WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part II

Transcription

1 WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part II Emre A. Yavuz 6.0 WCDMA The WCDMA scheme has been developed as a joint effort between ETSI and ARIB during the second half of 1997 [30]. The ETSI WCDMA scheme and the ARIB WCDMA scheme have been developed from the FMA2 scheme in Europe [31 37] and the Core-A scheme in Japan [38 43], respectively. The uplink of the WCDMA scheme is based mainly on the FMA2 scheme, while the downlink is based on the Core-A scheme. In this section, main technical features of the ARIB/ETSI WCDMA scheme are presented. Table 2 lists the main parameters of WCDMA [2]. Channel bandwidth Downlink RF channel structure Chip rate TABLE 2. Parameters of WCDMA 1.25, 5, 10, 20 MHz Direct spread (1.024) a /4.096/8.192/ Mc/s Roll-off factor for chip shaping 0.22 Frame length Spreading modulation Data modulation Coherent detection Channel multiplexing in uplink Multirate 10 ms/20 ms (optional) Balanced QPSK (downlink) Dual channel QPSK (uplink) Complex spreading circuit QPSK (downlink) BPSK (uplink) User dedicated time-multiplexed pilot (downlink and uplink); no common pilot in downlink Control and pilot channel time multiplexed I & Q multiplexing for data and control channel Variable spreading and multicode Spreading factors 4256 Power control Spreading (downlink) Open and fast closed loop (1.6 khz) Variable length orthogonal sequences for channel seperation Gold sequences 2 18 for cell and user seperation (truncated cycle 10 ms) WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

2 TABLE 2. Parameters of WCDMA Spreading (uplink) Handover Variable length orthogonal sequences for channel seperation Gold sequences 2 41 for user seperation (different time shifts in I and Q channel, truncated cycle 10 ms) Soft handover, Interfrequency handover a In the ARIB WCDMA proposal a chip rate of Mc/s has been specified, whereas in the ETSI WCDMA has not. 6.1 Carrier Spacing and Deployment Scenarios The carrier spacing has a raster of 200 khz and can vary from 4.2 to 5.4 MHz. The different carrier spacings can be used to obtain suitable adjacent channel protections depending on the interference scenario. Figure 15 shows an example for the operator bandwidth of 15 MHz with three cell layers. Larger carrier spacing can be applied between operators than within one operator s band in order to avoid inter-operator interference. Interfrequency measurements and handovers are supported by WCDMA to utilize several cell layers and carriers. FIGURE 15. Frequency utilization with WCDMA 6.2 Radio - Interface Protocol Architecture The general system architecture of UMTS/IMT-2000 includes user equipment (MS), UMTS terreterial radio-access network (UTRAN) and a core network. The functional layering of this system introduces the concepts of access stratum and nonaccess stratum [44]. Access Stratum, is a 3GPP term used to identify the protocol layers directly involved in interactions between the infrastructure and the subscriber equipment. Typically, it includes the Radio Resource Control sublayer, Layer 2 and Layer 1. The corresponding 3GPP2 term is Lower Layers. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

3 Nonaccess Stratum, is a 3GPP term used to identify the protocol layers and functionality related to the core network. Typically, it includes the Call Control and Mobility Management layers. The corresponding 3GPP2 term is Upper Layers. The radio interface of UMTS/IMT-2000, which is based on WCDMA, is layered into three protocol layers [44]: the physical layer (L1), the data link layer (L2), network layer (L3). Layer 1 comprises the WCDMA physical layer while layer 2 is composed of medium access control (MAC), radio link control (RLC-C for the control plane and RLC-U for the user plane), Broadcast/Multicast Control (BMC) and Packet Data Convergence (PDCP) protocols, as well as the link-access control (LAC) protocol. MAC and RLC belong to the access stratum and terminate within UTRAN whereas it is proposed that LAC belongs to the nonaccess stratum and terminates in the core network. The network layer of the control plane is split into the radio resource control (RRC) sublayer and the mobility management (MM) and connection management (CM) sublayers. CM and MM belong to the nonaccess stratum while RRC belongs to the access stratum. Like RLC, Layer 3 is also divided into Control (C-) and User (U-) planes. In the C-plane, Layer 3 is partitioned into sublayers where the lowest sublayer, denoted as Radio Resource Control (RRC), interfaces with layer 2 and terminates in the UTRAN. The next sublayer, denoted Duplication avoidance, terminates in the core network, is part of the Access Stratum; it provides the Access Stratum Services to higher layers. The functions and services of each protocol layer will be exemplified in the following section [45]: Physical Layer Physical layer offers information transfer services to the MAC layer. These services are denoted as transport channels, which will be mentioned later. The physical layer comprises at least the following functions: forward error-correction coding, interleaving, and rate matching; measurements and indication to higher layer (e.g. FER, SIR, interference power, transmission power, etc...); error detection on transport channels; macrodiversity distribution/combining and soft handover execution; multiplexing of transport channels and demultiplexing of coded composite transport channels; mapping of coded composite transport channels; WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

4 modulation and spreading/demodulation, and despreading of physical channels; frequency and time (chip, bit, slot, and frame) synchronization; closed-loop power control; power weighting and combining of physical channels; radio frequency (RF) processing MAC Layer The MAC layer offers data transfer service to RLC and higher layers and it comprises at least the following functions: selection of appropriate transport format (TF, basically bit rate), within a predefined set, per information unit delivered to the physical layer; service multiplexing on RACH, FACH, and dedicated channels; priority handling between data flows of one user as well as between data flows from several users-the latter being achieved by means of dynamic scheduling; access control on RACH; address control on RACH and FACH; contention resolution on RACH RLC Layer The RLC layer offers the following services to the higher layers: layer 2 connection establishment/release; transparent data transfer, i.e., no protocol overhead is appended to the information unit received from the higher layer; assured and unassured data transfer. The RLC layer comprises at least the following functions: segmentation and assembly; transfer of user data; error correction by means of retransmission optimized for the WCDMA physical layer; sequence integrity (used by at least the control plane); duplicate detection; flow control RRC Layer The RRC layer offers the core network the following services: WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

5 general control service, which is used as an information broadcast service; notification service, which is used for paging and notification of a selected MS( s); dedicated control service, which is used for establishment/release of a connection and transfer of messages using the connection. The RRC layer comprises at least the following functions: broadcasting of system information; radio resource handling (e.g., code allocation, handover, admission control, and measurement reporting/control); control of requested quality of service; 6.3 Types and Structures of Channels There are three types of channels defined in WCDMA for FDD mode: transport, logical and physical channels Transport Channels Transport channels are defined by how they are transmitted over the radio interface. They are the services mapped onto physical channels and offered by Layer 1 to the higher layers. Each transport channel has a set of characteristics and transports logical channels. The following is a list of the characteristics of the information to be sent over the radio interface: Format: Encoding (convolution, block or turbo codes), Interleaving, Bit rate Framing / Multiplexing: How the information is multiplexed if it is composed of several sources. General Characteristics: - Uplink or Downlink - Power control characteristics - Risk of collision or not - Mobile Station identification method (in-band or inherent) - Possibility of beam forming - Data rate variations - Broadcast area (entire cell or selected sector only) A general classification of transport channels is into two groups: - Dedicated transport channels (where the MSs are identified by the physical channel, i.e. code and frequency for FDD and code, time slot and frequency for TDD). There exists only one type of dedicated transport channel, the Dedicated Channel (DCH). WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

