Cellular Networks and Frequency Reuse Concepts

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Questions and Answers

What is the formula used to calculate the cluster size N based on i and j?

N = i^2 + 2ij + j^2

N = i^2 + ij + j^2 (correct)

N = i^2 + j^2 - ij

N = i + j

In the context of fixed channel assignments, what happens when all channels in a cell are occupied?

The call is automatically prioritized above others.

Calls will be rerouted to a different network.

The channels become available after a certain time.

The call is blocked and no service is provided. (correct)

What is one characteristic of dynamic channel assignments?

Channels are permanently assigned to cells.

Channels are assigned whenever a call request occurs. (correct)

Channels are allocated only based on user priority.

Channels may be borrowed from neighboring cells.

What role does the Mobile Switch Center (MSC) play in fixed channel assignment strategies?

It supervises borrowing of channels from neighboring cells. Signup and view all the answers

Given i = 3 and j = 2, what is the calculated cluster size N?

19 Signup and view all the answers

What was one of the primary limitations of the first mobile radio-telephone services?

They could not share bandwidth effectively. Signup and view all the answers

Which organization proposed the cellular telephony concept in 1968?

Bell Labs Signup and view all the answers

What is the primary benefit of frequency reuse in cellular networks?

Higher capacity without increased bandwidth. Signup and view all the answers

What geometric shape is commonly used to represent the layout of cellular networks?

Hexagon Signup and view all the answers

What determines the total number of duplex channels (capacity) in a cellular system?

The cluster size N and the number of channels k. Signup and view all the answers

Which of the following statements about cluster size is true?

Larger clusters result in lower capacity but weaker interference. Signup and view all the answers

What is the frequency reuse factor in most current networks?

1 Signup and view all the answers

What does a higher value of N in cluster size typically indicate?

Weaker interference but potentially lower capacity. Signup and view all the answers

Which of the following correctly describes what happens when a decrease in cell size occurs?

Allows for a higher M, thus higher capacity. Signup and view all the answers

One of the effects of increased frequency reuse is...

Increased power requirements for mobiles. Signup and view all the answers

What characterizes a soft handoff?

Connection is made to a new base station before dropping the old one. Signup and view all the answers

What is the purpose of a guard channel in handoff prioritization?

To ensure some channels are always available for handoffs. Signup and view all the answers

Which of the following describes co-channel interference?

Interference from cells using the same set of frequencies. Signup and view all the answers

What is a significant source of interference in cellular radio systems?

Other mobiles in the same cell. Signup and view all the answers

What is a primary effect of interference on voice channels in cellular systems?

Cross talk between calls. Signup and view all the answers

What is the primary purpose of handover in cellular radio systems?

To maintain call quality when moving out of range Signup and view all the answers

Which type of handover is characterized by a mobile switching center (MSC) managing the process based on base station reports?

Network Controlled Handoff Signup and view all the answers

What could occur if the handoff threshold (Δ) is set too small?

Calls dropping due to inadequate handoff time Signup and view all the answers

In which scenario does intra-cell handover typically occur?

When a mobile device changes frequency due to interference Signup and view all the answers

What is a key characteristic of hard handoff?

Only allows connections to one base station at a time Signup and view all the answers

What role does the mobile device play in Mobile Assisted Handoff (MAHO)?

It measures surrounding signal strength and reports back Signup and view all the answers

Why is dynamic channel assignment considered more complex?

It must operate in real time to reduce call drops Signup and view all the answers

Which type of handoff allows for a break-before-make connection system?

Hard handoff Signup and view all the answers

What does a small co-channel reuse ratio (Q) indicate about cluster size and capacity?

Small cluster size and large capacity Signup and view all the answers

What is the effect of a large co-channel reuse ratio on transmission quality?

Improved transmission quality Signup and view all the answers

In terms of the formula $Q = \frac{D}{R} = \sqrt{3N}$, what does the variable N represent?

Number of co-channels Signup and view all the answers

What is the primary purpose of the trade-off in cellular design?

Balancing capacity with transmission quality Signup and view all the answers

Which factor primarily affects the SINR according to the information provided?

Propagation factor and distance Signup and view all the answers

How is co-channel interference expressed mathematically in relation to power?

