Processing Procedure of WCDMA System
- Source coding can increase the transmitting efficiency.
- Channel coding can make the transmission more reliable.
- Spreading can increase the capability of overcoming interference.
- Through the modulation, the signals will transfer to radio signals from digital signals.
Bit, Symbol, Chip
- Bit : data after source coding
- Symbol: data after channel coding and interleaving
- Chip: data after spreading
Processing Procedure of WCDMA System – Channel coding & Interleaving
The Wideband Code Division Multiple Access (WCDMA) system is a 3G mobile communication technology that utilizes CDMA principles. It employs a complex processing procedure involving multiple stages to facilitate efficient communication. Here’s an overview of the processing procedure in a WCDMA system:
1. Channel Coding:
– Source Encoding: The data to be transmitted is encoded using channel coding techniques like Turbo coding or Convolutional coding. This adds redundancy to the data, enabling error detection and correction at the receiver.
– Interleaving: The coded data is interleaved to spread out burst errors caused by fading and other impairments. Interleaving rearranges the data to reduce the impact of consecutive errors during transmission.
2. Spreading and Multiplexing:
– Symbol Mapping: The encoded and interleaved data is mapped onto complex symbols using Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM) techniques. Symbol mapping helps in efficient data representation.
– Spreading: The symbols are spread by multiplying them with a spreading code unique to each user. Spreading codes are typically orthogonal or pseudo-random sequences, allowing multiple users to share the same frequency band simultaneously.
– Channelization: The spread symbols are further separated into different channels using specific channelization codes. This enables multiple logical channels to coexist within the same frequency band.
3. Power Control:
– Closed-Loop Power Control: The base station measures the received signal quality from each mobile device and sends power control commands to adjust the transmission power of individual users. This ensures that all users transmit at appropriate power levels, compensating for differences in channel conditions and mitigating interference.
– Orthogonal Variable Spreading Factor (OVSF): In WCDMA, OVSF codes are used to orthogonalize the channels of different users. OVSF codes are binary codes of varying lengths that ensure minimal interference between different users, improving system capacity and reducing cross-talk.
5. Multipath Channel Equalization:
– Rake Receiver: At the receiver side, the Rake receiver is employed to mitigate the effects of multipath propagation. It combines multiple multipath components, each experiencing different delays and attenuations, to improve the received signal quality.
6. Signal Detection and Demodulation:
– Despreading: The received signal is despread using the same spreading code used for spreading at the transmitter. This removes the spreading effect and extracts the original symbols.
– Channel Estimation: Channel estimation techniques are employed to estimate the channel characteristics and mitigate the effects of fading and interference. This allows for better demodulation and decoding performance.
7. Channel Decoding:
– Channel Demapping: The demodulated symbols are mapped back into bits, reversing the symbol mapping process.
– Channel Decoding: The received bits undergo channel decoding, which includes error detection and correction. Techniques such as Turbo decoding or Viterbi decoding are employed to recover the original data from the encoded symbols.
8. Deinterleaving and Source Decoding:
– Deinterleaving: The decoded bits are deinterleaved, reversing the interleaving process.
– Source Decoding: The deinterleaved bits are finally decoded back to the original data, reversing the source encoding process.
The above steps represent a simplified overview of the processing procedure in a WCDMA system. It highlights the key stages involved in transmitting and receiving data, as well as the techniques employed to improve signal quality, mitigate interference, and ensure reliable communication.