Portlets are isolated from the http request. You'll have to think a bit about it, but the idea is that someone needs to start the 'chain of events'. In this case, you'd want a portlet to be the one. It would pass whatever params to itself with a render or action request. In that request, the params are available as render params, or action params.
you could even 'bookmark' such a URL if you need to start this 'conversation' from outside the portal with a request like : portal/index.html?ctrl:id=window.default.HelloWorldPortletWindow&ctrl:type=action¶m1=doSomething
* ctrl:id points to the portlet you want to address, and
* ctrl:type=action fires the action request, and
* all other params on the URL (like param1=doSomething) would be action params available in the action request in the portlet code....