Patent Application
20070195905
One of the biggest problems associated with designing and maintaining optical communications systems is compensating for the optical power loss and non-linearities, such as dispersion, that occur in the physical plant between an optical transmitter and optical receiver, and preserving a sufficient optical budget to maintain a reliable communications link. This problem often exacerbates itself over time due to degradation of the fiber medium and fiber connectors over time.
In the past, efforts to overcome optical power loss have mostly focused on improving the optical performance of the system. Such efforts included increasing the launch power of the optical signal, improving the quality of the optical transmitter's laser, improving jitter and timing characteristics at the optical receiver, improving the quality of the trans-impedance amplifiers and limiting amplifiers at the optical transmitter, and improving rise and fall times and the extinction ratio of the optical signal. The problem associated with each of these efforts is that each involves increases in equipment costs that are disproportionately expensive when compared to the amount of optical budget gained.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for economical systems and methods for improving optical budgets for communications networks.
The Embodiments of the present invention provide methods and systems for forward error correction in wideband digital RF transport systems and will be understood by reading and studying the following specification.
In one embodiment, a method for transmitting digital data in a wideband digital RF transport system is provided. The method comprises receiving a first digital signal, wherein the digital signal includes a stream of digital RF packets, each digital RF packet comprising one or more data samples representing a digitized wideband RF spectrum; encoding error correction data into the stream of digital RF packets based on an error correction algorithm to produce a stream of modified packets; and transmitting a second digital signal comprised of the stream of modified packets via a communications medium.
In another embodiment, a computer-readable medium having computer-executable program instructions for a method for communicating digital RF data in a communications network is provided. The method comprises inputting a stream of digital RF packets, wherein each digital RF packet comprises one or more data samples representing a digitized wideband RF spectrum; encoding each digital RF packet of the stream of digital RF packets with error correction data; and outputting a stream of error correction encoded data packets based on the digital RF packets.
In yet another embodiment, a communications network is provided. The network comprises means for encoding a stream of digital RF packets with error correction data based on a forward error correction algorithm, wherein each digital RF packet of the stream of digital RF packets comprises one or more data samples representing a digitized wideband RF spectrum, the means for encoding further adapted to output a stream of error correction encoded digital RF packets; and means for communicating a digital signal to a communication medium, the means for communicating responsive to the means for encoding, wherein the means for communication a digital signal is adapted to communicate the stream of error correction encoded packets to the communication medium.
In still another embodiment, a wideband digital RF transport system is provided. The system comprises a first communications network segment adapted to incorporate forward error correction data into one or more digitized data samples based on an error correction encoding algorithm, wherein the one or more digitized data samples represent a wideband RF spectrum signal, the first communications network segment further adapted to output a digital signal representing the forward error correction encoded data samples; a second communications network segment adapted to receive the digital signal and apply an error correction decoding algorithm to the error correction encoded data samples; and a communications medium coupled to the first communications network segment and the second communications network segment, the communications medium adapted to communicate the digital signal from the first communications network segment to the second communications network segment.
Embodiments of the present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the described embodiments and the following figures in which:
In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present invention. Reference characters denote like elements throughout figures and text.
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.
Embodiments of the present invention provide improved methods and systems to overcome power loss in wideband digital RF transport systems over optical fiber through the application of forward error correction on the physical layer of a communications network. Forward error correction is a system of error control for data transmission that enables a data receiving device to detect and correct data corrupted by transmission errors. Forward error correction can be implemented either on existing computer hardware or with low cost new hardware at an existing plant site. Thus, embodiments of the present invention provide a significant increase in terms of dBm available to a receiver of a digital signal without the need for modifying installed communications media used in a communication network's physical layer. Embodiments of the present invention are not error correction algorithm specific and taking advantage of future improvements in error correction algorithms are contemplated.
In embodiments of the present invention, forward error correction is accomplished by adding redundancy to wideband digital RF data using a predetermined algorithm. This redundancy allows systems receiving the wideband digital RF data to both detect whether an error has occurred, and if so, to restore the correct information. The particular forward error correction algorithm chosen is readily determined by one skilled in the art upon reading this specification, from currently existing or yet to be developed algorithms. As embodiments of the present invention are not forward error correction technique specific, embodiment taking advantage of future improvements in forward error correction algorithms and coding techniques are contemplated as within the scope of embodiments of the present invention.
