Multiple Access Protocols for Mobile Communications: GPRS, UMTS and Beyond Alex Brand, Hamid Aghvami Copyright  2002 John Wiley & Sons Ltd ISBNs: 0-471-49877-7 (Hardback); 0-470-84622-4 (Electronic) 10 PACKET ACCESS IN UTRA FDD AND UTRA TDD In this chapter, first a brief introduction to UMTS Terrestrial Radio Access (UTRA) matters is provided, such as fundamental radio access network concepts, basics of, for example, the physical and the MAC layer, and the types of channels defined (namely logical, transport and physical channels). This is followed by a discussion of certain UTRA FDD features, such as soft handover, fast power control and compressed mode operation. The main focus is on the mechanisms that are available for packet access on UTRA FDD and UTRA TDD air interfaces, as provided by release 1999 of the 3GPP specifications. Improvements being considered for further releases, currently mostly dealt with in 3GPP under the heading of High Speed Downlink Packet Access (HSDPA), are also discussed. For more general information on UMTS, the reader is referred to dedicated texts such as Reference [86]. 10.1 UTRAN and Radio Interface Protocol Architecture 10.1.1 UTRAN Architecture The UMTS terrestrial radio access network (UTRAN) consists of one or more Radio Network Subsystems (RNS), which in turn are composed of a Radio Network Controller (RNC) and multiple base stations. In UMTS terminology, base stations are referred to as node B ; in the following, both terms will be used. A single node B may serve one or more cells (e.g. different sectors served from one site). A node B is connected to its RNC via the I ub interface. The RNC is said to be the Controlling RNC (CRNC) of that node B. The RNC is connected to the Core Network (CN) via the I u interface (see Figure 10.1). To be precise, two variants of the latter are discerned, namely I u -CS, which provides the connection to the circuit-switched core network (i.e. to an MSC), and I u -PS, providing the connection to the packet-switched core network (i.e. to an SGSN). Equivalent interfaces in GSM are the A bis interface between a BTS and a BSC, the A interface between BSC and MSC, and the G b interface between BSC and SGSN. Compared to GSM, UTRA FDD supports two new handover types, namely soft handover and softer handover. In both cases, communication between a mobile terminal and the network takes place over two (or more) air interface channels concurrently. With softer handover, the two channels are associated with two different sectors served 350 10 PACKET ACCESS IN UTRA FDD AND UTRA TDD U u I ub I ur Node B Node B Node B Node B RNC RNC I u -PS I u -CS MSC/ VLR SGSN RNS RNS CN UE Figure 10.1 The UTRAN architecture by the same node B, which has only ‘local’ implications not affecting the fundamental UTRAN architecture. During soft handover, instead, the mobile terminal is connected to the network via multiple node Bs, which may not all be controlled by the same RNC. In this case, a means for communication between RNCs is required, which is the main reason why a new interface is defined in UMTS to connect two RNCs, namely the I ur interface. Being connected to multiple cells served by different antenna sites allows one to benefit from so-called macro-diversity, a technique which improves the transmission quality and helps, together with fast power control, to combat the near-far problem typical of CDMA systems. One RNC, the Serving RNC (SRNC), must ensure that the right signals are sent by the relevant node Bs on the downlink, and must combine the signals from multiple node Bs on the uplink in order to deliver only one signal stream onwards to the core network. If node Bs involved in the soft handover are controlled by other RNCs, then these are referred to as Drift RNC (DRNC). For further information on this subject, the reader is referred to 3GPP technical report 25.832 [276] on handover manifestations. The UTRAN architecture is shown in Figure 10.1. This figure shows also the so-called User Equipment (UE), which is the combination of a mobile terminal or Mobile Equipment (ME) with a Universal Subscriber Identity Module (USIM), the UMTS version of the well know GSM SIM. The radio interface, that is the interface between UE and node B, is denoted U u . In the following, we stick to the terminology known from GSM, i.e. we continue to refer to a UE as a mobile terminal or mobile station (MS). 