6 The Dedicated Channel (DCH) - is a downlink or uplink transport channel. The DCH is transmitted over the entire cell or over only a part of the cell using beam-forming antennas. The Dedicated Channel (DCH) is characterized by the possibility of fast rate change (every 10ms), fast power control and inherent addressing of MSs. - Common transport channels (where there is a need for inband identification of the MSs when particular MSs are addressed). There are six types of common transport channels: BCH, FACH, PCH, RACH, CPCH and DSCH. The Broadcast Channel (BCH) - is a downlink transport channel that is used to broadcast system and cell specific information. The BCH is always transmitted over the entire cell with a low fixed bit rate. The Forward Access Channel (FACH) - is a downlink transport channel. The FACH is transmitted over the entire cell or over only a part of the cell using beam-forming antennas. It uses slow power control and designed to carry control information specific to a mobile station when the network knows in what cell the mobile is located. The Paging Channel (PCH) - is a downlink transport channel that carries control information specific to a mobile station when the network does not know where the mobile station is located. In that case, the paging message is broadcasted over the entire network. A response from the mobile indicates its location cell. The transmission of the PCH is associated with the transmission of a physical layer signal, the Paging Indicator, to support efficient sleep-mode procedures. The Random Access Channel (RACH) - is a contention based uplink channel used for transmission of relatively small amount of data, e.g. for initial access or non-realtime dedicated control or traffic data. The RACH is always received from the entire cell, characterized by a limited size data field, a collision risk and by the use of open loop power control. The Common Packet Channel (CPCH) - is an uplink transport channel and only exists in FDD mode. The CPCH is a contention based random access channel used for transmission of bursty data traffic. CPCH is associated with a dedicated channel on the downlink which provides power control for the uplink CPCH. The CPCH is fast power controlled. The Downlink Shared Channel (DSCH) - is a downlink transport channel shared by several MSs carrying dedicated control or traffic data. The DSCH is associated with a DCH Logical Channels An information stream dedicated to the transfer of a specific type of information over the radio interface is called a Logical Channel. The Layer2/MAC sublayer provides data transfer services to higher layers on logical channels which are mapped onto transport channels. A set of logical channel types is defined for different kinds of data transfer services as offered by MAC and classified into: - Control channels (for transfer of control information) WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

7 - Traffic channels (for transfer of user plane information) The control channels are: Broadcast Control Channel (BBCH - DL): Broadcasts control information from base station (BS) to mobile stations (MS). Paging Control Channel (PCCH - DL): Transfers paging information from BS to MS. Common Control Channels (CCCH - DL & UL) - Forward Access Channel (FACH - DL): Carries control information from BS to MS when the network knows where the MS is located on the network. - Random Access Channel (RACH - UL): Channel that carries control information from the MS to the BS. (contention channel) Dedicated Control Channel (DCCH - DL & UL): Point-to-point bi-directional channel that transfers dedicated control information from a BS to a MS or vice versa. The Traffic channel is: Dedicated Traffic Channel (DTCH - DL & UL): Point-to-point bi-directional channel that carries user data. The mappings between logical channels and transport channels as seen from the MS and UTRAN sides are shown in figure 16 and figure 17 respectively. FIGURE 16. Logical channels mapped onto transport channels, seen from the MS side. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

8 FIGURE 17. Logical channels mapped onto transport channels seen from the UTRAN side Physical Channels Physical channels are defined differently for FDD and TDD. For FDD, a physical channel is defined by its carrier frequency, access code and in the uplink, by the relative phase of the signal (either the In-Phase or Quadrature component). Similarly, TDD defines a physical channel by its carrier frequency, access code, relative phase for the uplink and also by the time slot in which it is transmitted. Physical channels are used to carry the transport channels through the air interface. Being more widely used, FDD is the mode that is mainly discussed in this report. It requires the allocation of two frequency bands: one for the uplink and another for the downlink. It has the advantage of being able to transmit and receive at the same time. Furthermore, the size of the cell is not limited by propagation delays like in TDD because of the absence of time slots and guard periods, which also makes the timing sychronisation between base and mobiles less critical than TDD. Because it transmits and receive at the same time, FDD radio units need duplexers in order to separate the incoming and outgoing signals at the antenna. Duplexers are made of filters which increase the complexity and cost of the hardware. Moreover, FDD does not allocate efficiently the available bandwidth for all types of services. For example, Internet access requires more throughput on the downlink than on the uplink. Of course by adjusting the spreading factor, it becomes possible to use only the required data rate, but it is still impossible to trade uplink bandwidth for downlink bandwidth. A general classification of physical channels is into two groups: dedicated and common physical channels. Two types of dedicated physical channels exist: Dedicated physical data channel (DPDCH - UL/DL) : Carries data generated at Layer 2 and above. Dedicated physical control channel (DPCCH - UL/DL) : Carries data generated at Layer 1 (pilot bits, TPC commands, and optional transport-format information) Each connection is allocated one DPCCH and zero, one, or several DPDCH s. Common physical channels that are classified for downlink are: WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

9 Primary common control channel (primary CCPCH), DL - Fixed rate (32kbps, SF=256) channel, designed to carry the BCCH. Secondary common control channel (secondary CCPCH), DL - Carries the FACH and the PCH. Synchronization channel (SCH), DL - provides timing information and is used for handover measurements by the mobile station. Common Pilot Channel (CPICH), DL - is a fixed rate (30 kbps, SF=256) downlink physical channel that carries a pre-defined bit/symbol sequence. Physical Downlink Shared Channel (PDSCH), DL - used to carry the Downlink Shared Channel (DSCH), is shared by users based on code multiplexing. As the DSCH is always associated with a DCH, the PDSCH is always associated with a downlink DPCH. Acquisition Indication Channel (AICH), DL - is used to carry Acquisition Indicators (AI). Page Indication Channel (PICH), DL - is a fixed rate (SF=256) physical channel used to carry the Page Indicators (PI). The PICH is always associated with an S-CCPCH to which a PCH transport channel is mapped. Common physical channels that are classified for uplink are: Physical random access channel (PRACH), UL - carries the RACH. Physical Common Packet Channel (PCPCH), UL - is used to carry the CPCH. The mappings between physical channels and transport channels are shown in the table below. TABLE 3. Transport Channel to Physical Channel Mapping Transport Channels BCH FACH and PCH RACH CPCH DCH DSCH Physical Channels Common Pilot Channel (CPICH) Primary Common Control Physical Channel (P-CCPCH) Secondary Common Control Physical Channel (S-CCPCH) Physical Random Access Channel (PRACH) Physical Common Packet Channel (PCPCH) Dedicated Physical Data Channel (DPDCH) Dedicated Physical Control Channel (DPCCH) Synchronisation Channel (SCH) Physical Downlink Shared Channel (PDSCH) Page Indication Channel (PICH) Acquisition Indication Channel (AICH) WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

