In this talk, we provide novel mean value estimates for exponential sums related to the extended main conjecture of Vinogradov's mean value theorem, by developing the Hardy-Littlewood circle method together with a refined shifting variables argument. We obtain the sharp estimate for the cases of degree 2 and 3. Furthermore, for d>3, we obtain analogous results depending on a small cap decoupling inequality for the moment curves in R^d. This is joint work with Changkeun Oh.