$S/I = R^{-4}/[2(D-R)^{-4} + 2(D+R)^{-4} + 2D^{-4}]$ Signup and view all the answers

What causes adjacent channel interference?

Imperfect receiver filters allowing nearby frequencies to leak Signup and view all the answers

If the SINR is expressed as $\frac{S}{I} = \frac{P_r}{I + \sigma}$, what does I represent?

Interference power Signup and view all the answers

Study Notes

Cellular Concept

Developed by Bell Labs in the 1960s-70s
Divided geographical areas into cells
Cells can range from a few hundred meters in cities to tens of kilometers in the countryside
Each cell is served by a base station with a lower-power transmitter
Each cell receives a portion of the total available channels
Neighboring cells assigned different groups of channels to minimize interference

Frequency Reuse Concept

Adjacent cells are assigned different frequencies to prevent interference or crosstalk
Frequency reuse is essential for maximizing the network's capacity
Each cell is usually allocated between 10 to 50 frequencies
Transmission power is carefully controlled to minimize the signal's escape to adjacent cells
The number of cells required between two cells using the same frequency is a crucial consideration

Frequency Reuse

Each cell is allocated a group of channels (k)
A cluster contains N cells with unique and disjoint channel groups
N is typically 4, 7, or 12
Total number of duplex channels (S) is equal to kN
Clusters are repeated M times in a system
Total network capacity (C) is MkN or MS
Smaller cells lead to a higher M and C
Channel reuse allows for higher capacity and reduced power requirements for mobile devices
However, it also requires more base stations, frequent handoffs, and increased risk of 'hot spots'

Effect of Cluster Size (N)

Each cell within a cluster has unique channels, with repetition across clusters
Maintaining a consistent cell size:
- Large N: results in weaker interference but lower capacity
- Small N: leads to higher capacity but more interference, requiring a higher S/I level to maintain a desired signal quality
The frequency reuse factor is 1/N, meaning each cell within a cluster gets 1/N of the total available channels
Most current cellular networks have a frequency reuse factor of 1.

Designing the Cluster Size

Adjacent cells must connect without gaps to form a tessellation
N can be calculated using the formula: N = i2 + ij + j2, where i and j are non-negative integers
Example: i = 2 and j = 1, resulting in N = 7
To move within a cluster, move i cells along a chain or hexagon, then rotate 60 degrees counterclockwise and move j cells

Channel Assignment Strategies

Fixed Channel Assignments:
- Each cell is permanently allocated a specific set of voice channels
- If all channels in a cell are occupied, the call is blocked, and the subscriber receives no service
- A borrowing strategy allows a cell to borrow channels from neighboring cells if its own channels are occupied
- This is managed by the Mobile Switch Center (MSC), which connects cells to the wide area network, manages call setup, and handles mobility
Dynamic Channel Assignments:
- Voice channels are not permanently allocated to cells
- Each time a call request is made, the serving base station requests a channel from the MSC
- The switch then allocates a channel based on an algorithm considering factors like frequency reuse and cost
- Dynamic channel assignment is more complex but reduces the likelihood of blocking calls

Handover/Handoff

Reasons for Handoff:
- Moving out of range
- Load balancing
Handoff process involves interaction between the cell, Base Station Controller (BSC), and MSC
Handover scenarios:
- Intra-cell handover: Changing frequency due to narrowband interference
- Inter-cell, intra-BSC handover: Moving across cells in the same BSC
- Inter-BSC, intra-MSC handover: Moving across BSCs in the same MSC
- Inter-MSC handover: Moving across MSCs

Four Types of Handoff

Type 1: Mobile Station (MS) within a cell; only one Base Transceiver Station (BTS)
Type 2: MS moves between cells within the same Base Station Controller (BSC)
Type 3: MS moves between BSCs served by the same Mobile Switching Center (MSC)
Type 4: MS moves between MSCs (different switching areas)

Handoffs

Crucial for any cellular radio system
Goal is to make handoffs successful, infrequent, and undetectable to users
Involves identifying a new base station and allocating a channel in that base station
Handoffs have a higher priority than new call initiation requests (blocking new calls is preferred over dropping existing calls)

Handoff Threshold ( )

