The power level of an 802.16 OFDMA signal can vary from one symbol to the next within a frame, depending on the burst and zone configuration. Thus, it is important to understand how the power level in the instrument is set up for a given waveform, and what you really get when you set the output power of the instrument to a particular value.
For a single source configuration in the N7615A/B Signal Studio software, by default, the symbol(s) with the highest power level in the frame is set as the reference for the ALC in the instrument. Normally this will be the preamble, so this means that the RF output power will be equal to the selected amplitude setting during the preamble, and the power level in the rest of the waveform will be relative to this level based on the zone and burst configuration. The power levels can be viewed in several ways:
On the Waveform Setup window, click the Power Envelope tab. The red region is the current reference power level, considered to be 0 dB for the purpose of displaying the amplitude in other regions in the frame. By default, this is the highest power level in the frame.
On the Downlink or Uplink windows, click the Power Envelope tab under the frame graph (black rectangle with yellow line) to display the power envelope trace showing the relative power levels in the downlink or uplink subframe.
On both of the above, hold the mouse over a region for a few seconds to see the relative amplitude of that region, compared to the reference level.
You can change the default ALC reference power by going to the Power Envelope plot on the Waveform Setup window and clicking on a different portion of the waveform. The new region will be highlighted in red, and the value in the table above the Power Envelope display will change from 0 dB to show the power level of the new reference region, relative to the highest power in the waveform. The instrument will now set its ALC to this level, and the output power will now be equal to the set value during this portion of the waveform.
Signal Studio assumes that in the STC zone, the sum of the two antenna outputs is the same as the power in the non-STC zone, and that each antenna transmits half the power. Also, it assumes that the preamble and first DL-PUSC zone are only transmitted from antenna 0, and that antenna 1 only transmits the STC zone.
By default, the antenna 0 signal will have the ALC reference set to the preamble as 0 dB. The STC zone will be at a lower power than the preamble, let’s say –7 dB for example, so the ALC reference for antenna 1 will be at –7 dB because it only contains the STC zone. This means that if we set the master instrument for antenna 0 to output 0 dBm for the preamble, the slave instrument for antenna 1 needs to be set to –7 dBm. Signal Studio will automatically adjust the amplitude setting on the slave instrument to the correct value when it downloads the waveform, so it really only uses the master instrument’s amplitude setting. If you want to have a different power for the antenna 1 output, you will need to adjust the source’s amplitude setting after Signal Studio is done downloading.
For the antenna 1 (slave) source in a dual source configuration, by default, the software will choose the highest-power symbols in the antenna 1 waveform as the ALC reference. This reference can be changed by going to the Waveform Setup window, clicking on and and selecting a region for setting the ALC Reference.
Any manual setting of the ALC Reference region should be done after you have completed all of the zone and burst configurations. If the ALC Reference Offset has been set to a non-zero value, and then you change the frame configuration in a way that causes that value to be gone from the frame (for example, by applying power boosting to a burst in that region), the ALC Reference selection will be erased and you will need to select a new ALC Reference region.
Note that all of the power envelope information only applies to the signal without fading applied.
For each antenna’s waveform, markers (marker 4 by default) are placed in the symbols that have been designated as the ALC Reference, and marker routing is set up to use these markers for ALC Hold (see Waveform Setup). The instrument will use the ALC to level power when the markers are present, and put the ALC in hold when the markers are not present.
When fading is embedded in a waveform containing multiple frames, the ALC markers are only used on the first frame because the power level for a given symbol will be different in each frame due to the fading. Signal Studio uses a new firmware feature in the instruments to perform a power search at the appropriate RMS power level to enable it to reach the correct power level quickly at the start of the waveform playback.
Signal Studio provides information about the average power in the waveform on the Waveform Setup window under section 1, General Settings. refers to the power averaged over the complete waveform, while refers to the power averaged over the portion where the signal is present.