Patent Application
20080151985
The present invention generally relates to the field of wireless communications, and more particularly relates to optimizing BTS receiver performance with assistance from the network in wireless communication systems.
In WiMAX 802.16e and GSM/EDGE receivers, as an example, equalizers are often used in fighting against channel impairment. Many factors characterize a particular channel. For instance, the terrain surrounding the receiver and the transmitter will influence the channel behavior. A receiver located in a hilly or mountainous environment will perform differently than one located on relatively flat terrain. Additionally, a receiver located in an urban area, which has many interfering buildings and other competing signals, encounters different channel characteristics than one located in a rural environment.
However, the equalizers used on each receiver must accommodate a wide variety of channel conditions because the channel characteristics vary by location. These characteristics are generally not known a-priori. The variety in channel conditions is typically dealt with by attempting to measure some characteristic of the channel and then adapt the equalizer's characteristics to that channel in real time. The algorithms used to measure and adapt to the channel are subject to degradations caused by noise and interference unless the range of adaptation is constrained by a-priori knowledge of the channel characteristics likely to be seen in a given location. These degradations resulting from not constraining the adaptation to locally expected channel characteristics generally results in less than optimum adaptation of the equalizer and degraded performance when compared to an equalizer whose adaptation might be constrained to a subset of all possible channel conditions. To accommodate all possible channel conditions, the algorithms required by the equalizer in a receiver can result in a high complexity design which in turn can be costly to implement. One possible solution is to constrain the complexity. Since there are many different kinds of channel types, with a constrained complexity design, the equalizer is forced to use a set of compromised common parameters for all conditions without the knowledge of a specific channel condition. This often translated into compromised receiver performance.
Therefore a need exists to overcome the problems with the prior art as discussed above.
Briefly, in accordance with the present invention, disclosed is a wireless communication system, and method, for optimizing performance of a base station receiver. The method includes providing the base station with information about the local channel characteristics, selecting an interpolation matrix from a pre-defined set of interpolation matrices, used together with the real-time calculated channel estimates of pilot or synchronization information to compute the channel estimates for the received data, and applying these channel estimates to an equalizer as a set of weights to correct the channel-induced distortions in the received data.
Pre-defined interpolation matrices are available to the receiver's equalizer that provide for improved data channel estimation in one or more different channel conditions. The desired interpolation matrix is selected from the pre-defined set of interpolation matrices in one of two ways. In one embodiment of the present invention, by receiving channel profile information for the base station receiver and selecting an optimal interpolation matrix from a pre-defined set of interpolation matrices, a matrix is selected that corresponds to the received channel profile information. In another embodiment of the present invention, the desired interpolation matrix may be selected based on a reception of either a known test signal or regular traffic data and a determination of which interpolation matrix optimizes a performance metric for the base station receiver or the system operation.
The performance metric can be a bit error rate, a symbol error rate, a frame error rate for a known received data signal transmitted by a mobile during a test drive, or standard network statistics such as the number of dropped calls, percentage of calls dropped, number of access failures, and percentage of access failures for normal received data signal during operation, or other relevant system performance metrics that are affected by the equalizer operation, or combinations of the above.
The received data signal may be either a known test signal or normal network traffic. Additionally, the base station receiver uses at least one of CDMA, TDMA (e.g., GSM, EDGE, and GPRS), FDMA, and OFDM (e.g., WiMAX) protocols or any air interface that advantageously utilizes a receiver channel equalizer to enhance the receiver's performance.
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.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely examples of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one of ordinary skill in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention.
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. 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.
The term wireless communication device is intended to broadly cover many different types of devices that can wirelessly receive signals, and optionally can wirelessly transmit signals, and may also operate in a wireless communication system. For example, and not for any limitation, a wireless communication device can include any one or a combination of the following: a cellular telephone, a mobile phone, a smartphone, a two-way radio, a two-way pager, a wireless messaging device, a laptop/computer, automotive gateway, residential gateway, and the like.
Wireless Communications System
According to an embodiment of the present invention, as shown in
Further, in this example, the communications standard of the wireless communications network 102 shown in
The wireless communications network 102 supports any number of wireless communication devices 104, 106, 132, 134. The support of the wireless communications network 102 includes support for mobile telephones, smart phones, text messaging devices, handheld computers, pagers, beepers, wireless communication cards, or the like. A smart phone is a combination of 1) a pocket PC, handheld PC, palm top PC, or Personal Digital Assistant (PDA), and 2) a mobile telephone. More generally, a smartphone can be a mobile telephone that has additional application processing capabilities. The wireless communication cards 132, 134, in one embodiment, reside within an information processing system as shown by the dashed lines. The information processing system, in one embodiment, can be a personal computer, a personal, digital assistant, a smart phone, and the like.
