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Digital Design of Signal Processing Systems: A Practical Approach by Shoab Ahmed Khan

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13

Digital Design of Communication Systems

13.1 Introduction

This chapter covers the methodology for designing a complex digital system, and an example of a communication transmitter is considered.

Major blocks of the transmitter are the source coding block for voice or data compression, forward error correction (FEC) for enabling error correction at the receiver, encryption for data security, multiplexing for adding multiple similar channels in the transmitted bit stream, scrambling to avoid runs of zeros or ones, first stage of modulation that packs multiple bits in a symbol and performing phase, frequency, amplitude or a hybrid of these modulations, and digital up-conversion (DUC) to translate a baseb and modulated signal to an intermediate frequency (IF). This digital signal at IF is passed to a digital-to-analog (D/A) converter and then forwarded to an analog front end (AFE) for processing and onward transmission in the air.

The receiver contains the same components, cascaded together in reverse order. The receiver first digitizes the IF signal received from its AFE using an A/D converter. It then sequentially passes the digital signal to a digital down-converter (DDC), demodulator, descrambler, demultiplexer, decryption, FEC decoder and source decoder blocks. All these blocks re-do whatever transformations are performed on the signal at the transmitter.

Receiver design, in general, is the more challenging because it has to counter the noise introduced on the signalon its ...

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