ath signals trans- mitted from two cell sites allow mobile radios to conduct "soft handoffs" from one cell site to another. Although much more complex, this system is inherently more robust than conventional narrowband radios using traditional FDMA and TDMA approaches. Perhaps most important is its robustness against multipath fad- ing. It also allows more flexibility in the application of antennas for sector- ization, being able to use fixed and adaptive multibeam antennas to increase capacity dramatically and further reduce radio power requirements. 3.3.2 Comparison of the IS-95, IS-54, and GSM The first U.S. digital cellular standard, IS-54, and the European digital cel- lular standard, GSM, are both based on narrowband modulations, with TDMA as the basic multiple-accessing technique. The second U.S. digital cel- lular standard, IS-95, differs fundamentally in its use of direct-sequence spread-spectrum modulation, using CDMA for multiple access. All three sys- tems overlay these basic access systems with further channelization using FDMA. Table 3.2 summarizes the key features of these three digital cellu- lar standards [20]. Like existing analog cellular systems, these digital cellular systems use sep- arate uplink and downlink frequency bands, with frequency division duplex- ing (FDD) to achieve full-duplex operation. Some level of power control is Table 3.2. Comparison of digital cellular systems Feature IS-54 IS-95 Multiple access Frequency band United States Europe United States Uplink (MHz) 869-894 935-960 869-894 Downlink (MHz) 824-849 890-915 824-849 Channel spacing 30 kHz 200 kHz 1.25 MHz Modulation DQPSK BPSK/QPSK Maximum Tx power 600 mW 600 mW (mobile handset) Average Tx power 200 mW 125 mW Variable Speech rate 8 kbps 13 kbps 1-8 kbps Number of channels Variable Channel bit rate 48.6 kbps 270.3 kbps 1.25 Mcps A New Paradigm for Designing Radio Networks used by all three systems, but the IS-95 system uses the tightest dynamic power control since power control plays a more critical role in CDMA sys- it