Making MIMO test work

The drive to increase wireless data rates within the limited radio frequency (RF) spectrum is leading to radios with capabilities beyond a single-input single-output (SISO) topology. SISO radio devices use one transmitter and one receiver to send data over a single RF channel. Recently introduced wireless protocols have adopted multiple-input multiple-output (MIMO) topologies that use two or more transmitters and two or more receivers to send data simultaneously over the same RF bandwidth. For example, the IEEE 802.11n/ac WLAN and IEEE 802.16e WiMAX standards include MIMO functionality.

This analysis focuses on MIMO RF topologies and the implications of MIMO on radio frequency integrated circuit (RFIC) test. Because MIMO topologies make use of multi-path signal transmission in a highly-scattered open-air environment, there are implications when testing MIMO RFIC devices in a cabled RF environment. In this analysis, we use IEEE 802.11 WLAN to illustrate the details of MIMO test equipment setup and operation for a specific protocol.

Overview of MIMO
A MIMO RF system uses multiple transmitters and multiple receivers to send data simultaneously over a single RF band. For clarification, the input and output terminology are in reference to the RF channel. For example, the input (the SI or MI portion) is driven by the transmitter(s), and the output (the MO or SO portion) feeds the receiver(s). Figure 1 shows the four input-output topologies. In overview, the four topologies are used in different applications as follows:
•    SISO is the most common transmission mode using a single transmitter and single receiver.
•    SIMO or receive diversity is when a single transmitter feeds multiple receivers. Although there is no increase in data rate, the multiple receivers reduce multipath fading and enhance signal-to-noise ratio (SNR).
•    MISO or transmit diversity is when multiple transmitters feed identical data to a single receiver. Similar to receive diversity, the duplicated transmitters reduce multipath fading.
•    MIMO involves multiple transmitters sending unique data content to multiple receivers using spatial multiplexing. MIMO does increase data rates and requires better SNR than an equivalent SISO transmission.


Whereas multipath interference degrades a SISO channel by causing channel fading, MIMO topologies compensate for and benefit from multipath effects. In MIMO, phased sets of antennas take advantage of the differences in the spatial propagation paths to improve signal robustness or to send multiple data sets over a single frequency band. In general, having multiple antennas offers three potential use cases: diversity, beamforming, and space division multiplexing (SDM).

Diversity
Using either SIMO or MISO configurations, diversity techniques are used in RF systems to improve signal quality and coverage. In diversity mode, duplicate data is sent in all data streams, so there is no increase in data rate. Instead, the multiple receivers or multiple transmitters reduce multipath fading and enhance SNR.

Fading occurs when there are multiple transmission paths between a transmitter and receiver due to reflections and scattering in a wireless environment. The different transmission paths combine at the receiver to create a superposition of multiple copies of the original signal. The resulting constructive or destructive interference is defined as multipath fading. Fading can be overcome using multiple antennas at either the receiver or transmitter. If the antennas are separated by at least a half wavelength, a highly scattered multipath environment creates relatively independent paths to or from the different antennas [1].

In a SIMO receive diversity configuration, there are different methods used to combine the signals captured at the receive antennas. The three common receiver combining methods include:
•    Selection combining, which uses a switch to select the received signal with the greatest SNR.
•    Equal gain combining, which uses an equally weighted combination of all received signals.
•    Maximal ratio combining, which uses a weighted combination of the received signals based upon SNR. With this technique, SNR improves on average by a factor of N, where N is equal to the number of receivers.

In a MISO transmit diversity configuration, it is possible to achieve the same SNR improvement with two transmit antennas as can be achieved using maximal ratio combining with two receive antennas [2]. Transmitting the identical signal simultaneously does have unwanted directionality effects caused by beamforming. Space Time Block Coding (STBC) is used to overcome the directionality effects by inserting a time delay into one of the transmission paths. The time delay for STBC is typically within the 50 to 200 ns range. STBC is prevalent in wireless systems design because it is often more feasible to have multiple transmit antennas at the base station due to size and power constraints at the mobile device.

Beamforming
Beamforming is used to control the shape and directionality of transmitted or received signals. This technique combines elements in an antenna array such that signals at particular angles experience constructive interference and signals at other angles experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. This is useful to extend the range of an RF channel in a particular direction, while simultaneously avoiding signals from other directions.

Space division multiplexing
SDM is similar to diversity, but it is used to achieve higher data rates instead of improved signal quality. In a highly scattered multipath wireless environment, SDM uses spatial multiplexing where different data streams are simultaneously transmitted and received over the same RF bandwidth. SDM requires a MIMO configuration with multiple antennas at both transmit and receive sides. Figure 2 shows an N x N MIMO configuration with signal path coefficients shown as hXY. These signal path coefficients represent the magnitude and phase response of the signal path between each transmitter and each receiver. The definition of an SDM channel includes all of the simultaneous data transmissions on the set of MIMO antennas.


The best MIMO channels have strong, well-separated spatial propagation paths. Similar to diversity, antennas that are separated by at least one-half wavelength will provide good spatial separation.
In order for the receiver to recover and separate the individual data streams, an estimate of the MIMO channel response must be predetermined. Typically, channel estimation is accomplished during a training sequence where all transmitters generate a known training signal. Signal processing at the receivers is used to estimate the signal path responses to this known training signal. Mathematically, the MIMO channel can be represented as a matrix of signal path coefficients as shown in Figure 3.


Using the inverse of the MIMO channel matrix (H) that is estimated during the training sequence, signal processing at the receivers can spatially demultiplex the original transmit data streams as:
where T, H and R are the matrices in Figure 3 and H-1 is the matrix inverse of H.

The singular values of the MIMO channel matrix provide a measure of the strength and separation of the MIMO data streams. The best spatially separated MIMO data streams have large singular values that are approximately equal in magnitude. When this is the case, the MIMO channel has good spatial separation on the paths to/from the different antennas and robust SDM data transmission is possible.