Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/339,969 filed Dec. 19, 2008, which is a continuation of U.S. patent application Ser. No. 11/103,202 filed Apr. 11, 2005, which issued as U.S. Pat. No. 7,483,473 on Jan. 27, 2009, which is a continuation of U.S. patent application Ser. No. 09/766,875 filed Jan. 19, 2001, which issued as U.S. Pat. No. 6,904,079 on Jun. 7, 2005, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/181,071 filed Feb. 8, 2000, which are all incorporated by reference as if fully set forth. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of wireless digital communications and more particularly to a technique for encoding access channel signals. 
     The increasing use of wireless telephones and personal computers has led to a corresponding demand for advanced wireless communication services which were once thought only to be meant for use in specialized applications. In particular, wireless voice communication first became widely available at low cost through the cellular telephone network. The same has also become true for distributed computer networks, whereby low cost, high speed access to data networks is now available to the public through Internet Service Providers (ISPs). As a result of the widespread availability of both technologies, the general population now increasingly wishes to be able to access the Internet using portable computers and Personal Digital Assistants (PDAs) over wireless links. 
     The most recent generation of wireless communication technologies makes use of digital modulation techniques in order to allow multiple users to share access to the available frequency spectrum. These techniques purportedly increase system capacity for a radio channel of a given available radio bandwidth. The technique which has emerged as most popular within the United States is a type of Code Division Multiple Access (CDMA). With CDMA, each transmitted radio signal is first encoded with a pseudorandom (PN) code sequence at the transmitter. Each receiver includes equipment that performs a PN decoding function. The properties of the PN codes are such that signals encoded with different code sequences or even with different code phases can be separated from one another at the receiver. The CDMA codes thus permit signals to be transmitted on the same frequency and at the same time. Because PN codes in and of themselves do not provide perfect separation of the channels, certain systems have added an additional layer of coding, and/or use modified PN codes. These additional codes, referred to as orthogonal codes, and/or modified PN codes encode the user signals so that they are mathematically exclusive in order to further reduce interference between channels. 
     In order for the CDMA code properties to hold true at the receiver, certain other design considerations must be taken into account. One such consideration involves the signals traveling in a reverse link direction, that is, from a field unit back to the central base station. In particular, the orthogonal properties of the codes are mathematically optimized for a situation where individual signals arrive at the receiver with approximately the same power level. If they do not, interference between the individual signals which arrive at the base station increases. Precise control over the level of each signal transmitted on the reverse link is thus critical. 
     More particularly, most CDMA systems are structured such that the forward link channels, that is, the channels carrying information from the base station towards the field unit, are different from the reverse channels. The forward link typically consists of three types of logical channels known as the pilot, paging, and traffic channels. The pilot channel provides the field unit with timing and phase reference information. Specifically, the pilot channel contains a sequence of data bits that permits the field unit to synchronize its PN decoding function with the PN coding used in the base station. The pilot channel is, therefore, typically transmitted continuously by the base station to facilitate the field units demodulation of the other forward link channels. 
     The paging channel is used to inform the field unit of additional information needed to communicate. Such information is typically management information which informs the field unit of which traffic channels it may use, for example. Other types of paging messages are used to communicate system parameters, access parameters, neighbor lists and other information needed for the field unit to manage its communication in such a way that it does not interfere with other field units transmissions. 
     The forward traffic channels are used to transmit user data and/or voice signaling information from the base station to the field unit. 
     On the reverse link, there are typically at least two types of logical channels, including an access channel and traffic channels. The access channel is used by the field unit to send a message to request access to traffic channels when it has data to communicate to the base station. The field unit thus uses the access channel to make requests for connection originations and to respond to paging messages. The traffic channels on the reverse link serve the same purpose as the traffic channels on the forward link, namely, to transmit user data and/or digitized voice payload information. 
