Mobil kommunikation

Question 1

Explain the main differences between contention-free and contention-based access protocols.

Answer 1

Contention free protocol:
In a contention free protocol:

• Each terminal sends packets using predetermined resources
• A controller scheduler coordinates the transmissions of different terminals.
• There is no contention or competitions
• Efficient in QoS guarantee, improved network throughput and reduced network response for heavy loaded networks

Contention based protocol:
In a contention based protocol

• The terminals have access to channels randomly when they have packets to send.
• Every user decides its channel access based on its own observations, thus leading to collisions

Advantages:
- Low complexity

• Inevitable package collisions
• Good performance in low traffic networks.

Question 2

Describe the hidden terminal and the exposed terminal problems. Furthermore, suggest ways to mitigate those problems.

Answer 2

Hidden terminal problem:
[A –> [B] <– C]

Both A and C sense that B is free because A can’t sense C and vice versa. Package collisions will occur when A and C sends at the same time

Exposed terminal problem:
[A <– [B C] –> D]

B wants to send to A and C to D, if B sends to A, C will sense that channel is busy, leading to half channel capacity

Can be solved with request to send/clear to send (RTS/CTS). They send RTS and if other terminal is ready they send back CTS

Question 3

Describe two scheduling algorithms (at your choice) and discuss potential advantages and disadvantages.

Answer 3

Proportional fair scheduling:
It’s designed to achieve a balance between fairness and efficiency in allocating radio responses.

It prioritises users with better channel conditions while still providing for users with poorer channel conditions.

Result is improved overall system throughput but does not guarantee strict fairness

MAX C/I scheduling:
It aims to maximise signal quality for users by selecting transmissions that provides the highest carrier to interference ratio.

Helps to mitigate interferences and improves overall capacity. but does not consider fairness

Question 4

What is a heterogeneous network? What are possible advantages and disadvantages from a heterogeneous network deployment?

Answer 4

A heterogenous network is a network consisting of multiple type of access points to give a seamless connectivity and good performance

Advantages:
1. Improved capacity

2. Enhanced network performance.
3. Cost effective

Disadvantages:
1. Increased complexity

2. Interference challenges

Question 5

Explain what handover is. Describe potential problems that can appear during a handover and describe what measures are taken in practice to improve handover’s performance.

Answer 5

A handover is whenever a terminal has to change its association from one base station to another due to the limited range of the base stations

Potential problems:
Late handover - Handover occurs too late

Premature handover - Handover occurs to early

Ping Pong effect - When a terminal switches associations frequently between base stations, which could cause large signal burden on the system.