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Providing unequal protection for compressed data using hierarchical modulations

The GOES-R series, with a first launch planned around 2012, is currently under development by NOAA. It will be the first in a series of new geostationary environmental satellites to provide greater capabilities needed for weather, atmosphere, climate, and ocean monitoring. An extremely high volume of multi to hyper/ultra spectral remote sensing data is generated from both the onboard sensors and the rebroadcast ground station, with rates in the range of 25 to 140 Mbps. However, the bandwidths allocated to GOES-R are limited, therefore, data compression is necessary. When data are compressed, the information contained in some segments, such as the frame header and sync mark, are much more important than that in other segments. In order to provide unequal protection to different segments of the streaming data, this paper presents a hierarchical modulation that utilizes a 16QAM constellation to transmit three bits per channel use by applying a rate 1/2 forward error correcting code to the least significant bit (LSB). When those bits that require high protection are placed at the LSB positions, the robustness can be significantly enhanced by several orders of magnitude. Depending on the performance requirements of the LSB and the other two bits, the constellation can be optimized so that a minimum amount of transmitted power is needed. The peak-to-average power ratio (PAR) of a 16QAM constellation requires a power back-off when a nonlinear power amplifier, e.g., traveling wave tube or solid-state power amplifier, is used. The power back-off is needed in order to avoid signal distortion. In this paper, the bit error rate (BER) performances of this hierarchical modulation, 8PSK, and rate 3/4 coded 16QAM, which have the same bandwidth efficiency, are compared, taking into account the required power back-off. It is shown that the hierarchical modulation outperforms 8PSK. When the unequal protection is considered, the paper also shows an advantage of using this hierarchical modulation over the rate 3/4 coded 16QAM.