Wimax standards and Security The Wimax

Transmitter Spectral Flatness

During normal system operation, all uplink and downlink transmissions in WiMAX begin with a preamble. The preamble is used to synchronize the receiver with the transmitter and perform various channel estimation and equalization processes. The preamble uses QPSK modulation and has no embedded BPSK pilots. Owing to this property, the preamble is ideally suited to specify the spectral flatness across all the subcarriers. According to the 802.16-2004 standard, the data shall be taken from the “channel estima- tion step’’ (which happens to be the preamble), and the absolute difference between adjacent subcarriers shall not exceed 0.1 dB.
For the measurement of transmitter spectral flatness, the DUT is set to transmit at valid output with a frame structure that has a proper preamble. The DUT spectral flatness is measured with the test equipment (VSA or spec- trum analyzer) and the relative amplitude of each subcarrier with adjacent subcarriers is compared [11].

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   Transmitter Constellation Error
   

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Transmitter constellation error test is a measurement of the transmitter mod- ulation accuracy. Accurate transmitter modulation is necessary to ensure that the receiver can demodulate the signal with minimal decode errors [11]. This measurement is quite similar to error vector magnitude (EVM) used in the industry for other digital communications technologies. The IEEE 802.16 stan- dard introduced a new term called relative constellation error (RCE), which determines the magnitude error of each constellation point and RMS (root mean squared) averages them across multiple symbols, frames, and packets. Each of the seven burst profiles (modulation/coding types) used in WiMAX has a specification for RCE expressed in dB. For example, RCE for the profile (QPSK, 1/2) is 16.0 dB.
For the measurement of RCE, the DUT is set to transmit valid subframes (preambles followed by data bursts). The DUT RCE is then measured with test equipment (such as spectrum analyzer with support for WiMAX analysis). The above procedure is repeated for various band edges for RF frequencies, modulation types, and power levels.

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