1 Two-Stage Dynamic Uplink Channel and Slot Assignment for GPRS Author: Ying-Dar Lin, Yu-Ching Hsu, Mei-Yan Chiang Reporter: Chen-Nien Tsai

2 Outline GPRS Background Introduction Two-Stage Dynamic Channel and Slot Assignment Results Summary

3 GPRS Background GPRS Introduction GPRS Network Architecture GPRS Air Interface GPRS Logical Channels Mapping Logical Channels to Physical Channels Packet Data Transfer Operations

4 GPRS Introduction GPRS: Stands for General (or generic) Packet Radio Services developed by European Telecommunication Standard Institute (ETSI) is one of the standards of Global System for Mobile communications (GSM) Phase 2+ is designed as a packet switching system

5 GPRS Network Architecture It fits in with the existing GSM PLMN Two new network elements Serving GPRS Support Node (SGSN) Gateway GPRS Support Node (GGSN) Many new interfaces G b, G i, G n, etc.

6 GPRS Network Architecture

7 GPRS Air Interface (1/3) GPRS uses the existing GSM resources. GPRS uses a two-dimensional access scheme (FDMA and TDMA). Total 25 MHz bandwidth 125 carrier frequencies of 200 kHz bandwidth GPRS users will share the same TDMA frame with GSM voice users. GPRS air interface will dynamically allocate resources (timeslots).

8 GPRS Air Interface (2/3) TDMA Frame 8 timeslots Period = ms

9 GPRS Air Interface (3/3) 52 Multiframes

10 GPRS Logical Channels PDCH is the generic name for the physical channel allocated to carry packet logical channels

11 Mapping Logical Channels to Physical Channels (1/3) Logical channels are carried on physical channels. Multiple logical channels can be mapped onto the same physical channels. Three possible combinations: PBCCH+PCCCH+PDTCH+PACCH+PTCCH PCCCH+PDTCH+PACCH+PTCCH PDTCH+PACCH+PTCCH

12 Mapping Logical Channels to Physical Channels (2/3)

13 Mapping Logical Channels to Physical Channels (3/3)

14 Packet Data Transfer Operations Before data can transfer GPRS Attachment PDP (Packet Data Protocol) context activation. After these two steps, the mobile can access the network, request resources, and send data.

15 Uplink Packet Data Transfer Request resources

16 Uplink Packet Data Transfer Fixed radio block allocation According to bit map

17 Uplink Packet Data Transfer Dynamic radio block allocation USF (Uplink State Flag) is attributed to each MS.

18 Outline GPRS Background Introduction Two-Stage Dynamic Channel and Slot Assignment Results Summary

19 Introduction Two-stage assignment Stage-1: BS assigns several PDCHs to an MS. Stage-2: BS selects one of the multiplexed MSs in a PDCH to use the radio resource. Objective of this paper Load balance in stage-1 Good prediction in stage-2

20 Outline GPRS Background Introduction Two-Stage Dynamic Channel and Slot Assignment Results Summary

21 Two-Stage Dynamic Channel and Slot Assignment Stage 1 Multiple PDCHs with the corresponding USFs are assigned to an MS. Stage 2 To utilize the radio resource, the BS has to predict who has data to send and the assign the following time slot to that MS.

22 Stage-1 Channel Assignment After receiving the Packet Channel Request, BS must decide the number of as well as which specific PDCHs to be assigned to the MS. Deciding which PDCHs to assign is more critical. (load balance) Two load measurement methods Number of Assigned Flow Effective transmission over last cycle

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24 Number of Assigned Flow (NoAF) The number of multiplexed MSs within a PDCH is chosen as the load measurement metric. This scheme can be considered a frequency-wise and PDCH-wise balance.

25 Effective Transmission over Last Cycle (EToLC) The load metric employed by EToLC is defined as the number of transmissions occurred during the previous PRR (Pure Round-Robin) cycle.

26 Stage-2 Slot Assignment The BS has to predict who has data to send. If the selected MS has no data impending, the slot is wasted. Three schemes are considered: Pure Round-Robin Round-Robin with Linearly-Accumulated Adjustment Optimal

27 Pure Round-Robin (PRR) Each multiplexed MS in a PDCH is round-robined to use the uplink channel. All MSs are assumed having impending data to send. A PRR cycle equals the number of MSs multiplexed in this PDCH. The highest mis-selection rate.

28 Round-Robin with Linearly- Accumulated Adjustment (RRLAA) Basis principle to reduce the transmission chance for the MSs that failed to utilize the last assigned slot, and increase the chance for those who had. For RRLAA, a Penalty cycle and a Reward cycle are defined and appear alternately.

29 Penalty Cycle (1/2) A Penalty cycle is derived from PRR cycle by skipping MSs who waste their last assigned timeslots in Penalty cycles. An MS will be skipped in n successive penalty cycles when it wastes n successive assigned timeslots in Penalty cycles. When the MS begins to send packets, the penalty accumulation is reset.

30 Penalty Cycle (2/2) M: number of multiplexed MS in a PDCH

31 Reward Cycle (1/2) An MS is authorized to transmit during the following Reward cycle if it transmits data in the previous Penalty cycle. An MS will be rewarded n timeslots in a Reward cycle when it successively employs the assigned timeslots in n penalty cycles. An MS will be selected to send data at most once in a Penalty cycle but possibly multiple times in a Reward cycle.

32 Reward Cycle (2/2) Infinite loop?

33 Optimal (OPT) Assume that whether an MS has data to send or not is known in advance. For compare performance.

34 Outline GPRS Background Introduction Two-Stage Dynamic Channel and Slot Assignment Results Summary

35 Results Comparison between Load Balancing Schemes for Stage-1 Channel Assignment Comparison between Selection Schemes for Stage-2 Slot Assignment

36 Comparison for Stage-1 Channel Assignment (1/3) FNoP-NoAF is designed to be compared with other three models and is considered as the most load-balanced case. RND: RandomFNoP: Fixed Number of PDCH (?) NoAF: Number of Assigned Flow EToLC: Effective Transmission over Last Cycle

37 Comparison for Stage-1 Channel Assignment (2/3) Standard deviation of PDCH utilization

38 Comparison for Stage-1 Channel Assignment (3/3) System throughput

39 Comparison for Stage-2 Slot Assignment (1/3) Mis-selection rate

40 Comparison for Stage-2 Slot Assignment (2/3) System throughput throughput ≈ offered load * ( 1 – mis-selection rate)

41 Comparison for Stage-2 Slot Assignment (3/3) Average Packet Queuing Delay

42 Outline GPRS Background Introduction Two-Stage Dynamic Channel and Slot Assignment Results Summary

43 Summary (1/2) Two load-balancing schemes for stage- 1 channel assignment. Number of Assigned Flow (NoAF) Effective Transmission over Last Cycle (EToLC) EToLC outperforms NoAF.

44 Summary (2/2) One selection scheme for stage-2 slot assignment. Round Robin with Linearly-Accumulated Adjustment (RRLAA). Reward and penalty cycles. RRLAA has the lower mis-selection rate, better system throughput, and lower packet queuing delay.