What 802.11n tech specs mean

'We established in the last newsletter that you need multiple transmit and receive antennas working in parallel (a.k.a. "MIMO" technology) to conduct spatial multiplexing. We also learned that spatial multiplexing is a mandatory component of the 802.11n standard and is the primary means to 802.11n's increased throughput promise. (Compare Enterprise Wireless LAN products.) But what's the ultimate difference in so-called 2x2 and 2x3 architectures, if they both support two spatial streams?

Bottom line, the extra receive antenna increases the range at which you might enjoy a given throughput. Or, it increases throughput at a given range. But let’s take it from the top.

You’ll recall that spatial multiplexing involves multiple antennas separately sending different flows of separately encoded signals over the air at the same time. By multiplexing the signals over a wireless path, more data gets through. Simplistically, N transmitting antennas send to N receiving antennas, and each receiver detects a unique stream, resulting in an N-fold increase in throughput.

The numbers in the “NxN” jargon represent, respectively, the number of transmitting (Tx) antennas and the number of receiving (Rx) antennas involved in the MIMO-based, spatially multiplexed transmission.

To date, there are systems on the market using 2x2 MIMO supporting 2 spatial streams, as well as those using 2x3 MIMO, which also support two spatial streams. How does having a different N at the Tx and Rx ends affect the transmission?

“When you have more receive antennas, you have what is called ‘combining gain,’” explains Paul Petrus, director of architecture at wireless LAN chipmaker Atheros. “In other words, you have more copies of the same signal, and…the greater the signal-to-noise ratio,” which strengthens the signals.

Simulated and real-world performance tests documented in an Atheros white paper show about a 20% increase in average performance when moving from a 2x2, 2-stream system to a 2x3, 2-stream system in the uplink direction in a 20MHz channel. The same paper shows that average uplink throughput rates when transmitting across 40MHz channels (two, 20-MHz bonded channels, which the 11n standard allows) can be up to 40% greater in the 2x3 configuration than in the 2x2 configuration over distances of 30 to 40 feet and 20% greater in the 60- to 100-foot range.