direct conversion receiver





The superheterodyne radio has clearly been the dominant architecture in cellular radio receivers. Therefore, it is discussed as an introduction at the beginning of this chapter. Next, the direct-conversion architecture and its properties, both benefits and drawbacks, are explained and recent implementations reported in the literature summarized. The other radio receiver architectures are not discussed since they are beyond the scope of this thesis. The purpose of a radio receiver is to detect the potentially weak desired signal in the presence of noise and unwanted signals. The power of the other signals might be many orders of magnitude larger than the power of the desired signal channel. Because of the harsh environment, a high selectivity is required. The channel selection at radio frequency (RF) would require filters with very high quality factors and selectivity. In UTRA/FDD, the channel bandwidth and carrier frequency are approximately 4MHz and 2GHz, respectively. The channel-select filter quality factor would be 500 and the adjacent channel attenuation at a 5MHz frequency offset should be at least 33dB in a cellular phone . In GSM, the signal bandwidth is only 200kHz, which increases the required quality factor to 4500, when the carrier frequency is 900MHz. The filter order should be at least five. Since such filters are not available, the problem has to be circumvented . The solution to the problem is heterodyning, in which the RF signal is downconverted to an intermediate frequency (IF) using a local oscillator (LO) signal at a different frequency from the carrier. At a lower IF, the requirements for the channelselect filter become easier to achieve After selecting the desired signal channel, the transmitted information must be detected. In modern cellular systems, which use digital modulation and coding, the detection is performed digitally. Most of the signal processing is implemented in the digital domain where the limitations of the analog domain can be avoided. However, the direct digitization at RF is not technically possible at the moment, nor will it be in the near future because of the lack of appropriate ADCs. In the future, the analog front-end of the radio receiver will therefore remain necessary in order to reduce the dynamic range and maximum signal frequency before the analog-to-digital conversion. However, this interface is moving towards the antenna as a result of developments in analog-to-digital conversion techniques.





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