WIRELESS NETWORKS AND MOBILE COMMUNICATIONS

Introduction of Wireless Networks

Contents

• 1. Content Description
• 2. Fundamentals of Wireless Networks
• 3. Mult-Access And Modulation
• 4. Cellular Concept

1. Content Description

Wireless and Mobile Communications is the invisible force that connects our world through mobile phones to the satellites orbiting the Earth. The world is rapidly tending towards an era of unprecedented connectivity, driven by rapid advancements in wireless technology. This material serves as a comprehensive guide to engineering students building their foundational knowledge or even professionals seeking to understand the latest trends in network evolution.Wireless and Mobile Communications is the invisible force that connects our world through mobile phones to the satellites orbiting the Earth. The world is rapidly tending towards an era of unprecedented connectivity, driven by rapid advancements in wireless technology. This material serves as a comprehensive guide to engineering students building their foundational knowledge or even professionals seeking to understand the latest trends in network evolution. This content provides a foundational introduction to principles of wireless networks and mobile communications. Participants will comprehend the fundamental concepts of signal propagation, which include attenuation, multipath, and fading. The content delves into the key technologies that enable mobility and high data rate communication. 

2. Fundamentals of Wireless Communication

• An introduction to wireless systems: Evolution of Wireless networks, its evolution, types, and an overview of the electromagnetic spectrum
• Concept of the wireless channel: Propagation mechanisms, Path loss (Both small-scale and large-scale), Fading, Doppler effect, and its impact

3. Multiple Access and Modulation Techniques

• Multi-access schemes: FDM, TDMA, CDMA, OFDMA, and how they compare
• Digital modulation: Review modulation basics, explain the Digital Modulation techniques (ASK, FSK, PSK, QAM). 

4. Cellular Concept

• Cellular systems: The cellular concept, cellular geometry and hexagonal cells, channel interference, and calculation of Signal-to-Noise Ratio (SNR)

 

 

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WIRELESS NETWORKS AND MOBILE COMMUNICATIONS

RADIO PROPAGATION

Contents

• 1. Reflection
• 2. Diffraction
• 3. Scattering
• 4. Large Scale Path Loss Models

A wireless channel is a physical medium that transports signals from a transmitter to a receiver. Unlike a wired channel, which is predictable and steady, a wireless channel is extremely dynamic. It is distinguished by phenomena such as path loss, fading, and multi-path propagation, which can substantially decrease signal quality and make reliable communication difficult. Radio wave propagation is the study of how electromagnetic waves travel from a transmitting antenna to a receiving antenna. This is a complex process influenced by the environment between the two points. The interaction of radio waves with their surroundings is categorized into three main mechanisms:Reflection, diffraction, and scattering

1. Reflection

Reflection occurs when a radio wave strikes a large, flat surface and bounces off it. For reflection to happen, the size of the reflecting object must be significantly larger than the wavelength of the radio wave. This is a common mechanism in urban environments where waves bounce off large structures like buildings and the ground. The angle of the reflected wave is equal to the angle of the incident wave, following the principle of Snell's Law. This phenomenon can create multiple signal paths, which can lead to constructive or destructive interference at the receiver.

Snell's Law

Snell's Law (Law of Refraction) describes the relationship between the angles of incidence and refraction for a wave, such as light or a radio wave, as it passes through the boundary between two different media.

Figure 2: Snell's Law

2. Diffraction

Diffraction is the bending of radio waves around an obstacle's sharp edges as shown in Figure 2 below. This method enables radio signals to areas where there are no direct line-of-sight paths available. For example, diffraction allows a signal to reach a receiver behind a building or a hill. The degree of diffraction is determined by the wave's wavelength as well as the obstacle's size and form. The "knife-edge" diffraction model is a standard method for analyzing this effect. Without diffraction, mobile connectivity in cities with towering buildings would be impossible

Figure 3: The Knife Edge

3. Scattering

Scattering happens when traveling waves encounter a change in the wave impedance and then reflect partially. And if the reflection is not total, it will also partially transmit into the new impedance. This is called scattering of the traveling wave. This is called when a radio wave strikes an item of the same size as its wavelength or when an object's surface is rough.

Reflection and diffraction are linked with large-scale obstacles, while scattering affects smaller objects. When a wave strikes these small objects, it scatters in multiple directions. The scattering often reduces the main signal, which is necessary in complicated indoor and outdoor contexts because it provides extra signals, although weaker, that help the receiver pick up the signal. 

