How 802.11n Works
You must have heard or read about new wireless LAN systems being referred to as 3×3 capable or 2×2 capable, or some combination of the two. What does all this mean? Let’s decode the true meaning of these terms.
Products based on the latest and greatest wireless standard, 802.11n, use a technology known as MIMO. MIMO (Multiple In, Multiple Out) utilizes multiple radio chains (and hence multiple antennas) at both the transmitter and the receiver to help increase the throughput and transmit larger amounts of data over the wireless link. At least two chains are required for MIMO functionality; but most systems have more 2 chains. Additionally MIMO utilizes a technique called SDM (Spatial Division Multiplexing) that takes advantage of the multiple transmit and receive radio chains making it possible to send multiple streams of data simultaneously on the same channel, thereby increasing the data rate and overall throughput. All products certified by the WiFi Alliance for 802.11n must support at least two spatial streams. The IEEE 802.11n specification offers options for up to four spatial streams, though as of now there are no systems available with this feature.
In the industry, 802.11n products are typically described in terms of their MIMO attributes, denoted by TxR where “T” is the number of transmit radio chains and “R” is the number of receive radio chains. (I think that this description is not adequate and generally leads to confusion, but more on that later in this blog). Most of the 802.11n enterprise APs are either 2×3 or 3×3 systems while most of the early 802.11n clients are 2×2 systems. Other combinations are also possible. In addition to the radio chains (and respective antennas), every 802.11n device must have multiple spatial streams, which is rarely talked about or referred to. Rather than the number of radio chains or antennas, the number of spatial streams is the key factor in determining the capability of the wireless device. The number of streams is a property of the radio chipset. All commercially available chipsets on the market today support a maximum of 2 streams for the access point and clients with the exception of Intel 5300 that support 3 streams for the clients. (Intel does not provide chipsets for the Access Points).
Assuming a clear signal, a two spatial stream link will achieve twice the throughput of a single spatial stream in the same channel. Each spatial stream provides data rate up to 150 Mbps while a Draft 2.0 802.11n system with two spatial streams will support up to 300 Mbps. Key point here is that as long as the system supports 2 spatial streams, you can achieve 300 Mbps data rates, regardless of the number of radios chains.
All the systems depicted next have 2 spatial streams and support 300Mpbs data rates with various combinations of transmit and receive chains.
The picture below shows a 3×3 MIMO system with 3 transmitters and 3 receivers on both the AP and the client and 2 spatial streams (denoted by dotted yellow lines). However since the system supports only 2 spatial streams, one pair of antenna is not used for transmission / reception and maybe used for diversity.
This picture shows a 3×2 MIMO system with 3 transmitters and 2 receivers, still supporting 2 spatial streams.
This picture shows a 2×3 MIMO system with 2 transmitters and 3 receivers with 2 spatial streams.
This picture shows a 2×2 MIMO system with 2 transmitters and 2 receivers, again with 2 spatial streams.
As you can see, to describe true capabilities of the system, both the number of radio chains and the number of spatial steams are important to know. While its not the norm, I think that 802.11n products should be described in terms of their MIMO attributes, denoted by TxR:S where T is the number of transmit radio chains, R is the number of receive radio chains and S is the number of spatial streams. Using the S attribute in addition to the T and R attributes, will create more clarity for the end user and help choosing the right product
Partnered with Meru Networks, AirJuice designs and installs a single channel virtual cell Access Point Solution for 802.11a/b/g/n. We are also proud to provide Meru Assure to our customers.