Crystal Oscillator Error Compensation in Software Defined Radios for 5G Network Testbeds

While Software Defined Radios (SDR) feature superior flexibility and ease of use, their sampling clock offset is also their main limitation, since compensating sampling rate errors incurs a significant complexity increase. In particular, a wireless cellular network Base Station is required a clock stability better than ±0.05ppm, while the TCXO found in most SDRs feature a ±2ppm stability. Therefore, an external time reference (e.g. GPSDO) is often employed to compensate clock errors. However, this solution might become costly in SDR-based experimental testbeds, since GPS is not always available and error compensation becomes imperative. Thus, this paper proposes a simple error measurement and compensation technique for SDRs without external reference sources. First, in the measurement phase, a GPSDO-locked device is used as reference. Then, the error correction method is implemented by modification of the Universal Hardware Driver (UHD) for Ettus USRP devices. In order to validate our calibration technique, we tested it on several low cost B210 SDRs, and we were able to setup a stable 5G SA FDD network without any external synchronization reference.