 The echoes may also be exposed to Doppler shift, resulting in a time varying channel model.The effect can be counteracted by applying some diversity scheme, for example OFDM (with subcarrier interleaving and forward error correction), or by using two receivers with separate antennas spaced a quarter-wavelength apart, or a specially designed diversity receiver with two antennas.Such a receiver continuously compares the signals arriving at the two antennas and presents the better signal.