10.1.2 Radio Interface Protocol Architecture As in GSM, three layers are relevant for the radio interface, namely the physical layer (layer 1 or PHY), the data link layer (layer 2) and the network layer (layer 3), the last two featuring several sub-layers. However, in contrast to the rather confusing situation in GSM depicted in Figure 4.3, the radio interface protocol architecture has been rationalised in UMTS — at least as far as terminology is concerned. The lowest three (sub-)layers 10.1 UTRAN AND RADIO INTERFACE PROTOCOL ARCHITECTURE 351 are uniformly referred to as physical layer, MAC, and RLC, the last two being sub- layers of layer 2. In the so-called control-plane or C-plane dealing with signalling, the Radio Resource Control (RRC) sits on top of the RLC. The RRC is the lowest sub- layer of layer 3, and is the only sub-layer of layer 3 fully associated with and terminated in the UTRAN. In the user-plane or U-plane, additional sub-layers may be required at layer 2 depending on the services supported, namely the Packet Data Convergence Protocol (PDCP) in the ‘packet domain’, which replaces the LLC and the SNDCP known from GPRS, and the Broadcast/Multicast Control Protocol (BMC). The UMTS protocol architecture is illustrated in Figure 10.2. As shown, the PHY offers its services to the MAC in the shape of transport channels, and the MAC to the RLC in that of logical channels. A transport channel is characterised by how the information is transferred over the radio interface, while a logical channel by the type of information transferred. This distinction is not made in GSM, where the PHY offers logical channels to the upper layers. Layer 2 provides radio bearers to higher layers. The C-plane radio bearers provided by the RLC to the RRC are signalling radio bearers. The RRC interfaces not only the RLC, but also all other layers below it for control purposes, quite like RR in GSM. For more information on the radio interface protocol architecture, the reader is referred to 3GPP technical specification 25.301 [277]. One reason why the GSM protocol architecture is somewhat confusing is that the system was designed initially for circuit-switched services, in particular voice, so MAC and RLC with associated header overheads were not really required at first and only added later for GPRS. In UMTS instead, for consistency, MAC and RLC are always defined, but they can both be operated in different modes, depending on what MAC and RLC features are U-plane radio bearers PHY RLC MAC RRC BMC PDCP Signalling radio bearers Control-plane User-plane Transport channels Logical channels Control L1 L2 L3 Figure 10.2 Protocol architecture on the radio interface 352 10 PACKET ACCESS IN UTRA FDD AND UTRA TDD required for a specific service. For instance, when no MAC header is required, the MAC operates in transparent mode. Before delving into some of the details pertaining to PHY, MAC, and RLC, let us reiterate a definition hidden in a footnote in Chapter 4 and add a new one, both listed in Reference [213]. A Protocol Data Unit (PDU) of protocol X is the unit of data specified at the X-protocol layer consisting of X-protocol control information and possibly X- protocol layer user data. A Service Data Unit (SDU) of protocol X is a certain amount of information whose identity is preserved when transferred between peer (X + 1)-layer entities and which is not interpreted by the supporting X-layer entities. In simple terms, taking as an example layer X to be the MAC and X + 1 the RLC, a MAC PDU is composed of a MAC header and an RLC PDU. From a MAC perspective, the RLC PDU represents the MAC SDU. 10.1.3 3GPP Document Structure for UTRAN The 3GPP Technical Specifications (TS) relevant for UTRAN are the 25-series of spec- ifications. Documents numbered 25.1xy deal with radio frequency matters, 25.2xy with the physical layer of the air interface, 25.3xy with radio layers 2 and 3 (i.e. MAC, RLC and RRC) and 25.4xy with the radio access network architecture. Additional information can be found in Technical Reports (TR) numbered 25.8xy and 25.9xy. The information presented in the following was mostly derived from 25.2xy and 25.3xy documents, in some cases complemented by 25.8xy and 25.9xy reports, as referenced in the text. For further information on the 3GPP document structure, refer also to the appendix. 10.1.4 Physical Layer Basics 10.1.4.1 Physical Layer Functions The physical layer performs numerous functions as listed in TS 25.201 [278]. Among them are: • macro-diversity distribution/combining and soft handover execution; • FEC encoding/decoding of transport channels, error detection on transport channels and indication of errors to higher layers; • multiplexing of transport channels onto so-called Coded Composite Transport CHan- nels (CCTrCH) at the transmit side, demultiplexing from CCTrCHs to transport chan- nels on the receive side; • mapping between CCTrCHs and physical channels; • modulation/spreading and demodulation/despreading of physical channels; • frequency and time synchronisation, the latter on the level of chips, bits, slots, and frames; • measurement of radio characteristics including FER, SIR, interference power, etc., which are then reported to higher layers; and • inner or closed-loop power control. 