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3rd Generation Systems Review of Cellular Wireless Networks UMTS.

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1 3rd Generation Systems Review of Cellular Wireless Networks UMTS

2 Cellular Wireless Network Evolution
First Generation: Analog AMPS: Advance Mobile Phone Systems Residential cordless phones Second Generation: Digital IS-54: North American Standard - TDMA IS-95: CDMA (Qualcomm) GSM: Pan-European Digital Cellular DECT: Digital European Cordless Telephone

3 Cellular Evolution (cont)
Third Generation: T/CDMA combines the functions of: cellular, cordless, wireless LANs, paging etc. supports multimedia services (data, voice, video, image) a progression of integrated, high performance systems: (a) GPRS (b) EDGE (c) UMTS

4 Cellular Concept Geographical separation Capacity (frequency) reuse
Backbone connectivity BS Backbone Network

5 AMPS (Advanced Mobile Phone System): FDMA
G Frequencies are not reused in adjacent cells In each cell, 57 channels each for A-side carrier and B -side carrier Channels are divided into 4 categories: 1. Control (base to mobile) to manage the system. 2. Paging (base to mobile) to alert mobile users to incoming calls. 3. Access (bidirectional) for call set up and channel assignment. 4. Data (bidirectional) for voice, FAX, or data

6 Handoff Handoff: Transfer of a mobile from one cell to another
Each base station constantly monitors the received power from each mobile. When power drops below given threshold, base station asks neighbor station (with stronger received power) to pick up the mobile, on a new channel. The handoff process takes about 300 msec.

7 Digital Cellular: IS-54 TDMA System
Second generation: digital Same frequency as AMPS Each 30 kHz RF channel is used at a rate of 48.6 kbps 3 TDM slots/RF band 8 kbps voice coding 16.2 kbps TDM digital channel 4 cell frequency reuse Capacity increase per cell per carrier 3 x 416 / 4 = 312 (instead of 57 in AMPS) Additional factor of two with speech activity detection.

8 IS-54 slot and frame structure
1944 bits in 40 ms( b/s) G6 R6 DATA16 SYNC28 DATA122 SACCH12 DVCC 12 MOBILE TO BASE DATA130 SACCH 12 RSVD 12 G:GUARD TIME R:RAMP TIME DVCC: DIGITAL VERIFFICATION COLOR CODE RSVD: RESERVE FOR FUTURE USE BASE TO MOBILE

9 GSM (Group Special Mobile)
Pan European Cellular Standard Second Generation: Digital Frequency Division duplex ( MHz Upstream; MHz Downstream) 125 frequency carriers Carrier spacing: 200 Khz 8 channels per carrier (Narrowband Time Division) Speech coder: linear predictive coding (Source rate = 13 Kbps) Modulation: phase shift keying (Gaussian minimum shift keying) Multilevel, time division frame structure Slow frequency hopping to overcome multipath fading

10 GSM functions - TDMA access technique
BURST TRANSMITTED BY TDMA FRAME (4.6 ms) MOBILE 1 MOBILE 2 The GSM has adopted the TDMA solution for the access scheme, hence each connection is assigned a frequency and a slot along the TDMA frame. As said, in this case the bandwidth is shared among the cells according to a pre-defined scheme: the adopted frequency plan. Within each cell, a second level of sharing takes place between mobile terminals roaming in that cell. The figure shows how eight mobile terminals can share the same carrier by making use of the eight frame time slots. It will be clarified that some of the available slots among those available in a cell are devoted to realise some logical channels carrying signalling information (both system and connection related information). MOBILE 8 TIME TIME-SLOT: 577 ms SIGNAL BURST: 546 ms

