Patent Application
20090245337
The present invention relates generally to reducing data required to convey information including channel quality information that is sent in OFDMA systems and, in particular, in ranking channels according to channel conditions or identification numbers and reducing the bits required to convey the rankings.
In certain wireless communication systems that are based on orthogonal frequency division multiple access (OFDMA), system bandwidth is grouped into channels and sub-channels known as physical resource blocks (PRBs.) User equipments report the channel quality information (CQI) on the different PRBs to the network. These CQI reports are generated frequently and are used by the network to select modulation and coding schemes that are used for communicating with the user equipment and the PRBs to allocate to different user equipments. As the reports are frequently provided for multiple PRBs, it places a burden on the uplink signaling load.
According to known methods of reporting CQI in wireless communication systems, such as 3GPP Long Term Evolution (LTE), each user equipment in the system reports the CQI on its best M PRBs, where M is a constant designating the desired number of PRBs in the report. This and other methods reduce the size of messages and uplink signaling load by selecting a given number of PRBs that are acceptable. Accordingly, the signaling load can be reduced by using delta encoding for PRB identifications (IDs) when the CQI is reported in the order of the PRB IDs. For example, if the total number of PRBs is 16, the user equipment reports the ID of the first PRB using a given number of bits. For the second PRB, the user reports the difference between the first and second PRB IDs. Similarly, the third PRB ID can be reported using the difference between the second and third PRB IDs.
CQI reporting for OFDMA system and LTE require reporting over multiple PRBs. The differences provided, however, cannot be lower than a particular value because any of the 16 PRBs can be the best, second best etc. What is needed, therefore, is a method that further reduces the uplink signaling load.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to an efficient method of reducing the data required to convey the channel quality information for a set of PRBs in OFDMA systems. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
In this document, relational terms such as first and second, top and bottom, and the like may be 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 “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of an efficient method of reducing the data required to convey the channel quality information for a set of PRBs in OFDMA systems described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of the method to perform an efficient method of reducing the data required to convey the channel quality information for a set of PRBs in OFDMA systems. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.
The present invention is directed to a method that includes determining a distribution of channel conditions across a plurality of PRBs and in an example for each of the best to worst PRBs. The best PRB is the PRB that experiences the highest signal to noise ratio and the worst PRB is the PRB that experiences the least signal to noise ratio. The distribution of channel conditions can be determined using order statistics. The method includes dividing the distribution of channel conditions into a plurality of levels, which can be modulation and coding scheme levels. The method also includes ordering the PRBs according to a ranking of the instantaneous channel conditions for each of the plurality of PRBs. In an embodiment the first ranking can correspond to the best channel conditions and the subsequent rankings are in a successive order of channel conditions. The method continues by selecting an MCS level that best represents the current channel condition of the PRB from the plurality of MCS levels representative of the distribution of the channel conditions for a given PRB.
In addition, the method includes reporting a mean value of the channel conditions across all PRBs and a representation of the range, which can include a bit value for the selected level for each of the ranked PRBs. In an embodiment, one of the levels is represented by a minimum number of bits required to represent that level. The method also includes reducing the number of bits required to report a successive rank. For example, if the total number of levels spanned by the distribution of the best PRB is 8 and the first rank was level 4, it is represented by 3 bits and the next rank must be level 4 or less and can be represented using 2 bits. In other words, the representation of the ranking includes a minimum number of bits for each of the rankings.
In an embodiment of the method, the channel conditions are conveyed by channel quality information representative of the signal to noise ratio of the plurality of channels, and the MCS levels are an equal division of the channel conditions. Moreover, the representation of the range comprises one of the designated levels of the channel conditions.
In another embodiment of the principles described, the method includes determining a distribution of channel conditions for each of the best to worst PRBs, which can be achieved by using order statistics. In addition, the method includes dividing the distribution of channel conditions into a plurality of MCS levels and ranking each of the plurality of PRBs or channels according to the instantaneous channel conditions for the PRBs. A level of the channel conditions can be selected from the plurality of levels representative of the distribution of the channel conditions for a PRB and based on the observed channel condition for the ranked PRBs. The method includes reporting the identity of the best PRB from the observed rankings of channel conditions and a minimum number of bits to report the CQI for the selected first rank using the mean value of the channel conditions. The number of bits for the selected first rank is equal to the ceil(log2 N), where N is the number of MCS levels within which most of the probability of the best PRB lies, given the mean value of the channel conditions across all PRBs. In addition the method includes reporting the identity of the PRB with a subsequent or successive rank to the first rank from the rankings of channel conditions and the observed CQI on the subsequent or successive ranked PRB based on the minimum number of bits for the selected level for the first rank.
