|
Editor's Note: This is a multi-part excerpt from Agilent Technologies' Application Note: Mobile WiMAX PHY Layer (RF) Operation and Measurement.
Part 1 of this article introduces the WiMAX standard and provides in-depth explanation of the IEEE 802.16 profiles.
Part 2 profiles control mechanisms and the mobile station.
Part 3 covers the RF Test Suite and transmitter measurement set up.
Part 4 covers transmitter power, including spectral flatness, spurious emissions, and adjacent channel power.
.
Part 5 covers modulation and frequency tests.
This Part covers transmitter power, including spectral flatness, spurious emissions, and adjacent channel power.
Transmitter power
Transmit power on UL : how pilot and data subcarrier signals combine
In this section, we discuss how the pilots and data subcarriers contribute to transmitted UL power. We also discuss how to measure the power on pilots and data subcarriers with the VSA.
OFDM is the sum of many narrow-band signals in frequency, measured over a certain number of symbols in time. The signal power is that of pilots (un-boosted in the UL) and data subcarriers. In turn, this is dependent upon the burst type and any power boosting being applied.
Total data power (dBm)
= DPwr (dBm) + 10log{10 (number of active data subcarriers})
Where
DPwr = The power per data subcarrier
Total pilot power (dBm)
= PPwr (dBm) + 10log{10 (number of active pilot subcarriers)}
PPwr = The power per pilot subcarrier
Symbol Power (Data + Pilot) = 10log {10 (Data power/10) + 10 (Pilot power/10)}
The ability of the receiver to demodulate the signal will depend predominantly on the part of the power of the received data subcarriers, and consequently the energy per bit. However, the data subcarrier power itself cannot be measured by a broadband power measurement because data subcarriers are always combined with pilots. The most powerful and convenient measurements are made using the OFDMA demodulator, because it is capable of isolating the data subcarriers from the pilot subcarriers and measuring their respective power. Subcarrier powerData is shown as part of the demodulation result as DPwr in Agilent's VSA and MXA. Typically it will read tens of dB less than the total signal power.
The power on the data burst measured using a power meter or MXA is the sum of the pilots and data subcarriers. In the downlink signal only, this power is related to the preamble, depending on the boosting used for different functions, described later.
Measuring broadband average power provides a traceable route for checking the correct calibration and operation of the transmitter and the performance of the receiver.
Power control
Power control is a mandatory part of ranging on the UL as specified in the WiMAX Forum Mobile System Profile Release 1.0 Approved Specification document. There are two modes, closed loop and open loop. In closed loop, the BS emulation instructs the MS to adjust the transmit power via MAC layer commands. The power control is over a 45 db dynamic range and minimum power steps of 1 dB, with relative accuracy of ±0.5 db plus the associated measurement uncertainty on power measurement. In open loop, the MS is responsible for setting its own transmit power level, based on calculated link loss and the SNR required at the BS receiver for a specific modulation and coding scheme. The MS changes transmit power immediately with changes in modulation. The MS is required to report to the BS the transmit power it is using. When the burst allocation is defined, this reading should be equivalent of the gated broadband measurement on the UL signal, which is the same as a gated channel power measurement.
Measurement of power
A WiMAX frame will exhibit different average power levels for different symbols throughout the frame. There are three ways to measure this power: using a power meter, spectrum analyzer, or a VSA. Triggering and measurement gating are required for the power meter and spectrum analyzer.
Mobile WiMAX has channel bandwidths from 3.5 to 10 MHz (potentially 20MHz in the future). The measurement tool needs to have the required bandwidth, frequency, and dynamic power range to make these measurements. Power meters provide the best absolute accuracy for measuring total power. As seen in the previous section, the burst design needs to be known to relate broadband power to subcarrier power. The second power measurement uses the MXA, with a one button function for channel power in dBm/10 MHz and power spectral density expressed in dBm/Hz.
The measurement capture in demodulation mode with the VSA, as illustrated in Figure 11, shows the time waveform, and time-gated band power in the top trace. The lower trace uses signal demodulation to show the power of pilots, data subcarriers, and in this example, a CDMA burst.
In the measurement shown in Figure 11 on an UL signal, is a PUSC zone with a QPSK burst. The total power is +20.9 dBm. The power of each pilot subcarrier (PPwr) is "8.3 dBm, as is the power of data subcarriers (DPwr). This is expected, because the pilots are not boosted in the UL. As discussed earlier in Section 4.2.1, the VSA can also provide a total power reading by scaling power of data and pilot subcarriers by the number of data and pilot subcarriers.
11. OFDM burst (UL) with information on pilot power, data subcarrier power, and total power.
We mentioned the differences between UL and DL in Section 4.1.1. Let us now describe power measurement on a DL that has a PUSC zone. This is necessary because we will be discussing receiver measurements on the mobile later in this application note. DL power measurements are a pre-cursor to making receiver measurements.
A DL signal has subcarriers associated with the preamble that occupy one symbol and pilot/data subcarriers associated with data bursts. To make DL power measurements, one can gate over the preamble or other parts of the DL signal and obtain the power. Alternatively, the power can be computed using the power measured from RSSI, pilot subcarrier, and data subcarrier provided by the VSA demodulation measurement. One ratio that is important is that between the preamble power (measured as RSSI) and the combined pilot and data power for a fully allocated burst. This preamble is 3.86 dB higher.
|
