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Direct-conversion radio architecture is a low-cost, small-size design for the analog front-end subsystem in a wireless communication transceiver. This architecture, however, induces radio impairments such as I-Q imbalance and dc offset that, along with frequency offset, incur severe degradation in communication performance. This paper aims to improve the performance of estimation and compensation for cascaded transmitter and receiver radio impairments in the MIMO-OFDM (multiple-input, multiple-output orthogonal frequency-division multiplexing) systems. First, a novel two-stage compensation scheme is proposed which is applicable to a general form of MIMO operations with any number of transmit and receive antennas. Second, with a periodic training, an improved low-complexity joint estimation and compensation algorithm for radio impairments is proposed. Numerical results show the superiority of proposed method over the existing ones in bit error rate (BER) performance and training overhead.

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The issue of channel estimation is investigated for the orthogonal frequency-division multiplexing (OFDM) relaying system with amplify-and-forward (AF) operation mode. Channel estimation in the AF mode is unique in that both the source-torelay and relay-to-destination channels are needed at the destination in order to perform optimum combining of the received signals from the direct (source-to-destination) and relay paths if a cooperative diversity transmission is employed. Until now, however, there have been only literatures to study the estimation of the composite channel of the relay path. In this work, an iterative expectation-maximization (EM) algorithmbased channel estimation is proposed for respective estimations of the source-to-relay and relay-to-destination links at the destination. Computer simulations show that the iterative channel estimator performs satisfactorily and converges rapidly.