MIMO SCALABILITY

One of the benefits of MIMO technology is its ability to scale data transmission speed with the number of antennas and radio and signal processing hardware. When coupled with the increasing integration levels governed by Moore’s law, it provides a communications roadmap to the future.

The data rate of a SISO system is determined by:

R = ES * BW

Where R is the data rate (bits/second or bps),
ES is the spectralefficiency (bits/second/Hertz or bps/Hz),
and BW is the communications bandwidth (Hz).

For instance, for 802.11a, the peak data rate is obtained by:
BW = 20MHz
ES = 2.7 bps/Hz
yielding R = 54Mbps

SISO systems obtain greater performance by using greater
bandwidth. For instance, Atheros’ Turbo® mode allows for:
BW = 40MHz
ES = 2.7 Bps/Hz
yielding R = 108Mbps

Using MIMO, an additional variable is introduced – the number of independent data streams, NS, that are communicated simultaneously in the same bandwidth, in different spatial paths. The spectral efficiency is now measured per-stream as ESS. The data rate of a MIMO system becomes:
R = ESS * BW * NS


For the current 802.11n proposal, there are 10, 20, and 40MHz modes allowed, yielding peak rates with the following parameters
BW = 10, 20, or 40MHz
ESS = 3.6 bps/Hz (BW = 10 or 20)
ESS = 3.75 bps/Hz (BW = 40)
NS = 2, 3, 4
yielding R=144Mbps (20MHz, Ns = 2)
yielding R=300Mbps (40MHz, Ns = 2)
yielding R=600Mbps (40MHz, Ns = 4)

Thus peak data rates ranging from 144Mbps to 600Mbps can be obtained by modifying the bandwidth and number of spatial streams.