lk about multiplying bit values, as shown in Eq. (8.1): +1 = +1 +1 1 = -1 (8.1) -1 x - +1 = -1 -1 x = +1 If a digital radio engineer, in casual conversation, refers to bits as zeroes, then it will not be the first time. There is a bit of schizophrenia about - 1 and 0 depending on which notation makes the most sense in context. This arithmetic is being done with high-speed transistor gates that have two voltages for the two bit values and that have no idea what formulas we wrote on paper to describe them. Simply sending the same bit over and over again improves the robustness of a digi- tal channel, but if all we wanted was a more robust digital channel for a single user, forward error correction would do a better job. Instead, we take the sequence of redun- dant chips and multiply them by a sequence of random-looking +1 and values. We call this multiplying sequence a pseudorandom sequence or a pseudonoise (PN) code in the next section. Suppose that we have a digital signal with few enough bits and a radio channel with wide enough bandwidth that we can maintain acceptable E/No with more interference than signal, perhaps a lot more interference than signal. We can take a data stream of x bits per second and send it on a digital radio channel at x bits per second, or we can spread it k times with a PN code and send each bit with k chips on a digital radio channel at kx chips per second. Assuming that the transmit power is the same in both cases, the processing-gain principle tells us that the x-bit-kx- chip-per-second modulation will have the same Eb/No as the plain old, ordinary x-bit- per-second modulation. Why would we want to do this? Because we can now send another call on the same channel, a call with a different PN code with no particular relationship to the first PN code. These two calls look like noise to each other. The spread kx-chip channel allows other calls with other PN codes to form noiselike interference, whereas the ordinary x-bit-per-second channel does not. This spread-spectrum technology is call