n eight noninterfer- ing channels, but it does not require synchronized chips. The forward link is naturally Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The CDMA Principle Key Radio Concepts synchronized with a common transmitter sending all the signals and can support eight simultaneous orthogonal codes in H8. 6 The reverse link is naturally unsynchronized, and it would be very difficult to stagger the user terminal transmit times for simulta- neous arrival of the chips at the base station receiver. Thus the eight-to-three strategy may make more sense. The logic described for H8 extends naturally to H64, where we have 64 orthogonal codes. This gives is 64 noninterfering channels forward and 64-to-6 processing gain in the reverse direction in cdmaOne systems. 8.4 Multipath and the Rake Filter Life is simple when there is a single, direct path from transmitter to receiver. Every- thing that is not signal is noise. Radio is more complicated when there are multiple paths arriving at the receiver at multiple times, as shown in Fig. 8.3. We can divide multipath into three rough categories, measured by the length of time between the arrival of the first signal and the last signal, which we call the delay spread. The delay spread can be roughly nanoseconds, microseconds, or milliseconds. In digital radio, these three work out to fading, chip confusion, and echo. It is not the actual delay that is the issue; rather, it is the difference in delay, which we call delay spread, that concerns us. The term delay spread is used both for the distribution of de- lay and for the time duration containing most of the distribution. In the wireless telephone environment, echo is almost never a problem because ra- dio paths rarely have such long delays. Echo is a major concern for communication links of 1000 km (600 miles) or longer. We'll let the long-haul mi