     Pilot channels are not typically used on the reverse link. There are perhaps several reasons for this. For example, the most widely deployed CDMA systems, such as the IS-95 compatible system as specified by the Telecommunications Industry Association (TIA), use asynchronous reverse link traffic channels. It is typically thought that the overhead associated with allowing each field unit to transmit on its own dedicated pilot channel is not necessary. It is also thought that the overhead associated with decoding and detecting a large number of pilot channels back at the base station would not justify any perceived increase in performance. 
     SUMMARY OF THE INVENTION 
     In general, pilot signals are advantageous since they provide for synchronous communication. If the communications on the reverse link traffic channels can be synchronized among various field units, parameters can be better optimized for each link individually. It would therefore be advantageous to make pilot signals available for use on the reverse link. 
     Furthermore, the use of pilot channels on the reverse link would assist in combating effects due to multipath fading. Especially in urban environments where many tall buildings and other surfaces may reflect radio signals, it is common for not just one version of each transmitted signal to arrive at a receiver. Rather, different versions of a particular transmitted signal, each associated with a particular delay, may be actually received. Having additional synchronization timing information available at the base station can help properly decode reverse link signals which have experienced a multipath fade. 
     The present invention is a technique for efficient implementation of pilot signals on a reverse link in a wireless communication system encompassing a base station which services a large number of field units. According to one aspect of the invention, an access channel is defined for the reverse link such that within each frame or epoch, a preamble portion of the frame is dedicated to sending only pilot symbols. Another portion of each access channel frame, called the payload portion, is then reserved for sending data symbols. In this payload portion of the frame, additional pilot symbols are interleaved among the data symbols. 
     In the preferred embodiment, the pilot symbols are inserted at predictable, regular intervals among the data symbols. 
     The preamble portion of the access channel frame allows for efficient acquisition of the access signal at the base station, and provides a timing reference for separating the data and pilot symbols in the payload portion, as well as a timing reference for, optionally, dealing with the effects of multipath fading. This is accomplished by feeding the preamble portion to a pilot correlation filter. The pilot correlation filter provides a phase estimate from the pilot symbols in the preamble portion, which is then used to decode the data symbols in the payload portion. 
     An access acquisition portion of the receiver then uses these phase estimates provided by the pilot correlation filter to process the output of a data symbol correlation filter. 
     The additional pilot symbols embedded in the payload portion are preferably used in a cross product modulator to further undo the effects of multipath fading. 
     The preamble portion of the frame may be defined by Barker sequences, which further assist with properly aligning the timing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a block diagram of the system which uses embedded pilot symbol assisted coherent demodulation according to the invention. 
         FIG. 2  is a detailed view of the format of data framing used on the access channel. 
         FIG. 3  is a high level diagram of the pilot symbol assisted demodulation process. 
         FIG. 4  is a more detailed view of the pilot symbol assisted coherent demodulators. 
         FIG. 5  is a still more detailed view of an access acquisition portion of the coherent demodulator. 
         FIG. 6  is a more detailed view of a data detection portion of the coherent demodulator. 
     
    
    
     DETAILED DESCRIPTION 
     Turning attention to the drawings,  FIG. 1  is a generalized diagram showing a wireless data communication system  10  that makes use of an access channel having embedded pilot symbols in order to effectuate coherent demodulation. The system  10  consists of a base station  12  and a field unit  20 . The base station  12  is typically associated with a predetermined geographic region  14  in which wireless communication service is to be provided. 
     The base station  12  contains several components, including a radio transmitter  15 , receiver  16 , and interface  17 . The interface  17  provides a data gateway between the base station  12  and a data network  18  such as the Internet, a private network, a telephone network, or other data network. 
     The field unit  20  consists of a corresponding receiver  21 , transmitter  22 , and interface  23 . The interface  23  permits the field unit  20  to provide data signals to and receive data signals from computing equipment  24  such as a laptop computer, Personal Digital Assistant (PDA), or other computing equipment. The interface  23  may be a PCMCIA bus, USB port, or other standard computer interface. 
