Coherent OFDM transmission at 60 GHz

The use of coherent OFDM is an efficient way to circumvent the need for an equalizer in high-speed wireless data transmission links. In a time variant channel, fast and robust channel estimation is necessary when no differential modulation schemes are used. Furthermore, the use of coherent OFDM paves the way for higher order modulation schemes for subcarriers such as 16/64-QAM thus enabling bandwidth efficient transmission. Channel estimation is necessary for coherent reception. It is achieved by linear interpolation between consecutive pilot carriers, which are uniformly inserted into the OFDM symbol. About 10% of the bandwidth is dedicated to pilots which are also used for frequency correction purposes. This combination is possible through the insertion of fractionally spaced triple pilots. The proposed system has been simulated and implemented on a DSP-based modem using 512-OFDM and different modulation schemes for subcarriers. Simulations as well as measurements with a 60 GHz indoor transmission system have been carried out. Burst data rates of 50 Mbit/s (QPSK) and 100 Mbit/s (16-QAM) are achieved using 35 MHz bandwidth. 16-QAM is only possible with at least one directed antenna whereas QPSK is able to cope with the non-LOS case and 2 omnidirectional antennas