Digital compensation of front-end non-idealities in broadband communication systems

The wireless communication industry has seen a tremendous growth in the last few decades. The ever increasing demand to stay connected at home, work, and on the move, with voice and data applications, has continued the need for more sophisticated end-user devices. A typical smart communication device these days consists of a radio system that can access a mixture of mobile cellular services (GSM, UMTS, etc indoor wireless broadband services (WLAN-802.11b/g/n), short range and low energy personal communications (Bluetooth), positioning and navigation systems (GPS), etc. A smart device capable of meeting all these requirements has to be highly flexible and should be able to reconfigure radio transmitters and receivers as and when required. Further, the radio modules used in these devices should be extremely small so that the device itself is portable. In addition, the device should also be economical in terms of costs and energy requirements. In short, building a compact, low cost, flexible and reconfigurable radio for present and future wireless systems is an extremely challenging task. An attractive hardware solution to limit the size and the cost of the radio is to use only small and necessary analog electronic components in the front-end. This means that most of the processing that was traditionally performed in the analog domain is now pushed into the digital domain. A direct conversion architecture is a suitable radio front-end for such systems. However, a direct conversion radio module may be very sensitive to various mismatches and imperfections of its analog components. These imperfections are due to manufacturing defects, varying operating temperatures etc, and may result in front-end non-idealities such as in-phase/quadrature-phase (IQ) imbalance, phase noise, carrier frequency offset (CFO), etc. If these front-end non-idealities are not properly understood and compensated, they can easily become a limiting factor to the quality and performance of the radio device and the entire communication link. Multicarrier systems such as OFDM, one of today?s dominant transmission scheme is considered to be particularly more sensitive to these non-idealities. Rather than reducing the effects of these non-idealities by using expensive analog electronic components, it is easier and more flexible to tolerate these effects in the analog domain and then compensate them digitally. In this dissertation, the effects and the compensation solutions of two of the most essential front-end non-idealities encountered in direct conversion radio transceiver design and implementation are investigated, namely mirror frequency interference due to IQ imbalance and the CFO. Various training based digital compensation solutions for these front-end non-idealities are proposed. The proposed algorithms are assessed based on an OFDM system similar to the WLAN IEEE 802.11a standard. However, the proposed techniques are equally applicable to other systems operating in a similar environment. The most challenging architectural case of a wideband multicarrier transceiver scenario in which a wide collection of carriers are subjected to different amounts of IQ imbalance is analyzed. Thus IQ imbalance is considered to be frequency selective in nature with variation over different carriers. The IQ imbalance is also considered to be present at both transmitter and receiver front-end. The dissertation also considers the case of OFDM transmission with an insufficient cyclic prefix length, i.e., the case where the cyclic prefix is unable to completely accommodate the combined transmitter/receiver filters and the channel impulse response. In addition, the dissertation also considers different multiple antenna scenarios, from single-user multiple-input multiple-output (MIMO) systems and space time coded systems to multiple-user MIMO systems. The proposed solutions allow for low complexity implementations and can adequately compensate for non-ideality levels much higher than those observed in today?s transceiver designs. This guarantees robustness/effectiveness of the proposed schemes also for next generation systems where the effects of front-end non-idealities are expected to be much more severe. The presented compensation solutions allow to relax the analog requirements for low cost, small, flexible and highly reconfigurable radios in broadband communication systems.