To provide practical transport in system 100, there must be a sufficient optical power margin within the system to overcome signal losses caused by communication medium 130. Physical components necessary to provide the necessary transport budget can be of significant cost and complexity depending on the digital method of transport. To reduce this cost and increase the loss budget, forward error correction (FEC) is encoded into the digital transport signal by first communications network segment 110 prior to transmitting the signals on communication medium 130. As would be appreciated by one skilled in the art, forward error correction techniques, which can often be implemented using existing hardware at many installations, allow the use of less expensive physical components for system 100 with an equivalent loss budget as that of non-forward error correction systems utilizing more expensive physical medium transport components.
In one embodiment, a wideband RF spectrum signal enters first communications network segment 10 and is converted into the digital signal by analog to digital converter 112. In one embodiment, the digital signal comprises a string of digital RF packets distributed across a plurality of bands of the wideband digital RF transport. In one embodiment, each digital RF packet includes a complex data sample of the wideband RF spectrum signal. In one embodiment, the digital RF packets comprise a 16-bit complex data sample of the wideband RF spectrum signal. First communications network segment 110 further comprises a forward error correction encoder (FEC-EN) 115, a wideband signal multiplexer 120 (MUX), and a signal transmitter 125.
In one embodiment forward error correction encoder 115 (FEC-EN) inputs the wideband digital signal from analog to digital converter 112 and evaluates the contents of the digital RF packets traveling though each band of the wideband digital signal. Forward error correction encoder 115 executes a predetermined forward error correction encoding algorithm that inputs the digital signal from analog to digital converter 112, calculates forward error correction data, and incorporates the forward error correction data into each of the digital RF packets to produce an FEC modified packet.
In one embodiment, forward error correction encoder 115 applies an error correction encoding algorithm based on one or both of block coding and convolutional coding. Examples of block coding include, but are not limited to Reed-Solomon coding, Golay coding, BCH coding and Hamming coding. Examples of convolutional coding include, but are not limited to the Viterbi algorithm. In one embodiment, forward error correction encoder 115 applies an encoding algorithm based on turbo coding, a scheme that combines two or more relatively simple convolutional codes and an interleaver to produce a block code. Examples of turbo coding include, but are not limited to 1xEV-DO (TIA IS-856). Other forms of forward error correction algorithms which may be utilized by embodiments of the present invention include, but are not limited to check bits, digital fountain code, differential space-time code, erasure codes, binary Golay codes, ternary Golay codes, Hagelbarger code, Hamming code, a low-density parity-check (LDPC) code, Reed-Muller code, and space-time trellis codes (STTCs). As would be appreciated by one skilled in the art upon reading this specification, the choice of which forward error correction algorithm can be readily determined by based on network specifics.
In one embodiment, multiplexer 120 inputs the FEC modified packets received from forward error correction encoder 115 and multiplexes the FEC modified packets into a serial stream of packets for serial transmission on communications medium 130. In an alternate embodiment, instead of locating forward error correction encoder 115 before multiplexer 120 in the signal path, multiplexer 120 multiplexes the digital RF packets from A/D 112 into a serial stream of packets and a forward error correction encoder (FEC-EN) (shown at 116) evaluates the contents of the digital RF packets traveling though serialized stream of packets to calculate forward error correction data as described above with respect to forward error correction encoder 115, and incorporates the forward error correction data into each of the digital RF packets to produce an FEC modified packet. Signal transmitter 125 transmits the stream of FEC modified packets to the second communications network segment 150 via communications medium 130.
In the embodiment shown in
In one embodiment, the second communications network segment 150 comprises a signal receiver 135, a wideband signal demultiplexer 140 (DE-MUX), and a forward error correction de-coder (FEC-DC) 145, and a digital to analog converter 142.
