Today's radio devices use a single-input, single-output (SISO) configuration with one transmitter and one receiver and information sent over a single-channel. Multiple-input, multiple-output (MIMO) transmission transmits information over multiple radio channels, but only occupies the bandwidth of a single channel. MIMO presents one of the most significant changes to happen to radio architectures in recent history. This technology can now be used in a wide range of commercial communications devices including mobile phones, PDAs and laptops and is an integral part of the 802.11n and WiMAX Wave 2 standards.

Along with the benefits of increased bandwidth, multisignal transmission and reception adds more layers of complexity. For instance, the transmitters must be aligned in time and phase and must have a high degree of isolation from each other. Thus, moving from SISO to MIMO-based systems presents a number of unique testing challenges that test engineers must consider.

The SISO configuration (Figure 1) is used in almost all contemporary radio designs. Sometimes, there may be an extra antenna for spatial diversity that is constantly switched for the best signal path. However, this is still considered to be a SISO system because there is a single upconverter and a single downconverter, a single demodulator/modulator, and a single datastream in the higher levels of the product's communications stack.

Multipath effects can degrade a SISO transmission. For example, a Bluetooth signal with a symbol rate of 1M symbols per second must receive a symbol within a window of one microsecond. If multipath effect delays the signal by more than this, a significant symbol error will occur. MIMO systems, on the other hand, require multiple paths. If two signals are transmitted with known characteristics, for instance a header, at the receiver end, one can assume what the signal should look like and create a model of the channel effects. When the unknown signal comes, i.e. the data, subtracting the channel effects can solve for the transmitted symbols. The key to a MIMO system, and why it is different from SISO, is that the behavior of the channel is critical and must always be understood.

Three ways to transmit data using a MIMO configuration, include:

1. The spatial multiplexing technique transmits different data on each channel, thus increasing the throughput.

2. Spatial diversity transmits the same data on each channel. This redundancy in effect increases the robustness of the signal and improves the transmission coverage.

3. Beam forming. This technique improves the throughput and coverage by controlling the directionality and the shape of the transmitted signal.

A typical MIMO configuration can range from a 2 × 2 system, containing two transmitters and two receivers, to a 4 × 4 system with four transmitters and four receivers (Figure 2). Many commercial wireless LAN (WLAN) devices today employ a 3 × 2 configuration of three transmitters and two receivers. In the future, beam-forming based systems could have up to 8 × 8 configurations.

Perhaps the greatest testing challenge for MIMO systems involves synchronization with good channel isolation in the transmitter and the receiver. Transmission of multiple signals requires accurate synchronization of multiple channels in phase and sampling alignment. This means that RF test equipment such as signal analyzers and generators must have precise alignment and excellent isolation between channels in order to make accurate and repeatable measurements.

For most test engineers, a major challenge is the ability to transition smoothly from single-channel to multichannel testing and, therefore, choosing instruments that provide a clear and easy upgrade path to MIMO. For example, moving from WiMAX SISO to the MIMO versions based on Matrix A, B and even C, the highest 4 × 4 configuration, can significantly lower test costs. Test engineers should also consider whether there is a clear upgrade path beyond the 4 × 4 Matrix C configuration.

Another major concern is keeping the cost of test per channel low while maintaining good performance, especially with respect to maintaining excellent channel isolation. This is important because measuring the channel characteristics is fundamental to verifying any MIMO device. The test equipment should ideally have independent transmitters and receivers for the best channel isolation and at least 14-bit or better amplitude resolution for good dynamic range.

Bandwidth is another important consideration. For mobile WiMAX, the subcarrier spacing is fixed at 10.94 kHz. The standard allows for FFT sizes from 128 to 2048, which means that the maximum signal bandwidth will be in excess of 20 MHz — so test equipment needs to have at least 20 MHz of bandwidth. If working with WLAN, then 40 MHz of bandwidth is even better for the 802.11n MIMO standard.

Instrument usability, or its user friendliness, is an often overlooked but equally important consideration. Intuitive displays are essential for debug-ging complex radio systems, especially when dealing with multiple signals. Going beyond the constellation diagram, users need to see how modulation quality behaves over time and over subcarriers.

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