UMTS Multirate Virtual Laboratory

Adaptive Multi-Rate (AMR) Codec Simulation for 3G Communications

Undergraduate Communication Engineering 3G/UMTS Technology

Laboratory Objectives

Primary Learning Outcomes

  • Understand the concept of Adaptive Multi-Rate (AMR) speech coding in UMTS networks
  • Analyze the trade-off between speech quality and network capacity in variable rate coding
  • Examine the eight AMR codec modes and their corresponding bit rates (4.75 - 12.2 kbps)
  • Investigate the effects of channel conditions on codec mode selection

Technical Skills

  • Simulate AMR codec operation under different radio channel conditions
  • Calculate speech quality metrics (MOS scores) for different bit rates
  • Analyze bandwidth requirements and network capacity implications
  • Understand C/I (Carrier-to-Interference) ratio impact on codec selection

Expected Outcomes

Upon completion of this laboratory, students will be able to explain how UMTS networks dynamically adjust speech coding rates to maintain voice quality while maximizing system capacity. Students will understand the relationship between radio link quality, codec mode selection, and end-user perceived speech quality.

Theoretical Background

UMTS AMR Speech Codec

The Adaptive Multi-Rate (AMR) codec is the mandatory speech codec for UMTS (Universal Mobile Telecommunications System) networks. It was standardized by 3GPP to provide high-quality speech transmission while efficiently utilizing radio resources.

AMR is an ACELP (Algebraic Code Excited Linear Prediction) based codec that operates at eight different bit rates between 4.75 kbps and 12.2 kbps. The codec adapts its rate based on radio channel conditions and network capacity requirements.

Key Features:

  • • ACELP (Algebraic Code Excited Linear Prediction) algorithm
  • • 8 different source coding modes
  • • 20 ms speech frames (160 samples at 8 kHz)
  • • Channel coding with 1/3 rate convolutional code
  • • Voice Activity Detection (VAD) and Discontinuous Transmission (DTX)

AMR System Architecture

Speech VAD Detection AMR Encoder (ACELP) Mode Control Channel Coding Air

Laboratory Procedure

1

Study AMR Codec Modes

Review the eight AMR codec modes and their characteristics. Note the source coding rates, total bit rates including channel coding, and expected MOS scores for each mode.

2

Set Channel Conditions

In the simulation panel, adjust the C/I (Carrier-to-Interference) ratio slider to simulate different radio channel conditions. Observe how the recommended codec mode changes based on the channel quality.

3

Select Codec Mode

Manually select different codec modes (4.75 kbps to 12.2 kbps) and observe the effects on:

  • Speech quality (MOS score)
  • Bandwidth utilization
  • Network capacity
  • Frame structure visualization

4

Analyze Quality vs. Capacity Trade-off

Use the calculator to determine how many users can be supported at each codec mode given a fixed bandwidth. Plot the relationship between speech quality (MOS) and system capacity.

5

Simulate Rate Adaptation

Enable automatic rate adaptation and observe how the codec switches between modes as you vary the channel conditions. Record the switching thresholds and hysteresis behavior.

6

Document Results

Record all observations, calculations, and plots in your lab report. Include analysis of the optimal codec mode selection strategy for different deployment scenarios.

Time Allocation

30 min
Theory Review
60 min
Simulation & Analysis
30 min
Report Writing

AMR Codec Simulation

Channel Conditions

-5 dB (Poor) 25 dB (Excellent)
Auto Adaptation

Select Codec Mode

12.2 kbps
Mode 0
Best Quality
10.2 kbps
Mode 1
7.95 kbps
Mode 2
7.4 kbps
Mode 3
6.7 kbps
Mode 4
5.9 kbps
Mode 5
5.15 kbps
Mode 6
4.75 kbps
Mode 7
Lowest Rate

Real-time Metrics

Speech Quality (MOS) 4.14
Total Bit Rate 24.4 kbps
Frame Size
488 bits
Users per MHz
~82

Frame Structure Visualization

Class A (Critical)
Class B
Class C

Quality vs Bit Rate

Speech Signal Representation

0 ms 10 ms 20 ms

Network Capacity Calculator

System Parameters

10%

Calculation Results

~164
Simultaneous Users
~33
Users per MHz
3.3
Erlangs/MHz
4.14
Expected MOS

Capacity Comparison Chart

Capacity vs Quality Comparison

Codec Mode Source Rate Total Rate MOS Score Users (5 MHz) Relative Capacity

Laboratory Report Guidelines

Required Report Sections

  1. 1 Title Page: Course name, experiment title, student name, ID, date
  2. 2 Objectives: List the learning objectives of this laboratory
  3. 3 Theory: Brief explanation of AMR codec and rate adaptation
  4. 4 Equipment/Software: UMTS Multirate Virtual Laboratory
  5. 5 Procedure: Step-by-step description of experiments performed
  6. 6 Results: Tables, graphs, and screenshots from simulation
  7. 7 Discussion: Analysis and interpretation of results
  8. 8 Conclusion: Summary of key findings
  9. 9 References: Citations and bibliography

Data to Include

  • Table of all 8 AMR modes with bit rates and MOS scores
  • Graph of MOS vs. Codec Mode
  • Graph of System Capacity vs. Speech Quality
  • C/I ratio thresholds for mode switching
  • Frame structure diagrams for at least 3 modes
  • Network capacity calculations
  • Comparison of fixed-rate vs. adaptive-rate systems

Discussion Questions

  1. 1. Why does AMR use different protection levels for Class A, B, and C bits?
  2. 2. Explain the trade-off between speech quality and system capacity.
  3. 3. How does the rate adaptation mechanism improve network performance?
  4. 4. Compare AMR with fixed-rate codecs (G.711, GSM-FR).
  5. 5. What are the advantages of ACELP algorithm in AMR?

Grading Rubric

Theory Understanding 20%
Simulation Results 30%
Data Analysis 25%
Discussion Quality 15%
Presentation 10%

Important Notes

  • • All graphs must be properly labeled with titles, axis labels, and units
  • • Include screenshots of the simulation with your student ID visible
  • • Show all calculations step-by-step in the results section
  • • Use IEEE citation format for references
  • • Report should be 8-12 pages including figures and tables
  • • Submit both PDF and printed copy (if required by instructor)