Interpretation of 1xEV-DO Tx and Over Air Analyzer 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. Access Terminals already using the site can remain on the air because they derive their timing from the signals transmitted by the access network. However, access terminals 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.

PN Offset

Verify that 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 smallest amount of Time Offset. If an incorrect PN Offset is displayed, the Time Offset will likely be very small.

Time Offset

Time Offset is a measure of the time of the arrival of the pilot signal from the access network with respect to GPS time. It is expressed in microseconds.
 The 1xEV-DO standards specify a maximum time offset of 10 µs. Generally 5 µs is a recommended maximum, although many access network manufacturers specify that timing must be within 3 µs. If the Time Offset is large enough, an "island cell" can occur. The "island cell" effect was mentioned already when discussing Frequency Error.
An access terminal 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 access terminal 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 access terminal, causing the handoff to fail.
Another cause of timing error is a bad GPS receiver or timing distribution network within the access network. To test your access network'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 access network tests.

Non Idle Power

When making channel power measurements, make sure you have accounted for the loss of the test cable and high power attenuator you are using. If you do not know the loss of your test cable and high power attenuator configuration, perform an insertion loss measurement and enter the insertion loss as the RF In Loss value. For more information see "Two Port Insertion Loss"
 If the channel power is lower than expected, verify you have a good connection to the RF output of the access network. Also, verify the cable you are using to connect to the access network is not faulty. A low channel power may also indicate a bad power amplifier.
Inaccurate channel power (high or low) may indicate an incorrect power setting at the access network.

Pilot + MAC Power

The Pilot + MAC Power is a measurement of the average power during the Pilot and MAC periods. This is when the transmitter is only sending overhead messages in a burst mode and is not transmitting any data to users. When the Idle slot activity goes to zero (no idle slots), the Pilot + MAC and a couple of the other measurements blank (as they have no meaning when all slots are active).

On/Off Ratio

The On/Off Ratio is a ratio of the Pilot + MAC Power to the Idle Data Power. The recommended minimum On/Off Ratio for idle slots was set to 7dB in the standard to allow Access Network manufacturers to protect the power amplifiers from large output power swings between On portions of the signal during bursts and Off portions of the signal in idle slots. Each manufacture may set this value to different levels. This parameter is loaded into the radios from the Switch.
 When the Idle slot activity goes to zero (no idle slots), the On/Off Ratio and a couple of the other measurements blank (as they have no meaning when all slots are active).

Idle Data Pwr

This is the average power level in the data portion, averaged across all idle slots in the acquired capture, and can contribute to the overall Noise Floor in the system. A high reading here combined with a high reading in the CDP Noise Floor can indicate a problem with the I/Q Modulator or indicates the amplifier is being overdriven. When the Idle slot activity goes to zero (no idle slots), the Idle Data Power and a couple of the other measurements blank (as they have no meaning when all slots are active).

Idle Slots

This reading is a measure of the number of idle slots, measured as a percentage of the total number of slots available. This reading can be used as a flag to add more capacity at the site as the reading approaches 0%.

Est Pilot Rho

The 1xEV-DO access network standard specifies that Rho must be greater than 0.912. Typical values for a healthy access network are greater than 0.94.
 Rho failures can indicate problems in:
 Because the uncorrelated power appears as interference to the access terminals, poor Rho performance will affect the sector's capacity. The added interference can require that the signal on traffic channels be raised 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.

Est MAC Rho

The 1xEV-DO access network standard specifies that Rho must be greater than 0.912. Typical values for a healthy access network are greater than 0.94.
 Rho failures can indicate problems in:
 Because the uncorrelated power appears as interference to the access terminals, poor Rho performance will affect the sector's capacity. The added interference can require that the signal on traffic channels be raised 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.

Max Inactive Ch

According to the 1xEV-DO standard, the CDP in all inactive channels must be at least 27 dB below the total MAC power.
 If this value exceeds 27 dBc, compression may be occurring in the base station power amplifier or there may be errors in the base station IQ modulator.

Noise Floor

The light gray bars on the CDP trace display are noise. While they appear to be channels on the trace and are referred to as noise channels, they are not real 1xEVDO channels. They represent the noise energy in the inactive displayed code channel. The average level of the top of these noise bars represents the CDP Trace noise floor, and is the figure reported here.
 The noise floor level can be influenced by many conditions one of which can be the existence of high power from another CDMA base station or such things as loose RF connections. It is desirable to reduce the noise floor as much as possible before making measurements. If you have a strong signal on the metrics display but still show a high noise floor, then a problem may exist in the base station.

Data Modulation Type

The Data Modulation Type measurement returns the highest modulation detected within the measurement interval. There are three types of modulation, and hence four possible measurement results: QPSK, 8PSK, and 16QAM, and blank when no modulation is detected. QPSK is the lowest type of modulation, 8PSK is the next higher, and the highest modulation type is 16QAM. Only if all the slots within the measurement interval are idle will the result be blanked out. The number of slots over which the acquisition is made is determined by the Update Interval, which itself is determined by the Meas Time setting.
 A reading of 16QAM indicates high throughput for the access network. If only QPSK is shown over time, then there may be a problem in the transmit path of the acess network.