IEEE C802.16m-07/220
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
An addition to the legacy frame structure to save BS power during periods of low traffic
Date
Submitted
2007-11-07
Source(s)
Stefan Lindgren, Sten Sjoberg
Ericsson AB
Re:
IEEE 802.16m-07/040 - Call for Contributions on Project 802.16m System Description
Document
Abstract
An addition to the legacy frame structure that introduces gaps in the sequence of frames is
described and it is proposed that the 802.16m SDD includes a section on generic frame structure
in a Physical layer chapter.
Purpose
Inclusion of a section on generic frame structure in a Physical Layer chapter of the 802.16m
SDD. The generic frame structure section should in addition to a basic description of, or
reference to, the legacy frame structure include proposed 802.16m additions including the
proposal in this document.
Notice
Release
Patent
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E-mail:
stefan.lindgren@ericsson.com
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An addition to the legacy frame structure to save BS power during
periods of low traffic
Stefan Lindgren, Sten Sjoberg
Ericsson AB
Introduction
Minimizing the energy consumption in telecommunication systems has become very important to reduce
operating cost and to protect the environment. Network operators are increasingly aware of the energy efficiency
of their networks not only from an operating cost point of view but also as a part of their environmental and
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IEEE C802.16m-07/220
social responsibilities and a part of their image. The energy efficiency of a network is multi-faceted, and it
depends e.g. on network planning, site efficiency and the power efficiency of individual units at a site.
An addition to the legacy frame structure is proposed to enable power saving in a base station during periods of
low traffic e.g. night time by using a concept similar to sleep mode of mobile stations.
Section 6.11 of the 802.16m System Requirements Document (SRD) [1] requires enhanced power saving which
applies to the access network devices as well. Section 8.4 of the SRD asks for self-optimization for optimization
of network efficiency.
The frame structure is one of the basic elements of the Physical Layer (PHY) and it is proposed to include the
presented proposal as a part of the Frame Structure section of the PHY chapter of the SDD. It is suggested that
the generic frame structure is described as the first part of the PHY chapter.
Contribution
It is suggested that the Physical Layer is a chapter of the SDD and that the generic frame structure is described
as the first subsection of the Physical Layer chapter.
After a description of the legacy WirelessMAN-OFDMA Reference System frame structure with Figure 1 from
the 802.16 standard or suitable references to the 802.16 standard, additions to the legacy frame structure should
be described.
Figure 1: Frame structure used in legacy IEEE 802.16 WirelessMAN-OFDMA (with only mandatory zone).
Frame
#N
Frame
#N+1
Frame
#N+2
Frame
#N+3
Frame
#N+4
Figure 2: A standard sequence of frames.
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Frame
#N+5
IEEE C802.16m-07/220
The frame length is 5ms in the WirelessMAN-OFDMA Reference System.
The Frame Number is a 24-bit number given as a part of the OFDMA PHY Synchronization field in the DLMAP. It is incremented by one each frame and eventually wraps around to zero.
In the WirelessMAN-OFDMA Reference System the base station (BS) is supposed to transmit a preamble
followed by a Frame Control Header (FCH) and a DL-MAP (or compressed MAP) which includes the Frame
Number.
At periods of very low traffic intensity it is a waste of energy to keep the BS alive and sending preamble, FCH
and DL-MAP every five milliseconds. In the mobile stations (MS) the concept of sleep mode is used; where the
MS is only awake a fraction of the time at certain predefined instances to save energy.
It is suggested that the frame structure includes an option to introduce gaps in the frame sequence consisting of a
number of frame intervals on a periodical basis, for example the BS transmits one or two frames followed by a
gap of a number of frames and then transmits one or two frames again as long as there is no traffic or the traffic
can be handled with the frame sequence with gaps. At the end of the gap the frame number counter should be
incremented by a number corresponding to the length of the gap. Please see Figure 3 for an example.
The change to or from the power saving mode could be configurable from the management system or initiated
by the BS based on traffic sensing in a self-optimizing system.
The frame gap can be announced in the DL-MAP via the frame duration code in the OFDMA PHY
Synchronization field (codes 9-255 are reserved in the legacy system) or via a new Frame Gap IE.
The Downlink Channel Descriptor (DCD) should announce the periodic on-off pattern used at the BS in a new
TLV and the same TLV could be used in the MOB_NBR-ADV message to announce the on-off patterns of
neighboring BSs.
Legacy MS that cannot read the protocol additions can be handled by assigning them a sleep mode pattern that
is synchronized with the BS on-off pattern.
There should be a limit on the length of the frame gaps to ensure initial network discovery within a reasonable
time.
It is suggested that the details of control and announcement of the pattern with gaps in the frame sequence
should not be included in the generic frame structure description part of the SDD but saved for protocol parts of
the SDD or the actual 802.16m amendment specification.
In addition to the primary intended use of BS power saving during periods of low load, the use of frame
sequences with gaps could be used to create a “noise limited” environment for initial deployment of cells with
reduced capacity by using staggered on-off patterns of neighboring cells.
Frame
#N-1
Frame
#N
Gap x frames
Frame
long…………. #N+1+x
Frame
#N+2+x
Gap x frames Frame
long………… #N+3+2x
Figure 3: A power saving mode sequence of frames. The example shows a pattern of two frames followed by a
gap of x frames.
Recommendation
It is proposed that the mechanism for allocation and announcement of gaps in the sequence of frames be
included in the section of the SDD pertaining to the Frame Structure.
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IEEE C802.16m-07/220
Proposed SDD Text
[Chapter] Physical Layer
…………..
[Section] Frame Structure
[A short description of the legacy WirelessMAN-OFDMA Reference System frame structure with Figure 1 from
the 802.16 standard or suitable references to the 802.16 standard]
The BS may choose to allocate and announce gaps in the sequence of frames, typically in a periodic pattern. The
Frame Number counter is incremented as if frames were transmitted in a non-interrupted sequence.
[Other additions]
……………….
References
[1] IEEE 802.16m System Requirements IEEE 802.16m-07/002r4
[2] IEEE Std 802.16-2004: Part 16: IEEE Standard for Local and metropolitan area networks: Air
Interface for Fixed Broadband Wireless Access Systems, June 2004
[3] IEEE Std 802.16e-2005 and IEEE Std 802.16-2004/Cor1-2005 (Amendment and Corrigendum to IEEE Std
802.16-2004), “IEEE Standard for local and metropolitan area networks, Part 16: Air Interface for Fixed
and Mobile Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control
Layers for Combined Fixed and Mobile Operation in License Bands,” Feb 28, 2006
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