Telecom Techniques Guide


Purpose of Channelization Code & Scrambling Code in WCDMA

Channelization Codes in WCDMA:

Channelization codes, also known as Walsh codes, serve a critical purpose in WCDMA (Wideband Code Division Multiple Access) networks. These codes are used for the spreading of user data over a wide frequency band, which is a fundamental principle of CDMA-based systems like WCDMA. The primary purpose of channelization codes is to differentiate and separate multiple users’ transmissions in both the forward and reverse links.

In the forward link, channelization codes are employed to distinguish between different users’ data streams transmitted simultaneously over the same frequency band. Each user is assigned a unique code, allowing multiple users to share the same frequency resources without interfering with each other. This spreading and separation of data are vital for achieving the spectral efficiency and capacity gains that are characteristic of CDMA systems. In the reverse link, channelization codes help the base station to identify and demodulate the signals from various mobile devices, enabling the network to receive and process multiple user transmissions simultaneously. In essence, channelization codes are the cornerstone of the CDMA concept in WCDMA, enabling efficient and simultaneous communication among multiple users.

Purpose of WCDMA Channelization Code

  • Channelization code is used to distinguish different physical channels of one transmitter
  • For downlink, channelization code ( OVSF code ) is used to separate different physical channels of one cell
  • For uplink, channelization code ( OVSF code ) is used to separate different physical channels of one UE
  • For voice service (AMR), downlink SF is 128, it means there are 128 voice services maximum can be supported in one WCDMA carrier;
  • For Video Phone (64k packet data) service, WCDMA downlink SF is 32, it means there are 32 voice services maximum can be supported in one WCDMA carrier.

Scrambling Codes in WCDMA:

Scrambling codes play a crucial role in WCDMA networks, primarily in the uplink or reverse link direction. These codes are applied after the spreading process and serve several important purposes.

Firstly, scrambling codes are used to randomize user data before transmission. This randomization enhances the security of the communication since the transmitted signal appears as noise to unintended receivers, making it difficult for eavesdroppers to decipher the information being transmitted.

Secondly, scrambling codes help mitigate the near-far problem, a common challenge in CDMA systems where a mobile device closer to the base station may transmit at a much higher power level than a more distant device. By applying different scrambling codes to users’ data, the base station can distinguish between the varying signal strengths, ensuring fair and reliable reception of signals from all devices, regardless of their proximity to the base station.

Purpose of WCDMA Scrambling Code

  • Scrambling code is used to distinguish different transmitters
  • For WCDMA downlink, scrambling code is used to separate different cells in one carrier
  • For WCDMA uplink, scrambling code is used to separate different UEs in one carrier

In addition to spreading, part of the process in the transmitter is the scrambling operation. This is needed to separate terminals or base stations from each other.

In summary, both channelization codes and scrambling codes are pivotal in WCDMA networks. Channelization codes enable efficient differentiation and separation of user data, while scrambling codes enhance security and address signal strength imbalances, contributing to the overall functionality and reliability of the network.

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