11 GSM network architecture and functions
Signalling channels BCCH: Broadcast Control Channel point-to-multipoint unidirectional control channel broadcasting system information to MS CCCH: Common Control Channel up-link: RACH (Random Access CHannel) down-link: PCH (Paging Channel) AGCH (Access Grant CHannel) DCCH: Dedicated Control CHannel point-to-point bidirectional control channel SACCH (Slow Associated Control CHannel) FACCH (Fast Associated Control CHannel) SDCCH (Stand Alone Dedicated Control CHannel) In order to drive the various functions needed to manage connections and mobility, one (or more) slots of the TDMA frame are devoted to carry logical channels for signalling and control purposes. BCCH carries in a point-to-multipoint manner all the system information needed to the mobile terminals roaming in that cell CCCH on the up link carries the random access channel, accessed with contention by all the mobiles roaming in the cell; on the down link, two logical channels are time multiplexed on CCCH, namely: the paging channel, addressing the mobile which is requested by an incoming call the access grant, carrying the acknowledgements to the mobile access. DCCH carries in both up and down links, control channels that are either signalling-dedicated or associated control channels; the former are carrying the call signalling in the phases when the traffic channel has not yet been activated for that call; the latter are active in conjunction with any channel activated (either signalling or traffic channel) and are basically devoted to transport measurement and handover-related information. SDCCH carries the signalling procedure otside the activation of a traffic channel SACCH and FACCH carry the control information associated to traffic (TCH) and signalling channels (SDCCH).

12 UMTS (Universal Mobile Transport Service)
Requirements 384 Kbps for full area coverage 2 Mbps for local area coverage variable bit rate packet traffic support flexibility (eg, multiple, multimedia streams on a single connection)

13 Third generation services
2M 384K 64K 32K 16K 9.6K 2.4K 1.2K video conference remote medical service video catalogue shopping video on demand mobile TV video conference electronic newspaper ISDN internet telephone conference voice mail distribution services (voice) mobile radio pager electronic publishing distribution services (data) Third generation mobile services can be grouped according to a bi-dimensional scheme characterising the service bandwidth and its connectivity behaviour. Wherever the de-regulation process has reached an advanced phase (USA, Canada, UK) new service delivery conditions are emerging. For instance, the ability of an operator to be active in both fixed and mobile service context, is strictly dependent on the regulatory constraints adopted in a particular country. These conditions are likely to modify significantly the service grouping and delivery configurations in the near future. The user is interested in being offered an easy way to access services and an easy way to use the relevant features. Possibly, the user requires also that advanced solutions such as single subscription, single number are available together with a simplified process in contacting the Operator/Service Provider. On the other hand, as for the service development process, Operator requirements and economical interests are rapidly changing, mainly due to the growing level of competition. In particular, mobile Operators are more and more concerned with: the reduction of operations costs the reduction of service time to market the capability of developing technology independent services. This is just to say how the pure bit rate (or delay and connectivity) requirements are not enough to identify optimal solutions for mobile systems. telephone FAX bidirectional unidirectional multicast point to point multipoint

14 Third generation bandwidth assignment (I)
ITU IMT-2000 IMT-2000 MSS MSS 1885 1920 1980 2010 2025 2110 2170 2200 MHz EUROPE DECT IMT-2000 MSS IMT-2000 MSS 1880 1900 1980 2010 2025 2110 2170 2200 MHz A basic issue is then represented by the spectrum requirements for third generation mobile services. The overall bandwidth assigned by WARC ‘92 to the world regions is shown in the figure. The bandwidth assigned to IMT-2000 systems includes both symmetric and asymmetrical portions. JAPAN IMT-2000 PHS MSS IMT-2000 MSS 1885 1895 1918.1 1980 2010 2025 2110 2170 2200 MHz

15 UTRAN (UMTS Terrestrial Radio Access Net) Architecture
Core Network Iu Iu UTRAN RNS Iur RNS RNC RNC Iub Iub Iub Iub B-node B-node B-node B-node Now, let’s consider in a more detailed way the possible architectural choices within the Radio Access Network. For the RAN, a scheme of the kind sketched in the figure, is more or less universally adopted (the depicted architecture is that adopted by ETSI for the UMTS Terrestrial Radio Access Network - UTRAN). UTRAN is composed by several Radio Network Subsystems (RNSs) connected to the Core Network through the Iu interface. The RNSs can be directly interconnected through the Iur interface. Every Radio Network Subsystem is composed of a Radio Network Controller (RNC) and one or more “B nodes”. These entities are responsible of the radio resource control of the assigned cells; A B-node may contain a single BTS or more than one (typically 3) controlled by a site controller. RNC is responsible for the local handover process and the combining/multicasting functions related to macrodiversity between different B-nodes. The above architecture is adopted as a model of the radio access segment to analyse the main function allocation and architectural problems arising in the definition of third generation mobile systems. Site Contr BTS Site Contr BTS Site Contr BTS Site Contr BTS

