Gain Imb (802.16 OFDMA)

IQ Gain Imbalance (dB) is the amplitude deviation between I&Q branches and compares the gain of the I signal to the gain of the Q signal.

The output will show "***" when a valid data result is not possible. This is a function of the number of active subcarriers which are symmetric across DC.

When performing data burst analysis, the burst geometry and permutation algorithm define the active subcarriers for a given measurement. If there are not enough active symmetric subcarriers (i.e. subcarriers active on both sides of DC, like +10 and -10) to perform the IQ impairment estimation, "***" will be displayed on the summary table in place of a valid result. This may be more noticeable with UL Up Link (reverse link: from cell phone to base station)-OPUSC and UL-AMC Adaptive Modulation and Coding zone types due to the lack of subchannel rotation for those permutation types. Reconfiguring the geometry of the analyzed burst(s) will allow this result to be computed.

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Because Gain Imbalance is expressed as a logarithmic value, it may have a positive or negative value. Gain Imb is computed and stored as a linear ratio equal to (I Gain/Q Gain) and then displayed in dB equal to [20 Log (I Gain/Q Gain)].

Gain imbalance for OFDMA demodulation differs from gain imbalance for single carrier modulation in that gain imbalance does not show up in the constellation diagram as a width different than height. That's because gain imbalance is in the time domain signal, but OFDMA constellations are in the frequency domain. In an OFDMA signal, gain imbalance causes each subcarrier to interfere with its opposing subcarrier, causing each constellation point to spread into a miniature picture of the entire constellation.

Inaccurate IQ Measurement Results

The accuracy of the IQ Mismatch Impairments, including IQ Gain Imbalance, IQ Quadrature Error and IQ Timing Skew are dependent on the accurate detection of a reference signal. The accurate detection of a reference signal is dependent on the data modulation format, as well as the cumulative impairments observed during data subcarrier detection. If the incorrect data modulation format is used, IQ mismatch impairment estimates will be incorrect. This can be avoided by selecting Manual Data Tone Modulation from the Advanced tab of the demodulation properties dialog box.

The following table shows the approximate level of IQ Gain Imbalance which can be tolerated before estimates become inaccurate. In this example the IQ Gain Imbalance is the only impairment (high SNR Signal-to-Noise Ratio when only AWGN Additive White Gaussian Noise - Statistically random radio noise characterized by a wide frequency range with regards to a signal in a communications channel. and distortion are considered).

FCH DL-MAP downlink map: A MAC message that defines burst start time for both time division multiplex and time division multiple access (TDMA) by a subscriber station (SS) on the downlink. Burst format	Auto Data Tone Modulation (IQ Gain Imbalance)	Manual Data Tone Modulation (IQ Gain Imbalance)
2 sym QPSK Quadrature phase shift keying	1.5 dB*	8 dB*
10 sym QPSK	1.5 dB*	8 dB*
10 sym 16QAM	1.6 dB*	3 dB*
10 sym 64QAM	n/a	1.3 dB*
Legend: * Maximum measured IQ Gain Imbalance to insure accurate data results.

Similar behavior can be observed for IQ Timing Skew and IQ Quadrature Error estimates.