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2 UMTS Air Interface 2.1 Introduction Universal Personal Communications (UPC) establishes the new concept of personal mobility and personal numbering [1]. In the UPC environment the fixed association between terminal and user identification is removed. This establishes the basis for personal mobility. Personal communications involves providing an essentially transparent connection so that a practical range of services can be automatically provided to people on the move [2]. Both wired and wireless access can, and should be involved, with existing infrastructures forming the basis of service delivery to a person rather than to a place. The goal of third-generation mobile systems is to provide users with world-wide coverage via handsets that have the capability to seamlessly roam between multiple networks (fixed and mobile, cordless and cellular) across regions, which currently use different technologies. This wireless and wired mobility clearly complements UPC, giving the user total mobility across both types of networks. Third-generation mobile systems are one step beyond the digital cellular and cordless systems that are now into service. At the global level regarding the third-generation of mobile systems, in ITU (International Telecommunication Union) there is already an initiative, IMT-2000 [3–7], settling the frame- work of the future telecommunication infrastructure. IMT-2000 will provide wireless access to the global telecommunication infrastructure through both satellite and terrestrial systems, serving fixed and mobile users in public and private networks. It is being developed on the basis of the ‘family of systems’ concept designed to be able to connect different radio transmission modules to the same core network equipment. The radio interfaces defined are based on different access technologies. The access technol- ogy not only defines how the users access the system. The structures defined in Figure 2.1 apparently define radio interfaces that are supported by two different access technologies, but in fact, what is defined, is a hybrid access technique. This is the case, for example, for IMT-TC (IMT-Time Code) where the uplink and downlink and the different users are separated based on transmission on different time slots and on the spreading sequences. Thus W-CDMA (Wide- band Code Division Multiple Access) is the access technique defined for three of the interfaces, IMT-DS (IMT-Direct Spread), IMT-MC (IMT-Multi Carrier) and IMT-TC. TDMA (Time Division Multiple Access) also supports IMT-TC, as has already been mentioned, IMT-SC (IMT-Single Carrier) and IMT-FT (IMT- Frequency Time). IMT-FT is also supported by a hybrid access technique based on FDMA (Frequency Division Multiple Access) and TDMA. Broadband Wireless Mobile: 3G and Beyond. Edited by Willie W. Lu Copyright  2002 John Wiley & Sons, Ltd. ISBN: 0-471-48661-2 There is a convergent effort towards the standardization of third-generation mobile systems that support ITU proposals that will accelerate the IMT2000 standardization activities. Differ- ent standards development organizations (SDOs), ARIB (Japan), CWTS (China), TIA (USA), TTA (Korea), TTC (Japan), T1 (USA) and ETSI (Europe) are participating in the development of these new standards. For that, consortiums and partnerships are being created among them whose results are standards for the radio interfaces (Figure 2.2). Broadband Wireless Mobile: 3G and Beyond12 Figure 2.1 Radio interfaces defined for IMT-2000. Figure 2.2 SDOs working for radio interfaces standardization. This chapter focuses on the radio interfaces defined with W-CDMA access technology: IMT-DS, IMT-TC and IMT-MC. The radio interfaces defined with W-CDMA are being developed thanks to the creation of the 3G partnership as a multilateral collaboration among SDOs, aiming to facilitate the development of global technical specifications for 3G mobile systems as an evolution of the present mobile architectures: GSM and ANSI/TIA/EIA-41. The 3G partnership is divided into two projects, 3GPP supported by the SDOs that are actually involved in specification of GSM systems and its evolution, and 3GPP2 that will comprise the ANSI/TIA/EIA-41 network evolution, involving the corresponding SDOs. The evolution of both technologies does not imply a convergence in the same solution, given that, in addition to the partnership projects, work should be done in the direction of setting the interoperability procedures in order that the ‘family concept’, which is the basis for IMT-2000, is a reality. This includes not only considering a harmonization in the air interface specifications, but also a definition of the network interfaces (Figure 2.3). The present mobile technologies are evolving in such a way that 3GPP is working towards the definition of IMT- DS and IMT-TC and 3GPP2 for IMT-MC. The radio interfaces are better known as UMTS- FDD, UMTS-TDD and cdma2000. 