Optimizing Shared Channels for Efficient Packet Data Transmission in W-CDMA

2. SHARED CHANNELS FOR PACKET DATA TRANSMISSION IN W-CDMA

3. Outline • Introduction • Strategy • UMTS Packet Data Implementation • Advantages of Shared Channel • Benefits of Fat Pipe • Downlink Shared Channel (DSCH) • Limitations of Packet Modeling Techniques • Uplink Shared Channel (USCH) • Conclusions and Recommendations

4. Don’t Transmit Packets on Circuits • Current UMTS approach looks more like fast circuit than packet switching. • For short packets, RACH is used. • For long packets, RACH sets up a brief circuit connection • Resource requirements changing continuously. • Not possible to negotiate appropriate data rate a priori. • Data rate is determined by Packet Size X Interarrival Time. • UTRAN must estimate the source data rate based on packet arrivals. • Internet/Intranet will be terminus for most data services. • Employ common IP packet scheduling. • “Random Early Detection” (RED) for congestion avoidance • “Weighted Fair Queuing” for packet scheduling • Adopting IP Techniques will insure compatibility with new Internet applications.

5. Strategy • Interference Management for Packet Channels • Provide uniform composite interference of all packet users across the cell • Schedule packet data burst intelligently to satisfy power and interference constraints of the cell in question • Maximize statistical multiplexing gain • Maximize peak transfer rates to a single mobile • Allocate a high rate channel to a single user rather than multiple low rate channels to multiple users • Minimize the access and paging delay for quick allocation of resources • Efficiently multiplex small packets from/to multiple mobiles

6. UMTS Packet Data Implementation • Shared Channel maximizes statistical multiplexing gain • Assign the fattest possible data pipe to a user so that overall delay experienced is minimized • Downlink Shared Channel (DSCH) • Power and code resource is shared between users • Overcomes the problem of downlink OVSF code shortage • Uplink Shared Channel (USCH) • Limited power resource which is shared between users • Problem of code shortage does not exist

7. Advantages Of Shared Channel • Advantages of Shared Channel over Dedicated Channels (DCH’s) controlled by RRC • Resource more fully used in every frame ( provided there are packets to transmit) • Facilitates efficient shared access to a large data pipe • Highest priority packets gets served first, irrespective of which UE the packets are going to/from. This improves QoS. • Average packet call completion times improved. • The data rate of the shared channel can be dynamically varied in response to rapid change in conditions. • No reliance on imperfect packet call admission control which with DCH approach can result in inappropriate data rate assignment. • For the case of downlink • Shared channel provides an efficient method to access limited downlink OVSF codes • Proportion of power assigned for carrying packet connections could be packed more efficiently when shared channel is used

8. MAC Scheduling at the CRNC • Perform MAC scheduling in the CRNC on shared channels as opposed to RRC scheduling at the SRNC onto DCH’s • By only making short leases on the radio resource a light-weight protocol can be exploited • Perform scheduling on MAC instead of RRC in order to minimise signaling and processing overhead • Enable CRNC to perform scheduling (as opposed to SRNC) in order to reduce message exchanges across Iur and to thereby facilitate fast scheduling onto the fat pipe • Resource Allocations for each frame are signaled in each frame • Therefore no need for acknowledged mode signaling • More efficient resource usage, improved packet call completion times • Faster scheduling

9. Benefits of Fat Pipe • Findings published in Motorola Contribution to SMG2 UMTS-L23 534/98 dated 12/9/98 • Preferable to allocate the total packet bandwidth allocation to a single user than to allocate an equal total packet bandwidth of multiple narrower band channels to simultaneous users.

