des use 3 to 1 convolutional coding for low rates and 2 to 1 turbo coding for high rates. For any required data rate, the system uses a spreading gain to expand the result to near 3.84 Mchips per second and then repeats some bits or punctures the bit stream to get the desired data rate to within 100 bits per second of the target rate. The TDD mode has no continuous pilot because there is no continuous transmission. Instead, each data burst has a set of bits for coherent demodulation. These bits are called a midamble because they are placed in the middle of the burst. The data bursts are 2560 chips at 1500 per second, just a little bit longer than a GSM time slot. The TDD mode also does not support soft handoff. Instead, all handoffs are mobile-assisted hard handoffs. Also, the spreading gains used are lower, ranging from a maximum of 16 to a minimum of 1 (QPSK with no spreading gain). 10.3.5 W-CDMA reverse channel coding The reverse channel chip rate is 3.84 Mchips per second, the same as the forward di- rection. All the W-CDMA channels have pilot bits included at 1500 per second, also the same as the forward direction. This ensures coherent detection in both forward and re- verse directions. The reverse carrier uses OVSF to allow spreading gains from 256 for voice and low data rates down to 4 for high data rates. Speech and data can be multiplexed onto a single channel with 1500 time slots per second. Packet mode uses a shared channel. Time slots are assigned using MAC mes- sages sent on the same shared channel using a slotted ALOHA protocol. 14 High-priority We say radio channel because engineers with a data-processing background would describe it differently. We would see the time division supporting multiple voice and data channels through a single device for a single user. This would allow, for example, two different applications on a per- sonal digital assistant (PDA) to use time division duplexing to carry both a voice call and e-mail over the wireless channel. We have a single radio channel, a single subs