Abstract:
Multiple antennas at the transmitter and receiver increase the capacity in a wireless communication system. Received signal quality can be enhanced through diversity by transmitting the same symbol on multiple antennas. Data rate can be increased through multiplexing by transmitting different symbols on the transmitting antennas. Operating in one particular mode only is generally not suitable. Choosing diversity or multiplexing by taking into account the location of a user can enhance both throughput and reliability.

Description:
TECHNICAL FIELD 
     Technical field of present disclosure relates to method, apparatus and system for switching between multiplexing and diversity, and in particular to switching between diversity and multiplexing based on a location of the user. 
     BACKGROUND 
     Multiple antennas at the transmitter and receiver increases the capacity in a wireless communication system. By transmitting the same symbol on the transmit antennas, the received signal quality can be increased due the diversity gain. On the other hand, by transmitting different symbols on the transmitting antennas, the data rate of the system can be increased due to multiplexing. In general, one particular mode is not always suitable in a wireless system. Thus, it is often a good practice to switch between diversity and multiplexing. 
     Conventionally, there are various methods to which to use diversity or multiplexing at any given moment. For example, expected spectral efficiency and/or capacity can be computed at the base station from pilot measurements for each individual communication mode—diversity, adaptation, or multiplexing. Based on the computed efficiency/capacity, it can be decided whether to use diversity or multiplexing. 
     However, a problem associated with the conventional method is that simply computing the efficiency/capacity at the base station does not fully take into account the environmental conditions that can affect the wireless communication between a UE and the base station. 
     SUMMARY 
     A non-limiting aspect of the disclosed subject matter is directed to a method to communicate between a base station and a user equipment of a wireless communication system. The method may be performed at a base station capable of transmitting over multiple antennas. In the method, it is determined whether the user equipment is located within a threshold edge distance from an edge of a cell corresponding to the base station. When the user equipment is determined to be located within the threshold edge distance, diversity is chosen. 
     Another non-limiting aspect of the disclosed subject matter is directed to a base station of a wireless communication system arranged to wirelessly communicate with a user equipment. The base station is capable of transmitting over multiple antennas. The base station includes a communication unit, a UE location determination unit, and a communication mode determination unit. The communication unit is configured to communicate with the user equipment, the UE location determination unit is configured to determine whether the user equipment is located within a threshold edge distance from an edge of a cell corresponding to the base station, and the communication mode determination unit is configured to choose diversity for transmission from the base station to the user equipment when the user equipment is determined to be located within the threshold edge distance. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects features, and advantages of the disclosed subject matter will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale. 
         FIG. 1  illustrates an example of a wireless communication system layout with base station, corresponding cells, and user equipments; 
         FIG. 2  illustrates a throughput of a cell sector for different modes of communication; 
         FIG. 3  illustrates a throughput at cell edge for the different modes; 
         FIG. 4  illustrates a flow chart of an example process for switching between diversity and multiplexing; 
         FIG. 5  illustrates an example of a cell divided into sectors; 
         FIG. 6  illustrates a flow chart of an example process for determining whether a user equipment is close to an edge of a cell; 
         FIG. 7  illustrates a flow chart of another example process for determining whether a user equipment is close to an edge of a cell; 
         FIG. 8  illustrates a flow chart of yet another example process for determining whether a user equipment is close to an edge of a cell; 
         FIG. 9  illustrates a flow chart of another example process for switching between diversity and multiplexing; 
         FIG. 10  illustrates a flow chart of an example process for adaptively choosing between diversity and multiplexing; 
         FIG. 11  illustrates a cell divided into different regions; 
         FIG. 12  illustrates an example embodiment of a base station. 
     
    
    
     DETAILED DESCRIPTION 
     For purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, and so on. However, it will be apparent to those skilled in the art that the technology described herein may be practiced in other embodiments that depart from these specific details. That is, those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the described technology. 
     In some instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description with unnecessary details. All statements herein reciting principles, aspects, embodiments and examples are intended to encompass both structural and functional equivalents. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform same function, regardless of structure. 
