Predicting Apple’s next steps for the iPad is a relative walk in the park compared to nailing the details for the iPhone 5, which I wrote about Tuesday.

There are only three major variables you need to consider in plotting what will be available in the iPad for the next 20 months:

• 1. When will LTE be available?
• 2. When will the screen resolution double from 768 x 1024 to 1536 x 2048?
• 3. When will NFC (near-field communication) be available?

LTE: The only comment Apple made about LTE was in early January when Tim Cook simply said the company couldn’t make LTE happen for Verizon’s iPhone 4. That’s it; no disagreement there as nobody expected Apple to be first in delivering a new radio technology.

That said, at this stage of the maturing game of smartphones and tablets, Apple probably also cannot afford to be too far behind the rest of the industry in launching a new radio technology, and that means LTE if you are introducing a new product in September for Verizon and AT&T.

Conventional wisdom has it that the reason one should expect Apple to take longer to deliver on a new radio technology is battery life. As the argument goes, companies such as HTC, Samsung, LG and recently even Motorola are more willing to cough up Android smartphones and tablets with poor battery life, if necessary, to hit the market as quickly as possible. In contrast, Apple cares more about the battery life experience, and is therefore willing to wait longer.

There are two reasons why this argument is a moot point now:

1. Apple’s iOS is very efficient compared to Google’s Android, especially because it polices multitasking. Compare the iPhone 4 and the iPad 2 to any Android smartphone or tablet, and you will find that Apple now beats all Android competitors in the battery life game.

2. On the iPad, the battery is already so large that the impact of a less efficient (LTE) radio will matter less as a percentage of the user experience. The impact on the iPhone may still be worth it, but it is larger as a percentage of the typical draw.

The first LTE tablets — presumably from Motorola and Samsung — will hit the market in July or August, both on Verizon, for starters. Considering that LTE smartphones are already on the market, making an LTE tablet is relatively easy: less battery concern, and no circuit-switched voice complexity to bake into the overall mix.

What does this mean for Apple? With multiple LTE tablets in the market by the end of August, why wouldn’t Apple make an LTE version of the iPad by September or October? I don’t see any reasonable engineering obstacle for this.

Doubling the iPad’s screen resolution: As with LTE, everybody knows this is coming — it’s only a matter of when. Samsung has stated its intent to double its large tablet resolution some time in 2012. I doubt Apple would be able to do so much earlier than Samsung, and it also takes a new more powerful processor to drive such a high-resolution display. It is therefore unlikely to happen this year.

NFC: Research In Motion  will have NFC in its 3G and 4G PlayBook tablets in the September-November 2011 time frame. Google will probably have it starting in November and onwards. There is no good reason Apple shouldn’t also have it by this September or October.

What is the bottom line here? All the logic points to Apple making an LTE iPad available no later than September or October. Perhaps it will be called the iPad 3, or perhaps iPad 2 LTE. Apple likes to keep things simple, which would point to iPad 3, but on the other hand nothing, or at least not much, would change to the WiFi-only version of the iPad 2. Perhaps including NFC in addition to LTE would point to calling it the iPad 3.

Later, some time between March 2012 and October 2012 — probably later rather than sooner — Apple would make the iPad 4 available, and its major feature would be the doubling of the screen resolution from 768 x 1024 to 1536 x 2048.

So there you have it: For the iPad, LTE could be available in a model refresh to happen in the next 90 days, whereas a doubling of the screen resolution will more likely follow one year after that. NFC could go either way, but market logic would suggest sooner rather than later.

I make it clear that all of this is pure speculation based on publicly available sources. I have not spoken to anybody at Apple, Apple’s carrier partners, or Apple’s suppliers.

[Cross-posted at The Street]