Raghu Ganesan

Quantization noise improvement of Time to Digital converter (TDC) for ADPLL

A number of communication applications are moving to digitally motivated architectures for their radio frequency module. This includes GSM-EDGE, WLAN, Bluetooth, GSM-GPRS, WiMAX. The All Digital PLL(ADPLL) forms the core of this architecture. The objective of the ADPLL is to generate a clean carrier frequency ƒc, based on a input reference frequency ƒref. As part of the phase error measurement of the PLL, a Time to Digital converter(TDC) is used to measure the delay between ƒref clock edge and carrier clocking edge. An inverter chain is used to measure this delay as a integer number of basic inverter delay. This measurement error is termed TDC quantization error and effects the phase noise present in the final carrier. Due to the coarse delay of the basic inverter available, TDC introduces large quantization noise at the output of the PLL. This is too high for systems operating at high carrier frequencies or systems which have a tight phase noise requirement. This paper presents techniques to improve TDC quantization noise.

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.

Robust channel estimation for 802.11n (MIMO-OFDM) systems

In a receiver for multiple input multiple output (MIMO) 802.11n system, the residual carrier frequency offset (CFO), after synchronization with training symbols in the preamble of a packet, affects the accuracy of channel estimation. The residual CFO causes inter-carrier interference as well as inter-spatial stream interference. A decision directed algorithm to estimate and correct the residual CFO before channel estimation is presented. A 802.11n packet transmitted with high-throughput mixed mode (HT-MM) packet format has both legacy and high-throughput (HT) training fields. Typically, the channel is estimated only with HT training fields. In this paper, a channel estimation technique which uses both the legacy and high-throughput training field is proposed. Simulation results demonstrating the improvement in channel estimation and packet error rate (PER) performance are also presented.