5G not only offers even higher data rates, but also reduced latency, increased connectivity and a wider range of mobility and reliability. In particular, three main usage scenarios have initially been defined by the ITU (Recommendation ITU-R M.2083):

• Enhanced Mobile Broadband (eMBB) to deal with hugely increased data rates, high user density and very high traffic capacity for hotspot scenarios as well as seamless coverage and high mobility scenarios with still improved used data rates.
• Massive Machine-type Communications (mMTC) for the Internet of Things (IoT), requiring low power consumption and low data rates for very large numbers of connected devices.
• Ultra-reliable and Low Latency Communications (URLLC) to cater for safety-critical and mission critical applications.

These three usage scenarios support the development of critical communication applications that were not possible with previous generation technologies. 

These new capabilities are enabled thanks to a set of new technologies, described below.

ITU defines an Advanced Antenna System (AAS) as a collection of technologies for enhancing wireless communication in terms of coverage, capacity, and end-user throughput [1],[2]. These technologies are implemented starting from 5G. Simply put, these are antennas that adjust themselves according to the devices looking for a connection. They rely on a technology to create very precise communication beams to a device. This is a major advance over previous generations, which emitted communication streams in a more static way. This results in reduced loss, greater device coverage, and improved interference, which is very useful in urban environments.

A Base Station (BS) is a fixed transmission and reception location in charge of handling radio traffic. It usually consists of one or more receive/transmit antenna, microwave dish and electronic circuitry. User Equipment (UE) is any device employed by an end-user to communicate with a BS (e.g., a smartphone). An air interface is the radio portion of the circuit between the UEs and the active BS.

Beamforming (or spatial filtering) is a signal processing technique for directional signal transmission or reception.

MIMO or spatial multiplexing is a method for transmitting multiple data streams, using the same time and frequency resource, where each data stream employs beamforming. This technique is known to previous generations, but the introduction of AAS in 5G enables more advanced MIMO methods. In Multi-User MIMO (MU-MIMO) or Massive MIMO, the AAS increases the network capacity by simultaneously sending different layers to different users in separate beams using the same time and frequency resources. MU-MIMO is possible only when the system finds two or more users who need to transmit or receive data simultaneously. Also, for efficient MU-MIMO, the interference between the users should be kept low, which can be achieved thanks to generalised beamforming with null forming.

4G and previous generations employ different frequencies to transmit or receive information between the UEs and the BS. This technique is known as Frequency Division Duplex (FDD). 5G proposes a Time Division Duplex (TDD) instead, where both uplink and downlink use the same spectrum frequencies but at different moments in time. In this scenario, the operator is the one who defines how the 5G antennas adapt over time (e.g., in Luxembourg, operators consider 25% uplink and 75% downlink).

5G also achieves an increased end-user throughput by using higher-frequency radio waves than previous generations. However, higher-frequency radio waves have a shorter range. A 5G network will be composed of different types of cells (e.g., small cells vs macrocells), each requiring particular antenna designs and providing a different trade-off between download speed and distance. The faster 5G cells employ an RF band from 30 to 300 GHz, which the ITU defines as Extremely High Frequency (EHF). Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band, and radiation in this band is called millimetre waves (mmWaves).

The sixth generation (6G)

As introduced above, each generation of cellular communication takes about 10 years to be designed, standardised and implemented before being commercialised and made available to the public. In addition to the gradual roll-out of 5G, initial work on 6G is progressing towards a network with enriched capabilities, relying on wider frequency bands (up to Terahertz bands) and offering far greater capacity and performance than 5G. This new generation will work in conjunction with artificial intelligence techniques to deliver a distributed network that dynamically adapts to the needs of its users, with reduced deployment costs and a single point of contact to provide increasingly personalised services.

Future mobile wireless generations are targeting two main objectives: to incorporate the previous technologies in order to take advantage of each generation’s potential, and to exploit new approaches, such as:

• Joint communication and sensing. The 6G experience requires more data as well as more environmental sensing and awareness—and joint communications and sensing explores combining them. Autonomous vehicles, for example, have incredibly sophisticated sensing systems powered by machine-learning algorithms fusing data from an array of cameras, lidar, and radar sensors. The advanced communications systems in these vehicles use cellular networks for streaming infotainment, environment and performance data, and vehicle-to-everything communications. The extent to which these two traditionally separate functions merge will depend on regulatory and technical factors, but the combination could potentially define 6G.
• Sub-THz bands. The perpetual demand for more data bandwidth is pushing researchers to explore underutilized spectrum in the sub-THz frequency bands. Frequency bands between 90 GHz and 300 GHz offer many times the amount of spectrum currently used for cellular communications. 3GPP already has identified 21.2 GHz above 100 GHz for possible 6G consideration. 
• Evolution of MIMO. With potential across many different use cases as well as frequency bands, MIMO continues to build on popular multiantenna techniques. Beamforming is key to overcoming sub-THz pathloss challenges. Distributed MIMO, which disaggregates large antenna arrays into multiple smaller, geographically separated radio heads, is especially interesting for higher frequencies.
• AI/ML. The fourth technology sure to play a significant role is artificial intelligence and machine learning (AI/ML). As complexity increases and we seek to squeeze every bit of bandwidth out of the available spectrum, it becomes increasingly difficult to optimize the communications system with traditional signal-processing methods. Machine learning offers one way to deal with this complexity.

[1] Zaidi, A. A.-C. (2017). Designing for the future: the 5G NR physical layer. Ericsson Technology Review, 1-13.

[2] Dahlman, E. a. (2020). 5G NR: The next generation wireless access technology. Academic Press. (n.d.).

Credits: 

• OpenAirInterface: https://openairinterface.org/ 
• srsRAN: https://docs.srsran.com/en/next/app_notes/source/5g_nsa_zmq/source/index.html