The Effects on $S$, $T$, and $U$ from Higher-Dimensional Fermion   Representations
  Inspired by a new class of walking technicolor models recently proposed using higher-dimensional technifermions, we consider the oblique corrections from heavy non-degenerate fermions with two classes of higher-dimensional representations of the electroweak gauge group itself. One is chiral SM-like, and the other is vector-like. In both cases, we obtain explicit expressions for $S$, $T$, $U$ in terms of the fermion masses. We find that to keep the $T$ parameter ultraviolet-finite there must be a stringent constraint on the mass non-degeneracy of a heavy fermion multiplet. 
Hong-Hao Zhang, Yue Cao, Qing Wang