Terminology » Mobile Communication » WCDMA DPCH: Downlink Dedicated Physical Channel Explained

WCDMA DPCH: Downlink Dedicated Physical Channel Explained

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This page covers the Downlink Dedicated Physical Channel (DPCH). The DPCH is a key component in the WCDMA/UMTS system for transmitting dedicated data to individual users.

There’s only one type of downlink dedicated physical channel in WCDMA, simply called the Downlink Dedicated Physical Channel (downlink DPCH).

The downlink DPCH carries dedicated data originating from Layer 2 and above – specifically, the dedicated transport channel (DCH). This data is time-multiplexed with Layer 1 control information. This control information includes:

  • Known pilot bits (for channel estimation and synchronization)
  • TPC commands (Transmit Power Control, for adjusting uplink power)
  • An optional TFCI (Transport Format Combination Indicator, to signal the data format)

WCDMA downlink DPCH

DPCH Characteristics

Here’s a breakdown of the key characteristics of the WCDMA downlink DPCH:

  • Channel Coding: Employs CRC (Cyclic Redundancy Check) for error detection and either convolutional, or Turbo codes for error correction, selected based on the required Quality of Service (QoS) on DPDCH(Dedicated Physical Data Channel).
  • Symbol Rate: Supports a range of symbol rates: 7.5, 15, 30, 60, 120, 240, 480, and 960 ksps (kilo-symbols per second).
  • Spreading Factor: Uses Orthogonal Variable Spreading Factor (OVSF) codes with spreading factors of 4, 8, 16, 32, 64, 128, 256, and 512. A lower spreading factor means a higher data rate and vice-versa.
  • Modulation: Utilizes Quadrature Phase Shift Keying (QPSK) modulation.
  • Spreading: Achieved using OVSF codes to maintain orthogonality between users.
  • Scrambling: Applies a Gold sequence with a chip rate of 3.84 Mcps (Mega-chips per second) and a period of 10 ms to distinguish between different base stations.
  • Power Control Period: Power control is performed every 0.625 ms.
  • Pilot Symbols: Included in the DPCH for coherent demodulation.
  • TFCI Bits: Included to indicate the transport format combination.
  • TPC Bits: Included to provide transmit power control commands to the UE (User Equipment).