What Is 4G Wireless?

4G wireless is the fourth generation of mobile cellular technology, defined by the ITU under the IMT-Advanced framework, that provides broadband mobile internet access at peak downlink rates of up to 1 Gbps for low-mobility users and 100 Mbps for users moving at vehicular speeds. The ITU finalized the IMT-Advanced requirements in 2012, and two radio access technologies, Long-Term Evolution Advanced (LTE-Advanced) standardized by 3GPP and WirelessMAN-Advanced (the IEEE 802.16m standard), received official IMT-Advanced designation. In practice, 4G networks worldwide are almost universally built on the LTE family, with WirelessMAN-Advanced remaining a niche deployment. Earlier LTE releases, branded as "4G" by operators despite not fully satisfying IMT-Advanced peak rate requirements, provided the infrastructure base onto which LTE-Advanced capabilities were layered.

4G represents a clean break from the 3G architecture in its use of an all-IP network design. Voice calls, which 3G handled through a parallel circuit-switched domain, are carried in 4G entirely over IP as Voice over LTE (VoLTE), using the IP Multimedia Subsystem (IMS). The radio access network was also flattened: the 3G Radio Network Controller function was distributed into the base stations themselves (called eNodeBs in LTE), reducing handover latency and simplifying the network hierarchy.

LTE and LTE-Advanced

LTE Release 8, the first commercialized version, launched in Norway and Sweden in 2009. It uses OFDMA (Orthogonal Frequency Division Multiple Access) in the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) in the uplink, with channel bandwidths from 1.4 MHz to 20 MHz. LTE-Advanced (Release 10) added carrier aggregation, which binds up to five component carriers together to achieve wider effective bandwidths, along with enhanced MIMO spatial multiplexing supporting up to eight downlink layers. Subsequent releases extended carrier aggregation to include combinations of licensed and unlicensed spectrum (LTE-U and LAA in Releases 12 and 13) and introduced massive MIMO precoding techniques in the 3D-MIMO feature of Release 13. The ITU announcement confirming LTE-Advanced as an IMT-Advanced 4G technology documents the formal recognition of the LTE-Advanced specification as meeting the 4G requirements.

OFDM and MIMO Radio Interface

The choice of OFDM as the 4G downlink multiple access scheme was central to 4G's spectral efficiency improvements over 3G CDMA. OFDM divides the channel into many narrow parallel subcarriers spaced 15 kHz apart, each carrying a low-rate modulated symbol stream, allowing efficient use of fragmented spectrum and simple equalization in frequency-selective fading channels using the cyclic prefix. Paired with MIMO spatial multiplexing, where independent data streams are transmitted simultaneously from multiple antennas to a receiver equipped with multiple receive chains, OFDM-MIMO formed the foundation for the spectral efficiencies of up to 30 bps/Hz achievable in favorable channel conditions. ITU resources on the IMT-Advanced framework describe the spectral efficiency and coverage requirements that motivated the adoption of OFDM-based radio interfaces.

Network Architecture and Evolved Packet Core

The 4G core network, called the Evolved Packet Core (EPC), consists of a small number of logical functions including the Mobility Management Entity (MME), the Serving Gateway (S-GW), and the Packet Data Network Gateway (P-GW), all communicating over standardized IP interfaces. This architecture supports fine-grained quality of service through the concept of EPS bearers, allowing operators to differentiate handling of VoLTE voice packets, video traffic, and best-effort data within the same radio connection. The 3GPP Release 15 overview on IEEE Spectrum provides context for how the EPC evolved into the 5G core network as the transition to the next generation proceeded.

Applications

4G wireless technology supports a broad range of services and connected devices, including:

• Consumer mobile broadband for smartphones, tablets, and home broadband routers
• Voice over LTE (VoLTE) providing high-definition voice calls and simultaneous data
• Mobile video streaming and real-time video conferencing on handheld devices
• Cellular IoT connectivity for smart meters, asset tracking, and connected vehicles
• Public safety broadband communications built on the FirstNet LTE network in the United States