4G: LTE and WiMAX
Fourth generation mobile networks brought all-IP architecture, OFDMA modulation, and data rates up to 1 Gbps.
The LTE Revolution
Long Term Evolution (LTE) was not merely an incremental improvement over 3G — it represented a complete architectural shift. Unlike its predecessors which carried both voice and data on circuit-switched networks, LTE was designed from the ground up as an all-IP (Internet Protocol) network. This "voice over LTE" (VoLTE) approach meant that all services, including voice calls, were treated as data packets.
The choice of OFDMA (Orthogonal Frequency Division Multiple Access) as the downlink modulation scheme was crucial. Unlike CDMA used in 3G, OFDMA allows multiple users to share the spectrum efficiently by assigning each user a subset of subcarriers. This provided better spectral efficiency, reduced interference, and enabled higher data rates.
Technical Specifications
- Peak Download: 300 Mbps (Release 8) to 1 Gbps (Release 10+)
- Peak Upload: 75 Mbps
- Latency: 10–30 ms (compared to 100–500 ms on 3G)
- Channel Bandwidth: 1.4, 3, 5, 10, 15, 20 MHz
- Multiple Antenna: 2×2 MIMO standard, up to 8×8 in Release 10
- Modulation: DL: OFDMA with QPSK/16-QAM/64-QAM. UL: SC-FDMA with QPSK/16-QAM/64-QAM
OFDMA Downlink and SC-FDMA Uplink
LTE uses OFDMA for the downlink (base station → mobile), dividing the channel into thousands of narrow subcarriers (15 kHz spacing). Each user is assigned a subset of Resource Blocks (RBs), each containing 12 subcarriers (180 kHz) over 0.5 ms (1 slot). This allows fine-grained resource allocation and multi-user scheduling.
The uplink uses SC-FDMA (Single-Carrier FDMA) to maintain a lower Peak-to-Average Power Ratio (PAPR), which is critical for mobile device battery life. SC-FDMA pre-codes the OFDMA signal to reduce power peaks, extending battery life while maintaining similar performance.
LTE Frequency Bands (3GPP Release 17)
LTE defines over 70 frequency bands across FDD (paired) and TDD (unpaired) configurations. The EARFCN (E-UTRA Absolute Radio Frequency Channel Number) encodes the channel center frequency. FDD bands use paired uplink/downlink; TDD bands share a single frequency for both directions.
EARFCN Formulas:
Downlink: FDL (MHz) = FDL_low + 0.1 × (NDL − NOffs_DL)
Uplink: FUL (MHz) = FUL_low + 0.1 × (NUL − NOffs_UL)
Key LTE FDD Bands
| Band | Name | Duplex | Uplink (MHz) | Downlink (MHz) | BW (MHz) | EARFCN Offset (UL/DL) |
|---|---|---|---|---|---|---|
| 1 | 2100 IMT | FDD | 1920–1980 | 2110–2170 | 5/10/15/20 | 18000 / 0 |
| 2 | 1900 PCS | FDD | 1850–1910 | 1930–1990 | 1.4/3/5/10/15/20 | 18600 / 600 |
| 3 | 1800+ DCS | FDD | 1710–1785 | 1805–1880 | 1.4/3/5/10/15/20 | 19200 / 1200 |
| 4 | AWS-1 | FDD | 1710–1755 | 2110–2155 | 1.4/3/5/10/15/20 | 19950 / 1950 |
| 5 | 850 Cellular | FDD | 824–849 | 869–894 | 1.4/3/5/10 | 20400 / 2400 |
| 7 | 2600 IMT-E | FDD | 2500–2570 | 2620–2690 | 5/10/15/20 | 20750 / 2750 |
| 8 | 900 Extended GSM | FDD | 880–915 | 925–960 | 1.4/3/5/10 | 21450 / 3450 |
| 12 | 700 a Lower SMH | FDD | 699–716 | 729–746 | 1.4/3/5/10 | 23010 / 5010 |
| 13 | 700 c Upper SMH | FDD | 777–787 | 746–756 | 5/10 | 23180 / 5180 |
| 14 | 700 PS Upper SMH | FDD | 788–798 | 758–768 | 5/10 | 23280 / 5280 |
| 18 | 800 Lower | FDD | 815–830 | 860–875 | 5/10/15 | 23850 / 5850 |
| 19 | 800 Upper | FDD | 830–845 | 875–890 | 5/10/15 | 24000 / 6000 |
| 20 | 800 Digital Dividend | FDD | 832–862 | 791–821 | 5/10/15/20 | 24150 / 6150 |
