Abstract:
A method and apparatus for encoding digital communication signals is described herein. A wireless communication unit may comprise a processor configured to produce first symbols derived from data bits and parity bits. The wireless communication unit may transmit a wireless signal including the first symbols and reference symbols in a time interval including a plurality of symbol time intervals. In one of the symbol time intervals a plurality of the reference symbols may be transmitted in a predetermined pattern, which may have the reference symbols not adjacent to each other. A base station may receive a wireless signal including first and reference symbols. In one of the symbol time intervals a plurality of the reference symbols may be received in a predetermined pattern. The base station may then demodulate the first symbols using the reference symbols, wherein the first symbols are derived from data bits and parity bits.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/480,116, filed on Sep. 8, 2014, which is a continuation of U.S. patent application Ser. No. 13/311,151, filed Dec. 5, 2011, which issued as U.S. Pat. No. 8,830,977 on Sep. 9, 2014, which is a continuation of U.S. patent application No. Ser. 12/290,755, filed Nov. 3, 2008, which issued as U.S. Pat. No. 8,072,958 on Dec. 6, 2011, which is a continuation of U.S. patent application Ser. No. 10/874,101, filed Jun. 22, 2004, which issued as U.S. Pat. No. 7,447,187 on Nov. 4, 2008, which is a continuation of U.S. patent application Ser. No. 09/728,575, filed Nov. 30, 2000, which issued as U.S. Pat. No. 6,804,223 on Oct. 12, 2004, the contents of which are all hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to communications systems and in particular to a scheme for digital encoding of signals in a wireless system. 
         [0003]    Demand for wireless communications equipment and services continue to grow at an unprecedented rate throughout the world. Increasingly, such systems are commonly relied upon to provide voice and data communications to a growing sector of the public. While these systems originally depended upon analog signaling technology, there is essentially unanimous agreement that future systems will be based on various types of digital signal coding schemes. 
         [0004]    The typical wireless communication system is a point to multi-point type system in which a central base station communicates with a number of remote units located within a local geographic area of coverage known as a cell. This system provides for duplex communication such that signals may be sent in both a forward direction (from the base station to the remote unit) as well as in a reverse direction (from the mobile remote unit back to the base station). In order to support communication between the remote unit and networks such as the Public Switched Telephone Network (PSTN), or data networks such as the Internet, the wireless system must also provide for various logical components and functional entities. 
         [0005]    Consider the Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) digital systems presently in widespread use. Each of these systems provides for certain logical types of the radio channels that make up the forward link and reverse link. In particular, the forward link channels often include a pilot channel, paging channels, and multiple forward traffic channels. The traffic channels are used to carry the payload data between the base station and the mobile unit. A pilot channel is also typically required to allow the remote unit to maintain synchronization with the base station. The paging channels provide a mechanism for the base station to inform the remote unit of control information, such as the assignment of forward traffic channels to particular connections and/or subscriber units. 
         [0006]    Likewise, an access channel is provided in the reverse direction in addition to reverse traffic channels. The access channels allow the remote units to communicate control information with the base station, such as to send messages indicating the need to allocate or deallocate connections as required. 
         [0007]    Various environmental conditions will affect the performance of any wireless communications system. These elements include atmospheric signal path loss, which may often introduce fading and interference. Fading may include variations that are introduced as a result of the specific terrain within the cell, as well as other types of fading, such as multi-path fading, that occurs due to signal reflections from specific features, such as buildings that cause fluctuations in receive signal strength. Systems in which the remote unit may be a mobile unit, especially those potentially operating at higher speeds, such as the cellular telephones used in automobiles, are particularly susceptible to multi-path fading. In such an environment, the signal pathways are continually changing at a rapid rate. 
         [0008]    Certain techniques can be used to attempt to eliminate the detrimental effects of signal fading. One common scheme is to employ special modulation and/or coding techniques to improve the performance in a fading environment. Coding schemes such as block or convolutional coding add additional parity bits at the transmitter. These coding schemes thus provide increased performance in noisy and/or fading environments at the expense of requiring greater bandwidth to send a given amount of information. 
         [0009]    In addition, pilot signals may also be used to provide a reference for use in signal demodulation. For example, most digital wireless communications systems provide for a dedicated pilot channel on the forward link. This permits the remote units to remain in time synchronization with the base station. Certain systems, such as the IS-95 CDMA system specification promulgated by the Telecommunications Industry Association (TIA) use the pilot signals that include pseudorandom binary sequences. The pilot signals from each base station in such a system typically use the identical pseudorandom binary sequence, with a unique time offset being assigned to each base station. The offsets provide the ability for the remote stations to identify a particular base station by determining this phase offset in the forward link pilot channel. This in turn permits the remote units to synchronize with their nearest neighboring base station. Coding the pilot channel in this way also helps support other features, such as soft handoff for cell-to-cell mobility. 
