The following table summarizes stock-based compensation expense related to stock options, PSUs, RSUs and restricted stock for the years ended December 31, 2021, 2020 and 2019:


(In thousands)	2021			2020			2019
Stock-based compensation expense included in cost of revenue	$	543		$	426		$	369
Selling, general and administrative expenses		4,571			4,036			3,889
Research and development expenses		2,366			2,372			2,704
Stock-based compensation expense included in operating expenses		6,937			6,408			6,593
Total stock-based compensation expense		7,480			6,834			6,962
Tax benefit for expense associated with non-qualified stock options, PSUs, RSUs and restricted stock		(1,849	)		(1,629	)		(1,659	)
Total stock-based compensation expense, net of tax	$	5,631		$	5,205		$	5,303

At the annual meeting of stockholders held on May 13, 2020, the Company’s stockholders approved, upon recommendation of the Board of Directors, the adoption of the ADTRAN, Inc. 2020 Employee Stock Incentive Plan (the “2020 Employee Plan”) as well as the ADTRAN, Inc. 2020 Directors Stock Plan (the “2020 Directors Plan”). No additional awards will be granted under the Company’s previous stock incentive plans, the ADTRAN, Inc. 2015 Employee Stock Incentive Plan (the “2015 Employee Plan”) or the 2010 Directors Stock Plan (the “2010 Directors Plan”) subsequent to the stockholders’ approval of these new stock plans. Outstanding awards granted under the 2015 Employee Plan and the 2010 Directors Plan will remain subject to the terms of such plans, and shares underlying awards granted under such plans that are cancelled or forfeited will be available for issuance under the 2020 Employee Plan or the 2020 Directors Plan, as applicable.

Under the 2020 Employee Plan, the Company is authorized to issue 2.8 million shares of common stock to certain employees, key service providers and advisors through incentive stock options and non-qualified stock options, stock appreciation rights, RSUs and restricted stock, any of which may be subject to performance-based conditions. RSUs and restricted stock granted under the 2020 Employee Plan will typically vest pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date. Stock options granted under the 2020 Employee Plan will typically become exercisable beginning after one year of continued employment, normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and have a ten-year contractual term. Stock options, RSUs and restricted stock granted under the 2020 Employee Plan reduce the shares authorized for issuance under the 2020 Employee Plan by one share of common stock for each share underlying the award. Forfeitures, cancellations or expirations of awards granted under the 2015 Employee Plan increase the shares authorized for issuance under the 2020 Employee Plan, with forfeitures, cancellations or expirations of RSUs and restricted stock increasing the shares authorized for issuance by 2.5 shares of common stock for each share underlying the award. Forfeitures, cancellations or expirations of stock options from the 2015 Employee Plan increase the shares authorized for issuance under the 2020 Employee Plan by one share of common stock for each share underlying the award.

Under the 2020 Directors Plan, the Company is authorized to issue 0.4 million shares of common stock through stock options, restricted stock and RSUs to non-employee directors. Stock awards issued under the 2020 Directors Plan typically will become vested in full on the first anniversary of the grant date. Stock options issued under the 2020 Directors Plan will have a ten-year contractual term. Stock options, restricted stock and RSUs granted under the 2020 Directors Plan reduce the shares authorized for issuance under the 2020 Directors Plan by one share of common stock for each share underlying the award. Forfeitures, cancellations and expirations of awards granted under the 2010 Directors Stock Plan increase the shares authorized for issuance under the 2020 Directors Plan by one share of common stock for each share underlying the award.

