OFDM
Understanding OFDM
OFDM revolutionized wireless communications by solving the multipath problem that plagued single-carrier wideband systems. In a multipath environment, reflections create frequency-selective fading that distorts wideband signals. OFDM divides the wideband channel into hundreds or thousands of narrowband subcarriers, each narrow enough to experience flat (non-frequency-selective) fading.
How OFDM Works
- Subcarrier spacing: The wideband channel is divided into equally spaced subcarriers. Each is independently modulated (QPSK, 16-QAM, 64-QAM, etc.).
- Orthogonality: Subcarrier spacing equals 1/T_symbol, ensuring zero interference between subcarriers at the sampling points.
- Cyclic prefix: A guard interval that absorbs multipath delay spread, eliminating intersymbol interference.
- IFFT/FFT implementation: Modulation/demodulation performed efficiently using inverse FFT (transmit) and FFT (receive).
OFDM Challenges
- High PAPR: Peak-to-average power ratio is high because subcarrier signals can add constructively. Requires linear PA operation with backoff.
- Frequency offset sensitivity: Carrier frequency offset destroys subcarrier orthogonality. Requires accurate frequency synchronization.
Total bandwidth: BW = N x delta_f
Symbol duration: T_symbol = 1/delta_f
5G NR numerology:
mu=0: 15 kHz spacing (FR1)
mu=1: 30 kHz spacing (FR1)
mu=2: 60 kHz spacing (FR1/FR2)
mu=3: 120 kHz spacing (FR2)
Frequently Asked Questions
What is OFDM?
OFDM divides a wideband channel into many narrowband subcarriers, each independently modulated. This makes the system resistant to multipath fading because each subcarrier is narrow enough to experience flat fading. OFDM is used in Wi-Fi, 4G LTE, 5G NR, and digital broadcasting.
Why is OFDM better than single-carrier modulation?
In multipath environments, wideband single-carrier signals suffer frequency-selective fading that causes severe intersymbol interference. OFDM breaks the signal into narrow subcarriers that each experience flat fading, and the cyclic prefix absorbs multipath echoes. This enables reliable wideband communication in complex propagation environments.
What is the cyclic prefix in OFDM?
The cyclic prefix is a copy of the end of each OFDM symbol appended to the beginning. It acts as a guard interval that absorbs multipath reflections, preventing them from causing intersymbol interference. The cyclic prefix length must exceed the channel delay spread.