How Data Transmission in UL in LTE?

In UL, the UE has no dedicated resources to transmit directly when new data arrived in the buffer from higher layer.
It requests resources to transmit them.
It receives radio resources.
It transmits them.

The scheduling request is sent on the Physical Uplink Control Channel (PUCCH).

This channel carries radio signaling in uplink:

Scheduling Request to grant resources in UL
Radio measurement report from the UEs

Radio retransmission ack or nack

The UE can know from the SIB how to use the PUCCH.

The response, the UL grant, is sent on the PDCCH.

The Physical Uplink Share Channel (PUSCH) carries:

User data
User Signaling
The resources are dynamically assigned in time and frequency.
The PUSCH supports the H-ARQ.

When the UE transmits in UL, it transmits also UL reference signals.

Data demodulation reference signal (DM-RS)
Sent with each packet transmission in order to demodulate data
Occupies center SC-FDMA symbol of the slot, only sent over bandwidth allocated for data transmission

Sounding reference signal (SRS)

Used to sound uplink channel to support frequency selective scheduling
SRS parameters are UE specific and configured semi-statically
1 symbol in subframe used for SRS
Periodicity: {2, 5, 10, 20, 40, 80, 160, 320} ms
Bandwidth: typically transmitted over the entire PUSCH
bandwidth (does not include PUCCH region)
SRS is not sent when there is a scheduling request (SR) or CQI to be sent on PUCCH (to avoid multicarrier transmission)

How Data Transmission in UL (Uplink) Works in LTE?

Data transmission in the Uplink (UL) direction in LTE refers to the process of sending data from the User Equipment (UE) to the evolved NodeB (eNB). The UL is a critical part of the LTE system, handling services like voice, text, and data uploads from the device to the network.

Steps of Data Transmission in UL

1. Data Preparation: The data to be sent from the UE is prepared by the higher layers of the LTE protocol stack, including the PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), and MAC (Medium Access Control). The data is then passed to the MAC layer for scheduling.
2. Resource Scheduling: The eNB uses the UL scheduler to allocate radio resources to the UE. The scheduler determines the time and frequency resources based on factors like the UE’s channel quality and priority. The UL scheduler uses feedback such as CQI (Channel Quality Indicator) from the UE to make these decisions.
3. Transport Block Transmission: The data is transmitted in the form of transport blocks. These blocks are created at the MAC layer and are sent to the physical layer for transmission over the radio interface. Depending on the available resources, the size of these transport blocks may vary.
4. HARQ (Hybrid Automatic Repeat Request): In case of transmission errors, HARQ mechanisms ensure that the data is retransmitted. The eNB sends an acknowledgment (ACK) or negative acknowledgment (NACK) based on whether the data was received correctly. If a NACK is received, the data is retransmitted for better reliability.
5. Data Reception at the eNB: The data is received at the eNB, where it is processed and routed through the core network to its final destination (e.g., the internet or another user). The eNB also handles any necessary error correction using HARQ.

Why UL Data Transmission is Important

The Uplink is essential for sending data from devices back to the network. It supports a wide range of services like voice calls, social media uploads, and video conferencing. Efficient UL data transmission ensures that users can upload data quickly and reliably without affecting the overall performance of the network.