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Sub-Carrier Activity Factor

Question

“If only RS is transmitted (i.e., unloaded cell) the resulting sub-carrier activity factors would be x = 1/6 and x = 1/3 for one and two Transmit antennas, respectively” is there any formula about it?

Answer

The Sub-Carrier Activity Factor is a parameter to characterize the total amount of cell received power on the number of transmitted sub-carriers in the OFDM symbol carrying Reference Signal Power an on the other number of transmit antennas.

This impact can be expressed by:

Stot Definition

x is defined as the Sub-Carrier Activity Factor

To explain in details the Sub-Carrier Activity Factor let’s start from RSRP.

RSRP in formula is defined as

RSRP Formula

In the following figure, this REs are denoted with RE0. To improve the accuracy of the RSRP estimate, the UE may optionally also measure the RS transmitted from the second antenna port (RE1), if present. In case of four BTS transmit antennas, Reference Signals of the third and fourth BTS antenna ports are not used in the RSRP measurement.

Since all LTE UEs have at least two receive antennas, it is also mandated by that the RSRP must be equal or higher than the stronger of the two receive antennas’ individual measured RSRP.

X is defined as

xwhere RE indicates number of RE used and RB is equal to 12 RE.

So we have:

  • x = 1 indicates full load. It means that all sub-carriers of one transmit antenna are transmitted for the OFDM symbol carrying RE0.
  • If only RS (Reference Signal) is transmitted (i.e., unloaded cell) the resulting sub-carrier activity factors would be
    • x = 1/6 for one transmit antenna
    • x = 1/3 for two transmit antennas

RS

Figure 1: Reference Signals as seen by UE, shown for transmit antenna port 1 (RE0) and
transmit antenna port 2 (RE1) for one 1ms sub-frame and 12 sub-carriers.

When calculating for two transmit antennas, one should take into account that REs overlapping with adjacent antenna RS transmission are muted, and therefore, for example, in a fully loaded 2Tx cell the scaling factor is = 5/3, instead of two (It is assumed that all sub-carriers have the same power, i.e., there is no power boosting for any channel.)

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