Performance Assessment of Single Channel Relaying Echo Cancellers

Due to the increasing needs of spectral efficiency in modern wireless communication, the concept of transmitting and receiving data in the same band at the same time (called single-channel full-duplex wireless communication, or in-band fullduplex) is attracting the attention of both theoretical and applied research [1]. Despite the idea of cancelling interference from the transmitted signal of the same unit (called echo cancellation) has been one of the key technologies in wireline communication area for many decades [2]–[5], it is only nowadays that it is becoming of significant relevance in the wireless area. For several applications, such as digital video broadcasting - terrestrial/handheld (DVB-T/H), cellular and terrestrial trunked radio (TETRA) systems, where an high power amplifier (HPA) is employed, the echo effect results not only in a degradation of the relayed signal quality, but also in system instability and consequent ”burn in”. Thus, the study of an on-channel repeater (OCR) able to cancel (or reduce as much as possible) its own transmitted signal leaked to its receiver input (sometimes referred as self-interference rather than as echo) is becoming crucial in order to achieve reliable communications with high spectral efficiency. Recently, many works focused on the problem of self interference in full-duplex relaying by proposing different solutions (e.g., passive, active, analogical, digital etc.) and analyzing the corresponding performance in the terms of residual echo level. In general, analytical solutions are based on acoustic speech oriented works of the early Eighty [...], while digital ones consider theoretical performance limits not always corroborated by experimental results. Here we focus on the digital solution presented in [6] and [7], where the coupling-channel between the transmitting and receiving antennas is estimated via training sequences, and counteracted by a negative feedback constituted by a finite impulse response (FIR) filter that reproduces the characteristics of the coupling-channel. This kind of echo canceller has been first proposed and implemented for single frequency network (SFN) DVB-T/H repeaters in the second part of the last decade, but its general purpose design makes it attractive in the whole scenario of modern fullduplex radio relaying. Starting from the performance and stability analysis done in [7] (which is extended in [8] by considering the impact of quantization noise), in this paper we derive the general design criteria, which have been skipped there due to lack of space, but are fundamental to properly realize the next generation full-duplex relays (in the broadcasting area, as well as in the one of wireless network). In particular, trade-off between the two main relaying characteristics • echo cancellation performance; • maximum achievable gain of the HPA (that directly impacts on the coverage area); and two main costs • time duration of the coupling-channel estimation; • disturbance to the final user due to the emission of the local training sequences; are explicitly investigated. The design criteria derived can be addressed to optimize the choice of echo canceller parameters, with particular reference to the characteristics of the echo cancelling FIR filter (number of taps and positioning), to the shaping of the Rx and Tx filters and the over-sampling factor.