2.1.1 3GPP In December 1998 five market-driven SDOs, ARIB (Japan), ETSI (Europe), T1 (USA), TTA (Korea) and TTC (Japan), agreed to launch the 3rd Generation Partnership Project (3GPP) with the posterior incorporation of CWTS (China) in May 1999. The aim of this project is to cooperate for the production of globally applicable technical specifications for a 3rd genera- tion mobile system called Universal Mobile Telecommunication System (UMTS) [8], based on an innovative radio interface Universal Terrestrial Radio Access (UTRA) and evolution of the GSM core network. The technical specifications will be transposed into relevant standards by the participating SDOs using their established processes. UMTS Air Interface 13 Figure 2.3 Definition of architecture interfaces and interoperability. The technical specifications are focused on four main issues that comprise system and service aspects, terminals, the core network and the radio access network. In the first section of this chapter the UMTS radio access network is discussed in detail. 2.1.2 3GPP2 The partnership project 3GPP2 was launched in order to complement the evolution study of non-GSM systems. 3GPP2 is an effort by the International Committee of the American National Standards Institute’s (ANSI) to establish the evolution for ANSI/TIA/EIA-41 networks and their Radio Transmission Technologies (RTTs). 2.2 UMTS air interface The assumed UMTS architecture [9] defines three main functional entities: User Equipment (UE), UMTS Terrestrial Radio Access Network (UTRAN), and core network. The interfaces defined between the UE and the UTRAN is the radio interface (Uu) and the interface between the core network and the UTRAN is called Iu. The point to focus on are the interfaces: they have been clearly defined in order to set the interoperability of UEs of different providers with UTRANs from different telecommunica- tion operators. In particular, this chapter focuses on the radio interface. The radio interface is characterized through its protocols [9,10] where it can be defined by two main groupings according to the final purpose service: the user plane protocols and the control plane protocols. The first carry user data through the access stratum and the second is responsible for controlling the connections between the UE and the network and the radio access bearers. A general protocol architecture splits the radio interface in three layers: a physical layer or Layer 1, the data link layer (Layer 2) and the network layer or Layer 3. This hierarchical stratification provides a complete vision of the radio interface, from both the functionality associated with each of the structured layer to the protocol flow between them. The purpose of the protocol stack is to set the services to organize the information to transmit through logical channels whose classifying parameter is the nature of the informa- tion they carry (i.e. control or traffic information) and map these logical channels into trans- port channels whose characteristic is how and with what characteristic the information within each logical channel is transmitted over the radio interface. This how and with whatchar- acteristic means that for each transport channel there is associated one or more transport formats, each of them defined by the encoding, interleaving bit rate and mapping onto the physical channel. Each layer is characterized by the services provided to the higher layers or entities and the functions that supports them. Layer 1 [11],[12] supports all functions required for transmission of information on the physical medium offering information transfer services through the physical channels to higher layers. This includes preparing the transport channels to be sent through the physical medium, controlling hypothetical errors and measuring parameters related to the quality of the transfer service provided: frame error rate, signal to interference ratio, power measurements, etc. Layer 2 is subdivided into the medium access control (MAC) layer, the radio link control (RLC) layer, Packet Data Convergence Protocol (PDCP), and Broadcast /Multicast Control (BMC), each of them providing services to higher layers through their associated functions. Broadband Wireless Mobile: 3G and Beyond14 The MAC layer handles the transport channels, mapping the logical channels into transport formats and transferring to peer MAC entities protocol data units. The mapping is controlled by Layer 3 which is the one that determines the transport format set; consequently MAC layer should be able to select an appropriate transport format depending on the instantaneous source rate, the priority handling inside one UE and between UEs. If any multiplexing of protocol data units (PDUs) into transport blocks are needed it should be performed by the MAC layer since this functionality is not performed by Layer 1. RLC provides the transference of higher PDUs to the receiving entity and transfer of user data with quality of service settings. The transfer can be achieved in three modes: transparent, unacknowledged and acknowledged, performing for that segmentation, reassemble, conca- tenation and padding. PDCP provides transmission of higher PDUs in acknowledged, unacknowledged and transparent RLC mode, mapping the network protocol into an RLC entity. BMC provides a broadcast/multicast transmission service in the user plane for common user data; the broadcast service is supported by a