Method and system for sensing available spectrum in wireless communication systems
Managing RF spectrum usage by secondary, or unlicensed, devices in portions of the RF spectrum where licensed, or primary, services operate is a requirement of cognitive radio systems. To perform the necessary spectrum scanning, a transition period, such as a TTG or RTG can be extended and the scanning performed during the extended period. The presence of a primary service signal can also detected by detecting an energy signature of the primary service signal in a known sequence. In both cases, there is no need to define quiet periods during which secondary devices scan the RF spectrum, thus resulting in saving of resources and improved QoS.
1. A method of radio frequency (RF) spectrum scanning for wireless communications to determine whether a channel is being used by a primary service or is available for use, comprising:
sensing a signal transmitted in a channel defined within the RF spectrum;
identifying a portion of the signal containing a known sequence; and
determining the presence of a primary service on the channel, to determine whether the channel is available for use, by
estimating multipath distortion in the signal,
reconstructing a reconstructed sequence by applying the estimated multipath distortion to the known sequence,
determining the difference between the reconstructed sequence and the identified portion of the signal containing the known sequence, and
determining that energy indicative of the primary service signal is present in the difference.
2. The method of claim 1 further comprising reporting the presence of the primary service on the channel.
3. The method of claim 1 wherein determining that energy indicative of the primary service signal is present in the difference comprises determining that energy in excess of an expected noise contribution is present in the difference.
4. The method of claim 1 wherein identifying the portion of the received signal containing the known sequence comprises identifying a preamble of a frame.
5. The method of claim 4 wherein the preamble is identified at frame synchronization.
6. The method of claim 4 wherein the frame is an Orthogonal Frequency Division Multiple Access (OFDMA) frame.