Transport in WCDMA Radio Access Network
The WCDMA Radio Access Network nodes communicate with each other over a transport network. The 3GPP specification provides a very clear split between radio related (WCDMA) functionality and the transport technology, meaning that there is no particular bias to any technology. The transport network is initially based on ATM, but IP will soon be included as an option.
Radio Interface Overview
The protocol stack of the radio interface between WCDMA Radio Access Network and the handset consists of a number of protocol layers, each giving a specific service to the next layer above. The main purpose with each layer is as follows:
Layer 3: Signaling to control the connection to the handset.
Layer 2: If there is time for it, to retransmit packets which has been received in error.
Layer 1: To transmit and receive data over the radio, including basic protection against bit errors.
The Physical Layer (Layer 1) offers Transport Channels to the MAC layer. There are different types of transport channels with different characteristics of the transmission. Common transport channels can be shared by multiple handsets (e.g. FACH, RACH, DSCH, BCH, PCH). Dedicated transport channels (DCH) are assigned to only one handset at a time.The transmission functions of the physical layer include channel coding and interleaving, multiplexing of transport channels, mapping to physical channels, spreading, modulation and power amplification, with corresponding functions for reception.A frequency and a code characterize a physical channel. The specifications include two modes: the FDD mode (Frequency Division Duplex) and the TDD mode (Time Division Duplex). The FDD mode is the mainstream mode that operators are now deploying in WCDMA. The TDD mode may eventually be deployed as well, as a complement to the FDD mode. This document
does not describe the TDD mode.
The Medium Access Control (MAC) protocol (Layer 2) offers logical channels to the layers above.The logical channels are distinguished by the different type of information they carry, and thus include the Dedicated Control Channel (DCCH), Common Control Channel (CCCH), Dedicated Traffic Channel (DTCH),Common Traffic Channel (CTCH), Broadcast Control Channel (BCCH) and the Paging Control Channel (PCCH). The MAC layer performs scheduling and map-
ping of logical channel data onto the transport channels provided by the physical layer. Also, for common transport channels, the MAC layer adds addressing information to distinguish data flows intended for different handsets. One major difference to GSM is the possibility to dynamically switch one logical channel (data flow) onto different transport channel types, e.g.
based on the activity of the subscriber. This is called channel type switching.
The Radio Link Control (RLC) protocol (Layer 2) operates in one of three modes: transparent, unacknowledged or acknowledged mode. It performs segmentation/re-assembly functions and, in acknowledged mode, provides an assured mode delivery service by use of retransmission. RLC provides a service both for the RRC signalling (the Signalling Radio Bearer) and for the user data transfer (the Radio Access Bearer).
The Radio Resource Control (RRC) protocol (Layer 3) provides control of the handset from the RNC. It includes functions to control radio bearers, physical channels, mapping of the different channel types, handover, measurement and other mobility procedures. Because of the flexibility of the WCDMA radio interface, this is a fairly complex protocol.
Characteristics of UTRA (WCDMA)
The WCDMA standard has two modes for the duplex method. A Frequency Division Duplex (FDD) and Time Division Duplex (TDD). The frequency bands allocated for UTRA are shown in Figure . In UTRA there is one paired fre-quency band in the range 1920 -1 980 MHz and 21 10 -2 170 MHz to be used for UTRA FDD. There are two unpaired bands from 1900 -1 920 MHz and 2010 - 2025
MHz intended for the operation of UTRA TDD.
At the time when this work was developed, only the standard of the FDD mode developed by ITU were at an advanced stage of standardization. The TDD mode standard started later. For this reason this work assumes the FDD mode of operation for the receiver. Table lists the most important parameters of the UTRA FDD.As can be seen in Table , the chip rate for the WCDMA standard is 3.84 Mcps , and . Spreading consists of two operations. The first operation is the channelization operation where the spreading code is applied to every symbol in the transmitted data. Thus the bandwidth of the data signal is increased. In this channelization operation, the number of chips per data symbol is called the
Spreading Factor (SF). The second spreading operation is the scrambling operation,where a scrambling code is applied to the already spreaded signal. Both of the spreading operations are applied to the so called In-phase (I) and Quadraturephase (Q) branches of the data signal. In the channelization operation, the Orthogonal Variable Spreading Factor (OVSF) codes are independently applied to the I and Q branches and . The resultant signals on the I and Q branches are then multiplied by a complex-valued scrambling code, where I and Q correspond to the real and imaginary parts respectively.
For the channel coding, the standard suggests three options of coding for different Quality-of-Services (QoS) requirements. The three coding options are: Convolutional coding, Turbo coding, or no coding. The selection of one of the three options is done by the upper layers. In addition, bit interleaving is used to improve the Bit Error Rate (BER). The modulation scheme selected in 3GPP WCDMA standard is QPSK .
An important characteristic of the WCDMA system is that i t is an asynchronous system, i. e. there is no global synchronization between base stations in the system. This means that each user can transmit independently of other users or base stations transmissions . This eliminates the need for global clock similar to the IS-95 system proposed by the USA. IS-95 uses the Global Positioning System (GPS) clock as a global clock for synchronization between base stations.Since this dissertation deals with the baseband of the mobile terminal's receiver, I will only explain the structure of the downlink. The downlink is the communication path from the base station to the mobile terminal. The physical chan nels of the WCDMA systems are structured in layers of radio frames and timeslots. There is only one type of downlink dedicated physical channel, the downlink Dedicated Physical CHannel (downlink-DPCH) . The structure layout of the downlink dedicated physical channel (DPCH) of the WCDMA signal can be seen in Figure . As shown in the Figure, the time line of the signal is divided into frames of 10 ms each. Each frame is then divided into 15 slots, i.e. 2560 chipslslot at the chip rate of 3.84 Mcps. In addition, every 72 frames constitute one super frame. The frame is a time multiplexed data and control bits from the Dedicated Physical Data Channel (DPDCH) and Dedicated Physical Control Channel (DPCCH)'. The DATA 1 and DATA 2 are data bits that belong to DPDCH, while bits of Transmit Power Control (TPC), Transport Format Combination Indicator (TFCI), and Pilot belongs to the DPCCH. The number of bits in each field vary with the channel bit rate. The exact number of bits in each field is shown in . The TPC bits are used by the base station to command the mobile transceiver to increase or decrease the transmission power. TFCI bits are the indicators of slot format.The bit count shown in Figure 2-2 is the maximum possible number of data bits that can be transmitted in one slot. In a frame 15x 1 0 x 2 b~i ts can be transmitted in every slot, where k is an integer in the range from 0 to 7. The parameter k is related to the Spreading Factor (SF):
Thus the spreading factor SF may range from 512 down to 4.