In one embodiment, the wireless communications network 102 is capable of broadband wireless communications utilizing time division duplexing (“TDD”) as set forth, for example, by the IEEE 802.16e (WiMAX) standard. The duplexing scheme TDD allows for the transmissions of signals in a downstream and upstream direction using a single RF frequency channel by separating the upstream and downstream transmissions in time. It should be noted that the present invention is not limited to an 802.16e system for implementing TDD. Furthermore, the wireless communications system 100 is not limited to a system using only a TDD scheme. For example, Frequency Division Duplexing (“FDD”) systems use a different RF frequency for transmission in each direction between the base station and the wireless communication devices. One example, using TDMA and FDMA as in GSM networks may be used for a portion of the available communication channels in the system 100, while one or more schemes are used for the remaining communication channels.
The wireless communications system 100 also includes a group of cell sites 107, 109 that may be, for example, synchronized to a common synchronization scheme. The base stations 112, 114, in one embodiment, are connected to the wireless communication network 102 via an Ethernet connection 136, 138. However, it should be noted that other networking communication standards can be used. The synchronization, in one embodiment, is a time-based synchronization for transmitting and/or receiving wireless data. For example, in a wireless communications system using TDD (e.g. where transmitting and receiving is performed on the same RF frequency channel), synchronization between the base stations is necessary so that their respective wireless communication devices 104, 106, 132, 134 are not transmitting while the other wireless devices in the group are receiving and vice-versa. If this situation occurs, interference between the wireless devices 104, 106 can be created. Each cell site 107, 109, in one embodiment, includes a base station 112, 114 that provides wireless communication services to a coverage area associated with the cell site.
Each base station 112, 114 includes, in one embodiment, a transmitter 116 and a receiver 120. Each receiver 120 also includes at least one equalizer 118 for removing channel distortion. The equalizer 118 will be discussed in further detail below.
The wireless communication devices 104, 106, 132, 134, in one embodiment, are capable of wirelessly communicating data using the 802.16e standard or any other communication scheme that supports TDD. In another embodiment, the wireless communication devices 104, 106, 132, 134 are capable of wireless communications using other access schemes in place of or in addition to TDD. One example is using TDMA communication such as for a GSM/EDGE system application.
Each cell site 107, 109 can be located in a variety of environments, which will affect the quality of the data signals received. A large number of reflecting structures, such tall buildings or mountains, will increase the possible number of paths that a signal can reach the receiving antenna, thereby increasing the multi-path distortion. Additionally, other interfering signals in the area can distort the signal or cause further unwanted signal quality degradation. For instance, in
Base Station Information Processing System
Example Of Equalizer Operation
Channel distortion can be extracted from a received signal by utilizing pilot subcarriers of a signal. Because the pilot symbols are known, the pilot symbols contained in the received signal are compared to the known transmitted signal and the channel estimate is determined for that particular pilot subcarrier. But the unknown channel estimates at the data subcarrier locations (different time and frequency coordinates) need to be estimated based on the previously calculated pilot subcarriers channel estimates and the information regarding the channel condition. This can be achieved by using an interpolation matrix to interpolate the channel estimate of the actual received data subcarriers using the calculated channel estimates of the pilot subcarriers. These channel estimates of the received data carriers are then applied in the one-tap equalizer 306 to recover the actual transmitted data from the received signal.
For example,
Using a Minimum Mean-square Error (MMSE) implementation, the channel estimates for the data sub-carriers are calculated by:
where Rhp is the cross-correlation of the pilot vector with data at the desired tile position, SNR is the signal to noise ratio, Rpp is the pilot autocorrelation matrix, and β is a constant dependent only on the signal constellation. However, this implementation typically requires high computational complexity because it needs to estimate the SNR and some key channel characteristics, such as the maximum delay spread and delay profile and the maximum Doppler frequency. Therefore, this process typically is too complex to be implemented in current base station receivers at a commercially reasonable low cost.
In an embodiment of the present invention, a simplified channel estimation is made using a pre-calculated hybrid interpolation matrix (Mint) with the real-time channel estimates of the pilot subcarriers ĤLS. The following is an example of this channel estimation using a pre-defined matrix for the eight data subcarriers 404 as shown for the tile structure 400 in
Each different interpolation matrix will yield different performance results. No one single interpolation matrix can work well in all channel conditions.
