Market research company The NPD Group has released its findings on the smartphone industry for the second quarter of 2012. It states that consumer smartphone sales by unit-volume have risen 9% since the previous quarter. The company contributes the continued growth of the market almost entirely to the growing sales pre-paid smartphones. It finds that, compared to the same quarter in 2011, pre-paid smartphone sales have risen by 91%, while post-paid smartphone sales have remained steady.

“Prepaid smartphones are no longer just cheap, also-ran options, focused on older and less capable phones,” said Stephen Baker, vice president of industry analysis at The NPD Group. “As the smartphone market matures, and as growth slows, carriers have been smart to aggressively market some of their best current smartphones on a pre-paid basis to a new set of customers, in order to keep sales humming along.”

The NPD Group’s research shows that smartphones are becoming more affordable for less affluent Americans. They state that in the second quarter of 2012, one-third of smartphone buyers had household incomes of less than $35,000 per year.

It is well-known that Samsung is quickly becoming Apple’s major competitor in the smartphone market, and The NPD Group’s research shows just how heavily those two brands dominate the industry. Apple’s and Samsung’s combined smartphone sales have risen 43% over the past year, while all other brands’ combined sales fell 16%. The NPD Group lists Apple’s smartphone brand market share as 31% and Samsung’s as 24%. HTC and Motorola follow these heavyweights with 15% and 12% of the market, respectively. LG rounds out the top 5 with only 6% of the market.

“By concentrating on their best, flagship devices, while at the same time supplementing their volumes with lower priced alternatives, both Apple and Samsung are extending their lead over the other smartphone makers,” said Baker. “To be a share leader means participating in all segments of the market, in order to take advantage of pre-paid and other growth opportunities, while also providing the hero devices that drive customers to your brand.”