Signal receiver 135 receives the digital signal comprising a string of FEC modified packets and communicates those packets to wideband signal demultiplexer 140. Wideband signal demultiplexer 140 in turn demultiplexes the packets from the serialized stream back into a digital wideband signal. Forward error correction decoder 145 reads the FEC modified packets from the wideband signal output of demultiplexer 140 and evaluates the forward error correction data incorporated into each FEC modified packet to determine whether the digital RF packet arrived error-free. When the packet is determined to be error-free, forward error correction decoder 145 strips the forward error correction data to restore the original digital RF packet and outputs the restored digital RF packet. In one embodiment, forward error correction decoder 145 forwards the restored digital RF packet to digital to analog converter 142. In one embodiment, when an error is detected within an FEC modified packet, forward error correction decoder 145 reconstructs the original digital RF packet based on information contained within the forward error correction data as described above, and outputs a corrected digital RF packet. In one embodiment, digital to analog converter 142 digital RF packets received from forward error correction decoder 145 into a wideband RF spectrum signal.
In an alternate embodiment, instead of locating forward error correction decoder 145 after demultiplexer 140 in the signal path, a forward error correction decoder (FEC-DC) (shown at 146) evaluates the contents of the serialized stream of packets digital RF packets from communications medium 130 before demultiplexer 140 in turn demultiplexes the packets from the serialized stream back into a digital wideband signal.
In one embodiment, communications network 200 is a bidirectional network and as shown includes equipment for forward links (i.e., transmissions on forward logical channels from upstream communications networks 210 to subscriber units 260) and reverse links (i.e., transmissions on reverse logical channels from subscriber units 260 to communications networks 210). In one embodiment, base station 220 includes functionality, implemented in one or both of hardware and software, for digitally performing waveform processing to modulate and demodulate radio signals transmitted and received, respectively, by remote unit 240.
In the embodiment shown in
In the embodiment shown in
In one embodiment, in the forward direction, base station 220 receives data in the form of representations of voice/data signals from upstream communications network 210. Call processing software module 221 generates digital representations of the voice/data signals from upstream communications network 210 into a stream of digital RF packets and provides the digital RF packets to digital up converter 222. In one embodiment, call processing software 221 modulates the representations of voice/data signals to a predetermined intermediate (i.e. baseband) frequency producing digital RF packets each comprising baseband modulated RF data samples representing the voice/data signals from upstream communications network 210.
Digital up converter 222 inputs the baseband RF data samples and remodulates the RF data samples to a frequency for one of a plurality of RF bands. Digital up converter 222 outputs the remodulated digital RF packets as a wideband digital signal representation of the voice/data signals. In one embodiment, digital up converter 222 converts a set of baseband RF data samples from the baseband frequency into a digital stream of complex digital RF data samples modulated at a center frequency designated for broadcasting with a particular RF channel assigned to the particular forward logical channel used to transport the digital RF packets. Increasing the center frequency of digital RF data samples results in remote unit 240 transmitting an RF signal at a higher radio frequency, while decreasing the center frequency of digital RF data samples results in remote unit 240 transmitting an RF signal at a lower radio frequency.
To provide forward error correction for the digital RF packets traveling towards subscriber unit 260, in one embodiment forward error correction encoder 224 (FEC-EN) inputs the wideband digital signal from digital up converter 222 and evaluates the contents of the digital RF packets traveling though each band of the wideband digital signal. Based on the contents of the digital RF packets, forward error correction encoder 224 incorporates forward error correction data into each of the digital RF packets. In one embodiment, multiplexer 226 inputs the digital RF packets received from forward error correction encoder 224 and multiplexes the digital RF packets into a serial stream of packets for serial transmission on communications medium 230.
In an alternate embodiment, the relative positions of multiplexer 226 and forward error correction encoder 224 within the signal path are reversed. In that case, multiplexer 226 inputs the digital RF packets from digital up converter 222 and multiplexes the digital RF packets into a serial stream of packets before forward error correction encoder 224 inputs the digital RF packets. In that case, forward error correction encoder 224 inputs the serial stream of packets from multiplexer 226, evaluates the contents of the digital RF packets, and incorporates forward error correction data into each of the digital RF packets before outputting the stream of digital RF packets to signal transmitter 228. Signal transmitter 228 transmits the stream of FEC modified digital RF packets to remote unit 240 via communications medium 230. In one embodiment, when communications medium 230 includes an optical fiber media, signal transmitter 228 is an optical transmitter such as, but not limited to a laser transmitter.
As shown in
Remote unit 240 receives the serialized stream of FEC modified digital RF packets from host unit 220 via signal receiver 242. In one embodiment, when signal receiver 242 is an optical receiver, signal receiver 242 includes one or more of a trans-impedance amplifier, a limiting amplifier and any clock and data recovery functionality necessary to convert an optical signal into an amplified electronic signal.
