Concatenated encoding and decoding for multilayer communication protocol

BACKGROUND

Field

The present invention relates to communication systems. More particularly, the present invention relates to a system and method for reduction of decoding complexity in a communication system.

Background

Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.

Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the "TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in <patcit id="pcit0001" dnum="US4901307A"><text>U.S. Patent No. 4,901,307</text></patcit>, entitled "SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," and <patcit id="pcit0002" dnum="US5103459A"><text>U.S. Patent No. 5,103,459</text></patcit>, entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," both assigned to the assignee of the present invention.

A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in <patcit id="pcit0003" dnum="US5504773A"><text>U.S. Patent No. 5,504,773</text></patcit>, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the "3rd Generation Partnership Project" (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or "TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems" (the IS-2000 standard).

An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in co-pending application serial number <patcit id="pcit0004" dnum="US08963386B"><text>08/963,386</text></patcit>, entitled "METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION," filed November 3, 1997, with patent number <patcit id="pcit0005" dnum="US2003063583A"><text>US 2003 063583</text></patcit>, and assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.

In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.

The above described wireless communication service is an example of a point-to-point communication service. In contrast, broadcast services provide point-to-multipoint communication service. The basic model of a broadcast system consists of a broadcast net of users served by one or more central stations, which transmit information with a certain contents, e.g., news, movies, sports events and the like to the users. Each broadcast net user's subscriber station monitors a common broadcast forward link signal. Because the central station fixedly determines the content, the users are generally not communicating back. Examples of common usage of broadcast services communication systems are TV broadcast, radio broadcast, and the like. Such communication systems are generally highly specialized purpose-build communication systems. With the recent, advancements in wireless cellular telephone systems there has been an interest of utilizing the existing infrastructure of the - mainly point-to-point cellular telephone systems for broadcast services. (As used herein, the term "cellular" systems encompasses communication systems utilizing both cellular and PCS frequencies.)

The information signal to be exchanged among the terminals in a communication system is often organized into a plurality of packets. For the purposes of this description, a packet is a group of bytes, including data (payload) and control elements, arranged into a specific format. The control elements comprise, e.g., a preamble and a quality metric. The quality metric comprises, e.g., cyclical redundancy check (CRC), parity bit(s), and other types of metric known to one skilled in the art. The packets are then formatted to fit a into a frame in accordance with a communication channel structure. The frame, appropriately modulated, traveling between the origination terminal and the destination terminal, is affected by characteristics of the communication channel, e.g., signal-to-noise ratio, fading, time variance, and other such characteristics. Such characteristics affect the modulated signal differently in different communication channels. Consequently, transmission of a modulated.signal over a wireless communication channel requires different considerations than transmission of a modulated signal over a wire-like communication channel, e.g., a coaxial cable or an optical cable. In addition to selecting modulation appropriate for a particular communication channel, other methods for protecting the information signal have been devised. Such methods comprise, e.g., encoding, symbol repetition, interleaving, and other methods know to one of ordinary skill in the art. However, these methods increase overhead. Therefore, an engineering compromise between reliability of the information signal delivery and the amount of overhead must be made. Even with the above-discussed protection of information signal, the conditions of the communication channel can degrade to the point at which the destination station possibly cannot decode (erases) some of the packets. In data- only communications systems allowing a communication of a feedback from a destination terminal to the origination terminal, one cure is to re-transmit the non-decoded packets using an Automatic Retransmission reQuest (ARQ) made by the destination station to the origination station. However, under certain conditions, the ARQ may overload the communication system. Furthermore, as discussed in regards to broadcast communication systems, the subscribers do not communicate back to the base station. Consequently, other means of information protection are desirable.

