A symbol constellation diagram for both datastreams is shown in the lower part of the display (Figure 5). Each stream is transmitting 54 Mbps with OFDM and with a 64-quadrature amplitude modulation (QAM) constellation. The two green constellation points representing the four subcarriers carry binary phase shift keying (BPSK) modulated pilot tones. These pilots are used to create a continuous series of amplitude and phase references throughout the data frame. Demodulation is then performed relative to these pilot carriers and this allows for signal impairments to be corrected continuously throughout the burst. The 64 red constellation points represent 64-QAM symbol constellation measurements taken from each of the 48 OFDM data subcarriers and over many symbols. The constellation points of pilot tones are relatively well defined compared to constellation points of data subcarriers. I/Q amplitude mismatch results in the pilot tones separating mostly along the I axis, while I/Q phase mismatch results in the pilot tones separating mostly along the Q axis.

Phase noise affects both modulation accuracy and EVM. Phase noise is usually introduced during frequency conversion when a baseband signal is mixed with a local oscillator (LO) to translate to RF frequency. The LO phase noise consists of contributions from three main sources in a frequency synthesizer: 1) the frequency stability of the reference crystal oscillator, 2) the frequency stability of the free-running voltage controlled oscillator (VCO) used by the phase locked loop (PLL), and 3) the loop bandwidth and the noise from the PLL used in the frequency synthesizer. The impact of phase noise can be seen as a circular distortion of the signal points around the center of the symbol constellation diagram.

At the bottom right of the display as shown in Figure 5, there is a window with numerical results of transmitter power, EVM, carrier frequency error, and phase noise.

One of the important elements in a radio system is the RF power amplifier. To achieve maximum efficiency, the RF power amplifier is ideally operated close to its saturation point, but not above it. This fact directly affects product cost and quality since it requires a trade off between power consumption and amplifier bias. The gain of the power amplifier is compressed when maximum, or peak power, exceeds the amplifier's saturation point. When the amplifier operates into its saturation region, non-linearity of the amplifier can lead to many undesired effects such as harmonic distortion, intermodulation, spectral re-growth, cross modulation, and modulation inaccuracy. A MIMO-OFDM radio generally requires a greater degree of power back-off from the power amplifier saturation point because of its high peak-to-average power ratio. The effect of amplifier compression can be seen in the symbol constellation diagram as constellation points are spread to the point where decision errors are likely.

On the production line, fast, accurate, and low-cost test methodologies are critical to lowering the overall cost of devices and to enable MIMO technology to gain widespread acceptance. Transmitter EVM testing along with transmitter power and transmitter spectrum mask testing of each of the transmitters can be used as a pass-fail metric for the system. Figure 6 is the block diagram of a MIMO test solution for mass production from LitePoint Corporation. The radio signals from two independent transmitters are combined through an RF combiner and fed into a single VSA and analyzed by proprietary DSP software.

Figure 7 shows the measurement results from a composite signal on a MIMO DUT. The data rate transmitted from the DUT measures 108 Mbps total with a 54 Mbps datastream from each transmitter. Notice that spectral mask, transmitter waveform, average EVM, frequency error, phase noise and symbol constellation are all shown as a composite measurement. Any defect or failure on any of the individual MIMO channels will result in a failure report in the composite measurement.

Channel estimation results for the individual MIMO channels are shown in the middle right window of the display. The graph provides information on the channel flatness of each channel and transmitted power balance between channels.

These proven, fast and accurate test methodologies greatly simplify MIMO system test in a production line, guard quality and increase test throughput.

MIMO-OFDM technology offers a promising way for next-generation wireless systems to enhance their channel capacity and robustness of the link. Accurate and fast measurements in the frequency, time and modulation domain can help greatly shorten the design cycle times and improve overall product quality and profitability. However, traditional test systems are not designed to handle the multiple simultaneous transmitters and receivers in a real MIMO system. Two test methods were investigated here. The first method proposed consisted of multiple synchronized VSAs and VSGs capable of simultaneously measuring all key parameters for system measurements involving multiple radios, simplifying a complex task to a straightforward one. This first test method proposed provided a fast, accurate, scalable, and affordable test solution for MIMO product development environments.

The second test method proposed involved a one-box MIMO test solution, which used the composite measurement approach to greatly simplify MIMO test procedures and improve test throughput on the manufacturing floor allowing for a fast and low-cost solution for mass production environments. The advantage of this method is that it can be used with existing test instruments such as the LitePoint IQflex.

Fan Liang is a senior member of the technical staff at LitePoint Corp. Liang has extensive experience in wireless networks, RF/microwave circuits and system design. He has worked with numerous wireless standards and has designed multiple 802.11 test systems for WLAN applications. He earned an M.S. degree from Xi'an Jiaotong University in Electrical Engineering with a focus on wireless communications. He may be reached at Fan.Liang@litepoint.com.

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