whitespace devices can use unused television frequencies for transmission and reception of wifi ofdm signals . three contiguous bands , such as former channels 2 , 3 , and 4 , may be bonded together to define a whitespace band . in order to fit a wifi ofdm signal into this whitespace band , a whitespace device compresses the bandwidth of each wifi ofdm signal using a specific spectrum mask . very low transmission power is needed for the modified wifi ofdm signals , eliminating the need for high power amplifiers and most of the wifi ofdm designs such as phy and mac can be reused with minor modifications .