The main reason why HetNets are interesting and challenging as opposed to the traditional Hierarchical Cell Structure (HCS) approach is that the deployments of different sizes of cells generally takes place in the same frequency (co-channel) as opposed to different frequencies (inter-channel).
It would not take a lot of understanding to point out that this would give rise to interference and may make the network unusable. As you can guess, the standards are well aware of this. During the last few releases, 3GPP has been working hard to overcome this challenge of interference and make sure the network behaves as expected. We will look at some of these approaches below.
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X2 – the magical interface
Evolved or Enhanced ICIC (eICIC)
Cell Range Expansion or Extension (CRE)
Further enhanced ICIC (feICIC)
“The initial deployments of LTE did not place too much emphasis on X2 interface being present. Inter-operability was another issue for which X2 didn’t work very well. One of the major benefits of having an X2 interface is that different base stations or eNodeB’s (hereafter referred to as just eNB’s) can talk to each other and coordinate to make sure interference is kept at the minimum, especially on the cell edges.”
“There will nevertheless be pre Release-10 devices that are unaware of this interference management arrangement. These devices may experience significant interference while on the edge of the Small-cell.”