Deep Dive: E8B08
The correct answer is D: A digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference. Orthogonal frequency-division multiplexing (OFDM) is a digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference. The subcarriers are spaced so they're orthogonal (mathematically independent), preventing interference between them. OFDM divides the available bandwidth into many narrow subcarriers, each carrying part of the data. The subcarrier frequencies are chosen so they're orthogonal - the peak of one subcarrier aligns with the nulls of adjacent subcarriers, preventing intersymbol interference. This allows efficient use of bandwidth and makes OFDM resistant to multipath fading. OFDM is used in many modern digital communication systems including Wi-Fi, digital TV, and amateur digital modes.
Why Other Answers Are Wrong
Option A: Incorrect. OFDM isn't a frequency modulation technique. It's a digital modulation technique that uses multiple subcarriers, not FM. Option B: Incorrect. OFDM isn't specifically a bandwidth compression technique. It's a multiplexing technique that divides bandwidth into subcarriers. Option C: Incorrect. OFDM can be used for high-speed transmissions, not just narrow-band, slow-speed. It's used in many high-speed digital systems.
Exam Tip
OFDM = Digital modulation with orthogonal subcarriers. Remember: OFDM is a digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference. The subcarriers are orthogonal.
Memory Aid
**O**FDM = **O**rthogonal **S**ubcarriers **A**void **I**nterference (think 'OFDM = OSAI')
Real-World Example
You're using an OFDM digital mode. The signal is divided into multiple subcarriers, each at a specific frequency. These frequencies are chosen so the subcarriers are orthogonal - they don't interfere with each other. This allows multiple data streams to be transmitted simultaneously without intersymbol interference, making efficient use of bandwidth.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E8B
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E8B topic.