10. Benefits of Fat Pipe (cont’d) Note: Total delay time in Table 1 and 2 refer to packet call completion time

11. Overview of Downlink Shared Channel (DSCH) • Two methods for DSCH have been proposed • DSCH with Time Multiplexed Packet Users (proposed by Lucent, Sony and Nortel, Tdoc SMG2 UMTS-L23 159/98, 320/98, 266/98, 169/98) • DSCH with Fast Code Multiplexing (proposed by Nokia and Motorola, Tdoc SMG2 UMTS-L23 296/98, 533/98) • It was agreed in SMG2 that DSCH should utilize Fast Code Multiplexing (FCM). • Concept of DSCH included in ETSI’s document#XX03-130 • Two possibilities exists for carrying the control information for DSCH • Using a dedicated channel (DCH) • Using a common DSCH control channel (also called the ACCH)

12. DSCH with Fast Code Multiplexing • Segment of the Code Tree for Orthogonal Variable Spread Factor (OVSF) codes assigned to packet data services • The number of OVSF codes assigned for packet data services (and the number of UE’s served) can change on a frame by frame basis

13. Code Assignment for the DCH • A 384 kbps packet data service is assigned a SF = 8. • Seven 384 Kbps UEs at activity rate of 1/10 consume 87% of OVSF tree.

14. Code Assignment for the DSCH • All 384 kbps on the DSCH monitor the same ACCH at SF=64 and the SF=8 is assigned as needed. • The same seven 384 kbps UEs at activity rate of 1/10 consume only 14% of OVSF tree.

15. Signaling Options for DSCH • DSCH is associated with a DCH • Disadvantages • Less powerful coding on allocation messages (e.g. (32,6) Bi-orthogonal Code used for TFCI field) • Signaling resources consumed will be proportional to the number of users • DSCH is associated with a common control channel called Access Control Channel (ACCH) • Advantages • ACCH time multiplexes all assignments on a single, relatively low rate, OVSF code, thus reducing the overall OVSF codes used for control • ACCH is always synchronized to the frame timing of the current cell • Disadvantage • Fixed power allocation, does not use Fast Forward Power Control (FFPC) • Simulations show that ACCH will be more efficient when resources are needed the most.

16. Probability of N or more Simultaneous Packet Calls (Web Browsing) Dedicated Channel more efficient 100% 90% Common Channel 75% more efficient 80% 90% 70% 92% 95% 60% 50% 40% 30% 20% 10% 0% 0 10 20 30 40 50 N DSCH control channel efficiency

17. Common Control Channel vs. Dedicated Control Channel • Summary • For low channel utilization DCH is efficient • For high channel utilization or when resources are needed most ACCH is more efficient • Recommendation • Provide both methods in the specification

18. Limitations of Packet Modeling Techniques • Number of simultaneous users is sensitive to packet interarrival time. • Congestion elsewhere in the network may increase interarrival times • Confidence in the data models is modest at best? • Will all applications fit into the narrow data models for ftp, www, and email? • What are the correct proportions? • UMTS protocol must adapt to data traffic presented. A Common Control Channel makes no assumptions on data traffic patterns. • Maximum packet size is governed by the IP Maximum Transmission Unit (MTU). • Typical MTU is on the order of 500 bytes. • 1500 bytes is the practical maximum for the MTU • ETSI’s model specifies a maximum of 66,000 bytes • The total data transfers sizes will not be known a priori. Therefore, the dedicated channel may not be as effective as previous simulations suggest.

19. Details of the Common Control Channel for DSCH • Common Control Channel for DSCH (Access Control Channel (ACCH)) • Aggregates functions of • Uplink Power Control • Dynamic Persistence for RACH • Downlink OVSF Code Assignment • Uplink SF Assignment • Uplink Timing Event • ACCH provides a direct method for assigning resources of the shared channel • ACCH is not power controlled • ACCH is transmitted over the entire cell

20. Structure of the ACCH

21. ACCH Assignment Fields

22. Number of Assignments per Frame for Various SF and Coding Rates • With a Spreading Factor of 128 and using R=1/2 Convolutional or Turbo Code, ACCH can accommodate assignments for 5 UE’s in both direction or assignments for 10 UE’s in one direction simultaneously per frame.

23. Overview of Uplink Shared Channel (USCH) • USCH represents a shared power resource • USCH coordinates fast scheduling of uplink data packets • Insure an uniform interference power profile protecting voice users • Schedule “Budgeted Noise Rise” • Each active MS is assigned a fraction of total noise rise which translates to a Spread Factor (SF) assignment • Reassign the data rate on a frame by frame basis (functionally equivalent to downlink FCM) • UE synchronizes framing to the strongest BTS on the active set

24. USCH Details • Commonality with DCH • Identical PDTCH channel frame formats • Ability to perform fast and slow power control • May employ soft handoff if necessary • Differences from the DCH • Discontinuous uplink transmission requires a one frame preamble before the start of data transmission. • Performance is identical to DCH when frames are consecutive • The preamble will prime acquisition, channel estimation and power control. • Timing advance or guard band is required for large cell sizes • Transmission from an near to BTS UE may overlap the transmission from a far from BTS UE, resulting in excessive noise rise.

