Overview
Long Term Evolution (LTE), standardised by 3GPP from Release 8 (2008) and first commercially deployed by TeliaSonera in Stockholm and Oslo in December 2009, constituted a complete redesign of both the radio access and core network. Unlike 3G, which layered data capabilities onto a fundamentally voice-centric architecture, LTE was designed from the outset as an all-IP system: voice, video, and data all flow as IP packets. Circuit-switched voice was replaced by Voice over LTE (VoLTE), and the complex hierarchical radio network controller of 3G was eliminated in favour of a flat architecture where intelligence resides in the evolved Node B (eNB) itself.
The IMT-Advanced requirements set by ITU-R mandated 1 Gbps for stationary users and 100 Mbps for mobile users. LTE as originally specified technically fell short of these targets; LTE-Advanced (Release 10, 2011) met the full IMT-Advanced requirements through carrier aggregation, enhanced MIMO, and coordinated multi-point transmission.
OFDMA: The Core Radio Innovation
The foundational radio technique of 4G is Orthogonal Frequency Division Multiple Access (OFDMA), a significant departure from the CDMA techniques of 3G. OFDMA divides the available bandwidth into a large number of narrow sub-carriers (15 kHz spacing in LTE), each modulated independently. A 20 MHz LTE channel contains 1,200 usable sub-carriers (out of 2,048 in an FFT). These sub-carriers are grouped into Resource Blocks (RBs) — the smallest schedulable unit — each consisting of 12 sub-carriers over one 0.5 ms slot (one RB = 180 kHz × 0.5 ms).
OFDMA's key advantage is its robustness against frequency-selective fading. In a wideband channel, different frequencies experience different levels of attenuation due to multipath. By transmitting on many narrow sub-carriers simultaneously and adding a cyclic prefix (guard interval) longer than the channel's delay spread, OFDMA converts a frequency-selective channel into a set of parallel flat-fading channels — each easily equalisable with a simple one-tap frequency-domain equaliser.
The uplink uses SC-FDMA (Single-Carrier FDMA) rather than OFDMA, to maintain a lower peak-to-average power ratio (PAPR). A lower PAPR means the power amplifier in the handset can operate more efficiently, extending battery life — critical for uplink transmissions from a battery-constrained device.
MIMO in LTE
Multiple-Input Multiple-Output (MIMO) antenna technology is integral to LTE, not optional. The standard specifies multiple transmission modes, including:
- Transmit Diversity (TM2): Same data sent from multiple antennas using space-frequency block coding, increasing reliability in poor channel conditions.
- Spatial Multiplexing (TM3/TM4): Independent data streams sent simultaneously from multiple antennas to the same device, multiplying throughput. 2×2 MIMO doubles the theoretical peak rate; 4×4 quadruples it.
- Multi-User MIMO (MU-MIMO): Multiple users scheduled on the same time-frequency resource using different spatial beams — increasing cell capacity without increasing bandwidth.
- Beamforming (TM7/TM8): Using phase-shifted signals from an antenna array to concentrate energy toward a specific user, improving signal quality and reducing interference to others.
LTE-Advanced: Carrier Aggregation and Beyond
LTE-Advanced (Release 10 onwards) introduced carrier aggregation (CA), allowing up to five component carriers to be combined, each up to 20 MHz wide, for a total aggregated bandwidth of up to 100 MHz. This allows operators to combine fragmented spectrum holdings — for example, 10 MHz in the 800 MHz band with 20 MHz in the 2.6 GHz band — and present them to the device as a single wide channel. Cat. 6 devices (2×20 MHz CA with 2×2 MIMO) achieved 300 Mbps; advanced Cat. 15/16 devices (4CC CA with 4×4 MIMO and 256-QAM) approach 1 Gbps.
Further LTE-Advanced Pro (Release 13+) features included Licensed Assisted Access (LAA, using unlicensed 5 GHz spectrum), massive MIMO precursors, Narrowband IoT (NB-IoT) and LTE-M for low-power IoT devices, and enhanced inter-site coordination techniques (eICIC) for heterogeneous networks combining macro cells and small cells.
Evolved Packet Core (EPC)
The 4G core network — the Evolved Packet Core — is a flat, all-IP architecture with key functional elements: the Mobility Management Entity (MME) handles signalling and mobility; the Serving Gateway (S-GW) anchors the user-plane bearer; the PDN Gateway (P-GW) provides IP address allocation and connectivity to external packet data networks; and the Policy and Charging Rules Function (PCRF) manages QoS policy. The elimination of the Radio Network Controller from 3G meant that handover decisions between eNBs are coordinated directly between base stations via the X2 interface, reducing latency and improving handover efficiency.
VoLTE and IMS
Voice over LTE (VoLTE) carries phone calls as IP Multimedia Subsystem (IMS) sessions over the LTE packet network, using the AMR-WB (Adaptive Multi-Rate Wideband) codec at 12.65 kbps. This delivers higher voice quality ("HD Voice") than legacy circuit-switched calls by extending audio bandwidth from the 300–3400 Hz of narrowband PSTN to 50–7000 Hz. VoLTE also enables faster call setup times, lower battery consumption during voice calls (since the device need not maintain a simultaneous LTE data connection and a 3G fallback connection), and the foundation for video calling over cellular.