scheduled transmission of cell broadcast messages handling the radio resources needed. Layer 3 is the interface between the access stratum of the radio interface and the non-access stratum. It is subdivided into the radio resource control (RRC) layer which interfaces with Layer 2 and an upper layer looking after providing access service to higher layers in the non- access level. Layer 3 provides three different types of services: general control services, notification services and dedicated control services. The RRC layer handles the control plane signalling providing measurements reports and radio resource assignments to peer RRC layers and controlling through feedback information RLC, MAC and physical layers. Figure 2.4 refers. Given this brief introduction to the radio interface, next we describe in detail each of the components. 2.2.1 Layer 1 The physical layer (L1) access scheme is based on Wideband Direct-Sequence Code Division Multiple Access (WCDMA) technology with two duplex modes: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). In the FDD mode a physical channel is character- ized by the code and frequency. Additionally, in the uplink a physical channel is defined by the relative phase, i.e. if the physical channel is mapped in the quadrature or phase component of the QPSK modulation. In TDD mode a physical channel is characterized by the code and time slot. The chip rate is 3.84 Mchips/s, but a fixed chip rate does not imply a fixed service bit rate. Different symbol rates can be specified for each physical channel applying different spreading factors (SF) to each symbol. Table 2.1 presents the possible symbol rates for both duplex modes. L1 offers data transport services to higher layers. It has two open interfaces: one with MAC layer through transport channels and the other with RRC layer that controls the configuration of the physical layer. Each transport channel is characterized by its transport format set. To each transport format a physical processing is applied to define the physical channel. A physical channel is defined by a carrier frequency, channelization code, scrambling code, time interval (starting and stopping transmission time) and relative phase (uniquely in uplink) and, additionally in TDD, the timeslot and burst type. UMTS Air Interface 15 The physical layer operates with a hierarchical structure with a basic time interval of 10 ms called a radio frame which is subdivided into 15 slots. The interpretation of each slot is different for each of the duplex modes. In FDD within the whole frame is processed the basic Broadband Wireless Mobile: 3G and Beyond16 Figure 2.4 Protocol Stack for the Radio Interface. 3GPP TSs and TRs are the property of ARIB, CWTS, ETSI, T1, TTA and TTC who jointly own copyright in them. They are subject to further modification and are therefore provided to you ‘as is’ for information purposes only. Further use is strictly prohibited. Table 2.1 Service symbol rates FDD TDD Uplink Downlink Uplink Downlink SF Symbol rate (ks/s) SF Symbol rate (ks/s) SF Symbol rate (ks/s) SF Symbol rate (ks/s) Min. 4 960 4 960 1 3840 1 3840 Max. 256 15 512 7.5 16 240 16 240 unit provided by MAC, the transport block, or several transport blocks, and within each slot a similar substructure is applicable. In TDD each slot applies not only to determine the dupli- city needed for uplink and downlink, but also to separate different users. This means that, unlike FDD, in TDD not only the code domain is being used to separate different users, but also the time slot. This would somehow justify the high rates provided in TDD mode, since in this mode the SF does not determine the net physical channel symbol rate, as each physical channel is not being transmitted in every slot and consequently its average transmission rate is decreased. The advantages of TDD vs. FDD are that TDD presents an optimal implementation of asymmetrical services, where the bandwidth requirements are tighter in the downlink than in the uplink. 2.2.1.1 FDD In FDD mode, uplink and downlink transmissions use separate frequency bands. Next, four main points are treated in order to describe the flow of information from high layers to L1 and how this information is finally mapped into a physical channel. Logical, transport and physical channels Logical channels, which are organized based on what type of information is transferred, are mapped into transport channels through the MAC layer. Given that the classification of trans- port channels is based on how the information is transferred, there is not a univocal matching between them. This is clearer for example observing that the attribute ‘common’ and ‘dedicate’ appears in both logical channels and transport channels, but sometimes a logical dedicated channel is mapped to a common transport channel and vice versa. The explanation for that is that a logical dedicated channel contains information for a particular user and can be carried on a common transport channel together with the information of other users. One step further is the mapping from transport channels to physical channels. Physical channels are not only deter- mined by how the information is transmitted but also the