Karthik Ramasubramanian

Exploiting signal and noise statistics for fixed point FFT design optimization in OFDM systems

Scaling the different stage outputs in an FFT appropriately is crucial for getting a good Signal to Quantization noise ratio (SQNR) in fixed point FFT design. Traditional designs have either handled this through Convergent block floating point technique (CBFP), which has memory, computation and latency penalties or through time consuming simulations. In this paper, we consider the special case of FFT design for OFDM transceivers. We exploit the Gaussian nature of OFDM signals to predict the bit-growth of the signal through the various stages of the FFT and propose a technique to scale the signal appropriately. Additionally, we investigate the quantization error profile and propose a technique to improve SQNR by exploiting the presence of null tones at the band edges. With the proposed techniques, the performance comes close to the CBFP design, with no increase in complexity compared to existing static designs. Simulation results illustrating the performance improvements of the proposed technique are presented.

Spur Mitigation in High-Sensitivity GNSS Receivers

The presence of spurious tones in a Global Navigation Satellite Signal (GNSS) receivers RF and analog front-end signal spectrum leads it to compute receivers position erroneously. Addressing spurs is crucial in modern multiconstellation GNSS receivers as their wide bandwidth and coexistence on a shared chip or board with other radios and processors make them highly susceptible to corruption by dynamically varying spurs. Proposed here is a solution to mitigate a large number of spurs in GNSS receivers using a combination of limited number of frequency-tracked digital hardware filters, fast Fourier transform-based spur detectors, and a firmware algorithm predicting and weeding out potentially corrupt satellite measurements. Laboratory validation results from an experimental GNSS receiver in a CMOS system-on-chip are shown to demonstrate its effectiveness.

Techniques to enhance GNSS signal acquisition and tracking sensitivity

Enhancing GNSS acquisition and tracking sensitivity is critical to improving the GNSS user experience in indoor situations. This paper describes various techniques to enhance GNSS signal acquisition and tracking sensitivity and increasing the robustness and availability of position fixes. Specifically, a technique called Staggered Coherent Integration is proposed which enables up to 1 dB improvement in sensitivity compared to conventional techniques. Also discussed are challenges in tracking the weak signals seen in indoor conditions and techniques to improve sustained tracking under these conditions. Analysis and simulation results are shown to demonstrate the effectiveness of the techniques described.