In two previous posts-Multipath environments and how they affect Wi-Fi propagation and How to make the best of 802.11 multipath environments-I examined the somewhat nebulous topic of multipath fading and technologies that minimize the negative effects on Wi-Fi networks. Now I’m going to pull a 180 and talk about how important multipath propagation is.
One researcher and wireless pioneer, Dr. Greg Raleigh, was especially instrumental in determining how to use multipath environments to an advantage. One of the better known developments resulting from the research is Multiple Input/Multiple Output (MIMO) smart antenna technology.
How MIMO works
Qualcomm, a wireless chipset developer that acquired Airgo, the company founded by Dr. Raleigh, has the best definition of how MIMO works:
MIMO systems divide a data stream into multiple unique streams, each of which is modulated and transmitted through a different radio-antenna chain at the same time in the same frequency channel. A revolutionary technique that reverses 100 years of thinking about how radio signals are transmitted, MIMO leverages environmental structures and takes advantage of multipath signal reflections to actually improve radio transmission performance.
Through the use of multipath, each MIMO receive antenna-radio chain is a linear combination of the multiple transmitted data streams. The data streams are separated at the receiver using MIMO algorithms that rely on estimates of all channels between each transmitter and each receiver. Each multipath route can then be treated as a separate channel creating multiple “virtual wires” over which to transmit signals. MIMO employs multiple, spatially separated antennas to take advantage of these “virtual wires” and transfer more data. In addition to multiplying throughput, range is increased because of an antenna diversity advantage, since each receive antenna has a measurement of each transmitted data stream. With MIMO, the maximum data rate per channel grows linearly with the number of different data streams that are transmitted in the same channel.
That describes in a nutshell the basic tenants behind MIMO antenna systems.
Two distinct environmental conditions
To avoid confusion when discussing MIMO and multipath propagation, it’s important to define the two different yet related environmental conditions encountered by MIMO RF propagation: RF Line of Sight, and RF Non-line of Sight.
RF Line of Sight (LoS): Under this condition, RF signal propagation-regardless if Single Input/Single Output (SISO) or MIMO technology-will not encounter any physical interference along the link path. This eliminates any multipath advantage gained by MIMO technology and any multipath fading disadvantage seen by SISO technology.
Even with a level playing field, MIMO technology still has a distinct advantage, because it uses a process called spatial multiplexing. In explanation, if a SISO system and a MIMO system are being supplied with an identical data stream, the MIMO system’s data rate will be X times the data rate of the SISO system-where X is the number of receive/transmit antenna pairings. Even the minimal doubling of the data rate is quite significant when considering today’s bandwidth intensive applications.
RF Non-Line of Sight NLoS): This is a condition where the RF signal encounters significant physical interference along the link path and only altered RF signals reach the receiving antenna. These altered RF signals have the tendency to interfere with each other, often destructively which results in multipath fading, the bane of conventional radios using SISO technology.
Wireless pioneers like Dr. Raleigh decided to take advantage of the specific multipath phenomena in which received signals from one transmitting antenna will have different phase, timing, or signal strength characteristics from received signals transmitted by a different antenna. This line of thinking brought about one of those all too seldom “Ah ha!” moments. Using multiple transmit/receive antenna pairings to overcome multipath fading also compliments the concept of higher data rates being derived from having multiple RF streams.
The last piece of the puzzle and where MIMO technology finally came together was the advent of new and, in my opinion, amazing receiver technology. By using advanced digital signal processing hardware and very sophisticated algorithms that deal with space-time coding, it becomes possible to decipher the multipath differentiated RF signals even though they are all on the same frequency.
I’m amazed at how technology development timelines continue to shorten, with the recent developments in 802.11n, especially MIMO technology, being one example. Even more important is the fact that these advances will effectively change how everyone accesses data networks and the Internet.
The 802.11n standard has additional features and technical breakthroughs that are not as well publicized as MIMO. I’ll look at those attributes in my next post.