Rebecca Clary
Analyst · Morgan Stanley
Thank you, Dave, and good afternoon, everyone. As previously mentioned, we expect that 2019 was a transitional year in many respects, particularly when compared to the record-breaking financial performance of 2018. However, we are optimistic that growth will accelerate based on early indications of the market's reaction to the 2 core products that we introduced in the second half of 2019.
Commercial IoT continued to be the high point of our financial results during the fourth quarter with a 9% increase in ARPU, coupled with a 12% increase in the subscriber base when compared to the fourth quarter of 2018. Similar results were reported for the full year as Commercial IoT service revenue was up 26% from the prior year. The mix of rate plans among our subscribers generated higher ARPU in 2019 as our newest IoT product, the SmartOne Solar tracking device, generally results in higher usage and therefore higher service pricing than our legacy products. As the number of users of this device continues to increase, so does the blended ARPU of the base. New customers activating both the solar-powered product as well as legacy devices have expanded the Commercial IoT base, which now exceeds 400,000 subscribers.
Hardware revenue generated from Commercial IoT products was also up during the fourth quarter and full year periods due to a higher volume of sales across all of our product types. Solar device sales have been generally incremental to legacy device sales, which indicates that our products and solutions are expanding with the market opportunities. We also continue to pursue a meaningful pipeline of sales opportunities in the IoT space and are working to expand the capabilities of our devices in order to provide the products to our resellers that address the growing needs of our subscribers.
Earlier this year, we announced that we closed the acquisition of the intellectual property developed by Carmanah, the company that produced the SmartOne Solar. Anticipating growth and sales of this product, this acquisition is important as it will improve our profitability by increasing hardware margin by over 40%.
The improvements driven by Commercial IoT were not enough to offset the impact from the decline in our Duplex and SPOT subscribers. Which was the primary driver of the slight decreases in service revenue when comparing quarter-over-quarter and year-over-year results. However, as Dave covered in his remarks, we have initiatives underway to combat these issues. We have already started to see stabilization of our SPOT subscriber base as gross activations increased in 2019 compared to the prior year driven particularly by the success of our recently updated 2-way SPOT X device.
The changes in net income and loss during the period were driven by various financial statement items that aren't directly related to our core operations, including primarily noncash derivative gains and losses. After adjusting for these types of noncash items and other nonrecurring items that aren't representative of our core operating business, adjusted EBITDA was down 7% from 2018 and up 1% from the prior year's fourth quarter. These fluctuations reflect our earlier comments regarding 2019 being a rebuilding year. However, certain of our primary operating metrics began to show improvement in the fourth quarter, such as a 20% increase in SPOT activations and a 32% increase in the volume of Commercial IoT hardware sales.
And now turning to the most important thing that we have achieved as a company during the year: a broad-scale improvement to our capital structure, encompassing a refinancing of our senior BPI facility agreement; the issuance of a new second-lien facility; and as announced earlier this week, the anticipated conversion of Thermo's $138 million subordinated loan agreement into shares of our common stock.
I won't repeat the fulsome details of our refinancing that we provided in our call in early December. Instead, I will focus on where liquidity profile and balance sheet look like today. Our next scheduled principal payment of approximately $40 million is due in June of next year and is expected to be funded almost entirely by proceeds from the exercise of 2L warrants that expire in March 2021. We've reduced our total debt balance by $116 million from 1 year ago, including a $200 million decrease in senior debt outstanding. We also expect to have a fully funded business plan into 2025. Needless to say, we have a much stronger balance sheet, a simpler capital structure and more time to focus on running the business and monetizing our array of spectrum and network assets.
With that, I will turn the call over to the operator for Q&A.