     Thus, for example, it will be appreciated that block diagrams herein can represent conceptual views of illustrative circuitry embodying principles of the technology. Similarly, it will be appreciated that any flow charts, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     Functions of various elements including functional blocks labeled or described as “processors” or “controllers” may be provided through dedicated hardware as well as hardware capable of executing associated software. When provided by a processor, functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, explicit use of term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may include, without limitation, digital signal processor (shortened to “DSP”) hardware, read only memory (shortened to “ROM”) for storing software, random access memory (shortened to RAM), and non-volatile storage. 
     In this document, 3GPP is primarily used as examples for explanation purposes. However, the scope of this disclosure is not limited to the set of 3GPP wireless network systems and can encompass many domains of wireless network systems. 
     As noted in the background section, a problem with the existing method to switch between diversity and multiplexing is that the environmental conditions that can affect the wireless communication between a UE and a base station (BS). For example, it is generally known that noises and interferences impact the quality of the radio link between a BS and a UE. 
     To serve as an explanation,  FIG. 1 , which illustrates a layout of an example wireless communication system  100 , is provided. For simplicity of explanation, many of the elements and functions a typical wireless communication system are not illustrated. For example, elements such as the core network nodes, home location registers, gateways, and so on are not illustrated so as to minimize clutter. The figures in this disclosure are exemplary and should not be taken to be limiting. 
     As seen in  FIG. 1 , the system  100  includes multiple BSs  120 ,  122 ,  124  serving corresponding coverage areas or cells  110 ,  112 ,  114 . UEs  130 ,  135  are within cell  110 , and thus it can be assumed that the BS  120  is the serving BS for UEs  130 ,  135 . Similarly, it can be assumed that BSs  122 ,  124  are the respective serving BSs for UEs  132 ,  134 . For the moment, focus will be on the BS  120  and the UEs  130 ,  135  it serves bearing in mind that the discussions that follow will likely apply with equal validity to the BSs  122 ,  124  as well as to the UEs  132 ,  134 . 
     As mentioned above, noises and interferences impact the quality of the radio link between the BS and the UE. In  FIG. 1 , the UE  130  is illustrated to be located close to its serving BS  120 . Thus, the radio link between the BS  120  and the UE  130  is likely to be of good quality. That is, the signal-to-interference-and-noise ratio (SINR) of this radio link is likely to be high. 
     UE  135  on the other hand is illustrated to be located fairly close to the edge of cell  110 , i.e., close to the limits of the coverage area of BS  120 . As such, the radio link between BS  120  and UE  135  is likely to suffer from noises. For example, where UE  135  is located, interferences due to transmissions from neighboring base station  122 ,  124  may be substantial. The likely result is that the radio link quality, at least in the downlink direction, between BS  120  and UE  135  will be low, e.g., link&#39;s SINR will be low. Also, note that due to the proximity of the UE  134  to the UE  135 , there may be significant interferences in the uplink direction from the UE  135  to the BS  120  as well. 
     At the transmitter, e.g., at a BS on the downlink to a UE, using multiplexing will increase the data transmission rate over using diversity generally. This is acceptable when the link quality between the UE and the BS is good. For example, multiplexing may be used as the communication mode between BS  120  and UE  130  in  FIG. 1 . 
     This is consistent with  FIG. 2  which illustrates a throughput of a cell sector for different modes of communication—Alamouti, cyclic delay diversity (CDD), and open loop multiple-input multiple-output (MIMO). Alamouti space-time codes are typically used to achieve transmit diversity (rank 1). CDD is also a way to achieve single-rank transmit diversity. Open loop MIMO is generally seen as an adaptive way to achieve transmission ranks greater than one (i.e. multiplexing). Higher the rank, greater the degree of multiplexing, and hence, greater the peak rate. 
