Satellite CDMA Simulation Laboratory

ECE 514E - Code Division Multiple Access in Satellite Communications

Undergraduate Level

Electrical Engineering

Duration

2-3 hours

Required Knowledge

Digital Communications

Laboratory Introduction

Purpose of This Laboratory Session

This laboratory session introduces undergraduate electrical engineering students to the principles of Code Division Multiple Access (CDMA) in satellite communication systems. Through interactive simulations and theoretical explanations, you will explore how multiple users can share the same frequency band simultaneously using unique codes.

Learning Objectives

By the end of this laboratory session, students will be able to:

  • Explain the basic principles of CDMA in satellite communications
  • Differentiate CDMA from other multiple access techniques (FDMA, TDMA)
  • Understand how spreading codes enable multiple access
  • Analyze the relationship between number of users and interference in CDMA systems
  • Calculate signal-to-interference ratios in CDMA satellite links
  • Simulate a basic satellite CDMA system with multiple users

Background

Satellite communications require efficient use of limited bandwidth to serve multiple users. Code Division Multiple Access (CDMA) is a spread spectrum technique that allows multiple transmitters to send information simultaneously over a single communication channel. In satellite systems, CDMA provides advantages such as resistance to interference, increased capacity, and enhanced security.

In this lab, you will simulate a satellite CDMA system with multiple ground stations communicating with a single satellite. You'll adjust parameters like number of users, transmit power, and spreading factor to observe their effects on system performance.

CDMA Theory for Satellite Communications

Fundamental Principles

Code Division Multiple Access (CDMA) is based on spread spectrum technology where each user is assigned a unique spreading code. These codes are orthogonal or nearly orthogonal to each other, allowing multiple signals to occupy the same frequency band simultaneously.

In satellite CDMA systems:

  • Each ground station spreads its signal using a unique code
  • All signals are transmitted simultaneously in the same frequency band
  • The satellite receives the composite signal
  • Each receiver uses the correct code to despread and recover its intended signal

Spreading and Despreading Process

Step 1: Data Signal

Each user has a data signal with bit rate Rb (bps). For example: User 1: [1, -1, 1, 1]

Step 2: Spreading Code

Each user is assigned a unique spreading code with chip rate Rc = N × Rb, where N is the spreading factor. Example codes (N=8):

Step 3: Spread Spectrum Signal

The data signal is multiplied by the spreading code, creating a signal with wider bandwidth. This spreads the signal power over a larger frequency range.

Step 4: Composite Signal at Satellite

The satellite receives the sum of all spread signals plus noise: S(t) = Σ si(t) + n(t)

Step 5: Despreading at Receiver

The receiver multiplies the composite signal by the same spreading code used by the transmitter and integrates over the bit period to recover the original data.

Key Mathematical Relationships

Signal-to-Interference Ratio (SIR) in CDMA systems:

SIR = Pi / (Σj≠i Pj + N0B)

Where Pi is the power of the desired signal, Pj are powers of interfering signals, N0 is noise spectral density, and B is bandwidth.

The capacity of a CDMA satellite system can be approximated by:

M ≈ 1 + (Gp / (Eb/N0)) × (1 / (SIRreq))

Where M is number of users, Gp is processing gain (spreading factor), Eb/N0 is energy per bit to noise ratio, and SIRreq is required SIR.

Laboratory Equipment

Required Software/Tools

  • Web browser with JavaScript enabled
  • Calculator (physical or software)
  • Note-taking application or lab notebook

Virtual Equipment

  • CDMA Satellite Simulator (this application)
  • Signal visualization tools
  • Parameter adjustment controls

Safety Precautions

This is a virtual laboratory. However, when working with actual satellite communication equipment:

  • Always follow RF safety guidelines
  • Ensure proper grounding of equipment
  • Be aware of high voltage in amplifier systems
  • Use appropriate personal protective equipment

Satellite CDMA Simulation

This simulation demonstrates a satellite CDMA system with multiple ground stations communicating with a single satellite. Adjust the parameters below to see how they affect system performance.

