Performance

A complete 4x4 MIMAX demonstrator was developed at the UC's laboratories. This demonstrator allows testing the developed hardware and software parts in real time and under real scenarios, e.g., the baseband algorithms for beamformer selection and MIMO channel estimation. Furthermore, it enables a performance investigation of the concept.

The MIMAX emulator includes a digital beamformer for both transmitter and receiver sides, which can change the weights at the I/Q branches on a symbol-by-symbol basis. Besides, at the receiver side, a specific digital RSSI block has been implemented in order to provide a valid RSSI signal based on the combined I/Q received signal, the overrange bits of the AD converters and the VGA gain setting.

The setup of the MIMAX emulator is controlled by Matlab and it can be accessed and configured through the Internet.

A typical experiment based on the MIMAX emulator consist of three steps

• Transmit/receive a MIMAX frame II for MIMO channel estimation
• Estimate the MIMO channel and calculate the optimal TX/RX beamformers (weights)
• Transmit/receive one or multiple MIMAX frame I (data frames) with optimal/default weights

The transmission/reception of the training frame (MIMAX frame II) is a key part of MIMAX because it involves MIMO channel estimation, frequency offset calculation, etc. The MIMAX training symbols are transmitted using 16 TX and RX beamformers to estimate the 16 equivalent SISO channels for the 52 active subcarriers in 802.11a.

Once the training frame is received, the channel is estimated and the optimal weights are determined. These channel estimates are used to get the optimal beamformer weights, which are used in the following data transmission (MIMAX frames I).

In order to show the advantages of MIMAX in comparison to SISO, 20 OFDM symbol QPSK modulated data frame is transmitted. The first 10 symbols are transmitted/received using the optimal weights (MIMAX) and the last 10 using a set of default weights (SISO).

More information about the demonstrator and the testing environment can be found in the demonstrator document (PDF).