Patent Description:
There is also a need for keeping communications secure, including keeping communications private and/or free from tampering. Further, there is a need to keep communications from being intentionally or unintentionally blocked, jammed, or otherwise interfered with. Thus, various techniques, such as encryption, forward error correction (FEC), and the like have been implemented to accomplish these goals.

However, there is a constant need to improve communication transmission and reception, whether based on improved interception and interference by nefarious individuals, or simply to improve the efficiency and effectiveness of communications systems.

The document <CIT> discloses a method and an apparatus for channel equalization in high speed S-RIO based communications systems. In particular, the document discloses a BER monitoring function, which can be paused, and an adaptive DFE functionality.

Further, the document <CIT> discloses an example for a system and a method for forward error correction.

The invention is defined by independent claims <NUM> and <NUM>.

Preferred embodiments are stipulated in dependent claims thereof.

One embodiment illustrated herein includes a method of processing signals.

The method includes receiving a signal transmission with a nb/mb encoding scheme that maps n-bit words to m-bit symbols. In this scheme, m > n. The method further includes, for a first payload data word in the transmission, determining that the first payload data word corresponds to a valid payload data word, and as a result, assigning a first reliability metric to bits in the first payload data word. The method further includes for a second payload data word in the transmission, determining that the second payload data word does not correspond to a valid payload data word, and as a result, assigning a second reliability metric to bits in the second payload data word. The method further includes performing signal decoding using the assigned reliability metrics.

Embodiments illustrated herein are directed to accomplishing communication in a fashion that improves various encoding, encryption, and/or FEC techniques. In particular, data can be transmitted using an encoding technique where a signal transmission is encoded with a nb/mb encoding scheme that maps n-bit words to m-bit symbols, where m > n. One example of this is the 8B/10b encoding scheme.

When such a signal is received through a channel, channel effects may cause errors in the signal. For certain FEC schemes, a reliability metric, such as a log likelihood ratio (LLR) can be used to facilitate the FEC, where the reliability metric provides an indication as to likelihood of a bit (or symbol) being without error. Embodiments illustrated herein are able to assign a reliability metric for various bits based on evaluation of payload data words received in the signal.

In particular, a given transmission using a particular encoding scheme has expected payload data words and expected flag words. Expected payload data words are data words that are valid data words that might be sent in the data payload. That is, payload data words should contain the useful data that will be used by the recipient, rather than data that is used as part of the data transmission and reception process. In contrast, flag words are used for framing and other tasks, such as for marking the start and end of data frames, to transmit comma characters between frames for alignment, to transmit clock correction characters, to mark the start of a sub data frame, etc. Further, certain encoding schemes may be such that they exhibit a tendency to achieve DC-balance whereby bits are transmitted in a fashion so as to cause the mean amplitude of a waveform to be zero, and thus have no DC bias. A disparity error can be detected in a payload data word when the payload data word tends to move the signal away from DC-balance.

Embodiments illustrated herein can assign a relatively high reliability metric for one or more bits in a payload data word that matches an expected payload data word. Bits in a payload data word may be assigned a low reliability metric for one or more bits in a payload data word that matches a known flag word (when a data word is expected) or that does not match any expected payload data words, inasmuch as it can be determined that bits in the payload data word are obviously incorrect as they do not match an expected payload data word.

In some embodiments, an intermediate level reliability metric may be assigned for one or more bits in a payload data word for a word that tends to cause a disparity error, particularly when the disparity error is of a level that tends to indicate only a single bit error. Generally, disparity errors tend to be caused by payload data words that have a single bit error. Thus, most of the bits in the payload data word that causes a disparity error are likely to be correct.

Additional details are now illustrated. Referring now to <FIG>, an example system is illustrated. In the example illustrated in <FIG>, information is transported via a free space optical link from an optical transmitter <NUM> to an optical receiver <NUM>. For example, to transport information via a free space optical link at rates from <NUM> to <NUM> Gbits per second, embodiments may use commonly available networking optical components such as Small Formfactor Pluggable (SFP) modules as the electrical to optical transducers and optical to electrical transducers to convert electronic signals to optical signals and vice versa respectively. For example, the transmitter <NUM> comprises a transmitting transducer <NUM> that convers electrical signals to optical signals. The receiver <NUM> comprises a receiving transducer <NUM> that converts optical signal to electrical signals. The transmitting transducer <NUM> and receiving transducer <NUM> may be, for example, optical SFPs. Alternatively, or additionally, the receiving transducer <NUM> may include, for example, a photodiode and comparator.

