Interpretation of CDMA Over Air Measurement Results

This section contains expected result values and possible causes of error if the expected results are not met.

Frequency Error

The standards specify very tight Frequency Error performance. In the PCS bands, the 0.05 parts-per-million specification translates to only 99 Hz at a carrier frequency of 1980 MHz or 40 Hz at 850 MHz cellular frequencies. Frequency Error shows up as uncorrelated power that adds to the noise floor or shows up in other Walsh codes.
 If a particular site loses its reference to GPS time, its reference signals will begin to drift over time. Mobiles already using the site can remain on the air because they derive their timing from the signals transmitted by the base station. However, mobiles using other sites/sectors may be prevented from using the site because they are confused by the error in frequency. This creates what is known as the "island cell effect". By itself, the cell is still functional. To the rest of the system, it's inaccessible.
This island cell effect can be caused by a failure in the site's GPS receiver and timebase distribution network. Using the test set's Internal GPS receiver provides an independent time reference that will allow you to determine if this cell site is out of sync with the rest of the network ("island cell effect").

PN Offset

Verify the PN Offset is correct. If you are in Manual mode, the PN Offset will display the value you entered. Make sure the Time Offset is small (less than 3 µs). If the Time Offset is greater than expected, see the section on Time Offset below. If you are in Auto mode, the test set will tune to the PN Offset with the least amount of Time Offset. If an incorrect PN Offset is displayed, the Time Offset will likely be very large.

Time Offset

The CDMA standards specify a maximum offset of 10 µs; generally 5 µs is a recommended maximum. If the Time Offset is large enough, an "island cell" can occur. The "island cell effect" was mentioned already when discussing Frequency Error.
A phone moving outward toward the limit of its cell will need to acquire the adjacent cell in order to hand off. If the time offset of the target cell is too far from that of the current site, the handoff may not happen.
The dependence of the phone on correct system time limits the physical distance to a target cell's antenna. Each PN Offset is 52.08 µs; if the propagation delay is too long, the received PN Offset may be different from the value designated on the Sync channel. This difference can confuse the phone, causing the handoff to fail.
Another cause of timing error is a bad GPS receiver or timing distribution network within the base station. To test your base station's GPS and timing distribution system, you can use the internal GPS receiver to generate an accurate timing reference for the test set to use while performing base station tests.
 The Time Offset should be within the following limits:
 If the measured value of the Delay falls outside of the expected range, consider the following problems:

Estimated Rho

The CDMA base station standard specifies that Rho must be greater than 0.912. Typical values for a healthy base station are greater than 0.94. When measuring Rho over-the-air, these values can only be achieved under very good conditions for multipath power and pilot dominance. For example, a Multipath Power of < 0.1 dB and a Pilot Dominance of > 15 dB is required to measure Rho of 0.912. For more on the affects of pilot dominance and multipath power on Estimated Rho see "Criteria for Making Valid CDMA Over Air Measurements"
 Verify that the Estimated Rho meets the following criteria:
 Poor Rho performance affects the base station capacity because the uncorrelated power appears as interference to the mobiles. The added interference will require an increase in the traffic channel level to overcome the interference. This may, in turn, be seen as further interference. At some point, the site will have to shed calls in order to supply the remaining calls with enough signal versus the interference in the system.
 If Estimated Rho is lower than expected, check the following:

Carrier Feedthrough

A good Carrier Feedthrough level is less than -25 dB. The IS-97 standard does not specify Carrier Feedthrough; however, this measurement provides an additional tool to troubleshoot the base station's transmitter.
In the frequency domain, Carrier Feedthrough can show up as an uncorrelated energy spike that can be seen on the spectrum analyzer by closely spanning into the top of the CDMA signal. The effects of Carrier Feedthrough can also show up as higher noise levels on the Code Domain Power screen. The inactive Walsh codes will be pushing the -27 dB specification for noise. Carrier Feedthrough can be caused by the lack of isolation across the mixer and cavity of the transmitter's I/Q modulator.

Noise Floor

A good Noise Floor value is between -20 dB and -30 dB and depends on the quality of the signal. If the Noise Floor is above these values (for example, -20 dB), the modulator may be the source of the added noise. Another possibility is a problem with the base station's channel card. The channel card generates the individual Walsh codes.

Channel Power

If the Channel Power is lower than expected, verify you are in a location that meets the criteria to provide valid measurements.
 Inaccurate Channel Power (high or low) may indicate an incorrect power setting at the base station.

Pilot Power

If the Pilot Power has varied significantly from previous readings at the same location, it may indicate the following:

Pilot Ec/Io

A high value of Ec/Io means there is less interference from other base stations and noise and is an indicator of base station performance. When Ec/Io is mapped, it can be used to determine coverage quality.

Delta Page Power

If the measured value of Delta Page Power is more than +/-0.5 dB different from previous measurements at the same location, it may indicate that the digital gain setting for the Paging Channel has been changed from the intended setting.

Delta Sync Power

If the measured value of Delta Sync Power is more than +/- 0.5 dB different from previous measurements at the same location, it may indicate that the digital gain setting for the Sync Channel has been changed from the intended setting.

Delta Quick Page Channel Power

If the measured value of Delta QPCH Power is more than +/-0.5 dB different from previous measurements at the same location, it may indicate that the digital gain setting for the QPCH has been changed from the intended setting.

Amplifier Capacity

The Amplifier Capacity metric provides an instantaneous reading of how much of the amplifier's capacity is currently being used. More meaningful information is provided by the Peak and Average Amplifier Capacity metrics gathered over a 10 to 15 minute interval.

Peak Amplifier Capacity

If the Peak Amplifier Capacity is greater than 100%, the traffic at this base station has caused the base station amplifier to exceed its maximum power rating. This is not desirable as this problem can result in system degradation or damage to the amplifier.
 You can get an indication of how often the amplifier capacity is being exceeded by watching the Amplifier Capacity measurement. This is an instantaneous measurement of the amplifier capacity.

Average Amplifier Capacity

If the Average Amplifier Capacity is greater than 85%, the base station power amplifier is close to its maximum power limit. Further increase in traffic on this base station could cause system performance problems or damage to the power amplifier. You should notify your RF Engineering Department as soon as possible.

Utilization

The Utilization metric provides an instantaneous reading of the percentage of the traffic channels currently being used. More meaningful information is provided by the Peak and Average Utilization metrics gathered over a 10 to 15 minute interval.

Peak Utilization

If the Peak Utilization of this base station is greater than 65%, this is a warning that peak traffic rates at this base station are getting very high. It is possible that calls could be getting blocked or dropped.

Average Utilization

If the Average Utilization of this base station is greater than 45%, the base station is carrying a great deal of traffic. It may be time to consider adding another carrier or another base station.