         [0010]    The pilot signal, having a predictable frequency and rate, allows the remote units to determine the radio channel transfer characteristics. By making such determinations, the receiver may in turn further compensate for the distortion introduced in the channel during the process of estimating symbols being received. 
         [0011]    However, it is generally considered to be impractical to use pilot signals in the reverse link. In particular, this would lead to a situation where pilot signal channels would have to be dedicated for each remote unit. While this would not necessarily pose a problem in a point to point system, in point to multi-point systems such as a cellular telephone network, the architecture would quickly lead to inefficiency in use of the available radio spectrum. In addition, it is generally thought that the overhead associated with a system that assigned individual pilot channels to each remote unit would unnecessarily complicate the base station receiver processing. 
         [0012]    An alternative to allocating individual pilot channels is to make use of a sequence of pilot symbols. The pilot symbols are interleaved with data symbols on the traffic channel. This technique is generally referred to as pilot symbol assisted modulation. In such a system, the transmitter encodes the data to be sent on the traffic channel as a series of symbols. A pilot symbol interleaver then inserts a sequence of predetermined pilot symbols within the data symbol sequence. The pilot symbol augmented sequence is then modulated and transmitted over the radio channel. At the receiving station, a decimeter or deinterleaver and filter separate the pilot symbols from the data symbols. 
       SUMMARY OF THE INVENTION 
       [0013]    What is needed is a way to integrate pilot symbol assisted modulation techniques with block encoding schemes in a way which maximizes the probability that data and pilot symbols will be correctly received. 
         [0014]    The invention accomplishes this with a pilot symbol insertion scheme that proceeds as follows. The source data bits are first augmented with periodically inserted pilot symbols. In a preferred embodiment, the pilot symbols are inserted at a position corresponding to a power of two, such as for example, every fourth, eighth, sixteenth, or thirty-second symbol. Next, this pilot symbol augmented data sequence is presented to a deterministic block coder. Such a block coder may, for example, be a sub-rate two dimensional turbo product coder. 
         [0015]    The symbols of the resulting encoded block are then rearranged such that the pilot symbols will be in a predictable location. Because the pilot symbols are always in a known place in the input block coding matrix, their positions are therefore also known in the output block coding matrix. The encoded output pilot symbols can therefore be rearranged such that they are evenly distributed through the output coded space, prior to modulation and transmission. 
         [0016]    An optional embodiment makes use of an interleaving scheme in which parity symbols are interleaved with data and pilot symbols. In such a scheme, all symbols from the coded space, with the exception of the pilot symbols, are placed in a temporary storage area by row. Data is then read out of the temporary storage area to provide the interleaved output, by reading data from the temporary array in column order. For example, a first pilot signal is selected, a row is read out, a second pilot signal is selected, a second row is read out, and so on. As a result, the pilot symbols are output at predetermined positions preferably located within symbol positions which are a power of two away from each other. 
         [0017]    In an alternate embodiment, the symbols may be composed of pairs of input data bits, to form complex -valved symbols, which can then be modulated using Quadrature Phase Shift Keyed (QPSK) schemes. In this embodiment, the data, parity, and pilot bits are processed in pairs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram of a communication system which encodes pilot symbols according to the invention; 
           [0019]      FIG. 2  is a more detailed diagram of a transmit encoder and receive decoder; 
           [0020]      FIGS. 3A and 3B  illustrate how a deterministic block encoder, such as a one-quarter rate turbo product encoder, distributes data and parity bits in an output matrix; 
           [0021]      FIGS. 3C and 3D  illustrate how the pilot inserter and block encoder operate according to the invention; 
           [0022]      FIG. 4A  illustrates how a first type of interleaver outputs pilot, data, and parity bits; 
           [0023]      FIGS. 4B and 4C  illustrate how a second type of interleaver may order the data, parity, and pilot bits; and 
           [0024]      FIG. 4D  illustrates how a third type of interleaver may order data, parity and pilot bits for use with a Quadrature Phase Shift Keyed (QPSK) type modulator. 
           [0025]    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. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    A description of preferred embodiments of the invention follows. 
         [0027]    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. 
         [0028]      FIG. 1  is a block diagram of a communication system  10  that interleaves pilot symbols with data symbols and uses a systematic block coder to ensure that the pilot symbols are located in predetermined locations. In the following description of a preferred embodiment, the communication system  10  is described such that the shared channel resource is a wireless or radio channel. However, it should be understood that the techniques described here may be applied to allow shared access to other types of media such as telephone connections, computer network connections, cable connections, and other physical media to which access is granted on a demand driven basis. 
