Cellular Mobile Radio Communication Systems

CELLULAR MOBILE RADIO COMMUNICATION SYSTEMS

Good morning everyone.
Today we are going to begin with one of the most important topics in modern wireless communication—Cellular Mobile Radio Communication Systems. This lecture will introduce what a cellular system is, how it operates, and the key objectives behind its design.

Introduction

What is a Cellular Radio?

A cellular radio system is a type of mobile communication system that divides the service area into smaller sections called cells.
Unlike traditional mobile radio systems that use one high-power transmitter to cover a large region, cellular systems use many low-power transmitters, each serving its own cell.

This approach allows more efficient use of the radio spectrum and supports many more subscribers. In simple terms, cellular radio is the foundation of modern mobile communication.

FCC Definition of a Cellular System

The Federal Communications Commission, or FCC, provides a very precise definition of a cellular system.

According to the FCC, a cellular system is a high-capacity land mobile system where the available spectrum is divided into discrete channels.
These channels are assigned to geographic cells, and the same channels can be reused in different cells as long as the cells are far enough apart.

This concept of channel reuse is what makes cellular systems highly efficient and scalable.

Key Concept – Cell & Frequency Reuse

The idea of dividing an area into cells is central to the cellular system.
Each cell has its own set of radio channels.
Cells that are far apart can reuse the same channels without causing interference.

This technique—called frequency reuse—greatly increases the overall capacity of the system.
It allows more people to use mobile communication services without requiring additional spectrum.

Design Objectives of Cellular Systems

Now let’s move on to the main objectives behind designing cellular systems.
Engineers had several goals in mind to make mobile communication practical, reliable, and accessible to the public.

Objective 1 – Large Subscriber Capability

The first objective is to support a large number of subscribers.
A cellular system should be capable of serving thousands of users within a limited frequency band.

This is achieved through the use of many small cells and the concept of frequency reuse.
By dividing the area and reusing channels smartly, the system can accommodate more concurrent users.

Objective 2 – Spectrum Utilization

The radio spectrum is a limited natural resource.
Hence, one of the most important objectives is to use it efficiently.

By reusing the same frequencies in different cells, cellular networks can multiply their capacity without requiring extra spectrum.
This makes cellular technology far more efficient than previous mobile communication systems.

Objective 3 – Nationwide Compatibility

The next objective is nationwide compatibility.
Mobile users should be able to use their phones anywhere in the country, even if they have moved outside their home network.

This capability is called roaming, and users who move outside their home area are known as roamers.
Roaming ensures seamless service even when traveling between different cellular systems.

Objective 4 – Adaptability to Traffic Density

Cellular traffic is not uniform.
Some areas—like city centers—experience heavy traffic, while rural areas may experience lower usage.

A good cellular system must adapt to these variations.
Techniques like cell splitting, sectoring, and dynamic channel allocation help the system handle different traffic densities efficiently.

Objective 5 – Quality of Service & Affordability

For a cellular system to be successful, it must provide service that is comparable to regular telephone networks in terms of clarity, reliability, and coverage.

At the same time, the service must be affordable so that it can be widely adopted by the general public.
Balancing performance with cost is a crucial design objective of mobile communication systems.

Summary

To summarize, cellular mobile radio systems are designed to provide:

• High subscriber capacity

• Efficient spectrum utilization

• Seamless nationwide coverage

• Flexibility to handle varying traffic loads

• High quality and affordable service

These principles form the foundation of all modern mobile communication technologies, including 2G, 3G, 4G, and even 5G.

Cellular Geometry

Cellular Geometry

Good morning everyone.
In today’s session, we will explore one of the core concepts in cellular mobile communication—Cellular Geometry.
This topic helps us understand why cellular networks look the way they do and how the shapes of cells affect coverage, interference, and network expansion.

What Are Cells?

A cell is an individual service area in a cellular system.
Each cell has a specific set of radio channels assigned to it from the overall spectrum.

As demand increases or the network expands geographically, new cells can simply be added to the system.

However, we must place the cells such that they are far enough apart to avoid co-channel interference, but still have a small overlap area so that handoff for a moving subscriber is smooth and uninterrupted.

This balancing act is the key reason why we use geometric patterns for cell planning.

