Guide to UMTS Modulation, Spreading and Scrambling Techniques
UMTS uses various techniques like modulation, spreading, and scrambling to facilitate efficient data transmission. Modulation determines the signal format, spreading distributes the signal across a wider bandwidth, and scrambling adds an extra layer of security. Mastering these concepts helps in fine-tuning network parameters for better performance and reduced interference.
Modulation in UMTS
• UMTS uses Quadrature Phase Shift Keying (QPSK) for downlink transmission and Binary Phase Shift Keying (BPSK)
for uplink transmission.
• QPSK Modulation: Each symbol represents two bits, and phase shifts are used to differentiate between symbols,
allowing higher data rates with relatively good noise immunity.
• DPCCH and DPDCH are BPSK modulated.
Spreading in UMTS
• Spreading is the process of multiplying the original data with a higher-rate spreading code,
resulting in a spread spectrum signal with increased bandwidth.
• The spreading factor controls the bandwidth and data rate trade-off.
• Spreading provides robustness against interference and allows for multiple users
to share the same frequency band.
Scrambling in UMTS
• Scrambling is applied after spreading to further differentiate user data and reduce cross-correlation between signals.
• Each base station (Node B) uses a unique scrambling code, enabling UEs to identify signals from specific base stations.
• Scrambling does not change the bandwidth but provides additional layer of separation between users.
• Scrambling does not change the bit rate. It only helps in separating signals from different sources.
• Gold codes are used for this purpose as they have good correlation properties.
Long and short type of scrambling codes are available. In the uplink it separates different UEs
and in the downlink it s used for separation of cells/sectors.
Process Flow:
➨Data is first spread using the channelization code (spreading code).
➨The spread signal is then scrambled using the scrambling code.
➨Finally, the scrambled signal is modulated using QPSK or BPSK before transmission.
Synchronization using synchronization code:
These codes will have good auto correlation functionality hence are used for initial cell search and
synchronization in the downlink. Primary sync. codes are derived using Golay sequence and secondary words
from Hadamard sequence.
Conclusion
Understanding UMTS modulation, spreading, and scrambling techniques is essential for optimizing data transmission and ensuring high-quality service in 3G networks. It provides a solid foundation for addressing challenges related to signal integrity and transmission efficiency.