Small cells are versatile. They can be used indoors and out of doors, for coverage extension and capacity enhancements, in the same frequency as surrounding cells or on different frequencies. If an LTE operator has an RF issue, chances are pretty good that a small cell can solve the problem. That’s not to say that deploying a small cell is necessarily the best choice in all cases, or that the operator can just plop the small cell down without considering the potential side effects to the surrounding cells and the nearby subscribers.

Take the case of an intra-frequency outdoor small cell intended to address a capacity hot-spot within the normal macrocell coverage area, such as a park or a nightclub area where a lot of people congregate. The macrocell signal in these areas may be perfectly fine, but the sheer number of users may overwhelm the capacity of the macrocell. A well-positioned small cell can offload some of that traffic, freeing up capacity on the macrocell for other users and providing a better experience for nearby subscribers.

This scheme only works if the small cell can pull in enough users to justify the effort and expense of deploying the cell, but not so many that the small cell itself becomes overloaded. The tricky part for the engineers is that, in many cases, the macrocell’s RF signal may still be significantly stronger than the small cell’s. If the operator does not compensate for this, the UEs will just lock on to the macrocell and never move to the small cell. What needs to be done to encourage UEs to move to the small cell instead?


Clowns to the Left of Me

The process starts when the UE powers up or otherwise appears in the LTE network. In idle mode cell selection, the UE determines which LTE cell it should camp on, based on its RSRP measurements of the surrounding cells and the selection criteria it reads from the System Information Blocks (SIBs) broadcast over the air. The UE evaluates the cell it is currently on, and then determines if it should actually move somewhere else. The UE makes two basic checks when selecting or reselecting a cell: is the cell good enough, and is it the best cell?

“Good enough” is determined by seeing if the measured RSRP value of the cell is greater than the minimum criteria for that cell. In other words, does the cell meet Srxlev = (RSRP – (Qrxlevmin + Qrxlevminoffset) – Pcompensation) > 0, where RSRP is the cell’s measured Reference Signal Received Power and Qrxlevmin is the cell’s configured minimum acceptable RSRP value. Qrxlevminoffset is used to steer UEs based on the supported networks in the cell, while Pcompensation adjusts the calculation according to the UE’s maximum power level; both of these values are typically set to 0 dB.

In the example illustrated below, both the macrocell and the small cell are configured with a Qrxlevmin value of -120 dBm. Since the UE measures the macrocell at -92 dBm and the small cell at -103 dBm, both cells meet the minimum criteria, and are therefore suitable for use.

Jokers to the Right

So which one will the UE actually choose? That depends on their relative rank.

The rank of the cell the UE is currently on (the serving cell) is calculated as Rs = RSRPs + Qhysts, where RSRPs is the serving cell’s RSRP value and Qhysts is the serving cell’s configured hysteresis value. In this example, Rs = -92 dBm + 2 dB = -90 dBm. Conversely, the rank of a neighboring cell is calculated as Rn = RSRPn – Qoffsetn, where RSRPn is the neighbor’s RSRP value and Qoffsetn is the configured offset value for that neighbor. In effect, the current cell is made to look better than it really is, while the neighbors look worse than they really are, so that when the UE finally decides to switch cells, it tends to stay there for a while.

Note that offsets may be different for each neighbor, and can be positive or negative values. (Hysteresis values must be greater than or equal to 0 dB.) In this example, Rn = -103 dBm – (-15 dB) = -88 dBm. Whichever cell has the highest rank will be selected. Here, the small cell wins, even though the macrocell’s signal strength is actually 11 dB higher.

By selecting appropriate values for Qhyst and Qoffset, the operator can steer idle UEs into the underlying small cell, without having to play games with the actual radio signal. As long as the small cell’s rank remains higher than the macrocell’s, the UE will remain camped on the small cell, and will use the small cell to request connections in order to deliver the user’s services.

In the next segment of this discussion, we’ll look at the corresponding calculations that take place when the UE is connected. What has to be done to move an active UE between a small cell and a macrocell? Stay tuned…