Abstract:
A method and system are directed to enabling a detector to perform relatively accurate channel estimates on blocks of data that include relatively short training sequences in wireless communication signals. Channel re-estimation is performed by transferring the estimation calculations from the time domain into a channel state domain. In one embodiment, information obtained from known training sequences and unknown data may be combined to generate an initial channel estimate. An embodiment of a new recursive Expectation Maximization (EM) process generally i) initializes the EM process with the initial channel estimate; ii) determines updates for the channel estimates; iii) produces a reduced observation vector; iv) generates a special sequence of channel states; v) rearranges each channel state estimate in the reduced observation vector; vi) generates a low-dimensional channel model matrix inversion based on the generated special sequence of channel states; and vii) generates channel estimates based on the generated matrix inversion.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to receiving wireless communications, and more particularly, to a detector that improves joint channel estimation and short block detection for received wireless communications. 
     BACKGROUND OF THE INVENTION 
     A multiple access wireless communication system enables multiple users to simultaneously access and use the system. These systems employ multiple access schemes, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). For FDMA, the multiple access wireless communication system&#39;s spectrum is divided into fixed frequency slots and users are allocated one or more slots for communication. 
     For CDMA, the multiple access wireless communication system assigns a unique code to each user that is used to modulate a signal from the user and that modulation identifies the user to the system. In CDMA systems, all users transmit and receive code-modulated data simultaneously, and the data for a given user is distinguished at the receiver using the code assigned to that user. 
     For TDMA, the multiple access wireless communication system divides time into fixed slots and users are allocated one or more time slots for communication over the system. Additionally, Global System for Mobile communications (GSM), is another type of multiple access wireless communication system that employs narrow band TDMA to allow at least eight simultaneous telephone calls on the same radio frequency. 
     Typically, a forward communication channel is used to transmit data signals from a base station to a mobile station; and a reverse communication channel is employed to transmit data signals from the mobile station to the base station. However, the transmission of a signal through a wireless medium can have arbitrary attenuation and delay due to hilly terrain, atmospheric conditions, outages and other factors. Also, the Doppler frequency shift in wireless signals transmitting from a fast moving vehicle can create fast fading channels where channel parameters can vary within each received block of data. To compensate for these less than ideal conditions, a base station&#39;s detector will employ various methods to estimate the attenuation and delay introduced in communication channel so that it can detect and identify each user&#39;s signal in the received signals. 
     For example, TDMA wireless communication systems often employ a training sequence that is included in each transmitted block of data to estimate channel. These methods usually employ an inverse of a matrix formed from the training sequence data. Also, since the training sequence can be known in advance, the inverse is typically precalculated and not separately performed for each received block of data. Similarly, CDMA wireless communication systems employ substantially the same methods based on a pilot signal that is included in each block of data. 
     Recently it has been shown that schemes exploiting several uncorrelated transmitting antennas are very promising for wireless communications. For example, delay diversity transmission (DDT), the simplest transmit diversity scheme in which delayed copies of the modulated signal are transmitted from multiple antennas, can significantly improve the performance of receiving devices (detectors). However, the transmit diversity gain is obtained by artificially introduced multipath requiring more channel parameters to be estimated given the same training sequence. Hence, the transmit diversity gain may not be fully utilized due to degradation of channel estimation accuracy. For example, accuracy of channel estimates based only on GSM training sequences (originally designed for channels without transmit diversity) may be not enough for transmit diversity schemes that in turn deteriorates the receiver performance. 
     The receiver performance may be improved by utilizing semi-blind detection methods, which combine information obtained from the known training sequence and unknown data to improve channel estimates. In particular, data-aided iterative channel estimation (ICE) or joint channel estimation and symbol detection based on the Expectation-Maximization (EM)-algorithm along with other iterative methods may be used. However, application of these methods implies a complicated inverse of a matrix formed by all transmitted data in a block. Since this matrix includes random transmitted together with known training sequence, then computational demanding matrix inverse should be performed for each transmitted block. Thus, there is a need for a low complexity matrix inverse that may be embedded into the EM (or some other iterative process) based receiver. 
     SUMMARY OF THE INVENTION 
     The present invention is directed at addressing the above-mentioned shortcomings, disadvantages and problems, and will be understood by reading and studying the following specification. 
     According to one aspect of the invention, a method is provided to improve the performance of a wireless signal detector that employs a modified Expectation Maximization (EM) method for joint channel estimation and data detection. Channel re-estimation is simplified by transferring the estimation calculations from the time domain into a principal subspace (channel state) domain. In doing so, high-dimensional online matrix inversions can be replaced with the inventive pre-calculated low-dimensional matrix inversions. Semi-blind detection methods are employed to combine information obtained from known training sequence and unknown data to improve channel estimates. By embedding the less complex algorithm for channel inverse into EM iterations, the EM convergence is improved for blocks of data with short training sequences. 