Note that while the transmitter <NUM> and receiver <NUM> appear in <FIG> to be in very close proximity, in practice, the transmitter <NUM> and receiver <NUM> may be quite distant from each. For example, embodiments can be implemented where the transmitter <NUM> is hundreds, or even thousands (e.g., in free-space optical cross links used in satellite communications) of miles from the receiver <NUM>.

Some embodiments use on/off keying modulation and demodulation paired with Low-Density Parity Check (LDPC) forward error correction for free space optical communications. In some example embodiments illustrated herein, nb/mb (e.g., 8b/10b) decoding errors can be used for assigning, determining and/or calculating reliability metrics for LDPC decoders. Additionally, using of 8b/10b k characters for the wave form sync-word and idle sequence can provide relatively quick lock/relock which is beneficial in deep fading environments, such as those caused by atmospheric scintillation. Some embodiments illustrated herein are designed with an ability to transport data over free space using low cost integrated optical fiber to electrical modules intended for fiber optic networks. 10Gbit SFP modules are one example of such modules. In some embodiments, sync patterns can be optimized for both usefulness in a high scintillation and ease/cost of implementation.

In some embodiments, using free space optical communications, systems can implement communications that are protected in that they have a low probability of detection, low probability of interception, low probability of jamming, and/or low probability of geo location. In particular, free space optical communications can be implemented in a fashion where spread of the signal is extremely low as compared to radio communications. Due to the low spread, a nefarious entity would have to physically locate the very small space which a signal is being transmitted to detect, intercept and/or jam a signal. Further, due to the on/off nature of free space communications, such communications may be time limited as well. This becomes akin to the literal needle in a haystack problem. For example, if a free space optical signal were transmitted from New York City to Washington DC (a distance of nearly <NUM> miles), the signal spread would only be about a <NUM> foot spread. In contrast, signals transmitted using traditional radio signals, assuming no radiation shaping, would have a spread that is over <NUM> times the <NUM> mile distance, and emitted in every direction, thus making the signal very easy to intercept and jam, absent other countermeasures. Thus, embodiments may use free space optical links in useful ways as described previously.

Illustrating now additional details, modulation and PHY layer framing are described for some embodiments of the invention. In some embodiments, modulation is performed by a modulator <NUM> modulating the transmitting transducer <NUM>. The modulator modulates using on/off keying such that the transmitting transducer <NUM> produces presences of light which, in one positive logic encoding scheme, is interpreted as a <NUM>, while no light is interpreted as a zero.

Data is encoded by an encoder <NUM> using nb/mb encoding, which as discussed above, in some embodiments is 8b/10b. This encoded data is provided to the modulator <NUM>, which modulates the transmitting transducer <NUM> as described above.

In one such example, flags and idle sequences comprise one 8b/10b k character (e.g., K28. <NUM>) followed by three normal symbols (e.g., D9. <NUM>) In some embodiments, flags have a negative beginning running disparity. This allows idles to be removed and added without altering the beginning running disparity of the preceding word.

Using nb/mb encoding (and in particular in some embodiments 8b/10b encoding) can be advantageous for several reasons. For example, this allows framing and alignment of the on/off keying (OOK) demodulator to be done by an existing Field-Programmable Gate Array (FPGA) SERDES for use with standardized SFP modules.

Alternatively, or additionally, this encoding syncs up very quickly as compared to other encoding methods. Free space optical links are impacted by fading due to differences in atmospheric pressure. Such fades can be greater than 35db, ideally. The clock data recovery will be able to "coast" thru such fading. In the event that it is unable to, and clock lock is lost, having an encoding scheme that locks quickly limits the impact of the deep fades.

Alternatively, or additionally, this encoding is advantageous in generation of reliability metrics, such as log likelihood ratios (LLRs), also commonly revered to as soft data. LDPC error correction works better when a probability of the received value of each bit can be determined. As will be illustrated in more detail below certain conditions may be used for assignment and/or calculation of reliability metrics. In particular, nb/mb disparity errors, not in table errors, and presence of a k-character when a data payload word is expected, can be used to generate reliability metric data.

In particular, attention is now directed once again to <FIG>, which illustrates additional details regarding the receiver <NUM>. The receiver <NUM> receives signals from the transmitter <NUM>. In particular, the receiving transducer <NUM> receives signals transmitted by the transmitting transducer <NUM> and converts the optical signal to electrical signals. Note that the signals may have been corrupted between the transmitter <NUM> and receiver <NUM>, or in some other location along the transmission path. This corruption can be detected and/or corrected using FEC techniques. In some embodiments, those FEC techniques can be aided by assigned and/or computed reliability metrics used in soft decoding techniques.