         [0029]    The communication system  10  includes a number of Personal Computer (PC) devices  12 - 1 ,  12 - 2 , . . .  12 -h, . . .  12 -l, corresponding Subscriber Access Units (SAUs)  14 - 1 ,  14 - 2 , . . .  14 -h, . . .  14 -l, and associated antennas  16 - 1 ,  16 - 2 , . . .  16 -h, . . .  16 -l. Centrally located equipment includes a base station antenna  18 , and a Base Station Processor (BSP)  20 . The BSP  20  provides connections to and from an Internet gateway  22 , which in turn provides access to a data network such as the Internet  24 , and network file server  30  connected to the network  22 . The system  10  is a demand access, point to multi-point wireless communication system such that the PCs  12  may transmit data to and receive data from network server  30  through bi-directional wireless connections implemented over forward links  40  and reverse links  50 . It should be understood that in a point to multi-point multiple access wireless communication system  10  as shown, a given base station processor  20  typically supports communication with a number of different subscriber access units  14  in a manner which is similar to a cellular telephone communication network. 
         [0030]    The PCs  12  may typically be laptop computers  12 -l, handheld units  12 -h, Internet-enabled cellular telephones or Personal Digital Assistant (PDA)-type computers. The PCs  12  are each connected to a respective SAU  14  through a suitable wired connection such as an Ethernet-type connection. 
         [0031]    An SAU  14  permits its associated PC  12  to be connected to the network file server  30  through the BSP  20 , gateway  22  and network  24 . In the reverse link direction, that is, for data traffic traveling from the PC  12  towards the server  30 , the PC  12  provides an Internet Protocol (IP) level packet to the SAU  14 . The SAU  14  then encapsulates the wired framing (i.e., Ethernet framing) with appropriate wireless connection framing. The appropriately formatted wireless data packet then travels over one of the radio channels that comprise the reverse link  50  through the antennas  16  and  18 . At the central base station location, the BSP  20  then extracts the radio link framing, reformatting the packet in IP form and forwards it through the Internet gateway  22 . The packet is then routed through any number and/or any type of TCP/IP networks, such as the Internet  24 , to its ultimate destination, such as the network file server  30 . 
         [0032]    Data may also be transmitted from the network file server  30  to the PCs  12  in a forward direction. In this instance, an Internet Protocol (IP) packet originating at the file server  30  travels through the Internet  24  through the Internet gateway  22  arriving at the BSP  20 . Appropriate wireless protocol framing is then added to the IP packet. The packet then travels through the antenna  18  and  16  to the intended receiver SAU  14 . The receiving SAU  14  decodes the wireless packet formatting, and forwards the packet to the intended PC  12  which performs the IP layer processing. 
         [0033]    A given PC  12  and the file server  30  can therefore be viewed as the end points of a duplex connection at the IP level. Once a connection is established, a user at the PC  12  may therefore transmit data to and receive data from the file server  30 . 
         [0034]    The reverse link  50  actually consists of a number of different types of logical and/or physical radio channels including an access channel  51 , multiple traffic channels  52 - 1 , . . .  52 -t, and a maintenance channel  53 . The reverse link access channel  51  is used by the SAUs  40  to send messages to the BSP  20  to request that traffic channels be granted to them. The assigned traffic channels  52  then carry payload data from the SAU  14  to the BSP  20 . It should be understood that a given IP layer connection may actually have more than one traffic channel  52  assigned to it. In addition, a maintenance channel  53  may carry information such as synchronization and power control messages to further support transmission of information over the reverse link  50 . 
         [0035]    Similarly, the forward link  40  typically includes a paging channel  41 . The paging channel  41  is used by the BSP  20  to not only inform the SAU  14  that forward link traffic channels  52  have been allocated to it, but also to inform the SAU  14  of allocated traffic channels  52  in the reverse link direction. Traffic channels  42 - 1  . . .  42 -t on the forward link  40  are then used to carry payload information from the BSP  20  to the SAUs  14 . Additionally, maintenance channels carry synchronization and power control information on the forward link  40  from the base station processor  20  to the SAUs  14 . 
