3.6 Channel Coding

It is not practicable simply to serialize raw data in a shift register for transmission, except over relatively short distances. Practical systems require the use of a modulation scheme, known as a channel code, which expresses the data as waveforms which are self-clocking in order to reject jitter, to separate the received bits and to avoid skew on separate clock lines. The coded waveforms should ideally be DC-free or nearly so to enable slicing in the presence of losses and have a narrower spectrum than the raw data both for economy and to make equalization easier.

Jitter causes uncertainty about the time at which a particular event occurred. The frequency response of the channel then places an overall limit on the spacing of events in the channel. Particular emphasis must be placed on the interplay of bandwidth, jitter and noise, which will be shown here to be the key to the design of a successful channel code.

Figure 3.10 shows that a channel coder is necessary at the transmitter, and that a decoder, known as a data separator, is necessary at the receiver. The output of the channel coder is generally a logic-level signal that contains a 'high' state when a transition is to be generated. The waveform generator produces the transitions in a signal whose level and impedance are suitable for driving the channel. The signal may be bipolar or unipolar as appropriate.

Figure 3.10: The major components of a channel coding system. See text for details.

One of the fundamental parameters of a transmission code is the efficiency. This can be thought of as the ratio between the Nyquist rate of the data (one-half the bit rate) and the channel bandwidth needed. Efficiency is measured in bits/sec/Hz. Another way of considering the efficiency is that it is the worst-case ratio of the number of data bits transmitted to the number of transitions in the channel.

Some codes eliminate DC entirely. Some codes can reduce the channel bandwidth by lowering the upper spectral limit. This permits greater efficiency (measured in bits/sec/Hz), usually at the expense of noise and jitter rejection . Other codes narrow the spectrum, by raising the lower limit. A code with a narrow spectrum has a number of advantages. The reduction in asymmetry will reduce peak shift and data separators can lock more readily because the range of frequencies in the code is smaller. In theory the narrower the spectrum, the less noise will be suffered, but this is only achieved if filtering is employed. Filters can easily cause phase errors that will nullify any gain.

A convenient definition of a channel code (for there are certainly others) is 'A method of modulating real data such that they can be reliably received despite the shortcomings of a real channel, whilst making maximum economic use of the channel capacity.' The basic time periods of a channel-coded waveform are called positions or detents, in which the transmitted voltage will be reversed or stay the same. The symbol used for the units of channel time is T _d .

As jitter is such an important issue in digital transmission, a parameter has been introduced to quantify the ability of a channel code to reject time instability. This parameter, the jitter margin, also known as the window margin or phase margin ( T _w ), is defined as the permitted range of time over which a transition can still be received correctly, divided by the data bit- cell period ( T ).

Since equalization is often difficult in practice, a code having a large jitter margin will sometimes be used because it resists the effects of intersymbol interference well. Such a code may achieve a better performance in practice than a code with a higher efficiency but poor jitter performance.

A more realistic comparison of code performance will be obtained by taking into account both efficiency and jitter margin.