Patent Application
20140029425
The present invention relates generally to communications and, in particular, to inter-carrier load balancing in wireless communication systems.
This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
In a cellular network, a user (wireless device) is typically served by the base station to which it has the strongest signal. If a base station communicates over multiple carriers (e.g., two 10 MHz carriers for LTE (Long Term Evolution)), it would typically transmit at similar powers on each carrier and a given user would perceive comparable averaged signal strengths on each of these carriers. Hence, a typical inter-carrier load balancing algorithm would balance the number of users on each carrier using methods such as hashing. Alternatively, a typical load balancing algorithm may attempt to equalize the expected traffic carried on each carrier, where users may have different traffic demands.
Recently, there has been great interest in overlaying metro cells with macro cells, both using the same spectrum. Because macro cells transmit at much higher powers, the coverage area for metro cells is very small. Cell selection bias can increase the serving area of metro cells, but bias values must be kept small in order to maintain control channel performance. Almost Blanked Subframes (ABS) will be introduced in LTE Rel 10, but simulations have shown that bias values will still be limited because interference from Cell-Specific Reference Signals (CRS) remains and can be significant at high bias values. Hence, relatively few users are served by metro cells and associated gains may be modest.
We have proposed an alternative solution called Soft Reuse which employs power management. Soft reuse is attractive because it is available to legacy UEs (and thus could yield immediate benefits), allows high biases, and yields comparable or even higher gains than ABS-based techniques, depending on the scenario. With soft reuse, transmission powers from a given base station (macro or metro) may be vastly different over each carrier. The typical inter-carrier load balancing algorithm that attempts to equalize the number of users on each carrier and/or the expected traffic on each carrier will likely provide a sub-optimal end user experience in a Soft Reuse environment. Thus, new approaches and techniques that are able to improve inter-carrier load balancing would meet a need and advance wireless communications generally.
Specific embodiments of the present invention are disclosed below with reference to
Simplicity and clarity in both illustration and description are sought to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. One of skill in the art will appreciate that various modifications and changes may be made to the specific embodiments described below without departing from the spirit and scope of the present invention. Thus, the specification and drawings are to be regarded as illustrative and exemplary rather than restrictive or all-encompassing, and all such modifications to the specific embodiments described below are intended to be included within the scope of the present invention.
Various methods and devices are provided to address the need for improved inter-carrier load balancing. In one method, depicted in logic flow 100 of
Many embodiments are provided in which the method above is modified. For example, in many embodiments, such as depicted in logic flow 200 of
Depending on the embodiment, the network equipment may also determine that a wireless device is a member of a restricted class, based on a certain characteristic (such as a level of mobility or what applications are being used on the wireless device, e.g.). The network equipment may then assign a carrier to the wireless device, as required by the restricted class, before performing inter-carrier load balancing for the other wireless devices needing carrier assignments.
Various network equipment architectures may be used to implement inter-carrier load balancing, depending on the embodiment. For example, the network equipment may include a single device or multiple devices, such as one or more base stations and/or other network devices, the devices acting either individually to perform certain functionality or in a distributed manner (such as in a cloud computing architecture).
To provide a greater degree of detail in making and using various aspects of the present invention, a description of our approach to inter-carrier load balancing and a description of certain, quite specific, embodiments follows for the sake of example. Our approach to inter-carrier load balancing attempts to address a unique problem that is introduced with soft reuse. Mainly, with soft reuse, transmission powers from a given base station (macro or metro) may be vastly different over each carrier. As a result, the downlink signal-to-interference-plus-noise ratios (SINR) that a wireless device or UE (user equipment) perceives on each carrier of a given serving cell may be quite different. Consequently, in order to achieve a more optimal end user experience, the load balancing method employed by the cell needs to take into account the channel quality experienced by users on different carriers associated with that cell. Intra-cell load balancing algorithms that factor in the number of users or the sum traffic, but do not account for user channel quality, are likely to exhibit distinctly inferior performance in heterogeneous networks employing soft reuse.
In several embodiments of the present invention, intra-cell, inter-carrier load balancing is performed in a manner that considers user channel quality, resource availability, and the number of users. Without loss of generality, consider a base station with two carriers, f1 and f2. In 3GPP LTE, a carrier may take on different bandwidths ranging from 1.4 MHz to 20 MHz. Let the bandwidth of f1 and f2 be W1 and W2, respectively. Further, assume that N users are served by this base station and their channel quality (signal-to-noise+interference ratio, SINR, for example) on f1 and f2 is Qi(1) and Qi(2), respectively, for i=1, 2, . . . , N.
The channel quality, Qi(1) and Qi(2), is converted to a spectral efficiency for all users, where Si(1)=f(Qi(1)) and Si(2)=f(Qi(2)). Experts in the art will appreciate that this can be performed in many ways, from employing Shannon capacity, log2(1+SINR) to using the rate tables used by link adaptation schemes in cellular systems.