4. Large Scale Path-Loss Models

Path loss is the reduction in power density of an electromagnetic wave as it

propagates through space. But large-scale path are used to predict the average signal strength over a large area, for distances measuring over 1 kilometer. Some of these loss models

• Free-Space Path Loss Models

The Free-Space Path Loss Model is considered the simplest loss model that assumes; direct line-of-sight  without any any obstacles to cause reflection, diffraction, or scattering. The model is useful for scenarios like satellite communication or microwave links. 

The path loss (Lfs​) is given by the formula:

Lfs​(dB)=10log10​(Pr​Pt​​)=20log10​(λ4πd​) 

Here, Pt​ is the transmitted power, Pr​ is the received power, d is the distance between the antennas, and λ is the wavelength of the signal. The received power in this model decreases with the square of the distance (d2).

• Two-Ray Ground Reflection Model

The Two-Ray Ground Reflection Model considers two signal paths arriving at the receiver direct line-of-sight ray and a ray that reflects off the ground.  It is considered more accurate.  The received signal is the vector sum of the two rays. Due to the phase shift upon reflection and the difference in path length, these two rays can interfere. At distances much greater than the antenna heights, the received power (Pr​) simplifies to:

Pr​=d4Pt​Gt​Gr​ht2​hr2​​

where

Gt​ and Gr​ are the gains of the transmitting and receiving antennas, and ht​ and hr​ are their respective heights. This model shows that for large distances, the received power decreases with the fourth power of the distance (d4), which is a much steeper drop-off than in free space.

 

 

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WIRELESS NETWORKS AND MOBILE COMMUNICATIONS

SMALL SCALE FADING

The statistical properties of small-scale fading are often described by probability distributions. The two most common models are Rayleigh and Rician fading.

1. Rayleigh Fading 

Rayleigh fading is a statistical model that applies when there is no direct line-of-sight (LOS) path between the transmitter and receiver. This is common in congested urban environments or indoor settings where the signal must bounce off numerous things before reaching the receiver.

The received signal is the sum of several reflected, dispersed, and diffracted waves, each of which has nearly equal power.
The amplitude of the received signal's envelope has a Rayleigh distribution.
This model is frequently regarded as a worst-case situation since the lack of a strong LOS path makes the signal extremely vulnerable to deep and frequent fades.

2. Rician Fading

Rician fading is a broader model that takes into account both multipath components and a strong line-of-sight (LOS) path. This is frequent in suburban or open rural locations where a straight path exists. The received signal consists of a strong, consistent LOS component and several lesser, dispersed components.
The amplitude of the received signal's envelope has a Rician distribution. The Rician K-factor, which is the power in the LOS component divided by the power in the scattered components, determines the severity of the fading. A high K-factor indicates that the LOS path is dominating, and fading is less severe. As the K-factor approaches 0, the LOS path's power decreases and the model approaches a Rayleigh distribution.

The Doppler Effect and Coherence Time
The mobility of the transmitter, receiver, or objects in the environment adds another layer of complexity to small-scale fading. This movement causes the Doppler effect.

1. The Doppler Effect
The Doppler effect is the change in frequency of sound waves as they reflect from moving objects, increasing in frequency when approaching and decreasing when moving away, allowing for the determination of flow direction and velocity.

A signal path from a moving object approaching the receiver will experience a positive Doppler shift or higher frequency, whereas a signal path from a moving object moving away will experience a negative Doppler shift or lower frequency. And the collective effect of these individual shifts is a spread of the received signal's spectrum, known as the Doppler spread (BD​). A larger Doppler spread indicates that the channel is changing more rapidly over time.

2. Coherence Time
Coherence time (Tc​) is a measure of how long the wireless channel's characteristics remain statistically constant. It is a crucial parameter for understanding the nature of fading.

The coherence time is inversely proportional to the Doppler spread.
Tc​≈fm​1​, where fm​ is the maximum Doppler shift.
The relationship between coherence time and the duration of a transmitted symbol (Ts​) determines whether the channel experiences slow fading or fast fading:

a) Slow Fading: If the symbol duration is much shorter than the coherence time (Ts​<<Tc​), the channel's state is essentially constant for the duration of one or several symbols. The signal experiences a single fade level during the transmission.
b) Fast Fading: If the symbol duration is longer than the coherence time (Ts​>Tc​), the channel's characteristics change significantly within the time it takes to transmit a single symbol. Different parts of the same symbol experience different fading levels, leading to signal distortion

 

WIRELESS NETWORKS AND MOBILE COMMUNICATIONS

 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. 

Disadvantages:

• 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. 

Disadvantages:

• 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.

Disadvantages:

• 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.

Figure 4: Orthogonal Frequency Division Multiple Access (OFDMA)

Application: It is the foundation of 4G LTE and 5G technologies. 

Advantages:

1. 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.
2. 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.
3. 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.
4. 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. 
5. 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.