Cellular Network Techniques and Challenges

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Questions and Answers

What is a primary technique used to reduce adjacent channel interference?

Using higher frequency bands

Careful filtering and channel assignments (correct)

Reducing the number of channels

Increasing signal density

Why is cell splitting primarily implemented in cellular networks?

To enable lower call quality

To facilitate better signal strength

To increase the capacity of a cell (correct)

To reduce infrastructure costs

What characterizes smaller cells resulting from cell splitting?

Higher antenna weight

Higher transmission power

Greater risk of co-channel interference (correct)

Longer coverage distance

What advantage does cell splitting provide in terms of frequency usage?

It enhances frequency reuse in adjacent channels (D) Signup and view all the answers

In urban areas, why does signal strength weaken?

High user density (B) Signup and view all the answers

What is a limitation associated with cell splitting?

Infrastructure cost increases (C) Signup and view all the answers

How does careful channel assignment help in cellular networks?

It prevents adjacent channel interference (D) Signup and view all the answers

What is one consequence of having smaller cells in a cellular network?

Increased handoff overhead (A) Signup and view all the answers

What is the primary purpose of using directional antennas in cell sectoring?

To focus signals within specific sectors and reduce interference (C) Signup and view all the answers

Which of the following configurations divides a cell into six sectors?

6-sector configuration (A) Signup and view all the answers

What is a significant challenge associated with cell sectoring?

Careful antenna alignment and potential sector boundary interference (B) Signup and view all the answers

How does cell splitting enhance capacity?

By creating smaller cells with new base stations (A) Signup and view all the answers

One of the advantages of cell sectoring is the ability to:

Isolate sectors to prevent overlap and enhance clarity (D) Signup and view all the answers

What is the primary advantage of limiting the coverage of each base station to a small area called a cell?

Increases the quality of service (B) Signup and view all the answers

Which of the following describes a limitation of conventional mobile systems?

Inability to effectively utilize frequency spectrum (D) Signup and view all the answers

What major approach does the cellular concept utilize to enhance communication?

Multiple low power transmitters to minimize interference (D) Signup and view all the answers

What common issue arises from a large number of subscribers in conventional mobile systems?

High blocking probability (A) Signup and view all the answers

What mechanism helps ensure uninterrupted processing in a cellular telephone system?

Handoff mechanism (B) Signup and view all the answers

Which of the following is NOT a characteristic of cellular systems?

Offers limited user capacity (C) Signup and view all the answers

What approach helps achieve high capacity in cellular systems?

Reusing frequency bands in different locations (D) Signup and view all the answers

What was a significant drawback of conventional mobile systems regarding call management?

Need to reinitialize calls when moving between zones (C) Signup and view all the answers

What is the formula for calculating the co-channel reuse ratio?

Q = 1/√3N (B) Signup and view all the answers

Which channel assignment strategy allows a cell to use channels from a neighboring cell when all its own channels are occupied?

Fixed Channel Assignment (A) Signup and view all the answers

What must a system designer determine to optimize handoff operations?

An optimal signal level for initiating a handoff (C) Signup and view all the answers

What is the primary goal of handoff in a cellular system?

Ensure imperceptible transitions for the user (C) Signup and view all the answers

Which of the following statements is true regarding fixed channel assignment?

It results in more frequent call blocks. (C) Signup and view all the answers

What is the distance to the nearest co-channel cell represented as?

D = 7R (C) Signup and view all the answers

In dynamic channel assignment, what does MSC do upon receiving a call request?

Request that the BSC allocate a channel for the call. (C) Signup and view all the answers

What is the estimated radius of the cell when N=4?

R ≈ 6.92 (D) Signup and view all the answers

What is the primary limitation in the performance of cellular radio systems?

Interference (D) Signup and view all the answers

What is the relationship between co-channel interference and cluster size?

Smaller cluster size increases co-channel interference (C) Signup and view all the answers

What is the effect of increasing the ratio of DIR in a cellular system?

Improves isolation of RF energy from co-channel cells (C) Signup and view all the answers

Which of the following is a source of interference in cellular systems?

Multiple mobile users in the same cell (A) Signup and view all the answers

What occurs when a user moves away from a base station at a very slow speed?

