IEEE C802.16m-10/0395

Project / IEEE 802.16 Broadband Wireless Access Working Group <
Title / Methods for Improved Power Save during Sleep Operationin IEEE 802.16m (16.2.16)
Date Submitted / 2010-03-05
Source(s) / Elad Levy, Maruti Gupta, Aran Bergman, Shantidev Mohanty
Intel Corporation / E-mail:
Re:
Abstract / This contribution proposes methods using which AMS in sleep mode can improved power saving in IEEE 802.16m.
Purpose / To be discussed and adopted by TGm for 802.16m Letter Ballot 30b.
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Methods for Improved Power Save during Sleep Operation in IEEE 802.16m (16.2.16)

Elad Levy, Maruti Gupta, Aran Bergman, Shantidev Mohanty

Intel Corporation

  1. Introduction

The subframe level sleep concept is defined in IEEE 802.16m to enable an AMS to sleep in the sub-frames where no DL/UL traffic is expected. However, this sub-frame level sleep operation does not provide efficient mechanism for additional power saving that can be achieved when the Listening Window is extended for HARQ retransmissions. In IEEE 802.16m standard, the concept of Listening Window extension is used to extend the duration of the default listening window so that during this extension an AMS in sleep mode can send/receive traffic without disabling Sleep mode.Currently, an AMS in sleep mode has to remain awake for the entire duration of the extended listening interval. However, during this extended listening interval the AMS may send/receive traffic only in some of the sub-frames. In many cases, the exact sub-frames during the extended listening interval can be easily determined. Therefore, instead of remaining awake for the entire extended listening interval, it is sufficient for the AMS to remain awake only during the specific DL/UL sub-frames of the extended listening interval where the AMS may send/receive traffic.

Similarly, for HARQ retransmissions a sleep mode AMS may have to remain awake not only in the sub-frames where it has HARQ retransmission related traffic (data and control) but also in other sub-frames. AMSs availability in the sub-frames where no HARQ retransmissions is present results in unnecessary power consumption.

This contribution proposes methods using which AMS in sleep mode can sleep in some of the sub-frames of the extended listening interval.As an example, VoIP traffic is used to show how enabling listen window extension at the sub-frame level can give significant power savings. VoIP traffic is periodic where the AMS has maximum one DL and/or UL VoIP packet every 20 ms. Using sub-frame level sleep option, the listening window is designed in such a way that the AMS remains awake only for a set of DL and UL sub-frames that are indicated using a bitmap.

Now, since VoIP traffic is fairly deterministic, we can use the option of either extending the Listen window (setting Listening Window Extension Flag (LWEF) = 1 or not LWEF = 0.

The following method shows how this can be done with each of the settings of LWEF.

MS negotiates a sleep cycle with one frame listening window and Listening sub-frame bitmap that indicates it is awake for 1 DL subframe and 1 UL subframeTowards this, this contribution proposes the following method.

Proposed Method:

If LWEF = 1, then the Listen window is implicitly extended for any pending HARQ retransmissions. Thus, the sequence of events can be shown to be as follows:

  1. MS negotiates asleep cycle with one frame listening window and Listening sub-frame bitmap that indicates it is awake for 1 DL subframe and 1 UL subframe during the default Listening window
  2. If MS gets UL allocation in the UL subframe, MS needs to be awake in the relevant DL subframe to hear the HARQ ack/nak

However, if the first DL transmission is not successful and HARQ retransmission is required, then the Listening window is extended by a certain duration, say, a frame to receive the DL retransmission. Since DL HARQ is asynchronous, the AMS does not know which DL sub-frame the retransmission will arrive in and must remain awake for the entire duration of DL sub-frame. So in this case, instead of the AMS being awake for the entire extended frame, the proposal is that AMS instead stays awake only for the sub-frame during which the DL HARQ retransmission was received and the UL sub-frame when it sends the ack/nak for it. This sub-frame may be same as the bitmap used for default window or a different bitmap altogether. If a different bitmap is used for the extended listening interval, then this bitmap can be negotiated between the AMS and its serving ABS. Using the same bitmap is more efficient since it saves signaling overhead.

Since UL HARQ is synchronous, the AMS does not need to be given a bitmap, it can just wake up at the given sub-frame and send the retransmission. However, it still needs to know which DL sub-frame it receives the DL ack/nak, for which it can again use the bitmap information.

  1. Text proposal for inclusion in the P802.16m/D4

======Start of Proposed Text ======

[Editor’s Note: Add the following lines after line 48, pg 301.in 6.2.16.2.3.2 Listening Window extension”].

Listening window can be extended in units of frames. For each extension, the AMS or ABS has the option to specify a bitmap that indicates the sub-frames the AMS needs to be actually awake in for receiving/sending traffic. This sub-frame level bitmap may be same as the one used for the default Listening window or it may be different. This is especially useful for deterministic periodic traffic such as VoIP.

======End of Proposed Text ======

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