Understanding CP-OFDM Transmitter with DFT-Spreading in LTE and 5G NR
Understanding the CP-OFDM Transmitter with Optional DFT-Spreading in LTE and 5G
Modern wireless technologies like LTE and 5G New Radio (NR) use Orthogonal Frequency Division Multiplexing (OFDM) to maximize spectrum efficiency and ensure speedy data transmission. Specifically for uplink transmissions, there's a variation called DFT-Spread OFDM (DFT-s-OFDM), which blends the strengths of Single Carrier (SC) and Multi-Carrier (MC) modulation. This combination boosts power efficiency and helps lower the Peak-to-Average Power Ratio (PAPR).
The diagram above illustrates the transmitter block diagram for CP-OFDM with optional DFT-Spreading — a straightforward yet effective way of showing how signals are prepared and sent out in LTE and 5G networks.
Let’s break down each block step-by-step so we can see how it all works and why DFT-spreading is such a vital part of today’s wireless communication.
Overview: CP-OFDM and DFT-Spreading
Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM)
CP-OFDM is a popular modulation method that splits available bandwidth into several orthogonal subcarriers, each carrying a piece of user data. To prevent inter-symbol interference from multipath propagation, a cyclic prefix (CP) is added.
The Transmitter Block Diagram Explained
The diagram displays four major blocks:
Transform Precoding (optional)
Subcarrier Mapping
Inverse Fourier Transform (IFT or IFFT)
Cyclic Prefix (CP) Insertion
Each of these stages is crucial for how we generate and shape the signal before it gets transmitted.
Transform Precoding (Optional DFT-Spreading)
What It Does:
The Transform Precoding block carries out a Discrete Fourier Transform (DFT) on the incoming symbols. This step spreads the user data symbols over multiple subcarriers, making sure that each transmitted sample represents a mixture of all the data symbols, not just one.
Why It’s Important:
It cuts down on PAPR, enabling power amplifiers in mobile devices to run more efficiently.
It keeps the frequency diversity and robustness associated with OFDM.
It provides a single-carrier-like signal structure in the time domain.
When It’s Used:
Optional in uplink (UL) — present in LTE uplink and optionally in 5G NR uplink.
Not applied in downlink (DL) since base stations have more powerful amplifiers and don’t need to limit PAPR.
Mathematical Step:
If the input data symbols are x[n], after the DFT operation we get:
X[k]=∑n=0N−1x[n]e−j2πkn/N
These transformed symbols X[k] are then mapped to subcarriers.
Subcarrier Mapping
Once the DFT-spreading (if it was done) is completed, the transformed data is mapped to specific subcarriers within the OFDM spectrum.
Purpose:
Assigns user data to specific Resource Blocks (RBs).
Makes sure each user occupies only its assigned chunk of the frequency domain.
Maintains orthogonality among different users in a multi-user setup.
Types of Mapping:
Localized Mapping: Adjacent subcarriers are given to the same user.
Distributed Mapping: Subcarriers are spaced out for frequency diversity.
Example:
If a user is allocated 12 subcarriers per Resource Block (RB), those subcarriers will be organized in frequency before moving on to the IFFT stage to transition into the time domain.
Inverse Fourier Transform (IFT / IFFT)
The Inverse Fast Fourier Transform (IFFT) translates frequency-domain symbols (from subcarrier mapping) into time-domain samples. This step converts discrete frequency data into a continuous time waveform that’s ready for transmission.
Key Functions:
Combines all subcarriers into one composite time-domain signal.
Preserves orthogonality between subcarriers.
Produces the actual waveform that gets sent over the air.
Mathematical Expression:
x(t)=∑k=0N−1X[k]ej2πkt/T
Here, X[k] represents the subcarrier symbols, and T is the OFDM symbol duration.
Output:
The IFFT output creates an OFDM symbol in the time domain, composed of contributions from all assigned subcarriers.
Cyclic Prefix (CP) Insertion
Purpose:
Before the signal gets transmitted, a Cyclic Prefix (CP) is attached to each OFDM symbol. The CP is essentially a copy of the end part of the symbol, which is put at the beginning.
Benefits:
Eliminates Inter-Symbol Interference (ISI) that can happen due to multipath propagation.
Changes the linear convolution of the channel into a circular one, making frequency-domain equalization easier.
Offers a guard interval between consecutive symbols.
Types of CP:
Normal CP: Used for most LTE and 5G operations.
Extended CP: Suitable for larger cell sizes or areas with long delay spreads.
Example:
If an OFDM symbol lasts 66.7 μs, a CP of 4.7 μs could be added, bringing the total symbol duration to 71.4 μs.
Output: Transmitted Signal
After adding the CP, the final time-domain signal is set for transmission through the antenna. In uplink, this signal moves from the User Equipment (UE) to the base station (gNB or eNodeB). In the downlink, the same principles, minus the DFT-spreading, are applied, sending signals from the base station back to the UE.
Advantages of DFT-s-OFDM (Transform Precoded OFDM)
Feature Benefit Low PAPR Makes better use of power amplifiers in UEs. High Spectral Efficiency Ensures orthogonality and full use of bandwidth. Single-Carrier Characteristics Reduces distortion in power-limited transmitters. Robustness Works well in fading channels that vary in frequency. Backward Compatibility Easily integrates with existing LTE and 5G systems.
In 5G New Radio (NR):
Uplink: Both CP-OFDM and DFT-s-OFDM are supported.
Usually, DFT-s-OFDM is utilized for the PUSCH (Physical Uplink Shared Channel).
CP-OFDM may be used for PUCCH or SRS transmissions.
Downlink: Exclusively employs CP-OFDM for all physical channels.
This flexibility allows 5G to optimize based on different device capabilities and deployment needs.
Conclusion
The CP-OFDM transmitter with optional DFT-Spreading is fundamental to the design of modern wireless communication uplinks. By integrating Transform Precoding, Subcarrier Mapping, IFFT, and Cyclic Prefix Insertion, networks find a balance between spectral efficiency, power efficiency, and signal robustness.
While CP-OFDM is the go-to for downlink due to its simpler transmitter design, DFT-s-OFDM remains essential for uplink transmission, ensuring efficient performance for billions of mobile devices worldwide.
Understanding how this transmitter chain operates equips telecom professionals with insights into how LTE and 5G systems deliver reliable, high-speed communication, even under tough real-world conditions.