Signal strength remains above the Ho threshold (C) Signup and view all the answers

What is adjacent channel interference?

Interference from signals adjacent in frequency (A) Signup and view all the answers

What mathematical expression is used to calculate the signal to interference ratio (S11) for a mobile receiver?

S11 = S/I = ΣJi / CDIR (B) Signup and view all the answers

How can co-channel interference be reduced effectively?

By increasing the physical separation of co-channel cells (D) Signup and view all the answers

What happens when the margin Δ is set too small for handoff firm?

Drop calls can occur due to excessive delays in handoff assignments. (B) Signup and view all the answers

Which factor is NOT relevant in determining the necessity of a handoff?

Current weather conditions. (D) Signup and view all the answers

What is the 'dwell time' in the context of cellular calls?

The maximum time a call can be maintained without handoff. (B) Signup and view all the answers

How does the guard channel concept prioritize handoff requests?

It allocates a fraction of total channels exclusively for handoffs. (A) Signup and view all the answers

What limitations does the guard channel concept impose?

Reduced available traffic capacity for originating calls. (D) Signup and view all the answers

What is a practical effect of high-speed vehicles on cell coverage?

They pass through a coverage region very quickly. (C) Signup and view all the answers

What condition leads to cell dragging?

Strong signals received by pedestrian users to the base station. (C) Signup and view all the answers

What technique provides coverage for both high-speed and low-speed users?

Umbrella cell approach. (B) Signup and view all the answers

Study Notes

Cellular Telephone System

Provides wireless connection to the PSTN (Public Switched Telephone Network)
Coverage area is divided into cells
Each cell has a low-power transmitter
Cells reuse frequencies to accommodate more users

Advantages

Accommodates numerous users in a large geographic area within a limited frequency spectrum
Provides high-quality service
Achieves high capacity by dividing the coverage area into smaller cells
All processing is uninterrupted, due to handoff mechanism

Limitations of Conventional Mobile System

Limited Service Capability:
- Large cells required high power transmitters
- Users had to re-initiate calls when moving into a new cell, leading to call drops
- Number of active users limited by the number of channels available
Poor Service Performance: Large numbers of subscribers created high blocking probability during busy hours
Insufficient Frequency Spectrum Utilization: Conventional systems did not utilize the spectrum effectively, as each channel could serve only one user at a time

Cellular Concept

Replaces large, high-power transmitters with multiple low-power transmitters (small cells)
Each small cell provides coverage to a specific portion of the service area

Cellular System Concepts

Frequency Reuse: Different cells reuse the same frequencies, but with proper spacing to avoid interference
- Hexagonal model is commonly used to represent cell structures for frequency reuse
Frequency Planning: A technique used to assign frequencies to cells to minimize interference

Performance Criteria

Voice Quality: Measured using Customer Motivation Scores (CMS)
Service Quality: Measured by grade of service, number of dropped calls, call waiting, call forwarding, and other special features

Frequency Reuse and Frequency Planning

Cell: A small geographical area
Base Station: Each cell has a base station with its allocated set of radio channels
Channels: Channels are assigned to cells in groups. Adjacent cells have different channel groups.

Channel Assignment Strategies

Fixed Assignment: Each cell is assigned a predetermined set of channels. Calls within a cell use unused channels. Blocked calls are not served.
Dynamic Assignment: Channels are not permanently assigned to each cell. The switching office assigns channels to calls as needed, thus avoiding the need to borrow channels from a neighbouring cell.

Handoff Strategies

Handoff: The automatic transfer of a call to a new cell/channel when the user moves between cells.
Important factors that drive handoff processes include the minimization of call interruption, quickness of transition between cells, and optimal user experience/quality.

Interference and System Capacity

Co-channel interference: Interference between signals from cells using the same frequencies
- Frequency reuse ratio is a factor used to manage co-channel interference
Adjacent channel interference: Resulting from signals adjacent in frequency to the desired signal
- Can be reduced by careful frequency separation between channels and using proper signal filtering techniques

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Description

This quiz explores key concepts in cellular network design, including cell splitting and sectoring techniques. Participants will learn about the advantages, limitations, and interference management strategies used in mobile communication. Test your knowledge of how these techniques enhance network capacity and performance in urban environments!