The surrounding environment, such as the terrain and the effects of other received signals, characterizes a particular channel. Channels with similar surroundings will perform in a like manner (e.g., Typical Urban (TU), Rural Area (RA), Hilly Terrain (HT), Vehicular A (VA), etc.). Thus, by characterizing the channel behavior according to its location and surrounding environment, an optimal interpolation matrix can be selected based on the channel profile.
Turning now to
Alternatively, the best interpolation matrix 216 can be found by initiating a training session to optimize the performance of the base station receiver 120. In this way, the base station receiver 120 can be fine-tuned in the field, either during the initial installation or as a part of routine maintenance and calibration.
Turning now to
In order to make the performance metric statistically valid, enough metric samples should be compiled and analyzed. So if more metric samples are needed, at step 608, the steps from 602 to 606 may be repeated. The initial optimal interpolation matrix is set to the default interpolation matrix and the initial optimal metric should be set to the current metric, at step 610. If the method is being performed as part of a test drive session, the data signal received may be a known test signal and a calculated error rate (e.g., bit error rate, symbol error rate, or frame error rate) can be monitored as a desired indicator of a system performance metric. However, normal network traffic can also be monitored during routine operation to achieve the same results. In this case, standard network statistics, such as a number of dropped calls, a percentage of calls dropped, a number of access failures, or a percentage of access failures, can additionally be used in the performance metric. The performance metric can include more than one measure of performance according to the needs of the system. By way of example, the system performance metric may include a combination of a dropped call rate and a bit error rate, each with a predefined weighting in the makeup of the overall system performance metric according to the system needs.
Then, at step 612, the matrix optimizer 214 selects a different interpolation matrix from the set of pre-defined matrices 218. The receiver 120 receives another data signal, at step 614. Next, the new interpolation matrix is used in the channel estimation and subsequent equalization process, at step 616, and the selected performance metric 212 is monitored, at step 618. As before, if more metric samples are needed, at step 620, the steps from 614 to 618 may be repeated. If, at step 622, the monitored performance metric 212 is better with the new matrix than with the old matrix, that is, than with the matrix currently set as the optimal interpolation matrix, then the new matrix is selected as the optimal interpolation matrix and the new metric as the optimal metric, at step 624. Otherwise, the optimal interpolation matrix and optimal metric are not changed. This process is repeated, at step 626, for each interpolation matrix in the set of pre-defined matrices 218 until the optimal interpolation matrix 216 for use with the equalizer 118 is determined. This assures the best performance for the particular receiver equalizer 118 in the base station with a particular surrounding environment.
In view of the present discussion, performance of a base station receiver 120 can be optimized by selecting parameters from a pre-defined set of parameters associated with a plurality of channel conditions, applying the parameters in the channel estimation process, and using the results of the channel estimation in subsequent processing of a received data signal by the equalizer 118. The selection of parameters can be done by receiving channel profile information for the base station receiver 120 and selecting the parameters from a predefined set of parameters, where the selected parameters correspond to the received channel profile information. In one embodiment, the optimization of performance can be done by receiving a data signal, using parameters in a channel estimation and applying the results of the estimation to the equalizer processing, monitoring a performance metric for received data quality, and selecting optimal parameters from a predefined set of parameters based on the monitored performance metric. The base station receiver 120 can then use the selected optimal parameters in the overall channel estimation and equalization process for that particular base station 120 for the received data signal in normal network operation. In one case, the parameters can be elements of an interpolation matrix, such as in an OFDMA system application. In another case, the parameters can describe the channel characteristics, such as in a GSM/EDGE system application. Of course, some system applications may be able to take advantage of both types of parameters. The selection of parameters can include selection of parameters that are specifically tailored for particular channel characteristics from a pre-defined set of parameters. The performance metric, in one embodiment, includes one or more of the following metrics: bit error rate, symbol error rate, frame error rate, number of dropped calls, dropped call percentage, number of access failures, percentage of access failures, percentage of handover failures, and maximum capacity provided. Other metrics that can be used should become obvious to those of ordinary skill in the art in view of the present discussion. Also, the received data signal can be normal network traffic or a known test signal.
The improved receiver sensitivity performance attained by the implementation of the present invention often translates to improvements in coverage area, a need for fewer base stations during deployment, better voice quality, and higher data throughput.
Non-Limiting Examples
Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiments, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.
For example, as has been discussed above,
