Introduction of Transceiver Section:
The baseband signal processing at transmitter and receiver is done in discrete time. The transmitter consists of an encoder, a spreader, a pulse shaping transmitter filter, a D/A. The converter and a IF-RF up-converter. The data stream generated by different user is encoded and interleaved into data sequence , then the data sequence is spread by a PN code and digitally modulated onto a chip pulse shape. The signal is then D/A converted and modulated onto an analogue carrier for transmission. In a system with different active users, each user is represented by such a transmitter.
The base station receiver contains the front end of the receiver, despreaders, each one for each user, multiuser detector and sinks. brief summary on modulation, spreading, and spreading codes.
There are two families of spreading codes: orthogonal codes and pseudorandom (also called pseudo-noise [PN]) codes. These have different properties and both types of codes are used in the UTRAN system. Spreading means increasing the bandwidth of the signal beyond the bandwidth normally required to accommodate the information. The spreading process in UTRAN consists of two separate operations or steps: channelization and scrambling. The uses of spreading codes are somewhat different in the uplink and in the downlink. Channelization transforms each data symbol into several chips. The ratio (number of chips/symbol) is called the spreading factor. Data symbols on I- and Q-branches are multiplied with a channelization code. Channelization codes are orthogonal codes, meaning that in an ideal environment they do not interfere with each other. However, orthogonality can only be achieved if the codes are time synchronized. Thus it can be used in the downlink to separate different users within one cell, but in the uplink only to separate the physical channels of one user. It cannot be used by the base.
stations to separate different users, as all mobiles are unsynchronized in time,and thus their codes cannot be orthogonal. However, note that in the TDD mode it is possible to have a finely time-synchronized uplink; Note that in the TDD mode the uplink is in any case crudely time synchronised, as UE transmissions must fit into their own time slots, but this is still not the same thing as chip level synchronisation that is required for code orthogonality.In the scrambling procedure, the I- and Q-phases are further (after channelization) multiplied by a scrambling code. These scrambling codes have good autocorrelation properties. In the uplink, different users have different long code offsets, and the network can recognize different users from their offsets. And once the synchronization is achieved, various services of the user can be separated using orthogonal codes.
In the downlink, pseudorandom scrambling codes are used to reduce inter-base-station interference. Each Node B has only one unique primary scrambling code, and this is used to separate various base stations.
The modulation scheme in the UTRAN is quadrature phase shift keying (QPSK), and also 16 QAM on the HS-PDSCH channel. Modulation is a process where the transmitted symbols are multiplied with the carrier signal.
The modulating symbols are called chips, and their modulating rate is 3.84 Mcps.
Transport and Physical Channel Mapping :