UMTS Radio Transmission Theory
RF Optimization Policy
RF Adjustment and Network Simulation
UMTS Radio Transmission Theory
Mobile Communication Environments
- Low antenna of UE
- Transmission paths are always influenced by terrains and man-made environments; various terrains and complex buildings, forests and so on make signals received as overlap of scattering signals and reflected signals.
- Mobility of UE
- UE is always moves, or the peripheral environments change. This makes a transmission path between a base station and an UE change all the time. In addition, the difference of direction and speed of an UE relative to the base station also causes changes of signal levels.
- Signal levels change at random
- Signal levels change with time and position; it can be described only with probability distribution of random process.
- Waveguide effect exists in urban environment
- Serious man-made noises
- Man-made noises include noises in starting motor vehicles, power line noises and industrial noises.
- Serious Interference
- Generally, there are co-frequency interference, adjacent-channel interference, intermodulation interference, local to remote ratio interference. co-frequency interference and adjacent-channel interference are the main factors.
Types of Radio Wave
Transmission
- Types of radio wave transmission: Direct wave, reflected wave, diffracted wave and scattering wave
- Sight distance and non-sight distance transmission, multi-path environments of complex forms
- Loss through buildings/vehicles
Radio
Signal Presentation Methods
- A signal is a random value, so it must be characterized jointly by a median and a transient value. An actually received signal is a median overlapped with a transient value. The median is called slow fading and the transient value is called quick fading.
- m(x) is slow fading, or local average, or long-term fading.
- r0(x) is quick fading, or Rayleigh fading, or short-term fading.
- The two methods for presenting signal field strength are used in different occasions: The signal presented in a time function is used for studying signal fading; while a signal presented in a distance function is used for studying transmission loss curve. Variation of the median level of a received signal with time is far less than that with location.
Statistical
Features of Slow Fading
- Definition of slow fading
- It is the average of attenuated signals received, that is, average (or field strength value or loss value) of signal levels attained in a specified length L. The value of L is 40 wavelengths, with 36~50 signals for test.
- Cause of slow fading
- Slow fading is caused by changes of terrains and man-made environments on transmission paths.
- Probability density function and accumulation probability distribution function of slow fading
Statistical Features of
Quick Fading
- Definition of quick fading
- It is the transient value of fading signals received.
- Cause of quick fading
- When transmission is reflected due to obstruction by scattering objects (mainly buildings) or natural obstacles (mainly forests) in the vicinity (within 50~100 wavelengths) of an UE, there will be multi-path wave interference on the ground, leading to a standing wave field. When the MS passes the standing wave field, the received signals presents quick fading, and the field strength fluctuates.
- Probability density function and accumulation probability distribution function of quick fading
Other Features of Signal
Transmission
- Time delay extended width
- Related bandwidth
- Inter-code Interference
Transmission Theory
- Definition of Transmission Theory
- For a radio link, the loss (or fading) value of power level of a signal from the output end of a transmitting antenna through certain transmission paths to the input end of the antenna. Usually, it is expressed in dB .
- Common Relations between Transmission Theory and Distance
- In mobile communication, the greater the transmission distance is, the greater the transmission loss will be. Within 1~20 km, roughly 40dB/dec. dec is 10 times the distance; in case of greater distance, it will be increased to 50~60dB/dec.
Common Types of
Transmission Theory
- Free Space Transmission Theory
- Diffraction Loss
- Reflection Loss
- Building Penetration Loss
- Human Body Loss
- In-vehicle Loss
- Vegetation Loss
Fresnel Region and
Transmission clearance
- Fresnel Region
- An area between curves satisfying f(n) and f(n-1) is called the nth Fresnel region. When N=1, it is called the 1st Fresnel region, an ellipsoid; the 1st Fresnel region contains 1/2 of the transmitting energy. In addition, tests and theories demonstrate that, if the gap is greater than 0.577 time of the radius of the 1st Fresnel region, the loss will be equal to the loss of the free space.
- Transmission Gap
- 0.577 time of the 1st Fresnel radius.
RF Optimization Policy
Common RF Optimization
Process
Single Station Check
- Confirm site information
- Longitude and latitude, configuration, height above sea level, peripheral environments and so on.
- Confirm antenna feeder information
- Antenna type, azimuth, down-tile angle and height.