<FIG> illustrates that the receiver includes a demodulator <NUM>. The demodulator <NUM> demodulates electrical signals from the receiving transducer <NUM> to produce an nb/mb encoded signal. This nb/mb encoded signal is provided to a decoder <NUM>, which has a primary function of decoding the nb/mb encoded signal into flags or valid data words.

<FIG> further illustrates a reliability metric generator <NUM>. The reliability metric generator <NUM> is coupled to the decoder <NUM>. The reliability metric generator <NUM> and decoder <NUM> work in concert to produce reliability metrics based on the nb/mb words received by the receiver <NUM>. In particular, the receiver receives a signal transmission, the signal transmission having been encoded with a nb/mb encoding scheme that maps n-bit words to m-bit symbols, where m > n. For a first payload data word in the transmission, the decoder <NUM> and/or the reliability metric generator <NUM> determine that the first payload data word corresponds to a valid payload data word. As a result, the reliability metric generator <NUM> assigns a first reliability metric to bits in the first payload data word. The reliability metric is a probability that the first payload data word is correct, i.e., that the first payload data word is the same data word sent by the transmitter <NUM>.

For a second payload data word in the transmission, the decoder <NUM> and/or the reliability metric generator <NUM> determine that the second payload data word does not correspond to a valid payload data word. As result, the reliability metric generator <NUM> assigns a second (typically lower) reliability metric to bits in the second payload data word. Signal decoding is then performed at the receiver using the assigned reliability metrics. Signal decoding may be, for example, LDPC decoding, which makes use of the reliability metrics using soft decoding techniques can be performed.

In some embodiments, determining that the first payload data word corresponds to a valid payload data word includes matching the first payload data word to a payload data word in a list of known payload data words. For example, as illustrated in <FIG>, the receiver <NUM> may include an expected payload data words repository <NUM> which contains a dictionary or index of all valid data words that are expected by the receiver <NUM>. A received word <NUM> can be compared to the entries in the expected payload data words repository <NUM>. If the received word does not match any of the entries in the expected payload data words repository <NUM>, then assignment of a reliability metric to the received word <NUM> can be assigned based on this determination. In particular, typically a received word <NUM> will be assigned a lower reliability metric if no entry for the received word is found in the expected payload data words repository <NUM> than if an entry for the received word is found in the expected payload data words repository <NUM>. Thus, in some embodiments, determining that the second payload data word does not correspond to a valid payload data word comprises failing to match the second payload data word using comparisons to payload data words in a list of known payload data words.

In some such embodiments, a determination can be made as to how many bit or symbol flips need to be performed to make a received word match a valid data word in the expected payload data word repository <NUM>. The more symbol or bit flips that need to be performed to make a received word match a valid data word, the lower the value of the reliability metric that can be assigned to the bits of the received word. For example, a received word that only requires a single bit flip to create a valid data word will have a higher reliability metric assigned than a received word that requires multiple bit flips to create a valid data word.

Illustrating now an alternative method of determining reliability metric using bit flips, Embodiments can determine various bit flips that, when performed on the received word <NUM>, would lead to a valid data word (such as a data word included in the expected payload data words repository <NUM>) and then use information about which bits can be flipped to assign reliability metrics for the individual bits of the received word <NUM>. For example, if there are three different valid symbols that would be obtained by flipping one bit, the decode of those three symbols is used to set the reliability metric for the individual bits of the received word. This approach can be expanded to look for valid symbols from <NUM>, <NUM> or more bit flips.

In some embodiments, determining that the second payload data word does not correspond to a valid payload data word comprises matching the second payload data word to a known flag word using comparisons to flag words in a list of known flag words. For example, <FIG> illustrates the receiver <NUM> may include an expected flag data words repository <NUM> which contains a dictionary or index of all valid flag data words for the nb/mb encoding that are expected by the receiver <NUM>. A received word <NUM> can be compared to the entries in the expected flag data words repository <NUM>. If the received word matches any of the entries in the expected flag data words repository <NUM>, then assignment of a reliability metric to the received word <NUM> can be assigned appropriately. In particular, typically a received word <NUM> will be assigned a lower reliability metric if an entry for the received word is found in the expected flag data words repository <NUM>, as this indicates that it is not a valid data word.

