3.7 Half-Chip Delay

On the reverse link, there is one final modification made to the mobile's "Q" signal path prior to modulating the "Q" phased carrier, which is a half-chip delay inserted just ahead of the modulator. ^[3] Without this delay, any time a symbol is followed by another symbol that lies two quadrants away, the final amplifier in the transmitter will be fed two symbols 180 degrees out of phase, causing the current in the amplifier to shut off momentarily. For example, if we send a "00" symbol followed by a "11" symbol, the phase of the carrier goes from the first quadrant to the third, by passing through 0 (shutting off the current for a moment). Although this is doable, inserting a half-chip delay onto the "Q" branch (i.e., staggering the two branches by a half-chip time), would effectively ensure that no sequence of symbols causes the carrier to shut down in transition. There are several advantages to this approach: it minimizes the production of "inter-mod" products, since shutting off the final amplifier forces it into a severe nonlinear mode (as mentioned previously, any time signals are forced through any nonlinear media, they tend to mix, producing "inter-mod" products and spurious noise; see Appendix A and C). The second advantage is that staggering the "I" and "Q" lines enables us to use more economical amplifiers in the mobile transmitters, helping to keep the costs down, while also helping to maintain a cleaner, less noise-prone transmitter with a...