One of the roles for small cells in an LTE network is to provide additional capacity in localized hot-spots. In intra-frequency outdoor scenarios, however, the RF signal from the overlying macrocell can easily overwhelm the small cell, preventing it from pulling in enough traffic. In the previous discussion (Stuck in the Middle with You – Part 1), we talked about how key RF configuration parameters can be used to steer idle UEs in and out of small cells.

What happens with the UE is connected, rather than idle? The problem is essentially the same: without making the appropriate adjustments, UEs served by the small cell see the macrocell as a much stronger (and therefore better) signal, and will request to be handed over, even though they are still well within range of the small cell.

Fortunately, the solution is also the same: change the definition of “better”, so that the UE stays on the small cell until the user leaves the small cell’s coverage area.

 

Trying to Make some Sense of it All

Connected UEs are configured by the serving cell with one or more sets of measurement criteria, identifying what the UE should measure and when it should report the results. In intra-frequency handover scenarios, the UE is typically set up to report A3 events, which occur when the UE detects a neighbor which is some number of dB stronger than the serving cell.

Take the case of a UE currently on a small cell underneath a more powerful macrocell. In the illustration below, for example, UE A is well within the small cell’s coverage area, but the macrocell has the stronger signal (an RSRP value of -95 dBm, versus the small cell’s RSRP of -101 dBm). The hysteresis and offset values typically used for A3 events only require the neighbor cell to be 2 to 5 dB stronger than the serving cell before the event is reported. In this situation, UE A would report the A3 event and would be immediately handed over to the macrocell. The small cell would be emptied of traffic within a few seconds.

Instead, the trick is to make the small cell look more attractive (or, conversely, make the neighboring macrocell look less attractive) so that the handover isn’t triggered until the UE reaches the edge of the small cell. In this example, the hysteresis value for the small cell (Qhyst) is set to 10 dB, while the offset for the macrocell (Qoffset) is set to 2 dB. Note that these hysteresis and offset values are separate from the ones used for idle cell selection and reselection, although they have the same meaning and use.

Let’s look at the results. UE A takes the small cell’s RSRP value and adds Qhyst to it (-101 dBm + 10 dB = -91 dBm), and takes the macrocell’s RSRP and subtracts Qoffset from it (-95 dBm – 2 dB = -97 dBm). Since the adjusted value for the macrocell is less than the adjusted value for the small cell (-97 dBm < -91 dBm), the A3 event is not triggered, the UE does not send a Measurement Report to the small cell, and no handover occurs. UE A remains on the small cell, precisely the desired behavior.

UE B at the edge of the small cell does exactly the same thing, but its results are different. The small cell’s adjusted measurement is -104 dBm + 10 dB = -94 dBm, while the macrocell’s adjusted measurement is -91 dBm – 2 dB = -93 dBm. Since the macrocell is now perceived to be stronger than the small cell, event A3 is triggered, UE B sends a Measurement Report to the small cell identifying the macrocell as a handover target, and the UE is handed over to the macrocell. Again, we have achieved the desired result. Despite the macrocell’s stronger signal levels, UEs are able to remain on the small cell until they move out of the intended coverage area.

Similar results are achieved in the other direction (from the macrocell to the small cell) by choosing appropriate Qhyst and Qoffset values for the macrocell. In this case, Qhyst would be relatively small (say 2 dB), while Qoffset would be a larger negative value (such as -10 dB), in order to make the small cell more attractive at the proper time and place.

 

Here I Am

LTE provides the tools the operator needs to steer traffic in and out of small cells, even when the macrocell’s signal dominates the coverage. The challenge for the operator is to determine what the appropriate settings for each cell must be, in order to move enough users into the small cell and keep them there, without exceeding the capacity of the small cell.