In Iwahori-Matsumoto's paper the Iwahori Hecke Algebra for $G=GL_n(F)$ is generated by $X_{s_0}, X_{s_i},i\in\{0,...,n-1\}$ and $ X_{\rho}$ with the relations:

$
1) (X_{s_{i}}-q)(X_{s_{i}}+1)=0\:,\;\;\;i=0,1,...,n-1 \\
2) X_{\rho}^n=1, \\
3) X_{\rho}X_{s_i}=X_{s_{i+1}}X_{\rho} \\
4) X_{s_i}X_{s_j}X_{s_i}=X_{s_j}X_{s_i}X_{s_j}, i\equiv \pm1 \mod n \\
5) X_{s_i}X_{s_j}=X_{s_j}X_{s_i}, i\not\equiv \pm1 \mod n
$

**Quetsion:**

If I consider the Steinberg representation $\pi$ as the unique irreducible subquotient $V$ of $Ind_B^G1$ (using normalized induction here) then this corresponds to the sign character of $H(G,J)$ acting on the one dimensional space $V^J$ i.e. $X_{s_i}(v)=-v$ for every simple reflexion $S$. My question is what is the eigenvalue of $X_{\rho}$? Of course it has to be an $n$-th root of unity but which one is it? Iwahori and Matsumoto define the sign character by sending $X_{\rho}\to 1$ but is this the only choice possible?