         [0036]    In the preferred embodiment, the logical channels  41 - 43  and  51 - 53  are defined by assigning each channel a unique pseudorandom channel (PN) code. The system  10  is therefore a so-called Code Division Multiple Access (CDMA) system in which channels assigned to unique codes may use the same radio carrier frequency. The channel may also be further divided or assigned. Additional information as to one possible way to implement the various channels  41 ,  42 ,  43 ,  51 ,  52 , and  53  is provided in Patent Cooperation Treaty Application No. WO 99/63682 entitled “Fast Acquisition Of Traffic Channels For A Highly Variable Data Rate,” assigned to Tantivy Communications, Inc., and published Dec. 9, 1999. In a preferred embodiment, the channel codes are a type of PN code which repeats at a code length of 2 N . One such orthogonal PN code scheme is described in U.S. patent application Ser. No. 09/255,156, filed Feb. 23, 1999, entitled “Method and Apparatus for Creating Non-Interfering Signals Using Non-Orthogonal Techniques”, assigned to Tantivy Communications, Inc. 
         [0037]    Turning attention now to  FIG. 2  there is shown a generalized block diagram of the encoding process at the transmit side and decoding process at the receive side according to the invention. It should be understood that the invention is implemented on the reverse link  50 , so that the transmitter  100  may typically be one of the SAUs  14  and the receiver is the Base Station Processor (BSP)  20 . However, in other implementations it is possible for the invention to be applied on the forward link  40 , in which case the transmitter is implemented in the BSP  20  and the receivers is the SAUs  14 . 
         [0038]    In any event, a transmitter  100  is implemented with a pilot inserter  110 , block encoder  120 , pilot interleaver  130 , channel coder  140  and radio frequency (RF) modulator  150 . The receiver  200  includes an RF demodulator  250 , channel decoder  240 , pilot deinterleaver  230 , block decoder  220 , pilot removal  210 , and pilot reference generator  205 . 
         [0039]    It should be understood that the receiver  200  performs the inverse functions of the corresponding portions of the transmitter  100 . In such an instance, the RF demodulator  250  performs the inverse radio frequency to modulation process, the channel decoder  240  decodes the channel codes reversing the operation of the channel coder  140 , the pilot deinterleaver  230  performs the inverse function of the specific pilot interleaver  130  implemented in the transmitter, and the block decode process  220  also undoes the block encode process  120 . The pilot removal process  210  uses a pilot reference signal generator  205 , for example, to multiply the received data in pilot stream via reference pilot signal to further aid in the recovery of the data. A pilot inserter  110  typically makes sense in the reverse link  50  given and that pilot symbols are preferably inserted with the data symbols or bits in this same channel. This is opposed to an arrangement where there are separate pilot channels devoted separately for simply sending pilot signals, which is typically more practical on the forward link  40 , in which case a single pilot channel can be associated with and be shared by numerous SAUs  14 . 
         [0040]    Before discussing the details of the pilot inserter  110  and block encoder  120  in more detail, it is instructed to consider the operation of a typical error coding process. In particular, consider an example situation in the use of a turbo product code which is to encode data at the rate of ¼. (We assume in the discussion of this first embodiment that data is real-valued only such that a “symbol” is a single data bit, and discuss a situation with complex-valued data symbols later on.) In the case shown in  FIG. 3A , the input data bits data  1 , data 2  . . . data 16  may thought of as being placed in the upper left hand corner of a matrix encoding space. Because the code is a ¼ rate code, the matrix encoding space consists of a matrix which is four times the size of the input data matrix space. In the current example the input data matrix space is 4×4, and the coded space is a matrix of 8×8. 
         [0041]    For a typical prior art block coding operation, that is one without supplementation with pilot symbols according to the invention, the 16 input data bits are placed in an upper left hand corner of the 8×8 encoded space as shown in  FIG. 3A . 
         [0042]    The encoded matrix is then presented to the block encoder to calculate and create the parity bits for an encoded space. In the example being discussed, in the case of a ¼ rate code, three times as many parity bits as data bits are calculated and created as shown in  FIG. 3B . This type of systematic turbo product code, is considered to be deterministic in the sense that input data bits appear in the same position in the output matrix as they do in the input matrix, with all of the parity bits taking up the other spaces in the matrix. 
         [0043]    Returning attention to  FIG. 2 , the pilot inserter  110  and block encoder  120  can now be understood more particularly. In the pilot insertion scheme employed by the pilot inserter  110 , some of the input data symbols are replaced with pilot symbols. In the preferred embodiment, the goal is to have pilot symbols make up approximately 6.25% of the data symbols sent on the channel after encoding. That means for every 64 channel symbols there needs to be 4 pilot symbols inserted into the information space. 
