channels are available, with cordless telephones "smart enough" to select the clearest channel among them. This somewhat resembles "intelligent" frequency hopping in that the radio selects the clearest channel, hops to it, and stays there. With frequency hopping, which indiscriminately hops over the total band, the voice channel may be hit with occasional high bursts or errors owing to multipath and inter- ference in different hop bands. Digital video conferencing has the most demanding requirements in both types of radio, requiring higher data rates and full-duplex operation. Probably the most important functionality to be compared between direct-sequence and frequency-hopping spread-spectrum radios is the poten- tial overall capacity achievable in areas with many such radios. In Section 3.5, we address this question for a high-density network of many synchro- nized cells. 3.2.4 Multipath and Diversity For most mobile applications, multipath often constitutes the most serious limitation on performance. Many studies have been conducted which clearly prove that diversity techniques can dramatically improve performance against multipath, with antenna diversity the most common method. For mul- tipath delays longer than a chip-time interval, direct-sequence spread-spec- trum radios can use RAKE processing to take advantage of multipath (see Part 2, Chapter 1, Section 1.7). Coding with interleavers, which has been dis- cussed extensively as a means of combatting jamming, is also ideal for mul- tipath if the interleaver time span covers independent fades. Antenna diversity is effective against multipath for both direct-sequence and frequency-hopping spread-spectrum radios. Coding and interleaving techniques are effective for frequency hopping if coded bits are taken from different hop bands. With commercial slow-frequency hopping radios, this is generally impractical because the interleaving span (memory) is too large. For direct-sequence radios, the interleaver span must cover a time interval Commercial Applications t