To mitigate the problem of frequency-selective fading due to multi-path propagation, some transceiver systems use a scheme known as antenna diversity. In this scheme, two antennas are used. Each time the transceiver enters the RX mode, the receive signal strength from each antenna is evaluated. This evaluation process takes place during the preamble portion of the packet. The antenna with the strongest received signal is then used for the remainder of that RX packet. The same antenna will also be used for the next corresponding TX packet. Using a two story house to illustrate, the position of Antenna 1 allows for good upstairs coverage but the downstairs is heavily shadowed. The orientation of Antenna 2 covers downstairs and the attic very well, but not the upstairs floor. With antenna diversity the entire house has 99% coverage; unlike in a single antenna application in which the end-users may experience poor performance due to the lack of coverage. The second diagram illustrates that the signal that comes from the transmitter will hit the receiver antenna either in a low or a high portion of the sine wave resulting in either good reception or bad reception. In the event of a moving transmitter, the receiver moves in and out of the sine wave peak. If antenna diversity is used and the antennas are one quarter wavelength apart, one of the antennas is always in the strong part of the signal. Antenna diversity also helps in multipath environments. Multipath interference is a phenomenon where a signal comes not only directly from the transmitter to the receiver, but also from reflections, with a slight delay. In this instance there are peaks and troughs in the signal, but there are also slight time lagged echoes. Antenna diversity can be used to combine or evaluate the data from each source.