TABLE OF CONTENTS The purpose of the physical layer is to condition the digital data from higher layers so that it can be transmitted over a mobile radio channel reliably. In the transmit direction, it performs such functions as channel coding, interleaving, scrambling, spreading, and modulation. In the receive direction, these functions are reversed so that the transmitted data is recovered at the receiver. The MAC layer delivers user data and signaling over a number of transport channels.
The physical layer maps each of these channels into a physical channel and transmits the information over the radio interface. A physical channel is characterized by its associated carrier frequency, scrambling, and channelization codes, the radio frame length, and the relative phase angle when meaningful. A radio frame is 10 ms long and consists of 15 time slots. Because the chip rate is 3.84 Mc/s, there are 38,400 chips in a frame and 2,560 chips in a slot. Except where otherwise indicated, the description of this section applies only to the FDD mode of UMTS.
The physical layer of the UMTS can be best explained in terms of the functions performed by a transmitter. Figure 6-5 shows a functional block diagram of a transmitter in the UE. In UMTS, the data arrives on a transport channel in blocks. We begin by encoding each of these blocks of a transport channel into a cyclic redundancy check (CRC) code and then serially concatenating all encoded blocks of that channel.