Initially, assume all users are on f1. Compute Ti=Si(1)/Si(2) for i=1, 2, . . . , N and order them from lowest to highest values of Ti. Begin with the user having the lowest value. Compute the effective rate that the user would receive on carrier 1 and carrier 2 by factoring in the user's spectral efficiency on each carrier and the expected bandwidth to be received. Without loss of generality, we assume that each user on a carrier will receive equal bandwidth and hence its expected bandwidth on carrier j=Wj/Nj, where Nj is the number of users on carrier j assuming the given user is assigned to carrier j. Hence, the expected rate on carrier j for user i is
and the user selects
argmax1Rj.
In another embodiment of this invention, different bandwidth allocations are allocated to users on a given carrier, and may be different for the same user across multiple carriers.
After a user is assigned to a carrier j, consider the user with the next lowest metric. Note that as more users are considered, the advantage in spectral efficiency of selecting the second carrier diminishes. Further, Nj increases and the available resource must be shared with more users. At some point, users will receive a higher rate by remaining on the first carrier.
Simulation results have shown that this load balancing technique paired with power management can potentially provide up to a 3 times gain in edge and median user throughput over the baseline heterogeneous network scenario without this technique.
In order to highlight the crux of this approach, the embodiments described above deliberately assumed a rather simple scenario with a single class of users who get an equal share of the bandwidth associated with the carrier to which they are assigned. It is to be noted, however, that this approach is not limited to such scenarios. Those familiar with the art can apply this basic framework to more complex scenarios. The following is a brief description of some scenarios likely to be encountered in practice and how one might adapt some of the embodiments described above to those scenarios.
In this implementation, there are K user classes such that if two users, one belonging to class m and the other to class n, are assigned to the same carrier on a cell, the share of the bandwidth they receive will be in the proportion km: kn. The idea here is that if class m has a higher priority than class n, km will be greater than kn so that class m users, in general, will be likely to receive preferential treatment compared to class n users. In this case, a simple tweak to the previously described approach is needed to carry out the assignment of users to carriers: Assume for simplicity that the cell carrying out user assignment has two carriers. As in the previously described approach, we begin by assuming that all users (indexed by i) are on carrier f1 and place them in an increasing order of Ti=Si(1)/Si(2). In calculating Rj(i), the rate a user, say i, receives on carrier j (j=1 or 2) when that carrier has Nj users assigned to it (including user i), we use the formula
instead of the one given above. In the present formula, c(u) denotes the class associated with user u. For a given user i, once the rate values Rj(i) have been calculated for all carriers (namely, j=1 or 2 in the present example), it is assigned to the carrier that corresponds to the largest value of Rj(i). That is, user i is assigned to the carrier jmax(i) where
Note that the approach in this case with multiple priority classes is essentially the same as before, except that the formula used for rate calculation has been modified to account for the fact that users belonging to different priority classes receive different shares of a carrier's bandwidth. Those familiar with the art can thus make similar modifications to the basic approach outlined above to account for the way priorities are implemented in a particular system.
The above examples illustrate how this basic approach can be applied to many of the scenarios one is likely to encounter in practice. Those familiar with the art can make similar modifications to suit the specific requirements of their implementation without violating the core concepts behind our approach. Also note that, although illustrated with two user classes, this approach is applicable to scenarios with more than two classes. In fact, extending it to the more general case is quite straightforward.
The detailed and, at times, very specific description above is provided to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. In the examples, specifics are provided for the purpose of illustrating possible embodiments of the present invention and should not be interpreted as restricting or limiting the scope of the broader inventive concepts.
A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions where said instructions perform some or all of the steps of methods described herein. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks or tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of methods described herein.
Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments of the present invention. However, the benefits, advantages, solutions to problems, and any element(s) that may cause or result in such benefits, advantages, or solutions, or cause such benefits, advantages, or solutions to become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims.
As used herein and in the appended claims, the term “comprises,” “comprising,” or any other variation thereof is intended to refer to a non-exclusive inclusion, such that a process, method, article of manufacture, or apparatus that comprises a list of elements does not include only those elements in the list, but may include other elements not expressly listed or inherent to such process, method, article of manufacture, or apparatus. The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. Unless otherwise indicated herein, the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. Terminology derived from the word “indicating” (e.g., “indicates” and “indication”) is intended to encompass all the various techniques available for communicating or referencing the object/information being indicated. Some, but not all, examples of techniques available for communicating or referencing the object/information being indicated include the conveyance of the object/information being indicated, the conveyance of an identifier of the object/information being indicated, the conveyance of information used to generate the object/information being indicated, the conveyance of some part or portion of the object/information being indicated, the conveyance of some derivation of the object/information being indicated, and the conveyance of some symbol representing the object/information being indicated.
