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发表于 2006-3-16 23:10:00
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<DIV>还有一片文章，也给大家看一下</DIV><DIV>Combining CCDF and EVM to assess 802.11 transmitters</DIV><DIV>
Sep 1, 2004 12:00 PM By Christian Olgaard </DIV><DIV> </DIV><DIV><P>Multicarrier orthogonal frequency division multiplex (OFDM) signals are employed by 802.11a/g to support the highest data rates under the 802.11a/b/g standards, but they pose challenges to the wireless LAN (WLAN) product designer and manufacturer.</P><!--end paragraph--><!--begin paragraph--><P>OFDM signals are characterized by fast-varying amplitudes with high peak-to-average power ratios. To minimize power consumption and yield the highest efficiency, the transmitter's RF power amplifier must be operated with some degree of compression. Finding the optimum operating point is complicated because most power sensors used in typical power meters are not fast enough to track the quickly varying OFDM signals with any accuracy. Moreover, the power meter reading must be gated in order to avoid averaging the gaps between packets.</P><!--end paragraph--><!--begin paragraph--><P>Test instruments that incorporate a vector signal analyzer (VSA) and are tailored to handle 802.11a/b/g signals provide the necessary tools to assess OFDM transmit signals. These instruments do not use power sensors; instead, they downconvert and digitize the complete signal spectrum over a wide bandwidth, capturing the entire signal energy. For instance, LitePoint's IQview and IQflex are tailored for this application. With linear receivers and calibrated gains, LitePoint test instruments provide accurate measurements of peak and average power for all 802.11a/b/g signal types.</P><!--end paragraph--><!--begin paragraph--><P>With such test instruments, two key measurements of 802.11a/g OFDM transmit signals can be made: the error vector magnitude (EVM) and the complementary cumulative distribution function (CCDF) of the transmit output power. EVM is a direct measure of modulation accuracy and transmitter performance, embodying all the transmitter impairments in a single number. The dominant contributors to poor EVM performance are phase and amplitude mismatches, phase noise, and non-linearity of the transmitter. During product development, one must identify these contributors and find ways to reduce them. During production, however, measurement of the EVM is relatively time-consuming and can be minimized in favor of measuring the CCDF.</P><!--end paragraph--><!--begin paragraph--><P>For a well-designed WLAN transmitter, the EVM performance will be limited by compression in the transmitter. With too much compression, the transmit signal quality will be degraded; with too little compression, a more expensive RF power amplifier will likely be required in order to achieve the desired average output power. Simply measuring the output power does not reveal the level of compression in the transmitter, but measuring the output power CCDF reveals the degree of compression and can be directly correlated to the EVM in a well-designed transmitter.</P><!--end paragraph--><!--begin paragraph--><P>The CCDF of the transmit output power is the probability that the signal power is greater than a given peak-to-average power ratio. In other words, if PAR is the peak-to-average power ratio (the magnitude squared of the instantaneous signal normalized by the average power level), and PAR0 is a specific peak-to-average power ratio value, then the CCDF is defined as:</P><!--end paragraph--><!--begin paragraph--><P>CCDF<NUM><SUB>power</SUB></NUM> (PAR) = Pr [PAR > PAR<NUM><SUB>0</SUB></NUM>].</P><!--end paragraph--><!--begin paragraph--><P>In the case of a compressed signal, clipping reduces the peak-to-average power ratios, and the CCDF curve shifts to the left as large peak-to-average power ratios become less likely.</P><!--end paragraph--><!--begin paragraph--><P>An uncompressed OFDM signal approximates a white Gaussian noise process by the central limit theorem as the number of OFDM subcarriers increases. The peak-to-average power ratio for such a Gaussian noise-like signal is itself a random process obeying the chi-square distribution with one degree of freedom. The CCDF for this process is well known, equal t</P><!--end paragraph--><!--begin paragraph--><P>CCDF<NUM><SUB>power (AWGN)</SUB></NUM> (PAR) = exp [- (PAR) ],</P><!--end paragraph--><!--begin paragraph--><P>or, if the peak-to-average ratio is expressed in dB,</P><!--end paragraph--><!--begin paragraph--><P>CCDF<NUM><SUB>power (AWGN)</SUB></NUM> (PARdB) = exp [ - (10^(PARdB / 10)) ].</P><!--end paragraph--><!--begin paragraph--><P>The CCDF corresponding to this Gaussian noise input provides a reference curve the CCDF curve of a compressed signal will fall to the left of this reference, as illustrated by the figures above of a 54 Mbps 64-QAM OFDM signal and a 6 Mbps BPSK OFDM signal. As can be seen, the lower data rate signal is more compressed than the higher data rate signal because it can tolerate a greater (worse) EVM level.</P><!--end paragraph--><!--begin paragraph--><P>By establishing a test where the same data pattern is transmitted with every data packet, measurement of the CCDF can be used to adjust a transmitter's output power to the desired compression level rather than to a specific power level. Measuring CCDF is significantly faster than measuring EVM, making it well suited for production testing. With IQview and IQflex, CCDF and EVM measurements can be made, allowing the EVM to be verified as desired to ensure test reliability.</P></DIV> |
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