16 Access techniques for mobile communications
FDMA (TACS) P F TDMA (GSM, DECT) T ATDMA (UMTS) P F CDMA (UMTS) T The figure summarises the basic concepts driving the possible radio access schemes: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) e CDMA (Code Division Multiple Access). For each, the related radio resource usage mechanisms are highlighted in the time, frequency and power domains. In the light of the most recent developments, the most innovative access technique in the field of mobile services is the Code Division Multiple Access (CDMA) technique. In January 1998 ETSI took an important decision about the radio access technique to be used for UMTS: for the asymmetrical portion of the spectrum available for UMTS, a hybrid solution based on a TDMA + narrow band CDMA scheme has been chosen for the symmetrical portion of the spectrum, a wide band CDMA scheme has been adopted. The former solution is thought mainly for in-door applications, the latter for public/outdoor/high mobility services. The Code Division Multiple Access (CDMA) belongs to the family of spread spectrum techniques. The following figures are recalling just some of the basic concepts on which the CDMA technique is based; this is done just to supply some background information to facilitate the comprehension of the subsequent network considerations. P F P - Power T - Time F - Frequency T

17 W-CDMA (Wide Band CDMA)
Key features Improved capacity and coverage (over second generation CDMA); backward compatible High degree of service flexibility: multiple, parallel services per connection; efficient pkt access Operator flexibility: asynchronous interstation operation; hierarchical cell structures (HCS); adaptive antenna arrays (enabled by uplink pilot symbols); TDD (Time Division Duplex) mode for asymmetric traffic and uncoordinated environments.

18 Radio Interface - protocol architecture
C-plane U-plane L3 RRC LAC L2/LAC LAC LAC Logical channels RLC The relationship between the mobile and the network is normally seen through the radio interface. It specifies how the information that has to be exchanged between those two entities (both user data and signalling information) has to be organised in a layered framework. Note that the radio interface is not directly specifying the end points of each protocol layer in the network: the allocation of the protocol end-points termination obeys network efficiency and performance criteria that will be discussed. The physical layer implements all the services and functions needed for the transfer of signalling and data information; from a radio resource viewpoint, the basic role of L1 is to map the transport channels to the radio physical channels. The MAC layer offers the transport service to the upper layers, in line with the defined attributes. The Radio Link Control Protocol (RLC) offers services and functions that are ensuring the correctness of the data transfer (ARQ functions) and performs the data segmenting/re-assembling features. This protocol could in principle be embedded in the MAC protocol. The presence of a Layer 2/LAC (Link Access Control) level is still under discussion in ETSI. Its role could consist in supplying retransmission features mainly in relation to handover and macrodiversity requirements, whenever they cannot be supplied by the RLC Protocol. The option of having a second ARQ layer depends on the archirectural solutions that will be adopted for the handover and macrodiversity resolution. Layer 3 contains, at least in the radio access segment on the C-Plane, services and functions related to the control of the radio resources. RLC RLC RLC L2/MAC MAC Transport channels L1 Physical Layer

19 Layer 1 - up link physical channels (W-CDMA)
Dedicated Physical Data Channel Data 0.667 ms Dedicated Physical Control Channel Pilot Feedback indicator Transmit power control Transport format ind. Layer 1 services refer typically to the transport role; a transport channel is specified by the way (e.g. bit rate and delays) adopted to transport the data offered by the upper layers; this role is played by making use of physical channels, directly defined on the physical structure of the radio interface. The example represents the up link physical channels under discussion in ETSI for the W-CDMA scheme. On the up link, two type of physical channels are identified, namely dedicated physical channels and common channels. The dedicated channels are: the Dedicated Physical Data Channel (DPDCH) used to carry the data (user or control data) generated by the MAC and upper layers the Dedicated Physical Control Channel (DPCCH) used to carry control information generated at Layer 1; specifically: pilot symbols for the channel estimate; transmission power control information and the transport format indicator (the latter specifies the configuration of encoding, interleaving, bit rate and physical channel mapping applied to the transport channels); in addition, DPCCH carries the feedback information channel, controlling the transmission diversity of the base station. A single common channel is defined on the up link, namely the physical random access channel (PRACH), offering the early access capabilities to the MSs. The PRACH is based on a Slotted Aloha concept; the access slots are defined on the depicted frame structure and the broadcast channel reports the needed information about the PRACH that can be accessed in that particular cell. Both DPDCH and DPCCH obey the frame structure depicted in figure; the frame is a 10 ms frame, split in 15 slots, each one lasting ms, corresponding to a power-control period. The number of bits inserted in each slot depends on the spreading factor adopted in that particular instant. Slot#1 Slot#2 Slot#i Slot#15 Frame#1 Frame#2 Frame#i Frame#72 10 ms

