ited CDMA system. It is estimated that in a typical two-way conversation, the average data rate is 3 kbps, which, with power control, increases the battery life of mobile radios. To overcome rapid multipath fading and shadowing, a time interleaver with a 20-msec span is used with the error-control coding. The time span used is the same as that in the time frame of the voice compression algorithm. Also a RAKE processor is used in these radios to take advantage of multi- path delays greater than 1 usec, which are common in large cellular net- works. (See Part 1, Chapter 2, Section 2.2.8 for historical origins of RAKE and Part 2, Chapter 1, Section 1.7 for technical details of this unique diver- sity technique.) Key IS-95 system features for each 1.25-MHz band CDMA system are as follows: All signals use an unmodulated "carrier," a direct-sequence binary phase- shift-keying (BPSK) signal using a 15-state pseudorandom (PN) sequences with a 32,768-chip period, with each cell using a different phase (time shift) of this PN sequence. Thus, each cell has its own unique PN carrier, which is used as a common carrier by all radios active in the cell. A downlink pilot channel consists of the cell's unique PN carrier, which helps mobile units acquire and track cell-site signals. The mobile unit essentially acquires the strongest unmodulated direct-sequence BPSK sig- nal it finds by ranging over the time shifts of the PN code. Each cell also transmits a low-bit-rate, low-power synchronization chan- nel, which allows mobile radios to time-synchronize to the network. Each CDMA downlink supports up to 62 paging and traffic channels. Commercial Applications The downlink channels use orthogonal Walsh codewords assigned uniquely to each mobile unit active in the cell. These codewords, further modulated by coded data bits, are superimposed on the PN carrier for the cell. Each active uplink mobile radio signal uses a unique non-orthogonal PN code of 242 chips on top of the PN carrier. Taking advantage of the RAKE processor, fake multip