Timing recovery of PAM signals using baud rate interpolation

The timing recovery function of a digital communications receiver has to set the sampling instants of the analog-to digital (A/D) converter. The natural implementation is by modifying the clock signal that controls the A/D (e.g. by using a voltage controlled oscillator). However, this is an expensive solution, and it is usually preferred that the A/D will sample with a free-running clock, and timing modifications will be done by digital interpolation of the A/D output samples. Such interpolation is practical only if the sampling rate is high enough such that a data sample can be reconstructed from several neighboring samples. However, fast digital data communications standards (such as Gigabit Ethernet) usually use baud rate sampling (one sample per symbol), which is below the Nyquist rate of the signal, and certainly cannot be used with standard interpolation techniques. This paper suggests to exploit the structure of pulse amplitude modulation (PAM) communications signals in order to perform interpolation at baud-rate sampling. With such interpolation, only a simple and cheap control of the A/D clock is required.