Patent Application
20100002681
In wireless networks, persistent scheduling takes advantage of the traffic characteristics of Voice over IP (VoIP) to increase the number of VoIP users in systems such as those conforming to the Institute for Electronic and Electrical Engineers (IEEE) 802.16 standard based WiMax, 3GPP LTE and 3GPP3 UMB systems. The periodic nature of the packet arrivals from a VoIP source allows the needed resource to be allocated persistently for the period of an active talk spurt. Consequently, base stations (BS) can send the allocation information once at the beginning of a talk spurt and avoid sending the allocation information anew for each subsequent packet. Significant resources that would otherwise be occupied by unnecessary overhead can now be used to accommodate VoIP packets from more users. Because VoIP packets are usually small in size, the savings from reduced overhead can significantly boost the overall system capacity. The capacity gain from persistent scheduling, however, can be compromised significantly due to dynamic link adaptation, which is a very common technique in wireless systems to adjust to dynamic variation in wireless channel quality.
Thus, a strong need exists for techniques for enhanced persistent scheduling with an efficient link adaptation capability.
The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the preset invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device, that manipulate and/or transform data represented as physical (e.g., electronic) quantities within the computer's registers and/or memories into other data similarly represented as physical quantities within the computer's registers and/or memories or other information storage medium that may store instructions to perform operations and/or processes.
Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. For example, “a plurality of stations” may include two or more stations.
Embodiments of the present invention provide a persistent scheduling mechanism that can maintain VoIP capacity improvement by adding efficient link adaptation capability in 802.16m WiMax systems. Link adaptation poses an important challenge to the benefits of persistent scheduling. When link adaptation is used in 802.16m WiMax systems, the number of resource blocks (RBs) needed for a VoIP packet depends on the Modulation and Coding Scheme (MCS) selected to transmit the packet over the wireless link to the pertaining user. As the channel quality of the user changes from frame to frame, the MCS selected for the packet is adjusted and thus, the number of RBs has to be changed accordingly. If users are persistently allocated on an individual basis, this change in the resource allocation due to MCS change has to be conveyed explicitly incurring message overhead that persistent scheduling is aimed to avoid. Therefore, the benefit of persistent scheduling starts to diminish when MCS change becomes frequent due to link adaptation. This conjecture has been verified by simulation experiments, where as much as 40% of persistent allocations in a frame are affected by MCS changes even in a low mobility scenario.
Current persistent scheduling proposals do not provide a very efficient mechanism to deal with link adaptations. Individual and group based schemes are the two types of proposals laid out for persistent scheduling. Individual persistent scheduling allocates resources for each user individually. The proposals in this category advocate de-allocating and re-allocating the persistently allocated resources in case an MCS change occurs from one persistently scheduled frame to the other for a particular user. On the other hand, previous group based persistent scheduling, which offers better resource packing and multiplexing gains compared to those from individual persistent scheduling proposals, is also vulnerable to a significant increase of overhead from MCS changes. One possible way to avoid this overhead could be to fix the MCS to a very conservative value to cancel out the effect of link adaptation. However, this will lead to a very sharp decrease in spectral efficiency and will drastically reduce the system capacity as a result. Therefore, there is a need for a persistent scheduling scheme that can deal with link adaptations with a minimal effect on the overhead.
Embodiments of the present invention provide a group based persistent scheduling mechanism where a novel grouping scheme for VoIP users is introduced along with an intelligent bitmap mechanism to compactly represent persistent allocations for the users within a group. In order to form the groups, an embodiment of the present invention starts by defining MCS clusters that are composed of neighboring MCSs selected from the pool of MCSs occurring in the system. Neighboring MCSs are those MCSs that are more likely to be selected for any particular user when link adaptation occurs due to changing channel conditions. When a persistent allocation is scheduled for a user, it is assigned to the group that represents the MCS cluster that the user's current MCS belongs to. The user will have to be switched to another group only if its link adaptation makes switching between MCS clusters necessary. A group change for a user will necessitate sending control information to remove the user from the previous group to the next group.
However, since the MCS clusters are formed intelligently to keep this switching probability at a very low level, a user will be less likely to change groups. Any persistent scheduling proposal will encounter significant control overhead every time the MCS changes for a user. By keeping the group change frequency to a very low level, the scheme of an embodiment of the present invention may avoid a significant amount of control overhead. A bitmap is included within a group's allocation information when that group's allocation has to be updated. The bitmap indicates a 2 bit representation of each user's MCS, although the present invention is not limited in this respect. There is a one-to-one relationship between the MCS and the number of RBs needed for that MCS. This relationship is known to the users who also get information on their position in the bitmap during their initial allocation to the group. Therefore, a user can find the starting RB of its allocation in the group by scanning the bitmap and from the knowledge of its position in the bitmap. Since the starting RB location for a group will be known to the users belonging to that group, a user will thus be able to find the absolute RB location of its allocation in the frames that the user is scheduled in. It is noted, but not limited in this respect, that a 2 bit representation of the MCS of a user enables 4 MCSs to be clustered for a group.
It is noted that a VoIP user may be scheduled only in every nth frame, where n is the frame duration of the VoIP codec. Therefore, the persistently scheduled user monitors the allocation information pertaining to its group only at every nth frame. The VoIP codec's frame duration essentially becomes the persistent allocation period for the users using that VoIP codec. If a user's MCS changes due to link adaptation, the change in most of the cases can be reflected by simple update of the two bits representing the user's new MCS in the corresponding group's bitmap.
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Embodiments of the present invention further provide a channel quality and timer thresholds mechanism that will prevent oscillation (ping-pong) of a user among adjacent groups. Such oscillation may occur, albeit in rare situations due to some specific propagation environment around a user. To avoid this ping-pong effect, in an embodiment of the present invention, the base station could use a threshold, i.e., if the channel condition of the mobile station becomes higher/less than the threshold over/under the channel condition of lower/upper bound range of the higher/lower MCS group then you can move the mobile station to the group with higher/lower MCS. The ping-pong effect can be controlled by adjusting the threshold.
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Embodiments of the present invention have the advantages of providing significant reduction in control overhead and improving the VoIP capacity by efficient handling of link adaptation during persistent scheduling. Simulation experiments show that the present invention may achieve nearly 30 percent more VoIP capacity compared to the capacity achievable from individual persistent allocation schemes. Compared to existing group based mechanism, the present invention may gain an extra 13% of VoIP capacity.
While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61134188 | Jul 2008 | US |
