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The Adaptive Permission Reservation Protocol for Wireless Communications

The Adaptive Permission Reservation Protocol for Wireless Communications. In IEEE IPCCC’97, pp. 483-489, by L.-S. Koh and M.-T. Liu. Introduction. More and more portable devices (e.g., handset and PDA) will proliferate in the near future.

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The Adaptive Permission Reservation Protocol for Wireless Communications

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  1. The Adaptive Permission Reservation Protocolfor Wireless Communications In IEEE IPCCC’97, pp. 483-489, by L.-S. Koh and M.-T. Liu

  2. Introduction • More and more portable devices (e.g., handset and PDA) will proliferate in the near future. • MAC (multiple-access control) must effectively use the wireless bandwidth. • TDMA/FDMA/CDMA • PRMA (packet reservation multiple access) • based on TDMA, easy to implement • This paper observes some weakness of PRMA and proposes a modified adaptive PRMA scheme.

  3. Background: Cellular System • Cell structure: • radius < 1km • downlink: from BS to terminals • used only by BS • uplink: from terminals to BS • shared by voice terminals, through MAC protocol

  4. Speech Pattern: principal talking spurt mini-spurt mini-gap listening state To detect speech activity, it is suggested to use a “Speech Activity Detector”: slow detector: can detect principal talk spurt can reduce traffic by 43% fast detector: can detect mini-spurt can further reduce traffic to 36% Background: Voice Characteristic

  5. Background: FP-PRMA • FP = fixed probability • Intuition: contending a time slot with a fixed probability. • Time is slotted. • A number of slots are organized as a frame. • After each slot, a voice terminal can listen to the feedback from the BS regarding the state of the slot (busy/idle). • To begin a new talk: • find an empty slot i • in the next frame, compute a random number x • if (x > pre-defined fixed probability P), • then send in slot i and listen to the feedback • if succeed, all subsequent slot i are reserved for this call.

  6. When BS finds nothing is sent in slot i in frame k, • declare it as empty • other talks can contend for slot i in the frame k+1. • For voice transmission: • A packet is dropped if it is not sent within 32 ms. • Voice quality must be maintained at a certain level. • Problem: • How to select P? • No flexibility with the pre-defined probability P

  7. An Example of PRMA (a) 6,8 contending (b) 8 contending (c) 8 succeeds, R1 drops (d) 6’s random # too small (e) 6 contending (f) 6 succeeds ** How to dynamically adjust the value of sending probability P?

  8. Disadvantage of FP-PRMA • When there are many contending senders, a large P (e.g., 0.8) cannot solve the contending problem. • When there are few contending senders, a low sending probability (e.g., 0.2) will waste the bandwidth. • Goal: APR (Adaptive Permission Reservation Protocol) • to estimate and adjust the sending probability • by observing the contending traffic • as there are typically little change on the contending calls • factors to affect contention • newly arrival calls • dropping from the contending calls

  9. ARP (Adaptive Reservation Protocol) • Similar to FP-PRMA, but will estimate the number of contending voice terminals. • Estimation Scheme: • BS will broadcast the state of each slot through downlink. • A terminal can monitor the downlink for a period of time for the following slot status: • slot[i] = EMPTY/OCCUPIED • ack[i] = SUCCESS/COLLIDE/EMPTY

  10. Basic idea: • net-collision = (# of collisions) - (# of successful calls) • Approach: • if ACK[i] = SUCCESS, then num_collision -- • if num_collision < 0, then num_collision = 0; • if ACK[i] = COLLIDE, then num_collision ++ • if num_collision = 1, then num_collision = 2; • if ACK[i] = EMPTY, then num_collision -- • if num_collision < 0, then num_collision = 0; • Selection of P: • P = 1/num_collision

  11. slow speech activity model fast speech activity model Simulation Model

  12. Other Models to Be Compared • Ideal PRMA: • a hypothetical model • assuming a perfect centralized queue • a packet is scheduled in a FIFO manner • Thus, there is NO contention problem at all. • OP-PRMA: • The exact number of contending terminals, k, can somehow be known. • sending probability = 1/k • (also hypothetical)

  13. Simulation Parameters • System Parameters: • frame length = 20 or 40 slots • Voice Traffic: • Mean Principle Talk Spurt Duration = 1.0 sec • Mean Principle Silent Gap = 1.35 sec • Mean Minitalk Spurt Duration = 0.275 sec • Mean Mini-silent Gap = 0.05 sec • Max Tolerable Speech Delay = 0.032 sec

  14. Simulation Results:Slow Speech Activity • Ideal PRMA < OP-PRMA < Adaptive PRMA < FP-PRMA (with P=0.3)

  15. Simulation Results:Fast Speech Activity • Similar Trend: Ideal PRMA < OP-PRMA < Adaptive PRMA < FP-PRMA (with P=0.3)

  16. Conclusions • Fixed probability v.s. Dynamic probability • proposing a good estimating heuristic for the current number of contenders • performance simulation shows the scheme works close to ideal PRMA models (ideal PRMA and OP-PRMA)

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