Abstract:
A technique for encoding a signal used in a digital communication system in which individual traffic channel data rates may be adapted to specific channel conditions. In particular, a forward error correction coding rate is adapted for individual channels while at the same time maintaining a fixed block size independent of the FEC coding rate. This allows the system data rate to adapt to the channel conditions experienced by a specific user. Thus, users experiencing good communication conditions with low multipath distortion may be allocated higher capacity, whereas users with significant multipath distortion may make use of lower rate (higher levels of coding) error codes to maintain high quality. Messages are sent from a transmitter to a receiver to inform the receiver of the coding rate implemented at any given point in time. These parameters may be adjusted independent of transmitted power level through the expedient of ensuring that size of a transmitted frame remains constant, while permitting the ability to change FEC coding rates and FEC block sizes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/890,485 filed Jul. 13, 2004 which issued as U.S. Pat. No. 7,426,241 on Sep. 16, 2008, which is a continuation of U.S. patent application Ser. No. 09/447,022 filed Nov. 22, 1999 which issued as U.S. Pat. No. 6,785,323 on Aug. 31, 2004, which are incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to wireless communication systems, and more particularly to a technique for providing variable data rate connections over digitally encoded radio channels. 
     BACKGROUND 
     The first generation of personal wireless communication devices, such as cellular radio telephones, operated by allocating distinct individual radio carrier frequencies to each user. For example, in an Advanced Mobile Phone Service (AMPS) type cellular mobile telephone, two 30 kiloHertz (kHz) bandwidth channels are allocated to support full duplex audio communication between each subscriber unit and a base station. The signals within each such channel are modulated using analog techniques such as frequency modulation (FM). 
     Later generation systems make use of digital modulation techniques in order to allow multiple users to access the same frequency spectrum at the same time. These techniques ostensibly increase system capacity for a given available radio bandwidth. The technique which has emerged as the most popular within the United States is a type of Code Division Multiple Access (CDMA). With CDMA, each traffic signal is first encoded with the pseudorandom (PN) code sequence at the transmitter. The receivers include equipment to perform a PN decoding function in such a way that signals encoded with different PN code sequences or with different code phases can be separated from one another. Because PN codes in and of themselves do not provide perfect separation of the channels, certain systems have an additional layer of coding referred to as “orthogonal codes” in order to reduce interference between channels. 
     In order for the PN and orthogonal code properties to operate properly at a receiver, certain other design considerations must be taken into account. For signals traveling in a reverse link direction, that is, from a mobile unit back to a central base station, power levels must be carefully controlled. In particular, the orthogonal properties of the codes are optimized for the situation where individual signals arrive at the receiver with approximately the same power level. If they do not, channel interference increases. 
     The forward link direction presents a different problem. In particular, a signal traveling from the base station to a subscriber unit may interfere with another signal in an unpredictable way as a result of the so-called near far problem. For example, faraway mobile units require relatively high power in order to be detected properly whereas close-in mobile units require lower power. The stronger signals may interfere with proper operation of mobile units located closer to the base station which typically operate with lower power levels. Unfortunately, this behavior depends upon the specific operating environment of the mobile communications system, including the topology of the surrounding geography, the juxtaposition of the subscriber units with respect to one another, and other factors. 
     In the past, it has been possible to set power levels individually to optimize each forward link channel so that interference is minimized. In particular, it has been suggested that each power level can be adjusted to affect an optimum received power level at the subscriber unit which tends to minimize interference. 
     In addition, coding algorithms such as forward error correction (FEC) type algorithms using convolutional, Reed-Solomon, and other types of codes, may be used to increase effective signal-to-noise ratio at the receiver. While such codes do provide increased performance in terms of lower bit error rates in noisy environments, by themselves they do not improve the difficulties associated with co-channel interference. 
     SUMMARY 
     The present invention provides an additional degree of freedom by permitting individual traffic channel data rates to adapt to specific channel conditions. In particular, a forward error correction (FEC) coding rate may be adapted for individual channels. At the same time, a fixed number of FEC symbols is maintained per transmitted frame, independent of the FEC coding rates and power levels. This allows a different FEC rate or even a different FEC code to be assigned to each user channel, depending upon channel conditions, without changing the effective transmitted power levels. 
     For example, if the channel is experiencing relatively good propagation conditions, the FEC coding rate may be reduced and the number of input bits per FEC frame may be increased without changing transmit power levels. Because the overall information rate then depends upon the ratio of the raw data rate divided by the code rate, a higher information rate is obtained without producing greater interference to other user channels. 