10.1 UTRAN AND RADIO INTERFACE PROTOCOL ARCHITECTURE 353 10.1.4.2 Basic Multiple Access Scheme and Physical Channels The basic multiple access scheme employed in UTRA is direct-sequence code-division multiple access (DS-CDMA), with information spread over approximately 5 MHz of bandwidth, which is why this scheme is also referred to as wideband CDMA (WCDMA). Two duplex modes are supported, namely frequency-division duplex (FDD) and time- division duplex (TDD), the basic multiple access scheme of the latter also referred to as TD/CDMA. In both cases, a 10 ms radio frame is divided into 15 regular slots, at a chip-rate of 3.84 Mchip/s each slot measuring 2560 chips. The UTRA modulation scheme is quadrature phase shift keying (QPSK). In UTRA FDD, a double-length (i.e. 5120 chips) access slot format is also defined, with 15 access slots fitting into two radio frames. The physical layer makes use of physical channels for the delivery of data over the air interface. In FDD mode, a physical channel is characterised by the code, the frequency and in the uplink also the relative phase, either I for in-phase,orQforquadrature-phase. In TDD mode, in addition, the physical channel is also characterised by the time-slot. UTRA supports variable Spreading Factors (SF): • UTRA FDD from 256 to 4 on the uplink and from 512 to 4 on the downlink; • UTRATDDfrom16to1oneitherlink. Accordingly, the information rate of the channel is also variable. Signals are first spread using channelisation codes, after which a scrambling code is applied at the same chip-rate as the channelisation code; hence scrambling does not alter the signal bandwidth. This is illustrated in Figure 10.3. Channelisation codes are used to separate channels from the same source (i.e. on the downlink different channels in one sector or cell, on the uplink different dedicated channels sent by one mobile terminal). Scrambling codes are used to separate signals from different sources. The channelisation codes are based on the Orthogonal Variable Spreading Factor (OVSF) technique, which allows mutually orthogonal codes to be chosen from a code- tree, even when codes for different spreading factors are used simultaneously. It indeed makes sense to invest some effort in choosing orthogonal codes to separate channels from the same source. In ‘benign’ propagation conditions, in fact, this orthogonality is largely maintained at the receiving side. The number of codes available per tree is fairly limited though; it is equal to the spreading factor if all codes use the same spreading factor. An example with spreading factors from one (root of the tree) to eight is shown in Figure 10.4. The leaves of the tree at SF = 8 represent the available codes, if only SF = 8isusedina cell. However, if a code at SF = 2 is assigned, then the tree is essentially pruned at that Channelisation code Scrambling code Data Bit rate Chip rate Chip rate Figure 10.3 SpreadingandscramblinginUTRA 354 10 PACKET ACCESS IN UTRA FDD AND UTRA TDD 'Pruned' sub-tree Assigned code at SF = 2 Root of tree Cannot be used when code below root assigned SF = 1 SF = 2 SF = 4 SF = 8 Figure 10.4 Example of an OVSF code tree code, codes with higher spreading factors in the sub-tree below that specific code are not available anymore. More precisely, a code can be assigned to a mobile terminal if and only if no other code on the path from that code to the root of the tree or in the sub-tree below that code is assigned [279]. Without introducing special measures such as very tight synchronisation between different users, the orthogonality of signals sent by different sources would be lost at the receiving side even if orthogonal codes were selected at the transmitting side. This is why rather than orthogonality, other criteria such as the number of available codes and their auto-correlation properties were more important for the choice of suitable scrambling codes. In UTRA FDD, there are two types of scrambling codes, Gold codes with a 10 ms period (i.e. 38 400 chips) and so-called extended S(2) codes with a period of 256 chips, the latter optional and only applicable on the uplink. In UTRA TDD, the code-length of scrambling codes is 16. For details on modulation and spreading, refer to TS 25.213 [280] (for FDD) and to TS 25.223 [281] (for TDD). 10.1.4.3 Transport Channels offered by the Physical Layer to the MAC Various types of transport channels are offered by the PHY to the MAC. Transport chan- nels are unidirectional channels. They can be classified into two groups, namely: • common transport channels, where there is a need for inband identification of mobile terminals if a particular terminal is to be addressed; and • dedicated transport channels, where, by virtue of a channel being dedicated to a particular communication, the terminal is identified by the physical channel it uses. Common transport channels supported in R99 are: • the Random Access CHannel (RACH) on the uplink; 10.1 UTRAN AND RADIO INTERFACE PROTOCOL ARCHITECTURE 355 • the Forward Access CHannel (FACH) on the downlink; • the Downlink Shared CHannel (DSCH); • the Common Packet CHannel (CPCH) on the uplink, only defined for UTRA FDD; • the Uplink Shared CHannel (USCH), only defined for UTRA TDD; • the Broadcast CHannel (BCH) on the downlink; and • the Paging CHannel (PCH), also on the downlink. There is only one type of dedicated transport channel defined in R99, namely the Dedicated CHannel (DCH). 