Ram Sunil Kanumalli

Active Digital Cancellation of Transmitter Induced Modulated Spur Interference in 4G LTE Carrier Aggregation Transceivers

Mobile user equipments supporting Long Term Evolution (LTE) - carrier aggregation (CA) face several new unpredicted issues when operating in frequency division duplex mode. This paper addresses the transmit (Tx) modulated spur problem, a new issue, that appears in LTE-CA transceivers. Due to the presence of several mixers and dividers to support different bands and CA scenarios, many harmonics are generated on the chip. Unavoidable crosstalk between these harmonics can generate an unwanted continuous wave spur around the Tx frequency. Once this spur appears at the chip area where the receive (Rx) local oscillator resides, it downconverts the undesired portion of the Tx signal which leaked through the duplexer to the Rx baseband. This interference, referred to as Tx modulated spur, causes a severe degradation of the signal-to-noise ratio (SNR) of the wanted signal. To mitigate such, we propose an active digital cancellation architecture which utilizes the known information of the Tx signal. An equivalent baseband model of the Tx modulated spur is presented from which the digital cancellation architecture is derived. Simulations show that the proposed architecture cancels the Tx interference and improves the SNR of the wanted signal significantly.

Adaptive self-interference cancelation in LTE-A carrier aggregation FDD direct-conversion transceivers

Modern frequency division duplex radio frequency transceivers experience transmitter-to-receiver leakage due to the limited isolation of the duplexer. In Long Term Evolution-Advanced (LTE-A) carrier aggregation receivers the coupling between the local oscillators creates harmonics on the chip which can lead to the downconversion of this leakage signal to the receive (Rx) baseband. Thereby, this so-called modulated spur interference reduces the signal-to-noise ratio of the Rx signal. In this paper, the modulated spur interference is modeled and several adaptive algorithms are compared regarding their convergence and cancelation performance. To maximize the data throughput, the adaptive filter is required to converge within the short time period of one orthogonal frequency-division multiplexing (OFDM) symbol. Out of the investigated concepts the proposed variable step-size least-mean-square algorithm turns out to be the most favorable choice. It satisfies the required constraints of convergence time and cancelation performance, and it features a reasonable low complexity.

Digitally controlled oscillator gain estimation for RF-DPLLs in 4G LTE polar transmitters

The gain of a digitally controlled oscillator (DCO) represents a crucial parameter for wideband phase modulators utilized in polar transmitters supporting SC-FDMA in LTE uplink. Accurate normalization of the DCO gain is necessary for a two-point modulation of a RF digital phase-locked loop (DPLL) to not degrade the in-band modulation quality and out-of-band emissions. The DCO gain can change due to process, voltage and temperature effects and therefore has to be estimated during runtime. A common way to estimate the DCO gain is by minimizing the residual error due to phase modulation signal content present in the error feedback signal of the DPLL control loop. This paper derives a simple discrete time, single-rate model of the DCO gain estimation and investigates the behavior and impact on modulation quality for different resource block (RB) allocation scenarios found in LTE uplink transmission. It is shown that the DCO gain estimation is sensitive to modulation signals with contiguous, sparse RB allocation and problems may arise in certain transmission scenarios.