     The base station  12  communicates with the field unit  20  by exchanging radio signals over various radio channels. The present invention is of particular advantage in a system  10  which uses Code Division Multiple Access (CDMA) modulation to define the channels. In the specific embodiment discussed herein, it is therefore understood that a specific pseudorandom (PN) code (which may or may not be augmented with orthogonal codes) is used to define each of the various logical channels on a given radio carrier frequency. 
     The forward link  30  consists of various types of logical channels, including at least a pilot channel  31 , a paging channel  32 , and one or more traffic channels  33 . The forward link  30  is responsible for forwarding data signals from the base station  12  towards the field unit  20 . 
     The pilot channel  31  contains typically no baseband information, but rather a stream of bits that are used to permit the field unit  20  to synchronize to the signals sent in the other forward link logical channels such as the paging channel  32  and traffic channels  33 . 
     The paging channel  32  is used to transmit messages from the base station  12  to the field unit  20  that control various aspects of communication, but most importantly, control assignment of various traffic channels  33  for use by each field unit  20 . 
     The forward traffic channels  33  are used to transmit data voice or other signaling messages from the base station  12  towards the field unit  20 . 
     Signals are also carried from the field unit  20  towards the base station  12  over a reverse link  40 . The reverse link  40  contains several logical channel types including at least an access channel  41 , a synchronization (sync) channel  42 , and one or more traffic channels  43 . 
     For the reverse link  40 , the access channel  41  is used by the field unit  20  to communication with the base station  12  during periods of time when the field unit  20  does not have a traffic channel  43  already assigned. For example, the field unit  20  typically uses the access channel  41  to originate request for calls as well as to respond to messages sent to it on the paging channel  32 . 
     The sync channel  42  on the reverse link may assist in or with the traffic channels  43  to permit the field unit  20  to efficiently send data to the base station  12  using synchronous modulation techniques. 
     The present invention relates to the formatting and use of the reverse link access channel  41 . Specifically, the invention uses an access channel  41  that contains within it certain formatting such as certain symbols used to convey pilot signal information. 
     The access channel  41  signal format is shown in more detail in  FIG. 2 . An epoch or frame  50  consists of a preamble portion  51  and payload portion  52 . The preamble  51  is further defined as a series of symbols including a pilot block  53  and Barker code block  54 . Multiple pilot blocks  53  and Barker code blocks  54  make up the preamble  51 ; in the illustrated preferred embodiment, a pilot block  53  and Barker code block  54  are repeated four times in each frame  50 . The Barker code blocks  54  assist in allowing the receiver to determine where the start of a frame  50  is. 
     Each pilot block  53  consists of a number of repeated pilot symbols. In the preferred embodiment, 48 pilot symbols are repeated in each pilot block  53 . The pilot blocks  53  are used to assist with timing reception and decoding of the information symbols which make up the access channel  41 . 
     The second portion of each frame  50  is the payload portion  52 . The payload portion  52  includes a data portion consisting of the information to be sent from the field unit  20  to the base station  12 . As shown in  FIG. 2 , pilot symbols  53  are inserted in the data portion of the payload portion  52 . A pilot symbol, for example, may be inserted every eight payload symbols. As will be discussed in greater detail later, these pilot symbols embedded in the payload portion  52  further assist with the coherent demodulation process of the information contained in the data portion. 
     The pilot symbols  53  typically consist of a series of positive data bits only. Therefore, they do not in and of themselves contain timing information. 
     The Barker code blocks  54  may consist of predetermined patterns of bits, as shown in  FIG. 2 . Binary Phase Shift Keyed (BPSK) bit encoding may be used to indicate a Barker sequence consisting of three positive bits followed by three negative bits, followed by a single positive bit, a pair of negative bits, a positive bit, and then a negative bit. The positive logic Barker sequence +B may be alternately sent with the negative of the Barker sequence −B to further assist in aligning the beginning of each frame  50  at the receiver  16 . 
     The use of multiple pilot blocks  53  and Barker code blocks  54  permit an averaging process to be performed in the acquisition of each access channel  41  is described further below. 
