In spite of attention given to fourth-generation (4G) Long Term Evolution (LTE) cellular networks, many third-generation (3G) rollouts around the world have not been completed. To guide 3G network providers through part of this process, Analog Devices offers an application note titled, "Multicarrier WCDMA Feasibility." Authored by the firm's Brad Brannon and Bill Schofield, it provides insight into how to implement a multicarrier 3G transceiver. The 22-page document also examines the required performance of major subsystems.
This work begins by providing a block diagram of a wideband multicarrier transceiver. The flexible radio architecture can be used to implement a variety of air standards, such as WCDMA, CDMA2000, and TD-SCDMA. Among the potential variations to the architecture's receiver section are low intermediate-frequency (IF) sampling and direct conversion. Direct RF modulation in the transmitter supports most applications, provided some amount of in-phase/quadrature (I/Q) balance is provided. If an application calls for high performance without an I/Q balance network, superheterodyne IF upconversion is an optimal choice.
The note offers numerous assembly options. They include system-level partitioning by means of separate receive and transmit boards as well as using separate RF and baseband boards. Chip partitioning is another example, such as the availability of MxFE functions that incorporate receive and transmit functions in a single package. For low-capacity systems, this option can facilitate higher integration and lower cost. As another alternative, combining the analog-to-digital converter (ADC) and receive signal processor (RSP) can lower cost and raise integration while avoiding import/export restrictions.
The application note focuses on the requirements for a wide-area base station (BS) as defined by the Third Generation Partnership Project's (3GPP's) TS 25.104 V6.2.0 (section 7) technical specification (TS). The examination of key WCDMA specifications includes a review of reference sensitivities, deployment band, and blocking requirements. The WCDMA study separates requirements into receiver and transmitter sections. The receive portion, for example, delves into receiver operating conditions and assumptions as well as requirements for the ADC signal-to-noise ratio (SNR), spurious-free dynamic range (SFDR), and third-order intercept.
The transmit section first examines the factors that impact signal quality. It then investigates different architectures, assuming that all have a channel filter at the output of the transmit power amplifier (PA) that is sharp enough to not desensitize the receive path. It also should ensure that spurious emissions, when co-located, are filtered sufficiently. Performance parameters covered in this section include frequency error, peak-to-average ratio and power reduction, PA linearization, transmit modulation, and superheterodyne single upconversion. This note shows how the firm's own products can satisfy direct-conversion architectures. Yet it also provides a very solid, in-depth overview of the requirements for implementing a multicarrier 3G transceiver.
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