Abstract:
A Coded Orthogonal Frequency Division Multiplexing (COFDM) communication systems is implemented in which each subcarrier data stream is individually FEC encoded instead of FEC encoding the overall input data stream as implemented in conventional COFDM systems. Specifically, each subcarrier is independently encoded using pseudo orthogonal QPSK M-QAM FEC modulators, transmitted, and decoded using pseudo orthogonal QPSK M-QAM FEC demodulators. Multiplexers and demultiplexers randomize subcarrier symbols across all subcarriers to prevent contiguous subcarrier data errors due to fading or corruption by narrowband interference. This technology can be applied to many wireless and wired communication systems including wireless underwater RF communications.

Description:
RELATED INVENTION 
     The present invention claims priority under 35 U.S.C. §119(e) to: “COFDM using Pseudo Orthogonal QPSK Coding Method and Apparatus” Provisional U.S. Patent Application Ser. No. 61/861,999, filed 3 Aug. 2013 which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the field of communications. More specifically, the present invention relates to a Coded Orthogonal Frequency Division Multiplexing (COFDM) wireless communication systems. 
     BACKGROUND OF THE INVENTION 
     Coded systems, which combine both Orthogonal Frequency Division Multiplexing (OFDM) and channel coding techniques, are used in communication systems to efficiently transmit high rate signals in fading channels. Due to the wide bandwidth of these signals they would normally suffer from severe frequency selective fading or isolated narrowband interference. 
     This is avoided in an OFDM system by dividing the data into multiple parallel data streams or channels, one for each subcarrier, where the transmission in each individual subcarrier experiences only flat frequency fading. Each subcarrier is modulated with a conventional modulation scheme such as quadrature amplitude modulation (QAM) at a low symbol rate, maintaining total data rates similar to conventional single carrier modulation schemes in the same bandwidth. Forward Error Correction (FEC) coding is used to combat errors resulting from noise, faded OFDM subcarriers, or isolated narrowband interference. 
     One of the primary disadvantages of OFDM is that it possesses a high peak-to-average-power ratio (PAPR) that requires the use of linear power amplifiers which are less efficient than nonlinear ones. Amplifier nonlinearity exhibits amplitude and phase distortions which cause loss of orthogonality among the subcarriers and introduces inter-carrier interference in the transmitted signal which increases bit error rate (BER). A number of techniques have been postulated to reduce PAPR but all suffer from either high complexity or low PAPR reduction. Amplitude clipping is the simplest technique but increases BER. 
     Accordingly, it is the object of the present invention to disclose methods which provide improved COFDM wireless communication system performance by reducing required signal-to-noise-ratio (SNR) and increasing immunity to amplifier nonlinearity. 
     SUMMARY OF THE INVENTION 
     Briefly, to achieve the desired object of the present invention, pseudo orthogonal QPSK coding will be applied to each OFDM subcarrier instead of applying FEC coding to the overall source data as in current COFDM systems. Pseudo orthogonal QPSK coding is described in commonly owned U.S. Pat. Nos. 8,098,773 and 8,437,431 which are incorporated herein by reference. 
     Pseudo orthogonal QPSK encodes message data using rate ½ Viterbi encoding. Symbols are decoded using an efficient sequential decoding algorithm in which all paths through the decoder tree are retained until it is certain that a particular path cannot be the correct path. It is a Shannon bound approaching code with superior error correction ability that can be used on short data packets. 
     Other objects and advantages of the present invention will become obvious as the preferred embodiments are described and discussed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a generic block diagram of a typical COFDM system. 
         FIG. 2  illustrates multiple OFDM orthogonal subcarriers. 
         FIG. 3  illustrates an OFDM signal in the time domain along with its PAPR value. 
         FIG. 4  illustrates the CCDF of a 64 subcarrier OFDM signal. 
         FIG. 5  illustrates the effect of clipping on a high SNR 16-QAM OFDM constellation. 
         FIG. 6  illustrates typical BER degradation due to clipping for a 16-QAM OFDM signal. 
         FIG. 7  illustrates a functional block diagram of the present invention CODFM system. 
         FIG. 8  illustrates a RAM implementation of a COFDM symbol multiplexer. 
         FIG. 9  shows data packet format prior to M-QAM FEC encoding, after encoding, prior to decoding, and after decoding. 
         FIGS. 10   a  and  10   b  show versions of 16-QAM and 64-QAM constellations respectively for pseudo orthogonal QPSK M-QAM FEC modulation. 
         FIG. 11  illustrates typical attenuation of sea water in an underwater RF communication system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The primary goal of the present invention is to provide improved COFDM wireless communication system performance by reducing required SNR and reducing the effects of nonlinear amplifiers on BER. 
