limited by its bandwidth and its signal-to-noise ratio (SNR). The SNR of a signal corresponds to the ratio of the signal power to the power contained in the noise at a specific point of the transmission system. Shannon's capacity formula can be expressed as C=Blog2(1+SNR) (2.4) where C is the capacity of the channel in bits per second, B is the bandwidth of the channel in Hertz, and SNR is the signal-to-noise ratio. This formula represents the theoretical maximum achievable datarate. Baseband Data Transmission Baseband data transmission refers to the transfer of data without modification. Figure 2.7(a) shows the waveform of a possible digital baseband signal. After transmission over the restricted bandwidth of a channel, the signal might look like Figure 2.7(b). Fig. 2.7 Binary data transmission (a) before the channel and (b) after the channel. Chapter 2 Data Transmission Roll-off = 1 Roll-off = 0.5 Roll-off = 0.1 Fig. 2.8 Raised cosine filter response for different roll-offs. Another effect known, as Intersymbol Interference (ISI), is also observed in Figure 2.7(b). This effect is due to the spreading in time of the pulses, provoking their overlapping. ISI can be reduced if the minimum transmission bandwidth allows all frequencies from zero to 1/2 T and rejects all the others (T is the time spacing between symbols). This was proved by Nyquist (1928). Then, if a brick-wall filter with a sharp cutoff is used before introducing the data signal to the channel, an ISI-free signal can be observed at the receiver side. However, this ideal case is not possible. As a consequence, errors will be observed at decision timing. An alternative is to use a raised cosine function in the equalizer block. With this, the rectangular pulses are shaped to obtain ISI-free transmission. Figure 2.8 shows the raised cosine response in time of a filter that is fed by a rectangular pulse. The shape of the filter corresponds to different raised cosine roll-offs. For a binary sequence, a single decision threshold is required. Each amplitude 