Synchronization, Equalization and Diversity Algorithms for Sparse Aeronautical Telemetry Channels
Wireless aeronautical telemetry systems are crucial when aircraft, missiles, or spacecraft are tested. Telemetry from an onboard flight test instrumentation system is the primary source of real-time measurement and status information transmission. The air-to-ground wireless downlink channel in aeronautical telemetry suffers from multipath interference. In addition, the complexity of airborne test articles has grown significantly over the last several decades which demands higher data rates (10-20 Mbits/s). Consequently, the bandwidth of the modulated carrier has grown up to such an extent that essentially all the multipath fading are frequency selective which remains as the dominant cause of link outage in aeronautical telemetry. However, these multipath channels are sparse in their equivalent baseband representations.
To alleviate multipath interference, spatial diversity and/or equalization can be used. Minimum mean squared error (MMSE) type equalizers are a viable solution to mitigate the frequency selectivity of the aeronautical channel. As MMSE equalizer filter coefficients are a function of the carrier frequency offset (CFO), equivalent discrete-time channel and noise variance, a reliable estimate of those parameters, are required. In this dissertation, we present the joint estimation of CFO, channel, and noise variance by exploiting the sparseness of the channel. Next the performances of the zero-forcing (ZF) and MMSE equalizers have been numerically studied using the test channels. These channels are derived from channel sounding experiments at Edwards AFB, CA and Cairns Army Airfield, AL. Frequency domain equalizer with MMSE criterion is also explored and found to have significantly lower computational complexity compared to its time-domain counterpart.
For the implementation of diversity using multiple transmit antennas, a generalized time-reversed space-time block code (GTR-STBC) is proposed. The generalization of the proposed GTR-STBC involves an unequal power allocation parameter which is optimized by minimizing the post-equalizer mean-squared error for a MMSE equalizer. GTR-STBC is applied to the measured channel impulse responses and a simple statistical channel model. Linear equalizers of sparse multipath channels are known to be nearly sparse or highly compressible. Since aeronautical telemetry channels are sparse, to exploit spatio-temporal diversity with low complexity, we address the use of GTR-STBC with sparse equalizers via optimizing its power allocation parameter. The effectiveness of the proposed algorithms are demonstrated by analysis and numerical results using practical aeronautical telemetry channels.