r- eral transport blocks are fed from the MAC layer to the vices). Rate matching by means of puncturing or repeti- physical layer, which performs coding, interleaving, tion of the coded bits is used to fine-tune the code rate multiplexing, spreading, etc., prior to data transmission. and multiple coded and interleaved transport channels Thus, for WCDMA, the TTI is the time which the can be multiplexed together, forming a single stream of interleaver spans and the time it takes to transmit the bits to be spread to the chip rate and subsequently transport block over the radio interface. A larger TTI modulated. For the downlink, QPSK modulation is used, implies better time diversity, but also a longer delay. In while the uplink uses BPSK. The resulting stream of the first release, WCDMA relies on TTI lengths of 10, modulation symbols is mapped to a physical channel and 20, 40, and 80 ms. As will be seen later, HSPA introduces subsequently digital-to-analog converted and modulated additional 2 ms TTI to reduce the latency. onto a radio-frequency carrier. In principle, each physical To support different data rates, the MAC can vary the channel corresponds to a unique spreading code, used to transport format between consecutive TTIs. The trans- separate transmissions to/from different users. port format consists of several parameters describing how The spreading operation actually consists of two steps: the data shall be transmitted in a TTI. By varying the spreading to the chip rate by using orthogonal channeli- transport-block size and/or the number of transport zation codes with a length equal to the symbol time blocks, different data rates can be realized. followed by scrambling using non-orthogonal scrambling At the bottom of the protocol stack is the physical sequences with a length equal to the 10 ms radio frame. layer. The physical layer is responsible for operations such as coding, spreading and data modulation, as well as modulation of the radio-frequency carrier. PDCP, RLC, MAC, and physical layer, are confi