     On the other hand, if a particular channel is in a relatively bad or marginal transmission environment, other steps can be taken to reduce the overall information rate. Specifically, rather than increasing the power level of the transmission, the effective FEC coding rate can be increased, and the number of input bits per FEC frame reduced. This then permits the channel to be more robust without increasing the transmit power level. 
     In a preferred embodiment, the FEC coding rate is changed by periodically sending a message to the intended receiver which indicates the coding rate to be used in future transmissions on each given channel. For example, in a typical implementation, a rate message may be sent on the forward link paging channel or sync channel directed to a particular receiver. 
     There are several advantages to the present invention. In a Code Division Multiple Access (CDMA) system, especially in environments where multipath fading or other poor channel conditions exist, power levels need not be adjusted in order to optimize the overall system information rate. 
    
    
     
       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 high-level diagram of a wireless communication system in which the invention may be used; 
         FIG. 2  is a more detailed diagram of the forward link portions of the system  15  which implements variable rate coding according to the invention; 
         FIG. 3  shows a particular set of framing circuits and corresponding coding circuits; and 
         FIG. 4  is a chart of bit error rate versus received signal strength as measured in energy per bit versus spectral noise power for different coding rates. 
     
    
    
     DETAILED DESCRIPTION 
     Turning attention now to the drawings more particularly,  FIG. 1  is a block diagram of a system  10  for providing high speed data service over a wireless connection such as, for example, a digitally modulated wireless service known as Code Division Multiple Access (CDMA). The system  10  consists of one or more base station processors  12  and multiple subscriber access units  14 - 1 , . . . ,  14 - n , . . . ,  14 - m  (collectively access units  14 ).  FIG. 1  illustrates one base station  12  and three access units  14  by way of example only and for ease of description of the invention. The invention is applicable to systems in which there are typically many more subscriber units communicating with one or more base stations. 
     The access units  14  provide wireless data services and can connect devices such as, for example, laptop computers, portable computers, personal digital assistants (PDAs) or the like, through base station  12  to a network  15  which can be a Public Switched Telephone Network (PSTN), a packet switched computer network, or other data network such as the Internet or a private intranet. The base station  12  may communicate with the network  15  over any number of different efficient communication protocols such as primary rate ISDN, or other LAPD based protocols such as IS-634 or V5.2, or even TCP/IP if network  15  is an Ethernet network such as the Internet. The access units  14  may be mobile in nature and may travel from one location to another while communicating with base station  12 . 
     It is also to be understood by those skilled in the art that  FIG. 1  may be similar to a standard cellular type communication system in which radio channels are assigned to carry signals between the base stations  12  and access units  14 . This invention, however, applies more particularly to non-voice, digital data transmissions of varying bandwidths. In a preferred embodiment, the system  10  uses code division multiple access (CDMA) principles for modulating the transmitted signals. However, it is also to be understood that the invention is not limited to using standardized CDMA protocols such as IS-95, or even newer emerging CDMA protocols such as CDMA-One or WCDMA. The invention is applicable to other multiple access modulation techniques. 
     In order to provide data and voice communication between the access units  14  and the base station  12 , a limited number of radio channel resources are provided via forward communication channels  16 - 1 , . . . ,  16 - n , and reverse communication channels  17 - 1 , . . . ,  17 - n . The invention provides for precise management of a way in which these channel signals are encoded on an as-needed basis for each access unit  12 . It should also be understood that data signals travel bi-directionally across the radio channels  16  and  17 , i.e., data signals originating at the access units  14  are coupled to the network  15 , and data signals received from the networks are coupled to the access units  14 . 
       FIG. 2  shows certain elements of the base station processor  12  and remote access unit  14  in more detail. The base station processor  12  and access unit  14  communicate at least in a forward direction over one or more of the forward link channels  16 - 1 , . . . ,  16   n . It should be understood that the base station  12  and access unit  14  may also communicate with one another in a reverse link direction, although the details of such are not shown in  FIG. 2 . The principles discussed herein for the forward link  16  implementation could also be used in implementing reverse link direction communications. 
     In a CDMA system, the signaling on a given forward link  16 - n  shares a common radio carrier frequency and time slot with signaling intended for other forward links  16 - m . Therefore, it is entirely possible that the signaling sent over a given forward link  16 - n  intended only for a specific access unit  14 - n  may in some way interfere with the signaling transmitted over another forward link  16 - m  and intended for another access unit  14 - m.    