In another embodiment, the method includes determining a distribution of channel conditions for each of the best to worst PRBs; dividing the distribution of channel conditions into a plurality of MCS levels and ranking each of the plurality of PRBs or channels according to the instantaneous channel conditions for the PRBs. A level of the channel conditions can be selected from the plurality of levels representative of the distribution of the channel conditions for a PRB and based on the observed channel condition for the ranked PRBs. The channels are selected such that the selected channels have a higher CQI than the ones that are not selected (i.e., top M channels are selected based on the observed instantaneous channel conditions), and then ordered according to the PRBs IDs. The method continues with reporting the channel identity of the first rank from the rankings of identifications of the channels using a minimum number of bits for the identification of the channel and reporting the value of the channel condition observed on that channel, and reporting the channel identity of the subsequent ranked channel from the rankings of identification of channels using a minimum number of bits for the identification of the channel based on the first identification.
Turning to
The probability distribution function of the SNR of a particular PRB is known and can be assumed to be independent and identically distributed over the various PRBs. For example, the distribution of the SNR is exponential for a user on a given PRB when the channel fading follows a Rayleigh distribution. The mean of the exponential distribution will not be known however, and is conveyed as part of the channel quality reporting. Turning to
In addition,
The MCSs are chosen to represent given SNR values of the CQI reports and to simplify the reporting of the CQI report. In an embodiment, the signaling can be further reduced when the user equipment reports the CQI of its best PRB by restricting the candidate MCS levels to those that are spanned by the probability distribution function of that best PRB. The probability distribution function of the best PRB is known based on the knowledge of the mean MCS level across all PRBs. In other words, the user equipment need only report one MCS level for its best PRB among the number of MCS levels spanned by the probability distribution function of the best PRB rather than among all of the MCS levels. For example, one MCS level among the MCS level range 2-8 can be used to represent the SNR range of the best PRB 202 and one MCS level among the MCS level range 2-7 can be used to represent the SNR range of the second PRB 204. The CQI report will include the identities of the top M PRBs in rank order, and the MCS level for each of the ranked PRBs.
As shown in
In an embodiment, more optimization is available based on the knowledge that a PRB is assigned a specific MCS level for the designated SNR range. In other words, a ranked PRB is represented by a specific, or one, level within the range of the SNR values for a particularly ranked PRB, which is based on the observed channel conditions for the ranked PRB. This further removes the availability of all MCS levels that are higher than that MCS level for a lesser or subsequently ranked PRB. To the extent that the MCS levels overlap for different PRBs, the knowledge that a specific MCS level is being used by a higher ranked PRB limits the availability of all MCS levels that are higher than that MCS for the successively or subsequently ranked PRBs.
For example, and still referring to
It is noted that the above description has been limited to the SNR distribution of a single user. It is possible that different users in a cell have different distributions and values. That results in a large SNR range. Accordingly, the probability distribution functions of the ranked PRBs will be different for different users. Moreover, for the same user the mean SNR or MCS level across all PRBs may vary with time. This may require that user equipment report the mean SNR or MCS level when it changes so that network 100 can use appropriate probability distribution functions to decode absolute MCS levels.
By using the probability distribution functions for the ranked PRBs, it has been shown that the number of MCS levels can be restricted. In another form of CQI reporting, a fixed number of bits per PRB can improve the granularity of the CQI reports. The restricted SNR ranges of the various ranked MCSs and the associated PRBs can be shown using the number of available bits. Instead of using a given number of bits to designate PRBs and associated MCS levels where many levels are not used by the PRB, the bits can be associated with the known MCS levels for that ranked probability function. For example, if 4 bits are being used to represent the MCS levels for a given ranked PRB, those 4 bits can represent 16 levels. As shown in
From
The above description has shown uses when the complete PRB ID specified for each of the ranked PRBs is used in reporting. According to the principles described, the signaling load can be also reduced by reporting the CQI in the ranked order of the PRB IDs and then applying the principles of encoding to those PRB IDs. This is different than reporting the difference in PRB IDs as part of the ranking. Rather, the principles of the invention lessen overhead by using information regarding higher ranked IDS to reduce the number of bits for successively or subsequently ranked PRBs. In this embodiment, CQI values need to be reported in full i.e., one MCS level among all possible MCS levels need to be specified for each PRB. For a given number of bits used to represent the PRB IDs, as the rank is decreased the number of bits needed to represent that ranked PRB ID is also reduced. For example, the highest rank PRB can use all of the bits while a lower ranked PRB need only use the number of bits required for its rank based on the information already conveyed for a higher ranked PRB.
