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

2.6 Sampling Frequency Offset

2.6.1 Description

Like the carrier frequencies, transmitter and receiver sampling frequencies used by the DAC and ADC are generally slightly mismatched. This impairment is known as SFO and will degrade the system performance.

Due to the SFO the received signal x(t) is sampled at n(T_s + ΔT) instead of the ideal nT_s:

2.99

The SFO is defined as

2.100

in which F_s is the ideal sampling frequency, F_s = 1/T_s, and ΔT/T_s is the relative sampling clock offset. Because ΔT/T_s is generally specified in ppm and then much smaller than one, we can approximate Equation 2.100 as

2.101

in which δ is the normalized relative sampling clock accuracy/precision generally specified in ppm. For example, 50 ppm precision for a sampling clock running at 100 MHz means a possible SFO of 5 kHz.

2.6.2 Impact of SFO in OFDM

Let us suppose that the receiver is impaired by an SFO but perfectly synchronized on the symbols; in this case the OFDM baseband signal is expressed as

2.102

in which N is the total number of subcarriers within the OFDM symbol (including DC and guard bands), s_k is ...