Exploiting modulation scheme diversity in multicarrier wireless networks

The pursuit of high speed wireless communication is pushing wireless networks toward wider channels. Narrowband interference and frequency- selective fading significantly impact the performance of broadband wireless communication. To cope with the harsh channel conditions, orthogonal frequency division multiplexing (OFDM), which divides a band of spectrum into numerous subcarriers that carry data in parallel, is widely adopted. Because subcarriers experience different channel qualities in a wide band of spectrum, different modulation schemes may be used for different subcarriers. This paper exploits the modulation scheme diversity as rich information to assist data transmission. In this paper, we propose encoding each modulation scheme with a bit pattern that occurs frequently when allocating data bits to subcarriers. If the bits to be allocated on a subcarrier match the bit pattern defined on the subcarrier, the receiver is informed by the transmitter through subcarrier nulling. Because the bit pattern usually consists of more bits than that can be represented by a modulation symbol in low-density modulation schemes, the total transmission time is shortened. Further, detecting a null subcarrier is more reliable than decoding a modulation symbol. Therefore, the known bits represented by the selected bit patterns help improve decoding performance. Through experiments on USRP1 we show that the throughput is increased due to shortened transmission time and reduced bit error rates.