10 The DCHs are coded and multiplexed, and the resulting data stream is mapped sequentially (first-in-first-mapped) directly to the physical channel(s). The mapping of BCH and FACH/PCH is equally straightforward, where the data stream after coding and interleaving is mapped sequentially to the Primary and Secondary CCPCH respectively. Also for the RACH, the coded and interleaved bits are sequentially mapped to the physical channel, in this case the message part of the random access burst on the PRACH. 6.4 Physical Channels in Detail Uplink Physical Channels There are two dedicated channels and two common channels on the uplink. User data is transmitted on the dedicated physical data channel (DPDCH), and control information is transmitted on the dedicated physical control channel (DPCCH). The Random Access Channel (RACH) is a common access channel. and is based on a Slotted ALOHA approach with fast acquisition indication. The MS can start the transmission at a number of well-defined time-offsets, denoted access slots. There are 15 access slots per two frames and they are spaced 5120 chips apart. Timing information on the access slots and the acquisition indication can be found in [46]. Figure 18 shows the access slot numbers and their spacing to each other. Information on what access slots are available in the current cell is given by higher layers. FIGURE 18. RACH access slot numbers and their spacing Before the transmission of a random access request, the mobile terminal should carry out the following tasks: Achieve chip, slot, and frame synchronization to the target base station from the synchronization channel (SCH) and obtain information about the downlink scrambling code also from the SCH Retrieve information from BCCH about the random access code(s) used in the target cell/sector WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

11 Estimate the downlink path loss, which is used together with a signal strength target to calculate the required transmit power of the random access request It is possible to transmit a short packet together with a random access burst without settting up a scheduled packet channel. No separate access channel is used for packet traffic related random access, but all traffic shares the same random access channel. More than one random access channel can be used if the random access capacity requires such an arrangement. The performance of the selected solution is presented in [47]. The Common Packet Channel (CPCH) transmission is based on DSMA-CD (Digital Sense Multiple Access with Collision Detection) approach with fast acquisition indication. The MS can start transmission at a number of well-defined time-offsets, relative to the frame boundary of the received BCH of the current cell. The access slot timing and structure is identical to RACH. The structure of the CPCH random access transmission is shown in figure 19. The CPCH random access transmission consists of one or several Access Preambles [A-P] of length 4096 chips, one Collision Detection Preamble (CD-P) of length 4096 chips, a [10] ms DPCCH Power Control Preamble (PC-P) and a message of variable length Nx10 ms. FIGURE 19. Structure of CPCH random access transmission The principle frame structure of the uplink Dedicated Physical Data Channel (DPDCH) shown in figure 20 [2]. FIGURE 20. WCDMA uplink multirate transmission. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

12 Each DPDCH frame on a single code carries 160 x 2 k bits (16 x 2 k Kb/s), where k = 0,1,..., 6, corresponding to a spreading factor of 256/2 k with the Mc/s chip rate. Multiple parallel variable rate services (= dedicated logical traffic and control channels) can be time multiplexed within each DPDCH frame. The overall DPDCH bit rate is variable on a frame-by-frame basis. In most cases, only one DPDCH is allocated per connection, and services are jointly interleaved sharing the same DPDCH. However, multiple DPDCHs can also be allocated (e.g. to avoid a too low spreading factor at high data rates). The Dedicated Physical Control Channel (DPCCH) is needed to transmit pilot symbols for coherent reception, power control signaling bits, and rate information (transport format indicator, TFI) for rate detection. A certain transport format (TF) defines how the layer 2 data carried on the DPDCH( s) is multiplexed and coded and what spreading factor is used etc. Two basic solutions for multiplexing physical control and data channels are time multiplexing and code multiplexing. A combined IQ and code multiplexing solution (dualchannel QPSK) is used in WCDMA uplink to avoid electromagnetic compatibility (EMC) problems with discontinuous transmission (DTX). The major drawback of the time multiplexed control channel are the EMC problems that arise when DTX is used for user data. One example of a DTX service is speech. During silent periods no information bits need to be transmitted, which results in pulsed transmission as control data must be transmitted in any case. This is illustrated in figure 21 [2]. Because the rate of transmission of pilot and power control symbols is on the order of 1 to 2 khz, they cause severe EMC problems to both external equipment and terminal interiors. This EMC problem is more difficult in the uplink direction since mobile stations can be close to other electrical equipment, like hearing aids. FIGURE 21. Illustration of pulsed transmission with multiplexed control channel. The IQ/code multiplexed control channel is shown in figure 22 [2]. Now, since pilot and power control are on a separate channel, no pulse-like transmission takes place. Interference to other users and cellular capacity remains the same as in the time multiplexed solution. In addition, link-level performance is the same in both schemes if the energy allocated to the pilot and the power control bits is the same. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

13 FIGURE 22. Illustration of parallel transmission of DPDCH and DPCCH Downlink Physical Channels In the downlink, there are seven common physical channels. The dedicated channels (DPDCH and DPCCH) are time multiplexed. The EMC problem caused by discontinuous transmission is not considered difficult in downlink since (1) there are signals to several users transmitted in parallel and at the same time and (2) base stations are not so close to other electrical equipment, like hearing aids. In the downlink, time multiplexed pilot symbols are used for coherent detection. Since the pilot symbols are connection dedicated, they can be used for channel estimation with adaptive antennas as well. Furthermore, the connection dedicated pilot symbols can be used to support downlink fast power control. Figure 23 shows the frame structure of the downlink DPCH. Each frame of length 10 ms is split into 15 slots, each of length T slot = 2560 chips, corresponding to one power-control period. A super frame corresponds to 72 consecutive frames, i.e. the super-frame length is 720 ms. FIGURE 23. Frame structure of downlink DPCH. The Primary Common Control Physical Channel (P-CCPCH) carries the BCCH channel and a time multiplexed common pilot channel which can be used for coherent detection. It is of fixed rate and is mapped to the DPDCH in the same way as dedicated traffic channels. The primary CCPCH is allocated the same channelization code in all cells. A mobile terminal can thus always find the BCCH, once the base station s unique scrambling WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

14 code has been detected during the initial cell search. Figure 24 shows the frame structure of the Primary CCPCH. The frame structure differs from the downlink DPCH in that no TPC commands, no TFCI and no pilot bits are transmitted The Primary CCPCH is not transmitted during the first 256 chips of each slot. Instead, Primary SCH and Secondary SCH are transmitted during this period. FIGURE 24. Frame structure for Primary Common Control Physical Channel. The Secondary Common Control Physical Channel (S-CCPCH) carries the PCH and FACH in time multiplex within the super frame structure. The rate of the S-CCPCH may be different for different cells and is set to provide the required capacity for PCH and FACH in each specific environment. The channelization code of the secondary CCPCH is transmitted on the primary CCPCH. There are two types of Secondary CCPCH: those that include TFCI and those that do not include TFCI. It is the UTRAN that determines if a TFCI should be transmitted, hence making it mandatory for all MSs to support the use of TFCI. The frame structure of the Secondary CCPCH is shown in figure 25. FIGURE 25. Frame structure for Secondary Common Control Physical Channel WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