       FIG. 3  is a generalized block diagram of the portion of the receiver  16  used by the base station  12  to demodulate the reverse link access channel  41 . As shown, the access channel receiver consists of two functions including access acquisition function  60  and data decoding  62 . In a preferred embodiment, multiple data decoding blocks  62 - 1 ,  62 - 2 , . . .  62 - n  may be used as individual rake receiver portions, or receiver “fingers,” tuned to different timing delays. 
     In general, the preamble pilot symbols are first processed by the access acquisition function  60 . These provide generalized timing information which is then fed to the data decoding function  62 , along with the payload portion containing the data symbols and embedded pilot symbols. Each of the individual fingers  62 - 1 ,  62 - 2 , . . . ,  62 - n  make use of the timing information provided by the access acquisition function  60  to properly decode the data in the access channel. 
     This receiver signal processing can now be understood more readily by reference to  FIG. 4 , which is a more detailed diagram of both the access acquisition function  60  and data decoding function  62 . In particular, the access acquisition function  60  is seen to include a Pilot Correlation Filter (PCF)  70  as well as an integration function  72 . As will be discussed in more detail below, the PCF  70  is a matched digital filter having coefficients matched to provide an impulse response to input preamble pilot signals. 
     The integration function  72  operates on successive outputs of the pilot correlation filter  70  to provide a smoothed estimate of timing information inherent in the pilot symbols. 
     The data decoding portions  62  each include a data matched filter  80 , a selection function  82 , a dot or “cross” product function  84 , integration functions  86 , and delay  88 . A summer  90  operates on the outputs of the individual data decoders  62 - 1 ,  62 - 2 , . . . ,  62 - n  to provide an estimate of the payload data. Briefly, each of the data decoders  62  operates as a synchronous demodulator to provide an estimate of the data symbols for a given respective possible multipath delay. Although three data decoders  62  are shown in  FIG. 4 , it should be understood that a smaller number of them may be used depending upon the anticipated number of multipath delays in the system  10 . 
       FIG. 5  is a more detailed block diagram of the access acquisition function  60 . This circuit includes the previously mentioned pilot correlation filter  70  in the form of a pair of pilot correlation matched filters (PCMFs)  700 - 1 ,  700 - 2 , and a corresponding pair of vector infinite impulse response (IIR) filters  710 - 1  and  710 - 2 . In addition, the integration function  72  is provided by the pair of magnitude squaring circuits  720 - 1  and  720 - 2 , a summer  722 , and threshold detector  724 . 
     In operation, the access channel  41  signal is fed to the pilot correlation matched filter (PCMF) sections  700 - 1  and  700 - 2 . The pair of PCMFs  700  are used in a ping pong arrangement so that one of the PCMFs may be operating on received data while the other PCMF is having its coefficients loaded. In the preferred embodiment, the access channel is encoded using 32 PN code chips per transmitted symbol. At the receiver, 8 samples are taken per chip (e.g., 8 times the chip rate of 1.2288 megahertz (MHz)). The pilot correlation matched filter  700  must not only be matched to receive the pilot symbols, but also to the particular pseudorandom noise (PN) code used for encoding the access channel. A controller is used to control the operation of the two portions of the access acquisition function  60 , both the top half and bottom half, as illustrated. 
     Continuing with the discussion of the Pilot Correlation Filter  70 , the vector IIR filter  710 - 1  receives the output of the PCMF  700 - 1  in the form of in-phase (I) and quadrature (Q) samples. As shown in the signal diagram  750  next to the output of the PCMFs  700 , the output tends to be a series of peaks spaced apart in time, with the peak spacing, depending upon the multipath delays experienced on the reverse link. For example, a peak occurring at a first time T1 may be associated with the most direct signal path taken. A second peak may occur at a time T2 associated with a portion of the signal which follows an alternate path. Finally, a third peak may be associated with a time T3 which follows yet a different path from the field unit  20  to the base station  12 . The series of peaks are output for each of the 48 symbols in the pilot burst. The function of the vector IIR filter  710 - 1  is thus to average these pilot bursts to provide a more well defined set of peaks  760  which represents the outputs of the PCMF  700 - 1  averaged over time. The averaging process implemented by the vector IIR filter  710 - 1  may, for example, eliminate a false peak, such as that occurring at time T4, which is attributable to a noise burst and not to an actual multipath signal portion. 
