Compensation for Impairments of Frequency Converters in Millimeter Wave Vector Signal Generators
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Date
2018-09-05
Authors
Chung, Arthur
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
The upcoming fifth generation (5G) of wireless communications aims to utilize millimeter wave (mm-wave) frequencies in its infrastructure to alleviate the crowded spectrum problem below 6 GHz. At higher frequencies, modulation bandwidths of several hundreds of MHz can be utilized to increase system capacities. However, the radio frequency (RF) frontends can exhibit significant amounts of impairments over these wide bandwidths, thereby limiting the achievable output signal quality and capacity. In this work, two signal generation architectures and the accompanying compensation schemes to mitigate the impairments are proposed for the generation of wideband modulated signals at mm-wave frequencies.
The frequency dependent IQ imbalance effects in conventional direct conversion signal generation architectures over ultra wide bandwidths are first investigated. For that, a new interleaved multi-tone test signal based identification and compensation scheme is proposed. This scheme was experimentally validated by using an off-the-shelf IQ mixer operating at 30 GHz driven with an interleaved multi-tone signal with 4 GHz of modulation bandwidth and achieving a reduction in the normalized mean squared error (NMSE) from -14 dB to -38 dB. Subsequently, a low-complexity pruned Volterra series based digital predistortion (DPD) scheme was devised to mitigate the nonlinear distortions exhibited by the power amplifier stage and maximize the signal quality of orthogonal frequency division multiplexing (OFDM) signals with modulation bandwidths up to 800 MHz. After compensation of the system with 66 DPD coefficients, the OFDM signal with 800 MHz of modulation bandwidth exhibited an NMSE of -32.4 dB and an adjacent channel power ratio (ACPR) of 45 dBc.
However, the challenges associated with the implementation of traditional direct conversion architectures exacerbate as the operating frequency increases. For instance, the performance of high frequency active building blocks, e.g. mixer and amplifiers, deteriorates as the operating frequency approaches the maximum oscillation frequency of the semiconductor technology. To address this challenge, a signal generation system utilizing frequency multipliers to replace the mixer and facilitate frequency upconversion is proposed. A novel Volterra series based behavioural model is also devised to predict the nonlinear behaviour of frequency multipliers and to form the basis for synthesizing a DPD scheme capable of obtaining acceptable signal quality when driven with wideband modulated signals. Various frequency multiplier based signal generation systems were implemented using off-the-shelf frequency doublers, triplers, and quadruplers to serve as proof of concept prototypes. Experiments confirmed the ability to generate modulated signals with competitive error vector magnitudes (EVM) and ACPR levels with low complexity DPD schemes.
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Keywords
Wireless Communication, Digital Predistortion, mm-wave, IQ imbalance, Frequency Multipliers