15 The parameter k in figure 25 determines the total number of bits per downlink Secondary CCPCH slot. It is related to the spreading factor SF of the physical channel as SF = 256/ 2 k. The spreading factor range is from 256 down to 4. The values for the number of bits per field are given in table 4 and table 5, which are taken from [46]. The channel bit and symbol rates given in Table 16 are the rates immediately before spreading. The pilot patterns are also given in [46] as well. The FACH and PCH can be mapped to the same or to separate S-CCPCHs. If FACH and PCH are mapped to the same S-CCPCH, they can be mapped to the same frame. The main difference between a CCPCH and a downlink dedicated physical channel is that a CCPCH is not inner-loop power controlled. The main difference between the Primary and Secondary CCPCH is that the P-CCPCH has a fixed predefined rate while the S-CCPCH can support variable rate with the help of the TFCI field included. Furthermore, a P-CCPCH is continuously transmitted over the entire cell while a S-CCPCH is only transmitted when there is data available and may be transmitted in a narrow lobe in the same way as a dedicated physical channel (only valid for a S-CCPCH carrying the FACH). TABLE 4. Secondary CCPCH fields with pilot bits Slot format Channel Bit Channel SF Bits/ Bits/ N data N pilot N TFCI #i Rate (kbps) Symbol Rate (ksps) Frame Slot * * * * * * If TFCI bits are not used, then DTX shall be used in TFCI field. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

16 TABLE 5. Secondary CCPCH fields without pilot bits Slot format Channel Bit Channel SF Bits/ Bits/ N data N pilot N TFCI #i Rate (kbps) Symbol Rate (ksps) Frame Slot * * * * * * If TFCI bits are not used, then DTX shall be used in TFCI field. The Synchronization Channel (SCH) consists of two subchannels, the primary and secondary SCHs. Figure 26 illustrates the structure of the SCH radio frame. The SCH applies short code masking to minimize the acquisition time of the long code [48]. The SCH is masked with two short codes (primary and secondary SCH). The unmodulated primary SCH is used to acquire the timing for the secondary SCH. The modulated secondary SCH code carries information about the long code group to which the long code of the BS belongs. In this way, the search of long codes can be limited to a subset of all the codes. FIGURE 26. Structure of the synchronization channel (SCH) The primary SCH consists of an unmodulated code of length 256 chips, which is transmitted once every slot. The primary synchronization code is the same for every base station in the system and is transmitted time aligned with the slot boundary, as illustrated in figure 26 [2]. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

17 The secondary SCH consists of one modulated code of length 256 chips, which is transmitted in parallel with the primary SCH. The secondary synchronization code is chosen from a set of 16 different codes depending on to which of the 32 different code groups the base station downlink scrambling code belongs. The secondary SCH is modulated with a binary sequence of length 16 bits, which is repeated for each frame. The modulation sequence, which is the same for all base stations, has good cyclic autocorrelation properties. The SCH is nonorthogonal to the other downlink physical channels. The frame structure of the Common Pilot Channel (CPICH) is shown in figure 27. There are two types of Common pilot channels, the Primary and Secondary CPICH. They differ in their use and the limitations placed on their physical features. FIGURE 27. Frame structure for Common Pilot Channel. The Primary Common Pilot Channel has the following characteristics [46]: The same channelization code is always used for this channel Scrambled by the primary scrambling code One per cell Broadcast over the entire cell The Primary CPICH is the phase reference for the following downlink channels: SCH, Primary CCPCH, AICH, PICH. The Primary CPICH is also the default phase reference for all other downlink physical channels. A Secondary Common Pilot Channel characteristics [46] : Can use an arbitrary channelization code of SF=256 Scrambled by either the primary or a secondary scrambling code WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

18 Zero, one, or several per cell May be transmitted over only a part of the cell A Secondary CPICH may be the reference for the Secondary CPCCH and the downlink DPCH. If this is the case, the MS is informed about this by higher-layer signalling. The Physical Downlink Shared Channel (PDSCH), is used to carry the Downlink Shared Channel (DSCH), is shared by users based on code multiplexing. As the DSCH is always associated with a DCH, the PDSCH is always associated with a downlink DPCH. The frame and slot structure of the PDSCH are shown on figure 28. FIGURE 28. Frame structure for the PDSCH. To indicate for MS that there is data to decode on the DSCH, two signalling methods are possible, either using the TFCI field, or higher layer signalling. The PDSCH transmission with associated DPCH is a special case of multicode transmission. The PDSCH and DPCH do not have necessary the same spreading factors and for PDSCH the spreading factor may vary from frame to frame. The relevant Layer 1 control information is transmitted on the DPCCH part of the associated DPCH, the PDSCH does not contain physical layer information. The channel bit and symbol rates for PDSCH are given in table 6. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

19 Slot format #i Channel Bit Rate (kbps) TABLE 6. PDSCH fields Channel Symbol Rate (ksps) For PDSCH the allowed spreading factors may vary from 256 to 4. If the spreading factor and other physical layer parameters can vary on a frame-by-frame basis, the TFCI shall be used to inform the MS what are the instantaneous parameters of PDSCH including the channelisation code from the PDSCH OVSF (Orthhogonal Variable Spreading Factor) code tree. A DSCH may be mapped to multiple parallel PDSCHs as well, as negotiated at higher layer prior to starting data transmission. In such a case the parallel PDSCHs shall be operated with frame synchronization between each other. The Acquisition Indicator channel (AICH) is used to carry Acquisition Indicators (AI). Acquisition Indicator AI i corresponds to signature i on the PRACH or PCPCH. Note that for PCPCH, the AICH is either in response to an access preamble or a CD preamble. The corresponding to the access preamble AICH is the AP-AICH and the corresponding to the CD preamble AICH is the CD-AICH. The AP-AICH and CD-AICH use different channelization codes. SF Bits/ Frame Bits/ Slot N data FIGURE 29. Structure of Acquisition Indicator Channel (AICH). Figure 29 illustrates the frame structure of the AICH. Two AICH frames of total length 20 ms consist of 15 access slots (AS), each of length 20 symbols (5120 chips). Each access WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

20 slot consists of two parts, an Acquisition-Indicator (AI) part and an empty part. The empty part of the access slot consists of 4 zeros. The phase reference for the AICH is the CPICH. The frame structure of the Page Indicater Channel (PICH) is illustrated in Figure 30. One PICH frame of length 10 ms consists 300 bits. Of these, 288 bits are used to carry Page Indicators. The remaining 12 bits are not used. 6.5 Spreading FIGURE 30. Structure of Page Indicator Channel (PICH) The WCDMA scheme employs long spreading codes. Different spreading codes are used for cell separation in the downlink and user separation in the uplink. In the downlink, Gold codes of length 218 are used, but they are truncated to form a cycle of a 10-ms frame. The total number of available scrambling codes is 512, divided into 32 code groups with 16 codes in each group to facilitate a fast cell search procedure. In the uplink, either short or long spreading (scrambling codes) are used. The short codes are used to ease the implementation of advanced multiuser receiver techniques; otherwise long spreading codes can be used. Short codes are VL-Kasami codes of length 256 and lond codes are Gold sequences of length 241, but the latter are truncated to form a cycle of a 10-ms frame. FIGURE 31. IQ/code multiplexing with complex spreading circuit. For channelization, orthogonal codes are used. Orthogonality between the different spreading factors can be achieved by the tree-structured orthogonal codes. IQ/code multiplexing leads to parallel transmission of two channels, and therefore, attention must be paid to modulated signal constellation and related peak-to-average power ratio (crest factor). By using the complex spreading circuit shown in figure 31 [2], the WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

