o-called cyclic prefix (also known as guard interval) is removed. If the length of this interval A is longer than the maximum channel delay, all reflections of previous symbols are caught within the cyclic prefix and orthogonality is pre- served. The cost of this solution is the extra time spent repeating the symbol cyclic prefix, resulting in less time available for transmitting information and therefore a lower data rate. The channel response can be measured by inserting known "pilot" subcarriers at regular intervals in time or frequency at the transmitter. The subcarriers are monitored at the receiving station and compared to their known expected value. By dividing the received value by the expected one, the channel response for that pilot time-slot and pilot subcarrier can be obtained. Each individual time-slot and subcarrier channel responses can be found by interpolation. The original signal can be determined by dividing through the interpolated channel influence. The principal advantage of dividing up the frequency band into many narrowband carriers in OFDM is that the effect of wireless channel impairments can be reduced. The most critical of these problems, multipath fading, is the result of receiving a single transmit- ted signal multiple times at the receiver antenna. This is due to reflections caused by the multiple paths that a radio signal can travel as it meets physical obstructions while it propa- gates through the air. Variations in amplitude, phase, and arrival times are all consequences of reflections. Multipath can also result in frequency-selective fading, which is a degradation of specific frequencies or frequency ranges across a channel, as particularly out-of-phase sig- nals cancel each other out. While this can have a dramatic effect on a wideband single car- rier signal, OFDM counters this problem relatively easily. OFDMA is an enhanced version of OFDM which divides a signal into subchannels (i.e., groups of subcarriers), allowing different subscribers to be allocated different subchanne