On the Performance of OFDM downlink FDD channels under Tx Leakage using zero-IF receivers

Transmitter Leakage has a significant impact on frequency division duplex transceivers in mobile devices using zero-IF receivers. In this contribution, the statistical properties of the Tx Leakage impact in zero-IF receivers is derived, in the concept of Uplink and Downlink OFDM transmission. In order to design suitable algorithms for the digital compensation of Tx Leakage in OFDM communication systems, an accurate modeling of the corresponding performance loss is required. We therefore provide a single statistical description of Tx Leakage effects on OFDM signals, which is used to compute bit error rates and mutual information. I. I NTRODUCTION In wireless, frequency division duplex (FDD) handset devices a duplexer connects the transmit (Tx) and receive (Rx) chain with the antenna. Since both chains are operating simultaneously, the duplexer has to provide a high TxRx isolation. However, a sufficient Tx-Rx isolation is very challenging and not always achievable. Thus, a significant part of the transmitted Uplink (UL) signal is leaking through the duplexer in the Rx chain, causing severe interference to the received Downlink (DL) signal. Tx Leakage (TxL) is of considerable importance especially in difficult receive conditions (e.g. at cell edges), where the demodulation of the weak DL signal is difficult, because the TxL significantly reduces the receiver sensitivity. Focusing on zero-IF receivers, the nonlinearity of the I/Q down converter generates 2nd order intermodulation products (IM2) of the leaking UL signal, which is partly located around DC and thus severely interferes the down converted DL signal [1]. Currently, tight requirements in terms of linearity and TxRx isolation are set on the analog font end in order to avoid severe TxL interference. However, meeting these requirements is very challenging for modern terminals, since several communication standards must be supported in parallel at even reduced terminal sizes. Compensating the TxL interference in the digital domain relaxes the requirements on the analog front end and thereby reduces development costs. In order to determine the conditions, under which a digital TxL compensation is required, the performance degradation caused by TxL must be derived. In this contribution we therefore analyze the statistical properties of the TxL interference, considering an OFDM transmission in the UL and DL. Based on the obtained results, we evaluate the impact of Tx Leakage on the performance of OFDM DL communications over the frequency selective fading channel. Specifically we will show, that the TxL interference on the OFDM subcarriers can be modeled as a white gaussian noise process, whose variance can be calculated for each subcarrier by a closed form expression. The outline of the paper is as follows. The next section presents the system model. In Sec. III the power spectral density (PSD) of the TxL interference is derived. In Sec. IV the performance of the DL OFDM transmission impaired by TxL is derived in terms of bit error probability and mutual information. In Sec. V the analytical results are verified with simulation results and in Sec. VI conclusions are drawn. II. SYSTEM MODEL This paper focuses on the Tx Leakage (TxL) present at the mobile terminal, as depicted in Fig. 1. After