des use the orthogonal variable spreading factor (OVSF) described in Sec. 8.3.2, so different channels on the same carrier can use different spreading gains. cdma2000 3x supports 256 Walsh codes with spreading gains as high as 256 and as low as 4. With 2 to 1 turbo coding FER, can support data rates over 1 Mbps. The cdma2000 pilot, synchronization, and paging channels are similar to cdmaOne. Recent work has developed the potential of the cdma2000 1x carrier. The 1xEV-DO (1x EVolution Data Only) carrier is specifically designed for packet data. For user ter- minals close to the base station (picocell service), 1xEV-DO achieves a peak forward di- rection rate of 2.45 Mbps in only 1.25 MHz of radio spectrum. In its highest data rate configuration, 1xEV-DO abandons QPSK and CDMA and goes into TDMA with higher- order constellations in its modulation, 8-ary and 16QAM as demand and radio condi- tions allow. It also uses turbo forward error correction, described in Sec. 17.2.2, which is designed for large data blocks. Small changes in data rate are accomplished by re- peating sequences of bits to stretch them out or by adding a few extra bits into the stream, not enough to break the forward error correction scheme. This technique is called puncturing the bit stream. Other recent work has developed even higher-order modulation schemes in the cdma2000 1.25-MHz camp. 1Xtreme uses 64QAM, 6 bits per symbol, as the key to even higher speeds, with claims of packet data rates of up to 5.2 Mbps and CDMA data throughput of 1.2 Mbps. 10.2.5 cdma2000 reverse channel coding In contrast to cdmaOne, the cdma2000 reverse link supports multiple channels for a single user terminal. The reverse channel set includes a pilot that allows coherent de- modulation at the base station and more accurate measurement for power control and the rake filter.9 The multiple reverse channels of a cdma2000 link are spread with or- thogonal Walsh codes to keep them separate. The pilot uses a Walsh code of all zeroes, the dedicated control channel uses an ei