Roberto Rimini

A compensation scheme to allow full duplex operation in the presence of highly nonlinear microwave components for 4 G systems

Current and future generation radio transceivers have to share spectrum in increasingly tight frequency space. Front end nonlinearities create spectral expansion which leads to radio interference in adjacent bands. The problem is greatly exacerbated for full duplex receivers with closely spaced Tx/Rx channels. This work proposes a novel radio architecture that can take care of out of band (OOB) emissions and allow interference free full duplex radio operation using appropriate nonlinear interference cancellation schemes. We propose that nonlinear operation and the resulting spectral re-growth can be handled without incurring any extra analog hardware in the RF front-end. This is accomplished at the cost of extra base band DSP workload. We then demonstrate that such an operation is indeed possible by setting up an experiment with a commercial RF front end chipset. This can possibly lead to a wide range of applications where interference cancellation can either facilitate or enable radio co-existence in upcoming multi-radio transceivers.

A PA-noise cancellation technique for next generation highly integrated RF front-ends

The evolution of efficiency in RF power amplifiers has been very gradual. Even today, they dissipate a lot of energy as heat which reduces the efficiency and increases the noise level. This PA noise is generally attenuated down to the thermal noise floor of the receiver by using highly selective duplexers. High selectivity necessarily carries high insertion loss and hence the PA output power requirement further increases. In the next generation of radios, efforts are underway to substantially reduce duplexer size and selectivity specifications. This will lead to a significant increase in the PA noise falling into the receive band. This work looks at a new technique to mitigate this receive-band noise by means of a mixed signal architecture using analog and digital components. We demonstrate that the cancellation of this noise can be accomplished by the receiver, and SNR gains can be demonstrated using commercial RFICs.

Interference cancellation for odd harmonics of envelope tracking RF power amplifier systems

In building highly efficient transmitters of today, one is forced to sacrifice linearity for efficiency. Some of the highest PA efficiency figures are reported by envelope tracking amplifiers. These amplifiers can generate strong higher order harmonics which can lead to interference with receivers operating at the harmonic frequencies. Using nonlinear interference cancellation, we can help to remove the interference being caused in those receivers. This paper looks at the problem of modeling the third harmonic emission from an envelope tracking amplifier. It derives the nonlinear kernel for estimating such interference. This kernel has been rigorously expanded to show its correlation with the third harmonic and its effectiveness in predicting the harmonic content. We then set up an envelope amplifier test-bench to capture the third harmonic and cancel it using the derived kernel model. The experiment yields excellent agreement with theory and provides a validation of the system and concept.

All digital compensation scheme for spur induced transmit self-jamming in multi-receiver RF frond-ends

In radio receivers employing diversity reception, receiver LO frequencies are closely separated. Different LO frequencies can couple through the substrate to the nonlinear components in the phase locked loops thus generating spurs. Such a spur, if it falls nearby the local transmit frequency, can inadvertently demodulate the strong local transmit signal and hence degrade the received SNR severely. In this paper, we investigate a novel method of compensating for transmitter self-jamming by carefully mimicking this mechanism in the received signal path and thus allowing cancellation. It has been shown that with certain simplifying assumptions one can realistically cancel the spur-induced transmit self-jamming interference. We outline the theoretical feasibility of this approach and then setup an RF test-bench to demonstrate these ideas on actual RF chipsets. Performance results from prototype measurements are reported.