WCDMA Basic Theory
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This document defines and explains key symbols and concepts related to UMTS networks. It discusses: 1) Symbols such as Ec, Eb, Io, No, and their meanings in relation to energy, power, interference, and noise. 2) The concept of pilot pollution which occurs when there are too many strong pilot signals within coverage but none is dominant, negatively impacting network performance. 3) Causes of pilot pollution including unreasonable cell layout, antennas being too high, improper antenna azimuth or tilt, and ambient environmental effects. 4) Ways to reduce pilot pollution such as optimizing the network plan, proper site selection, and adjusting antenna parameters and transmit powers.
WCDMA Basic Theory
- 2. Symbol Explanation • Ec – Average energy per Chip – Not considered individually, but used for Ec/Io – Pilot Ec is measured by the UE (for HO) or the Pilot scanner, in the form of Received Signal Code Power (RSCP) – For CPICH Ec: • Depends on power and path loss. • Constant for a given power and path loss. Ec is not dependent on load – For DPCH Ec: • Depends on power and path loss
- 3. Symbol Explanation • Eb – Average energy per information bit for the PCCPCH, SCCPCH, and DPCH, at the UE antenna connector. – Typically not considered individually, but used for Eb/Nt – Depends on channel power (can be variable), path loss, and spreading gain (Gp) – Constant for a given bit rate, channel power, and path loss – Can be estimated form Ec and processing gain • Speech 12.2kbps example • Ec = -80 dBm • 12.2kbps data rate => Processing gain = 24.98 dB • Eb~ -80 + 24.98 = -55.02 dBm
- 4. Symbol Explanation • Io – The total received power spectral density, including signal and interference, as measured at the UE antenna connector. – Similar to UTRA carrier Receive Strength Signal Indicator (RSSI), at least for practical consideration (SC scanner) • RSSI in W or dBm • Io in W/Hz or dBm/Hz – Measured by the UE (for HO) or Pilot scanner in the form of RSSI – Depends on All channel power, All cells, and path loss – Depends on same-cell and other cell loading – Depends on external interferences
- 5. Symbol Explanation • No common RF definition – Thermal noise density – Typically not considered individually, but used for Eb/No – Can be calculated • No = KT – K is the Bolzman constant, 1.38*10^-23 – T is the temperature, 290 K • No = 174 dBm/Hz under typical conditions – Typically the bandwidth noise and the receiver noise figure are also considered • No = KTBNF, where NF is noise figure – To avoid confusion, NF should be used when referring to thermal noise
- 6. Symbol Explanation • No for WCDMA system – Total one-sided noise power spectral density due to all noise sources – Typically not considered individually, but used for Eb/No – Defined this way, No and Io are substituted for one another: • On the uplink the substitution is valid • On the downlink, differentiating between Noise and Interference is more challenging
- 7. Symbol Explanation • RTWP – Received Total Wide Bandwidth power – To describe uplink interference level – When uplink load increase 50%, RTWP value will increase 3dB • RSSI – Received Signal Strength Indicator – To describe downlink interference level at UE side
- 8. Symbol Explanation • RSCP – Revived Signal Code Power (Ec) – Ec/Io = RSCP/RSSI, to describe downlink CPICH quality • ISCP – Interference Signal Code Power; can be estimated by: • ISCP = RSSI – RSCP
- 9. Concept of Pilot Pollution • Concept of pilot pollution – Pilot pollution means that there are too many strong pilots within the coverage, but none of the pilots is dominant • Criteria of pilot pollution – There are more than 3 pilots with Ec > -95 dBm – The level difference between the strongest pilot and the fourth strongest pilot is < 5 dB
- 10. Effect of Pilot Pollution • Pilot pollution is specific to the CDMA system CDMA and greatly affects the network performance • Effect of pilot pollution – High BLER – Low system capacity – High call drop rate due to frequent handover – Low access success rate due to no dominant cell
- 11. Causes of Pilot Pollution • The causes of the pilot pollution includes: – Unreasonable cell layout – Too high site or antenna – Unreasonable azimuth or tilt angle of an antenna – Effect of the back lobe of an antenna – Effect of the ambient environment of the coverage
- 12. Unreasonable cell layout • The distances among the Site A, B and C are not balanced, and the location relation between the Sites is distinctly different from an equilateral triangle Site A Site C Site B
- 13. Too high site or antenna • The antennas of A and C are too high, so it is hard to control overshooting Pilot pollution
- 14. Unreasonable azimuth of an antenna • The antenna azimuth of the sector with scramble of 100 is unreasonable
- 15. Unreasonable tilt angle of an antenna • The too small antenna tilt angle results in overshooting
- 16. Effect of the back lobe of an antenna • Front-to-back ratio of the antenna does not meet the requirement, so the signal of the back lobe leaks
- 17. The Ways to Reduce Pilot Pollution • An area with pilot Pollution can be predicted in the planning simulation – Optimize the planned scheme to avoid the pilot Pollution • Optimal solution – excellent system design – Proper site – Proper azimuth and tilt angle of antennas – Proper transmit power and power ratio of sites