In one embodiment, a wideband signal demultiplexer 244 receives the amplified electronic signal from signal receiver 242 and de-multiplexes the serialized packets into a wideband signal. Forward error correction decoder 246 reads FEC modified digital RF packets from the wideband signal output of demultiplexer 244 and evaluates the forward error correction data from each FEC modified digital RF packet to determine whether the digital RF packet arrived error-free. When the packet is determined to be error-free, forward error correction decoder 246 strips the forward error correction data from the digital RF packet and forwards the digital RF packet to digital to analog converter 248. In one embodiment, when an error is detected within a FEC modified digital RF packet's RF data sample, forward error correction decoder 246 reconstructs a corrected RF data sample based on information contained within the forward error correction data as described above, and outputs a corrected digital RF packet to digital to analog converter 248. Digital to analog converter 248 converts digital RF packets received from forward error correction decoder 246 into an analog wideband RF signal which is wirelessly transmitted to one or more subscriber units 260 by wireless transmitter 250 via antenna 252.
In an alternate embodiment, forward error correction decoder 246 inputs the serialized stream of FEC modified digital RF packets received by signal receiver 242 and evaluates the forward error correction data from each digital RF packet to determine whether the digital RF packet arrived error-free, as described above. In that case, wideband signal demultiplexer 244 receives the digital RF packets from forward error correction decoder 246 and de-multiplexes the serialized packets into the wideband signal for processing by digital to analog converter 248 as described above.
In the embodiment shown in
In one embodiment, wireless receiver 270 receives RF signals via antenna 252 from one or more subscriber units 260. In one embodiment, wireless receiver 270 receives RF signals in multiple RF bands. Analog to digital converter 268 converts the analog RF signals into digital RF packets and outputs the digital RF packets as a wideband digital signal.
To provide forward error correction for the digital RF packets traveling towards upstream communications network 210, in one embodiment forward error correction encoder 266 (FEC-EN) inputs the wideband digital signal from analog to digital converter 268 and evaluates the contents of the digital RF packets traveling though each band of the wideband digital signal. Based on the contents of the digital RF packets, forward error correction encoder 266 incorporates forward error correction data into each of the digital RF packets.
In one embodiment, multiplexer 264 inputs the FEC modified digital RF packets received from forward error correction encoder 266 and multiplexes the digital RF packets into a serial stream of packets for serial transmission on communications medium 235.
In an alternate embodiment, multiplexer 264 inputs the digital RF packets directly from analog to digital converter 268 and multiplexes the digital RF packets into a serial stream of packets before forward error correction encoder 266 inputs the digital RF packets. In that case, forward error correction encoder 266 inputs the serial stream of packets from multiplexer 264, evaluates the contents of the digital RF packets, and incorporates forward error correction data into each of the digital RF packets before outputting the stream of FEC modified digital RF packets to signal transmitter 262. Signal transmitter 262 transmits the stream of FEC modified digital RF packets to base station 220 via communications medium 235. In one embodiment, when communications medium 235 includes an optical fiber media, signal transmitter 262 is an optical transmitter such as, but not limited to a laser transmitter.
As shown in
Signal receiver 288 receives the serialized stream of FEC modified digital RF packets from remote unit 240. In one embodiment, when signal receiver 288 is an optical receiver, signal receiver 288 includes one or more of a trans-impedance amplifier, a limiting amplifier and any clock and data recovery functionality necessary to convert an optical signal into an amplified electronic signal. In one embodiment, wideband signal demultiplexer 286 receives the amplified electronic signal from signal receiver 288 and de-multiplexes the serialized packets into a wideband signal. Forward error correction decoder 284 reads the FEC modified digital RF packets from the wideband signal output of demultiplexer 286 and evaluates the forward error correction data from each FEC modified digital RF packet to determine whether the digital RF packet arrived error-free. When the packet is determined to be error-free, forward error correction decoder 284 strips the forward error correction data from the digital RF packet and forwards the digital RF packet to digital down converter 282. In one embodiment, when an error is detected within a FEC modified digital RF packet's RF data sample, forward error correction decoder 284 reconstructs a corrected RF data sample based on information contained within the forward error correction data as described above, and outputs a corrected digital RF packet to digital down converter 282. Digital down converter 282 inputs the digital RF packets and remodulates RF data samples within the packets from an RF broadcast channel frequencies to a predetermined baseband frequency. Digital down converter 282 outputs the remodulated RF data samples to call processing software module 221. In one embodiment, call processing software module 221 generates voice/data signals from the RF data samples for transmission to upstream communications network 210.