In January 2015, the Board of Directors adopted the 2015 Employee Plan, which authorized 7.7 million shares of common stock for issuance to certain employees and officers through incentive stock options and non-qualified stock options, stock appreciation rights, PSUs, RSUs and restricted stock. The 2015 Employee Plan was adopted by stockholder approval at our annual meeting of stockholders held in May 2015. PSUs, RSUs and restricted stock granted under the 2015 Plan reduce the shares authorized for issuance under the 2015 Employee Plan by 2.5 shares of common stock for each share underlying the award. Options granted under the 2015 Employee Plan typically become exercisable beginning after one year of continued employment, normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and have a ten-year contractual term. Expiration dates of options outstanding as of December 31, 2021 under the 2015 Employee Plan range from 2025 to 2026.

In January 2006, the Board of Directors adopted the ADTRAN, Inc. 2006 Employee Stock Incentive Plan (the “2006 Plan”), which authorized 13.0 million shares of common stock for issuance to officers and certain employees through incentive stock options and non-qualified stock options, stock appreciation rights, RSUs and restricted stock. Options granted under the 2006 Plan typically become exercisable beginning after one year of continued employment, normally pursuant to a four-year vesting schedule beginning on the first anniversary of the grant date and had a ten-year contractual term. The 2006 Plan was replaced in May 2015 by the 2015 Employee Plan. Expiration dates of options outstanding as of December 31, 2021 under the 2006 Plan range from 2022 to 2024.

In May 2010, the Company’s stockholders approved the 2010 Directors Plan, under which 0.5 million shares of common stock have been reserved for issuance. This plan replaced the 2005 Directors Stock Option Plan. Under the 2010 Directors Plan, the Company may issue stock options, restricted stock and RSUs to our non-employee directors. Stock awards issued under the 2010 Directors Plan become vested in full on the first anniversary of the grant date. Options issued under the 2010 Directors Plan had a ten-year contractual term. All remaining options under the 2010 Directors Plan expired in 2019.

The following table is a summary of our PSUs, RSUs and restricted stock outstanding as of December 31, 2020 and 2021 and the changes that occurred during 2021:


	Number of shares (In thousands)			Weighted Average Grant Date Fair Value
Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2020		1,846		$	11.49
PSUs, RSUs and restricted stock granted		748		$	20.17
PSUs, RSUs and restricted stock vested		(552	)	$	13.67
PSUs, RSUs and restricted stock forfeited		(112	)	$	11.70
Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2021		1,930		$	14.11

The following table details the significant assumptions that impact the fair value estimate of the market-based PSUs:


	2021			2020			2019
Estimated fair value per share	$26.07			$14.43			$9.53 to $18.05
Expected volatility		53.27	%		51.88	%	32.7% to 38.9%
Risk-free interest rate		0.85	%		0.24	%	1.6% to 2.46%
Expected dividend yield		1.63	%		2.85	%	2.3% to 4.09%

For market-based PSUs, the number of shares of common stock earned by a recipient is subject to a market condition based on ADTRAN’s relative total shareholder return against all companies in the NASDAQ Telecommunications Index at the end of a three-year performance period. Depending on the relative total shareholder return over the performance period, the recipient may earn from 0% to 150% of the shares underlying the PSUs, with the shares earned distributed upon the vesting. The fair value of the award is based on the market price of our common stock on the date of grant, adjusted for the expected outcome of the impact of market conditions using a Monte Carlo Simulation valuation method. A portion of the granted PSUs vests and the underlying shares become deliverable upon the death or disability of the recipient or upon a change of control of ADTRAN, as defined by the 2020 Employee Plan. The recipients of the PSUs receive dividend credits based on the shares of common stock underlying the PSUs. The dividend credits vest and are earned in the same manner as the PSUs and are paid in cash upon the issuance of common stock for the PSUs.

During the first quarter of 2021 and 2020, the Company issued 0.6 million performance-based PSUs under the 2015 Employee Plan to its executive officers and certain other employees as noted. The grant-date fair value of these performance-based awards is based on the closing price of the Company’s stock on the date of grant. Subject to the grantee’s continued employment, the grantee has the ability to earn shares in a range of 0% to 142.8% of the awarded number of PSUs based on the achievement of a defined performance target at the end of a three-year period. If the Company achieves the performance target at the end of the first or second year during the vesting period, the grantee will be entitled to the target number of performance shares, which will be issued at the end of the three-year period. Equity-based compensation expense with respect to these awards will be adjusted over the vesting period to reflect the probability of achievement of the performance target defined in the award agreements.

