Mitigation of harmonic-downmixing images using digital signal processing techniques

Abstract

There is a growing demand for wireless communications at increased data rates. This has necessitated the development of new communication standards. New electronic devices must support a growing number of wireless standards and their various frequency bands, as the new standards do not replace the older ones. Together with the desire for small portable devices, this means that a single-chip solution supporting all existing wireless standards and featuring a wide tuning range, is highly desirable. In receive mode, a frequency synthesizer comprising a high frequency oscillator and programmable digital output divider generates the required wide tuning range. Highly linear switching mixers perform the frequency translation, thereby generating the baseband signals for the embedded processor. However, owing to the pulse-like nature of the synthesizer output and the switching action of the mixer, additional frequency bands are mixed down to baseband together with the desired signal, causing interference. These additional frequency bands, termed harmonic images, are related to the harmonics of the desired frequency. In regular receive applications, harmonic image interference corrupts the desired signal causing bit errors to occur. Harmonic images are also problematic in spectral sensing applications; they cause signal energy to be detected when there actually is none. This thesis presents a method, based on interference cancelling, to significantly reduce the interference caused by the dominating harmonic image. A harmonic rejection radio front-end, which offers up to 40 dB of rejection, is used to generate two complex baseband signals. An interference estimate is generated based on these baseband signals. After equalization to remove amplitude and phase differences, the interference estimate is subtracted from the contaminated received signal, thereby producing a received signal containing less interference. This method was successfully tested using two different front-ends, one built using off-the-shelf components and one fully integrated 65nm CMOS front-end. Using the latter front-end more than 80 dB of harmonic rejection was observed for the dominating harmonic image. A method for dealing with harmonic image interference was proposed for spectrum sensing applications. The method consists of an analog front-end that uses two quadrature down-converters to generate two complex baseband signals and a digital subband cross-correlator. The second down-converter is tuned Δf Hz higher than the first, resulting in the desired signal band experiencing a shift of Δf, while the harmonic images shift by nΔf, where 'n' is the harmonic image number. This second baseband signal is shifted by -Δf Hz in the digital domain, thereby spectrally re-aligning the desired signal band with the first baseband signal. The harmonic images, however, experience the same -Δf Hz shift, irrespective of their harmonic number. Therefore, all harmonic images in the second baseband signal are not spectrally aligned with respect to the first baseband signal. The end result is that only energy found in the desired signal band will produce an output at the cross-correlator, while the harmonic images are rejected. The effectiveness of this method was shown by experimental simulations.