Blind synchronization for NC-OFDM — When “channels” are conventions, not mandates

Recent efforts in making licensed spectrum available for secondary use have opened up new opportunities and has redefined the meaning of sharing spectrum. Sharing spectrum requires aggregation of multiple non-contiguous bands of varying width to communicate as a network. Rather than limiting spectrum access to fixed width narrowband channels, they should be treated as conventions that are highly flexible and allow for simultaneous multi-user communication among a variety of heterogeneous devices with different transceiver capabilities. Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) is a physical layer technique that can be utilized to achieve this goal. Unlike the contiguous channelized access model, wideband non-contiguous access posses a critical challenge of synchronization. In this paper, we propose a practical algorithm and hardware implementation to overcome this challenge. Equipped with this blind synchronizer, we propose a Medium Access Control (MAC) layer design to enable flexible channel access while achieving co-existence with the incumbent and other secondary heterogeneous networks. The blind synchronization technique vastly simplifies channel rendezvous in the secondary network and provides faster migration to a vacant spectrum. Through extensive simulations under varying signal-to-noise ratio (SNR) and spectral occupancy, we show significant improvement over existing algorithms employed for NC-OFDM synchronization in cognitive radios and make wideband cognitive radio networks a distinct possibility in the near future.