20 Layer 1 - down link physical channels (W-CDMA example)
DPCCH DPDCH Pilot TFI Data TPC 0.667 ms Slot#1 Slot#2 Slot#i Slot#15 frame Also for the down link, dedicated and common physical channels are foreseen. In the dedicated channels, data information generated at Layer 2 or above, are time-multiplexed with the control information generated at Layer 1 (pilot, power control and transport format indication). The frame structure is the same as for the up link. Whenever the total bit rate of a connection exceeds the maximum allowed bit rate for a single DPCH, a multi-code transmission is adopted, employing more than one DPCH for that connection; if a single spreading factor is used for each DPCH, the Layer 1 control information is associated to one only of the physical channels: the other does not transmit in the corresponding slot period. The common physical channels defined on the down link are: the primary Common Control Physical Channel used to carry the Broadcast Channel (BCH) the secondary Common Control Physical Channels, used to carry the Forward Access Channel (FACH) and the Paging Channel (PCH); FACH is used to transmit mobile specific control information and short packets. Both primary and secondary common channels obey the frame structure described for the dedicated channels; the only difference is that for the Common Channels, the single slot includes only two data fields, containing the pilot and the data information respectively; both primary and secondary control channels adopts a fixed bit rate, which is defined at system level for the former and at a cell level for the latter (the cell BCCH informs the roaming mobiles about the secondary channel bit rate). A primary and a secondary synchronisation channels are defined on the down link and transmitted with specific codes synchronously with each BCH slot; the former is unique for all the system, the latter identifies the scrambling code associated to the specific station. Frame#1 Frame#2 Frame#i Frame#72 superframe 10 ms

21 Transport channels (example)
Dedicated Channel (DCH): fast change of bit rate (10ms) fast power control inherent MS addressing Random Access Channel (RACH) - up link: collision open loop power control explicit MS addressing Broadcast Control Channel (BCH) - down link Forward Access Channel (FACH) - down link: slow power control Paging Channel (PCH) - down link: use of sleep modes The figure lists the transport channels that are mapped onto the physical channels previously introduced; the main characteristics of each transport channel are summarised in the figure. Each transport channel is associated to a transport format (represented by the transport format indicator); the transport format belongs to a set of possible transport formats that can be associated to that transport channel.

22 Multiplexing transport channels onto physical channels
trasport channels multiplexing DCH interleaving coding matching rate DCH interleaving matching rate interleaving multiplexing At Layer 1, the Dedicated Transport Channel, in general undergoes some processing before being mapped to physical channels and transmitted (after spreading); the basic process steps are shown in the figure. Several transport channels can be multiplexed before being mapped to one or more physical channels. Channel coding is performed in general per each transport channel independently; several options are available for the coding algorithm (convolutional, Reed-Solomon, turbo coding) that can possibly be use in conjunction. Different transport channels can possibly be interleaved. The static rate matching is performed at a slow pace, typically when a transport channel is added or removed from the pool of jointly transmitted transport channels. This process is done in order to fulfil the transmission quality requirements of each transport channel, by maintaining at the same time a uniform bit energy per each channel. Another role of the static rate matching is to adjust the (coded) channel bit rate in accordance with the available bit rate of the dedicated physical channels. The dynamic rate matching is performed on the up link, after multiplexing, to adapt the instantaneous bit rate of the multiplexed transport channels to the bit rate of the dedicated physical channel(s); on the down link, DTX is used to cope with bit rate misalignments; both static and dynamic rate matching use unequal repetition techniques. The transport channel multiplexing is performed on a time-division basis, within a radio frame. Inter frame interleaving is performed within a single transport channel over more than one frame for those services that require this kind of mechanism; intra frame interleaving is performed on a single frame and can apply to multiplexed transport channels. DCH dynamic (up link) intra frame interleaving interleaving coding matching rate interleaving DCH static inter frame interleaving

