*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 is particularly effective in environments where intersymbol interference (ISI) is a concern. Here's why the correct answer is correct and why the other choices are not:

1. **Correct Answer: D**  
   OFDM uses multiple subcarriers that are orthogonal to each other, meaning they do not interfere with one another. This orthogonality allows the subcarriers to be packed closely together without causing ISI, which is interference between successive symbols. This makes OFDM highly efficient for data transmission over channels with multipath propagation, such as wireless communication systems.

2. **Why Other Choices Are Incorrect:**
   - **Choice A:** "A frequency modulation technique that uses non-harmonically related frequencies" is incorrect because OFDM is not a frequency modulation technique. It is a digital modulation scheme that uses multiple closely spaced subcarriers.
   - **Choice B:** "A bandwidth compression technique using Fourier transforms" is misleading. While OFDM does use Fourier transforms (specifically the Fast Fourier Transform, FFT) to modulate and demodulate the subcarriers, its primary purpose is not bandwidth compression but rather efficient data transmission with reduced ISI.
   - **Choice C:** "A digital mode for narrow-band, slow-speed transmissions" is incorrect because OFDM is typically used for broadband, high-speed data transmission, such as in Wi-Fi and LTE networks.

**Key Points:**
- **Orthogonality:** The subcarriers in OFDM are orthogonal, meaning their peak values do not overlap, minimizing interference.
- **FFT:** The use of FFT allows efficient modulation and demodulation of the subcarriers.
- **ISI Reduction:** By using multiple subcarriers, OFDM reduces the effects of ISI, making it suitable for environments with multipath propagation.

Understanding these concepts will help you recognize why OFDM is a powerful technique for modern digital communication systems.