Why Geometry Matters?

The shape of each cell plays a major role in the design of cellular networks.

If we use irregular cell shapes, planning becomes much more complicated:

• The initial layout would be difficult to configure.

• Upgrading or adding new cells becomes a challenge.

• The network loses scalability.

Hence, we use standard geometric shapes that help in simple, predictable, and expandable network planning.

Why Not Circular Cells?

You may wonder: since radio waves propagate roughly in circular patterns, why not use circles?
Well, circles cannot fit together without leaving gaps.

If we attempted to cover the entire service area with circles, we would either end up with:

• Gaps where there is no coverage, or

• Large overlaps, which waste power, spectrum, and cause interference.

So, although circular propagation is ideal theoretically, circles are not suitable for cellular layout.

Suitable Cell Shapes (Regular Polygons)

To cover an entire region without gaps or overlaps, we need shapes that tessellate—meaning they fit together perfectly.
The regular polygons that can do this are:

• Equilateral triangles

• Squares

• Rectangles

• Hexagons

Among these, the hexagon is the most preferred for cellular networks.

Why Hexagonal Cells Are Preferred

Hexagons offer several practical advantages:

1. Maximum coverage for a given radius.

2. Minimum number of cells required to cover a given area, leading to fewer base stations.

3. Lower installation and maintenance costs.

Hexagons provide 30% more coverage area than a square cell and 100% more area than a triangular cell.

This makes the hexagon the most efficient shape for planning real-world cellular networks.

Hexagonal Cell Layout

In a hexagonal layout, each cell fits perfectly with its neighboring cells.
This uniform pattern is ideal for:

• Proper frequency reuse

• Predictable handoff boundaries

• Easy expansion by simply adding more hexagons

This is why almost all planning models for mobile systems represent cells as hexagons, even though real terrain may cause slight irregularities.

Coordinate System for Hexagonal Cells

Hexagonal systems are best represented using a coordinate system with axes separated by 60 degrees.
Each cell is represented by a pair of coordinates, (u, v), based on this grid.

To calculate the distance between two cells located at (u₁, v₁) and (u₂, v₂), we use this formula:

D = √[(u₂–u₁)² + (v₂–v₁)² + (u₂–u₁)(v₂–v₁)]

This formula works accurately because of the 60-degree geometry of the hexagonal lattice.

Simplified Hexagonal Distance Formula

If we assume the origin (0, 0) is the center cell, and the destination cell has coordinates (i, j), the equation simplifies to:

D = √[(i + j)² – i·j]

Here, i and j are whole numbers.
This simplified form is extremely useful in cluster size calculations and frequency reuse analyses.

Adjacent Cell Distance

In a hexagonal grid, the normalized distance between two adjacent cells is always 1.
This corresponds to coordinate pairs:

• (i = 1, j = 0) or

• (i = 0, j = 1)

This standard distance helps in planning handoff regions and interference boundaries.

Summary

To summarize today’s session:

• Cellular geometry is essential for efficient network planning.

• Shapes like circles are not suitable for real-world use.

• Hexagons are ideal because they cover the largest area with the least number of cells.

• The 60-degree coordinate system makes it easy to calculate distances between cells.

• Using hexagonal geometry minimizes cost, improves coverage, and enhances system efficiency.

This geometric foundation supports all higher-level concepts in cellular networks, including cluster formation, frequency reuse, and mobility management.

Determination of Number of Cells per Cluster

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Determination of Number of Cells per Cluster

Good morning everyone.

In this lecture, we will study an important concept in cellular system design—determination of the number of cells per cluster.

This topic is crucial because it directly affects frequency reuse, system capacity, and co-channel interference in a cellular network.

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Introduction

In cellular systems, the same set of frequencies is reused in different cells to increase system capacity.

However, cells using the same frequency—called co-channel cells—must be separated by a sufficient distance to limit interference.

This separation is controlled by the number of cells per cluster, denoted by N.

So, determining the correct value of N is a key task in cellular planning.”

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Adjacent Hexagonal Cell Distance

Let us begin with the geometry of a hexagonal cell.

Let R be the distance from the center of the hexagon to one of its vertices.