A co-pending application serial number <patcit id="pcit0006" dnum="US09933912B"><text>09/933,912</text></patcit>, entitled "METHOD AND SYSTEM FOR UTILIZATION OF AN OUTER DECODER IN A BROADCAST SERVICES COMMUNICATION SYSTEM," filed August 20, 2001, with publication number <patcit id="pcit0007" dnum="US2003035389A"><text>US 2003 035 389</text></patcit>, and assigned to the assignee of the present invention, discussed in detail utilization of an outer decoder in a broadcast system. As described in the co-pending application serial number <patcit id="pcit0008" dnum="US09933912B"><text>09/933,912</text></patcit>, the bit stream of information to be transmitted is first encoded by an outer decoder and the encoded stream is then encoded by an inner encoder. As illustrated in <figref idref="f0001"><b>FIG. 1</b></figref><b>,</b> the bit stream of information to be transmitted <b>102,</b> originating at higher layers, is provided to a transmit buffer <b>104</b>. The transmit buffer is illustrated in more detail in <figref idref="f0002"><b>FIG. 2</b></figref><b>.</b> Referring to <figref idref="f0002"><b>FIG. 2</b></figref><b>,</b> the bits fill the systematic portion <b>204(1)</b> of the transmit buffer <b>104</b> (of <figref idref="f0001"><b>FIG. 1</b></figref>) row by row from left to right. The systematic portion <b>204(1)</b> comprises k <b>rows 208</b> of length <i>L</i>. Referring back to <figref idref="f0001"><b>FIG. 1</b></figref><b>,</b> once the systematic portion <b>204(4)</b> (of <figref idref="f0002"><b>FIG</b>. <b>2</b></figref><b>)</b> is full, the outer block encoder <b>106</b> is activated to perform column-wise encoding of the bits in the systematic portion <b>204(1)</b> (of <figref idref="f0002"><b>FIG. 2</b></figref><b>)</b> to generate (n-k) additional rows <b>210</b> (of <figref idref="f0002"><b>FIG. 2</b></figref><b>)</b> of parity bits. This column-wise operation is performed column by column for binary outer code, i.e., <i>m</i> = 1. For non-binary code, i.e., <i>m</i> &gt; 1, every <i>m</i> adjacent columns in a row are treated as a <i>m</i>-bit symbol. The <i>m</i>-bit symbols along the top <i>k</i> rows are read by the outer encoder to produce n-k <i>m</i>-bit symbols that fill the corresponding lower n-k rows of these columns.

The outer encoder comprises, e.g., a systematic Reed-Solomon (R-S) encoder. Referring back to <figref idref="f0001"><b>FIG. 1</b></figref><b>,</b> the content of the transmit buffer <b>104</b> is then provided to a physical layer <b>108.</b> At the physical layer <b>108,</b> the individual frames are encoded by an inner encoder (not shown), which results in encoded frames. The structure of the inner decoder may is well known to one of ordinary skills in the art. The systematic rows and the parity rows of the buffer may be interlaced during transmission to reduce the chance of large number of systematic rows erased when the total number of inner code erasure exceeds the outer code's correcting capability. The frames are further processed in accordance with a selected modulation scheme, e.g., cdma2000, WCDMA, UMTS, and other modulation schemes known to one of ordinary skills in the art. The processed frames are then transmitted over a communication channel <b>110.</b>

The transmitted frames are received at the destination station and provided to a physical layer <b>112.</b> At the physical layer <b>112,</b> the individual frames are demodulated and provided to an inner decoder (not shown). The inner decoder decodes each frame, and if the decoding is successful, outputs a correctly decoded frame; or if the decoding is unsuccessful, declares an erasure. The success or failure of decoding must be determined with a high accuracy, achieved e.g., by including a long (for example, 16-bit) cyclic redundancy check (CRC) in the frame after outer encoding and before inner encoding. The included CRC obtained from the decoded frame is compared with a CRC calculated from the bits of the decoded frame, and if the two CRCs are identical, the decoding is declared successful.

If the inner decoder cannot decode the frame, the decoder declares an erasure, and provides an outer block decoder <b>116</b> with an indication that the frame is missing. The process continues until there are as many parity frames received correctly and passed to a parity portion <b>114(2)</b> of a receive buffer <b>114,</b> as there are erased systematic frames. The receiver stops the reception of any remaining frames and the outer decoder (not shown) is activated to recover the erased systematic frames. The recovered systematic frames are passed to the upper layer.

