With the continued massive rollout of Long Term Evolution (LTE) and more and more operators becoming obsessed with staying up to date, the issues of limited spectrum remains. Multi-layer heterogeneous network layout including small cell base stations are key for further enhancements of the spectral efficiency achieved in mobile communication networks. As operators move forward with this plan, inter-cell interference is becoming the limiting factor when it comes to achieving not only high average user satisfaction, but also when trying to achieve satisfaction for as many users as possible. Therefore, according to Nomor Research, researchers are focusing on inter-cell interference coordination (ICIC) when defining next generation mobile communication standards, such as LTE-A (LTE Advanced). But ICIC LTE itself is an issues that comes as no surprise.
3GPP is no dummy – they were ready for this. ICIC was originally introduced for LTE in 3GPP Release 8. eICIC was introduced in 3GPP’s Release 10, as part of LTE-A. So what is the difference? The WirelessMoves blog breaks it down:
3GPP Release 8 LTE ICIC: This method is optional and is used to decrease interference between neighboring macro base stations. This is achieved by lowering the power in parts of the subchannels in the frequency domain which then can only be received close to the base station. These subchannels do not interfere with the same subchannels used in neighboring cells, allowing data to be sent faster on those subchannels to mobile devices that are close to the cell.
3GPP Release 10 LTE-Advanced eICIC: This is part of the heterogeneous network (HetNet) approach, where macro cells are complemented with pico cells inside their coverage area (hotspots in shopping centers, at airports, etc.). While the macro cells emit long range high power signals, the pico cells only emit a low power signal over short distances. To mitigate interference between a macro cell and several pico cells in its coverage area, eICIC coordinates the blanking of subframes in the time domain in the macro cell. In other words, there is no interference in those subframes from the macro cell so data transmissions can be much faster. When several pico cells are used in the coverage area of a single macro cell overall system capacity is increased as each pico cells can use the empty subframes without interference from the other pico cells. The downside is of course that the macro cell capacity is diminished as it can’t use all subframes. Therefore, methods have to be put in place to quickly increase or decrease the number of subframes that are assigned for exclusive use of in pico areas when traffic patterns change.
“In other words, ICIC is a macro cell interference mitigation scheme, while eICIC has been designed as part of HetNet to reduce interference between the macro and pico layer of a network (once pico cells are rolled out to increase coverage and system capacity).” [1]
But, LTE-A is not anywhere near ready. According to 3GPP, work on LTE Advanced is ongoing. Release 10 of the standard already introduces significant technological enhancements to meet the ambitious requirements set forth by 3GPP. “Besides features to boost overall peak data rates like carrier aggregation or higher-order spatial multiplexing for uplink and downlink, enhancement of inter-cell interference coordination (ICIC) has moved into the focus of intensive research and standardization. The reason is that the evolution of the physical layer has reached a level, where operation very close to information theoretic bounds on achievable spectral efficiency for a given signal-tointerference-and-noise (SINR) ratio is feasible. Significant increases in spectral efficiency can therefore only be achieved by improving the SINR through reduction /avoidance of the interference. The basic goal of ICIC in practice is the provision of a more homogeneous service to users located in different regions of the network, i.e. mostly to promote the cell-edge performance. This aspect has gained even more importance with the introduction of multi-layer heterogeneous networks.”
Sever Inter-Cell Interference Scenarios
To successfully launch multi-layer HetNet networks applications for LTE and LTE-A, it’s advised to build up the network of not only a single type of eNodeB (homogeneous network), but to deploy eNodeBs with different capabilities, most importantly different Tx-power classes. Commonly referred to as macro eNodeBs (MeNB), pico eNodeBs (PeNB) and femto/home eNodeBs (HeNB), these nodes are intended for basic outdoor, outdoor hot-zone and indoor/enterprise coverage, respectively. “The concept of HetNets supported by SON procedures offers great opportunities to enable large scale, low cost deployment of small base stations below roof top to boost user experience at hotspots or to improve network coverage. The traffic offloading effect (or cell-splitting gain) can be leveraged by cell range expansion for PeNBs that lets a larger number of users connect to the small cells. On the other hand, HetNets is introducing new challenges regarding inter-cell interference coordination and load balancing as well as user mobility in idle and active mode.”
Heterogeneous network layouts introduce two predominant inter-cell interference scenarios: The macro-pico scenario with cell range expansion (CRE) depicted. The other is a macro-femto scenario with closed subscriber groups (CSG). The two interference scenarios are dual in many ways. For example they both experience severe interference situations occurring for UEs served by the PeNB. Procedures to alleviate the interference there can often be applied in a dual manner in the macro-femto scenario. [1]
[1] What’s the Difference Between LTE ICIC and LTE-Advanced eICIC?, WirelessMoves, http://mobilesociety.typepad.com/mobile_life/2012/03/whats-the-difference-between-lte-icic-and-lte-advanced-eicic.html
[2] Inter-Cell Interference Coordination for LTE Advanced Heterogeneous Networks, Nomor Research, http://www.nomor.de/home/technology/white-papers/icic-for-lte-a-hetnets
