LTE: Cell Range Extension (CRE)

In a Heterogeneous Network (HetNet) introduced in 3GPP Release 10, low power nodes (LPNs) such as RRUs/RRHs, Pice cells, femto cells, and relay nodes are deployed in a macro cell.

          Small cells are primarily added to increase capacity in hot spots with high user demand and to fill in areas not covered by the macro network – both outdoors and indoors. They also improve network performance and service quality by offloading from the large macro-cells. The result is a HetNets with large macro-cells in combination with small cells providing increased bitrates per unit area.

Cell Range Extension
A major issue in Hetnet planning is to ensure that the small cells actually serve enough users. One way to do that is to increase the area served by the small cell, which can be done through the use of a positive cell selection offset to the the received power of the small cell. Extending the coverage of a cell by means of connecting a UE to cell that is weaker than the strongest detected cell is referred to as cell range extension.

In a network, with all cells using the same frequency (frequency reuse = 1), the UE normally camps on a cell with strongest received DL signal (SSDL­­­); hence the border between two cells is located at the point where SSDL is the same for both the cells. In homogeneous networks, this also typically coincides with the point of equal path loss for the UL (PLUL) in both cells.

In a Hetnet, macro cell transmits at a very high power as compared to LPN. So, as shown in the figure below, there are two borders, a Downlink cell border which is defined by SSDL­­­ and an Uplink cell border which is defined by PLUL. In the Downlink, SSDL observed from the macro cell and the pico cell are equivalent at a location that is closer to the pico cell, which forms the Downlink cell border. On the other hand, the location where path loss to macro and pico cells is equivalent is far from the pico cell. This is due to the fact that the UE transmission power is same whether the UE is being served by macro cell or pico cell.


In a normal case without CRE feature, the serving cell choice for the UE is based on the DL received power. With CRE, a positive cell selection offset is added to the SSDL of the small cell to increase the range served by small cell. So, more UEs are attracted by small cell as compared to the case without CRE which greatly increases HetNet efficiency.

          In CRE (gray region) in the figure above, the UE does not necessarily be served by the eNB that has the strongest DL received power.

In addition to the advantages of increasing HetNet efficiency, offloading macro cell’s traffic, CRE has another advantage of reduced Uplink interference in the system. A UE without CRE, served by the macro cell closer to the pico cell border, would be transmitting at a higher power and interfering with the uplink of small cell. With CRE, the same UE is likely to be served by closer cell (small cell) with lower path loss reducing the need for the UE to transmit with such a high power.

As discussed already, the range of the small cell is expanded by implementing a selection offset (UEs in idle mode) or handover threshold in measurement configuration (UEs in connected mode) in the favor of the small cell.

UEs in Connected Mode
A connected mode UE may be configured with cellIndividualOffset as defined within measObjectEUTRA corresponding to the frequency of the neighbor cell. Measurement events that are influenced by this parameter are A3, A4, A5 and A6.

This parameter is applied individually to each neighbor cell with load management purposes. The higher the value allocated to a neighbor cell, the “more attractive” the cell will be.

As an example for event A3, (not considering other offsets and hysteresis), the event entering condition is fulfilled when;
 where Mn and Ms ­­­are measurement result of the neighboring and serving cells respectively. So, the more the value of cellIndividualOffset, the early the UE sends measurement report indicating neighbor cell (small cell) is better than the serving.

UEs in Idle Mode
In Idle mode, mainly, parameter q-OffsetCell within IntraFreqNeighCellInfo (in SIB4) and within InterFreqNeighCellInfo (in SIB5) used for cell range extension purpose. This parameter is mainly used in cell-ranking criterion for neighbor cell.

Having discussed the benefits of CRE feature, a major drawback of this feature is that in the CRE zone, the UEs are forcibly being served by the small cell, but in reality, the downlink received power from the macro cell is higher than the small cell. So, there will be severe downlink inter-cell interference from the macro cell to UEs receiving transmissions from a pico cell i.e., the SINR at the UEs in CRE which is being served by small cell is below 0 dB. This may impact the reception of the DL control channels in particular.

A number of features added to the 3GPP LTE specifications can be used to mitigate the above-mentioned interference problem in HetNets with small cells. These features will be discussed in detail in further posts. Usually CRE is jointly designed with ICIC/eICIC/FeICIC schemes/features to mitigate this interference problem.

Reference: 3GPP TS 36.300, 36.331, HetNet

1 comment:

  1. AOA,
    Sir at one place your mention that "UE transmission power is same whether the UE is being served by macro cell or pico cell"........ while in other place you mention that " A UE without CRE, served by the macro cell closer to the pico cell border, would be transmitting at a higher power and interfering with the uplink of small cell."... so which one is correct? user transmitting with same power or different power in uplink???