In the first installment of this blog I said I’d adjust the topic based on reader feedback. As we prepare for 4G, certain RF testing topics keep coming up, so I’m going to derail our topic thread.

By the way, while I do appreciate your emails (most of them, anyway… as to the others, you know who you are...), some good questions and comments might have stimulated useful conversations had they been shared publicly. We ask for contact info when you comment, but we won’t release that to anyone. Also, unless I have your clear permission, I will not publicly share anything that’s emailed to me. So in the interest of sharing information and questions, I’d urge you to please use the Comments section below.

Our last posting [Weighing the Options – Three Approaches for Testing Real-World A-GPS Performance] discussed approaches for testing real-world A-GPS performance, ending with a recommendation for a lab-based simulation approach. With its repeatable environment, easy setup, fast test times, automated result collection and enhanced handset performance benchmarking capabilities, lab simulation certainly sounds like a great idea, right? Well actually, of the three approaches we discussed, lab simulation is not the most widely used. Why is this?

For starters, real-world lab simulation is really hard to do correctly. Accurately simulating a handful of moving satellites transmitting wirelessly to a moving receiver while signals are distorted, reflected, and obstructed by moving objects and changing atmospheric conditions is enormously complex. In fact, it has been Spirent’s business for the past 20 years or so to do exactly this, and we’ve struggled with it too!

It’s time to answer a question from a couple of weeks back: “if fading is so random, how can we control and quantify what it’s doing?” In other words, how do we express it, record it and eventually repeat it?

One key concept is the idea of Level Crossing Rates (LCR). At a high level, all this means is that a) we choose a level below which we say a fade is a “deep fade”, and b) count the rate at which fades dip below this level.