| 25 | 1900+ Extended PCS | FDD | 1850–1915 | 1930–1995 | 1.4/3/5/10/15/20 | 26040 / 8040 |
| 26 | 850+ Extended Cellular | FDD | 814–849 | 859–894 | 1.4/3/5/10/15 | 26690 / 8690 |
| 28 | 700 APT | FDD | 703–748 | 758–803 | 3/5/10/15/20 | 27210 / 9210 |
| 30 | 2300 WCS | FDD | 2305–2315 | 2350–2360 | 5/10 | 27660 / 9770 |
| 66 | 1700/2100 Extended AWS | FDD | 1710–1780 | 2110–2200 | 1.4/3/5/10/15/20 | 131972 / 66436 |
| 71 | 600 Digital Dividend | FDD | 663–698 | 617–652 | 5/10/15/20 | 33122 / 68586 |
Key LTE TDD Bands
| Band | Name | Duplex | Uplink/Downlink (MHz) | BW (MHz) | EARFCN Offset |
|---|---|---|---|---|---|
| 38 | 2600 IMT-E | TDD | 2570–2620 | 5/10/15/20 | 37750 |
| 40 | 2300 S-Band | TDD | 2300–2400 | 5/10/15/20 | 38650 |
| 41 | 2500 BRS | TDD | 2496–2690 | 5/10/15/20 | 39650 |
| 42 | 3500 CBRS | TDD | 3400–3600 | 5/10/15/20 | 41590 |
| 43 | 3700 C-Band | TDD | 3600–3800 | 5/10/15/20 | 43590 |
| 48 | 3600 CBRS | TDD | 3550–3700 | 5/10/15/20 | 45540 |
MIMO: Multiple-Input Multiple-Output
LTE supports various MIMO configurations to boost throughput:
- 2×2 MIMO: Two antennas at base station and mobile — standard configuration, doubles peak rate
- 4×4 MIMO: Four antennas — doubles throughput again (200 Mbps at 20 MHz)
- 8×8 MIMO: Release 10 — up to 300 Mbps at 20 MHz
- Beamforming: Precoding to focus energy toward specific users, improving cell-edge performance
Carrier Aggregation
A key feature introduced in LTE-Advanced (Release 10) was carrier aggregation, allowing operators to combine multiple frequency bands for even higher data rates. By aggregating 5 carriers of 20 MHz each (100 MHz total), theoretical peaks of 1 Gbps became possible. This technique is now standard in 4G networks worldwide.
Carrier aggregation can be:
- Intra-band contiguous: Adjacent carriers in the same band — simplest to implement
- Intra-band non-contiguous: Carriers in the same band but separated by other operators' spectrum
- Inter-band: Carriers from different bands (e.g., 700 MHz + 2600 MHz) — most common, enables coverage + capacity combination
VoLTE: Voice over LTE
The transition to all-IP networks required new solutions for voice. VoLTE(Voice over LTE) emerged as the standard, using AMR-WB (Adaptive Multi-Rate Wideband) codec for HD voice quality with 50–7000 Hz audio bandwidth (vs. 300–3400 Hz for traditional telephony). VoLTE calls connect in under 1 second (vs. 5–10 seconds for circuit-switched fallback).
Carriers initially deployed circuit-switched fallback (CSFB), temporarily switching to 3G for voice calls, before fully implementing VoLTE networks. VoLTE uses IMS (IP Multimedia Subsystem) with SIP (Session Initiation Protocol) for call signaling.
WiMAX: The Competing Standard
While LTE became the global standard, WiMAX (IEEE 802.16) was its primary competitor in the early 2000s. WiMAX offered similar capabilities and was deployed by providers like Clearwire in the United States. However, LTE's backing by virtually all major GSM carriers and its evolution path eventually made it the dominant technology. WiMAX was effectively phased out by 2015, though its influence on broadband wireless persists.
Global Impact
By 2015, LTE had become the fastest-growing mobile technology in history, with over 1 billion subscribers. The smartphone ecosystem truly exploded with 4G — the iPhone 5 (2012) was the first to support LTE, and by the iPhone 6, LTE was standard. Apps requiring constant connectivity — video streaming (Netflix, YouTube), social media, real-time gaming — all flourished on 4G networks.