10.1.4.4 Transport Channel Characteristics The basic information unit delivered by the MAC on a transport channel to the physical layer is a transport block. Every so-called Transmission Time Interval (TTI), the MAC delivers either one or a set of transport blocks to the PHY for a given transport channel. Within a transport block set, all transport blocks are equally sized (but the block size can change from TTI to TTI). The TTI can assume integer multiples of the minimum interleaving period, which is 10 ms. More precisely, possible values are 10, 20, 40 or 80 ms. The TTI determines the interleaving depth, hence robustness against fading can be adjusted according to the delay constraints of the service to be supported. The characteristics of a given transport channel are determined by its transport format, with attributes such as the transport block size, the number of transport blocks in a transport block set, the TTI, the error protection scheme to be applied (type and rate of channel coding), and the size of the CRC. Transport channel characteristics can be defined in terms of a transport format set. Some of the transport format attributes, such as those regarding the error protection scheme, must be the same within a transport format set. However, different transport block set sizes, optionally even different transport block sizes, can be chosen for transport formats within a transport format set. These two parameters affect the instantaneous bit-rate, and thus provide the means for a transport channel to support variable bit-rates. At every TTI, the MAC delivers the transport block set for a given transport channel to the PHY with the Transport Format Indicator (TFI) as a label, which indicates the transport format picked by the MAC from the transport format set. Layer 1 can multiplex one or several transport channels onto a coded composite trans- port channel, each of them with its own transport format picked from its transport format set. However, not all possible permutations of these combinations are allowed. Rather, only a set of authorised Transport Format Combinations (TFC) may be used so that, for instance, the maximum instantaneous bit-rate of all transport channels added together can be limited. On the transmit side, the physical layer builds the Transport Format Combi- nation Identifier (TFCI) from the individual TFIs, which is then appended to the physical control signalling. This is illustrated in Figure 10.9 provided in the next section on UTRA FDD. By decoding the TFCI on the physical control channel, the receiving side has all the parameters needed to decode the information on the physical data channels and deliver them to the MAC in the format of the appropriate transport channels. In UTRA FDD, if only a limited set of transport format combinations is used, then the receiving side may be in a position to perform blind detection, in which case TFCI signalling may be omitted. 356 10 PACKET ACCESS IN UTRA FDD AND UTRA TDD More details on these matters including suitable illustrations are provided in the next few sections. Further information can also be found in TS 25.302 [282]. 10.1.5 MAC Layer Basics 10.1.5.1 MAC Layer Functions The MAC layer is specified in TS 25.321 [283]. Functions performed by the MAC include: • the mapping between logical channels and transport channels; • the selection of appropriate transport formats for each transport channel depending on the instantaneous source rate; • various types of priority handling, be this between data flows from one terminal or from different terminals; • the identification of mobile terminals on common transport channels; and • the multiplexing of higher layer PDUs onto transport blocks to be delivered to the PHY on the transmitting side and demultiplexing of these PDUs from transport blocks delivered from the PHY on the receiving side. Regarding the selection of appropriate transport formats (within the transport format sets defined for each transport channel), note that the assignment of transport format combination sets is done at layer 3. Therefore, the MAC has only a limited choice of transport formats, namely from the permitted combinations contained in the transport format combination set. 10.1.5.2 Logical Channels offered by the MAC to the RLC Logical channels can be classified into two groups, namely control channels for the transfer of C-plane information, and traffic channels for the transfer of U-plane informa- tion. Except for the last one, the types of control channels defined for UMTS R99 will be familiar in name from GSM: • the Broadcast Control CHannel (BCCH), a downlink channel used for broadcasting system control information; • the Paging Control CHannel (PCCH), a downlink channel used to transfer paging information, when the network does not know the MS location at cell level or when the MS is in sleep mode; • the Common Control CHannel (CCCH), a bi-directional channel used for transmitting control information; • the Dedicated Control CHannel (DCCH), a point-to-point bi-directional channel used for the transmission of dedicated control information between an MS and the network; and • the SHared Channel Control CHannel (SHCCH), a bi-directional channel defined for UTRA TDD only, which is used to transmit control information between MS and network relating to shared uplink or downlink transport channels. 