     For a UE located close to a cell center such as the UE  130  in the cell  110 ,  FIG. 2  shows that the adaptation mode results in a better throughput than either of the individual modes. That is, multiplexing results in a better throughput than the single-rank modes. The multiplexing rank can be adapted generally such that as the UE is located closer to the cell center, the transmission rank can be correspondingly increased. 
     But when there is significant amount of noise and interference on the radio link, the data throughput may actually be better when a lower rank transmission is used. At the extreme, for example at the cell edge, diversity may result in a better overall throughput than multiplexing. As mentioned above, high rank transmissions can achieve high peak rates. However, high rank also results in the coverage being correspondingly reduced. To put in another way, lower rank transmissions result in greater coverage. Thus, at the edge of the cell  110  where the UE  135  is located, rank 1 (diversity) transmissions from the BS  120  will likely be the best in terms of throughput. This is confirmed in  FIG. 3  which shows that at the cell edge, diversity has the best throughput performance. 
     It is seen the location of the user, i.e., the location of the UE, is a significant factor that should be taken into account in choosing the communication mode—diversity (rank=1) or multiplexing (rank&gt;1)—at least to achieve optimum throughput. In the present disclosure, an exemplary scheme is proposed in which the communication mode between a BS and a UE is chosen based on the UE&#39;s location. That is, the user location is considered in the proposed scheme. This is unlike the existing method in which the user location is not considered. When the user location is not considered, the cell edge user throughput can be negatively impacted as seen in  FIG. 3 , which in turn reduces the coverage. The proposed scheme can replace the existing method. It is also contemplated that the proposed scheme can be combined with the existing method. 
       FIG. 4  illustrates a flow chart of an example process or method for choosing a communication mode between a base station and a user equipment. The method  400  may be performed at the base station of a wireless communication system, e.g. the BS  120 , to wirelessly communicate with the user equipments it serves, e.g. UE  130 ,  135 . It is assumed that the BS  120  is capable of transmitting over multiple antennas, i.e., the BS  120  is capable of performing diversity (rank=1) or multiplexing (rank&gt;1) transmissions. 
     Note that the communication mode between a base station and one UE can be independent of the communication mode between the same base station and a different UE. For example, diversity mode may be chosen for a radio link between the BS  120  and the UE  135  (referred to as the “first radio link” for ease of reference) while multiplexing may be chosen for a radio link between the BS  120  and the UE  130  (referred to as the “second radio link” also for ease of reference). More generally, the ranks of radio links can be independent of each other. For example, the ranks of both the first and second radio links may indicate multiplexing (rank&gt;1), but differ in the rank level. Thus, the method  400  should be taken as being independently applicable for each BS-UE radio link. 
     Further, the communication mode is assumed to be dynamic in that the mode can change depending on the circumstances. For example, as the UE  130  moves away from the BS  120 , the first radio link&#39;s rank can be reduced. Thus, the method  400  should also be taken as being independently applicable for a particular BS-UE radio link at different times. 
     For simplicity of explanation, the method  400  will be described with reference to the BS  120  and the UE  130 . In the method  400 , the BS  120  determines the location of the UE  130  (step  410 ). The BS  120  then determines whether the UE  130  is located within a threshold edge distance from the edge of the cell  110  (step  420 ). When the BS  120  determines that the UE  130  located within the threshold edge distance, the BS  120  chooses diversity (rank 1) for the first radio link at least for the downlink transmission to the UE  120 . Conversely, when the BS  120  determines that the UE  130  not located within the threshold edge distance, the BS  120  chooses multiplexing (rank&gt;1) for the first radio link at least for the downlink transmission. 
     As seen in  FIG. 1 , the cell  110  corresponds to the radio coverage area of the BS  120 . The threshold edge distance is some distance from the cell&#39;s edge in which diversity is enforced. Note that the threshold edge distance can be particular to each cell, i.e., it need not be the same for all cells. For example, the threshold edge distance for a relatively small cell (e.g., corresponding a base station in an urban area) may be small as compared to the distance for a relatively large cell (e.g., corresponding a base station in a rural area). 