The center-to-center distance between two adjacent hexagonal cells is given by:

√3R, which can also be written as 2R cos 30°.

This relationship comes directly from hexagonal geometry and forms the base for further cluster calculations.________________________________________

Concept of a Cluster

A cluster is a group of cells in which each frequency channel is used exactly once.

Once a cluster is completed, the same frequency set is reused in another cluster.

The number of cells in one cluster is denoted by N.

By fixing N, we can determine how far apart co-channel cells will be located.

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Co-Channel Cells

Now consider the cellular structure shown in Figure

Cells labeled A represent co-channel cells.

The center cell A has six nearest co-channel cells, all using the same frequency set.

These six co-channel cells lie at the vertices of a larger hexagon whose radius is D, known as the co-channel distance.

The vectors from the center cell to these co-channel cells subtend angles that are multiples of 60 degrees, due to the symmetry of the hexagonal layout.

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Geometry of the Larger Hexagon

To locate a co-channel cell, we move i steps along one hexagonal axis and j steps along another axis.

Using the law of cosines, the square of the co-channel distance D is given by:

D² = 3R² (i² + j² + ij) —— Equation (i)

This equation connects the geometry of the cell layout with the reuse pattern defined by i and j.

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Area of the Large Hexagon

The area of a hexagon is proportional to the square of its radius.

So, the area of the larger hexagon enclosing the co-channel cells is:

A_large = k × (3R²)(i² + j² + ij) —— Equation (ii)

Here, k is a proportionality constant depending on hexagon geometry.

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Area of the Small Hexagon

Similarly, the area of a single small cell is:

A_small = k × R² —— Equation (iii)

Now, taking the ratio of the two areas, we get:

A_large / A_small = 3(i² + j² + ij) —— Equation (iv)

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Determination of Cluster Size N

From the symmetry of Figure 3.2, we observe that:

• The large hexagon encloses the central cluster of N cells

• Plus fractional cells from six surrounding clusters

• Effectively enclosing an area equal to 3N cells

Since area is proportional to the number of cells:

A_large = 3N and A_small = 1

Substituting into the area ratio gives:

N = i² + j² + ij —— Equation (v)

This is a very important formula for determining the cluster size.

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Co-Channel Reuse Ratio

Now, combining equations (i) and (v), we obtain:

D / R = √(3N)

This ratio D/R is called the co-channel reuse ratio.

It tells us how far apart co-channel cells are relative to the cell radius and is widely used for estimating co-channel interference.

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Practical Example

Let us consider a commonly used cluster size:

N = 7

Then,

D / R = √(3 × 7) = √21 ≈ 4.6

So, for a 7-cell cluster, the co-channel reuse ratio is approximately 4.6.

This value offers a good balance between capacity and interference and is therefore widely used in practical systems.”

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Importance of Cluster Size Selection

The choice of N involves a trade-off:

• Larger N

o Greater separation between co-channel cells

o Lower interference

o Reduced capacity

• Smaller N

o Higher capacity

o Increased co-channel interference

Hence, N must be chosen carefully based on system requirements.

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Summary

To summarize this lecture:

• Cluster size is given by:

N = i² + j² + ij

• Co-channel reuse ratio is:

D/R = √(3N)

• Hexagonal geometry enables systematic frequency reuse

• The reuse ratio plays a key role in interference estimation and capacity planning

This concept is fundamental to the design and optimization of cellular mobile networks.

Frequency Reuse in Cellular Systems

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Frequency Reuse

Good morning everyone.

In this lecture, we will discuss one of the most important concepts that makes cellular communication practical and efficient—Frequency Reuse.

Without frequency reuse, it would be impossible to serve millions of mobile users with the limited radio spectrum available.

________________________________________Introduction to Frequency Reuse

Frequency reuse allows the same radio channels assigned to one cell to be reused in another cell, provided the cells are separated by sufficient distance.

This separation ensures that co-channel interference remains within acceptable limits.

The basic idea is simple: reuse frequencies intelligently rather than increasing spectrum.

________________________________________Need for Frequency Reuse

Radio spectrum is scarce and costly.

Cellular systems overcome this limitation by dividing large service areas into smaller cells and using low-power transmitters.

