 same pseudo-random code. Modulation 75 This is easily achieved by ensuring that both transmitter and receiver use the same algorithms to generate these sequences. The drawback of using a pseudo-random code is that the codes are not orthogonal and as a result some data errors are expected when regenerating the original data. Another form of modulation that is becoming more frequently used with the rise in levels of integration is called orthogonal frequency division muliplex (OFDM). A form of this known as coded OFDM or COFDM is used for digital radio (DAB) broadcasts that are being made and is applicable for use in the cellular telecommunications environment. COFDM has forward error correction applied to the signal before transmission and this enables it to overcome errors caused by interference and lost carriers due to selective fading or reflections to be overcome. As a result of its resilience to interference and reflected signals that introduce data delays, it is an ideal medium for broadcasting digital radio. It caters equally for those with hi-fi tuners at home and those in motor vehicles that are on the move and require omnidirectional antennas. These mobile antennas can pick up the signals that may be arriving from a variety of directions due to reflections, and hence may not all arrive at the same time. OFDM is a form of transmission that uses multiple carriers. The data to be transmitted is spread across the carriers SO that each one carries a low data rate stream. The carriers are spaced very closely to preserve the bandwidth. Normally they would interfere with one another but the signals are made orthogonal to each other to prevent this occurring. This is achieved by making the data bit rate equal to the carrier spacing. In this way the nulls in the sidebands occur at the point where the next signal occurs. This arises because the spectrum of the signal from each carrier has a sin x/x shape, and the next carrier is slotted in where the null in the spectrum of the first carrier exists. Summary There a