23 MS physical layer up-down link example of multiplexing
DCH DCH DCH DCH DCH DCH decoding and demultiplexing Coding and multiplexing Down link mapping TFI transmitted on the control channel multiplexing The multiplexing technique is applied to Dedicated Channels only, all the common channels (RACH, BCH, PCH and FACH) are independently coded (on both the up and down link) and each is mapped on a single physical channel. The configuration of coding and multiplexing of the transport channel(s) is given by the Transport Format Indicator which identifies the instant (10ms) configuration within a set of pre-defined transport configurations; it was already said that this indicator is carried on both the up and down link over the Dedicated Physical Control Channel (DPCCH) The differences between up and down link are due to the soft handover mechanism; moreover, on the receiving side, the down link Dedicated Physical Channel, Data and Control part are time multiplexed, as already stated. The effect of macrodiversity on the receiving side is shown in the figure; the equivalent physical channels (carrying the same information bits through different cells) are combined in the MS receiver; also TPC and TFI can be combined in the MS if they bear exactly the same information through different cells. In the example, on the MS receiving side, all the three cells involved in the active set use the same TFI, hence, in this case, it could be combined in the MS. It may happen that different services of the same call are using differently the macrodiversity mechanism, for instance, along the same call, it may happen that for one connection a macrodiversity state is active while for another connection no macrodiversity is allowed (for radio transmission reasons); in this case, the TFIs of the various cells are in general different from each other and may not be combined. phy ch phy ch phy ch phy ch phy ch Cell 1 phy ch phy ch Cell 2 phy ch phy ch Cell 3 Up link

24 MAC Services and Functions
set-up, release of logical channels data transfer service on logical channels allocation/re-allocation of radio resources measurement report Functions mapping phy ch DCH Coding and multiplexing mapping phy ch DCH Coding and multiplexing Selection of the transport format Handling of priority within one user/between users Scheduling of control messages (broadcast, paging, notification) Multiplexing/de-multiplexing of higher layers PDUs on/from common or dedicated transport channels Contention control on the random access channel The MAC layer manages the mapping of the logical channels to the transport channels by controlling the mapping/multiplexing mechanisms previously introduced. The transport format is chosen within the set of available transport formats. Priorities are handled by choosing the appropriate transport formats; for instance, data with high priority can use an high bit rate format; the same solution can be adopted for data which have accumulated a high delay in the upper layer queues. On the down link, still the transport formats can be used to fulfil different priority requirements arising between different users. Most of the functions listed in the figure may require an exchange of signalling information between MAC layers belonging to MS and the relevant end point in the access network. Contention control is related to the collision event that may arise on the random access channel (the contention is detected by Layer 1 and possibly resolved by Layer 3).

25 Retransmission Protocol - services and functions
Layer 2 connection set-up and release transparent data transfer unacknowledged data transfer acknowledged data transfer Services Functions connection control segmentation and re-assembly error detection/recovery and in-sequence delivery transfer of user data flow control duplicate detection QoS adaptation RCLP PDU RCLP PDU RCLP PDU The layer 2 functions residing above the MAC layer, take the role of controlling the data transfer of any connection; to do so they supply error correction, retransmission, combining and multi-casting when needed. These roles can be split for the U-plane between the Link Access Control (if present) and the Radio Link Control (RLC) functions. RLC controls the transfer of user data; in sequence delivery; segmentation and re-assembly mechanisms; Automatic Repeat Request (ARQ) functions. Through the above mechanisms, RLC supports upper layers with: acknowledged mode operation (ARQ and variable bit rate) unacknowledged mode operation (variable bit rate) transparent mode operation (no RLC service) An example of segmentation function is reported in the figure; in this case the RCL service allows to transmit with different rates, RCL data PDUs, according to the need of the traffic source; the bit rate can be changed every frame period (10 ms). If present, the LAC implements all the functions tied to the connection control and the Layer 2 algorithms: establishment and release of LAC connections; ARQ; flow control; segmentation, re-assembly. The above services are used to supply assured, unassured and transparent transport services to layer 3 PDUs. 160 bit 160 bit 160 bit 10ms 10ms 32kbit/s 16kbit/s

