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Case Study
RF Engineering & Training Services
Evaluating the System Uplink Between RTWP/RSSI Reference Points for an Ericsson UMTS 1900 System
RTWP Reference Point
RSSI Reference Point UMTS System Radio Unit
Filter Unit
Dual TMA Diplexer
Antenna
Feeder LNATMA
LNAFU
Antenna System Loss (ULAttenuation) Net UL Gain
Date: June 2012 Document Number: 1801-00201-EN
By Jim Bryant Bryant Solutions, Inc. +1.925.968.9265
[email protected] www.bryantsolutions.com
INTRODUCTION This case study examines the uplink (UL) path between the Receive Total Wideband Power (RTWP) and Receive Signal Strength Indicator (RSSI) reference points for an Ericsson UMTS 1900 system with a tower mounted amplifier (TMA). Existing system UL parameters are used to determine if the actual RTWP (RTWPACT) is equal to the system calculated RTWP (RTWPCALC) using the measured RSSI. Uplink parameters will be changed (as necessary) to ensure the calculated RTWP value is accurate. Impact on mobiles, system performance and site statistics will be considered.
ANALYSIS OF EXISTING PARAMETERS AND THEIR IMPACT The diagram of the SUT below illustrates the RSSI/RTWP reference points and loss/gain definitions. RTWP Reference Point
RSSI Reference Point UMTS System Radio Unit
Dual TMA Filter Unit
Diplexer
Antenna
Feeder LNATMA
LNAFU
Antenna System Loss (ULAttenuation) Net UL Gain
The following UL parameters are defined for the SUT: • • • •
ULGain: 12 dB (Set by User) ULAttentuation: 3.5 dB (Set by User) Filter Unit Gain (FU_Gain): Automatically Set by System to Achieve Net UL Gain Below. Net UL Gain: Defined by System Manufacturer for Carrier.
For the SUT, ULGain represents the gain for the low noise amplifier (LNA) in the TMA, and ULAttenuation should represent the defined system loss and additional external system gains between the RTWP reference point and the input to the UMTS system. Based on the ULGain and ULAttenuation settings, the UMTS system will automatically attempt to set the gain of the LNA in the filter unit (FU) to achieve a pre-defined net UL gain between the RTWP and RSSI reference points. The first step is to verify the parameter ULAttenuation accurately reflects the actual system loss. A system loss of 6.7 dB was calculated for the SUT using the following: • • • •
Connections: 7 * 0.07 dB ≈ 0.50 dB Feeder: 1.13 dB/100’ *330’ = 3.7 dB (Based on an antenna height of 300’) Jumpers: 3.25 dB/100’ * 30’ ≈ 1 dB (Based on 10’ jumpers) Filters: ~1.5 dB
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The system can now be analyzed. To do this, an arbitrary value of -104 dBm will be selected for RTWPACT. Based on RTWPACT, RSSI can be calculated using the actual system losses and gains. The system then uses the measured RSSI value to calculate RTWPCALC. The actual and calculated RTWP should be equal. RTWPACT → RSSI Calculation RSSI = RTWPACT + LNATMA - SysLoss + LNAFU = -104 dBm + 12 dB – 6.7 dB + LNAFU = -98.7 dBm + LNAFU RSSI → RTWPCALC Calculation RTWPCALC = RSSI - LNAFU + ULAttenuation - ULGain = (-98.7 dBm + LNAFU) - LNAFU + 3.5 dB - 12 dB = -107.2 dBm Therefore RTWPACT > RTWPCALC.
IMPACT OF AN IMBALANCED UPLINK The proof below shows that having RTWPACT > RTWPCALC causes mobiles to initially power up too low. Though it is desirable to have a mobile initially power up low, this should be achieved with the parameter constantValueCprach, while having RTWPACT equal to RTWPCALC. In addition, the reported RTWP will always be 3 dB lower than it should be, potentially masking problems. L_PCPICH MOBILE RSCP P_PRACH
BASE STATION RTWP CPICH constantValueCprach
CPICH - L_PCPICH = RSCP P_PRACH - L_PCPICH = RTWP + constantValueCprach Therefore P_PRACH = RTWP + constantValueCprach + CPICH - RSCP Givens: CPICH = +33 dBm RSCP = -90 dBm constantValueCprach = -22 dB If RTWP = -104 dBm then P_PRACH = -104 dBm - 22 dB + 33 dBm - (-90 dBm) = -3 dBm If RTWP = -107 dBm then P_PRACH = -107 dBm - 22 dB + 33 dBm - (-90 dBm) = -6 dBm
RTWPACT = RTWPCALC The goal is to achieve a net uplink gain where RTWPACT is equal to RTWPCALC. This can be obtained by changing the parameter ULAttenuation to reflect the actual system loss of 6.7 dB as shown below. RTWPCALC = -98.7 dBm + 6.7 dB - 12 dB = -104.0 dBm Bryant Solutions, Inc, Part Number: 1801-00201-EN, Rev. 2
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CONCLUSION The uplink path between the RTWP and RSSI reference points for an Ericsson UMTS 1900 system was analyzed. The goal was to verify whether the actual RTWP (RTWPACT) was equaled to the RTWP calculated by the system (RTWPCALC). For the system analyzed in this case study, RTWPACT was greater than RTWPCALC, which could cause a mobile’s initial power level (P_PRACH) to be too low; however, in this case, there will more than likely be no measureable impact to network performance given only one or two power steps are required for the site to hear the mobile; however, this would also depend on the setting for the parameter constanValueCprach. This scenario will cause RTWP to be inaccurately reported by 3 dB, which could mask interference issues. In general, a mobile’s initial power level will be impacted as follows: •
•
•
RTWPACT = RTWPCALC: o Best scenario. o Minimum number of preambles given constantValueCprach set properly. RTWPACT > RTWPCALC: o P_PRACH may be too low. o May require too many preamble steps. o Reported RTWP inaccurate. RTWPACT < RTWPCALC: o P_PRACH will be high. o Mobiles may need to ratchet down power. o Could cause runaway situation during a special event. o Reported RTWP inaccurate.
A Receive Antenna Interface Tray (RXAIT) or Remote Radio Unit (RRU) can be compensated for in the ULAttenuation parameter, which is why it is sometimes a negative number. In addition, the RTWP reference point will change when a system does not have a TMA. Overall, it is important to ensure the calculated and actual RTWP values are equal.
ABOUT BRYANT SOLUTIONS Bryant Solutions offers technical training, accredited certifications, and a wide range of solutions and services designed to drive higher Quality of Service and increased revenue for our clients in the Telecommunications Industry. Our company also designs and develops proprietary systems to augment its training courses in solidifying the learning experience. These proprietary systems simulate various real world conditions and problems students will encounter on the job. Since 2002, Bryant Solutions has focused on developing training courses for wireless carriers, tower contractors, local government agencies and the military. For more information visit www.bryantsolutions.com.
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