type of information transmitted. The main grouping for transport channels is related to the exclusive use that is made of this channel; for that we consider dedicated channels (DCH) and common channels. The common channels are subdivided into Broadcast Channel (BCH), Forward Access Channel (FACH), Paging Channel (PCH), Random Access Channel (RACH), Common Packet Channel (CPCH) and Downlink Shared Channel (DSCH). Two logical channels are mapped into the DCH transport format, the Dedicated Traffic Channel (DTCH) and the Dedicated Control Channel (DCCH). The first is used for the transfer of user information and the second for control information. The DCH is an uplink or downlink channel that can be transmitted over the entire cell or over just part of the cell using beamform- ing. The DCH would set the transport format for the associated physical channels. Associated with the DCH, in the uplink there are Dedicated Physical Data Channel (DPDCH) and the Dedicated Physical Control Channel (DPCCH) corresponding to a mapping in the quadrature (Q) and phase (I) component, respectively, of the QPSK modulation. In the downlink, the physical channel is not characterized by the phase modulation in the QPSK so there is just one Downlink Dedicated Physical Channel (Downlink DPCH) where both control information and data information are multiplexed in time within the radio frame. The control information that is UMTS Air Interface 17 transported in the dedicated physical channels is generated at L1. It consists of known pilots for channel estimation, transmit power control commands, and feedback information. RACH channel is an uplink transport channel that is always received from the entire cell. Its main characteristic is that it is characterized by an initial collision risk and open loop power control. This main characteristic will be useful for the transportation of different logical channels: Dedicated Control Channel (DCCH), Common Control Channel (CCCH) and Dedicated Transport Channel (DTCH). The RACH is mapped into the Physical RACH (PRACH) based on a slotted ALOHA with fast acquisition indication. The random access transmission can be started at defined time intervals and it consists of one or several pream- bles of 4096 chips and the message part of duration one or two radio frames. The structure of the message part is similar to the dedicated physical channel, since the type of information they transport is the same; what changes is the way the radio resource is accessed. As in the uplink dedicated physical channels, the data information is mapped in the I component and the control information inserted in the Q component by L1. CPCH carries information from DTCH and DCCH logical channels. It is an uplink trans- port channel associated with a downlink dedicated channel and characterized by the initial collision risk and inner loop power control. The physical channel with the transport format associated with the CPCH channel is the Physical CPCH (PCPCH). The transmission is based on CSMA-CD (Carrier Sense Multiple Access-Collision Detection). As in PRACH the transmission can be started at defined time intervals and the transmission structure is one or several preambles, a collision detection preamble, a power control preamble and N multi- ples of the frame duration containing the message. The logical channel that broadcasts system control information and cell specific informa- tion is the BCCH (Broadcast Control Information). One of the transport channel that carries this information is the BCH, which main characteristic is that it is always transmitted over the entire cell with a single transport format. The physical channel associated is the Primary Common Control Physical Channel (P-CCPCH). The transmission rate is fixed within this channel with a SF ¼ 256. The radio frame structure defined by frames of 10 ms divided in 15 slots is generated here by multiplexing in time the primary and secondary Synchronization Channel (SCH). The P-CCPCH is transmitted in every slot in the last nine symbols of each slot. The other transport channel that carries BCCH information is the FACH. But the transport characteristics of this channel, transmission over the entire cell or over part using beamform- ing, the possibility of fast rate change (each 10 ms) and slow power control, match the requirements for the transportation of many other logical channels: DCCH, CCCH, Common Traffic Channel (CTCH) and DTCH. FACH mapping into a physical channel is made to the Secondary Common Control Physical Channel (S-CCPCH) that is described next. A special control common logical channel is the Paging Control Channel (PCCH). It is a downlink channel that transmits paging information when the network does not know the cell location of the UE or the UE is in connect mode using sleep mode procedures. This channel is mapped to the PCH transport channel which at the physical level corresponds to S-CCPCH. Both transport channels have similar characteristics although the PCH is always transmitted over the entire cell and the FACH could be transmitted over just part of the cell. The S- CCPCH has a frame structure based on the 10 ms frame with 15 slots. Within each slot