     The base station processor  12  more particularly includes a controller  30  and signal processing circuits which generate the various signals making up the forward link  16  transmitted signals. These include circuits for implementing functions such as a pilot channel  32 , paging channel  34 , and one or more traffic channels  36 . As it is known in the art, the pilot channel  32  is responsible for generating known continuous pilot signals that permit receiver circuits in the access unit  14  to properly synchronize to signals transmitted by the base station processor  12 . The paging channel  34  sends control signals to the access unit  14  to, for example, allocate traffic channel capacity over the forward link  16 . For example, the paging channel  34  is used to send messages to the access unit  14 - n  when it is necessary to allocate a traffic channel on the forward link  16 - n  when messages need to be sent to the access unit  14 - n.    
     The traffic channel  36  provides a physical layer structure for sending payload data over the forward links  16 . In a preferred embodiment, CDMA encoding is used to define the pilot channels  32 , paging channels  34 , as well as the traffic channels  36 . 
     More specifically, the traffic channel circuitry  36  includes symbol framing function  40 , forward error correction logic  42 , a demultiplexer  44 , a summer  50 , and radio frequency (RF) up converters  52 . 
     Data which is to be sent over the forward link  16  is first fed to the framing function  40 . The framing function  40  packages input sized groups referred to as frames. The size of these pre-encoded frames will vary depending upon the particular forward error correction (FEC) coding scheme selected at any given time by the FEC encoder  42 . What is important is that the combination of the framers  40  and FEC encoder  42  produce a fixed number of output FEC symbols in each given transmitted frame. 
       FIG. 3  is a diagram showing how the framers  40  and FEC encoders  42  are selected in pairs to accomplish this end result. The fixed output FEC frame size in the illustrated embodiment is 4096 symbols. This embodiment uses four different FEC symbol encoders  42 - 1 ,  42 - 2 ,  42 - 3 , and  42 - 4  providing, respectively, a ¼, ⅓, ½, and ⅞ rate encoding. The coding rate of each FEC symbol encoder  42  indicates the ratio of the number of input bits to the number of output bits. The actual codes used by the FEC encoders  42  may be any of a number of different types of error correction codes such as R, thus, a higher information rate is obtained with higher rate FEC code. 
     This embodiment also uses four framer circuits  40 - 1 ,  40 - 2 ,  40 - 3 ,  40 - 4  corresponding to the four FEC encoders  42 - 1 ,  42 - 2 ,  42 - 3 ,  42 - 4 . For example, the ¼ rate encoder  42 - 1  requires a ¼ rate framing circuit  40 - 1  which groups incoming bits into pre-encoded FEC groups of 1024 bits, producing the desired 4096 output symbols. Similarly, the ⅓ rate encoder  42 - 2  requires a ⅓ rate framer  40 - 2  to group incoming bits into pre-encoded sets of approximately 1365 bits. The ½ rate encoder  42 - 3  users a framer  40 - 3  with a pre-encoded set size of 2048, and ⅞ encoder  42 - 4  uses a framing circuit  40 - 4  with the pre-encoded size of 3584 bits. 
     Framing circuit  40  and FEC encoder  42  thus only utilize one of the specific framers  40 - 1 ,  40 - 2 ,  40 - 3 , or  40 - 4 , and one of the specific encoders  42 - 1 ,  42 - 2 ,  42 - 3 , or  42 - 4  at any given point in time. Which particular framing circuit  40  and FEC encoder  42  is activated is controlled by coding rate control signal  60  input to each of the framing circuits  40  and encoder  42 . The code rate select signal  60  is generated by the controller  30 . 
     Returning attention now to  FIG. 2 , a given connection may require multiple traffic channels to be allocated at a particular time. For circuits  46 - 1  and channel modulators  48 - 1 , which impress not only a quadrature phase shift keyed (QPSK) modulation, but also the appropriate pseudorandom noise (PN) and/or Walsh or other coding in order to produce multiple CDMA channel signals  49 - 1 ,  49 - n . These multiple CDMA traffic signals are then summed by the summer  50 , together with the pilot channel signal produced by the channel pilot circuits  32  and the paging signal produced by the paging channel circuit  34 . The output of summer  50  is then fed to the RF up converter  52 . 
     The controller  30 , which may be any convenient suitable microcontroller or microprocessor, also has among its software programs a process referred to as the capacity manager  55 . The capacity manager  55  not only allocates one or more of the channel modulators  48  to a specific forward link  16  traffic channel connection, but also sets the value for the code rate select signals  60 . In addition, the capacity manager  55  sets power levels for particular forward link signals  16 . 
     A single capacity manager  55  in a base station processor  12  may manage  15  multiple traffic channel circuits  36 , each producing a number of forward link signals  16 . The capacity manager  55  sets the code rate select signal  60  according to observed conditions in a corresponding traffic channel. These adjustments to the channel physical layer characteristics are made preferably in response to determining a signal strength value, such as by measuring a ratio of the energy per data bit divided by a normalized noise power level (Eb/No) at the receiver. 
