A digital radio  (Page 8/8)

Use DSP when possible.

The digital signal processing portion of the receiver can perform the following tasks:

• downconvert the sampled signal to baseband
• track any changes in the phase or frequency of the modulating sinusoid
• adjust the symbol timing by interpolation
• compensate for channel imperfections by filtering
• convert modestly inaccurate recovered samples into symbols
• perform frame synchronization via correlation
• decode groups of symbols into message characters

A central task in Software Receiver Design is to elaborate on the system structure in  [link] – [link] to create a working software-defined radio that can perform these tasks. This concludes the illustrative design at the outer,most superficial layer of the onion.

Use DSP to compensate for cheap ASP.

The complete onion

This section provides a whirlwind tour of the complete layered structure of Software Receiver Design . Each layer presents the same digital transmission systemwith the outer layers peeled away to reveal greater depth and detail.

• The Naive Digital Communications Layer: As we have just seen, the first layer of the onion introduced the digital transmission of data, and discussedhow bits of information may be coded into waveforms, sent across space to the receiver, and then decodedback into bits. Since there is no universal clock, issues of timing become important, and some of themost complex issues in digital receiver design involve the synchronization of the received signal. The systemcan be viewed as consisting of three parts:
  1. a transmitter as in [link]
  2. a transmission channel
  3. a receiver as in [link]
• The Component Architecture Layer: The next two chapters provide more depth and detail by outlining a complete telecommunicationsystem. When the transmitted signal is passed through the air using electromagnetic waves, it must takethe form of a continuous (analog) waveform. A good way to understand such analog signals is via the Fourier transform, and thisis reviewed briefly in [link] . The five basic elements of the receiver will be familiar to many readers,and they are presented in [link] in a form that will be directly useful when creating M atlab implementations of the various parts of the communication system.By the end of the second layer, the basic system architecture is fixed and the ordering of the blocks in the systemdiagram is stabilized.
• The Idealized System Layer: The third layer encompasses [link] through [link] . This layer gives a closer look at the idealized receiver—how things work when everythingis just right: when the timing is known, when the clocks run at exactly the right speed, when there are no reflections,diffractions, or diffusions of the electromagnetic waves. This layer also integrates ideas from previous systems courses,and introduces a few M atlab tools that are needed to implement the digital radio.The order in which topics are discussed is precisely the order in which they appear in the receiver:
  $$\begin{array}{c}\\ \text{channel}\\ \mathit{\text{Chapter}}\text{4}\end{array}\to \begin{array}{c}\text{frequency}\\ \text{translation}\\ \mathit{\text{Chapter}}\text{5}\end{array}\to \begin{array}{c}\\ \text{sampling}\\ \mathit{\text{Chapter}}\text{6}\end{array}\to$$
  $$\begin{array}{c}\underbrace{\begin{array}{c}\text{receive}\\ \text{filtering}\end{array}\to \text{equalization}}\\ Chapter\text{7}\end{array}\to \begin{array}{c}\underbrace{\begin{array}{c}\text{decision}\\ \text{device}\end{array}\to \text{decoding}}\\ Chapter\text{8}\end{array}$$
  $$\begin{array}{cc}\text{Channel}\text{impairments}\text{and}\text{linear}\text{systems}\hfill & \hfill \text{Chapter}\text{4}\\ \text{Frequency}\text{translation}\text{and}\text{modulation}\hfill & \hfill \text{Chapter}\text{5}\\ \text{Sampling}\text{and}\text{gain}\text{control}\hfill & \hfill \text{Chapter}\text{6}\\ \text{Receive}\text{(digital)}\text{filtering}\hfill & \hfill \text{Chapter}\text{7}\\ \text{Symbols}\text{to}\text{bits}\text{to}\text{signals}\hfill & \hfill \text{Chapter}\text{8}\end{array}$$
  [link] provides a complete (though idealized) software-defined digital radio system.
• The Adaptive Component Layer: The fourth layer describes all the practical fixes that are required in order to create a workable radio.One by one the various problems are studied and solutions are proposed, implemented, and tested. These include fixesfor additive noise, for timing offset problems, for clock frequency mismatches and jitter, and for multipath reflections.Again, the order in which topics are discussed is the order in which they appear in the receiver:
  $$\begin{array}{cc}\text{Carrier}\text{recovery}\hfill & \hfill \text{Chapter}\text{10}\\ \mathit{\text{the}}\mathit{\text{timing}}\mathit{\text{of}}\mathit{\text{frequency}}\mathit{\text{translation}}\hfill & \\ \text{Receive}\text{filtering}\hfill & \hfill \text{Chapter}\text{11}\\ \mathit{\text{the}}\mathit{\text{design}}\mathit{\text{of}}\mathit{\text{pulse}}\mathit{\text{shapes}}\hfill & \\ \text{Clock}\text{recovery}\hfill & \hfill \text{Chapter}\text{12}\\ \mathit{\text{the}}\mathit{\text{timing}}\mathit{\text{of}}\mathit{\text{sampling}}\hfill & \\ \text{Equalization}\hfill & \hfill \text{Chapter}\text{13}\\ \mathit{\text{filters}}\mathit{\text{that}}\mathit{\text{adapt}}\mathit{\text{to}}\mathit{\text{the}}\mathit{\text{channel}}\hfill & \\ \text{Coding}\hfill & \hfill \text{Chapter}\text{14}\\ \mathit{\text{making}}\mathit{\text{data}}\mathit{\text{resilient}}\mathit{\text{to}}\mathit{\text{noise}}\hfill & \end{array}$$
• The Integration Layer: The fifth layer is the final project of [link] which integrates all the fixes of the fourth layer into the receiver structure of the third layer to create afully functional digital receiver. The well-fabricated receiver is robust to distortions such as those causedby noise, multipath interference, timing inaccuracies, and clock mismatches.

Please observe that the word “layer” refers to the onion metaphor for the method ofpresentation (in which each layer of the communication system repeats the essential outline of the last, exposinggreater subtlety and complexity), and not to the “layers” of a communication systemas might be found in Bertsekas and Gallager's Data Networks . In this latter terminology, the whole of Software Receiver Design lies within the so-called physical layer . Thus we are part of an even larger onion,which is not currently on our plate.