WiMAX broadband mobile wireless communications systems promise expanded voice, data, and video services. To do so, however, they face a number of transmission hurdles, including limited bandwidth, interference, and multipath conditions. With advanced digital modulation schemes, the bandwidth can be put to optimum use. And with multiple antenna formats, such as multiple-input, multiple- output (MIMO) schemes, multipath can be a benefit.

A MIMO system uses multiple transmit and receive antennas and spatial diversity, transmitting multiple bit streams across the same physical channel. The system throughput can be increased by adding more transmitters and transmit antennas, without increasing the channel bandwidth. MIMO approaches are a part of a number of newer wireless communications standards, including WiMAX Wave 2 (IEEE 802.16e), WLAN (IEEE 802.11n), and 3GPP Long Term Evolution (LTE). These systems also employ orthogonal frequency division multiple access (OFDMA) modulation to achieve significant increases in data throughput without adding bandwidth.

In a conventional or single-input, single-output (SISO) communications system, radio signals travel a single path from a transmit antenna to a receive antenna. In urban areas, those signals are often reflected by buildings, with direct and reflected signals arriving at the receiver with in-phase and out-ofphase signal components, respectively. The constructive and destructive combining of the signal components results in fluctuations in signal strength at the receiver. Rather than simply overcome the multipath effects, MIMO approaches make use of multipath to increase data throughput. MIMO architectures use at least two transmit antennas and two receive antennas to send and receive unique bit streams over the same physical channel. Spectral efficiency increases as antennas are added.

Multiple bit streams in a MIMO system coexist in limited bandwidth. The receiver uses advanced signal processing, along with prior knowledge of the transmitted signals, to decipher the bit streams. Characterizing a WiMAX MIMO receiver thus involves measurements that evaluate how well it operates in terms of amplitude/gain, phase, and multipath/delay performance. A receiver can be tested at various points in the signal-processing chain, including at the RF signal stage and at baseband.

Because multiple signals with complex modulation in the form of in-phase (I) and quadrature (Q) signal components are used in the testing, a vector signal generator (VSG) is usually the source of test signals while some form of a propagation fading simulator is used to mimic multipath effects found in a MIMO channel. In addition, valid testing requires that any signals transmitted or received within a MIMO test setup be properly synchronized to simulate the conditions found in an actual MIMO operating environment, which calls for the use of some form of synchronization unit for RF and baseband signals. For transmitter testing, a vector signal analyzer (VSA) is typically used to take the place of a WiMAX MIMO receiver, for transmitter signal-quality evaluation.

For example, Agilent Technologies has developed its model V2920A RF Vector Signal Generator and model V2820A RF Vector Signal Analyzer to serve as a transmitter and receiver in a WiMAX MIMO test system, with signals synchronized by means of the model V2895A MIMO Synchronization Unit. The company also provides the model V2891A Upconverter to generate higher-frequency test signals as needed. The low-noise VSG is available with frequency ranges of 10 MHz to 4 GHz or 10 MHz to 6 GHz.

Based on an arbitrary-waveformgenerator architecture, the V2920A can download waveform files for playback and provides instantaneous bandwidths as wide as 80 MHz. It works with an external reference source from 1 to 60 MHz and can be linked directly to other instruments such as the V2820A VSA with trigger input lines and synchronization output lines. The V2920A VSG offers outputs from -130 to +13 dBm (with an option) and typical absolute amplitude accuracy of 0.6 dB at 1 GHz. The frequency switching speed is better than 300 s and the amplitude switching speed is better than 150 s for open-loop switching.