21 transmitter power amplifier efficiency remains the same as for QPSK transmission in general. Moreover, the efficiency remains constant irrespective of the power difference G between DPDCH and DPCCH. This can be explained with figure 32 [2], which shows the signal constellation for IQ/code multiplexed control channel with complex spreading. In the middle constellation with G = 0.5 all eight constellation points are at the same distance from the origin. The same is true for all values of G. Thus, signal envelope variations are very similar to the QPSK transmission for all values of G. The IQ/code multiplexing solution with complex scrambling results in power amplifier output backoff requirements that remain constant as a function of power difference. Furthermore, the achieved output backoff is the same as for one QPSK signal. FIGURE 32. Signal constellation for IQ/code multiplexed control channel with complex spreading. G is the power difference between DPCCH and DPDCH. 6.6 Channel Coding and Multiplexing A key feature of the WCDMA radio interface is the possibility to transport multiple parallel services (TrCH s) with different quality requirements on one connection. The basic scheme for the channel coding and transport channel multiplexing in WCDMA is drawn in figure 33 [45] Parallel TrCH s (TrCH-1 to TrCH-M) are seperately channel coded and interleaved. The coded TrCH s are then time multiplexed into a coded composite TrCH (CC-TrCh). Final intraframe (10 ms) interleaving is carried out after transport channel multiplexing. After service multiplexing and channel coding, the multiservice data stream is mapped to one DPDCH. If the total rate exceeds the upper limit for single code transmission, several DPDCHs can be allocated. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

22 FIGURE 33. Channel coding and transport-channel multiplexing. A second alternative for service multiplexing would be to map parallel services to different DPDCHs in a multicode fashion with separate channel coding/interleaving. With this alternative scheme, the power, and consequently the quality of each service, can be separately and independently controlled. The disadvantage is the need for multicode transmission, which will have an impact on mobile station complexity due to the increased envelope variations in the transmitted signal and the need for multiple RAKE receivers. Multicode transmission sets higher requirements for the power amplifier linearity in transmission, and more correlators are needed in reception Channel Coding Different coding and interleaving schemes can be applied to a TrCH depending on the specific requirements in terms of error rates, delay etc. The following channel coding schemes are used: Rate 1/2 convolutional coding is typically applied for low-delay services (like BCH, PCH, FACH and RACH transport channels) with moderate error rate requirements (BER = 10-3 ). A concatenation of rate 1/3 convolutional coding and outer Reed-Solomon coding + interleaving can be applied for high-quality services (like CPCH and DCH with BER between 10-3 and 10-6 ). Retransmissions can be utilized to guarantee service quality for non real-time packet data services. Turbo codes of rate 1/3 can also be applied for high rate high quality services (like CPCH and DCH with BER between 10-3 and 10-6 ). WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

23 6.6.2 Rate Matching After channel coding and service multiplexing, the total bit rate can be almost arbitrary. Rate matching adapts this rate to the limited set of possible bit rates of a DPDCH. Repetition or puncturing is used to match the coded bit stream to the channel gross rate. As shown in figure 33, there are two different rate-matching steps: static and dynamic rate matching.. Static Rate Matching - This kind of matching is carried out at the addition, removal and redefinition of TrCH s, i.e., on a very slow basis. The static rate matching is applied after channel coding and uses code puncturing to adjust the channel coding rate of each TrCh so that the maximum bit rate of the coded composite TrCh is matched to the bit rate of the physical channel. In general, code puncturing is chosen for bit rates less than 20 percent above the closest lower DPDCH bit rate. The static rate matching is applied both the uplink and downlink. On the downlink, the static rate is used to, if possible, reduce the coded composite TrCH rate to the closest lower physical channel rate (closest higher spreading factor) thus avoiding the overallocation of orthogonal codes on the downlink and reducing the risk for a code limited downlink capacity. The static rate matching should be distributed between the parallel TrCH s in such a way that the TrCH s fulfill their quality requirements at approximately the same channel signal-to-interference ratio (SIR), i.e., the static rate matching also performs SIR matching.. Dynamic Rate Matching - The dynamic rate matching is carried out once every 10 ms radio frame, i.e., on a very fast basis. The dynamic rate matching is applied after transport channel multiplexing and uses symbol repetition so that the instaneous bit rate of coded composite TrCH is exactly matched to the bit rate of the physical channel. The dynamic rate matching is only applied to the uplink. On the downlink, discontinious transmission (DTX) within each slot is used when the instantaneous rate of the coded composite TrCh does not exactly match the bit rate of the physical channel. In short, for the uplink, rate matching is based on both symbol repetition and code puncturing, while it s only based on code puncturing for the downlink since DTX is used otherwise. It should be noted that although the transport channel coding and multiplexing is carried out by the physical layer, the process is fully controlled by the radio resource controller, e.g, in terms of choosing the appropriate coding scheme, interleaving parameters, and rate matching parameters. 6.7 Radio Resource Functions Power Control WCDMA employs fast closed-loop power control in both the uplink and downlink. The basic power-control rate is 1600 Hz, and the power-control step can be varied adaptively according to the MS speed and operating environment. SIR-based power control is used, i.e., the receiver compares the estimated received SIR with a SIR target value and com- WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

24 mands the transmitter to increase or decrease the power accordingly. The power-control command increases or decreases the power of all physical channels on one connection. The target SIR values are controlled by an outer power control loop. This outer loop measures the link quality, typically, a combination of frame and bit error rates (BER s) depending on the service, and adjusts the SIR targets accordingly. Ensuring that the lowest possible SIR target is used at all times results in maximum capacity. In addition, the outer loop is used to independently control the relative power of different physical channels belonging to the same connection. As an example, the DPDCH and DPCCH power difference can be controlled by the outer loop to take into account the variations in DPDCH coding gain for different environments. Open-loop power control is used by the random-access procedure, where uplink path loss is estimated from downlink path loss. Also, common-channel packet transmissions depend on open-loop power control Random Access A fast and efficient random-access scheme is essential for the UMTS system since packet access is becoming more important in the third-generation systems. This will lead to an increased number of random-access attempts that need to be served quickly. The WCDMA random-access procedure is based on slotted ALOHA and works as follows. The MS achieves chip and frame synchronization to the target cell using the initial cellsearch procedure. The BCCH is read to retrieve information about the random-access code(s) used in the target cell. The downlink path loss is estimated, and the estimate is used to calculate the required transmit power of the random-access burst. A random-access burst is transmitted with a random time offset. The time offset is a multiple of 1.25 ms relative to the received frame boundary. The base station responds with an acknowledgment on the FACH. If the MS receives no acknowledgment, it selects a new time offset and tries again. The random-access procedure is described in more detail in [49] and [50] Initial Cell Search WCDMA base stations are, in general, mutually asynchronous, i.e., there is no universal time reference known to all base stations. To separate different cells, different downlink scrambling codes are used. During the initial cell search, the MS first searches for the strongest base-station cell. The MS then determines the scrambling code and the frame synchronization of that cell. The cell search consists of three steps. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