Because of smaller coverage areas and controlled power levels, the same frequencies can be reused at least two cells away, allowing efficient spectrum utilization with minimum interference.

________________________________________Clusters in Cellular Systems

To manage frequency reuse systematically, cells are grouped into clusters.

A cluster is a group of cells in which each frequency channel is used only once.

Typical cluster sizes are 4, 7, 12, or 21 cells.

Once all frequencies are used in one cluster, they are reused in the next cluster.

________________________________________Frequency Assignment in a Cluster

In a cluster, each cell is assigned a group of radio channels.

The same channels are never assigned to adjacent cells.

This ensures that interference is minimized.

The geographical area covered by a cell is known as its footprint.

________________________________________Practical Cell Shape

The hexagonal shape used in cellular planning is a theoretical model.

In real life, due to terrain, buildings, and propagation effects, cells appear as distorted circles.

However, the hexagonal model simplifies analysis of coverage, frequency reuse, and interference.

________________________________________Cell Cluster Geometry

Figure 3.3 shows a 7-cell cluster, which is very commonly used.

In this geometry:

• The distance between the centres of two adjacent cells is 1.732 times R,

where R is the distance from the centre to the vertex of a cell.

________________________________________Co-Channel Distance

For a 7-cell cluster, the distance between two co-channel cells is given by:

D = 4.6 R

The ratio D/R is extremely important:

• If D/R is reduced → Co-channel interference increases

• If D/R is increased → Traffic carrying capacity decreases

Hence, D/R represents a trade-off between interference and capacity.

________________________________________Co-Channel Interference Characteristics

As long as all cells transmit equal power, co-channel interference depends mainly on geometry and reuse distance—not on transmit power.

In the cellular layout, the six nearest co-channel cells are the main contributors to interference.

These cells form a larger hexagon around the central cell using the same frequency.

________________________________________Frequency Reuse Implementation

In practice, the total frequency spectrum is divided into two or more mutually exclusive channel groups.

Channel assignment depends on the signal-to-noise ratio of the mobile:

• Mobiles with high S/N ratio are assigned channels with lower reuse factor

• Mobiles with lower S/N ratio receive channels with higher reuse factor

Typically, mobiles near the cell centre get channels with low reuse factor.

________________________________________Base Transceiver System (BTS)

Each cell is served by a Base Transceiver System, or BTS.

It consists of the transmitter, receiver, and antenna system.

Initially, the BTS was located at the centre of the cell and used an omnidirectional antenna, radiating power equally in all directions.

________________________________________Co-Channel Interference Problem

In a hexagonal cell layout, interference always comes from the six adjacent cells.

The worst-case scenario occurs when a mobile is at the cell boundary.

Here, it receives the weakest signal from its own BTS and strong interference from neighboring cells.

As a result, the S/I ratio may fall below the required 18 dB.

________________________________________Methods to Reduce Co-Channel Interference

One possible solution is to increase the D/R ratio by increasing the cluster size to 9 or 12 cells.

However, this reduces system capacity.

A better and widely used solution is sectoring, which reduces interference without increasing cluster size.

________________________________________Sectorised Cells

In sectoring, each cell is divided into 3 or 6 sectors, each served by a directional antenna.

The most common configuration uses three 120-degree antennas.

This significantly reduces interference by limiting radiation to specific directions.

________________________________________Effect of Sectoring

Sectoring improves the average S/N ratio from about 18 dB to 24 dB.

Although trunking efficiency is slightly reduced because each sector has fewer channels, this is usually compensated by:

• Lower interference

• Smaller reuse factor

• Higher overall capacity

________________________________________Cell Splitting Concept

When traffic demand in a cell increases beyond acceptable limits, cell splitting is used.

A large cell is divided into smaller cells, each with its own BTS.

This increases capacity and improves Quality of Service.

________________________________________Impact of Cell Splitting

As cells become smaller, subscribers cross cell boundaries more frequently.

This results in:

• Increased number of handoffs

• Higher processing load on the system

Practically, reducing the cell radius by four times increases processing load by nearly ten times.

________________________________________Handoff in Cellular Systems

When a subscriber moves from one cell to another, the ongoing call must be transferred to a new channel.

This process is called handoff.

