Caption: Ofcom Bands (Source: ‘Laying the Foundations for Next Generation Mobile Services: Update on Bands Above 6 GHz’, Ofcom, April 2015)
In the first part of this blog, we discussed the requirements for 5G and the specification process being undertaken by the International Telecommunication Union (ITU) and the Third Generation Partnership Project (3GPP). Here, we continue the discussion by investigating the potential technical solutions.
Mobile telecommunication systems have traditionally operated in the UHF band, from 300 MHz to 3 GHz. This has given the best trade-off between the competing issues of coverage and capacity, which are greater at low and high frequencies respectively. Unfortunately the UHF band is now extremely fragmented and congested, so attention is being directed to the SHF and EHF bands, from 3 to 300 GHz. The diagram shows some bands that have been suggested in studies by the UK regulator Ofcom, while an agenda item to identify suitable bands has been adopted for the World Radiocommunication Conference in 2019.
Radio propagation in the SHF and EHF bands is somewhat different from that in UHF. The cell size in an urban environment appears limited to 200 metres or so, with this limit arising because the propagation is dominated by line-of-sight signals, and because the signal is attenuated by rainfall and atmospheric absorption. Penetration losses are large and appear likely to preclude outdoor-to-indoor communication. In addition, the use of radio signals with wavelengths of millimetres or centimetres implies that antenna arrays will be an important way to maximise the received signal power.
The use of antenna arrays suggests that multiple input multiple output (MIMO) antennas will play an important role in 5G. In the new technique of massive MIMO, each base station is equipped with hundreds of antennas, which it uses to communicate with tens of mobiles using the same time-frequency resource. If we consider the transmissions from the base station to a single target mobile, then the signals from the different base station antennas combine coherently at the target, but combine incoherently elsewhere. For M antennas serving K mobiles, the base station’s transmitted power is proportional to 1/M, while the received signal-to-interference ratio is proportional to M/K. The benefits to the spectral efficiency and energy efficiency of 5G are potentially large.
The most likely multiple access technology for 5G appears still to be OFDMA, but this issue is being investigated further as part of the 3GPP studies. Massive MIMO is more effective when used with time division duplex (TDD) rather than frequency division duplex (FDD), but attention is also being directed to full duplex communication, which involves simultaneous transmission and reception on a single carrier frequency. In a continuation of existing trends, 5G networks are likely to be heterogeneous, with a variety of cell sizes, and to make increasing use of relays and device-to-device communications. Finally, attention is being directed to centralization and virtualization of the radio access network, in which the base stations’ signal processing is first brought back to a central hub, and then implemented in software on general-purpose servers.