In an alternate embodiment, forward error correction decoder 284 inputs the serialized stream of FEC modified digital RF packets received by signal receiver 288 and evaluates the forward error correction data from each FEC modified digital RF packet to determine whether the digital RF packet arrived error-free, as described above. In that case, wideband signal demultiplexer 286 receives the digital RF packets from forward error correction decoder 284 and de-multiplexes the serialized packets into the wideband signal for processing by digital down converter 282 as described above.
It would be appreciated by one skilled in the art that embodiments of the present invention, such as communications network 200, are not limited to communications networks based on any one RF modulation protocol. To the contrary, embodiments of the present invention handle multiple types of modulation protocols, and in different embodiments, one or more of the forward and reverse logical channels transmit data using a different modulation protocol than other forward and reverse logical channels. In one embodiment, base station 220 and remote unit 240 handle modulation protocols including one or more of, but not limited to, Advanced Mobile Phone System (AMPS), code division multiple access (CDMA), Wide-band CDMA (WCDMA), time division multiple access (TDMA), Global System for Mobile communications (GSM), Cellular Digital Packet Data (CDPD), Enhanced Data rates for GSM Evolution (EDGE), General Packet Radio Service (GPRS), Integrated Digital Enhanced Network (iDEN), Orthogonal Frequency Division Multiplexing (OFDM), High Speed Downstream Packet Access (HSDPA), or any other appropriate modulation protocol.
In one embodiment, to reduce the potential for errors before the FEC modified packets are transmitted on the communications medium 330, first communications network segment 301 comprises a pre-emphasis function 315 that processes FEC modified packets prior to their transmission by signal transmitter 320. Pre-emphasis boosts high frequency components of the voice/data signals represented by digital data while leaving the lower frequency components at their original levels, thus effectively increase the signal to noise ratio of the digital data prior to transmitting the FEC modified packets via communication media 330.
In one embodiment, second communications network segment 302 comprises a signal receiver 340 for receiving the FEC modified packets via a communications medium 330. In one embodiment, when signal receiver 340 is an optical signal receiver, signal receiver 340 includes a trans-impedance amplifier 342, a limiting amplifier 344 and a clock and data recovery function 346 necessary for converting an optical signal into an amplified electronic signal. In one embodiment, signal receiver 340 further includes a receiver equalization function 348 which processes the digital data received via communication media 330 prior to clock and data recovery function 346 as illustrated in
In one embodiment, the method continues at 440 with receiving the digital signal from the communications medium, and proceeds to 450 with decoding the stream of modified packets. Decoding the modified packets determines whether the stream of modified packets arrived error-free based on the error correction data encoded into the packets. In one embodiment, when a modified packet is determined to have been received error-free, the method removes the error correction data from the modified packet and outputs the original digital RF packet. When a modified packet is determined to have arrived with one or more errors, the method reconstructs a digital RF packet based on the error correction data encoded in the modified packet, and outputs a corrected digital RF packet. In one embodiment, the method continues with generating an analog wideband RF spectrum signal based on the decoded digital RF packets received from the communications medium. In another embodiment, data samples within the decoded digital RF packets are down-converted from a broadcast channel frequency to a baseband modulation frequency, as described with respect to
Several means are available to implement the forward error correction encoders and decoders, pre-emphasis functions and receiver equalization functions discussed above. These means include, but are not limited to, digital computer systems, programmable controllers, or field programmable gate arrays. Therefore other embodiments of the present invention are program instructions resident on computer readable media which when implemented by such processors, enable the processors to implement embodiments of the present invention. Computer readable media include any form of computer memory, including but not limited to punch cards, magnetic disk or tape, any optical data storage system, flash read only memory (ROM), non-volatile ROM, programmable ROM (PROM), erasable-programmable ROM (E-PROM), random access memory (RAM), or any other form of permanent, semi-permanent, or temporary memory storage system or device. Program instructions include, but are not limited to computer-executable instructions executed by computer system processors and hardware description languages such as Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL).
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.