Represents the minimum acceptable signal strength to maintain a call
Choosing the right  is crucial:
- Too small: Insufficient time for handoff completion, leading to call drops & more frequent handoffs
- Too large: Too many handoffs, burdening the MSC

Styles of Handoff

Network Controlled Handoff (NCHO):
- Used in first-generation cellular systems
- Base stations continuously monitor signal strength from mobiles in their cells
- The MSC decides if handoff is needed based on the measurements
- Mobile plays a passive role
- Places a significant burden on the MSC
Mobile Assisted Handoff (MAHO):
- Implemented in second-generation systems
- Mobile measures received power from surrounding base stations and reports to the serving base station
- Handoff is triggered when power from a neighboring cell exceeds the current value by a threshold or for a certain duration
- Faster than NCHO because mobiles take measurements, reducing the MSC's load
Mobile Controlled Handoff: The mobile itself manages the handoff decision and process.

Types of Handoff

Hard Handoff:
- Used in FDMA and TDMA systems
- Mobile has a radio link with only one base station at a time
- The old base station connection is terminated before the new connection is established
Soft Handoff:
- Used in CDMA systems
- Mobile has multiple radio links with multiple base stations simultaneously
- The new base station connection is established before the old one is broken
- Mobile remains in this state until one base station becomes dominant
Vertical Handoff: This involves switching between different wireless technologies or network types.

Prioritizing Handoff

Dropping a call is more disruptive than a busy line
Guard Channel Concept:
- Reserves channels specifically for handoffs
- Wastes bandwidth but provides flexibility
- Can be dynamically managed based on network conditions
Queuing Handoff Requests:
- Takes advantage of the gap between handoff initiation and drop
- Balances call drop probability and network traffic
Strategies to Reduce Handoff Burden:
- Cell Dragging: Deferring a decision to handoff, allowing the mobile to move closer to the new base station
- Umbrella Cells: Larger, overlapping cells that serve as temporary coverage for mobile devices during handoff

Interference and System Capacity

Interference significantly impacts the performance of cellular systems
Sources of Interference:
- Other mobiles in the same cell
- Calls in progress in neighboring cells
- Other base stations operating on the same frequency band
- Non-cellular systems leaking energy into the cellular frequency band
Effects of Interference:
- Voice channels: Cross-talk (undesired signal mixing)
- Control channels: Missed or blocked calls
Two main types of interference:
- Co-channel interference
- Adjacent channel interference

Co-channel Interference

Cells using the same set of frequencies are called co-channel cells
Interference between these cells is called co-channel interference
Co-channel reuse ratio (Q): Q = D/R = sqrt(3N)
- R: Radius of a cell
- D: Distance between the nearest co-channel cells
Small Q: Small cluster size (N), resulting in high capacity
Large Q: Good transmission quality
A tradeoff must be made in cellular design to balance capacity and quality

Co-Channel Interference: SINR

Power: Pr = Pt (d/d0)-α
- Pr: Received power
- Pt: Transmitted power
- d: Distance between the transmitter and receiver
- d0: Reference distance
- α: Path loss exponent
Power (dBm): Pr (dBm) = Pt (dBm) - 10α log (d/d0)
SINR: (S/I) = S/ (σ2 + Σ Ii)
- S: Desired signal power
- I: Interfering signal power
- σ2: Noise power
- Σ Ii: Sum of interfering signals
Example with AMPS (Advanced Mobile Phone System):
- α = 4, S/I = 18 dB, N must be greater than 6.49 (resulting in a small 1/N reuse factor)
Relationships: Co-channel interference, link quality, reuse factor
Example 3.2 provides a detailed example of analyzing co-channel interference

Worst-Case Interference

In worst-case scenarios, the S/I can be estimated as: S/I ~ R-4 / [2(D-R)-4 + 2(D+R)-4 + 2D-4]

Adjacent Channel Interference

Interference from signals that are adjacent in frequency to the desired signal
Caused by imperfect receiver filters that allow nearby frequencies to leak into the passband

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Description

This quiz explores the foundational concepts of cellular networks, including the division of geographical areas into cells and the principles of frequency reuse. Learn how these concepts work together to enhance network capacity and minimize interference. Test your understanding of the technology developed by Bell Labs and its implications in telecommunications.