6. Patent Assets

The following table presents the changes in the carrying value of patent assets (in thousands):


	December 31,
	2011			2010
Balance, beginning of period	$	126,508		$	82,759
Acquisition of patent assets		99,171			88,639
Reclass to assets held-for-sale		—			(80	)
Sale of patent assets		—			(500	)
Loss on sale of patent assets		—			(1,423	)
Amortization expense		(62,327	)		(42,887	)

Balance, end of period	$	163,352		$	126,508

The Company's acquired patent assets relate to technologies used or supplied by companies in a variety of market sectors, including consumer electronics, e-commerce, financial services, media distribution, mobile communications, networking, semiconductors, and software. The Company amortizes each acquired portfolio of patent assets on a straight-line basis over its estimated economic useful life. As of December 31, 2011 and 2010, the estimated economic useful lives of the Company's patent assets generally ranged from 24 to 60 months. As of December 31, 2011, the weighted average original estimated economic useful life was 49 months.

The following table summarizes the expected future annual amortization expense of patent assets as of December 31, 2011 (in thousands):


Year Ending
2012	$	64,382
2013		49,796
2014		27,137
2015		15,380
2016		6,657

Total estimated future amortization expense	$	163,352

Amortization expense was approximately $62.3 million, $42.9 million and $16.9 million for the years ended December 31, 2011, 2010, and 2009, respectively.

Structured Acquisitions

Structured acquisitions are transactions involving patent assets that may cost more than the Company is prepared to spend with its own capital resources or that are relevant only to a very small number of clients. In such transactions, the Company may work to acquire these assets with financial assistance from the particular clients against whom they are being or may be asserted. Such clients either pay amounts separate from their subscription fee or, less frequently, lend the Company funds to be used in the transaction. As discussed in the revenue recognition policy in Note 2, the Company may treat the contributions from the clients on a gross or net basis depending on the specific facts and circumstances of the transaction. In the event that such contributions are recognized on a net basis, the Company will capitalize the acquired asset that relates to its non-contributing clients. As a result, the cost basis of the acquired patent rights excludes the amounts paid by the contributing client.

Barter Transactions

During the year ended December 31, 2009, the Company entered into two transactions where it granted a subscription agreement to a client in exchange for patent assets. The Company accounts for non-monetary exchanges in accordance with ASC 845, Non-monetary Transactions ("ASC 845"), which requires non-monetary exchanges to be based on the fair value of the assets or services involved. ASC 845 further clarifies that the cost of the non-monetary asset acquired in exchange for another non-monetary asset is the fair value of the asset exchanged to obtain it. In each of the non-monetary transactions that the Company completed, the subscription agreement exchanged as consideration for the acquired patent assets was valued using the Company's standard fee schedule in effect, together with standard discounting practices offered at the time of the transaction. Given that this is the same pricing methodology that the Company uses in arm's-length monetary transactions, it considers the resulting value of the subscription agreement to be indicative of both its fair value and the fair value of the assets received. During the years ended December 31, 2011, 2010 and 2009, the total fair value of subscription services exchanged for patent assets was $120,000, $0 and $3.2 million, respectively. No gain or loss was recorded for either of these exchanges.

Sale of Patent Assets

During the year ended December 31, 2010, the Company entered into two patent asset sale transactions for total cash consideration of $500,000 resulting in a loss of $75,000. In October 2010, the Company committed to sell certain patent assets and accepted an offer from a third party to purchase these patent assets for $80,000 in November 2010 and reduced the carrying value accordingly. As a result, the Company recorded a $1.3 million loss, which is reflected in the consolidated statement of operations for the year ended December 31, 2010. This loss was partially offset by an $887,000 gain recognized in November 2010 from the forgiveness of a portion of the non-recourse financing secured by these patent assets. The Company recorded these patent assets as held for sale as a component of prepaids and other current assets on the consolidated balance sheet as of December 31, 2010 and the sale was consummated in January 2011.

No patent asset sales transactions occurred during the year ended December 31, 2011 and there were no assets held for sale at December 31, 2011.