Direct-conversion RF receiver design

The direct-conversion radio-frequency receiver architecture promises superior performances in power, size, and cost over existing superheterodyne-based receivers. The use of direct-conversion receiver (DCR) architecture, however, has been limited due to two well-known problems, namely, the 1/f noise and the direct-current offset noise, to which conventional architectures are less sensitive. This paper analyzes these noise effects on reception performance of a DCR with alternating-current (ac) coupling filter in the receive path. A mathematical treatment of the performance of a DCR is provided. A performance bound of a DCR given its 1/f noise roll-off frequency and ac-coupling filter cut-off frequency is first obtained using vector coding. Then, the performance of a more practical adaptive reception method using a linear equalizer is discussed. Adaptability is especially important in rapidly time-varying channels such as in the wireless environment. The linear equalizer is effective at signal-to-noise ratios (SNRs) below 9 dB, but its performance degrades significantly at larger SNR. To achieve high performance at large SNR (>9 dB), a spectrum shaping method using line codes for direct-conversion reception is proposed. This method achieves near-optimum direct-conversion reception at high SNR while maintaining low complexity and adaptability at the receiver.