In LTE-A (Long term evolution advanced) or LTE Rel. 10+, heterogeneous net-work deployments are supported. In a heterogeneous network, a macrocell provides basic coverage, complemented by multiple low-power pico-/femtocells where needed. Such deployments give scenarios of macro-pico/femto inter-cell interferences.
In different LTE releases, the inter-cell interference coordination (ICIC) mechanism may be used. In ICIC, the goal is to determine the resources (frequency, time and power) available in each cell, and schedule those resources to users such that the inter-cell interference can be avoided or reduced.
In a heterogeneous network (HetNet), base stations (BSs) or evolved Node Bs (eNBs) in terms of 3GPP (3rd Generation Partnership Project) LTE of pico-/femtocells can be operator-deployed or user-deployed. In the operator-deployed case, users of a macrocell are usually allowed to connect to the pico-/femtocell. This is known as open access. And in the user-deployed case, home BSs, femto, or Home eNBs (HeNBs) are installed by the users. Such low-power BSs are typically limited to a small set of user equipment (UE) (e.g. family members) which is called the closed subscriber group (CSG). A UE is usually not allowed to connect to the CSG if it does not have the access code. In this scenario, a non-CSG UE staying in close proximity to the CSG may be subject to strong interference from the HeNB and may not be able to access the macrocell at all.
In order to deal with these harsh interference problems, the so-called enhanced inter-cell interference coordination (eICIC) has been introduced, and the further eICIC (FeICIC) may be used as well. By using almost blank subframes (ABSs), the eICIC can schedule the frequency or time resource for macrocell and pico-/femtocell to avoid the interference.
Additionally, there is another scenario in which a pico-/femtocell UE is interfered by a strong macrocell. This setup occurs when it is preferable to have a UE connect to a low-power pico-/femtocell node even when it has significantly lower received power as compared to a high-power macro node. This setup may be useful when, for example, the strong cell has weak backhaul quality, or when it is desirable to enable traffic off-loading to the low-power nodes and to achieve true cell-splitting gains in the network. Especially in such HetNet situations, ICIC may not be fully effective in mitigating the inter-cell interference.
As ABSs still contain the cell-specific reference signals (CRSs), paging channel (PCH), physical broadcast channel (PBCH), and primary and secondary synchronization channels (PSS/SSS), etc, interference to the transmission at these positions cannot be avoided. Especially due to the large amount of CRSs distributed throughout the overall user bandwidth, which are primarily used for channel estimation, the impact of the interfering CRSs can be quite large. When the CRSs from both cells have different offset, the channel estimation is not affected by the interferer, but the interfering CRSs will wipe out some data tones. In this situation, muting of the interfered data tones is one solution for this problem. When CRSs from both cells have the same or similar offset, they will collide with each other. Based on the colliding CRSs, channel estimation accuracy will degrade severely and the demodulation performance suffers accordingly.