Navigate The AFE And Data-Converter Maze In Mobile Wireless Terminals
Before starting a mobile wireless design, you need to determine the data-converter performance requirements in your baseband-sampling radio architectures.
Mobile broadband wireless communication systems employ several techniques for improving spectral efficiency. To achieve high data rates, yield optimal system capacity, and ensure reliable quality-of-service, modern communication systems use wide variable channel bandwidths (BW = 1.25 MHz to 20 MHz), high-order modulation (16QAM to 64QAM), code-division or orthogonal multiple-access (CDMA, OFDMA), and scalable smart-antenna technology (MIMO, spatial diversity).
3GPP standards including UMTS, TD-SCDMA, and Long-Term Evolution (LTE) as well as others like IEEE 802.16e and IEEE 802.11n are examples of systems using these techniques. As an example, 4G systems like LTE using 64QAM modulation with subcarrier = 2048 OFDM, 20-MHz wide channel bandwidth and 2×2 MIMO architecture can achieve data rates greater than 100 Mbits/s with robust performance.
High-order modulation with OFDM, wide channel bandwidths, and MIMO architectures all conspire to demand higher performance from the receiver analog-to-digital converters (RX-ADCs) and transmitter digital-to-analog converters (TX-DAC). The data-converter requirements include faster sample rates, better dynamic range , improved spectral performance, and multiple channels. Furthermore, since the end-product communication equipment is mobile and battery-powered, the data converters must be low in cost and power with small footprints.
Because mobile wireless products are size, power, and cost sensitive, the preferred radio architecture is direct-conversion zero intermediate frequency (ZIF). Compared to heterodyne radios, t he ZIF architecture eliminates multiple IF components such as the mixer, LO synthesizer, and image reject filter, and that lowers cost and reduces size. Furthermore, in applications with variable channel bandwidth, like WiMAX and LTE, the ZIF architecture lends itself to programmable baseband filtering.
Figure 1 shows a typical ZIF radio lineup for a UMTS-WCDMA handset or data card application. The ZIF radio architecture requires a dual-channel RX-ADC and dual-channel TX-DAC used for in-phase and quadrature (I-Q) baseband signal sampling and construction. Other low-speed converters are used for RF front-end gain control and ancillary analog-signal measurements like temperature and transmitter power. The converter’s digital bus interfaces with an FPGA, DSP, or ASIC digital baseband processor. The digital baseband performs signal-processing functions like channel coding, modulation, and digital filtering. A single-mode WCDMA ZIF radio may require eight data- converter channels.
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