 the same ratio of forward and reverse time slots. Even if the cells are time-synchronized, radio signals travel at the speed of light, and this may be slow enough to interfere with other cells. These extra cross-interference terms are not modeled at all in the CDMA equations presented earlier. The correction is not simple. We model radio propagation in these equations and in planning tools described in Part 8 by calculating an array of radio signal path gains from every base station location B to a set of (x, y) coordinates. Adding a set of radio paths from every b to every other B is not a significant addition to the list, but adding a set of radio paths from every (x, y) to every other (x, y) is a huge increase in the prob- lem size. If there is a significant amount of TDD cross-interference as we described here, then these equations do not do an adequate job of helping us understand TDD CDMA. 28.3 Applying the Equations to the Forward Direction As we move into the specific details of forward and reverse CDMA radio paths, some of the mathematical elegance and symmetry are lost. Once we take pilots, soft handoffs, orthogonal codes, and multipath into account, the matrix M is no longer exactly the same in forward and reverse directions. However, we stick to our original claim that the mathematical structure is the same and that the fundamental structure of CDMA matrix M is the same. Because the forward link in cdmaOne uses coherent modulation and the reverse link uses noncoherent modulation (discussed in Sec. 1.10.2), a busy cdmaOne system is lim- ited by the reverse direction. The 3G systems with their more balanced modulation schemes are limited in the forward direction. Soft handoffs and multipath are greater burdens in the forward direction, and these burdens outweigh the relative advantages of a common pilot and orthogonal codes. 28.3.1 Forward pilots and signaling The forward pilot is a single source of radio power shared among all the user terminals. While it is an essential part of the receiving process,