System Parameters

Simulation Results

User 1: SIR = 15.2 dB
User 2: SIR = 12.8 dB
User 3: SIR = 10.1 dB
User 4: SIR = 8.5 dB

System Metrics

Total Capacity: 4 users

Processing Gain: 12 dB

Total Interference: 2.3 dBW

Performance Indicators

Average SIR: 11.7 dB

Min SIR: 8.5 dB

Max SIR: 15.2 dB

Simulation Observations

As you adjust the parameters, observe how the Signal-to-Interference Ratio (SIR) changes for each user. Note the following relationships:

  • Increasing the number of users typically decreases SIR due to increased interference
  • Higher spreading factors improve SIR by providing better interference rejection
  • Increased noise levels degrade all signals equally
  • Higher transmit power improves SIR but increases interference for other users

Experimental Procedure

Step 1: Baseline Simulation

Run the simulation with default parameters. Record the SIR values for each user and the average SIR.

Step 2: Vary Number of Users

Increase the number of users from 4 to 8. Observe how SIR changes. What happens when you exceed the theoretical capacity?

Step 3: Adjust Spreading Factor

Change the spreading factor to its minimum (4) and maximum (64) values. How does this affect system performance?

Step 4: Noise Effects

Increase noise level to 0.5. How does this affect the relationship between number of users and SIR?

Step 5: Power Control

Adjust transmit power to 5 dBW and then to 15 dBW. Does increasing power always improve performance for all users?

Data Recording

Record your observations in a table with columns for parameter settings and resulting SIR values.

Data Analysis & Questions

Analysis Questions

Question 1: CDMA vs. FDMA/TDMA

Based on your simulation results, what are the advantages and disadvantages of CDMA compared to Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) for satellite communications?

Question 2: Capacity Limitations

As you increased the number of users in the simulation, at what point did the system performance degrade significantly? How does this relate to the theoretical capacity formula presented in the theory section?

Question 3: Spreading Factor Impact

What effect did increasing the spreading factor have on the SIR? Explain this relationship in terms of processing gain and bandwidth expansion.

Question 4: Near-Far Problem

In your simulation, all users had equal transmit power. In a real satellite CDMA system, users at different distances from the satellite would have different path losses. How would this "near-far problem" affect system performance, and what techniques could mitigate it?

Question 5: Practical Implementation

Based on your simulation experience, what practical challenges would engineers face when implementing a CDMA satellite system, and how might they address these challenges?

Calculations

Using the data from your simulations, perform the following calculations:

  1. Calculate the theoretical capacity using the formula from the theory section with your simulation parameters.
  2. Determine the processing gain in dB for spreading factors of 16, 32, and 64.
  3. If the required SIR for acceptable performance is 10 dB, how many users could be supported with a spreading factor of 32 and transmit power of 12 dBW?

Laboratory Conclusions

Write a brief conclusion summarizing what you learned from this laboratory session about satellite CDMA systems. Include:

  • Key principles of CDMA operation in satellite communications
  • The relationship between number of users, spreading factor, and system performance
  • Advantages and limitations of CDMA for satellite applications
  • How this laboratory experience enhanced your understanding of multiple access techniques

References & Further Reading

Textbook References

1. Proakis, J. G., & Salehi, M. (2007). Digital Communications (5th ed.). McGraw-Hill. (Chapter 15: Spread-Spectrum Communications)

Satellite Communications

2. Pratt, T., Bostian, C., & Allnutt, J. (2003). Satellite Communications (2nd ed.). Wiley. (Chapter 9: Multiple Access)

CDMA Systems

3. Viterbi, A. J. (1995). CDMA: Principles of Spread Spectrum Communication. Addison-Wesley.

Online Resources

4. IEEE Xplore Digital Library: Search for "satellite CDMA" for recent research articles

5. ITU-R Recommendations: P.618 (Propagation data for satellite services)

Lab Report Guidelines

Your lab report should include:

  • Title page with experiment name and date
  • Introduction and objectives
  • Theoretical background
  • Simulation procedure and parameters
  • Results with tables and analysis
  • Answers to all questions
  • Conclusions and lessons learned

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