The easiest answer is to stick with a 5.1 surround sound system and put the surrounds at 90° to your sitting position. The rear surrounds in a 7.1 layout really should be a distance behind you, which you can't do when the couch is against the wall. If you really want to try it, you could install in-ceiling speakers above you, in-wall speakers behind you, or maybe use bipole/dipole speakers attached to the back wall. But neither solution is ideal, and you will probably be better off setting up an outstanding 5.1 system than trying to hack a 7.1 layout.