Base-station receivers for next-generation wireless systems must deliver higher performance at lower cost than their predecessors. The direct-conversion receiver architecture is a good candidate to satisfy these conflicting requirements. Although the approach has been applied to different designs in the past, performance has been compromised by limitations in available hardware, including demodulators. Fortunately, the improved performance of commercial integrated-circuit (IC) quadrature demodulators make direct-conversion receiver designs viable alternatives to traditional superheterodyne receiver architectures.

To better understand the benefits of direct conversion, it might make sense to compare the receiver approach to a superheterodyne system (Fig. 1), commonly used because of its high selectivity and sensitivity. In a superheterodyne receiver, the received RF signal is filtered by the first RF preselection filter to remove out-of-band signals, and then boosted by the low-noise amplifier (LNA). The second RF preselection filter at the LNA output provides additional filtering to attenuate undesirable signals at the image frequency. The resulting signal is then translated to a lower, intermediate frequency (IF) by a downconversion mixer in conjunction with a local oscillator (LO). The IF must be sufficiently high that the image channel falls is within the filter's stopband. Such image-rejection considerations usually dictate that the IF should be on the order of 10 percent of the carrier frequency. The RF preselection filters serve to remove out-of-band energy and reject the image-band signals. Since a superheterodyne receiver performs the channel-filtering function in the IF and baseband stages, aggressive dynamic-range requirements are imposed on the components in these stages.

For superhetero-dyne receivers in base stations, a fixed-gain LNA is typically used for initial amplification of the received signals. The entire passband, including noise, is translated in frequency to a fixed IF. For the frequency downconversion, a passive (diode) mixer is most often utilized in order to meet the dynamic-range requirements of high linearity and low noise, although high LO power (greater than +10 dBm) is needed to drive such a mixer. Poor LO-to-IF isolation, typical of passive mixers, complicates the LO filtering in the receiver's IF section. At the IF output of the mixer, the desired signal channel always resides at the center of the IF channel-select filter, which is used to remove unwanted adjacent or alternate channels.

Following the IF channel-select filter, the desired channel is boosted by a variable-gain amplifier (VGA) and then demodulated to baseband for further signal processing. The high-quality-factor (Q) IF channel-select filter passes desired signals and rejects unwanted signals, including larger-amplitude alternate-channel signals. Unfortunately, such selective filters are expensive and add undue cost to the superheterodyne receiver. Moreover, high-Q filters are typically accompanied by high insertion loss requiring additional gain in the LNA and mixer stages to offset the filter loss and lower noise figure in the VGA.

Since the LNA gain is fixed in the base-station receiver, the mixer in particular must achieve very high linearity to meet the system's strict dynamic-range requirements. Moreover, the IF channel-select filter has a frequency response precisely tuned to the required channel bandwidth. The inflexibility of the IF channel-select filter limits the receiver hardware to a single RF standard. Because of the proliferation of standards for wireless communications, however, new receiver systems must support a variety of different standards seamlessly and cost-effectively, with limited cost budget for any one standard.

The direct-conversion receiver architecture can achieve the goals of a superheterodyne design, but with considerably less complexity (Fig. 2). In this system, the received signals are amplified with a fixed-gain LNA after the first RF preselection filter. Subsequently, the RF signals are directly downconverted to in-phase (I) and quadrature (Q) baseband signals without an intervening IF stage. The requirements for the second RF preselection filter are less stringent than for the first, because there is no image channel. In practice, an inexpensive RF bandpass filter can prevent strong out-of-band signals from overloading the I/Q demodulator. [Without this filter, strong out-of-band signals may result in both in-band second-order and third-order intermodulation products, causing inter-symbol interference (ISI)]. After the RF signals are demodulated to baseband, individual channel selection is performed using a baseband channel-select filter. The baseband filter is more compact and less expensive than the superheterodyne receiver's IF channel-select filter. In addition, the baseband channel-select filter can be designed with variable bandwidth, facilitating multi-mode or multi-standard operations.

Although baseband channel-select filters offer a great deal of flexibility, the composite baseband signals contain all of the adjacent-channel blocking signals that are normally filtered before they reach the I/Q demodulator (see Fig. 1). As a result, the direct-conversion-receiver's I/Q demodulator must provide a dynamic range as wide as 80 dB.