10.1 UTRAN AND RADIO INTERFACE PROTOCOL ARCHITECTURE 357 Two types of traffic channels are distinguished: • the Dedicated Traffic CHannel (DTCH), a point-to-point uplink or downlink channel dedicated to one MS for the transfer of user information; and • the Common Traffic CHannel (CTCH), a point-to-multipoint unidirectional (downlink only) channel used for the transfer of dedicated user information for all or a group of specified mobile terminals. It is important to note that the DTCH can be mapped onto dedicated or common transport channels. This is owing to the distinction mentioned earlier between the type of information transferred (as defined by the logical channel, here the DTCH) and how the information is transferred over the radio interface at the level of transport channels. 10.1.5.3 Types of MAC Entities and MAC Modes Three different types of MAC entities are distinguished in TS 25.321, which handle different types of transport channels, namely: • the MAC-b handling the BCH (hence b for broadcast), at the network side, it is situated at the node B; • the MAC-c/sh handling all other common (or shared) transport channels, namely the DSCH, CPCH, FACH, PCH, RACH, and USCH; it is situated at the controlling RNC; and • the MAC-d handling the only dedicated transport channel defined, namely the DCH. The MAC-d is situated at the serving RNC. When logical channels of dedicated type are mapped onto common transport channels, then the MAC-d, which provides these logical channels to the RLC, must interact with the MAC-c/sh, e.g. pass data to be transmitted through common transport channels on to the MAC-c/sh. Obviously, the mobile terminal must support all different types of MAC entities. Certain MAC features are not always required. For instance, inband identification of mobile terminals through a suitable identity contained in a MAC header are, with a few exceptions, only required when a dedicated logical channel is mapped onto a common transport channel. The case where no MAC header is required is referred to as transparent MAC transmission in TS 25.301. 10.1.6 RLC Layer Basics The RLC provides three types of data transfer services to higher layers, namely trans- parent, unacknowledged, and acknowledged data transfer. In the case of transparent data transfer, higher layer PDUs are transmitted without adding any protocol information (e.g. RLC headers). In this transfer mode the ‘RLC barely exists’, although RLC segmentation and reassembly functionality may be used in transparent RLC mode. Unacknowledged data transfer means that higher layer PDUs are transmitted without guaranteeing delivery to the peer entity. However, the RLC performs error detection and delivers only SDUs free of transmission errors to higher layers. Finally, acknowledged data transfer implies 358 10 PACKET ACCESS IN UTRA FDD AND UTRA TDD error-free transmission (to the extent possible within specified delay limits, etc.). This is achieved by applying appropriate ARQ strategies. Both RLC acknowledged mode and unacknowledged mode imply the addition of RLC headers to higher layer SDUs. 10.2 UTRA FDD Channels and Procedures 10.2.1 Mapping between Logical Channels and Transport Channels All transport channels and logical channels listed in Subsections 10.1.4 and 10.1.5 respec- tively are defined for UTRA FDD, with the exception of the USCH and the SHCCH, which are only defined for UTRA TDD. The possible mapping between UTRA FDD logical channels and transport channels is depicted in Figure 10.5. As pointed out in the previous section, the DTCH can be mapped onto common or dedicated transport chan- nels, hence onto the RACH, the CPCH, the DSCH, the FACH and the DCH (the first two obviously only in uplink direction, the DSCH and the FACH only in downlink direction). More than one DTCH can be mapped onto a single DCH, but different DTCHs can also be mapped onto different DCHs, depending on how the relevant radio bearers are configured. 