     Although some received signals may be affected by attenuation, delay and other factors, a detector employing the inventive modified EM method can provide relatively accurate channel estimates. Also, the modified EM method&#39;s pre-calculated low-dimensional matrix inversion reduces a detector&#39;s demands on processing resources. Additionally, the modified EM method enables a detector to perform relatively accurate channel estimates on blocks of data that include relatively short training sequences. 
     In accordance with yet another aspect of the invention, an apparatus such as a base station may be employed to implement substantially all of the methods described above. The base station can be coupled to several different types of networks in different ways and enable wireless communication with mobile stations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a graph of the performance of a detector by plotting block error rate (BER) versus decibels (db) for EM and ICE methods; 
         FIG. 2  shows a graph plotting the accuracy of channel estimates calculated with an EM method without channel model inverse, with direct channel inverse and the proposed simplified channel model inverse calculations; 
         FIG. 3  illustrates performance of conventional and the proposed detectors for diversity delay transmission (DDT) in Enhanced Data rates for Global Evolution (EDGE) for a Typical Urban 50 km/h (TU50) channel; 
         FIG. 4   a  shows a generic k-stage linear recursive shift register for generating sequences; 
         FIG. 4   b  illustrates an exemplary linear recursive shift register for generating binary sequences with maximum length of 2 4 −1; 
         FIG. 5  shows a block diagram of a detector employing the modified EM method; 
         FIG. 6  illustrates an exemplary mobile IP network in which the invention may operate; 
         FIG. 7 , shows a block diagram of a base station that includes a detector with the modified EM method; and 
         FIG. 8  shows a flow chart illustrating a process for joint channel estimation and detection with a modified EM method that employs a channel model inverse in the channel state domain, in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanied drawings, which form a part hereof, and which is shown by way of illustration, specific exemplary embodiments of which the invention may be practiced. Each embodiment is described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a”, “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or is inconsistent with the disclosure herein. 
     The term “node” refers to a network element that monitors a load for a link within a path. The term “link load” refers to the load associated with the node. The term “flow” means a flow of IP packets. The term support node refers to both Gateway General Packet Radio Service Support Node (GGSN) and Serving General Packet Radio Service Support Node (SGSN) nodes. 
     The term “user” refers to any person or customer such as a business or organization that employs a mobile device to communicate or access resources over a mobile network. The term “operator” refers to any technician or organization that maintains or services an IP packet based network. The term “identifier” includes a Mobile Station International Subscriber Directory Number (MSISDN) number, an IP address, or any other information that relates to the location or identity of the user. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. 
     The term “router” refers to a dedicated network element that receives IP packets and forwards them to their destination. In particular, a router is used to extend or segment networks by forwarding IP packets from one logical network to another. A router typically operates at layer  3  and below of the Open Systems Interconnection (OSI) reference model for networking. However, some routers can provide additional functionality that operates above layer  3  of the OSI reference model. 
     The term “base station” refers to a relatively stationary wireless network device that can forward/route communications between distant mobile stations that are part of the same or another wireless network. Also, the base station can enable mobile stations to communicate with resources accessible on a wired network that is coupled to the base station. The base station may communicate wirelessly with other base stations and may enable the forwarding of mobile station communications over a wired network. The base station can perform handover management and various measurements such as triangulating the location of a mobile station. 
     Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or is inconsistent with the disclosure herein. 
     The invention improves the performance of a wireless signal detector with a modified Expectation Maximization (EM) algorithm (method) for joint channel estimation and data detection. Channel re-estimation is simplified by transferring the estimation calculations from the time domain into a principal subspace (channel state) domain. In doing so, high-dimensional online matrix inversions can be replaced with the inventive pre-calculated low-dimensional channel model matrix inversions. Semi-blind detection methods are employed to combine information obtained from known training sequence and unknown data to improve channel estimates. By embedding a less complex algorithm for channel model inverse calculations into EM iterations, the EM convergence is improved for blocks of data with short training sequences. In particular, the modified EM algorithm enables a detector to perform relatively accurate channel estimates on blocks of data that include relatively short training sequences. 
     Although some received wireless signals may be adversely affected by attenuation, delay and other factors, a detector employing the inventive modified EM method can provide relatively accurate channel estimates. Also, the modified EM method&#39;s pre-calculated low-dimensional matrix inversion reduces a detector&#39;s demands on processing resources. 
