LTE RSRP, RSRQ and RSSI Measurments

If your smart phone has to write its autobiography. The first thing your phone will describe is the amount of measurements it continues to read all the time. These measurement signals are the life blood of your phone , so that you can use it for the purposes you want—— be it to call someone , email, WatsApp etc.

How a UE communicates ? : Measurements and Signals

Among various measurements, the three most important ones are :

RSRP

RSRQ

RSSI.

All these are derived from Reference signals.

Reference signals are equivalent  to what  Pilot signals do in UMTS.

RSRP – Reference Signal Received Power 

RSRP is the water and wine for a UE, from the moment a UE is powered-on  to the point it goes into idle mode. RSRP measurements will be used by the UE at all times.

If analogy helps RSRP is the equivalent tof the UMTS CPICH Received Signal Code Power (RSCP).

What is the usage of RSRP?

RSRP measurements are used for

• Cell selection
• Cell reselection
• Handover
• Mobility measurements
• Estimate the Path Loss for power control calculations

How it is calculated?

RSRP is the average power received from a single cell specific Reference Signal Resource Element

1. The average RSRP is taken in linear units
2. Power measurement is based upon the energy received during the useful part of the OFDMA symbol and excludes the energy of the cyclic prefix.
3. Reference point for RSRP measurement is the antenna connector of the UE

Please note, that RSRP can be based upon the cell specific Reference signal transmitted by only the first antenna port or RSRP can be based upon the cell specific reference signal transmitted by first and second antenna ports

RSRP Measurement Reporting:

When RSRP value is reported back, its not that UE send the actual measurement right away. No , No, No.

In fact, a mapping is applied to RSRP measurements prior to including them within RRC messages.

The range of RSRP measurements  is defined from -140 dBm to -44 dBm  with one dB resolution,

while the reporting range is an integer value between 0 and 97.

Mapping between reported and measured RSRP values

What is the maximum RSPR value which can be reported?

The maximum reportable RSRP is based upon the -25 dBm maximum input power for a UE as specified by 3GPP.

1.4 MHz channel bandwidth has 72 Resource Elements in the frequency domain. RSRP is based upon the power of a single resource element so the maximum RSPR equals

-25 – 10xLOG (72) = -44 dBm

What is the minimum RSPR value which can be reported?

The minimum reportable value is based on the assumptions:

-a maximum path loss of 152 dB,

-a transmit power of 43 dBm

-a 5 MHz channel bandwidth (25*12 = 300 subcarriers)

The above assumptions lead to a minimum RSRP of

43-152-10xLOG (300) = -134 dBm.

Note: An addition of 6 dB has been subtracted to provide some margin

Typical field values for RSRP range from -75 dBm close to site to -120 dBm at the edge of LTE coverage

RSSI – Reference Signal Strength Indicator 

The RSSI is calculated as a linear average of the total power measured across OFDMA symbols which contain Reference Symbols transmitted from the first antenna port (if MIMO is not used). E.g., OFDM symbols 0 and 4 in a slot , in the measurement bandwidth over N resource blocks.

The total received power of the carrier RSSI includes the power from :

• co-channel serving cells
• non-serving cells
• adjacent channel interference
• thermal noise etc.
• Total measured over 12 sub-carriers including reference signals from serving cell and traffic in the serving cell.

RSSI provides information about total received wideband power including all interference and thermal noise

Simply we can write it as :

RSSI = wideband power =  noise + serving cell power + interference power

RSSI is a more traditional metric which has been used in other technologies such as GSM and CDMA1X etc.

RSRQ – Reference Signal Received Quality 

In order to get more details about Channel quality and whole bandwidth. A better metric to measure is Reference signal received quality. RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal.  The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision.

RSRQ is the equivalent of UMTS CPICH E_c/I_{o .}

Usage of RSRQ

RSRQ measurements are also used for

• Cell selection
• Cell reselection
• Handover
• Mobility measurements

How it is computed?

Mathematically RSRP is defined as:

RSRQ = RSRP / (RSSI/N)

Where:

N = # of resource Blocks over which the Received Signal Strength Indicator (RSSI) is measured.

Measurement can be done on Narrowband or Wideband:

Narrowband N = 62 sub carriers (6 Resource Blocks)

Wideband N = Full bandwidth (up to 100 Resource Blocks or 20 MHz)

Reference Point:

The reference point for RSRQ measurement is the antenna connector of the UE.

RSRQ Measurement Reporting:

Just like RSRP measurement reporting. Similarly, RSRQ measurements are not transmitted straight away instead a mapping is applied to RSRQ measurements.

The measurement of RSRQ is defined from -3 to -19.5 dB

Whereas the reporting range for RSRQ measurements have an integer value between 0 and 34.

Mapping between reported and measured RSRQ values

Maximum Reporting Value

The maximum reportable RSRQ is based upon the assumption that only the cell specific Reference Signal Resource Elements are occupied, in other words no traffic is transferred.

As there are 2 cell specific reference signal resource elements per OFDMA symbol so the calculation becomes:

RSRQ = RSRP / (RSSI / N) = RSRP / (2 x RSRP x N/ N) = 0.5 = -3 dB

During handover, operators can configure paramete for UE to do measurement of RSRP alone or RSRQ or both.

In this way, it can help UE to make better decisions for mobility measurements.

SUMMARY

 In a nutshell it can be concluded, that RSRP provides information about signal strength , while RSSI helps in determining interference and noise information. Therefore, this is why RSRQ (QUALITY) measurement and calculation is based on both RSRP and RSSI. 

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