25. Fast Power Control and Channel Estimation for USCH • Convergence of power control loop and the availability of good channel estimates are critical for operation of USCH. Two solutions are envisaged • Use of a low rate bi-directional link maintenance channel between packet burst • Unnecessary power resource is consumed when there are no packets to transmit • Increase in uplink noise rise • Maintaining a dedicated downlink channel for each uplink channel will worsen the code shortage problem • Preamble transmission using DPCCH before packet data transmission

26. Bi-directional Link Maintenance Channel

27. Preamble Transmission

28. Preamble Transmission (cont’d) • Three cases are considered in the Figure • No need for Preamble, if RACH is used before transmission of packets • Preamble used to converge uplink DPCCH (for power control, channel estimation and acquisition), before packet data transmission starts on DSCH • Preamble used to converge uplink DPCCH (for power control, channel estimation and acquisition), before packet data transmission starts on USCH

29. Cd Ad Switched off during preamble transmission DPDCH Cscramb QPSK Modulation Ap Cc DPCCH j Preamble Transmission

30. Consecutive Idle Frames within a Packet Call for Various Values of System Utilization Mean packet size = 480 bytes Rate = 384 Kbps

31. Timing Events Timing Events • If a far-end UE and a near-end UE is assigned a low SF code in consecutive frames, the last part of transmission from far-end UE may collide with the first part of transmission from near-end UE (due to propagation delay) resulting in excessive noise-rise in the cell in question. • UE’s need to retard their timing by an amount Dt to prevent collisions • Three methods are proposed for computation of Dt: • Method1 - Dt is computed based on a relative distance between the two UE’s w.r.t BTS • Method2 - Dt is computed based on a distance between a single UE and the BTS • Method 3 - Uses a fixed guard period

32. UE’s Propagation Delay w/o Timing Offset Correction • a is proportional to the range between the node B and UE#A • b is proportional to the range between the node B and UE#B • g is proportional to the range between the node B and UE#C

33. Signaling Methods for Timing Events • Method - 1 • TOA from UE#A to node-B - l • TOA from UE#B to node-B - m • UE#B retards its frame timing by an amount Dt2 = l-m

34. Signaling Methods for Timing Events (cont’d) • Method - 2 • At Frame#1 an offset Dt2 is broadcast using ACCH • UE#B transmits data packets using an offset Dt2 • TOA denoted by r between the Node B and UE#B is computed • Node B broadcasts offset Dt3 = Dt2+b using ACCH • UE#C transmits data packets using an offset Dt3 • Offset is reset after it reaches a set threshold e.g. 10000 ms • Method - 3 • UE’s uses a fixed guard period n (set to 100 ms for cell size of 16 km) • Dt3 = Dt2+n • Dt4 = Dt3+n

35. QoS for W-CDMA Packet • Base QoS on network and application standards • Internet QoS (End-to-end QoS support) • Guaranteed throughput and bounded delays • FER is irrelevant for most or manydata services, networks are effectively perfect. However, delay is related to operating FER. • QoS negotiation (analogous to call set-up) • Admission control for premium service levels • At L2 & MAC each mobile has QoS associated • Implications for the MAC scheduling • Need to signal multiple queue depths (per QoS level) during RACH • Scheduling based QoS level • May police mobiles with respect to negotiated QoS. • Must standardize method for representing mobile QoS with UTRAN.

36. Conclusions • Shared Channel maximizes statistical multiplexing gain • Resource fully used in every frame • Problem of downlink code shortage is mitigated using DSCH with Fast Code Multiplexing (FCM) • Limited power resource is shared between users using USCH • Provides fast power control, transmit diversity and soft-handoff • Recommendations for 3GPP specification: • DSCH and USCH • Provisions for DSCH and USCH to be associated with ACCH • Provisions for DSCH and USCH to be associated with DCH • Provisions for Preamble based transmission for uplink • Provisions for Link Maintenance for uplink