In some embodiments, determining that the second payload data word does not correspond to a valid payload data word comprises determining that the second payload data word comprises a disparity error. That is, in typical nb/mb coding, it is desired that DC-balance is achieved. This is achieved by causing bits to be transmitted in a fashion so as to cause the mean amplitude of a waveform to be zero, and thus having no DC bias. A disparity error can be detected in a payload data word when the payload data word tends to move the signal away from DC-balance. Thus, in some embodiments, determining that the second payload data word comprises a disparity error comprises identifying a DC-balance error.

In some embodiments, assigning a second reliability metric to bits in the second payload data word includes assigning a higher reliability metric, when assigned due to disparity error, that is larger than a reliability metric that would be applied if the second payload word failed to match payload data words using comparisons in a list of known payload data words or if the second payload word matched a known flag word in a list of known flag words. That is, if a reliability metric is assigned due to a disparity error, that reliability metric may be higher than one assigned due to a received word not matching expected payload data words or matching expected flag data words. Note that in some embodiments, the size of the disparity error may affect the assigned reliability metric. Thus, for example, a higher disparity error causing a higher DC bias may result in a lower reliability metric than a lower disparity error with a lower DC bias.

In many ways nb/mb encoding, such as 8b/10b overhead, may already be similar to FEC coding overhead, in that it costs some bandwidth but brings other benefits. Thus, leveraging the features of nb/mb encoding, FEC coding can be implemented using existing functionality of SERDES FPGAs and SFP modules.

Embodiments may be implemented where communication waveforms are made up of data frames, subchannel frames and idle sequences. Data and sub channel frames may include, for example, a sequence number, a type field, the data frame payload, an end of framer flag, etc. The payload of a data frame may be sized for appropriate encoding. For example, in one embodiment, data frames contain <NUM>8b/10b symbols to implement an <NUM> bit LDPC code block.

The data type field can be used, for example, to indicate different data types. In some embodiments, the data type field may be used to indicate different LDPC codes.

A second type of frame, as noted above, is the sub channel data frame or subdata frame. It is similar to the data frame but has a smaller payload. In one embodiment, the payload of the sub data frame is <NUM> symbols (<NUM> bits before 8b/10b encoding).

Sub channel and sub channel data frames can be used, for example, to carry control and status data between nodes. The payload of the sub channel data frames used for internode communication can be encoded in a strong LDPC code, such as ½ rate code.

Embodiments may implement certain frame spacing. For example, in some embodiments, a minimum of two idle sequences will follow any data or sub channel frame That is, any two frames are separated by at least two idle sequences. In some embodiments, idle sequences will be transmitted whenever there are no frames available for transmission.

Embodiments may be implemented where data and sub frames can be intermixed.

In one specific embodiment, 8b/10b not in table errors, disparity errors and unexpected is a k character assertions will be used in generating of the reliability metrics. A symbol that decodes as a valid 8b/10b symbol has a high probability of being correct. A symbol with a disparity error likely has one or more wrong bits. A symbol which decodes as not in table or as an unexpected k-character has a higher probability of multiple bit errors.

A simple reliability metric coding scheme is implemented using just this data. Consider the following rules in one simple example: (<NUM>) Bits that were decoded from a valid symbol are assigned a high reliability metric; (<NUM>) bits decoded from a symbol with a disparity error are assigned a medium reliability metric; (<NUM>) bits that are represented by a 'not in table' or an unexpected k character are set to the lowest reliability metric.

Some embodiments may assign reliability metrics based on information about a transmission path favoring one logic state over another. For example, in many optical embodiments, in the absence of significant ambient light near the communication wavelength or in the absence of optical amplification, the odds of light being detected in the wrong location (e.g., an invalid one, assuming positive logic) is much less than light not being detected where it should (e.g., an invalid zero, assuming positive logic). This can be used when generating the reliability metrics. That is, reliability metrics for a given received bit may be assigned differently depending on whether the received bit is logical one or a logical zero based on characteristics of the transmission path that tend to favor one logical state over the other.

In some embodiments, it may be advantageous to compute and/or assign reliability metrics based on a combination of operations performed in a particular order. For example, in some embodiments, a disparity error check can be performed. If a disparity error is discovered, then bit flipping is performed to determine if an expected payload data word can be recovered by only flipping one bit. If an expected payload data word cannot be recovered by only flipping one bit, then a relatively low (as compared to some metric, such as a metric that would be assigned absent bit flipping or a metric that would be assigned when a received word is an expected data word) reliability metric is assigned. If an expected payload data word can be recovered by only flipping one bit, then a relatively high reliability metric is assigned. Note that if the disparity error is sufficiently large to indicate that two or more bits are likely incorrect, then multiple bits can be flipped, and reliability metrics assigned accordingly.

