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Paired amps slash MIMO design costs

Posted: 16 Aug 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Mau-Chung Frank Chang? SST Communications Inc.? spotlight? communications? don't?

In communication theory, multiple input, multiple output (mimo) refers to radio links with multiple antennas at the transmitter and the receiver. MIMO uses spatial-multiplexing techniques to send multiple data streams on multiple antennas while using the same frequency channel.

Applying MIMO to current WLAN systems can increase the data rate achieved by IEEE 802.11a/g to 108Mbps with a dual-transmit antenna, as in a two-transmitter/two-receiver system being standardized by IEEE 802.11n.

One challenge in harnessing the performance attainable in MIMO is to maintain matching of amplitude and phase accuracy for each of the individual transmitters, while achieving better linearity and efficiency as required in a traditional single-input, single-output (SISO) system. It is very important for MIMO WLAN vendors to have a cost-effective solution with accurate amplitude and phase.

To achieve this, designers should evaluate trade-offs between performance and cost when selecting key components, such as power amplifiers. On the performance side, besides gain, linearity, stability margin, I/O matching tolerance and thermal stability as required in SISO systems, designers must pay attention to those characteristics affecting the transmission arm balance, amplitude and phase-control accuracy. On the cost side, the lowest total-ownership BOM is always desired. Here are some tips.

Do

  • Choose a power amplifier pair with factory-matched amplitude and phase characteristics. Due to process variations from lot to lot, wafer to wafer and even within a wafer, separately packaged power amplifier pairs show a larger mismatch.

  • Use power amplifiers having good linearity (~ 20dBm with
  • Consider on-chip power detectors. To achieve the lowest total-ownership cost, on-chip power detectors are crucial, especially in MIMO transmitters where the related external-component BOM will double compared with SISO transmitters. The on-chip power detector should exhibit dynamic range, sensitivity, decibel-wise linearization, temperature robustness and the most important load-impedance robustness. For the same reason, choose power amps with high load-impedance robustness.

  • Package power amplifier pairs with a compact form factor. A matched power pair housed in a single package (such as 3mm?) provides a more-compact solution, and saves 50 percent board space and cost compared with designs using separate power amplifiers.

  • Use digital or quasidigital on/off control with performance robustness over the operating temperature range. Evaluate the trade-off between amplifiers with digital control and quasidigital control. As long as the baseband signal can enable the on/off control, there will be no difference in the BOM between the digital and quasidigital on/off control solutions.

Don't

  • Use power amplifiers of low efficiency, since the actual power amplifier operating current will double in MIMO systems.

  • Use power amplifiers where the linearity is sensitive to load impedance, since the coupling between the arms in MIMO systems imposes the worst load condition on power amplifiers.

  • Use the on-chip detector of a power amplifier if it is sensitive to load impedance. Otherwise, variations in antenna environment or coupling between the arms will result in significant power control errors. In such a case, use separate directional couplers with high-directivity (~ 20dB) taping signal out for each arm, and direct the taped signal to Schottky diodes to form on-board power detectors.

  • Pursue unnecessary high transmission-arm isolation. Typical coupling between antenna units can be as high as -15dB. Isolation of -20dB between transmitter arms is acceptable.

  • Have an asymmetrical layout between individual transmission arms, since mismatch tolerance depends on specific system design.

- Mau-Chung Frank Chang
Vice Chairman and Chief Technology Adviser, SST Communications




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