*Question E8B08:
What describes orthogonal frequency-division multiplexing (OFDM)?*  

*Correct Answer is  D: A digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference*  

### AI Response: (may contain errors)  

Orthogonal Frequency-Division Multiplexing (OFDM) is a digital modulation technique that effectively utilizes multiple subcarriers to transmit data. Let's break down the correct answer and the other choices.

### Correct Answer: 
**D: A digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference.**

- **Explanation**: 
  - OFDM divides a single data stream into multiple smaller streams, each transmitted on a separate subcarrier frequency. 
  - The subcarriers are spaced apart in such a way that they are orthogonal to each other, meaning they do not interfere with one another. This orthogonality helps to minimize intersymbol interference (ISI), which can occur when symbols overlap in time.

### Why the Other Choices are Incorrect:

- **A: A frequency modulation technique that uses non-harmonically related frequencies**  
  - **Incorrect**: OFDM is not a frequency modulation technique; it is a digital modulation technique. It uses multiple subcarriers that are orthogonal, not just any non-harmonically related frequencies.

- **B: A bandwidth compression technique using Fourier transforms**  
  - **Incorrect**: While OFDM does utilize concepts from Fourier transforms, it is not primarily a bandwidth compression technique. Instead, it spreads the data across multiple frequencies to improve robustness against interference and multipath fading.

- **C: A digital mode for narrow-band, slow-speed transmissions**  
  - **Incorrect**: OFDM is typically used for high-speed data transmissions over wide bandwidths, such as in Wi-Fi and digital television. It is not limited to narrow-band or slow-speed applications.

### Useful Concepts:
- **Orthogonality**: The key feature of OFDM is that the subcarriers are orthogonal, which allows them to overlap in the frequency domain without causing interference.
- **Intersymbol Interference (ISI)**: This occurs when symbols interfere with each other, which OFDM aims to minimize through its design.

In summary, OFDM is a sophisticated digital modulation technique that effectively manages multiple data streams while avoiding interference, making it suitable for high-speed communication systems.