AdvancedTopic 88 of 101Signals, Control & Comms
OFDM
Splits data over many orthogonal subcarriers to fight multipath.
In depth
OFDM divides a high-rate stream into many low-rate orthogonal subcarriers spaced 1/T_s apart; implemented with IFFT/FFT. A cyclic prefix longer than channel delay spread removes inter-symbol interference and simplifies equalisation. Used in Wi-Fi, LTE, 5G, DVB, DSL. Drawbacks: high PAPR and sensitivity to frequency offset.
Key points
- Subcarrier spacing
- Pilot tones
- PAPR reduction
ExampleLTE downlink uses OFDMA with 15 kHz subcarriers.
What is OFDM?
Splits data over many orthogonal subcarriers to fight multipath.
Why is it important to understand OFDM?
OFDM splits data across many carriers, which makes it robust to multipath and efficient for high data rates.
What is an orthogonal subcarrier?
A carrier spaced so that its peak falls on the zeros of the others, so the subcarriers do not interfere and can be packed tightly.
Why does OFDM handle multipath well?
Each subcarrier sees a narrow band, which is nearly flat in frequency, so equalisation is simple.
How is OFDM used in practice?
Typical applications include providing Wi-Fi and 4G LTE data over a multipath radio channel, carrying broadcast digital television across buildings and using subcarriers that can be turned off in a crowded band.
What should I learn before OFDM?
Study "Multiplexing" first (topic 87 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
What are the key points of OFDM?
Subcarrier spacing. Pilot tones. PAPR reduction.
Can you give an example of OFDM?
LTE downlink uses OFDMA with 15 kHz subcarriers.
What comes after OFDM?
The next topic is "Spread spectrum", which builds directly on this one.
How difficult is OFDM?
It is classed as Advanced in Signals, Control & Comms (topic 88 of 101). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Signals, Control & Comms.