## Symbol timing recovery

#### The eye diagram is a very graphic way of highlighting the need for accurate symbol timing in the receiver in order to sample the received signal at the maximum eye opening.

#### Symbol timing can be derived by sending a timing reference alongside the data signal, for example in the form of a continuous tone at a multiple or sub-multiple of the symbol rate (Bateman, 1990), or in the form of a burst clock between message data transmissions. Most symbol timing systems, however, obtain their information from the message data itself, making use of the 'zero-crossing' information in the baseband (bipolar) signal. |

#### The problem of symbol timing is eased significantly for raised cosine filtered signals if the roll-off factor a is equal to 1. For this unique case, the zero-crossings of the filtered waveform always occur at a time Ts/2 before the optimum zero ISI detection point. Triggering a timer from these zero-crossings to sample the signal Ts/2 seconds later will thus give perfect symbol timing. When the data contains long strings of 1s or 0s, the sampling process must interpolate the correct sample times until the next zero-crossing occurs. Data 'scrambling' can be used to try to increase the frequency of zero-crossings in the data stream.