LTE : Data Transmission in DL
In Long-Term Evolution (LTE), data transmission in the Downlink (DL) involves the transfer of user data from the Evolved NodeB (eNB) to the User Equipment (UE). The LTE DL is designed to provide high data rates, low latency, and efficient spectrum utilization. Here’s an overview of the key aspects of data transmission in the DL:
- Logical Channels:
- Traffic Channels: Carry user data and are divided into Dedicated Traffic Channels (DTCH) and Shared Traffic Channels (STCH). DTCH is dedicated to a specific UE, while STCH is shared among multiple UEs.
- Control Channels: Carry control information. Examples include the Broadcast Control Channel (BCCH) and the Paging Control Channel (PCCH).
- Transport Channels:
- Dedicated Transport Channels (DTCH): Used for the transmission of user data between the eNB and a specific UE.
- Shared Transport Channels (STCH): Used for the transmission of user data that is shared among multiple UEs.
- Physical Channels:
- Physical Downlink Shared Channel (PDSCH): Transmits user data and control information to UEs in the cell.
- Physical Downlink Control Channel (PDCCH): Carries control information related to resource allocation and scheduling for UEs.
- Resource Blocks:
- LTE uses resource blocks (RBs) as the basic unit for resource allocation. Each RB consists of a certain number of subcarriers in the frequency domain and a certain number of symbols in the time domain.
- PDSCH is mapped onto one or more resource blocks for data transmission.
- Modulation and Coding:
- The data transmitted on PDSCH is modulated using Quadrature Amplitude Modulation (QAM). The modulation scheme and coding rate are determined based on the channel conditions to maximize data rate and reliability.
- MIMO (Multiple Input Multiple Output):
- LTE supports multiple antenna configurations for the DL, such as Multiple Input Single Output (MISO) and Multiple Input Multiple Output (MIMO). MIMO enhances the system capacity and reliability by using multiple antennas at both the eNB and UE.
- Transmission Modes:
- LTE supports different transmission modes, including Single-Input Single-Output (SISO), Transmit Diversity, Open-Loop MIMO, and Closed-Loop MIMO. These modes adapt the transmission strategy based on channel conditions and the capabilities of the UE.
- Scheduling and Resource Allocation:
- The eNB dynamically schedules and allocates resources to UEs based on their channel conditions, Quality of Service (QoS) requirements, and system load. This ensures efficient use of available resources.
- Harq (Hybrid Automatic Repeat reQuest):
- HARQ is used for error correction. If a UE detects errors in received data, it can request retransmission. HARQ combines Automatic Repeat reQuest (ARQ) with error correction coding for more efficient error recovery.
- Beamforming:
- LTE supports beamforming techniques to enhance DL performance by focusing the transmitted signal in the direction of the intended UE, reducing interference and improving signal quality.
In summary, LTE’s Downlink data transmission involves the use of logical channels, transport channels, and physical channels, along with techniques like modulation, MIMO, and resource allocation to achieve high data rates and efficient spectrum utilization. The system adapts dynamically to varying channel conditions and user requirements to provide optimal performance.
- The data in DL are sent on the PDSCH
- A minimum of 2 RBs are allocated by the eNodeBs.
- A RB is 12 sub-carriers on a slot (7 symbols).
- The eNodeB sends on the PDCCH the required information to allow the UE
decode the data on the PDSCH. - On which slot?
- On which Resource blocks?
- How are the data modulated?
The Physical Downlink Shared Channel (PDSCH) carries:
- User data
- User signaling
- Paging messages
- System information message
The PDSCH can use all the resources not used by the other channel:
- Synchronization channels
- Reference signal
- PDCCH
- PBCH
- PCFICH