Impact of asynchronous transmissions in noncontiguous OFDMA

Multiple point-to-point (p2p) communication links that share noncontiguously available spectrum via noncontiguous orthogonal frequency division multiple access (NC-OFDMA) are considered in this paper. NC-OFDMA divides the spectrum into orthogonal subcarriers and assigns nonoverlapping subsets of these subcarriers to each of the links. Such a setup is motivated by the need for dynamic and opportunistic access of a large pool of spectrum that is fragmented due to the presence of incumbent transmissions. The transmissions of the p2p links are assumed to be asynchronous, i.e., the start and end of the NC-OFDM symbol is not time synchronized across the links. In such a setting, this paper studies the inter-carrier-interference (ICI) arising from the loss of orthogonality between the subcarriers due to (a) the asynchronous nature of the communications and (b) the frequency offset between the links. An analytical characterization of this ICI reveals that in the absence of transmit power control, the impact of this ICI can be severe enough to disable the concurrent transmissions of the other p2p links. Experimental results using a Universal Software Radio Peripheral (USRP) platform validate our analysis and show that the negative effects of asynchronous transmission which are particularly significant at high SNR. To remedy this, a decentralized synchronization scheme is presented wherein beacons appended to the transmitted signal are used to enable timing coordination between two NC-OFDM links, thereby eliminating the ICI resulting from asynchronous transmission. Effectiveness of such a scheme is validated using experiments. In cases where such timing coordination cannot be enabled, our analysis is then used to provide guidelines on the choice of basic system parameters for the p2p links such as the number of contiguous subcarriers to be allocated to a link and length of guard band and range of operating SNRs to mitigate the effect of asynchronous NC-OFDM transmission.