It is well known in the art that a decoding/error correcting computation complexity increases with increased values of the number of rows in the transmit buffer <b>104.</b> Because the decoding/error correcting computation complexity affects hardware complexity at the receiving terminal as well as power consumption, there exists a need in the art for a method and system.

Further attention is drawn to the following document: <nplcit id="ncit0001" npl-type="s"><text>Lou, Podilchuk, Chio: "Progressive video streaming over 2G and 3G wireless systems", proceedings of the 11th IEEE international symposium on personal indoor and mobile radio communications, PIMRC 2000, vol. 2, 18-21 September 2000, pages 1550-1554</text></nplcit> (XP010520891). The article discusses a progressive video codec with video scalability. The article proposes an unequal protection scheme in the application layer for the video bit stream that works in concatenation with the channel codes in the physical layer of current cellular systems.

Attention is also drawn to the document <patcit id="pcit0009" dnum="US6047395A"><text>US 6 047 395</text></patcit>, which describes an error correction processor for correcting errors in binary data read from a disk storage medium, wherein the binary data comprises a first and second set of intersecting ECC codewords of a multi-dimensional codeword. The error correction processor comprises a data buffer for storing the ECC codewords read from the disk storage medium; a syndrome generator for generating ECC syndromes in response to a codeword in the second set; an error-locator polynomial generator for generating an error locator polynomial sigma (x) in response to the ECC syndromes; a selector for selecting between the error-locator polynomial sigma (x) and an erasure polynomial sigma (x)EP, wherein: (i) the erasure polynomial sigma (x)EP is generated while processing the first set codewords; and (ii) the erasure polynomial sigma (x)EP is used to correct at least two codewords in the second set; and an error corrector for generating correction values in response to either the error-locator polynomial sigma (x) or the erasure polynomial sigma (x)EP output by the selector, the correction values for correcting errors in the codeword in the second set.

SUMMARY

In accordance with the present invention an encoding method, as set forth in claim 1, a method, as set forth in claim 5, an encoding apparatus, as set forth in claims 9, and an apparatus, as set forth in claim 12, are provided. Embodiments of the inventions are claimed in the dependent claims.

Embodiments disclosed herein address the above stated needs by providing a method and a system executing the method by encoding systematic bits in each of a plurality of buffers with an outer code; multiplexing content of the plurality of buffers; and encoding said multiplexed content with an inner code to provide a set of frames.

In another aspect of the invention, the received set of frames is decoded by an inner decoder; the correctly decoded frames are de-multiplexed to a plurality of buffers; and the content of each buffer is further processed. If the systematic portion of a buffer has been decoded correctly, the processing comprises providing the content of the systematic portion to higher layers. Alternatively, if it is determined that decoding of the buffer content by an outer decoder recovers the systematic portion, the outer decoder is activated and the recovered content together with the correctly received content of the systematic portion are provided to higher layers.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="f0001"><b>FIG. 1</b></figref> illustrates prior art physical layer processing;

<figref idref="f0002"><b>FIG. 2</b></figref> illustrates a transmit buffer;

<figref idref="f0003"><b>FIG. 3</b></figref> illustrates physical layer processing in accordance with one embodiment of the invention;

DETAILED DESCRIPTION

Definitions

The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

The terms point-to-point communication is used herein to mean a communication between two subscriber stations over a dedicated communication channel.

The terms broadcast communication or point-to-multipoint communication are used herein to mean a communication wherein a plurality of subscriber stations are receiving communication from one source.

The term packet is used herein to mean a group of bits, including data (payload) and control elements, arranged into a specific format. The control elements comprise, e.g., a preamble, a quality metric, and others known to one skilled in the art. Quality metric comprises, e.g., a cyclical redundancy check (CRC), a parity bit, and others known to one skilled in the art.

The term access network is used herein to mean a collection of base stations (BS) and one or more base stations' controllers. The access network transports data packets between multiple subscriber stations. The access network may be further connected to additional networks outside the access network, such as a corporate intranet or the Internet, and may transport data packets between each access terminal and such outside networks.