data Transport Format Combination Indication (TFCI) field and pilot files are inserted and that is the key for the main difference between the P-CCPCH and the S-CCPCH; the latter supports Broadband Wireless Mobile: 3G and Beyond18 the different transport format necessary for the changing rate while the primary transport format is fixed. The DSCH is a downlink transport channel shared by different UEs. It is associated with one or several DCH and it carries information from DTCH and the DCCH. The DSCH could be transmitted over the entire cell or over part of it using beamforming. The associated physical channel is the PDSCH. The PDSCH is mapped in the 10 ms radio frame but in a radio frame different PDSCHs can be allocated using the channelization codes that are described in the corresponding section. It does not carry control information, but to indicate that there is information in the PDSCH for a UE either it uses the TFCI of the associated DPCH or higher layer signalling carried in the DPCH. The same way that the mapping between logical channels and transport channel is not biunivocal, there are some physical channels that do not have higher layer associated chan- nels, i.e. there is not a transport or logical channel that is mapped into them. Those channels are: Common Pilot Channel (CPICH), Synchronization Channel (SCH), Acquisition Indicator Channel (AICH), Paging Indicator Channel (PICH), Access Preamble Acquisition Indication Channel (AP-AICH), CPCH Status Indicator Channel (CSICH) and Collision-Detection/Channel Assignment Indicator Channel (CD/CA-ICH). All of them are downlink channels that support functionalities associated explicitly with L1. The CPICH is used for transmission diversity to provide the UE with a channel where measurements of the channel state can be performed and later fed back to the base station. The SCH is used for cell search, synchronization purposes and defining the scrambling codes for downlink channels. The functionality is performed through two sub-channels. The primary synchronization channel caries a primary code that is repeated within the associated radio frame structure in every slot. This code is the same for every cell in the system. The secondary channel carries a secondary spread over the whole frame. This secondary code identifies the scrambling sequence for the downlink channels. The synchronization can be made on a frame basis using the secondary code and on a slot basis using the primary. The AICH, AP-AICH and CD/CA-ICH are used to support the uplink channels with random access, PRACH and PCPCH. CSICH is used to carry CPCH status information and is directly associated with the AP-AICH. PICH carries paging indicators for the S- CCPCH. In Figure 2.5 all the mapping from logical channels to physical is detailed, specifying which channels are downlink, which uplink and which could be both. Multiplexing and coding of transport channels The multiplexing and channel coding functionality is a combination of error detection, error correction, rate matching, interleaving and multiplexing [30]. This functionality supports partly the transport services offered to MAC. Information arrives at the multiplexing and coding unit in transmission time intervals of 10 ms, 20 ms, 40 ms or 80 ms, in basic proces- sing units called transport blocks. The process the information follows until is ready to be sent through the channel can be clearly differentiated into two main parts. First is the processing associated with each transport channel, i.e. setting the information with the corresponding transport format; here the information is coded, and arranged in order to provide the next step, multiplexation, with uniform information coming from every transport channel. The multi- UMTS Air Interface 19 plexation is made over different transport channels. A code-composite transport channel (CCTCH) is generated and this would be mapped to one or different physical channels. The steps followed to set the transport format differ when referring to the uplink or down- link. There are three steps in the process that are common for both: Cyclic Redundancy Code (CRC), Transport Block Concatenation/Code Block Segmentation, and Channel Coding. In the uplink the following processes apply: radio frame equalization, first interleaving, radio frame segmentation and rate matching. In the downlink, after channel coding rate matching, first insertion of Discontinuous Transmission (DTX) indication bits, first interleaving and radio frame segmentation are performed. There are two main differences between uplink and downlink. The first is that, in the downlink, the transmission does not necessarily have to be continuous; discontinuous transmission bits can be inserted to determine in which time intervals there is no transmission. The second is that, in the uplink, the rate matching is made once first interleaving and radio frame segmentation is performed. In the multiplexing part the steps are transport channel multiplexing, second insertion of DTX indication bits (only in downlink), physical channel segmentation, second interleaving and physical channel mapping. In the CRC attachment a number of bits that are specified by