Label: WLAN chip spatial multiplexing
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The scary terminology of 802.11n includes mysterious terms such as Multiple Input Multiple Output (MIMO), Spatial Streaming, and Spatial Reuse, which can quickly numb the network manager's brain. What is the true meaning of these specifications in the context of a real 802.11n wireless network environment?
MIMO and spatial streams
In general (and in an oversimplified sense), the numbers of MIMO and spatial streams are used together to represent the throughput potential over a given distance. In general, the more the number of receiving antennas, the longer the transmission distance that can maintain a certain data transmission speed.
Of course, like most wireless technologies, the actual throughput and distance you experience when using 802.11n depends on your environment. For example, the floor plan layout is open or separated; the building materials of the building and windows are; the configuration of the client equipment involved in the transmission. Therefore, in this context, consider the figures for MIMO and spatial streams that appear in the vendor data sheets.
In fact, MIMO is only part of this equation. It refers to the number of transmit and receive antennas involved in exchanging wireless signals in a propagation channel ("propagation channel" is a fancy word used to represent the wireless path over which the signal travels in the air). For example, 2×2 MIMO means two antennas at the transmitting end and two antennas at the receiving end, which are the minimum requirements of the 802.11n draft standard. 2×3 MIMO means two transmit antennas and three receive antennas, and so on.
Spatial reuse is a mandatory component
Spatial multiplexing is a mandatory component of the 802.11n standard, while MIMO is required for spatial multiplexing. Therefore, the two are working together.
So, what is spatial multiplexing? It is actually a technique in which multiple antennas independently transmit different streams (so-called spatial streams) consisting of separately encoded signals. In effect, wireless media or "multiplexed" signals are reused to transfer more data in a given channel. At the receiving end, each antenna sees a different combination of signal flows. In order to accurately decode them, the receiving device must separate these signals (ie "demultiplexing").
Note that the number of spatial streams that can be wirelessly multiplexed depends on the number of transmit antennas. Therefore, although 2×3 MIMO has one more receiving antenna than 2×2 MIMO, only two spatial streams can be supported in both configurations. So why is the receiver configured with three antennas?
What is the difference between N×N?
The conclusion is that the extra receive antenna increases the transmission distance for a given throughput that you can enjoy. Or, it increases the throughput over a given distance. However, let us start from the beginning.
Spatial multiplexing involves multiple antennas simultaneously transmitting wirelessly different streams of independently encoded signals. By multiplexing the signals over the wireless path, more data is transmitted over the wireless path.
Briefly, N transmit antennas transmit data to N receive antennas, and each receive antenna detects a unique stream, resulting in an increase in throughput of N-1 times. The "N x N" numbers represent the number of transmit (Tx) antennas and the number of receive (Rx) antennas participating in MIMO based and spatial multiplexing transmission, respectively.
So far, there have been systems on the market that use 2x2 MIMO to support two spatial streams, and two spatial streams using 2x3 MIMO. What is the impact of different N pairs on the transmitter and receiver?
PaulPetrus, director of architecture at WLAN chip maker Atheros, explains: "When you have more receiving antennas, you get the so-called 'combined gain'.
In other words, you receive more copies of the same signal, and ... a larger signal-to-noise ratio, which increases the signal strength. â€
An analog and real performance test recorded in an Atheros white paper shows an average performance improvement of approximately 20% when migrating from a 2x2 system to a 2x3 system in the uplink direction of a 20MHz channel. The white paper also shows that when transmitting on a 40MHz (two 20MHz bonded channels, which are allowed by the 11n standard), the 2x3 configuration increases the ratio of the average uplink throughput to the 2x2 configuration. Up to 40% over a distance of 30 to 40 feet and a 20% increase over a distance of 60 to 100 feet.
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