IEEE C802.16m-07/258

Project / IEEE 802.16 Broadband Wireless Access Working Group <
Title / Resource Allocation Scheme for 802.16m
Date Submitted / 2007-11-15
Source(s) / Suresh Kalyanasundaram, Anand Bedekar, Steven Xu, Hua Xu
Motorola / Voice:
E-mail:
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Re: / IEEE 802.16m-07/040Call for Contributions on Project 802.16m SDD
Abstract / The document describes two recommendations for persistent scheduling signaling. One is to exploit ARQ and HARQ for persistent scheduling signaling. Another is to enable a one-time change to the persistent schedule, without revoking the persistent schedule for subsequent frames.
Purpose / For 802.16m discussion and eventual adoption for SDD
Notice / This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein.
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Resource Allocation Scheme for 802.16m

Suresh Kalyanasundaram, Anand Bedekar, Steven Xu, and Hua Xu

Motorola

1. Introduction

It is widely agreed that some form of persistent scheduling is needed to reduce the MAP overhead for applications that generate periodic traffic, such as, VoIP [1]. Such signaling information for persistent scheduling (persistent scheduling start) is transmitted once, along with either a validity period, or until revoked/modified by the BS using either a persistent scheduling stop or persistent scheduling modify messages. Because the persistent allocation is valid for several frames, it is important to ensure that the SSs receive this message. Otherwise, there will be wastage of resources. Moreover, fairly complex mechanisms are needed for the BS to realize that the persistent scheduling message is lost [2]. In this contribution, we recommend the use of HARQ and ARQ mechanisms to ensure reliability of the persistent scheduling messages. Thus we recommend that the persistent scheduling messages are not sent in the MAP, but instead as MAC management messages with ARQ and HARQ mechanisms to guarantee reliability.

In addition, current proposals for persistent scheduling primarily envisage three possible messages:

  • persistent scheduling start
  • persistent scheduling stop
  • persistent scheduling modify

Based on our analysis, there are some benefits from a one-time “fast modification” message that, for example, changes the MCS or the resource location of an SS that is valid only for one frame, without changing the persistent schedule of the SS for subsequent frames.

2. Detailed proposals

2.1Reliability of persistent schedule signaling

As mentioned earlier, we propose that the persistent schedule signaling messages (persistent scheduling start, persistent scheduling stop, and persistent scheduling modify) are sent as MAC management messages and use HARQ and ARQ mechanisms to guarantee reliable delivery. We propose that the BS starts effecting the decision conveyed in the persistent schedule signaling only after it receives an acknowledgment for this MAC management message. In the meantime, the BS can continue to send dynamic scheduling messages using the MAP to allocate resources for the SS. This dynamic scheduling message can point to the same resource as that allocated using persistent scheduling. Our proposal is illustrated in the following Figure.

Figure: Illustration of proposed persistent allocation signaling

As shown in the figure, in frame #1 the DL-MAP points to a location in the DL sub-frame, where both the first VoIP burst and the persistent scheduling start MAC management messages are sent. In addition, the UL-MAP allocates resources for the SS in frame #2 to transmit an ACK for the persistent scheduling start message. Meanwhile, in frame #2, the BS uses the DL-MAP to dynamically schedule another burst for the user. Once the ACK is received in frame #2, the BS no longer sends the scheduling information in the DL-MAP.

A similar mechanism can be used for UL persistent allocation signaling. In the UL, the SS using the allocated resource can also be used as an implicit ACK.

2.2 Fast modification message

Some companies have proposed the use of a bitmap to convey whether or not the persistent scheduling message is valid for a given frame [1]. But there are complications when there are periodic allocations with periods greater than one and when different SSs have different periods. The problems associated with changing MCS of users have also been recognized [3]. Any change in the MCS of one user that has a persistently allocated resource is likely to have knock-on effects on other users’ allocations. Moreover, it can be expected that some MCS changes can only be transient. For example, the SNR of the SS may have only improved marginally to allow the user to operate at a higher MCS, but the SNR is expected to correspond to the persistently scheduled MCS in the short-run. In such scenarios, the BS may want to make a modification to the persistent allocation for only a single frame, without revoking the persistent allocation for subsequent frames.

This can be achieved by enabling a “fast modification” message, which would be a modification of the DL-MAP_IE or UL-MAP_IE, which specifies a new allocation for the SS that is applicable for the current frame, along with a bit that indicates whether or not the persistent allocation is valid for that frame. Note that this bit can be ignored by those SSs that do not have a persistent allocation. It is understood that the SS continues to read the MAP messages even after the persistent allocation has taken effect. This is to enable it to read the fast modification messages.

References

[1] Beceem, “Allocations for Periodic Traffic to Reduce MAP Overhead,” WiMAX Rel 1x MAC-MAP overhead Reduction_Beceemv02.ppt, WiMAX Forum TWG contribution.

[2] Nortel, NextWave, “Harmonization Update on Periodic Assignment using Broadcast MAPs,” Summary of harmonization for periodic assignment on broadcast MAP v06.ppt, WiMAX Forum TWG contribution

[3] Samsung, “MCS-based Group Allocation,” WiMAX_Group_Persistence_Allocation_Samsung_v2.ppt, WiMAX Forum TWG contribution