     Even for one cell, the threshold edge distance may change over time. For example, the threshold distance may be set closer to the edge during times of low demand (e.g., during sleeping hours, less interference likely) and set further from the edge during times of high demand (e.g., during business day, more interference likely). The threshold edge distance can be set manually or be set automatically. 
     For the purposes of this disclosure, the term “cell” is to be taken broadly to mean a coverage area. This is explained with to  FIG. 5  which illustrates a cell divided into three sectors. For a variety of reasons, it is fairly common to sectorize a cell in which each sector is essentially treated separately even though all sectors are served by the same base station. Thus, in at least one aspect of the present disclosure, each sector can be viewed as a separate “cell”, i.e., as a separate coverage area. Thus, the threshold edge distance of sector  1  is not necessarily the same as the threshold edge distance of sector  2  or of sector  3 . 
     Referring back to  FIG. 4 , the BS  120  determines in step  420  whether or not the UE  130  is within the threshold edge distance (whether or not the UE  130  is close to the edge). If the UE  130  is GPS capable, it may provide its geographical location in a report to the BS  120  from which the BS  120  may make the determination. The geographical location may be determined in other ways such as through triangulation. 
     There are also ways in addition to geographical means to determine the UE&#39;s location and hence determine whether the UE is within the threshold edge distance.  FIG. 6  illustrates one example process in which the step  420  may be implemented. Generally, in the process illustrated in  FIG. 6 , the quality of the radio link between the BS  120  and the UE  130  is used as a basis to determine whether or not the UE  130  is close to the cell&#39;s edge. 
     As seen in the figure, the BS  120  receives a radio link report from the UE  130  regarding a radio link between the BS  120  and the UE  130  (step  610 ). Based on the radio link report, the BS  120  determines the link quality of the radio link (step  620 ). The BS  120  then determines whether the link quality is lower than a threshold link quality (step  630 ). When the link quality is determined to be is lower than the threshold link quality, the BS  120  determines that the UE  130  is located within the threshold edge distance (step  640 ). Conversely, when the link quality is not determined to be lower than the threshold link quality, the BS  120  determines that the UE  130  is not located within the threshold edge distance (step  650 ). 
     In  FIG. 6 , the threshold link quality is correlated to the threshold edge distance. Thus, just as the threshold edge distance of one cell is not necessarily dependent on the threshold edge distance another cell, the threshold link qualities of different cells need not be dependent as well. Also just as the threshold edge distance of one cell can by dynamic, the same can be said of the threshold link quality of the cell. 
     The radio link quality can be expressed in multitudes of ways. The above mentioned SINR is one measure of the link&#39;s quality. Others measures include SNR, error rates (bits, frames, packets, etc.), retransmission rates (e.g., related to ARQs), MCS, and even the rank assigned to the UE. The radio link quality can be expressed in any combination of these measures and any other individual quality measures. 
     In step  630 , it is indicated that whether or not the link quality is lower than the threshold link quality is determined. However, it should be noted that the direction of comparison should be adjusted or otherwise accounted for depending on the particular measurement. For example, if an error rate is used; then the inequality in step  630  should be reversed or the reciprocals of the error rate should be used as the link quality measurement. 
       FIG. 7  illustrates another example process for implementing step  420 . Like the process illustrated in  FIG. 6 , the process of  FIG. 7  also uses information the UE provides to determine whether or not the UE is located close to the cell edge. In this process, the number of neighboring BSs detected by the UE is used. The logic is as follows. Before a handoff occurs, a UE typically sends a measurement report or multiple of such reports to the serving BS, which indicates that the UE is close to the cell boundary and is likely to handoff to other cells. The measurement report typically contains information about the neighboring base stations, and may also include a measurement of the signal power emanating from the base stations. For example, in 3GPP, the UE can report to the serving eNB the RSRP levels of the neighboring eNBs as seen by the UE. A high number of detected neighboring BSs is an indication that the UE is close to the cell edge. 