         [0044]    Turning attention to  FIG. 3C , we see that in considering a group of 16 data symbols, 4 of the symbols will be replaced with pilot symbols such that the input matrix becomes as shown. Thus a pilot symbol, pilot 1 , is followed by three data symbols, data 1 , data 2 , data 3 . The next pilot symbol, pilot 2 , is followed by data symbols data 4 , data 5 , data 6  and so on. 
         [0045]    As in the case of the standard turbo product code, the matrix in  FIG. 3C  is then presented to the block encoder  120  to create the encoded space shown in  FIG. 3D . The parity symbols of this example will, in fact, be different from those for the situation where the non-inserted information space because the information space has changed between the two examples. In particular, of course, the information space in  FIG. 3C  is different from the information space in  FIG. 3A , and so the parity symbols parity 1 , parity 2  . . . parity 48  are different. What is important to note here is that the pilot symbols pilot 1 , pilot 2 , pilot 3  and pilot 4  are still in the same identifiable positions in the matrix. 
         [0046]    It is the job of the pilot interleaver  130  to rearrange the output matrix in such a manner that the pilot symbols are evenly distributed among the data and parity symbols in a manner which makes sense. In the simplest instance, the data and parity symbols can be placed on the channel in, more or less, the order in which they appear in the matrix. This situation is shown in  FIG. 4A . In particular, it is noted that the pilot symbols pilot 1 , pilot 2 , pilot 3  are redistributed through the matrix so that they are read out once every 16 bits, or 6.25% of the time, as desired. The matrix can thus be interpreted as a set of instructions for ordering the output bits, by reading along the first row, and then reading the bits out along the second row, and then also the third row, and so on. 
         [0047]    In certain other instances it is important to interleave the parity symbols among the pilot and data symbols as well. In this situation, the parity symbols and data symbols can be better distributed throughout the information space. In this approach, all except the pilot symbols may be placed in a temporary storage area  180  in row order fashion and then read out by column order. The goal in allocating rows and columns in the temporary storage area  180  is to remain as square as possible. Thus, in the example illustrated shown in  FIG. 4B , the data and parity symbols are first read out from a first row of the coded output matrix in  FIG. 4A , while saving the pilot symbols in another temporary storage area. The result is a matrix having the data 1 , data 2 , data 3 , parity 1 , parity 2 , parity 3  . . . parity 47 , parity 48  symbol arrangement as shown. Data is then read out of this temporary storage area  180  by reading out the non-pilot symbols in column order. Thus, for example, as shown in  FIG. 4C , a first pilot symbol is read out of the pilot matrix, and then fifteen symbols are read from the non-pilot storage area (data 1 , parity 4 , parity 7 , parity 10 , and so on) resulting in the order of symbols shown in the first row of  FIG. 4C . This results not only in the pilot symbols continuing to be distributed once every 16 symbols, but also in a situation such that the data symbols are more evenly disbursed throughout the encoded space. 
         [0048]    In yet another example of the implementation of the interleaver  130 , it may be advantageous to apply data to the channel with Quadrature Phase Shift Keyed (QPSK) format modulation. In this case, individual input data bits are read in pairs so that for example, 2 pilot bits are required to make up respectively the In-phase (I) and the Quadrature (Q) portion of a complex valued data symbol. In this case, pilot bits are also read out in pairs so that two pilot bits comprise a pilot symbol. The result, as shown in  FIG. 4D , is a situation in which pilot symbols (consisting of a pilot 1  and pilot 2  bit) still appear every 16 symbols or 6.25% of the time. 
         [0049]    Using the systematic block encoder, the position of the pilot, information, and parity symbols is always known in the output matrix. This creates a structure where pilot symbols can be repositioned in a known fashion, to ensure that they repeat in a regular pattern in the modulated output signal. 
         [0050]    For example, a system timing requirement may demand that the ratio of pilot symbols to the ratio of data and parity symbols remain at a power of two, so that clock phasing requirements are much easier to meet. In particular, even if a block encoder produces a number of data and parity symbols as a power of 2, the additional pilot symbol insertions would create an output sequence which is not an exact power of 2. This makes it difficult to insert pilot symbols in blocks which do not remain in phase, and therefore “roll” with respect to the PN sequences used with respective channel encoding  140 . For example, in a case where pilot symbols need to be inserted 6.25% of the time, a block of 4096 would require 4096 for data and parity, plus 6% of 4096, or 256 symbols for pilots for a total of 4352 symbols per block. Because no PN channel code is such a length, to maintain synchronization, the position of the PN code would change at the start of every symbol block. However, with the invention, this difficulty is avoided, and the output symbol blocks are easily contrived to be in groups of 2N, including both parity and pilot symbols. Thus, PN code synchronization timing, as required to maintain the proper spread spectrum characteristics, is easy.