1. What is 4G?
4G (LTE) launched in 2009. It delivered true broadband mobile internet — speeds up to 150 Mbps (LTE) and 1 Gbps (LTE-Advanced). 4G dropped the legacy circuit-switched voice network entirely; everything is IP-based.
Key Fact: LTE stands for Long-Term Evolution. The first commercial 4G LTE network launched in Sweden and Norway in 2009.
2. EPC Architecture
The Evolved Packet Core (EPC) replaced the older circuit-switched core:
- eNodeB – LTE base station (smarter than Node B; connects directly to core)
- MME (Mobility Management Entity) – Handles authentication, attach, paging, handovers
- SGW (Serving Gateway) – User-plane anchor for intra-LTE mobility
- PGW (PDN Gateway) – Connects to the internet; assigns IP addresses
- HSS (Home Subscriber Server) – Combined HLR/AUC for LTE
3. OFDMA Radio
LTE uses OFDMA (Orthogonal Frequency Division Multiple Access) in the downlink and SC-FDMA in the uplink. The spectrum is divided into resource blocks (12 subcarriers × 15 kHz = 180 kHz each).
| Channel BW | Resource Blocks | Max Speed (DL) |
|---|---|---|
| 5 MHz | 25 | 37 Mbps |
| 10 MHz | 50 | 75 Mbps |
| 20 MHz | 100 | 150 Mbps |
4. 4G vs 3G
| Aspect | 3G (HSPA+) | 4G (LTE) |
|---|---|---|
| Radio | W-CDMA | OFDMA / SC-FDMA |
| Max speed | 42 Mbps | 150 Mbps (LTE) |
| Latency | ~50 ms | ~10 ms |
| Voice | Circuit + VoIP | VoLTE only |
| Architecture | UTRAN + CN | E-UTRAN + EPC |