25 In the first step, the MS acquires slot synchronization to the strongest base station. This is done using a matched filter matched to the system-specific code used on the primary SCH. Each ray of each cell within hearable range will result in a peak in the signal output from the filter. The largest peak indicates the slot timing of the strongest cell. In the second step, the MS correlates the received slot synchronized signal with the available 16 codes used on the secondary SCH, followed by correlation with the 16 different cyclic shifts of the 16-b modulation sequence. The maximum of these 256 correlation values identifies the code group and the frame timing. The third cell-search step consists of an exhaustive search of all the scrambling codes in the code group identified in the second step. The search is done through symbol by symbol correlation over the primary CCPCH. Since frame synchronization was obtained in the second step, the start of the scrambling code is known. When the scrambling code has been identified, the cell and system specific broadcast information on the primary CCPCH can be read Handover The normal handover in WCDMA is soft intrafrequency handover, where the MS is connected to two or more cells simultaneously on the same frequency. The MS continuously searches for new cells, using the cell-search technique described above, but the search is limited to a list of neighboring cells broadcast from the network. The neighboring list tells the MS in which order to search for the scrambling codes, and it can also limit the search to a subset of all available codes. In soft handover, the uplink signals are combined in the network, and downlink combining of signals is done in the MS s RAKE receiver. When including a new additional base station in the active set, i.e., the set of base stations currently connected, the MS signals via the old link(s), how the new base station should adjust its DPCCH/DPDCH frame timing to minimize the received frame timing differences in the MS. This can be done since the MS from the cell search knows the relative frame timing of the primary CCPCH of the handover candidates. Timing adjustments of dedicated downlink channels (DPCCH/DPDCH) of the new base station relative to the primary CCPCH can be carried out with a resolution of one DPCCH/DPDCH symbol without losing orthogonality of downlink codes. The synchronization of the dedicated downlink signals from the two base stations with an accuracy of one symbol, enables the mobile RAKE receiver to collect the macro diversity energy from the two base stations. A special case of soft handover is the softer handover, where the MS is connected to two cells belonging to one base-station site. Instead of doing the uplink combining in the network, as is the case for soft handover, softer handover combining can be done in the base station. This makes it possible to use more efficient uplink combining, e.g., maximum ratio combining. In WCDMA, soft and softer handover use relative handover thresholds. By doing so, fewer MS s will be in soft or softer handover, compared to when absolute thresholds are WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

26 used, as is the case for current narrow-band CDMA systems, i.e., IS-95-A. Moreover, the adding and dropping of cells in the active set is load dependent for IS-95-A, while the active set updating in WCDMA is load independent. This behavior is illustrated in figure 34 [45]. FIGURE 34. Comparison of active set updating for absolute (IS-95-A) and relative (WCDMA) handover thresholds. A and B are candidate cells. Furthermore, as softer handover can employ more efficient combining in the uplink and has lower network transmission load, the handover margin for softer handover will typically be larger than for soft handover. The handover parameters are service and load dependent. Even though much of the handover functionality resides in the MS, the network can still put a veto on the MS s suggestion of cells to connect to. As mentioned, the normal handover in WCDMA is a soft intrafrequency handover. However, interfrequency handovers are also supported. Interfrequency handovers in the system are essential to support: hot-spot scenarios, where a cell uses more carriers than the surrounding cells; hierarchical cell structures, where macro, micro, and pico layers are on different frequencies; handovers between different operators; handovers to other systems, e.g., GSM. To support seamless interfrequency handovers, measurements on other frequencies must be possible without disturbing the normal data flow. Since the MS is receiving the downlink signal continuously, there is no time to carry out measurements on other frequencies using the ordinary receiver. Two methods are considered for interfrequency measurements in WCDMA: Dual receiver Slotted mode WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

27 The dual receiver approach is considered suitable especially if the mobile terminal employs antenna diversity. During the interfrequency measurements, one receiver branch is switched to another frequency for measurements, while the other keeps receiving from the current frequency. The loss of diversity gain during measurements needs to be compensated for with higher downlink transmission power. The advantage of the dual receiver approach is that there is no break in the current frequency connection. Fast closed loop power loop is running all the time. The slotted mode is considered attractive for the mobile station without antenna diversity. The information normally transmitted during a 10-ms frame is compressed time either by code puncturing or by changing the FEC rate. A 10-ms data frame can then be transmitted in less than 10 ms. The transmission is done with higher power than normal to compensate for the decreased processing gain. Using this technique, an idle period of up to 5 ms is created during which no data is to be received by the MS. This period can then be used to tune the receiver to other frequencies and signaal strength measurements on those. Base stations in WCDMA need not be synchronized, and therefore, no external source of synchronization, like GPS, is needed for the base stations. Asynchronous base stations must be considered when designing soft handover algorithms and when implementing position location services Interoperatibility Between GSM and WCDMA The handover between the WCDMA system and the GSM system, offering worldwide coverage already today, has been one of the main design criteria taken into account in the WCDMA frame timing definition. The GSM compatible multiframe structure, with a superframe multiple of 120 ms, allows similar timing for intersystem measurements as in the GSM system itself. Apparently the needed measurement interval does not need to be as frequent as for GSM terminal operating in a GSM system, as intersystem handover is less critical from intra-system interference point of view. Rather, the compatibility in timing is important that when operating in WCDMA mode, a multimode terminal is able to catch the desired information from the synchronization bursts in the synchronization frame on a GSM carrier with the aid of frequency correction burst. This way the relative timing between a GSM and WCDMA carriers is maintained similar to the timing between two asynchronous GSM carriers. FIGURE 35. Measurement timing relation between WCDMA and GSM frame structure. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

28 The timing relation between WCDMA channels and GSM channels is indicated in figure 35 [2], where the GSM traffic channel and WCDMA channels use similar 120 ms multiframe structure. The GSM frequency correction channel (FCCH) and GSM synchronization channel (SCH) use one slot out of the eight GSM slots in the indicated frames with the FCCH frame with one time slot for FCCH always preceding the SCH frame with one time slot for SCH as indicated in the figure 35. A WCDMA terminal can do the measurements either by requesting the measurement intervals in a form of slotted mode where there are breaks in the downlink transmission or then it can perform the measurements independently with a suitable measurement pattern. With independent measurements the dual receiver approach is used instead of the slotted mode since the GSM receiver branch can operate independently of the WCDMA receiver branch. For smooth interoperation between the systems, information needs to be exchanged between the systems, in order to allow WCDMA base station to notify the terminal of the existing GSM frequencies in the area. In addition, more integrated operation is needed for the actual handover where the current service is maintained, taking naturally into account the lower data rate capabilities in GSM when compared to UMTS maximum data rates reaching all the way to 2 Mb/s. The GSM system is likewise expected to be able to indicate also the WCDMA spreading codes in the area to make the cell identification simpler and after that the existing measurement practises in GSM can be used for measuring the WCDMA when operating in GSM mode. As the WCDMA does not rely on any superframe structure as with GSM to find out synchronization, the terminal operating in GSM mode is able to obtain the WCDMA frame synchronization once the WCDMA base station scrambling code timing is acquired. The base station scrambling code has 10-ms period and its frame timing is synchronized to WCDMA common channels. 6.8 Medium Access Control and Radio Link Control The MAC and RLC protocols are responsible for efficiently transferring data of both realtime and nonreal-time services. The transfer of nonreal-time data transfer includes the possibility of an optimized low-level automatic repeat request (ARQ) at the RLC layer, offering higher protocol layers reliable data transfer. In addition, the MAC layer controls the multiplexing of data streams originating from different services. A description of the MAC/RLC protocol(s) can also be found in [51] Data Flow In order to achieve the requirements mentioned above, the RLC layer segments the data streams into small packets, RLC protocol data units (RLC PDU s) suitable for transmis- WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

