Design and implementation of i/q imbalance compensation methods in uwb multi-gbps transmitters for point-to-point communications

Resumo

The demand for higher data capacity in wireless communication links is motivating an intense research in wideband systems in the millimeter wave (mmW) part of the spectrum. Designing circuits for this applications is very challenging, especially if both wideband and spectrally efficient communication need to be achieved with low cost and lower power. When the bandwidth is ultra wide, it is very hard to design analog circuits that meet the requirements to support the transmission of high-order modulated signals. This departure from the desired functionality and performance of the analog circuits falls below what is called radio-frequency (RF) impairments. In spectrally efficient ultra wideband (UWB) systems required to address the needs of future communication networks, RF impairments can become the main performance limiting factor. In order to compensate for these imperfections, a solution that is gaining momentum is to accept the RF impairments up to a certain degree and to compensate them digitally. This enables keeping power consumption, size and cost of the transceiver at desirable low levels. That is why in order to enhance performance and reduce costs, digital compensation has become a must in recent years. One of the most prominent impairments that degrades the system performance of UWB systems is the In-phase and Quadrature (I/Q) imbalance, caused by mismatches between the I and Q paths of the analog front-end. Although the literature offers some approaches to compensate for I/Q imbalance, the majority of them are either focused on the receiver side or are specific to narrowband or wideband systems. Ultra wideband systems are not commonly addressed in these works and these systems can present I/Q imbalance issues that depart from the classical gain and phase imbalance. I/Q imbalance encountered in narrowband systems can be regarded as non-frequency-selective (NFS), and it is mainly caused by the local oscillators (LO) used for quadrature modulation or demodulation. However, I/Q imbalance in wideband systems may also exhibit frequency-dependent or frequency-selective (FS) behavior due to mismatches between the analog filtering paths of the I and Q components caused by finite tolerances, as well as mismatches in the signal paths. Furthermore, especially in high-speed applications, it is common to use multiple parallel paths between the digital signal processor and the digital-to- analog (DAC) and analog-to-digital (ADC) converters, as well as separate converters for the I and Q datapaths. This configuration can present sample synchronization or delay imbalance problems between the I and Q datapaths. This research work provides frequency-selective I/Q imbalance and delay imbalance compensation techniques for UWB transmitters. These techniques provide robust and efficient solutions to address I/Q imbalance impairment in any kind of transmitter. Ultimately, the proposed solutions facilitate the implementation of more flexible, low cost radio devices for future UWB wireless systems.