What did LTE introduce, and why is it called "3.9G" instead of true 4G?

LTE introduced OFDMA and an all-IP architecture, delivering 100+ Mbit/s downlink. It was labeled 3.9G because it didn't initially meet the ITU's full 4G speed requirements.

* OFDMA hands each user a slice of subcarriers. — M7ammad, Public domain, via Wikimedia Commons. *

LTE (Long Term Evolution), 2009:

• Downlink: minimum 100 Mbit/s (with 4x4 MIMO: up to 326 Mbps).
• Uplink: minimum 50 Mbit/s (up to 75 Mbit/s).
• Latency: under 10 ms round-trip. A massive improvement over 3G's 100-200 ms.
• Cell sizes: 1 km to 100 km, very flexible deployment.
• Coexistence with legacy 2G/3G standards.

Key technical innovations:

• OFDMA (Orthogonal Frequency Division Multiple Access) for downlink, SC-FDMA for uplink. These replaced W-CDMA entirely.
• MIMO (Multiple Input, Multiple Output): using multiple antennas simultaneously.
• EPC (Evolved Packet Core): a purely IP-based core network. Voice was now just another data stream, no more circuit-switched voice.

The 3.9G controversy: The ITU defined "true 4G" as requiring 1 Gbit/s for stationary users. LTE didn't meet this, so purists called it 3.9G. Only LTE-Advanced (up 500 Mbit/s, down 1 Gbit/s) technically qualifies as IMT-Advanced (true 4G). Marketing won, though, and everyone calls LTE "4G."

Go deeper:

• LTE (telecommunication) (Wikipedia) — the 4G standard, its OFDMA/SC-FDMA design and all-IP EPC core.
• Orthogonal frequency-division multiple access (Wikipedia) — how OFDMA hands each user a slice of subcarriers, with diagrams.