Originally posted Dec 1, 2014
Maybe this time I’ll remember my own explanation
At times, my brain seems to have a non-stick coating when it comes to certain technical details. I usually feel confident in my grasp of things while getting an explanation from an expert, watching a video or looking at a diagram. But a month or two later I’ll struggle to remember some critical element or distinction, and heaven help me if I’m called on to explain it to someone else!
One distinction that has vexed me in this way is the difference between MIMO channels and streams. The literature on MIMO is packed with mentions of both terms, but it’s rare to see them both explained in context. This blog has also been guilty of casual treatment—see the first post on MIMO—and it’s time to explain a little more. A clear, intuitive understanding is worthwhile, since both channels and streams are important, and understanding them together can lead to better RF measurements.
As always, different explanations will gain traction with different readers. Some will gain optimal insight from a mathematical discourse. Others, like me, do better with a visual approach and a diagram of a MIMO transmitter chain is the best place to start:
Transmit chain example for 2×2 MIMO in an IEEE802.11n system. The spatial encoding or mapping block determines how streams become RF channel outputs.
Streams are the easiest element to understand from a digital point of view. In the 2×2 MIMO case, a single payload data stream is scrambled and interleaved, and error correction is added. The stream is then split in two and multiplexed to the 48 OFDM data subcarriers. Independently, the two streams each carry half of the data payload.
The streams then become I/Q values, and if the streams are separately sent to RF transmit chains—bypassing any spatial encoding or mapping—the distinction between streams and RF channels is trivial. This configuration is called direct mapping.
However, there are several reasons why direct mapping is not the best approach for some real-world conditions. I’ll explain more in a future post, but for now imagine a situation in which one RF channel is nearly perfect and the other is badly impaired. The error-correction overhead required to keep the bad channel functioning would be wasted on the good one, and total throughput would be suboptimal.
An elegant way to solve this problem is to convert streams to RF channels using a scheme that’s more complex than direct mapping. For example, the spatial encoder could add the I/Q values of the two streams and send the sum to one RF channel; it could simultaneously subtract one from the other and send that result to the other RF channel.
In this way—and with appropriate encoding and decoding—the effective impairments are averaged between the two RF channels. An efficient amount of error-correction overhead can be chosen for the channel pair, optimizing overall data transmission. Symmetrical decoding and de-mapping at the receiver recovers the streams from the two incoming RF channels.
To cement the distinctions in my mind, I view streams and channels like this: streams are payload data, transformed to I/Q values on the OFDM subcarriers; and channels are the actual transmitted RF signals. For the direct mapped case, the streams become channels using the modulation processing we are familiar with, and that’s a useful mode of operation for RF troubleshooting. However, it’s likely not the common operating mode and it’s important to understand the difference!
For explaining channels and streams, then, this is a start. I haven’t said much about the implications for RF measurements, but will get to that in future posts.