The measurements for SISO are similar to MIMO. For example, EVM is a key metric for establishing the quality of any digital signal. In a MIMO system, it is still important to understand the EVM performance of the system, i.e., the composite EVM. However, as part of the design process, it is also important to be able to understand the EVM performance of each channel, while it is in the presence of all the other channels. Here lies a significant challenge. For instance, if one of the transmitters is generating an in-band spurious signal, then the composite EVM would be degraded. The next step is to check the EVM of each channel or stream. In so doing, the engineer would notice that one of the streams has a degradation in EVM. This performance could be attributed to either time domain or frequency domain effects. By then observing the EVM of each OFDM carrier over the frequency, it will quickly become apparent that some in-channel distortion is causing the radios performance to be degraded (Figure 3).

Test engineers also need to see how the radio responds to changes in the channel, especially with different multipath models (Figure 4). Channel response shows how all the radio transmissions interact with each other in the channel. In a 2 × 2 system the interaction is between T×1 and R×1, T×2 and R×2, T×1 and R×2 and T×2 and R×2. As the channel or stream count increases, the number of channel interactions also increases. For example, in a 4 × 4 system, the measurement needs to process 16 streams or channel responses to determine how each channel interacts with the other.

Beam forming also presents many test challenges. Beam forming is a technique that helps increase receiver sensitivity to the desired signal and decreases the sensitivity to interference and noise. This is accomplished by creating a series of beams and nulls in the transmitted signal. Test equipment for beam forming should be capable of finite phase and amplitude adjustment to be able to effectively create and receive specific patterns of radiation from each antenna.

Solutions to testing

Keithley's slate of next-generation RF test instruments, including the new 4 × 4 MIMO RF test system, are designed to meet the challenges faced by designers and manufacturers of today's wireless technologies. They have the flexibility to test multiple signals and can be used in test configurations for SISO and 4 × 4 MIMO testing. They also provide the measurement accuracy required for product development, combined with high test speeds and repeatability for production test.

Furthermore, MIMO synchronization unit (2895) can synchronize up to four vector signal generators (VSGs) and vector signal analyzers (VSAs). The test system offers flexible capability for MIMO configurations from 2 × 2 to the most challenging 4 × 4 40 MHz applications, combined with support for a number of commercial standards including cellular (GSM/EDGE/W-CDMA/cdma2000), WiMAX, and SISO WLAN (802.11a/b/g/j). (Figure 5)

When faced with one of the industry's most demanding signals — the 40 MHz-wide OFDM WLAN signal defined by the 802.11n standard — 2820 VSA can measure a characteristic EVM of - 40 dB. It also features ±1 nanosecond (ns) signal sampler synchronization, less then one ns peak-to-peak signal sampler jitter, and less than one degree of peak-to-peak RF-carrier phase jitter when using the MIMO synchronization unit.

The 4 × 4 MIMO RF test solution uses a DSP-based software-defined radio (SDR) architecture that adapts to the quickly changing test requirements of the dynamic wireless market, giving the instrument added longevity by making it easily upgradable. It can generate or demodulate virtually any signal with up to 40 MHz of modulation bandwidth, important for many of today's devices and for tomorrow's new signal standards such as 3G long-term evolution (LTE) and ultramobile broadband (UMB) with only a simple software upgrade.

In addition, WLAN 802.11n MIMO signal analysis software (280111) is a PC-based analysis tool for either single-channel or multichannel analysis of 802.11x signals. It is equipped with an extensive measurement suite for analyzing all 802.11x signals and is also capable of supporting 4 × 4 MIMO channel configurations. The software user interface is easy to set up and comes with a SCPI command set to quickly and easily interface with test systems.

Thus, these models are MIMO-ready with the hardware and software required to configure them into a MIMO test system. Besides performing as stand-alone bench or rack units, with standard spectrum analysis and signal-generation capability, they can also be easily configured as a MIMO test system. Also, the initial configuration of a 2 × 2 system can be upgraded to three or four channels by adding standard 2820 or 2920 instruments without the need for extra calibration. This flexibility is useful for users who do not want to dedicate a large investment to a MIMO system, yet want the capability to do so when necessary.

ABOUT THE AUTHOR

Mark Elo is marketing director for Keithley Instruments. He joined the company in 2006 after working for Agilent Technologies in R&D and marketing positions. Elo holds a bachelor's degree in engineering with honors from the University of Salford, Lancashire, England, and an MBA from Herriot Watt University in Edinburgh, Scotland.

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