To realize the envisioned 5G services, significant enhancement
of per-user throughput and overall system
capacity is required, compared with those of the
4G system. Such an enhancement can be achieved
through advanced PHY/MAC/network technologies
and efficient methods of cell deployment and spectrum
management. In particular, utilizing a larger
amount of system bandwidth guarantees an increase
in the capacity by allocating more frequency
resources to each user in the system. Therefore, utilizing
the spectrum where huge bandwidth is available
can be considered to be the most critical issue
for the 5G system.
Currently, the 4G system specifies its operating frequency
bands and some of them are assigned to
cellular operators. These deployments can be beneficial
since the existing 4G system will not interfere
with other RATs.
However, obtaining the licensed spectrum requires
not only considerable financial investment, but also
a significantly long period of time spent on regulatory
procedures. More importantly, a substantial
portion of the licensed spectrum around 2 GHz is
already being used. Therefore, finding a bandwidth
that is wide enough to support the 5G system is very
challenging.
The recent trend in spectrum management is to
aggregate both the licensed and unlicensed spectrums
to extend available system bandwidth, as
shown in Figure 17.