UL Allocation (SC-FDMA) in LTE
- SC-FDMA utilizes single carrier modulation, DFT-spread orthogonal frequency multiplexing, and frequency domain equalization.
- LTE uses OFDMA on the DL. It allocates DL bandwidth to the user based on resource blocks, rather than individual subcarriers.
- When the signal is a combination of subcarriers, high amplitude power peaks can occur. The base stations power amplifiers are designed to handle the resulting high peak-toaverage power ratio.
- Although the eNodeB is fully capable of transmitting and receiving OFDMA symbols, the subscriber units must balance reduced complexity and lower transmit power requirements with support for high data rates and good QoS at cell boundaries. OFDMA is not used on the UL.
- Instead, SC-FDMA was selected for the UL to meet those requirements. SC-FDMA helps with peak-to-average power ratio
reduction by adding extra encoding steps. - In OFDMA, each data stream is transmitted in a separate subcarrier. However, with SCFDMA,
multiple data streams are transmitted in the same subcarrier and symbol time. - The “sub-symbols” are spread over multiple subcarriers. Each SC-FDMA symbol
contains 12 sub-symbols transmitted at the rate of 12 times 15 KHz. The following
diagram illustrates this.
OFDMA vs. SC-FDMA Subcarrier Coding
- Unlike OFDMA, the SC-FDMA signal appears to be more like a single carrier, with each data symbol represented by a wide signal spanning the 12 subcarriers of the resource block.
UL Allocation (SC-FDMA) in LTE
In LTE, the uplink (UL) transmission is achieved using Single-Carrier Frequency Division Multiple Access (SC-FDMA). SC-FDMA is used in the LTE uplink to provide efficient use of resources and to improve power efficiency, especially for mobile devices. Here’s how SC-FDMA works in UL allocation:
SC-FDMA Technology: SC-FDMA is a variation of OFDMA (used in the downlink), but it operates with a single carrier. It helps in reducing the peak-to-average power ratio (PAPR), which is important for mobile devices with limited power resources, ensuring better battery life.
Uplink Resource Allocation: In LTE, resources are allocated dynamically depending on the demand and network conditions. The eNodeB (evolved Node B) is responsible for allocating Resource Blocks (RBs) to the User Equipment (UE) for uplink transmission. These RBs consist of time slots and subcarriers, where SC-FDMA is used to map data over these physical resources.
Modulation and Coding: Depending on the channel conditions, SC-FDMA in the uplink can use different modulation schemes such as QPSK, 16-QAM, or 64-QAM. The network adapts the modulation and coding to ensure efficient transmission, considering the quality of the radio link.
Uplink Scheduling: The eNodeB schedules the uplink transmissions based on feedback from the UE, including channel quality indicators (CQI) and scheduling requests. The scheduler then allocates resources like RBs based on these inputs, ensuring efficient use of available resources and optimizing data throughput.
In summary, SC-FDMA in LTE uplink allocation ensures power-efficient transmission, reduces interference, and improves overall performance. Dynamic resource allocation and adaptive modulation help optimize data transfer, enhancing the overall network efficiency.