26 Radio Resource control - functions
Broadcast of information provided by the Core Network related to the access segment Set-up, maintenance and release of an RRC connection Set-up, maintenance and release of radio bearers on the user plane Assignment, reconfiguration and release of radio resources for the connection Arbitration of radio resource allocation between cells RRC connection mobility functions Quality of Service control and radio resource allocation among the cells Admission and congestion control Control of the MS measurement reporting The basic requirements for the Radio Resource control refer to: the need for broadcasting messages to the MSs in a given area (general control provided on the General Control SAP) the need for notifying paging messages and other mobile-specific information within a given area (notification control provided on the Notification SAP) the need for setting-up and releasing connections (point-to point or multiparty connections provided on the Dedicated Control SAP); for each control it must guarantee: the correct transfer of messages along the connection the correct management of priorities between signalling and non-signalling messages (e.g. short messages) sharing the same control channels. The RRC connection is a signalling connection between the MS and the Radio Access Network. The control of radio bearers includes the execution of the admission control algorithms and may include the setting-up of the radio parameters for Layers 1 and 2; this choice heavily conditions the splitting edge between radio dependent and independent functions that will be discussed in the following. The control of radio resources for the connection is tied to the allocation/de-allocation of resources also along the same connection. The RRC mobility functions refer also to handover and macrodiversity.

27 Orthogonal Variable Spreading Factor
= (1,1,1,1) 4,1 c = (1,1) 2,1 c = (1,1,-1,-1) 4,2 c = (1,-1,1,-1) The Orthogonal Variable Spreading Factor Codes ensure the complete ortogonality between the different channels. The use of OVSF codes follows the binary tree scheme of the figure and the choice of the code depends on the bit rate of the connection to be spread. The code sequence has to be repeated bit per bit of the original digital signal, hence the code depth is chosen accordingly. The choice of a given channelisation code inhibits the use of all the derived codes it is subtending in the tree scheme. Being the chip rate constant, the above allocation scheme implies that connections with different bit rates are transmitted with different spreading factors. 4,3 c = (1,-1) 2,2 c = (1,-1,-1,1) 4,4

28 Uplink Variable Rate 10 ms 1-rate 1/2-rate 1/4-rate 0-rate Variable
Different connections can then use different rates and the rate of a connection can be changed along the connection itself. The Dedicated Channel has to be maintained in any case, due to the information it is carrying. The change of bit rate may also imply the modification of the mapping between transport and physical channels, whose configuration is controlled through the Transport Format Indicator. The basic chip rate for the ETSI proposal is Mchips/s. Additional chip rates are specified for the FDD mode, namely Mchips/s and Mchips/s. These are for further evolution of the radio access towards services requiring a transmission capacity greater than 2Mbit/s. Variable rate R = 1 R = 1/2 R = 0 R = 0 R = 1/2 : DPCCH (Pilot+TPC+RI) : DPDCH (Data)

29 Downlink Variable Rate (DTX based)
0.625 ms 1-rate 1/2-rate 1/4-rate 0-rate On the down link, Data and Control channels are time multiplexed. With the same spreading factor, variable bit rate within the same Physical Channel is managed through a discontinuous transmission mechanism, as depicted in the figure. High bit rate connections require, as seen, multi-code transmission using more than one Physical Channel. : DPCCH-part (Pilot+TPC+RI) : DPDCH-part (Data)

30 TD-CDMA (Time Division Duplex)
Multiple access scheme TDMA/CDMA Channel spacing 5 MHz Charrier chip rate 3.84 Mchips/s Spreading factor 1-16 Frame length 10 ms Multirate concept multislot /multicode Modulation QPSK Burst Types burst 1 long delay spread burst 2 short The TD-CDMA logical channel structure is similar to that adopted for the W-CDMA case. Traffic channel and control channels are envisaged: traffic channel, used to transfer user data or layer 3 signalling data control channels, further divided in common control and dedicated channels Common Control Channels BCCH Broadcast Channel (DL) SCH Synchronisation Channel (DL) FACH Forward Access Channel (DL) PACH Paging Channel (DL) RACH Random Access Channel (UL) Dedicated Channel DCCH Dedicated Control Channel-ACCH and SDCCH (DL&UL) DTCH Dedicated Traffic Control (DL&UL) Detection Coherent, based on midamble

31 multiframe =24 frames (240 ms)
TDD - frame structure multiframe =24 frames (240 ms) 23 frame = 15 TS (10 ms) 15 UL DL codes DL>UL UL>DL The ETSI proposal for the TD-CDMA access scheme is based on: 15 slots per frame; 10 ms frame duration (the same as for the FDD case) 24 frames per multiframe; 240 ms multiframes switching points between up and down sessions to adapt to the varying UL/DL asymmetric traffic 16 orthogonal codes (Walsh) are available for each time slot. The code management within a given time slot can assign one or more orthogonal codes to a given transport channel. Multicode/multislot arrangements are then used to cope with high bit rate services. switching points BCCH RACH UL TCH DL TCH

32 Packet Data Service In W-CDMA, data packets can be transmitted in 3 ways: (a) RACH (Random Access Channel): used for small amount of data; no reservations, thus low latency; but, collisions and no power control (on RACH) (b) Request a dedicated channel (like VC setup): MS sends a Res Req msg (on RACH) with traffic specs; network returns a Req All + Cap All (with transport formats) on FACH, if resources are available; Cap_All may be issued separately (later) if the network load is high; MS transmits after receiving the Cap All.

