Low I.F. direct digital demodulation for reception and decoding of
The I.F. is directly sampled with a 6 bit precision using non orthogonal sampling methods. This paper on Direct I.F. Demodulation for automotive applications includes some introductory explanation of the technique. The six bit sampling at 9/7 pi of the I.F. phasing yields a 14 bit baseband precision for a 384 kHz I.F. bandwidth. Following the low resolution A/D, all signal processing is done digitally. The receiver coherent demodulator includes filtering to de-warp the aggregate group delay distortion of the [pre-warped] transmitted signal and the receiver I.F. filters. The coherent demodulator output (in digital form) is the instantaneous amplitude and phase information. The symbol locking generator includes a digital PLL which remains aligned to the symbol center.
I-Q and Ø-V direct digital synthesis for transmission of complex
The digital information can be modulated in an 8PSK or 16 QAM format. The baseband information is filtered to reduce the occupied bandwidth and pre-warped to remove the inter-symbol interference (ISI). Digital baseband signal processing is done in the complex frequency domain and then converted to I-Q [in-phase and quadrature] or Ø-V [differential phase and amplitude] vectors. A simple D/A converter is required at the output. The Ø-V signals can be used for many complex modulation systems, thus saving the cost of an I-Q modulator. A direct sequence spreading function may be included in the modulation. A separate direct sequence output is provided at the receiver such that it may be used in advance of the selective stages, thus improving the process gain [linearity] and dynamic range.
Two-dimensional forward error correction system to improve symbol
This FEC improves Eb/No ratio by adding product codes which cross correlate with the signal in two dimensions, overlapping adjacent symbol groups. We can maintain channel lock with an Eb/No within 2.4 dB of the Shannon channel limit adding only 27% overhead.
Algorithm and hardware for combinatorial / antenna diversity
A low cost two-antenna diversity system at the receiver is implemented with a combination of hardware and firmware. The net gain over a single element is 2.4 dB. No dropouts greater than 5 dB below the peak occur provided that the system is operated within the link margin of 15 dB. The system works by implementing dynamic phasing as well as polarization diversity.