cks, each matched to the appro- priate chip rate of the code being tracked. Before concluding this section, we also point out that quadriphase PN is not limited only to balanced QPSK but indeed can also be applied to unbalanced QPSK data modulation by a simple generalization of (2.96), namely, (2.98) Here, S1, the power in channel 1, is, in general, unequal to S2, the power in channel 2, and the data rates 1/T, 1/T2 of m1 and m2(t), respectively, are no longer restricted to be equal. Examples of such PN spread signals are typ- ical of many of the transmission modes of the Tracking and Data Relay Satellite System (TDRSS) [20]. Once again, depending on the relation between the two PN codes, either one or two DLLs are required to provide the necessary PN tracking as per the discussion for balanced QPSK. 2.10 FURTHER DISCUSSION To conclude this chapter, we briefly summarize some of the most recent con- tributions to the subject of PN tracking. The primary intent here is to acquaint the reader with the current status of effort in the area without going into the level of detail characteristic of the previous sections of this chap- ter. It is not intended that the reader come away with the idea that the con- tributions discussed here are therefore of lesser importance; rather, they should be looked upon as evidence of continued interest in the subject and a springboard for future endeavors. One idea for improvement in PN tracking loop performance is offered by Wakabayashi, Nakagawa, and Tsunogae [21], who suggest an improved delay-locked loop (IDLL) with an effective increased number of correlators, thus allowing one to shape the loop discriminator characteristic in such a way as to expand the loop's correlation range without affecting its lock range. Although the specific results in [21] are described for a coherent DLL, the concept is readily applicable to a non-coherent DLL. As an example, con- sider the non-coherent IDLL illustrated in Figure 2.31, which for N = 2 rep- resents a slight modification of the loop in Fi