If the ranked PRB IDs are 8, 7 and 5, at least 4 bits are needed to represent the highest ranked PRB ID 8 (assuming a total of 16 PRBs are present in the system), i.e. bit representation 1000, while three bits can be used to represent the next highest ranked PRB ID of 7, i.e. bit representation 111. If the highest ranked PRB is represented by the bit combination of 1001, the next value can be four bits long. If the first three bits of the next highest ranked PRB is 1000 then the subsequent value can also be three bits long because, if the three most significant bits of the next four digit value is 100, the fourth bit must be a 0, and therefore the fourth bit does not need to be sent. In the case where the first PRB ID is represented by 100100 (assuming a total of 64 PRBs in the system,) the next subsequent ranked PRB will be represented by 6 bits. But if the first three bits of the PRB ID is represented by 100, the fourth bit must be a 0 as 1 was previously used and therefore does not need to be sent and only the next 2 bits must be sent. The number of bits is therefore reduced to 5. In this way, knowledge concerning a higher ranked ID can reduce the number of bits needed to represent a subsequent ID.
Turning to
The PRBs are then ordered 306 according to a ranking of the instantaneous channel conditions for each of the plurality of PRBs. As seen in
In addition, the method includes reporting 310 the rankings of channel conditions. In an embodiment, rankings are reported by using a mean value of the channel conditions as a representation of the channel conditions as well as an identification for the reported PRB. The representation includes one of the designated levels of the channel conditions. In an embodiment, one of the levels is represented by a minimum number of bits required to represent that level based on the probability distribution function of the channel with a given rank. The method also includes reducing 312 the number of bits required to report a successive rank. For example, if the first rank was level 4, which is represented by 3 bits based on the knowledge of the span of MCS levels for the highest ranked channel, the next rank must be level 4 or less and can be represented using 2 bits. In other words, the representation of the ranking includes a minimum number of bits for each of the rankings. Therefore, the number of bits required to report the successively or subsequently ranked PRBs is based on the number of bits and the actual bits required to report the higher ranked PRB. Using the mean value and the known distribution of channel condition as well as the reported channel conditions, the network can determine the CQI information from the user equipment.
A level of the channel conditions can be selected 408 from the plurality of levels representative of the distribution of the channel conditions for a user. The method includes reporting 410 a mean value of the channel conditions, the PRB ID of the first ranked PRB from the rankings of channel conditions and the selected level corresponding to the observed SNR value on that PRB. The selected level is reported using a minimum number of bits for the selected first rank. In an embodiment, one of the levels within the range of levels of the first rank is used to represent that first rank. For example from
On the basis of the above description and method, the benefits of using order statistics of the probability distribution function for selecting MCSs can be shown. A number of bits are used within OFDMA to convey mean SNR for PRBs. In an embodiment, 5 bits can be used to designate the mean SNR of the PRBs. It has been shown that the dynamic range of the best PRB (PRB with the highest SNR) based on the range of SNR values can be reduced to 7 levels. In addition, it has been shown that the dynamic range of the second best PRBs based on the range of SNR values can be reduced to only 4 levels. As the rank of PRB is reduced the number of levels is also reduced and is not any higher than a higher ranked PRB. As can be appreciated, there are a corresponding number of bits required to convey the number of levels. Moreover, the number of bits is reduced as the rank is reduced. In other words, fewer bits are required for subsequent ranked levels. Based on the demonstrated dynamic range for the best and second best, etc., ranges, 3 bits is generally required to convey the 7 levels of the best ranked channel, 2 bits is generally required to convey the second best ranked channel as well as for each subsequently ranked channel or PRB.
Thus, it can be shown that the overhead is reduced by using the order statistics of the probability distribution function. When the PRB index requires 7 bits, the total number of bits needed for M best PRBs is shown by the equation (5+(3+7)+(2+7)(M−1)). In addition, the second best PRB is encoded based on the observed value of the previous ranked PRB such that the number of bits for the second best PRB can be approximated as 0.5*2+0.5*1=1.5 bits. Similarly for the third and subsequent channels the number of bits can be approximated to be 1.5 bits. Referring back to
In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