     As seen in the figure, the BS  120  receives a neighbor report from the UE  130  regarding other base stations neighboring the BS  120  (step  710 ). For example, the neighbor report may include information of the BSs  122  and/or  124 . Based on the neighbor report, the BS  120  determines the number of neighboring base stations (step  720 ). The BS  120  then determines whether the number of neighboring base stations is higher than a threshold neighbors number (step  730 ). When the number is determined to be is higher than the threshold neighbors number, the BS  120  determines that the UE  130  is located within the threshold edge distance (step  740 ). Conversely, when the link quality is not determined to be higher than the threshold neighbors number, the BS  120  determines that the UE  130  is not located within the threshold edge distance (step  750 ). 
     In  FIG. 7 , the threshold neighbors number is correlated to the threshold edge distance. Thus, threshold neighbors number of one cell need not be dependent on another cell and can dynamically change. 
     As indicated above, the neighbor report can also include the power level of the neighboring base stations as detected by the UE. Two different UEs at the same location may detect different number of base stations because one is more sensitive than the other. When this occurs, the neighbor reports from the two UEs may not be similar even though they are similarly located. If the total number of detected neighboring base stations is used for comparison in step  730 , the higher sensitive UE may be unnecessarily forced into a lower rank communication mode simply because it is able to detect weak signals that in reality may not be a significant source of interference. 
     Thus, in one aspect, only those neighboring base stations that contribute to the interference by more than a negligible amount are considered. That is, in one example implementation of step  720 , each neighboring base station in the neighbor report whose signal level is higher than a threshold neighbor level is counted towards the number of neighboring base stations and others are ignored. Again, it should be noted that the setting of the threshold neighbor level for at least one base station can be independently set from other base stations, and the level for at least one cell may be adjusted dynamically. 
       FIG. 8  illustrates yet another example process implement step  420 . In this process, the signal(s) from the UE is (are) measured at the BS to determine whether or not the UE is close to the cell edge. Generally, weaker signals indicate that the UE is farther away from the BS and consequently closer to the cell edge, and vice versa. 
     As seen in the figure, the BS  120  measures the UE signal level of the signal transmitted from the UE  130  (step  810 ), and determines whether or not the UE signal level is below a threshold UE level (step  820 ). If so, the BS  120  determines that the UE  130  is located within the threshold edge distance (step  840 ). Otherwise, the BS  120  determines that the UE  130  is not located within the threshold edge distance (step  850 ). The UE signal that is measured may be the UE&#39;s SRS or its pilot signal. 
     In many wireless communication systems, the base station typically controls the transmission power level of the UEs within its serving area so as to minimize interferences while maintaining some acceptable level of service. Thus, the strength of the measured signal by itself may not be an accurate indication of the UE&#39;s location. Thus, in one aspect, the BS  120  can also take into account the level at which the UE  130  transmits since the BS  120  can instruct the UE  130  to transmit at a particular level. 
     In another aspect, when a UE first enters a cell of a base station, no transmission control has yet taken place, and thus the UE will transmit its pilot signal, e.g., a sounding reference signal (SRS), at a default power level. In this aspect, the signal from the UE  130  may be measured in step  810  prior to the BS  120  instructing the UE  130  to adjust its transmission power, and that measured level may be used for comparison in step  820 . 
     In another aspect, the BS  120  may simply instruct the UE  130  to transmit at a predetermined power level (step  805 ) prior to measuring the UE signal level in step ( 810 ). 
     Referring back to  FIG. 4 , when the BS  120  determines that the UE  130  is close to the cell edge, diversity is strictly chosen. On the other when the BS  120  determines that the UE  130  is not that close to the cell edge, multiplexing is strictly chosen. When the UE&#39;s location is the main basis for choosing the communication mode, enforcing diversity at the cell edge is logical since  FIG. 3  indicates that at the limits of the coverage area, diversity results in a better throughput. 
