Fast frequency correction for DQPSK-OFDM in the 60 GHz indoor environment

The OFDM modulation scheme is known to be very sensitive to frequency offset. Using OFDM in a 60 GHz transmission system is an extremely crucial issue, since frequency deviations of only one ppm of the oscillators are within the range of the subcarrier spacing. In order to achieve a fast and robust frequency correction the insertion of a fractionally spaced training sequence in the frequency domain, with RLS-adaption to that sequence at the receiver, and final FIR-equalization is proposed. Proper work of the FIR equalization is ensured by using root-raised-cosine shaping at the transmitter in the time domain, thereby limiting inter channel interference. The proposed modulation and frequency correction scheme has been implemented on a digital signal processor array using 512-DQPSK-OFDM. Bit error rate measurements have been carried out with a 60 GHz indoor transmission system. Within that system the OFDM signal was supplied to the transmitter via a fibre optic link at the first IF stage. Measurements confirm the simulations to a high extent showing only about 2 dB loss compared to simulation results in the AWGN channel. Burst data rates of ≈45 Mbit/s have been achieved using a 35 MHz bandwidth, an omnidirectional antenna at the transmitter, and a directed antenna at the receiver