Since adjacent cells do not share frequencies, handoff must be fast and seamless to avoid call drops.

________________________________________Early Analog Cellular Systems

Early cellular systems were based on analog technology and were not globally compatible.

For example, the AMPS system in the USA used:

• 50 MHz bandwidth

• 832 channels of 30 kHz each

• FM for speech and FSK for signaling

However, these systems suffered from low capacity, poor security, and limited data support.

________________________________________International Incompatibility

Different countries used different frequency bands and standards.

This caused major problems for international roaming subscribers.

To overcome these issues, a globally compatible digital standard called GSM was developed.

GSM (Global System for Mobile Communication)

Good morning everyone.

Today’s lecture is on GSM – Global System for Mobile Communication.

In this session, we will study the evolution of mobile communication systems and then move on to the basic system architecture of GSM.

Understanding GSM is very important because it laid the foundation for modern digital mobile communication systems.

________________________________________Evolution of Mobile Communication – 1946

Let us begin with the history of mobile communication.

The first mobile telephone service started in 1946 in St. Louis, USA.

This system was manually operated, meaning calls had to be connected by human operators.

It covered only a very small area and was available to very few subscribers.

Due to high cost and limited capacity, it was not widely used.

________________________________________Evolution from 1950 to 1970

Between 1950 and 1970, mobile telephone systems evolved from manual operation to automatic operation.

Automatic switching systems were introduced, which reduced human involvement.

However, these systems still suffered from limited coverage, poor spectrum efficiency, and low subscriber capacity.

True mobility was still not achieved during this period.

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Cellular Concept – 1970s

A major breakthrough occurred in the 1970s when Bell Laboratories introduced the concept of cellular coverage.

The service area was divided into small cells, each with its own base station.

This allowed frequency reuse, which significantly increased system capacity.

The cellular concept became the backbone of all modern mobile communication systems.

________________________________________First Generation Systems – 1G

The first practical cellular system was AMPS – Advanced Mobile Phone Service, launched in the USA in 1979.

During the 1980s, other analog systems such as NMT and TACS were also introduced.

These systems used analog speech transmission and frequency modulation.

They provided nationwide coverage and supported hundreds of thousands of users.

All these analog systems are collectively called 1G or First Generation systems.

________________________________________Second Generation Systems – 2G

In the 1990s, digital mobile communication systems were introduced.

These systems are known as 2G or Second Generation systems.

Examples include GSM, DAMPS, and CDMA.

Unlike 1G, these systems used digital transmission, which improved voice quality, capacity, security, and spectrum efficiency.

Among these, GSM became the most successful and widely adopted standard worldwide.

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ETSI and GSM

The development of GSM was led by ETSI – European Telecommunications Standards Institute.

ETSI was responsible for defining technical standards for GSM.

The main objective was to create a common mobile communication standard that would work across different European countries.

This ensured interoperability, international roaming, and large-scale deployment.

________________________________________GSM System Architecture – Introduction

Now let us move to the GSM System Architecture.

The GSM system is designed as a set of system entities, where each entity performs a specific function.

This modular architecture simplifies network design, operation, and maintenance.

Together, these entities provide end-to-end mobile communication.

________________________________________Major GSM Entities

The GSM architecture is broadly divided into four main entities:

1. Mobile Station (MS) – used by the subscriber for communication.

2. Base Station Subsystem (BSS) – handles radio transmission between mobile and network.

3. Network Switching Subsystem (NSS) – manages call switching, mobility, and databases.

4. Operation and Support Subsystem (OSS) – supports network operation, maintenance, and monitoring.

Each entity plays a critical role in the GSM network.

________________________________________GSM Architecture Diagram

This slide shows the reference model of GSM system architecture.

Here, we can see how the Mobile Station communicates with the Base Station Subsystem, which is connected to the Network Switching Subsystem.

The OSS oversees the overall network performance and maintenance.

This architecture clearly separates radio functions from switching and control functions, which is a key strength of GSM.

________________________________________Conclusion

To conclude, GSM represents a major milestone in the evolution of mobile communication.

It introduced digital cellular technology, improved capacity, and enabled international roaming.

GSM also formed the foundation for later generations, such as 3G, 4G, and beyond.

