Robust methods for determining an investment portfolio are based on investment parameters which are assumed to be error bounded rather than precisely known values. The spool unit consists of a first end cap connected to a second end cap by at least one inner tape roll supporting cross member for rotatably supporting a first tape roll having a first inner diameter and a plurality of outer tape roll supporting cross members for rotatably supporting a second tape roll having a second inner diameter being greater than the inner diameter of the first tape roll. The method includes forming an estimate of the channels of the subcarriers for received OFDM data to provide frequency responses of the channels of the subcarriers, and forming a measure of quality for the channels of the subcarriers based on the relative strength of the frequency responses.