29 sion over the radio interface. In figure 36 [45], the data flow of the WCDMA system is shown. FIGURE 36. Segmentation and transformation of network layer protocol data units (N-PDU s). Network layer PDU s (N-PDU s) are first segmented into smaller packets and transformed into LAC PDU s. The LAC overhead typically consists of a service access point identifier, sequence number for a higher level ARQ, and other fields. The overhead is typically in the range of three octets. Then, the LAC PDU s are segmented into small packets, which are transformed into RLC PDU s. The RLC PDU header typically contains a sequence number. The sequence number is used for the optimized fast ARQ. The data flow of the WCDMA system is very similar to the data flow of GPRS [52]. However, one important difference is that in the GPRS system, an RLC PDU always consists of four bursts, while the code rate may vary. Hence, the number of information bits of the RLC PDU s in the GPRS system can vary. However, once a segment of the LAC PDU is transformed into an RLC PDU with a particular code, then, at a later time, this segment cannot be transformed into another RLC PDU with a different code. Thus, in case of retransmissions, the same segment of the LAC PDU will be retransmitted with the same code rate. In the WCDMA system, on the other hand, all RLC PDU s have the same size, regardless of the transmission rate. This means that since the transmission rate may change every 10 ms, the number of RLC PDU s transferred per 10 ms varies. This is illustrated in figure 36, where in the first transport frame two RLC PDU s can be conveyed. The second transport frame, which in time is equally long, can only convey one RLC PDU since the rate has been changed Model of Operation 1) Packet Data Services: In this section, the model of operation when packets are transmitted in the uplink is described.. For the downlink, packet transmission will be done in a very similar way. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

30 In the WCDMA system, packet data can be transmitted in three ways. First, if a layer 3 packet is generated, the MS may choose to transmit it on the RACH. The data is simply appended to the access burst. This method is illustrated in figure 37 [45]. FIGURE 37. Packet transmission on the RACH. Typically, this method, which is called common channel packet transmission, is chosen if the MS has only a small amount of data to transmit. No reservation scheme is used in the method, so the overhead necessary to transmit a packet is kept to a minimum. The MS does not need to get assigned a channel, thus, the access delay is kept small as well. The other method is illustrated in figure 38 [45]. FIGURE 38. Packet transmission on a dedicated channel. Here, the MS first sends a Resource Request (Res_Req) message. Typically, this is done when the packet is large. In this Res_Req message, an indication is given of what sort of traffic is to be transmitted. The network then evaluates whether the MS can be assigned the necessary resources. If that is the case, it transmits a Resource Allocation (Res_All) message on the FACH. A Res_All message consists of a set of transport formats (TF s). Out of this set, the MS will use a TF to transmit its data on the DCH. Exactly which TF the MS may use and at what time the MS may initiate its transmission is either transmitted together with the Res_All message or is indicated in a separate Capacity Allocation (Cap_All) message at a later time. In situations where the traffic load is low, probably the first alternative will be used, while the second alternative is used in cases where the load is high and the MS is not allowed to immediately transmit. In figure 39 [45], the second alternative is illustrated. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

31 FIGURE 39. Resource request and allocation on the RACH and FACH respectively, followed by transmission of data on the dedicated channel. This method of first requesting for resources before transmitting data is used in cases when the MS has large packets to transmit. The overhead caused by the reservation mechanism is then negligible. Due to the fact that the MS gets assigned a dedicated channel, data transfer will be more reliable than when it would have been transmitted on the RACH. The reason for this is that the dedicated channel is not a shared channel, thus no collisions are possible, and the MS uses closed-loop power control on the dedicated channel, whereas this is not the case on the RACH. The reason of having been assigned a set of TF s and not only one is that the TF can be changed during transmission. This can be useful for interference control. This change is done by means of a TF_Change message, which contains the new TF to be used. The TF_Change message is transmitted on the DCH. The third method of transmitting packets, illustrated in figure 40 [45], is when the MS already has a dedicated channel at its disposal. FIGURE 40. Packet transmission on the dedicated channel. The MS can then either transmit first a Cap_Req message on the DCH, in case when the MS has a large amount of data to transmit, or it can just start transmitting, in case the MS has just a small amount of data to transmit. The MS can already have a DCH at its disposal due to the fact that it uses it for another service. Another reason can be that the MS having just finished transmitting packets on the DCH, will then keep the DCH for a certain time. If in this time new packets arrive, the MS may immediately start transmission, using the TF that was used during the last data transmission. Between packets on the DCH, link WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

32 maintenance is done by sending pilot bits and power-control commands, ensuring that the packet transmission is spectrally efficient. If no packets have been generated for too long of a time, the MS will release the DCH. However, the MS will keep the TF set allocated in the Res_All message. Thus, when it has new packets to transmit, it only transmits a short-capacity request (Cap_Req) message on the RACH, after which it may receive a Cap_All message. 2) Real-Time Services: For real-time services, the allocation procedures are very similar. Once a MS has data to transmit, it first requests resources with a Res_Req message. This is done on the RACH, or, in the case where the MS already uses a DCH, it is transmitted on the DCH. As a result, the network now allocates the required resources, again by means of a set of TF s. In contrast to the packet data case, where the MS first waits for a Cap_All message, the MS immediately starts transmitting after it has received a Res_All message. Another difference from the packet data transmission is that the MS is now allowed to use any TF allocated in the Res_All message. In this way, the MS can support variable bit rate services such as speech, but also in this case the network can limit the capacity of the MS. By means of a resource limit message (Res_Limit), it can limit the previously allocated TF set. The consequence of this message is that a MS now may only use the TF s out of the limited TF set. If later on the capacity in the system is sufficient, determined, and indicated to the MS by the network, the MS is allowed to transmit with all TF s allocated in the TF set. 3) Mixed Services: The MAC should also be able to support multiple services. As mentioned previously, the physical layer is capable of multiplexing bit streams originating from different services. The MAC protocol controls this process by controlling the data stream delivered to the physical layer over the TrCH s. This control can particularly be important in the case of when there is a lack of capacity in the system. If a MS wants to transmit data of different services like, for example, a real-time service such as speech and a packet data service, then it has been assigned two sets of TF s. One set is assigned for the real-time service and one for the packet data service. As mentioned, in the single-service case, the MS may use any TF assigned for the real-time service, whereas it may only use one of the TF s of the TF set for the data service. In the multipleservice case, the MS may use any TF assigned to it for the speech service. In addition, the MS gets assigned a specific output power/rate threshold. The aggregate rate of both services must be below this threshold. The TF s used for the data service are chosen out of the allocated TF set in such a way that the aggregate output power/rate will never exceed the threshold. Thus, the TF s used for the data service fluctuate adaptively to the used TF s of the speech service. 6.9 Radio Network Aspects One major benefit of CDMA is the avoidance of frequency planning. Nevertheless, current second-generation narrowband CDMA systems have proven to be difficult to plan, mainly due to power planning. Thus, a great deal of effort has been put into reducing the network planning for WCDMA. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