     Thus, in addition to changing the power level of the individual modulated signals generated by the modulators  48 , it is also possible with a system according to the invention to control the Eb/No at the receiver by adjusting the value of code rate select signal  60  in order to select different code rates under different conditions. 
     For example, if a remote access unit  14  located deep inside of building is experiencing particularly adverse multipath or other distortion conditions, in the past it was thought necessary to increase the power level of the forward link  16 - n  in order to obtain an appropriate received signal level at the access unit  14 . However, with the invention, if a full maximum data rate is not needed, then the coding rate implemented by the FEC encoder  42  can be lowered. 
     And in other environments where multipath distortion is minimal, such as in a direct line of sight situation, the highest code rate generator  42 - 4  can be selected while at the same time reducing the radiated power level on forward link  16 - n  for that particular channel. This therefore maximizes the available data rate for a given user while also minimizing interference generated to other users of the same radio channel. 
     Thus, in environments where propagation is good, the system  10  can increase the data rate to a given user without introducing additional interference to other users. However, in a bad signaling environment, an advantage is also obtained since each particular user channel can be made more robust without increasing its power level. 
     Continuing to pay attention to  FIG. 2 , various components of the access unit  14  will be discussed in more detail. The access unit  14  consists of an RF down converter  60 , equalizer  62 , multiple rake receivers  64 - 1 , . . . ,  64 - n , multiple channel demodulators  66 - 1 , . . . ,  66 - n , a multiplexer  68 , an FEC decoder  70 , and framing circuit  72 . 
     The RF down converter  60  accepts the forward link signal  16 - n , producing a baseband digitized signal. The chip equalizer  62  provides equalization of individual chips of the received signal, fitting it to one of several rake finger and interference cancellation circuits  64 . These circuits cooperate with multiple channel demodulator  66  in a manner which is known in the prior art and a strip off the CDMA encoding on each channel. Pilot receiving circuit  74  and paging signal receiving circuit  76  similarly are adapted for receiving the pilot channel signal and the paging signal generated by the base station processor  12 . The multiplexer  68  reconstructs signals in the situation where multiple traffic channels were allocated to the particular connection. 
     A controller  80  executes programs which set various parameters of the components of the traffic channel circuit  58 . Of particular interest here is the fact that the controller  80  executes a management process  82  which determines the coding rate select signal  84  to be sent to the FEC decoder  70 . 
     Specifically, the coding rate selected by the FEC decoder  70  at the receiving access unit  14  must be the same as the coding rate of the FEC encoder  32  at the transmitting base station processor  12  in order for, the receiver framing circuit  72  to correctly reproduce the input data signal. Thus, in order for the system  10  to adapt to changing conditions in the RF link  16 , it is necessary for the base station processor  12  to communicate this information to the access unit  14  in some manner. 
     For example, if it is desired to allow the coding rate to change during the duration of a connection, which is the case in the preferred embodiment, the paging channel  34  may initially include, during a channel acquisition sequencing, a command to inform the access unit  14  not only of the different channels  36  on which it will be communicating, but also to inform it of the particular encoding rate that it will be using. Then, as a connection remains open and coding rates that are optimum change over time, additional control messages may be embedded in the traffic channel itself by embedding a command message within the received data which is fed back to the controller  80  via a command signal input  86 . 
     It should be understood that measures of link quality can also be determined by the controller  80  from the output signal  86  and periodically sent back to the controller  30  in the base station processor  12  via a command structure on a reverse link channel (not shown). This permits the controller  30  at the base station processor  12  to appropriately set optimum FEC coding rates to be used by the FEC encoder  42  and the FEC decoder  70  for particular connections. 
       FIG. 4  is a chart of bit error rate (BER) versus Eb/No in decibels (dB) for various combinations of framers  40  and FEC encoders  42 . The legend in the graph shows the performance of different rate turbo product codes normalized for the energy in a particular bit. For example, in a state indicated at point A, the particular channel may be operating with approximately ½ rate turbo product code and experiencing relatively low bit error rate of 0.05. Without adjusting the transmit power and by merely selecting a lower rate turbo product code, such as the approximately ¼ rate code (indicated by the rate 0.266 turbo product code) a state B is entered for the system in which the bit error rate is markedly decreased to approximately 0.0002. This is accomplished without adjusting the energy per bit or otherwise altering transmit power level. 
     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. 
     Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided herein may be implemented in a computer program, software, or firmware incorporated in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
     Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
     A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) or Ultra Wide Band (UWB) module.