Technology


A journeyman’s guide to MIMO


MIMO remains a seemingly arcane fi eld of wireless study, says Nigel Wright, of Spirent. But it forms the foundation for the astonishing data rates and coverage potential promised by tomorrow’s radiocommunications technologies


frequencies to transmit separate streams of information. Time Division Multiplexing (TDM) diff erentiates data streams by using a round-robin of slices of time. Since time is fi xed and radio spectrum is expensive, wireless engineers needed to fi nd a domain separate from the time and frequency domains: they needed to fi gure out a way to transmit diff erent information streams at the same time and at the same radio frequency. Figure 1 provides a hint as to the solution.


The wireless channel in the real world: radiocommunications signals may take a variety of indirect routes from the transmitter to the receiver, by way of refl ections from surfaces in the environment


A


nyone who follows wireless commu- nications these days has run across a funny-looking acronym: MIMO.


While Multiple-In Multiple-Out antenna tech- niques have not yet been a big story in business- to-business communications, they are already deployed in consumer communications such as WiMAX and LTE. As the lines begin to blur between PMR, PAMR and general wireless ac- cess, it is time to get a handle on the fundamen- tal concepts of MIMO. To begin, think about the wireless channel depicted in the diagram above. Wireless would


About the author


Nigel Wright is vice president of product marketing for Spirent, a provider of test systems for mobile devices and network equipment


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be relatively simple if radio waves travelled di- rectly from transmitters to receivers. T is is not the case. Radiated energy actually refl ects from buildings, hills, bodies of water, moving vehi- cles, etc. In some cases, there is no line-of-sight path. Every signal ‘seen’ by the receiver actu- ally includes refl ections from several diff erent directions. Since refl ected signals travel diff erent dis-


tances (therefore taking diff erent amounts of time to get to the receivers), the receiving an- tenna sees a series of time-shifted, attenuated copies of the same signal. In some cases, these refl ections can include components that par- tially cancel out some frequencies at the receiv- ing antenna. T is phenomenon is known as fading, and engineers have been wrestling with it since the time of Marconi. However, there is an upside to having a del-


eterious radio environment: it off ers another domain in which to create data ‘pipes’ between transmitter and receiver. In the past, radio signals have been multi-


plexed in several diff erent domains: Frequency Division Multiplexing (FDM) uses separate


Each received signal traverses a diff erent path in space. If a system could use spatial informa- tion to tell one path from the other, it would be able to diff erentiate them, allowing each to carry diff erent data. T e result would be multi- plexing in the spatial domain.


Separate paths T e key to making this work was a discovery made by some RCA telegraphy engineers in 1926. T ey discovered that, when receiving an- tennas were separated by distance or were phys- ically cross-oriented, the received signals were slightly diff erent since each had experienced diff erent environmental (refl ective) eff ects. Because it would be rare for multiple anten-


nas to experience a deep fade (signal loss) at the same time, the RCA engineers realized that they could use multiple antennas to improve signal reception. T is was the world’s fi rst re- ceive diversity system. Because the ability to recover a signal was


based on the diff erences in fading, the industry needed a metric to measure those diff erences. In this context, ‘correlation’ is a measure of the extent to which separate radio paths experience diff erences in fading.


Signal diversity At the top of the facing page are examples of high and low correlation between two received signals, shown as functions of time. When two signals are highly correlated at the receiver (i.e.


LAND mobile January 2012


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