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DL-SCH (Downlink Shared Channel)

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The Downlink Shared Channel (DL-SCH) is a channel in the Long-Term Evolution (LTE) wireless communication system that is responsible for transmitting downlink data from the base station to the user equipment. In LTE, the DL-SCH is the main data-bearing channel and is used to transmit both user data and control information.

In this article, we will discuss the DL-SCH in detail, including its structure, modulation and coding scheme, and transmission process.

DL-SCH Structure:

The DL-SCH is a shared channel, which means that multiple user equipment (UE) can receive data simultaneously on the same channel. The channel is divided into time-frequency resources called Resource Blocks (RBs), each consisting of 12 subcarriers and one slot in the time domain. Each slot is 0.5ms long and consists of 7 or 14 OFDM symbols, depending on the cyclic prefix length.

The DL-SCH uses the Hybrid Automatic Repeat Request (HARQ) protocol to improve the reliability of data transmission. The HARQ protocol allows the receiver to request retransmission of lost or corrupted data, which reduces the number of data packets that need to be retransmitted, thereby improving the overall efficiency of the system.

Modulation and Coding Scheme:

The DL-SCH uses a variety of modulation and coding schemes (MCS) depending on the channel conditions and the required data rate. The MCS determines the modulation scheme (QPSK, 16QAM, 64QAM, etc.) and the coding rate of the data transmitted on the channel.

Higher modulation schemes provide higher data rates, but are more susceptible to errors in the presence of noise and interference. Therefore, the MCS is selected based on the channel quality measured at the UE.

The coding rate determines the amount of redundancy added to the data to enable error correction. The coding rate is inversely proportional to the redundancy, meaning that higher coding rates provide higher redundancy and better error correction capabilities.

Transmission Process:

The transmission process of the DL-SCH can be divided into four main stages: Channel Encoding, Rate Matching, Modulation and Layer Mapping.

  1. Channel Encoding: The data to be transmitted on the DL-SCH is first encoded using a turbo encoder. The turbo encoder adds redundancy to the data to enable error correction. The turbo encoder uses a convolutional encoder followed by an interleaver and a second convolutional encoder to generate the redundant data.
  2. Rate Matching: The output of the turbo encoder is then rate-matched to match the number of bits to be transmitted with the available channel resources. The rate matching process involves selecting a sub-set of the encoded bits and mapping them onto the available RBs.
  3. Modulation: The rate-matched data is then modulated using the selected MCS. The modulation process involves mapping the bits onto the modulation symbols, which are then transmitted on the channel.
  4. Layer Mapping: The modulation symbols are then mapped onto the layers of the physical channel. The number of layers used depends on the channel conditions and the UE capabilities. For example, a UE with multiple receive antennas can receive data on multiple layers, thereby improving the data rate and reliability.

Conclusion:

In conclusion, the DL-SCH is a critical channel in the LTE wireless communication system that is responsible for transmitting downlink data from the base station to the user equipment. The channel uses a variety of modulation and coding schemes to provide high data rates and reliable data transmission. The HARQ protocol is used to improve the reliability of data transmission by allowing for retransmission of lost or corrupted data. The DL-SCH is a shared channel that allows multiple UEs to receive data simultaneously on the same channel, thereby improving the efficiency of the system.