10.2.2 Physical Channels in UTRA FDD AUTRAFDDphysical channel is characterised by the code, the frequency and in the uplink also the relative phase, either I for in-phase,orQforquadrature-phase.More precisely, the uplink modulation is a dual-channel QPSK, which means separate BPSK modulation of different channels on I-channel and Q-channel. Downlink modulation is ‘proper’ QPSK (i.e. a single channel is modulated onto both in-phase and quadrature phase). It means that the symbol-rate of an up- and a downlink channel at a given spreading factor are the same, but that the downlink physical channel bit-rate is double that of the uplink physical channel, for example 30 kbit/s as compared to 15 kbit/s at a spreading factor of 256. As well as physical channels, there are also physical signals, DCCH DTCH RACH CPCH DCH BCCH PCCH CCCH DCCH DTCH CTCH BCH FACH PCH DSCH DCH Logical channels CCCH Transport channels Logical channels Transport channels UPLINK DOWNLINK Figure 10.5 Mapping between logical and transport channels in UTRA FDD [...]... as either dedicated or common physical channels 10.2.2.1 Dedicated Physical Channels All dedicated physical channels feature a radio frame length of 10 ms, with each frame subdivided into 15 slots In the uplink direction, a Dedicated Physical Control CHannel (DPCCH) carrying layer 1 control information is code-multiplexed with the Dedicated Physical Data CHannel (DPDCH) In the downlink direction, there... Acquisition Indicator CHannel (AICH) used to carry Acquisition Indicators (AI) responding to PRACH preambles • The CPCH Access Preamble Acquisition Indicator CHannel (AP-AICH) carrying Access Preamble acquisition Indicators (API) responding to CPCH access preambles • The CPCH Collision Detection/Channel Assignment Indicator CHannel (CD/CA-ICH) carrying either Collision Detection Indicators (CDI), or, if... channel (PDSCH) Indicators AI Acquisition indicator channel (AICH) API Access preamble acquisition indicator channel (AP-AICH) PI Paging indicator channel (PICH) SI CPCH status indicator channel (CSICH) CDI/CAI Collision-detection/channel-assignment indicator Channel (CD/CA-ICH) Synchronisation channel (SCH) Common pilot channel (CPICH) Signals Figure 10.8 Mapping of transport channels and indicators to... Detection Indicators/Collision Assignment Indicators (CDI/CAI) in response to CPCH collision detection preambles • The CPCH Status Indicator CHannel (CSICH) signalling the availability of CPCHs through Status Indicators (SI) This channel is always associated with a CPCH APAICH, the AP-AICH making use of the first 4096 chips per access slot, the CSICH of the remaining 1024 chips The fifth downlink indicator... direction only) so-called feedback information bits used for closed-loop transmit diversity (see Reference [86] for details) In the uplink direction, the DPDCH spreading factor is variable from 256 to 4, while that of the DPCCH is always 256, giving 10 bits per slot for physical layer overhead at a channel bit-rate of 15 kbit/s1 Different slot formats are defined, on the DPDCH specifying the spreading... parameters to obtain a reservation Transmission time validity, Tvalidity , and time duration before retry, Tretry , are indicated to the mobile terminal when the DCH controlled by DRAC is established, and can be changed through radio bearer or transport channel reconfiguration Both parameters can assume values from 1 to 256 radio frames According to Reference [288], which recommends DRAC to be mandatory... multiple users to which data is directed on the respective DSCH Assume that the root code is at SF = 4 In one frame, data may be directed to a single terminal using the root channelisation code at SF = 4, in the next frame to two different terminals at the two codes below the root code, i.e at SF = 8 In the following frame, data could be directed to multiple users at different spreading factors, as long as... time-slot Up to eight different channelisation codes are available for RACH purposes on RACH time-slots, irrespective of the spreading factor to be used for this RACH, which can be either eight or 16 Splitting RACH resources into up to eight RACH subchannels is possible, so that an individual subchannel occurs only in a limited subset of radio frames Different RACHs must either be assigned to different time-slots... access slot indicates the required data-rate Again, this choice is based on the CSICH signalling, which in this case does not only indicate PCPCH availability, but also the smallest available spreading factor When the node B receives the collision detection preamble, it can assign explicitly one among possibly many PCPCHs supporting the requested data-rate, by sending a channel assignment indicator together... transport channels These indicators are either boolean (two-valued) or three-valued The mapping of transport channels and indicators to physical channels is illustrated in Figure 10.8 This figure also shows physical signals, which do not have transport channels or indicators mapped to them Transport Channels Physical channels DCH Dedicated physical data channel (DPDCH) Dedicated physical control channel . Detection Indicators (CDI), or, if channel assignment is used for the CPCH, Collision Detection Indicators/Collision Assignment Indicators (CDI/CAI) in. uplink direction) and on the FACH (in downlink direction). Small to medium amounts of data can be sent on the CPCH (in uplink direction). In the downlink direction,