What Are Radio Access Technologies?

Radio access technologies (RATs) are the physical and medium-access-control layer protocols that govern how user devices communicate with the base stations of a mobile network over the radio interface. Each RAT defines the multiple-access scheme by which spectrum is shared among simultaneous users, the modulation and coding formats used to encode data onto radio carriers, the duplexing method that separates uplink from downlink transmissions, and the signal processing techniques applied at transmitter and receiver. The choice of RAT determines a network's spectral efficiency, latency, capacity, and ability to serve heterogeneous device populations. Different generations of mobile standards have introduced successive RATs, with each generation substantially revising the physical layer to exploit improvements in signal processing, semiconductor capability, and spectrum availability.

The succession of RATs from first-generation analog systems to fifth-generation New Radio spans roughly four decades and reflects sustained improvements in information theory, antenna design, and digital signal processing that collectively increased spectral efficiency by several orders of magnitude.

First- and Second-Generation Multiple Access

First-generation analog systems used frequency division multiple access (FDMA), assigning each call an exclusive narrow frequency channel for its duration. Second-generation digital systems divided spectrum by both frequency and time, with GSM using a combination of FDMA and time division multiple access (TDMA): the available spectrum is divided into 200 kHz channels, and each channel is further divided into eight time slots that different users occupy in rotation. GSM's 200 kHz channel structure and GMSK modulation allowed frequency reuse across cells, supporting the first global standard for digital mobile telephony. The IS-95 CDMA standard, deployed primarily in North America, took a different approach: all users share the same wideband channel and are distinguished by unique spreading codes, with each user's signal appearing as noise to others. A comparison of these fundamental multiple access approaches is covered in the Electronic Design overview of FDMA, TDMA, CDMA, OFDMA, and SDMA.

Third-Generation CDMA and Wideband Extensions

Third-generation systems standardized through 3GPP and 3GPP2 adopted wideband CDMA (WCDMA) as the primary radio access technology in UMTS, expanding the channel bandwidth from 1.25 MHz in IS-95 to 5 MHz and adding a high-speed downlink packet access (HSDPA) enhancement that reached downlink rates approaching 42 Mbps with HSPA+. CDMA2000, the 3GPP2 track, deployed the 1xEV-DO variant to deliver comparable packet data performance over the same 1.25 MHz channels used by IS-95. Both 3G RATs relied on power control to manage the near-far problem inherent in CDMA: because all users share the spectrum, a user close to the base station would overwhelm distant users unless each device's transmit power is precisely regulated. The 3GPP specification body continues to maintain and version these standards, with the full technical specifications available through the 3GPP 5G system overview portal.

Fourth- and Fifth-Generation OFDMA

Long-Term Evolution (LTE), specified in 3GPP Release 8, replaced CDMA with orthogonal frequency division multiple access (OFDMA) on the downlink and single-carrier frequency division multiple access (SC-FDMA) on the uplink. OFDMA divides the channel into many narrow orthogonal subcarriers and assigns subsets of those subcarriers to different users, allowing frequency-selective scheduling that matches subcarrier assignments to each user's channel conditions. This approach, combined with MIMO antenna techniques, raised peak downlink rates to hundreds of megabits per second in LTE-Advanced. Fifth-generation New Radio (NR), specified from 3GPP Release 15, retained OFDMA but introduced a flexible numerology in which subcarrier spacing can be configured from 15 kHz to 240 kHz to support use cases ranging from wide-area coverage to millimeter-wave short-range links. The physical layer details of the 5G NR air interface, including OFDMA parameterization and waveform design, are described in the Systems Approach 5G radio transmission chapter.

Applications

Radio access technologies have applications in a range of fields, including:

• Consumer mobile broadband delivered through successive 3G, 4G, and 5G cellular RATs
• Industrial wireless connectivity using licensed-spectrum private LTE and 5G NR deployments
• IoT device connectivity using narrowband RATs such as NB-IoT and LTE-M
• Fixed wireless access providing broadband to underserved areas using 5G NR base stations
• Satellite-ground radio access for non-terrestrial network integration in 5G