I found the paper of Chriss and Khuri-Makdisi (Chriss, Neil; Khuri-Makdisi, Kamal.
On the Iwahori-Hecke algebra of a $p$-adic group. Internat. Math. Res. Notices 1998, no. 2, 85--100.) quite helpful.

You may also look at [Haines-Kottwitz-Prasad][1] and [Prasad][2].


  [1]: http://arxiv.org/abs/math.RT/0309168
  [2]: http://arxiv.org/abs/math/0504417