Understanding GSM is essential for anyone studying telecommunication or wireless systems.

Base Station Subsystem (BSS)

Good morning everyone.
In today’s lecture, we will study the Base Station Subsystem, commonly known as BSS.
The BSS is a crucial part of the GSM network because it forms the radio access part, connecting the Mobile Station to the core network.
It mainly handles all radio-related functions.

Components of BSS

The Base Station Subsystem consists of two main entities:

1. Base Transceiver Station (BTS)

2. Base Station Controller (BSC)

The BTS handles radio transmission, while the BSC performs control and management functions over multiple BTSs.

Base Transceiver Station (BTS)

The Base Transceiver Station, or BTS, is the equipment that facilitates wireless communication between the User Equipment, that is the mobile station, and the GSM network.
It forms the first network element that communicates directly with the mobile phone over the air interface.

Role of BTS

The BTS is responsible for sending and receiving radio signals.
It may also contain encryption and decryption equipment to secure communication.
Filtering tools such as band-pass filters are used to limit interference.
Antennas are considered part of the BTS because they enable radio transmission.
In sectorized cells, a BTS supports multiple sectors to improve coverage and capacity.

BTS Transceivers (TRXs)

A BTS typically contains several transceivers, known as TRXs.
Each TRX can handle a specific frequency or sector of the cell.
This allows the BTS to serve multiple users simultaneously.
Even though technologies evolve, the basic structure and function of BTS remains the same.

Units of BTS

In general, a BTS consists of the following units:

• TRX (Transceiver)

• Power Amplifier

• Combiner

• Duplexer

• Antenna

• Alarm Extension System

Each unit plays a specific role in ensuring efficient radio communication.

TRX – Transceiver

The TRX, also known as DRX (Driver Receiver), is the core unit of the BTS.
It performs transmission and reception of radio signals.
It also communicates with higher network entities, such as the Base Station Controller.
Without TRX, radio communication is not possible.

Power Amplifier and Combiner

The Power Amplifier increases the signal strength received from the TRX before transmission through the antenna.
In some systems, it may be integrated with the TRX.

The Combiner merges signals from multiple TRXs and sends them through a single antenna, thereby reducing the number of antennas required.

Duplexer and Antenna

The Duplexer allows both transmission and reception to occur through the same antenna by separating the uplink and downlink signals.
The Antenna radiates and receives radio frequency signals and is considered an integral part of the BTS.

Alarm Extension System

The Alarm Extension System collects alarms and status information from various BTS units.
These alarms are forwarded to Operations and Maintenance (O&M) monitoring stations.
This helps in quick fault detection and maintenance.

Functions of BTS

The BTS performs several important functions, such as:

• Radio transmission in GSM format

• Use of frequency hopping techniques

• Use of spatially diverse antennas

• Coding and decoding of radio channels

These functions improve signal quality and system reliability.

Functions of BTS – Continued

Additional functions of BTS include:

• Implementation of equalization algorithms to counter multipath effects

• Encryption of transmitted data

• Transmission of signaling messages

• Operations and Maintenance of the BTS

Together, these ensure secure and efficient communication.

Base Station Controller (BSC)

The Base Station Controller, or BSC, controls the activities of multiple BTSs within a geographical area, known as a cluster.
It acts as the central controller for the Base Station Subsystem.

Functions of BSC

The BSC performs the following key functions:

• Allocation and reservation of radio channel frequencies

• Control of handover between BTSs

• Paging of mobile stations for incoming calls

• Management of BTS resources

This ensures seamless communication during mobility.

BTS–BSC Relationship

Each BTS is controlled by a parent BSC through the Base Station Control Function (BCF).
The BCF may be implemented as a separate unit or integrated within a TRX, especially in compact base stations.

Base Station Control Function (BCF)

The BCF provides an Operations and Maintenance connection to the Network Management System (NMS).
It manages the operational states of TRXs, software handling, and alarm collection.
This helps in efficient monitoring and control of the BSS.

Conclusion

To conclude, the Base Station Subsystem is a vital part of the GSM network.
The BTS handles the radio interface, while the BSC manages and controls multiple BTSs.
Together, they ensure reliable wireless communication between mobile users and the GSM core network.