     At least in part, many of the implementation details to practice the modified EM method and ICE algorithms are described in Appendix A. In particular, steps six, seven and eight in Appendix A, pages A-8 through A-9, are directed to the inventive channel model inverse employed in the modified EM method. Also, Appendix B discusses the operation of k-stage linear recursive shift registers for generating sequences in the modified EM method. Additionally, Appendix C lists several publications that are referenced in Appendixes A and B and which provide background information regarding the modified EM method. 
       FIG. 1  illustrates a graph of bit error rate (BER) plotted against decibels (dB) for an EM based detector in a static typical urban (TU) channel with binary signaling. As shown in the figure, the use of adaptive EM and ICE detectors provides more than one decibels of gain. See Appendix A, page A-9, for a more detailed discussion of this graph. 
       FIG. 2  illustrates a graph showing the accuracy of channel estimates with and without the inventive channel model inverse for EM-based detector. The results are derived from a simulation of 10,000 blocks of length 100 symbols for three taps with CIR equal to 0.77, 0.55 and 0.33. The dashed line with stars shows the standard deviation estimates obtained via an EM algorithm without channel inverse. The other lines show the accuracy of channel state estimates where channel states are formed from estimated channel taps. In particular, accuracy of channel taps that are calculated for every transmitted block based on the direct channel model inverse are represented by the solid line. Also, channel taps that are calculated for transmitted blocks with the modified EM method (using the improved channel model inverse) are represented in the graph by the line with circles. From the figure, it is clear that the improved channel model inverse increases the estimation accuracy as compared to the EM algorithm that does not perform channel inverse calculations. See Appendix A, page A-9, for a more detailed discussion of this graph. 
       FIG. 3  illustrates a graph of Block Error Rate (BLER) versus decibels (dB) for diversity delay transmission (DDT) in EDGE for a TU50 channel with ideal frequency for a Modulation and Coding Scheme 6 (MCS6) coding scheme that employs rate ½ convolutional code. In the Figure, the use of ICE methods provides at least one decibel of improvement with or without DDT. See Appendix A, pages A-9 through A-10, for a more detailed discussion of this graph. 
       FIG. 4   a  illustrates a generic k-stage linear recursive shift register to generate periodic sequences. See Appendix B, page B-1, for a more detailed discussion of the operation of this register. Also, the k-stage linear recursive shift register operates in substantially the same manner as step six that is described in greater detail in Appendix A, page A-8. Additionally,  FIG. 4   b  illustrates a corresponding shift register whose operation is described in greater detail in Appendix B, page B-1. 
       FIG. 5  illustrates a functional block diagram of at least some of the modules employed in the operation of a detector that employs the inventive modified EM method. In particular, the identified blocks for semi-blind regularization, state space reduction, channel inverse and states calculation are employed to improve the performance of a detector that includes the modified EM method. 
     Operating Environment 
     With reference to  FIG. 6 , an exemplary mobile IP network in which the invention may operate is illustrated. As shown in the figure, mobile IP network  100  includes mobile station  105 , radio access network (RAN)  110 , SGSN  115 , core network  120 , routers  125   A-F , GGSNs  135   A-B , data network  140 , and data network  145 . To improve the detection of wireless signals in mobile IP network  100 , the inventive modified EM method (with the less complex channel model inverse) may be included in detectors for wireless devices such as mobile stations and base stations. 
     The connections and operation for mobile IP network  100  will now be described. Mobile station  105  is coupled to radio access network (RAN)  110 . Generally, mobile station  105  may include any device capable of connecting to a wireless network such as radio access network  110 . Such devices include cellular telephones, smart phones, pagers, radio frequency (RF) devices, infrared (IR) devices, integrated devices combining one or more of the preceding devices, and the like. Mobile station  105  may also include other devices that have a wireless interface such as Personal Digital Assistants (PDAs), handheld computers, personal computers, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, wearable computers, and the like. 
     Radio Access Network (RAN)  110  manages the radio resources and provides the user with a mechanism to access core network  120 . Radio access network  110  transports information to and from devices capable of wireless communication, such as mobile station  105 . Radio access network  110  may include both wireless and wired telecommunication components. For example, radio access network  110  may include a cellular tower and/or base stations that are linked to a wired telecommunication network. Typically, the cellular tower carries wireless communication to and from cell phones, pagers, and other wireless devices, and the wired telecommunication network carries communication to regular phones, long-distance communication links, and the like. 
     As shown in the figure, RAN  110  includes routers  125   A-C . In one embodiment, one or more of routers  125   A-C  may be configured as an internal router for a base station that is coupled to a wired telecommunication network and in communication with wireless devices such as mobile station  105 . 