The term base station is used herein to mean the hardware with which subscriber stations communicate. Cell refers to the hardware or a geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector has the attributes of a cell, the teachings described in terms of cells are readily extended to sectors.

The term subscriber station is used herein to mean the hardware with which an access network communicates. A subscriber station may be mobile or stationary. A subscriber station may be any data device that communicates through a wireless channel or through a wired channel, for example using fiber optic or coaxial cables. A subscriber station may further be any of a number of types of devices including but not limited to PC card, compact flash, external or internal modem, or wireless or wireline phone. A subscriber station that is in the process of establishing an active traffic channel connection with a base station is said to be in a connection setup state. A subscriber station that has established an active traffic channel connection with a base station is called an active subscriber station, and is said to be in a traffic state.

The term physical channel is used herein to mean a communication route over which a signal propagates described in terms of modulation characteristics and coding.

The term logical channel is used herein to mean a communication route within the protocol layers of either the base station or the subscriber station.

The term communication channel/link is used herein to mean a physical channel or a logical channel in accordance with the context.

The term reverse channel/link is used herein to mean a communication channel/link through which the subscriber station sends signals to the base station.

A forward channel/link is used herein to mean a communication channel/link through which a base station sends signals to a subscriber station.

The term soft hand-off is used herein to mean a communication between a subscriber station and two or more sectors, wherein each sector belongs to a different cell. The reverse link communication is received by both sectors, and the forward link communication is simultaneously carried on the two or more sectors' forward links.

The term softer hand-off is used herein to mean a communication between a subscriber station and two or more sectors, wherein each sector belongs to the same cell. The reverse link communication is received by both sectors, and the forward link communication is simultaneously carried on one of the two or more sectors' forward links.

The term erasure is used herein to mean failure to recognize a message.

The term dedicated channel is used herein to mean a channel modulated by information specific to an individual subscriber station.

The term common channel is used herein to mean a channel modulated by information shared among all subscriber stations.

The term physical layer is used exclusively herein to mean that part of the communication protocol between an origination terminal and a destination terminal that is responsible for the transmission an reception of data. The physical layer corresponds to Layer 1 in the International Standards Organization model for Open System Interconnection.

The term higher layer(s) is used exclusively herein to mean that part of the communication protocol between an origination terminal and a destination terminal that is above a physical layer. The higher layers correspond to Layers 2 through 7 in the international Standards Organization model for Open System Interconnection.

Description

In accordance with one embodiment of the present invention, as illustrated in <figref idref="f0003">FIG. 3</figref>, the bit stream of information to be transmitted 302, originating at higher layers, is de-multiplexed and provided to transmit buffers <b>304(i).</b> The bits fill the systematic portion <b>306(1)</b> of the transmit buffer <b>304(1)</b> row by row from left to right. The systematic portion <b>306(1)</b> comprises <i>k</i> rows of length <i><u>L.</u></i> In one embodiment, the length <i>L</i> of the buffer coincides with the length of a radio frame without the overhead (e.g., CRC to help the inner decoder and the tail bits for the inner encoder). Once the systematic portion <b>306(1)</b> of the transmit buffer <b>304(1)</b> is full, the procedure is repeated for the remaining transmit buffers <b>304(2) - 304(p).</b> Once the systematic portions <b>306(1)</b> of the transmit buffers <b>304(l)</b> are full, the outer block encoders <b>310(i)</b> are activated to perform column-wise encoding of the bits in the systematic portion <b>306(i)</b> to generate (n-k) additional rows of parity bits <b>308(i).</b> This column-wise operation is performed column by column for binary outer code, i.e., <i>m</i> = 1. For non-binary code, i.e., <i>m</i> &gt; 1, every <i>m</i> adjacent columns in a row are treated as a <i>m</i>-bit symbol. The <i>m</i>-bit symbols along the top <i>k</i> rows are read by the outer encoder to produce n-k <i>m</i>-bit symbols that fill the corresponding lower n-k rows of these columns.

