver, with a decrease in power over very long distances (several hundreds or thousands of meters). It is primarily caused by two factors:
- Path Loss: The signal power decreases as the distance between the transmitter and receiver increases. This is deterministically described by the Friis transmission equation as follows.
§ Shadowing: This is the phenomenon in which the received signal power varies due to objects obstructing the propagation route between the transmitter and receiver. These fluctuations are felt on local-mean powers, or short-term averages, which reduce fluctuations caused by multipath fading. They are random variations in received signal power due to obstructions blocking the line-of-sight path.
b) Small-Scale Fading (Multipath Fading)
Small-scale fading describes the rapid fluctuations in signal strength over very short time periods or travel distances. It is caused by the constructive and destructive interference of
multiple signal paths arriving at the receiver at slightly different times. This can be categorized into two types:
§ Rayleigh Fading: Occurs when the received signal is a sum of many reflected, scattered, or diffracted components.
§ Rician Fading: This fading occurs when, in addition to a strong line-of-sight path, there is the effect of the multipath components.
Mitigating the Fading Problem
a) Diversification: Send the same information over multiple channels that are likely to experience independent fading. This can be done as follows.
1. Spread the signals over different frequencies
2. Use multiple antennas at the receiver and/or transmitter to diversify space.
3. Sending the same data multiple times at different time slots.
b) Channel Coding and Interleaving: Using error-correcting codes and interleaving data packets to protect against signal loss during a deep fade.
c) Multiple-Input Multiple-Output (MIMO): Is a technique of space diversity where multiple antennas are employed at both the transmitter and receiver to improve data throughput and reliability.
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