VARIABLE RATE CODING FOR FORWARD LINK

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 tranmitted 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: VARIABLE RATE CODING FOR FORWARD LINK FIELD OF THE 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 OF THE INVENTION 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 OF THE INVENTION 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 which implements variable rate coding according to the invention. Fig. 3 illustrates a particular set of framing circuits and corresponding coding circuits. 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 OF A PREFERRED EMBODIMENT 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 W-CDMA. 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 processor 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 payload data into conveniently 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 1/4, 1/3, 1/2, and 7/8 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 codes. 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 1/4 rate encoder 42-1 requires a 1/4 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 1/3 rate encoder 42-2 requires a 1/3 rate framer 40-2 to group incoming bits into pre-encoded sets of 1331 bits. The 1/2 rate encoder 42-3 uses a framer 40-3 with a pre-encoded set size of 2048, and 7/8 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 that multiple traffic channels be allocated at a particular time. For example, the demultiplexer 44 accepts the signal produced by the FEC encoder 42and feeds it to multiple spreading circuits 36-1 and channel modulators 38-1 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 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 circuit 50 is then fed to the RF up converter 42. 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-n 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 a particular forward link signals 16-n. A single capacity manager 55 in base station processor 12 may manage 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 32 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 31 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 24 may initially include, during a channel acquisition sequencing, a command to inform the access unit 14 not only of the different channels 38 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 1/2 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 1/4 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.

Claims: CLAJMS What is claimed is: 1. A method for coding channels in a wireless communication system in which a digital signal is communicated from a transmitting station to a receiving station, the method comprising the steps of: grouping bits of the input signal into frames, a size of the frames being adjustable according to a predetermined frame size parameter; encoding bits of the frames to include an error correction code, a rate of the error correction code selected such that a number of encoded symbols in a transmitted frame remains constant, even if a number of bits in a frame changes; modulating the encoded symbols by a spreading code to produce a modulated encoded signal; and transmitting the modulated encoded signal over a wireless communication link. 2. A method as in claim 1 wherein the number of encoded symbols in each transmitted frame remains the same, even if a channel coding rate is changed for a given connection. 3. A method as in claim 1 wherein the error correction code is selected from a group consisting of convolutional codes, Reed-Solomon codes, product codes, and turbo codes. 4. A method as in claim 1 additionally comprising the step of: sending a message to the receiver station from the transmitter station, the message including an indication of the coding rate used in generating the encoded frames, thereby permitting the receiver station to determine a decoding rate required to properly decode a received frame. 5. A method as in claim 1 additionally comprising the step of coding each encoded symbol with a channel code to permit separation of encoded symbols from other encoded symbols transmitted on a given radio frequency carrier signal. 6. A method as in claim 1 wherein the communication link is a forward link transmitting from a base station transmitter station towards an access unit receiver station. 7. A method as in claim 1 wherein the communication link is a reverse link channel transmitting information from an access unit station towards a receiving base station. 8. A method as in claim 1 wherein an error correction code rate is chosen based upon observed link quality conditions in a radio channel. 9. A method as in claim 8 in which radio channels experiencing bit error rates cause selection of an error correction code rate which is lower, to maintain higher link quality. 10. A method as in claim 1 wherein encoding rates for different connections over a given radio carrier frequency have different error correction and framing rates.

Inventors: PROCTOR JAMES A JR, PROCTOR, JAMES, A., JR.
Applicants: TANTIVY COMM INC, TANTIVY COMMUNICATIONS, INC.
Application year: 2000
Main classification: H04L1
IPC codes: H04B1, H04B7, H04L1, H04Q7