0.14 Mix’n’match receiver design

The final layer is the final project of this chapter which integrates all the fixes of the adaptive component layer into thereceiver structure of the idealized system layer to create a fully functional digital receiver. The well-fabricatedreceiver is robust to distortions such as those caused by noise, multipath interference, timing inaccuracies,and oscillator mismatches.

Make it so.

Captain Picard

This chapter describes a software-defined radio design project called ${\mathcal{M}}^6$ , the Mix `n' Match Mostly Marvelous Message Machine . The ${\mathcal{M}}^6$ transmission standard is specified so that the receiver can be designed using thebuilding blocks of the preceding chapters. The DSP portion of the ${\mathcal{M}}^6$ can be simulated in M atlab by combining the functions and subroutines from the examples and exercises ofthe previous chapters.

The input to the digital portion of the ${\mathcal{M}}^6$ receiver is a sampled signal at intermediate frequency (IF)that contains several simultaneous messages, each transmitted in its own frequency band.The original message is text that has been converted into symbolsdrawn from a 4-PAM constellation, and the pulse shape is a square-root raised cosine.The sample frequency can be less than twice the highest frequency in the analog IF signal, but it must be sufficiently greater than theinverse of the transmitted symbol period to be twice the bandwidth of the baseband signal.The successful ${\mathcal{M}}^6$ M atlab program will demodulate, synchronize, equalize, anddecode the signal, so it is a “fully operational” software-defined receiver(although it is not intended to work in “real time”). The receiver must overcome multiple impairments.There may be phase noise in the transmitter oscillator. There may be an offset between the frequency of theoscillator in the transmitter and the frequency of the oscillator in the receiver.The pulse clocks in the transmitter and receiver may differ. The transmission channel may be noisy.Other users in spectrally adjacent bands may be actively transmitting at the same time.There may be intersymbol interference caused by multipath channels.

The next section describes the transmitter, the channel, and the analog front-end of the receiver.Then "A Design Methodology for the M 6 Receiver" makes several generic observations about receiver design, and proposesa methodology for the digital receiver design. The final section describes the receiverdesign challenge that serves as the culminating design experience of this book.Actually building the ${\mathcal{M}}^6$ receiver, however, is left to you. You will know that your receiver works whenyou can recover the mystery message hidden inside the received signal.

How the received signal is constructed

Receivers cannot be designed in a vacuum; they must work in tandem with a particular transmitter.Sometimes, a communication system designer gets to design both ends of the system.More often, however, the designer works on one end or the other, with the goal of making the signalin the middle meet some standard specifications. The standard for the ${\mathcal{M}}^6$ is established on the transmitted signal, and consists, in part, of specifications on the allowable bandwidth andon the precision of its carrier frequency.The standard also specifies the source constellation, the modulation, and the coding schemes to be used.The front-end of the receiver provides some bandpass filtering, downconversion to IF, and automatic gain control prior to the sampler.