LTE UE Physical Layer Measurements

Physical layer measurements in LTE UE and eNB are carried out on both the downlink and uplink. This process occurs at Layer 1 and is reported to higher layers for various functions, including:

• Intra- and inter-frequency handover
• Inter-RAT (Radio Access Technology) handover
• Timing measurements
• Functions to support RRM (Radio Resource Management)

LTE UE physical layer measurements are defined in 3GPP 36.214. The following are the key measurements performed in the UE. Because LTE UEs are mobile, they often need to work with GSM, CDMA2000, UMTS TDD, and UMTS FDD networks. Measurements related to these RATs are also necessary.

Reference Signal Receive Power (RSRP)

Reference Signal Receive Power (RSRP) represents the linear average power across the channel bandwidth for LTE reference signals (RSs). RS exists in only one symbol at any time. It’s measured using REs (Resource Elements) that contain cell-specific RS. The absolute limit range is from +/-6 dB to +/-11 dB.

Reference Signal Receive Quality (RSRQ)

Reference Signal Receive Quality (RSRQ) provides a measure of signal quality. It’s defined as the ratio of RSRP to E-UTRA carrier RSSI (Received Signal Strength Indicator). RSRQ is measured only during connected states. The absolute RSRQ accuracy range is from +/-2.5dB to +/-4dB, similar to the inter-frequency relative RSRQ range of +/-3dB to +/-4dB.

UTRA FDD CPICH RSCP

UTRA FDD CPICH RSCP (Received Signal Code Power) is derived from UMTS. It measures the absolute power of one code channel within the overall UTRA CDMA signal. More specifically, it measures the code power of the CPICH (Common Pilot Channel). This measurement is used for interworking between LTE and UMTS RATs, similar to RSRP in LTE. It’s used in LTE inter-RAT IDLE and CONNECTED states.

UTRA FDD carrier RSSI

UTRA FDD carrier RSSI is also derived from UMTS. It measures the total received power, including thermal noise and noise generated in the receiver, across the receiver pulse shaping filter bandwidth. It’s the same as the E-UTRA carrier RSSI used in RSRQ calculations.

UTRA FDD CPICH Ec/No

UTRA FDD CPICH Ec/No represents the ratio of CPICH signal energy to the total power density in the channel. If diversity is not enabled, it is equal to (CPICH RSCP/RSSI). Typically, it will be around -10dB. UTRA FDD CPICH Ec/No is used in LTE inter-RAT IDLE and CONNECTED STATES.

GSM Carrier RSSI

GSM Carrier RSSI is measured by the LTE UE when it needs to interwork with GSM/EGPRS systems. It’s measured on the BCCH (Broadcast Control Channel). This measurement is used when the UE is in IDLE and CONNECTED states.

UTRA TDD Carrier RSSI

UTRA TDD Carrier RSSI is used for interworking with UTRA TDD-based systems. It’s similar to other RAT RSSI measurements and is carried out when the UE is in inter-RAT IDLE and CONNECTED states.

UTRA TDD P-CCPCH RSCP

UTRA TDD P-CCPCH RSCP measures the code power of the Primary Common Control Physical Channel (P-CCPCH). It is used in LTE inter-RAT IDLE and CONNECTED states and is the same as UTRA TDD RSRP.

CDMA2000 1xRTT Pilot Strength

CDMA2000 1xRTT Pilot Strength is the RSRP equivalent for the CDMA2000 system. It’s based on the 1.22Mcps chip rate system. The CDMA2000 pilot is carried on Walsh code 0 and is typically about -7dB from the total downlink power.

CDMA2000 HRPD Pilot Strength

CDMA2000 HRPD Pilot Strength is defined in the time domain and exists for about 9.375% of the frame. It’s important when the UE is interworking with HRPD (High Rate Packet Data) systems and is another variant of LTE RSRP.

Reference signal time difference (RSTD)

Reference signal time difference (RSTD) is measured during inter- and intra-frequency RRC CONNECTED states. It’s the relative time difference between when the UE receives a subframe from one cell ‘j’ and another cell ‘i’ at the antenna connector.

UE GNSS Timing of Cell Frame for UE positioning

UE GNSS Timing of Cell Frame for UE positioning is applicable when the UE is in RRC CONNECTED intra-frequency state. It measures the timing between cell ‘j’ and a GNSS-specific reference time for a given GNSS (e.g., GPS/Galileo/Glonass system time).

UE GNSS code measurements

UE GNSS code measurements are not related to E-UTRAN/UTRAN/GSM signals and are therefore independent of the UE state. It measures the GNSS code phase (integer and fractional parts) of the spreading code of the ith GNSS compliant signal. The antenna connector of the UE is taken as the reference point.

UE Rx - Tx time difference

UE Rx - Tx time difference is carried out when the UE is in RRC CONNECTED intra-frequency. It’s defined as:

T UE-RX - T UE-TX

Where:

• T UE-RX is the UE received timing of DL frame #i from the serving cell, defined by the first detected path in time.
• T UE-TX is the UE transmit timing of uplink radio frame #i.

The reference point for the UE Rx - Tx time difference measurement is the UE antenna connector.