Design, implementation and characterization of an RF translator for TVWS communication

Abstract

This study describes a radio frequency (RF) frontend that enables direct RF translation of an IEEE 802.11 signal at 2.4GHz ISM band to UHF TV frequencies. Specifically, the intention is to allow 2.4GHz IEEE 802.11 or WiFi-compliant radios to be used for opportunistic communication at so-called TV white spaces (TVWS). In order to faithfully translate WiFi signals to TVWS frequencies, while preventing undue interference to the primary users in the TV band, the spurious response of the RF translator should be closely examined as part of the design considerations. We present the results of single tone spurious response and spectral mask tests in order to characterize the nonlinear performance of the translator. The tests are conducted for different 2.4GHz WiFi channels at the input, and for different target TV channel frequencies at the output of the translator. The performance of the translator was found to vary significantly across the 2.4GHz input frequencies with WiFi channel 11 (2.462GHz) providing the best results. This suggests that the choice of the input WiFi channel to the translator should be fixed to that which provides the best spurious response at the output. In addition, we evaluate system functionality by measuring TCP throughput with and without the translator, and compare the results with the spurious response tests. The results show that the signal distortion caused by the spurious response contributes to degrading both the measured RSSI and throughput levels. In particular, the lower RSSI and peak throughput values achieved at certain output TV channel frequencies is consistent with the high levels of spurious response that were measured in those cases. Furthermore, the variation in throughput performance across different cases is more apparent in the peak, rather than the average, throughput levels over time.