RF Analog Impairments Modeling for Communication Systems Simulation: Application to OFDM-based Transceivers

2.5 Carrier Frequency Offset

2.5.1 Description

In communication systems the carrier frequencies are generated from frequency synthesizers, commonly PLLs, using precise crystal oscillators whose performance is specified with a certain accuracy in parts per million (ppm). Because TX and RX carrier frequencies will be slightly different, after frequency down-conversion in reception the signal will have a residual frequency error, which is referred to as CFO.

Figure 2.29 illustrates this impairment. Due to the LO frequency difference between the transmitter and the receiver, after frequency down-conversion to low-IF or zero-IF the signal spectrum will be shifted by a frequency offset:

2.88

in which Δ_CFO is the spectral shift introduced by the CFO.

Figure 2.29 Carrier frequency offset after frequency mixing in reception

2.5.2 Impact of CFO in OFDM

By assuming a CFO Δ_CFO after frequency down-conversion in the receiver, the OFDM baseband signal can be written

2.89

in which N is the total number of subcarriers within the OFDM symbol (including DC and guard bands), s_k is the symbol modulating subcarrier k, and T_s is the sampling frequency. For data demodulation the OFDM receiver will apply ...