Cell-phone basestations are regularly updated to expand system capacity as well as implement new standards and data services. Complete replacement of equipment is too expensive, so most carriers look to vendors for system upgrades. Soon, many basestations will upgrade to systems that can support the new wideband 3G cell-phone systems. At the heart of the new and upgraded equipment are the data converters that translate between the analog wireless and digital processing worlds.
What are the current trends in basestation design? The newer equipment uses a broadband design that is essentially a software-defined radio (SDR). The receiver picks up the entire assigned multichannel band and digitizes at the earliest point in the receiver possible. Receivers with bandwidths to 30 MHz may receive part of the band or an entire 75-MHz band, which may include multiple types of cell-phone air standards. From there, all other receiver functions such as additional downconversion, filtering, demodulation, demulitplexing, and all baseband functions (decompression, etc.) are done digitally in a DSP. With such a design, fewer receivers are needed, reducing space and decreasing power consumption and heat.What does the architecture of a basestation transceiver look like? The receiver has an appropriate low-noise amplifier (LNA), input filters, and a mixer that downconverts the signal to the intermediate frequency (IF). Newer SDRs implement a single high-frequency IF. Typical values in the newer systems range from about 60 to 70 MHz to as high as 240 MHz. The IF is usually harmonically related to some baseband reference frequency. Some newer designs use zero-IF (ZIF) or direct-conversion designs. In the receiver, the signal is immediately downconverted to baseband by heterodyning the signal with a synthesizer on the same frequency. This greatly relaxes the ADC's conversion speed requirement. DSP takes over from there with filtering, demodulation, and other functions. In the transmitter, the baseband signal is sent to the DAC and immediately upconverted to the final output frequency before being sent to the PA.Click here to download the PDF version of this entire article.