This chapter addresses the spectrum requirements anticipated for 5G networks. Spectrum is a scarce finite resource; there is however a growing demand for spectrum due to the hungry data demand of consumers who want to be able to download videos in seconds, instantly connect with friends and families from various parts of the globe, share viral videos and favourite moments with one another, hashtag a political concern or even issues at work and finalize business deals remotely.
The success of 5G applications highlighted in the previous chapter is heavily dependent on the Governments and regulators support of a timely release of affordable spectrum. Within the Industry, there are several developments geared at providing spectrum and increasing the efficiency of the existing spectrum for 5G applications, termed spectral aggregation. This chapter therefore introduces the reader to the spectral aggregation methods developed for the 5G network.
2.1 Spectrum Re-farming
In order to make spectrum available, traditionally old bands are cleared of incumbents . Spectrum Re-farming involves the re-assignment of frequencies previously assigned for older generations like 2G and 3G and subsequent use for 5G. This is particularly true in countries like the US and East Asia where there are no 2G services available anymore . This is a cheaper and cost effective way of acquiring spectrum.
Furthermore, the digital switchover also contributed towards freeing up some spectrum; infact the 700MHz band was cleared of incumbents to provide some bandwidth for 5G. Besides 700MHz, some other bands will be re-farmed for 5G .
2.2 Spectrum Sharing
Spectrum sharing, as the name implies, involves sharing spectrum among various bands.
2.2.1 Spectrum Sharing – Unlicensed bands
Unlicensed spectrum (e.g. 2.4GHz) can support offloading from licensed bands. This is very important because 5G would involve a combination of different technologies like Long Term Evolution (LTE)/WiFi etc.
However, Quality of Service (QoS) cannot be guaranteed when using unlicensed spectrum; therefore spectrum sharing (with unlicensed bands) needs to be well planned and carefully integrated with licensed bands .
2.2.2 Spectrum Sharing – Under-utilized bands
Bands that are under-utilised in time or geography can also be utilised for 5G services e.g. TV white spaces in the US. This has led to specialized spectrum sharing techniques like Authorized Share Access (ASA) where the ASA spectrum holder only has access to the spectrum when it is not utilized by the incumbent 
Furthermore, certain Organisations like GSMA are pushing for regulators to encourage voluntary spectrum sharing agreements among operators or verticals, where possible .
2.3 New Spectrum from mm wave band
Traditionally, frequency bands used for cellular systems were within the sub-3GHz . However, the need for an increased spectrum for 5G services means that the higher frequencies (30-300GHz), termed mm wave, are now been considered for use in 5G.
These frequencies were traditionally being used for satellites and radar applications. And these frequencies are able to provide a large amount of bandwidth. As the higher the frequency, the more data that can be transmitted. The huge bandwidth/data capability of the mm wave frequencies is of utmost importance to 5G applications.
But these frequencies have different propagation characteristics and thus present numerous challenges such as large path loss, shorter transmission range, signal blocking/absorption by objects, low transmission capability etc. 
In order to combat these propagation challenges, base stations can be situated closer to the users (which reduces the loss) and makes a good case for citing various small cells indoors. However, an issue that could arise with citing base stations closer to the public is that it may increase the publics’ concern and worry about the impact of electromagnetic radiation on their health.
At these higher frequencies, the antennas are small due to the small wavelength dimensions (from the name mm-wave); hence the signals propagate very short distances, which is an issue for signal reception . Hence to maximize the signal reception, a large number of small antennas are combined within the transmitter and receiver to provide for spatial and multiplexing gain using a technique called Massive Multiple Input Multiple Output (MIMO).
Massive MIMO simply implies that the number of antennas is far greater than the number of data streams and requires hundreds/thousands of antennas, as shown below in Fig. 2.1 below .
Furthermore, adaptive beam-forming can also be used to target the radiation towards or away from the user, thereby reducing the interference, as depicted below in Fig. 2.2 below [7,8].
2.4 Dynamic Spectrum Management
Previously, spectrum assignment was static and regulators simply monitor that the spectrum assigned is being utilized for the right purpose. Research has shown this is inefficient and there are better ways to maximize the scarce spectrum resource. For example, dynamic spectrum allocation (e.g. Nominet Dynamic Spectrum Management) allows for the allocation of spectrum in real time using a geo-location database by checking for the usage, location of user and the demand . This increases the efficiency of the spectrum.
2.5 Spectrum Harmonization
From the foregoing, it is clear that 5G would rely on spectrum from the low band, mid band to the high band. For this reason, a new air interface, called 5G New Radio (NR), has been developed to support these wide range of bands, as depicted below in Fig. 2.3. The 5G NR will be discussed further in the next chapter. Fig. 2.3 shows that LTE works within the sub 6GHz whereas 5G NR would operate over a wide range of bands.
The effective use of frequencies in different bands is critical to the success of 5G services and therefore places huge emphasis on spectrum harmonization as the key to minimising cross border interference, facilitating international roaming agreements and reducing the cost of mobile devices .
It is clear from this chapter that the spectrum needed for 5G would be realized in different bands: Sub-1GHz for widespread coverage and Internet of Things (IoT) services, 1-6GHz for coverage and capacity and mm wave band for higher capacity; hence spectrum harmonization will vital in delivering the huge promises anticipated from 5G applications .
Furthermore, it has been noted that spectrum pricing in developing countries, are on average more than three times higher than in developed countries, after taken into considering the income levels within the country, in order to maximise revenues. This therefore leads to slower broadband services and worse coverage. These costs are often transferred to the consumers which therefore widens the digital gap. In order to accelerate the adoption of 5G services, regulators are advised to set affordable spectrum prices based on the market forces [10, 13].