higher layers are inserted in order to provide the transport block with a redundancy check. The number of bits inserted belongs to the set {0, 8, 12, 16, 24} and they are obtained through four different cyclic generator polynomials, respectively. Once the CRC is performed the serial concatenation of all the transport blocks to be processed might not give a total length adequate for the next step, coding. The channel coding defines a maximum size for the block to be coded (code block), depending on the coding that would be used. The possibilities are, convolutional coding with a maximum code block of 504 bits, turbo coding with a maximum code block of 5114 bits and no coding. If the Broadband Wireless Mobile: 3G and Beyond20 Figure 2.5 Mapping of logical channels into physical channels. [...]... coding scheme The following channel coding schemes can be applied to transport channels: † convolutional coding † turbo coding † no coding The coding schemes applied are the same as for the FDD mode already described Radio frame size equalization is padding the input bit sequence in order to ensure that the output can be segmented in Fi data segments of the same size The input bit sequence to the radio... corresponding maximum code block length, segmentation will be performed obtaining a number of code blocks of equal length Which of the possible coding schemes is used depends on the transport channel that is being processed (Table 2.2) Table 2.2.2 Coding schemes for transport channels in FDD Transport channel BCH PCH RACH FACH CPCH DSCH DCH Coding scheme Convolutional coding Turbo coding No coding Coding... information from different sources; in UMTS these sources are either different users, different channels associated with different users or base stations The spreading process in UMTS is divided into two main parts The first is the channelization operation where the bit sequence is spread with orthogonal sequences that preserves the orthogonality between sequences even when the SF are different The second... carrying dedicated control or traffic data † The Downlink Shared Channel (DSCH) is a downlink transport channel shared by several UEs carrying dedicated control or traffic data Indicators Indicators are a means of fast low-level signalling entities which are transmitted without using information blocks sent over transport channels The meaning of indicators is implicit to the receiver The indicator currently... the paging indicator Physical channels All physical channels take hierarchical structure with respect to timeslots, radio frames and system frame numbering (SFN) Depending on the resource allocation, the configuration of radio frames or timeslots becomes different All physical channels need guard symbols in every timeslot The time slots are used in the sense of a TDMA component to separate different user... transmitted using different channelization codes Operation with a single code with spreading factor 1 is possible for the downlink physical channels The spreading factors that may be used for uplink physical channels range from 16 down to 1 For each physical channel an individual minimum spreading factor SFmin is transmitted by means of the higher layers There are two options that are indicated by UTRAN:... each radio frame Thus, beacon channels must be present in each radio frame UMTS Air Interface 29 Multiplexing, channel coding and interleaving Data stream from/to MAC and higher layers (transport block/transport block set) is encoded/ decoded to offer transport services over the radio transmission link [33] The channel coding scheme is a combination of error detection, error correcting (including rate... randomizes the bit position of the different transport channels The last step is the physical channel mapping, at this point the control information needed by L1 for channel estimation TFI, etc is inserted and spreading and modulation is made accordingly to each physical channel characteristic Spreading and modulation FDD mode characterizes spreading and modulation distinguishing between uplink and... in one radio frame for the respective PhCH The bits wp,k are mapped to the PhCHs so that the bits for each PhCH are transmitted over the air in ascending order with respect to k Different transport channels can be encoded and multiplexed together into one Coded Composite Transport Channel (CCTrCH) There are two types of CCTrCH: † CCTrCH of dedicated type, corresponding to the result of coding and multiplexing... successive in-sync indications Node B shall indicate that the CCTrCH has re-established synchronization and the CCTrCH’s state shall be changed to the in-sync-state The specific parameter settings (values of T_RLFAILURE, N_OUTSYNC_IND, and N_INSYNC_IND) are configurable Discontinuous transmission (DTX) of radio frames Discontinuous transmission (DTX) is applied in up- and downlink individually for each . 21 Table 2.2.2 Coding schemes for transport channels in FDD Transport channel Coding scheme Coding rate Convolutional coding Turbo coding No coding CC TC BCH. radio access network. In the first section of this chapter the UMTS radio access network is discussed in detail. 2.1.2 3GPP2 The partnership project 3GPP2