Measurements

Although theoretical expressions and simulations are very useful, they only represent a simplified abstraction. Ultimately, only the performance in real world scenarios matters. However, testing new ideas in real world scenarios is often too costly. Therefore testbeds are employed. They can be seen as a test vehicle between simulations and real world scenarios and often provide new important insights.

My testbed measurements work, in principle, as follows: FBMC and OFDM signals are pre-generated off-line in MATLAB and the samples are saved on a hard disk. Then, a DAC together with a radio frequency hardware up-converts the signal to 2.5GHz respectively 60GHz. The receive antennas are relocated within an area of a few wavelengths, resulting in Rayleigh or Rician fading. Different SNR values are obtained by a stepwise attenuator at the transmitter. Time and frequency synchronization is guaranteed by a rubidium frequency standard, a GPS reference clock, and a backbone network to exchanging time-stamps. The receiver itself down-converts the signal and saves the samples on a hard disk. After the measurement, the received samples are again evaluated off-line in MATLAB. Such off-line evaluation represents a cost efficient way of emulating real world transmissions.

2.5GHz Testbed

The Vienna Wireless Testbed operates at a carrier frequency of 2.5GHz. The link distance between TX and RX is approximately 150m. The RX antenna is mounted on a xyφ-table, allowing to measure at different RX positions. This leads to Rayleigh fading.

Corresponding publications

High Velocity Measurements (of up to 400km/h)

The basic idea is to rotate the receive antenna around a central pivot. Compared to a linear guide we avoid repeated acceleration and deceleration steps so that high velocities can be achieved. The setup represents a scenario where the rotational speed is low relative to the transmission time, so that the antenna moves approximately linear during the transmission time. Assume for example a velocity of 400km/h (17.7 revolutions per second for a 1m arm) and a transmission time of 1ms. Then the antenna only moves by 17.7rps × 1ms × 360° = 6.4°, close enough to a linear movement. The rotation unit is portable and can be placed, for example, next to a rail track, emulating high velocity train communications.

Main features:

  • Repeatable
  • Fully controllable
    • Average SNR
    • Velocity
  • Movement resembles a linear translation
    • LTE subframe at 400km/h = 6.4°

Corresponding publications

60GHz Testbed

The indoor mmWave testbed operates at a carrier frequency of 60GHz. The link distance between TX and RX is approximately 5m. Again, the RX can be relocated, leading to Rician fading in case of LOS. Once the LOS component is blocked, the Rician K factor becomes very low, close to Rayleigh fading.

Corresponding publications