- Check antenna feeder link
- Standing wave ratio, primary set and diversity RSSI check, primary set and diversity lock balance.
- Confirm system parameters
- List of adjacent areas, overhead channel transmitting power, PN configuration, switching parameters.
- Check and test basic functions
- Basic call process, soft switching, softer switching.
- Check station coverage
Base Station Group
Optimization
- Spectrum scanning
- Load-free test
- Load test
Whole Network
Optimization
- Test on various radio indexes of the system
- Analysis on test results
- Confirm whole network adjustment scheme
Performance Test Indexes
- Voice quality--FER
- Call connection rate (call completion rate and paging response rate)
- Resource utilization—CPU utilization-
- Switching completion rate
- Call drop rate
- Network coverage rate
- Forward coverage
- Pilot coverage
- Service coverage
- Backward coverage
Common RF Problems
- Call Drop
- Discontinuity
- Access Failure
Call Drop Analysis
- Forward coverage is not satisfactory (Ec/Io and Ec)
- Improve the coverage of the points.
- List of adjacent areas is not complete
- Configuration of list of adjacent areas is not complete.
- Interference
- There is in-band interference source.
- Pilot pollution is serious
- Faults with base stations
- Incorrect connection of antenna feeders, GPS fault causes asynchrony between the time and the system, interruption of transmission.
- Hard switching takes place
Access Failure
- Interference
- Coverage over weak areas, blind zones or pilot pollution areas makes it impossible for signaling interaction between the base station and the mobile phone to be completed during the access.
- Mobile phone performance
RF Optimization Policy
- Adjust the antenna down-tilt angle
- Adjust the antenna directional angle
- Adjust the antenna height
- Change the antenna type
- Appropriately adjust the base station transmitting power
- Adjust the base station location
- Increase the base stations
- Antenna directional angle
- During optimization, attention should be paid to antenna directional angle, as shown in the figure on the right.
- If the antenna coverage area is a vast space of residence, and the buildings are of the similar structure, the antenna direction shall be alongside the direction of the buildings (as the red arrow on the left); if the antenna direction is the same as the arrow on the right, the quality of signals in the coverage area may not be good.
- RF Optimization Policy for Pilot Pollution
- Adjust the antenna down-tilt angle, so as to reduce the coverage area, and further reduce the number of pilots in the pilot pollution area.
- Appropriately reduce the transmitting power of the cell, so as to reduce the signal strength to narrow the coverage area, and also further reduce the number of pilots in the pilot pollution area.
- If the two measures are of no use, we can increase base stations in the pollution areas, so that there will be a master pilot signal, to solve the pollution. But be careful in taking this measure, as it may impose great influence on the entire network.
RF Adjustment and Network Simulation
Before Adjustment
- The diagram on the right shows part of the base stations of the Guangzhou MTNet Pilot Network.
- Where, the directional angle of the antenna in the DiTuChuBanShe is 30°, the mechanica down-tilt angle is 6° and the electronic down-tilt is 2 °.
- This is a pilot intensity simulation diagram: We can see that the pilot intensity is quite satisfactory as a whole.
- This is a pilot Ec/Io simulation diagram: We can see that the pilot Ec/Io in the middle (the yellow part) of the diagram is not so satisfactory
- This is a pilot pollution simulation diagram: We can see pilot pollution in the lower middle (the brown part) of the diagram. Taking the pilot Ec/Io simulation effect in the previous diagram into consideration, we should perform RF optimization here.
After Adjustment
- Analysis shows that adjustment of RF parameters in the DiTuChuBanShe may improve the current situation.
- Adjust the mechanical down-tilt of the antenna in the DiTuChuBanShe as 0°, and leave the electronic down-tilt angle unchanged as 2 °.
- Through this adjustment, the pilot intensity of the DiTuChuBanShe, where there is pilot pollution, is improved, and becomes the maste pilot, so that pilot pollution is improved and the pilot Ec/Io here is enhanced.
- This is the effect of pilot intensity simulation after adjustment. We can see that the pilot intensity after adjustment is much improved than that before adjustment.
- The effect of pilot Ec/Io simulation after adjustment. We can also see that the pilot Ec/Io after adjustment is much improved than that before adjustment.
- This is the effect of pilot pollution simulation after adjustment. We can see that big brown part (with pilot pollution) has been greatly reduced. This proves that the RF adjustment has fulfilled the optimization aims.
End of Course