CFO manifests itself as ICI and a symbol-dependent phase rotation common to all subcarriers. When the SCO is large, and a packet is long, subcarriers far away from DC will experience a substantial impairment. In OFDM systems SCO manifests itself as a subcarrier- and symbol-dependent phase rotation and inter-carrier interference (ICI). The inclusion of pilot subcarriers in the IEEE® 802.11™ standard allows for tracking and correction of SCO and CFO impairments. When the sampling clock at the receiver is running slower than the transmitter, this results in a larger sampling period and a positive sampling clock offset. This shift of sampling instants between receiver and transmitter is described as sampling clock offset (SCO). Due to this mismatch, carrier frequency offset (CFO) exists between the receiver and transmitter and the sampling instants at the receiver shift relative to the transmitter. The oscillators in the transmitter and receiver radios do not run at the exact same frequency. A WLAN radio typically uses a single oscillator to derive clocks for sampling and modulation.
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