33 Packet Data Service (cont)
(c) use existing dedicated channel (before it expires): if a DCH was recently used, go ahead and tx the unscheduled pkt on that channel. If timer expired, the MS can still omit the Res Req and issue just the Cap All

34

35

36 Real time services MS issues Res_Req on RACH (or on DCH if it has one going) Network issues Res All (with TF parameters) MS starts transmission immediately (no wait for Cap_All) Network may later reduce/restore the TF depending on load fluctuations

37 Congestion Control Congestion may occur even after careful admission control without cong. control, mobiles tend to increase their tx power, to combat interference, thus aggravating the problem solutions: (a) lower bit rate of users insensitive to delay; (b) perform interfrequency handovers ( c) remove connection(s) congestion control remedies activated by load thresholds

38 Handover modes Handover Functions basic feature for the RAN
hard seamless Handover Functions basic feature for the RAN architecture The figure summarises the main kinds of handover. Hard handover is typically done in second generation mobile like the GSM. It imples a time disjunction between the two alternative paths involved in the handover process. The seamless handover introduces a level of duplication of the radio resources before switching to the new path. This solution is typically implemented in TDMA systems adopting a Dynamic Channel Allocation scheme (like for instance the DECT system). The soft handover case is introduced with the CDMA access mode; it is based on the eventuality that a single connection can be simultaneously in progress with more than one base station (macrodiversity). The redundancy of the multiple connection paths is possible in CDMA schemes. The latter kind of handover shows a significant impact on the architecture of the radio access network. soft

39 Macrodiversity - active set
Cell A Cell B Ec/No Signal margin Time margin ADD threshold In line with its access characteristics, a CDMA system can implement the handover function in a completely asynchronous way; this “soft handover” functionality is enabled through the macrodiversity mechanism. As said before, a given connection which has been set-up between the mobile station and the network, may be active through more than one base station; this means that the same information data are transported on the radio interface through different channels (codes) which are activated with different stations. The figure highlights the mechanism typically adopted to determine the set of stations (the active set) carrying the connection; the activation/de-activation of the parallel paths is done in function of the signal level received by the mobile along its movements. In the following, the impact of this feature will be further analysed, but something can be argued since now: a single connection is active through more than one path, each routed independently in the network. Of course, soft handover and macrodiversity are strictly linked to each other: in the phase of the “channel change” along the handover process, the connection continuity is implicitly guaranteed through the paths multiplicity realised between the mobile terminal and the “bridging point” in the network. This mechanism relaxes the stringent time constraints that characterise the classical types of handover (e.g. the hard handover of a TDMA system) where the connection continuity is ensured through a change of frequency and time slot and a time disjunction between the activation of the two alternative paths. DROP threshold Soft handover region Cell C Time

40 The macrodiversity control
point two control points The macrodiversity mechanism drives the soft handover process which is typical of the CDMA access. This mechanism guarantees the connection continuity by doubling the the active channels between the mobile terminal and an equivalent bridging point in the network. This multiple path is maintained along the connection, so that the continuity is ensured through an asynchronous acquisition/release process which relaxes the stringent time constraints characterising the handover execution in the present systems. The set of BTSs involved in a connection which is in the macrodiversity status (active set) is changing dynamically with the movements of the mobile station. In conjunction with a change of the active set, the Radio Access Network must identify the combining/multicasting point recovering in the upper direction a single path from the multiplicity of equivalent paths in progress on the radio interface (combining) and, creating, in the downward direction, the necessary equivalent paths to be directed towards the mobile terminal (multicasting). Different combining levels may be in progress along the same connection: they may base on different levels of the quality information associated with the data flow. The quality information can be either associated to any single bit (soft information) or to a data packet (hard information). mobility control points mobility


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