n (3G) environment there is a large selection of services, and there is a cost per subscriber for each of them. We would like a cost per subscriber for each service. As described in Sec. 45.1, we can compute an allocated cost, the fair share of system cost per subscriber, or an incremental cost, the extra system cost for each sub- scriber added. The incremental cost is very important for marketing decisions about what services to promote. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Business-Case Models Business-Case Models 46.3 Return-on-Investment (ROI) Models Cost per subscriber, even with a time analysis, is only half the money equation. We want to make sure that our system is earning its keep, and this means that we need to analyze the relationship between revenue and cost. In particular, we want to ensure that the revenue is more than the cost. Investors in any business venture, and wireless telephony is no exception, want a positive return on their investment. In particular, investors have a time horizon dur- ing which they want to see that positive return. The time window for a positive return on invested capital depends, in general, on the risks involved in the investment and, in particular, on the specific industry. Let us consider the preceding example, where our investment in the wireless tele- phone service is $349.46 per subscriber. This includes the user terminal, channels, car- riers, sectors, cells, and switches. Let us suppose that the subscribers in this system are paying an average rate of $20 per month. For this discussion, we really do not care if they are paying a flat $20 per month or they are paying $10 per month and another $10 for busy-hour air-time charges and other services. If we take a simple ratio of system cost and monthly revenue per subscriber, then we get $349.46 divided by $20 per month, and we see a p