33 For WCDMA, power planning is less demanding, as the common downlink channels are assigned only about 10% of the base station s output power, a factor three less than for current narrow-band CDMA systems. Relative comparison of base stations signal strengths results in easier planning of soft/ softer handover zones, especially in areas covered by cells with different sizes. The cell coverage of CDMA is, in general, very dependent on the cell load. As WCDMA employs admission control and congestion control, the load can be controlled and, thus, also the coverage. In the following sections, the benefits and features of admission and congestion control will be described. To further reduce deployment cost, the WCDMA system is designed so that a reuse of old second-generation sites, e.g., GSM sites, is possible. WCDMA link budgets show that a coverage greater than that of GSM 1800 can be achieved for voice users. Furthermore, the link budgets also show that 384-kbps packet data services can be provided by WCDMA with the same coverage as voice service for GSM Consequently, a WCDMA system supporting wide-area coverage up to 384-kbps packet data can be deployed using only already existing GSM 1800 sites Admission Control Admitting a new call will always increase the interference level in the system. This interference increase will reduce the cell coverage, so-called cell breathing. In order to secure the cell coverage when the load increases, the admission control will limit the interference. The basic strategy is to protect ongoing calls by denying a new user access to the system if the system load is already high since dropping is assumed to be more annoying than blocking. In a highly loaded system, the interference increase may cause the system to enter an unstable state and may lead to call dropping. Hence, in addition to securing cell coverage, the admission control is used in order to achieve high capacity and still maintain system stability. Admission control is required in both links, since the system is capable of serving different services. Furthermore, different services demand different capacity as well as different quality. Hence, service-dependent admission control thresholds will be employed. These services-dependent thresholds should preferably depend on load estimates, for instance, the received power level at the base station as an uplink load estimate and the total transmitted power from a base station as a downlink load estimate. Since the received power level as well as the transmitted power level may change rapidly, event-driven measuring and signaling are preferred. The measurement values are obtained at the base station, where the admission decision ought to be made, unless global information is required. Arrivals of high-bit-rate users, particularly the ones that require a large amount of resources in the downlink, may demand global information in order to make an efficient admission decision. WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

34 6.9.2 Congestion Control Even though an efficient admission control algorithm and an efficient scheduling procedure are employed, an overloaded situation may still occur. When reaching overload, the output powers are rapidly increased by the fast closed-loop power control until one or several transmitters are using their maximum output power. The connections unable to achieve their required quality are considered useless and are only adding interference to the system. This is, of course, an unacceptable behavior. Hence, a procedure to remove the congestion is needed. The congestion problem is particularly severe in the uplink, where the high-interference levels may propagate in the system. The impact of the high-uplinkinterference level, due to overload, may be limited by integrating the uplink power control with the uplink congestion control procedure. This is achieved by slightly degrading the quality of the users in the overloaded cell during the time it takes to resolve the congestion. The congestion control consists of several steps: lowering the bit rate of one or several services that are insensitive to increased delays this is the most preferred method; performing interfrequency handovers; removing one or several connections. The congestion control is activated once the congestion threshold is exceeded. Thus, both the uplink and downlink thresholds correspond to a certain load. This means that the same measurements as in the admission control are used. However, to detect overload, these measurements have to be updated continuously since the considered values vary very rapidly when overload occurs. In order to make an efficient decision regarding which connections to deal with, i.e., minimizing the number of altered connections, the congestion control algorithm is likely to require global information. This information is obtained by event-driven signaling, triggered by the occurrence of overload. Once the connections to alter are identified, the required signaling is typically the same as for altering bit rates, performing an interfrequency handover or call termination Performance Both link and system level performance of WCDMA were investigated during the UMTS radio-interface evaluation phase in ETSI. In this section, some results from this evaluation, that are taken from [45], are presented. More simulation results and detailed descriptions of simulation assumptions are found in [50] and [53]. The document [54] sets the rules for the evaluation and describes the environments more in detail. The evaluation is based on the REVAL procedure so, e.g., the channel models can be found also in [55]. Dynamic system simulations were performed to translate the link-level results into system capacity and spectrum efficiency. The capacity was measured at the point where 98% of the users were satisfied (see [54] for definition). WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

35 The BER performance of uplink 8-kbps speech is plotted in figure 41 [45] for the outdoorto-indoor and pedestrian A (3 km/h) and vehicular A (120 km/h) environments. Twobranch antenna diversity is assumed. FIGURE 41. Link-level BER performance for 8-kbps speech. The speech data is coded using a rate 1/3 convolutional code and interleaved over 20 ms. No TFI is transmitted, i.e., blind-rate detection is assumed. It can be seen that the required E b /N o to obtain a BER of 10-3 is 3.3 and 5.0 db for the pedestrian and vehicular environments, respectively. The E b /N o values include all overhead, such as 8-b CRC, 8-b encoder tail, and the entire DPCCH. The corresponding downlink E b /N o values, where no antenna diversity is used, are 6.7 and 7.6 db, respectively. The system simulations for speech assume 50% voice activity. A Manhattan-like model is used for the pedestrian channel, while the vehicular channel is assumed in a classic threesector macrocell environment. In the Manhattan environment, the spectrum efficiency of speech is 189 kbps/mhz/cell in the uplink, and 163 kbps/mhz/cell in the downlink. The corresponding figures for the macrocell environment are 98 and 78 kbps/mhz/cell, respectively. In the macrocell, one cell is the same as one sector. Unconstrained delay data (UDD) services, i.e., packet services, were also evaluated in ETSI. The UDD packet services have characteristics modeling WWW browsing sessions and are defined in [ Selection procedures for the choice of radio transmission technologies of the UMTS (UMTS 30.03), ETSI Tech. Rep , version 3.1.0, Nov. 1997]. One packet service defined is the UDD 384 service. In this service, when packets arrive for transmission over the radio interface, the average bit rate of those packets is 384 kbps. However, packets do not arrive continuously. The time between packets can be used for transmission over the radio interface. This means that the average link level bit rate can be lower than 384 kbps and still meet the requirements. In WCDMA simulations performed, the link level bit rate used for the UDD 384 service is 240 kbps (the minimum rate allowed according to [54] is 38.4 kbps). WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,

36 The UDD 384 service was simulated for the outdoor to indoor and pedestrian A (3 km/h) channel in the Manhattan environment. Rate 1/2 convolutional coding is used to obtain the 240-kbps link level bit rate, with interleaving over 10 ms. In the link-level simulations, the block error rate (BLER) performance for 300-b blocks was studied. A BLER of around 10% is a good working point for the ARQ scheme [20]. The 10% BLER value is reached at an value of 0.2 db in the uplink with antenna diversity (see figure 42 taken from [45]). In the downlink without antenna diversity, the corresponding value is 3.2 db. As for speech, the values include all overhead, including a TFI field. FIGURE 42. Link-level BER and BLER performance for 240-kbps data. In the system simulations, an ARQ scheme with retransmissions of 300 bit blocks was used to find the UDD 384 performance. Simulations of the UDD 384 packet service, using the 240-kbps link, show a spectrum efficiency in the uplink and downlink of 470 and 565 kbps/mhz/cell, respectively. Using a 384-kbps link level bit rate will lead to similar spectrum efficiency numbers. WCDMA spectrum efficiency numbers are summarized below in table 7. TABLE 7. WCDMA Spectral Efficiency for Different Services and Environments Spectrum efficiency [kbps/mhz/cell] Service and env. Uplink Downlink Speech UDD 384 Manhattan Macrocell Manhattan WCDMA and cdma The Radio Interfaces for Future Mobile Multimedia Communications - Part IIDecember 15,