If these frequencies are not enough, it is necessary to increase the spectral efficiency — transmit more data in one channel in the same frequency range. In this case, 5G is better than 4G thanks to the updates in signal modulation and signal coding schemes, but radical progress is not to be expected. Modern modulation systems are already close to physical limits and the practically achievable spectral efficiency of the cellular networks in a single channel can’t be increased dramatically. By the way, during the transition from 3G to 4G, the growth of spectral efficiency of a single channel was more substantial than expected when switching to 5G. Nevertheless, there is still significant potential for increasing the efficiency of using the frequency range for the provision of services, but it is realized by the more complex means. We are talking about the frequency resource distribution between the network services, more efficient resource allocation between the uplink and downlink data channels, network self-organization, resource recombination and cell coordination, increases support of multi-frequency (carrier aggregation), and etc. All this will be actively used in 5G and will have a positive effect, but most of these approaches are already present and in use in the fourth generation.
If one can’t radically improve the efficiency of using a single channel, it is logical to try to organize simultaneous exchange of different data streams between the network and the subscriber or subscribers in the same frequency range. In other words, it is necessary to increase the re-use level of the frequency resource. For that purpose, data channels operating in the same frequency must be isolated from each other in order to avoid interference. There are several approaches to solving this problem that have been used extensively by the existing cellular networks and that have been gaining traction in 5G lately.
The growth of the cellular networks’ capacity has always been guaranteed by a combination of the three factors listed above: expansion of the spectrum being used, increase in spectral efficiency and increase in the level of re-use frequency. In the past two decades, the emphasis has been on the re-use techniques and the way they have contributed towards the capacity increase. By various estimates, over the course of the cellular network existence its capacity has grown by 3-4 times on account of the frequency resource, by 5-6 times — due to increase in spectral efficiency, and by 40-60 times due to improvement in the frequency usage.
A New Technology is not yet a Business
The most well-known example of re-using frequencies is installation of a large number of cells: the more cells, the more times one can use the same spectrum. The main limitation of this approach is crosstalk (interference) at the coverage areas’ boundaries. The greater the cell number is, the less area they cover and the greater is their proportion of areas with high interference. To combat this, a ton of various methods have been employed — from strategies for re-using frequency bands, depending on the mutual disposition of the base stations to complex algorithms for coordinating signals by power and phase at the boundaries of the coverage areas. As the number of cells increases, so does the complexity, and, of course, the cost of solutions that allow them to be shared.
In 5G, provisions have been made for improving the efficiency of using a large number of cells, but nevertheless this improvement is only evolutionary in nature. It would seem that the 5G ideology is oriented towards freedom from macro-architecture, but the same was said about 4G. At a time of the mass construction of 4G networks, the market anticipated that the small cells would take over the role of the main provider of communication services in the context of dense urban development, and the explosive growth of their production and consumption.
This is not to say that a lot of microcellular architectures and 3GPP-supported solutions, for which high-quality equipment is available on the market, haven’t been in high demand; it certainly hasn’t turned into the main method of establishing network coverage. Coverage outdoors continues to be established primarily by the macrocell sectors, while small cells are used for the most part as an additional tool that makes it possible to plug «holes» in the coverage and improve the network quality. A commonly-occurring model of using small cells today involves installing them in close proximity to the macrocells. At the same time, the network is parameterized in such a way that the subscribers near the macrocell (located in close to ideal conditions of radio signal propagation and able to connect at a very high speed) more often than not would be serviced by its «entourage» of small cells, whereby subscribers located at a distance, would receive more cell resources and improved communication quality and speed.
The main reason for the small cells maintaining the auxiliary role and the relatively slow growth in the number of cells is not in fact technical, but economic. The growth of the cellular traffic (for all the mobile operators may say about this) has occurred in the last decade, albeit very intensively, but more slowly than the market anticipated in the face of inflated expectations following the period of initial investment in 4G. And, more importantly, mobile operators haven’t learned how to profit off of this traffic. As early as the moment of the 3G’s debut, the cellular market was well aware of the threat of the mobile operators transforming into a «data pipe» with the rapidly plummeting cost of traffic transmission. If you look at the speeches of telecommunication company executives at industry conferences of those times, they unanimously proclaimed that in a few years mobile operators will be selling not just the basic services to the subscribers («voice», messages and data transfer), but the content and a multitude of useful services (media, communication, related to control, management and security, gaming, etc.). Experts predicted that this will become the main source of revenue for telecom operators.
Of course, even though the mobile operators offer lots of useful services these days, they still extract the lion’s share of their revenues from the same good old data, the «voice» and the messages. Only now the data has moved to the first place. Revenues from additional services and content remained just a pleasant addition. Hence the very conservative recently adopted approach to investments, which inevitably affects the cellular networks architecture. A large-scale implementation of small cells in the cities turned out to be economically unsound and it is not yet clear how 5G will be able to factor into this troubling trend.
Smartphones and MIMO
Another way to increase capacity and speed through re-using the frequency resource is a multi-channel reception and data transmission from one cell to one or more subscribers. This is a family of technologies, commonly referred to as MIMO (Multiple Input Multiple Output) and built on the principle of spatial multiplexing of the radio channels. In 5G, the so-called, Massive MIMO (M-MIMO) and the beamforming technology associated with it (in fact, these are the versions of the general approach that is based on the use of multi-element antennas), in theory allowing a tenfold increase of the total radio network bandwidth. This additional bandwidth can be used either to increase the number of simultaneously serviced subscribers in the coverage area, or to increase the data transfer rate for specific subscribers or groups of subscribers. Beamforming additionally increases the efficiency of using the network’s frequency resource, taking advantage of the fact that not all subscribers in the coverage area simultaneously need a full speed access. The principle of its operation lies in the dynamic redistribution of the signal’s power (by forming directed beams) in favor of those subscribers who currently need large amounts of data.
M-MIMO is a complex technology that requires the creation of phased antenna arrays with hundreds of elements at a reasonable price and in form factors that can be used in the cities (outside the city limits these systems are simply not needed). There is no doubt that M-MIMO will eventually be put to good use in the base stations. But it is very important to emphasize that on the part of the subscribers, the use of MIMO is limited by price, size, energy parameters and the permitted radiation power of portable devices. The practically achievable number of independent data transfer channels available in the smartphone is very limited, and the quality of their work depends heavily on the conditions of use and the distance to the base station. Therefore, if 5G network’s capacity can indeed be significantly increased by means of the M-MIMO, the data exchange rate between the network and the individual subscriber will grow much more slowly, limited by the capabilities of the subscriber devices, and will depend heavily on the conditions of use.