Multiple input/multiple output (MIMO) is the use of multiple antennas at both the transmitter and receiver to improve communication performance. Performance is the operative word here, in terms of capacity, bandwidth, throughput, range, or a host of other parameters that enhance the user experience.
Advanced wireless communications standards such as Wi-Fi (IEEE 802.11), WiMAX (IEEE 802.16), and LTE (Long- Term Evolution from 3GPP) use multiple transmit and receive antennas in different configurations to squeeze out more performance. All of these emerging commercial communications protocols have one key factor in common: As transmission and reception conditions change, they dynamically use the multiple transmit and receive antennas to achieve different performanceenhancing effects.
Thus, testing MIMO wireless communications takes on a new perspective from previous generations of short- and longrange wireless solutions. That’s because the most important factor is the ability to deliver performance, despite the conditions. Central to this testing is a tool called a channel emulator. It can reproduce dynamically changing transmission conditions to stress the MIMO scenario and ensure optimal performance and interoperability.
MIMO CONFIGURATIONS
MIMO exploits two key factors in wireless. First, thanks to semiconductor technology, it’s relatively easy and cost-effective to put two, three, four, or even more transmitters and receivers on a single chip. Second, at microwave frequencies, wavelengths are short (inches), making it easy to space antennas a wavelength or more apart to achieve the effects offered by MIMO.
This gives rise to a variety of configurations. The most common is probably 2 × 2 MIMO with two transmitters and two receivers (Fig. 1). Many other configurations are possible, such as 2 × 1, 2 × 3, 3 × 2, 3 × 3, 4 × 4, and other more elaborate schemes. In general, the greater the number of antennas involved, the greater the performance. The configuration decision is based on the type of equipment (basestation, handset, or laptop), the practical side of implementing the number of antennas, and cost.
MIMO TECHNIQUES
The broad category of MIMO encompasses three core performance-boosting techniques. Spatial multiplexing transmits different data from each antenna, but on the same frequency and time. The conditions for transmission on each antenna are different, enabling the receiving antennas to split the transitions out to discrete data streams at the receiving end. This technique generally doubles the throughput in the same RF space.
Another technique called diversity effectively combats fading. Diversity has two flavors: transmit (Tx) diversity and receive (Rx) diversity.
Tx diversity can transmit the same data on multiple antennas at the same time, but with different coding schemes. The receivers can then decode the streams with more confidence because they can be compared. Also, in many cases, if one stream contains an error, the second will be able to provide the correct data, greatly enhancing the reliability of the data.
Rx diversity captures the data transmitted from a single source on two receive chains. Each chain has a different antenna that may be positioned differently and have different characteristics. So as the two receive signals are added together, there’s a greater chance of the correct error-free data being received.
Finally, beamforming is an advanced technique that can steer the antenna array. Correctly phasing the antennas in the array will reinforce the waves (peaks and troughs will sum). What results is a stronger wave to be received in that line of transmission and a longer transmission in a specific direction. Correspondingly, beamforming can be used to reduce interference by phasing the antennas to cancel signals arriving from undesired transmitters (receive beamforming) or to prevent waves from being received strongly at an undesired receiver (transmit beamforming).
Each of the MIMO techniques will be most effective in different conditions (see the table). The net outcome of these techniques is to overcome the effects of overthe- air transmission and motion to deliver higher throughput, better coverage at the cell edge, and greater range.
CHANNEL EMULATION
Before MIMO, traditional wireless testing focused mostly on the quality of the receiver. The better the receiver and its ability to decode the lowest-power signal under the highest noise conditions, the better the system would operate. MIMO takes wireless to new extremes, though, so while receiver capabilities are important, that’s only a part of the system’s ability to deliver performance.
Constantly changing conditions in the real world bring new challenges for devices under test. Changing signal strength (amplitude), distance (amplitude and phase), and direction of the transmitter and receiver (angle of arrival and departure), as well as the two devices’ adaptibility to these conditions and ability to implement the best MIMO techniques at any given moment, all impact performance.
In the real world, the ability to operate while in motion is a basic requirement for mobile wireless broadband. But it isn’t easy to recreate it in a test lab so the test is stable, reproducible, and realistic and so the test bed represents the dynamic and changing conditions of a real-world scenario. This is where a MIMO channel emulator comes into play. It delivers three core capabilities, and depending on the quality of the solution, it can provide multiple additional test features:
• Fading: As a mobile device transmits to the receiver, the over-the-air transmission effects and obstructions in its path over the band will cause the signal strength to vary rapidly. Fading is frequencyselective; the signals may momentarily fade high at one point in the band, while a nulling effect (down fade) may simultaneously occur in another part of the signal bandwidth. Since the signal in broadband wireless orthogonal frequency- division multiplexing (OFDM) consists of multiple subcarriers over the band, the frequency-selective fading may negatively affect one carrier and have no effect on another. This scenario, called fast fading, is typically influenced by the type of environment and the speed of the user equipment.
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