     The output  760  of the vector IIR filter  710  thus represents an estimate of where the true multipath peaks occur in the reverse link access channel  41 . 
     Of ultimate interest is the signal level of the received pilot signal. To determine this level, the magnitude block  720 - 1  takes the magnitude of the vector IIR output signal  760 . The sum circuit  722  thus sums these signals as provided by each of the two ping pong branches  700 . A threshold detector  724  is then applied to the summed signal to provide an output similar to the plot  770 . The threshold detector is set at a predetermined amplitude TH so that an output appears as in plot  780 . 
     The points at which the summed signal output crosses the threshold TH indicate points at which rake fingers  62  will be assigned to processes the signal. In particular, the peaks occurring at times T1, T2 and T3 are examined, and each respective time is used and assigned to a respective data matched filter  80  and the corresponding finger  62 . These provide an estimate of possible phases from the pilot symbols which is in turn used in the data decoding process. 
       FIG. 6  illustrates how the data detection process of the three rake fingers  62 . Each finger  62  is identical. An exemplary rake finger  62 - 1  consists of a corresponding Data Correlation Matched Filter (DCMF)  80 - 1 , a peak sample detector  81 - 1 , a switch  82 - 1 , a vector IIR filter  83 - 1 , complex conjugate function  85 - 1 , and dot product circuit  84 - 1 . 
     In operation, the access channel signal is first fed to the Data Correlation Matched Filter (DCMF)  80 - 1 . This filter  80 - 1  is loaded with coefficients at a specific phase delay of the PN sequence. In this instance, the phase delay loaded is that data associated with the time T1 indicated from the output of the access acquisition function  60 . 
     The output of data correlation matched filter  80 - 1  will consist of a signal having a localized peak. As shown in the diagram next to the peak sample detector  81 - 1 , the peak sample detector  81 - 1  selects a predetermined number of samples around this peak for further processing. 
     These peak values are then fed to the switch  82 - 1 . The switch  82 - 1 , under the operation of the data decoder controller  790 , alternately steers the peak detected signal, depending upon whether it contains pilot symbols or pilot plus data symbols. The decoder controller  790  may be synchronized with a start of frame indication as determined by the received Barter symbols in the preamble portion, and therefore knows the position of pilot symbols in the payload portion. Thus, while receiving the payload or data portion  52  of the access channel frame  50 , the signal will be steered to the lower leg  88 - 1 , in the case of receiving a pilot symbol, or in the case of receiving a data symbol, will be steered to the upper leg  89 - 1 . 
     The pilot symbols of the payload portion  52  are processed in a manner similar to the pilot symbol processing in the preamble portion  51 . That is, they are processed by a vector IIR filter  83 - 1  to provide an average estimate of an estimate signal value [p]e j . The complex conjugate of this pilot estimate is then determined by the complex conjugate function  85 - 1 . 
     Data symbols steered to the upper leg  89 - 1  provide a data estimate signal x n e je . 
     The two estimate signals, data and pilot are then fed to the multiplier  84 - 1  to provide a cross product of the pilot symbols with the data symbols. This causes the phase terms of the complex signal to cancel more or less. That is, the phase estimate (theta) should be approximately equal to the measured phase theta of the pilot symbols. The output thus represents the pilot channel energy |p|x n . Given a pilot symbol normalized value of 1, the data is therefore recovered. 
     Returning to  FIG. 4 , the reader will recall that this is the output of only one rake finger  62 - 1 . Each rake finger output is, therefore, then fed through the integrators  86 ,  87 , additional dot product circuits  89 , and delays  88 - 1 , to the summer  90  to provide a final estimate of the data, X. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Technology Category: h