The fair value of RSUs and restricted stock is equal to the closing price of our stock on the grant date. RSUs and restricted stock vest ratably over four-year and one-year periods, respectively.

We will continue to assess the assumptions and methodologies used to calculate the estimated fair value of stock-based compensation. If circumstances change, and additional data becomes available over time, we may change our assumptions and methodologies, which may materially impact our fair value determination.

As of December 31, 2021, total unrecognized compensation expense related to the non-vested portion of market-based PSUs, RSUs and restricted stock was approximately $17.5 million, which is expected to be recognized over an average remaining recognition period of 2.9 years and will be adjusted for actual forfeitures as they occur.

As of December 31, 2021, 3.8 million shares were available for issuance under shareholder-approved equity plans in connection with the grant and exercise of stock options, PSU’s, RSU’s or restricted stock.

The following table is a summary of stock options outstanding as of December 31, 2021 and 2020 and the changes that occurred during 2021:


	Number of Options (In thousands)			Weighted Average Exercise Price (Per share)		Weighted Avg. Remaining Contractual Life in Years		Aggregate Intrinsic Value (In thousands)
Stock options outstanding, December 31, 2020		2,718		$	21.17		2.86	$	—
Stock options granted		—		$	—			$	—
Stock options exercised		(382	)	$	16.82			$	1,491
Stock options forfeited		—		$	—			$	—
Stock options expired		(615	)	$	28.89			$	21
Stock options outstanding, December 31, 2021		1,721		$	19.37		2.39	$	6,669
Stock options exercisable, December 31, 2021		1,721		$	19.37		2.39	$	6,669

All of these stock options were issued at exercise prices that approximated fair market value at the date of grant. As of December 31, 2021, there was no unrecognized compensation expense related to non-vested stock options.

The aggregate intrinsic values represent the total pre-tax intrinsic value (the difference between ADTRAN’s closing stock price on the last trading day of 2021 and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders had all option holders exercised their options on December 31, 2021. The amount of aggregate intrinsic value will change based on the fair market value of ADTRAN’s stock and was $6.7 million as of December 31, 2021.

The total pre-tax intrinsic value of options exercised during 2021, 2020 and 2019 was $1.5 million, $0 and $0.1 million, respectively. The fair value of options fully vesting during 2021, 2020 and 2019 was $0, less than $0.1 million and $0.9 million, respectively.

The following table further describes our stock options outstanding as of December 31, 2021:


	Options Outstanding						Options Exercisable
Range of Exercise Prices	Options Outstanding at December 31, 2021 (In thousands)		Weighted Avg. Remaining Contractual Life in Years		Weighted Average Exercise Price		Options Exercisable at December 31, 2021 (In thousands)		Weighted Average Exercise Price
$15.33 – $16.97		703		2.75	$	15.89		703	$	15.89
$16.98 – $18.97		463		2.77	$	18.96		463	$	18.96
$18.98 – $30.36		555		1.64	$	24.12		555	$	24.12
		1,721						1,721

The Black-Scholes option pricing model (the “Black-Scholes Model”) is used to determine the estimated fair value of stock option awards on the date of grant. The Black-Scholes Model requires the input of certain assumptions that involve judgment. Because our stock options have characteristics significantly different from those of traded options, and because changes in the input assumptions can materially affect the fair value estimate, existing models may not provide reliable measures of fair value of our stock options. The stock option pricing model requires the use of several assumptions that impact the fair value estimate. These variables include, but are not limited to, the volatility of our stock price and employee exercise behaviors.

There were no stock options granted in during the years ended December 31, 2021, 2020 or 2019.