CNA 7.0 (CDMA : Comprendre la capacité du système et le rôle du SIR)

Introduction to CDMA Techniques

Overview of Digital Transmission and Multiple Access Techniques

• The video introduces the series focused on digital transmissions, specifically exploring fundamental concepts in wireless communications.

• It highlights the challenge of multiple users transmitting simultaneously on the same radio channel, emphasizing interference as a limiting factor for bandwidth efficiency.

• The discussion transitions to various multiple access techniques developed over generations, including FDMA, TDMA, and CDMA.

Understanding CDMA (Code Division Multiple Access)

• The video focuses on CDMA as a unique method allowing multiple users to share the same radio channel using distinct codes.

• Unlike traditional methods like FDMA and TDMA, all users transmit simultaneously on the same frequency band but utilize pseudo-random codes for signal spreading.

Mechanism of Signal Separation

• Signals from different users intertwine without mixing due to code correlation; using the correct code allows extraction of individual signals.

• This technique significantly enhances mobile communication capacity and quality by optimizing radio resource management.

Applications of CDMA

Role in Satellite Navigation Systems

• CDMA is crucial in satellite navigation systems such as GPS and Galileo, where each satellite transmits a unique coded signal for accurate location determination.

• This coding enables receivers to distinguish between simultaneous signals from multiple satellites, ensuring precise positioning even amidst numerous signal sources.

Impact on Mobile Communications

• In mobile networks, CDMA has revolutionized 3G technology by improving data rates and coverage to meet increasing user demands.

Capacity and Performance Metrics in CDMA

Interference-Limited Capacity

• The capacity in a CDMA system is not fixed like FDMA or TDMA; it depends on managing interference levels among users sharing the same bandwidth.

Signal-to-Interference Ratio (SIR)

• SIR is critical for maintaining service quality; it measures useful signal power against unwanted interference. A higher SIR indicates better reception quality.

System Capacity Calculation

• The maximum theoretical capacity can be quantified using the ratio of energy per bit (EB) to noise spectral density (N0), which influences how many users can effectively share a channel while maintaining acceptable service quality.

CDMA Technology and User Capacity

Understanding CDMA Capacity Limits

• The capacity of a CDMA system is significantly affected by the level of interference from other users. For instance, at a moderate threshold of around 3 decibels, the system can support approximately 55 to 60 users, while a more stringent threshold of 10 decibels reduces this capacity to about 10 users.

• A fundamental aspect of CDMA technology is that its capacity is not constrained by bandwidth but rather by the interference caused by simultaneous users. This highlights the trade-off between required quality (EB/N0) and the number of concurrent users.

Characteristics of CDMA Systems

• CDMA operates as a multiple access technique where all users share the same frequency band simultaneously, differentiated by their unique spreading codes. This method allows for flexible and robust spectrum utilization but introduces mutual interference among users, known as background interference.

• Unlike FDMA or TDMA techniques that partition spectrum into segments, CDMA's performance limitations arise from cumulative interference generated by other active users. Thus, its capacity directly correlates with the EB/N0 ratio necessary to maintain acceptable service quality.

Conclusion on Quality vs. User Count

• As the required EB/N0 threshold increases, user capacity decreases, illustrating an essential compromise between link quality and simultaneous user count. This conclusion sets up further discussions in upcoming videos regarding how CDMA codes are generated.