Electronic Design

Test System Pushes MIMO Standards Into The Spotlight

The latest wireless standards get to market sooner if you can test them quickly and easily.

Multiple input/multiple output (MIMO) uses its multiple transmitters, receivers, and antennas to achieve greater link distance and reliability as well as higher data rates. So it shouldn't be much of a surprise, then, to tell you that MIMO is now an option in most of the latest wireless technologies.

Already, it's being used in 802.11g/n Wi-Fi wireless localarea networks (WLANs). It's also being picked up in some of the new WiMAX products. And there's no doubt that in the near future, we'll see it applied to Ultra-Wideband (UWB) and fourthgeneration cell phones using Long Term Evolution (LTE) and Ultra Mobile Broadband (UMB).

This hot technology isn't new, per se, but adoption has been slow because it's complex and difficult to test. Its complexity comes from the basic computationally intense MIMO reception process, as well as from the fact that MIMO usually is part of an orthogonal frequency-division multiplexing (OFDM) process.

OFDM is the modulation/multiplexing darling of wireless these days, since it's been adopted by the standards that also embrace MIMO, like LTE and UMB. While several MIMO test products are already out there, Keithley's MIMO RF Test System makes testing much faster and easier at a reasonable price.

If you aren't familiar with Keithley's name being tied to RF test, it's time to get acquainted. The company has slowly built an excellent line of RF test products, including the model 2920 vector signal generator (VSG) and the model 2820 vector signal analyzer (VSA). Add the model 2895 MIMO synchronization unit and powerful MIMO signal analysis software, and you get a complete and very capable MIMO test system.

A brief look
Most wireless applications still use single-input single-output (SISO), where one transmitter (Tx) sends a signal to a single receiver (Rx). This arrangement works fine, but at UHF and microwave frequencies, the signal is subject to the usual noise and interference as well as multipath fading. Reflections from buildings, cars, trees, people, and other obstacles cause multiple but delayed signals to arrive at the receiver, producing signal cancellation. Also, moving objects create Doppler shifts that produce fading.

Multipath interference has another negative effect called intersymbol interference (ISI). The modulated data generates symbol changes in phase and/or amplitude. If the data rate is fast enough, these symbol changes occur at a rate that can cause them to overlap if the same signal is received, but at different times over different paths.

With one symbol interfering with another, data recovery at the receiver produces errors or no usable signal. The problem is usually solved by slowing the data rate so the symbols don't overlap in the presence of multipath effects. Yet it defeats the purpose of the link because high speed is needed or desired. The way to have your cake and eat it too, so to speak, is to use MIMO.

MIMO uses multiple transmitters and receivers, with two transmitters and two receivers (2x2) being the common arrangement (Fig. 1). The data to be transmitted is divided into two parallel streams, and each is used to modulate a transmitter. The signals are also encoded, making them unique so that they can be recovered at the receiver. The two signals are transmitted on the same frequency.

At the other end of the link, each receiver picks up both signals in addition to any multipath signals. Due to the signal decoding and the multiple delayed versions of the signals, it's possible to recover each signal and reconstruct the data through mathematical algorithms.

The receivers recover all individual signals and combine them to create the final highly robust output. MIMO makes the previously undesirable multipath signals useful, greatly improving link reliability. This enables greater transmit range as well as the ability to retain a high data rate in the presence of noise and multipath effects.

The situation improves if you use even more transmitters and receivers. This increases system cost, but with today's small, low-cost transceivers, multiple transceivers are practical and affordable. Many modern WLANs use a 3x2 arrangement. But the complexity grows as the number of transceivers increases. A 4x4 MIMO will produce up to 16 signals to decode in the receiver. Thanks to powerful DSP processors, FPGAs, or ASICs, the process becomes realistic.

Adding to the complexity is use of the OFDM in most MIMO systems. In this case, high-speed data is transmitted by dividing the serial bit stream into multiple, parallel slower bit streams. Then those parallel signals are modulated on multiple carriers equally spaced throughout a relatively broad bandwidth.

For instance, the data in 802.11a/g Wi- Fi systems is divided among 52 separate carriers. In WiMAX systems, data can be divided among carriers numbering up to 1024 or 2048. The carriers are spaced by a frequency increment that makes them in phase quadrature with each other. This "orthogonality" prevents them from intertofering with one another despite their close adjacent spacing.

The multicarrier OFDM system is very spectrally efficient. That means OFDM can transmit more bits per second per hertz than other radio methods. It uses a wide bandwidth, but it can produce very high data rates. Also, this method is very tolerant of interference from other signals in the same band. Best of all, because of the widely varying carrier wavelengths, the signals will take different paths to the receiver. This makes OFDM highly tolerant of multipath effects.

OFDM is also complex, but DSPs have made it easy to implement. You generate the OFDM signals using an inverse fast Fourier transform (IFFT), which today is easily implemented in a programmable DSP chip or in an FPGA or ASIC. At the receiver, a standard FFT is implemented to recover the individual carriers and recombine them. Now mix OFDM and MIMO together, and you can understand why testing such systems is complex.

The MIMO solution
Keithley uses its 2920 VSG to create the OFDM signal that will become the test input. By employing additional VSGs, you can generate two, three, or four more signals on the same frequency with different data interto produce the final MIMO signal. The 2895 MIMO Synchronization Unit is needed to coordinate the inputs and produce the final MIMO output.

On the receive end of the chain, the 2820 VSA receives and recovers the signals. The VSA integrates the company's MIMO Signal Analysis software for signal recovery, measurement, and display (Fig. 2). As many as four VSGs and VSAs may be used together.

The MIMO test systems allow any MIMO configuration up to 4x4 (e.g., 2x3, 4x3, 2x1, etc.). Also, they support most commercial wireless standards, including WLAN, WiMAX, and cellular. They offer 1-ns signal sampler synchronization, less than 1-ns peak-topeak signal sampler jitter, and less than 1° of peak-to-peak RF carrier phase jitter. And, they supply -40-dB error vector magnitude (EVM).

The MIMO test signals start with the 2920 VSG. It can generate signals from 10 MHz to 6 GHz. An optional arbitrary waveform generator (AWG) lets you produce modulating signals for GSM, EDGE, WCDMA, cdma2000, SISO WLAN, and the very demanding 40-MHz wide 802.11n Wi-Fi signal. The 2920 also comes as a standalone test unit. The 2910 has similar specs but produces output frequencies up to 4 GHz.

The 2820 VSA receives signals up to 6 GHz; there's also a 4-GHz version. Bandwidth is 40 MHz. It can receive, analyze, and display most of the popular wireless standards, such as those mentioned for the 2920 (both SISO and MIMO).

To use these instruments in a MIMO test system, you need the 2895 MIMO Synchronization Unit. It synchronizes all of the VSGs and VSAs. The 2895 provides a common local oscillator (LO) output to all units as well as a 100-MHz digital clock and trigger signals to sync all of the units for the selected MIMO configuration. Finally, the PC-based MIMO Signal Analysis software lets you test and measure all WLAN configurations to 4x4 MIMO as well as other standards.

The 2920 VSG begins at $17,500, the 2820 VSA starts at $22,500, and the 2895 MIMO Synchronization Unit is priced at $9900. MIMO signal analysis software goes for $9500. Most products are available within weeks, with the 2895 available in December.

Keithley Instruments Inc. www.keithley.com

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