The companion model V2820A VSA has a standard frequency range of 400 MHz to 4 GHz with optional coverage to 6 GHz. It features 0.6 dB absolute amplitude accuracy and a displayed average noise level (DANL) of -150 dBm/Hz at 1 GHz when using a preamplifier. The analyzer features 250- s frequency switching speed and better than -140 dBc/Hz phase noise offset 400 kHz from a 2-GHz carrier. Multiple VSG and VSA units can be combined to test MIMO configurations as large as 8 x 8 using the model V2895A MIMO synchronization unit. It distributes a common local oscillator (LO), common stable clock signals, and precise triggers to all the signal analyzers or generators connected in the system. A single oscillator provides the frequency reference and a single synthesizer provides the LO reference to minimize phase jitter and frequency jitter. Three years ago, the company also announced a version of its N7615B Signal Studio software specifically for testing mobile WiMAX MIMO systems with the V2820A and V2920A instruments.

Anritsu produces the model MG3700A VSG based on a 160-MHz arbitrary waveform baseband generator, with waveforms produced by loading the appropriate files. It has a standard frequency range of 250 kHz to 3 GHz, with optional coverage to 6 GHz. It offers an internally produced modulation bandwidth of 120 MHz and as wide as 150 MHz using external in-phase (I) and quadrature (Q) ports. The absolute level accuracy is 0.5 dB. Output levels can be set from -140 to +13 dBm with 0.01-dB resolution. The frequency switching speed is better than 40 ms for changes greater than 3 GHz and better than 15 ms when the frequency change is less than 1 GHz. In support of the signal generator, the company has created the MX370105A Mobile WiMAX IQproducer application software to equip the MG3700A for downlink (receiver) performance testing according to mobile WiMAX IEEE 802.16e specifications. The MG3700A outputs the modulation baseband and RF signals using the waveform patterns generated by the MX370105A.

Tektronix offers the AWG5000B arbitrary waveform generator for WiMAX MIMO signal generation. Available with sampling rates of 0.6 and 1.2 GSamples/s and 14-b resolution, it generates RF output signals to 370 MHz with as many as four single-ended or two differential output signals. The signal source boasts a spurious-free dynamic range (SFDR) of -80 dBc and a 5 ns skew range that allows output signals to be adjusted in timing relative to one another with 50-ps resolution. When higher performance is needed, the firm also offers the AWG7000B instruments with sampling rates of 6 and 12 GSamples/s and RF outputs to 9.6 GHz.

National Instruments offers PXI board-level test solutions for WiMAX MIMO testing in the form of the PXI-5422 signal generator and PXI-5124 signal analyzer. They can be readily synchronized to facilitate MIMO testing. The PXI-5422 is a 200-MSamples/s, single-channel arbitrary waveform generator with 16-b resolution and as much as 512 MB of on-board waveform memory. The PXI-5124 signal analyzer operates at 200 MSamples/s with 12-b resolution and a real-time capture bandwidth of 150 MHz. The two-channel card has SFDR of -75 dBc and phase noise of -130 dBc/Hz.

In addition to signal generation and analysis tools, several companies offer channel emulators, including Azimuth Systems and Elektrobit. The model ACE 400WB channel emulator from Azimuth Systems is a bidirectional unit capable of exercising 4 x 4 MIMO systems. It can program a wide range of signal fading scenarios for 2.5, 3.5, and 5.0-GHz WiMAX MIMO testing. The EB Propsim F8 MIMO over-the-air (OTA) emulator offers a frequency range of 220 MHz to 6 GHz with modulation bandwidth to 125 MHz. It can create as many as 16 fading channels and as many as 24 fading paths per channel.

In some cases, such as the R&S SMU200A Vector Signal Generator from Rohde & Schwarz, most of the signal generation and fading capability for WiMAX MIMO testing have been packed into a single unit. The instrument (see figure) houses two independent signal generators in a single rack-mount enclosure, with frequency options of 10 kHz to as high as 6 GHz for the first path and to 2.2 or 3.0 GHz for the second path. The SMU200A also includes an I/Q modulator with 200-MHz bandwidth. It can be equipped with an optional fading simulator with as many as 40 fading paths for 2 x 2 MIMO testing. The signal generators feature excellent quality, with phase noise of typically -135 dBc/Hz offset 20 kHz from a 1-GHz carrier.