Chapter 13

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 baseband 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 signal on its way from the transmitter. Also, the receiver employs components that are running at its own clock, and frequency synthesizers that are independent of the transmitter clock, so this causes frequency, timing and phase synchronization issues. Multi-path fading also affects the received signal. All these factors create issues of carrier frequency and phase synchronization, and frame and symbol timing synchronization. The multi-path also adds inter-symbol interference.

For high data-rate communication systems most of the blocks are implemented in hardware (HW). The algorithms for synchronizations in the receiver require complex nested feedback loops.

The transmitter example in this chapter uses a component-based approach. This approach is also suitable for software-defined radios (SDRs) using reconfigurable field-programmable gate arrays (FPGAs) where precompiled components can be downloaded at runtime to configure the functionality.

Using techniques explained in earlier chapters, this chapter develops time-shared, dedicated and parallel architectures for different building blocks in a communication transmitter. MATLAB® code is listed for the design. Acritical analysis is usually required for making major design decisions. The communication system design gives an excellent example to illustrate how different design options should be used for different parts of the algorithm for effective design.

A crucial step in designing a high-end digital system is the top-level architecture, so this chapter first gives design options and then covers each of the building blocks in detail. For different applications these blocks implement different algorithms. For example, the source encoding may compress voice, video or data. The chapter selects one algorithm out of many options, and gives architectural design options for that algorithm for illustration.