In another embodiment, the length <i>L</i> of the buffer is equal to the number of bits the radio frame without the overhead divided by <i>m</i>, the dimension of the outer encoder code. In this embodiment, the first <i>m</i> rows from the transmit buffers <b>304(i)</b> are sent in the first radio frame, the second <i>m</i> rows of bits are sent in the second radio frame, until the entire buffer is transmitted. Once the systematic portion <b>306(1)</b> of the transmit buffer <b>304(1)</b> is full, the procedure is repeated for the remaining transmit buffers <b>304(2) - 304(p)</b>. Once the systematic portions <b>306(i)</b> of the transmit buffers <b>304(i)</b> are full, the outer block encoders <b>310(i)</b> are activated to perform column-wise encoding of the bits in the systematic portion <b>304(i)</b> to generate m(n-k) additional rows of parity bits <b>308(i).</b> This column-wise operation is performed column by column for binary outer code, i.e., <i>m</i> = 1. For non-binary code, i.e., <i>m</i> &gt; 1, every m-rows of a column form a m-bit symbol. The k symbols from the top <i>k m</i> rows in the column are read by the outer encoder to produce (n - k) <i>m</i>-bit symbols that fill the corresponding lower m(n-k) rows of this column.

In one embodiment the outer encoders <b>310(i)</b> comprise a systematic Reed-Solomon (R-S) encoder. The content of the transmit buffers <b>304(i)</b> are then provided to an multiplexer <b>312.</b> The multiplexer <b>312</b> cycles through the transmit buffers <b>304(1) - 304(p),</b> selecting a successive transmit buffer <b>304(1)</b> after a block of bits containing a pre-determined number of bits has been sent form a previous buffer <b>304(i-1).</b> In one embodiment, the pre-determined number of bits in a block equals <i>L</i>. This strategy intends to uniformly distribute corruption of data caused by disturbance of a physical channel <b>316</b> among the buffers <b>304(i).</b> However, one of ordinary skills in the art understands that other multiplexing strategies are equally applicable, and can be utilized without departing from the scope of the invention as defined in the appended claims. The multiplexed blocks of bits are provided to a physical layer <b>314</b> of the originating terminal. At the physical layer <b>314,</b> additional overhead bits (e.g., a CRC check) are added to each of the blocks of bits, and the resulting structure is encoded by an inner encoder (not shown), which results in encoded frames. The structure of the outer and inner encoders and the multiplexer may be, e.g., the structure of <figref idref="f0003"><b>FIG. 3</b></figref><b>.</b> The frames are further processed in accordance with a selected modulation scheme. In one embodiment, the processing is performed in accordance with the IS-2000 standard. The processed frames are then transmitted over a communication channel <b>316.</b>

The transmitted frames are received at the destination station (not shown), and provided to a physical layer <b>318</b> at the destination station. At the physical layer <b>318,</b> the individual frames are demodulated and provided to an inner decoder (not shown). In one embodiment, the inner decoder decodes each frame, and if the decoding is successful, outputs a correctly decoded frame; or if the decoding is unsuccessful, declares an erasure. The success or failure of decoding must be determined with a high accuracy. In one embodiment, the accuracy is achieved by including a long (for example 16-bit) cyclic redundancy check (CRC) in the frame after outer encoding and before inner encoding as discussed above. However, one of ordinary skills in the art recognizes that other mechanisms for frame quality indication may be used. The included CRC obtained from the decoded frame is compared with a CRC calculated from the bits of the decoded frame, and if the two CRCs are identical, the decoding is declared successful. Further processing at the physical layer proceeds in accordance with the result of the inner decoder decision.

The correctly decoded frames are provided to a de-multiplexer <b>320</b> that distributes the correctly decoded frames among the receive buffers <b>322(i)<i>,</i></b> utilizing an inverse method to the method used for multiplexing. If all the systematic <i>k</i> frames are correctly decoded by the inner decoder for a particular receive buffer <b>322(i),</b> the systematic frames from the systematic portion <b>324(i)</b> of the receive buffer <b>322(i)</b> are provided to higher layers.