     Some nodes may be General Packet Radio Service (GPRS) nodes. For example, Serving GPRS Support Node (SGSN)  115  may send and receive data from mobile stations, such as mobile station  105 , over RAN  110 . SGSN  115  also maintains location information relating to MS  105 . SGSN  115  communicates between mobile station  105  and Gateway GPRS Support Node (GGSN)s  135   A-B  through core network  120 . 
     Core network  120  is an IP packet based backbone network that includes routers, such as routers  125   D-F , to connect the support nodes in the network. Routers are intermediary devices on a communications network that expedite message delivery. On a single network linking many computers through a mesh of possible connections, a router receives transmitted messages and forwards them to their correct destinations over available routes. Routers may be a simple computing device or a complex computing device. For example, a router may be a computer including memory, processors, and network interface units. 
     GGSNs  135   A-B  are coupled to core network  120  through routers  125   A-C  and act as wireless gateways to data networks, such as network  140  and network  145 . Networks  140  and  145  may be the public Internet or a private data network. GGSNs  135   A-B  allow mobile station  105  to access network  140  and network  145 . 
     Furthermore, computers, and other related electronic devices may be connected to network  140  and network  145 . The public Internet itself may be formed from a vast number of such interconnected networks, computers, and routers. Mobile IP network  100  may include many more components than those shown in  FIG. 6 . However, the components shown are sufficient to disclose an illustrative embodiment for practicing the present invention. 
     The media used to transmit information in the communication links as described above illustrate one type of computer-readable media, namely communication media. Generally, computer-readable media includes any media that can be accessed by a computing device. Communication media typically embodies computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. By way of example, communication media includes wired media such as twisted pair, coaxial cable, fiber optics, wave guides, and other wired media and wireless media such as acoustic, RF, infrared, and other wireless media. 
       FIG. 7  illustrates a schematic diagram of exemplary base station  700 , which includes processor  710 , transceiver  702 , antennae module  716  and network interface  708 . Network interface  708  may be adapted to enable base station  700  to communicate with components on a network over a wired communication link and/or a wireless communication link. Transceiver  702  includes transmitter  704  and detector  706  for sending and receiving wireless signals, respectively, over antennae module  716 . Also, detector  706  includes modified EM component  710  for joint channel estimation and detection with signals received from wireless devices such as mobile station  714  and base station  712 . Although not shown, detector  706  with modified EM component  710  may be included in a mobile station for joint channel estimation and detection of wireless communication signals received from base stations and other mobile devices. 
     Joint Channel Estimation and Detection Process 
       FIG. 8  illustrates a general overview of the process for implementing actions for joint channel estimation and detection for each transmitted block in at least one wireless signal. Moving from a start block, the process transitions to block  802  where each block included in at least one wireless signal is received. Each block includes a training sequence and data. The process advances to block  804  where the training sequence in a received block is employed to generate an initial estimate for each channel tap. Next, the process moves to block  806  where each initial estimate of each channel tap is used to determine at least one channel state estimate. At block  808 , each determined channel state estimate is employed to initialize an Expectation Maximization (EM) process. 
     Stepping to block  810 , the EM process is employed to determine an update for each channel state estimate. At block  812 , a combining of channel estimates obtained from the EM procedure and from known training sequence is performed (e.g., see Equ(14) in Appendix A, page A-6) and a reduced observation vector formed from updated state estimates is generated. The reduced observation vector can be used to generate a correlation matrix. 
     At block  814 , the EM process is used to generate a result based on a channel model inverse in a channel state domain. To obtain the result, the proposed procedure includes the following actions: (a) a special sequence of channel states is generated in advance based on a channel memory and a symbol alphabet; (b) the ordering of each channel state estimate is rearranged in the reduced observation vector in relation to the sequence for branch realizations above; and (c) based on the data sequence from a) the channel model inverse is generated in the channel state domain to obtain channel taps estimates. These channel tap estimates are sent to the block  806  to generate a new update for each channel state estimate for another EM iteration. Also, each updated channel state estimate is employed to generate an update for each channel tap estimate. 
     The actions in blocks  806  though  814  except action (a) in  814  are recursively performed until the result of the EM process is less than a predetermined threshold. When the result of the EM process is less than the predetermined threshold, the result is used by the detector to detect each symbol in a received block for a channel. Next, the process moves to a return block to perform other actions. 
     Although not shown, it is understood that other iterative processes besides the EM process discussed above that could be used with the inventive channel model inverse in substantially the same manner. Additionally, the inventive channel model inverse can be used for iterative channel estimation or joint channel estimation and detection. 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.