     But further in from the edge, strictly choosing multiplexing may not always be optimal. This is because the level of interferences can change over time at a given location. Thus, in an alternative, rather than strictly choosing multiplexing when the UE is not near the edge of the cell, the choice between diversity and multiplexing may be made adaptively. This is illustrated in  FIG. 9 . The method  900  illustrated in  FIG. 9  is the same as method  400  of  FIG. 4  with the exception of step  940 , which is performed when the BS  120  determines that the UE  130  is not close to the edge, e.g., determines that the UE  130  is not within the threshold edge distance. In step  940 , the BS  120  adaptively chooses the operation mode. 
       FIG. 10  illustrates an example process to implement step  940  to adaptively choose between diversity and multiplexing. As illustrated, the BS  120  determines whether the UE  130  is located within a threshold base station distance from the base station  120  (step  1010 ). If so, the BS  120  chooses multiplexing for transmission from the base station  120  to the UE  130  when the UE  130  is determined to be located within the threshold base station distance (step  1020 ). Otherwise, the BS  120  chooses one of diversity and multiplexing based on spectral characteristics of a radio link between the BS  120  to the UE  130  (step  1030 ). Spectral characteristics can be the expected spectral efficiency and/or capacity computed at the base station from pilot measurements for each individual communication mode. 
     The logic behind the process illustrated in  FIG. 10  is explained with reference to  FIG. 11  which illustrates a cell represented by the large solid circle. To minimize clutter, the corresponding base is not shown, but it is assumed that the base station is centrally located. In  FIG. 11 , three regions are shown—the center region enclosed by smaller dashed circle, edge region defined between the outer solid circle and the larger dashed circle, and an intermediate region between the two dashed circles. 
     The cell edge region may be viewed as a region in which diversity (rank=1) is strictly chosen. As mentioned above, the interference level can change over time. Thus, it is possible that the interference, even within the cell edge region, will be low enough at some point so that multiplexing can be utilized. However, the cell edge region can be viewed as a region in which diversity is assumed to achieve the best throughput is sufficiently accurate such that incurring the cost of determining the spectral characteristics for choosing the best mode is not warranted. At the other extreme, there can be a region close to the center in which simply assuming that multiplexing can be used is sufficiently accurate such that the cost of determining the spectral characteristics need not be incurred. This is the center region in  FIG. 11 . But in intermediate region, the interference changes affect whether diversity or multiplexing can be used to a degree such that simply assuming one or the other will not be sufficiently accurate. 
     Thus, the cell edge region may be viewed to correspond to the YES branch in step  420  of  FIGS. 4 and 9 , the center region may be viewed to correspond to the YES branch in step  1010  in  FIG. 10 , and the intermediate region may be viewed to correspond to the NO branch in step  1010  in  FIG. 10 . 
     In the center region, multiplexing is chosen, i.e., the rank is at least two in this region. While not shown, it should be realized that the center region can be even more finely subdivided into minimum rank 2 region, minimum rank 3 region, and so on. 
       FIG. 12  illustrates an example arrangement of a base station such as the BS  120 . As seen, the BS  120  may include a communication unit  1210 , a UE location determination unit  1230 , and a communication mode determination unit  1240 .  FIG. 12  provides a logical view of the BS  120 , and thus, some or all units may be separate or may be combined physically in an integrated module. Also, the some or all units may be implemented through a combination of hardware, software, and firmware. The BS  120  may include one or more processing units executing program instructions stored in a storage unit  1220  to perform the functions of any one or more the units  1210 ,  1230 , and  1240 . The program instructions may be provided through a non-transitory storage medium or in programmed in firmware. 
     The roles performed by the different units of the BS  120  correspond to the methods and processes described above, and thus will not be repeated here. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the disclosed subject matter but as merely providing illustrations of some of the presently preferred embodiments. Therefore, it will be appreciated that the scope of the disclosed subject matter fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope is accordingly not to be limited. All structural, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed hereby. Moreover, it is not necessary for a device or method to address each and every problem described herein or sought to be solved by the present technology, for it to be encompassed hereby.