With a still-weak economy and financial trouble in Europe, markets are unstable, and investors are taking hits. This is all the more reason to find a recession-proof investment for your portfolio in such volatile times.

While no stock is completely recession-proof, there are stocks that seem to defy the odds and are less affected by the downturn. One counterintuitive name is Sirius XM Radio (Nasdaq: SIRI). The company, with its quality product offerings and fruitful partnerships, has been able to generate favorable results even during tough economic times.

Increasing subscriber base
About 65% of new cars manufactured in the U.S. come with a built-in Sirius XM satellite radio system. The subscription fees from these cars comprise a major part of its revenue. Despite a slowdown in the automobile industry, Sirius XM continues to increase its revenue by continuously partnering with new car manufacturers. The company added 334,000 subscribers in the third quarter of 2011, achieving a subscriber base of 21.3 million. The latest impressive subscriber growth figures are significant, especially after considering that its competitor Pandora Media (NYSE: P) offers free service to radio listeners.

Sirius XM signed a deal with General Motors in June 2011, under which a Sirius XM system will be installed in all the radio-equipped used cars sold by about 1,000 GM dealers, regardless of the brand of cars. The company is also planning to target used cars in the Canadian market. This step is likely to boost Sirius's revenue and will continue to strengthen its financials in the years to come.

Revenue growth
Sirius XM has devised another plan to ensure revenue growth. It has planned to increase its subscription rates from $12.95 to $14.49 per month for existing customers. While it is possible that some customers might unsubscribe because of the rate hike, it is unlikely to have a big impact like Netflix did, since the hike is only marginal. Netflix, in July 2011, had increased its subscription fee for some subscribers by 60%, which led to outrage among its subscribers.

Fool's bottom line
Sirius' robust performance, even during such turbulent times, is an indicator of how well it would perform when the economy begins to improve. Keep track of the latest developments by adding Sirius to your watchlist so that you don't miss out on any news about the company.