If the inner decoder cannot decode the frame, the decoder declares an erasure, and provides the de-multiplexer <b>324</b> with an indication that the frame is missing. The de-multiplexer <b>324</b> provides the information to the outer block decoder <b>328(i)</b> communicatively coupled to the received buffer <b>322(i)</b> to which the frame belonged. The process continues until there are enough systematic frames and correctly received parity frames accumulated in the systematic portion <b>324(i)</b> and the parity portion <b>326(i)</b> of the receive buffer <b>322(i),</b> or until the receive buffer <b>322(i)</b> is full. The outer decoder (not shown) is then activated to recover the erased systematic frames. The recovered systematic frames are provided to higher layers.

If the total number of correctly received frames in the receive buffer <b>322(i)</b> is less than <i>k</i>, in accordance with one embodiment the outer decoder is not activated since there is no guarantee that the decoding would be successful. The correctly received systematic frames together with identification of the missing bits are provided to the higher layers. In another embodiment, the receiver uses decoded bits from the inner decoder (which are unreliable as indicated by the failed CRC checks) to recover bits for the systematic bits. In accordance with one embodiment, the receiver decodes the unreliable bits from the inner decoder and finds the most likely codeword. In the another embodiment, the receiver uses measurement of the signal quality of the erased frames in the buffer to choose enough erroneously received frames with the highest signal to noise ratio to form a sub buffer with k rows. The receiver then performs bit flipping (changing a bit value of 0 to a bit value 1 and vice versa at one column at a time) and checks whether the bit flipping resulted in a codeword. In one embodiment, the bit flipping is first performed on the least reliable bits and continues with bits in the order of the bits' increasing reliability. The reliability of a bit may be determined in accordance with inner decoding metrics, e.g., a signal to noise and interference ratio during the frame, like the Yamamoto metric, the re-encoded symbol error rate, re-encoded energy metric, and other metrics known to one of ordinary skills in the art, or the metrics' combinations. If a codeword was not found, the bit flipping continues through all the remaining columns for all the unreliable rows. If a codeword was not found, the bit flipping continues with increased number of bits flipped (that is, changing 2 bits at a time, then 3 bits, until the maximum number of bits), until either a codeword is found or all combinations are exhausted. In another embodiment, the CRC from the unreliable rows are used to check the overall success of the decoding in this situation. The frames are provided to the higher layers only if the CRC from all.rows match; otherwise, only bits from reliable rows are provided to the higher layers.

To improve reliability of decoding, in another embodiment, the demodulation and inner decoding are performed for more than <i>k</i> correctly received frames in a buffer. In accordance in yet another embodiment the demodulation and inner decoding are performed for all frames in the buffer. In both embodiments, the outer decoding is performed on the <i>k</i> (or km) rows with the highest quality. The quality may be determined in accordance with inner decoding metrics, e.g., a signal to noise and interference ratio during the frame, like the Yamamoto metric, the re-encoded symbol error rate, re-encoded energy metric, and other metrics known to one of ordinary skills in the art, or the metrics' combinations. Use of quality metrics for quality estimation is disclosed in detail in <patcit id="pcit0010" dnum="US5751725A"><text>U.S. Patent No. 5,751,725</text></patcit> entitled "METHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATIONS SYSTEM" and <patcit id="pcit0011" dnum="US5774496A"><text>U.S. Patent No. 5,774,496</text></patcit> entitled "METHOD AND APPARATUS FOR DETERMINING DATA RATE OF TRANSMITTED VARIABLE RATE DATA IN A COMMUNICATIONS RECEIVER" and both are assigned to the assignee of the present invention.

One skilled in the art will appreciate that the number of transmit buffers <b>304</b>, consequently, receive buffers <b>322</b> is a compromise between processing overhead and amount of potential data loss. A small value of <i>k</i>, resulting in more transmit/receive buffers causes increased processing overhead. On the other hand, a large value of <i>k</i> resulting in less transmit/receive buffers causes the transmit buffer size to increase, which leads to discarding a large block of data if the content of the transmit buffer cannot be recovered due to more than (n-k) row erasures. A large transmit buffer size also increases the memory requirement at the destination terminal.

Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.