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LTE Basics: 4G Mobile Networks: Complete Beginner's Guide

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LTE (Long Term Evolution) is the 3GPP standard behind 4G: an all-IP packet-switched network carrying everything, including voice (VoLTE), as data packets. Its radio uses OFDMA on the downlink and SC-FDMA on the uplink, splitting a wide channel into hundreds of narrow orthogonal subcarriers.

LTE matters because it made the phone a true broadband device: ~10 ms latency and ~150 Mbps peaks in 20 MHz, roughly ten times 3G. Nearly every smartphone still connects over LTE, and its architecture is the foundation 5G was built on.

Key Concepts

  • Resource block (RB): the smallest scheduled unit: 12 subcarriers x 15 kHz = 180 kHz, by 0.5 ms (7 OFDM symbols).
  • Frame: 10 ms frame = ten 1 ms subframes; each subframe is one scheduling interval (TTI).
  • OFDMA vs SC-FDMA: OFDMA downlink resists multipath; SC-FDMA uplink has lower peak-to-average power, saving phone battery.
  • Bandwidths: 1.4 to 20 MHz, using 6, 15, 25, 50, 75 or 100 RBs.
  • Modulation: QPSK, 16-QAM, 64-QAM adapt to signal quality; better signal, more bits per symbol.
  • EPC core: flat all-IP core: eNodeB base stations, MME, S-GW/P-GW gateways, HSS subscriber database.
  • Carrier aggregation: LTE-Advanced bonds up to 5 carriers (100 MHz) to multiply peak rates.

Formulas

  • Occupied bandwidth: N_RB x 12 x 15 kHz. For 100 RBs: 18 MHz inside a 20 MHz channel; 2 MHz remain as guard bands.
  • Peak rate: subcarriers x symbols/second x bits/symbol x MIMO layers, minus overhead for pilots, control and coding.
  • Assumptions: peaks assume 64-QAM, full MIMO rank, a clean channel and one user holding every RB; real rates are lower.

Worked Example

Gross peak downlink of a 20 MHz LTE carrier, 64-QAM, 2x2 MIMO:

  1. Subcarriers = 100 RBs x 12 = 1200 (18 MHz occupied).
  2. Symbols/second per subcarrier = 14 per 1 ms x 1000 = 14,000.
  3. Resource elements/second = 1200 x 14,000 = 16.8 million.
  4. Bits: 16.8M x 6 (64-QAM) = 100.8 Mbps; x2 MIMO layers = 201.6 Mbps gross.
  5. Minus ~25% overhead for pilots, control and coding: ~150 Mbps practical peak, matching the published 3GPP figure.

Common Misconceptions

  • "LTE is true 4G." The original ITU bar was 1 Gbps; first-release LTE fell short, though ITU allowed the 4G label. LTE-Advanced genuinely qualifies.
  • "More bars mean faster internet." Bars show signal strength; speed also depends on cell load, bandwidth and assigned modulation.
  • "5G replaced LTE." 5G still uses LTE for coverage and as an anchor; LTE will carry traffic for years.

Real-World Applications

  • Smartphones/hotspots: mobile broadband for video, browsing, apps.
  • VoLTE: voice as IP packets with fast call setup.
  • IoT: NB-IoT and LTE-M serve meters and trackers on tiny bandwidth for years of battery.
  • Fixed wireless: home broadband over LTE where fibre is absent.

LTE is the all-IP 4G system that made mobile broadband universal: 180 kHz resource blocks, a 10 ms frame, adaptive modulation to 64-QAM, and about 150 Mbps from a 20 MHz carrier with 2x2 MIMO. Its flat core and carrier aggregation paved the direct path to 5G.

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