Technology
Correlation in the real world T e diagram below shows a MIMO system similar to that in previous diagram, except that in this case the receiving device is in a diff erent physical orientation relative to the transmitting antennas. T is could often be the case when one end of the system is a mobile device such as a cellular phone. In this orientation, the four RF paths shown
previously now traverse a very similar route. It is highly likely that all will experience very similar fading conditions. T is creates what the previous paragraphs described as a ‘high corre- lation’ scenario. Since it is now diffi cult for the system to
High and low correlation: diversity reception and MIMO techniques can offer a performance advantage if (as in the lower trace) the radio paths fade at different times
when both have experienced very similar envi- ronmental and fading eff ects), it is improbable that the system can combine or process those signals to improve reception. Diversity (and MIMO, for that matter) is most successful when there is low correlation between radio paths.
The birth of MIMO While diversity is not the same thing as MIMO, the discovery of per-path fading diff erences provided an opportunity for radio experts. T at opportunity was seized in 1996, when
Lucent’s Gerard Foschini led the creation of the fi rst MIMO system, the Bell Laboratories Lay- ered Space-Time (BLAST) system. T e system used m transmitting antennas and n receiving antennas, creating m × n spatially-separate ra- dio paths. T e diagram below depicts a ‘2 × 2’ MIMO system using two transmitting antenna ele-
ments and two receiving elements to create four separate radio paths. At a basic level, the system takes advantage
of multiple receiving antennas to diff erentiate between each of the transmitted signals based on fading, which is in turn a function of the spatial properties of the radio environment. T e system picks them out one by one and re- constructs separate data streams, even though all were transmitted at the same time and in the same frequency. Modern MIMO usage is a little more compli-
cated, mainly due to its prominence in mobile (cellular) radio systems. Implementation would be simpler if both transmitters and receivers re- mained motionless and were always oriented so that the RF paths were widely separated. In reality, most MIMO systems are meant
for mobile wireless, so these factors are not only out of the control of the design engineers, but they are constantly changing.
diff erentiate between RF paths, the MIMO system must back down (at least momentarily) and only attempt to deliver data at speeds simi- lar to a Single-Input Single-Output (SISO) sys- tem. In rough terms, the data rates off ered by a 2 × 2 MIMO system in this scenario would be only about half of the maximum theoreti- cal rates.
MIMO system in a ‘high correlation’ arrangement: here the MIMO technology can deliver no advantage because the four radio paths are identical
Antenna design also has signifi cant eff ects
on the correlation between radio paths. In ad- dition, the eff ects are not static: private research shows that in a typical mobile system, MIMO correlation changes drastically (from near 0 per cent correlation to nearly 100 per cent and back again) hundreds of times per second. T is cre- ates a series of challenges for the wireless pro- fessional, leading to specialized MIMO-related branches of wireless science and engineering.
Conclusion We are still in the infancy of the deployment of these important antenna techniques, and even this topical discussion hints at some of the complexities that need to be resolved in order for next-generation wireless networks to deliver on their full potential. Today’s WiMAX and LTE systems use 2 × 2
Topography of Lucent’s BLAST system: with two transmitting antenna elements and two receiving elements, four separate radio paths are available, offering a choice of routes
LAND mobile January 2011
MIMO to increase data rates and increase sys- tem capacities. Tomorrow’s systems will deploy 4 × 2 and 4 × 4 MIMO, and the technology continues to move quickly. Researchers have already built working prototypes of 8 × 2 and 8 × 4 MIMO systems to be deployed a few short years from now.
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