Referring now to <FIG>, a method <NUM> is illustrated. The method <NUM> includes acts for processing signals. The method <NUM> includes receiving a signal transmission, the signal transmission having been encoded with a nb/mb encoding scheme that maps n-bit words to m-bit symbols, where m > n (act <NUM>). For example, this may include receiving a signal transmission transmitted in the 8b/10b encoding scheme.

The method <NUM> further includes, for a first payload data word in the transmission, determining that the first payload data word corresponds to a valid payload data word, and as a result, assigning a first reliability metric to bits in the first payload data word (act <NUM>). For example, the first reliability metric may be an LLR.

The method <NUM> further includes, for a second payload data word in the transmission, determining that the second payload data word does not correspond to a valid payload data word, and as a result, assigning a second reliability metric to bits in the second payload data word (act <NUM>). Thus, reliability metrics are assigned based on whether or not a received data word corresponds to a valid payload data word.

The method <NUM> further includes, performing signal decoding using the assigned reliability metrics (act <NUM>). For example, LDPC or other FEC decoding can be performed based on the reliability metrics.

The method <NUM> may be practiced where determining that the first payload data word corresponds to a valid payload data word comprises matching the first payload data word to a payload data word in a list of known payload data words. Examples of this are illustrated in <FIG>, when a received word <NUM> is found in the expected payload data words repository <NUM>.

The method <NUM> may be practiced where determining that the second payload data word does not correspond to a valid payload data word comprises failing to match the second payload data word using comparisons to payload data words in a list of known payload data words. Examples of this are illustrated in <FIG>, when a received word <NUM> is not found in the expected payload data words repository <NUM>.

The method <NUM> may be practiced where determining that the second payload data word does not correspond to a valid payload data word comprises matching the second payload data word to a known flag word using comparisons to flag words in a list of known flag words. Examples of this are illustrated in <FIG>, when a received word <NUM> is found in the expected flag data words repository <NUM>. Thus, if a payload data word is expected, but a flag data word is found, it can be determined that the received word has errors, and thus an appropriate reliability metric can be assigned.

The method <NUM> may be practiced where determining that the second payload data word does not correspond to a valid payload data word comprises determining that the second payload data word comprises a disparity error. In many nb/mb encoding schemes, encoding is performed to eliminate DC bias by having a DC balance. Detecting DC bias detects a disparity error, and therefore, and error in the received word. Thus, in some embodiments, determining that the second payload data word comprises a disparity error comprises identifying a DC-balance error.

In some such embodiments, assigning a second reliability metric to bits in the second payload data word comprises assigning a higher reliability metric that is larger than a reliability metric that would be applied if the second payload word failed to match payload data words using comparisons in a list of known payload data words or if the second payload word matched a known flag word in a list of known flag words. Thus for example, a higher reliability metric is assigned for received words that match expected data words, an intermediate reliability metric is assigned for received words with disparity errors, and a lower reliability metric is assigned for received words that do not match expected data words.

The method <NUM> may be practiced where the signal transmission comprises a free space optical transmission.

The method <NUM> may further include performing one or more bit flips on the second payload data word to attempt to identify an expected data word, and wherein assigning the second reliability metric to bits in the second payload data word is based on the results of performing one or more bit flips. , single bit flip, or number of bit flips.

When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer properly views the connection as a transmission medium. Transmissions media can include a network and/or data links which can be used to carry desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer.

Claim 1:
A method (<NUM>) of processing signals the method comprising:
receiving (<NUM>) a signal transmission, the signal transmission having been encoded with a nb/mb encoding scheme that maps n-bit words to m-bit symbols, where m > n; wherein the method is characterized in that
for a first payload data word in the transmission, determining (<NUM>) that the first payload data word corresponds to a valid payload data word, and as a result, assigning a first reliability metric to bits in the first payload data word, wherein the first reliability metric indicates a probability that the first payload data word is correct;
for a second payload data word in the transmission, determining (<NUM>) that the second payload data word does not correspond to a valid payload data word, and as a result, assigning a second reliability metric to bits in the second payload data word, wherein second reliability metric indicates that a probability that the second payload data word is correct is lower than that the first payload data word is correct; and
performing (<NUM>) signal decoding using the assigned first reliability metric and the assigned second reliability metric.