     A generic functional block diagram  80  of a typical COFDM system is illustrated in  FIG. 1 . High speed data input  5  is FEC encoded and interleaved  10 . Interleaving is required to convert contiguous burst errors into separate single errors after final deinterleaving at the receiver. Encoded data  12  is serial to parallel converted  15  into parallel data streams  17  and functionally each data stream is modulated onto orthogonal subcarriers using M-QAM modulators  20 . The frequency domain modulated subcarriers  22  are Inverse Fast Fourier Transformed (IFFT)  25  into a time domain signal that is amplified and transmitted  30  as a high speed analog waveform through channel  35 . 
     M-QAM is m-ary quadrature amplitude modulation where m equal 2 is BPSK, m equal 4 is QPSK, m equal 16 is 16-QAM, and m equal 64 is 64-QAM. 
     Mathematically, modulating orthogonal waveforms and adding them is equivalent to taking a IFFT. This is because the time domain representation of OFDM is made up of different orthogonal sinusoidal signals which are nothing but an inverse Fourier transform. The IFFT can be thought of as converting frequency domain samples (modulation symbols created by M-QAM modulators  20 ) to time domain samples  26 . 
     The high speed time domain analog waveform from channel  35  is received using receiver  40  and Fast Fourier Transformed (FFT)  45  to convert it back to modulated subcarriers in the frequency domain. Each subcarrier is demodulated using parallel M-QAM demodulators  50 . Parallel to serial converter  55  converts the demodulated parallel subcarrier data back to high speed serial data which is deinterleaved and FEC decoded  60  to recover the original high speed data  65 . 
     All blocks, except transmitter  30 , channel  35 , and receiver  40  in  FIG. 1  are normally implemented digitally using high speed processors, Field Programmable Gate Arrays (FPGAs), Application Specific Integrated Circuits (ASCIs), or other digital means as is well known by those skilled in the art. 
     The multiple OFDC subcarriers are illustrated in  FIG. 2  in which all subcarriers  70  are orthogonal. The instantaneous amplitude of the transmitted signal is illustrated in  FIG. 3  which indicates peak amplitude  85 , average amplitude  90 , and PAPR  95 . 
     It is customary to use the Complementary Cumulative Distribution Function (CCDF) of the PAPR as a performance criterion. The CCDF of the PAPR is defined as the probability that the PAPR per OFDM symbol exceeds a certain clipping level. The CCDF for 64 OFDM carriers is illustrated in  FIG. 4 . This figure shows that the peak amplitude exceeds the average amplitude by 8 dB only approximately 10% of the time. If an 8 dB clipping level is implemented, then the PAPR would be 8 dB. 
     The effect of clipping on the constellation diagram for a high SNR 16-QAM OFDM subcarrier is illustrated in  FIG. 5 . Clipping can cause Amplitude Modulation (AM) to AM conversion whose effect is to scatter the received symbol constellation. Clipping can also cause AM to Phase Modulation (PM) conversion whose effect is to slightly rotate the received symbol constellation. Both scattering and constellation rotation are illustrated in  FIG. 5 . The end effect is BER degradation as illustrated by curve  97  in  FIG. 6  compared with curve  96  for 16-QAM without clipping. 
     Standard coding methods used in COFDM are designed to correct errors due to noise and faded subcarriers. However, their error correcting ability can be easily exceeded by the multitude of errors that can occur due to clipping as illustrated by the symbol constellation in  FIG. 5  where many symbols are in error or nearly in error even at high SNR. 
     A functional block diagram  100  of the present invention COFDM communication system is illustrated in  FIG. 7 . Again all blocks, except transmitter  30 , channel  35 , and receiver  40  in  FIG. 7  are digitally implemented using high speed processors, Field Programmable Gate Arrays (FPGAs), Application Specific Integrated Circuits (ASCIs), or other digital means. However, to facilitate understanding, each function is described and illustrated as a separate block even though some of the blocks can and will be combined and implemented digitally. 
     FEC encoder and interleaver  10  of  FIG. 1  and deinterleaver and FEC decoder  60  of  FIG. 1  are eliminated and high speed data input  5  is serial to parallel converted  15  into multiple slower parallel output data streams  17 . Parallel M-QAM FEC modulators  120  create M-QAM FEC symbols  125  for each data stream. These M-QAM FEC symbols  125  are multiplexed in symbol multiplexer  130  to spread the symbols from any one parallel data stream across all data streams to prevent contiguous errors on any particular channel due to a faded subcarrier or narrowband interference. 
     Parallel data streams of multiplexed M-QAM FEC symbols  135  in the frequency domain each modulate an OFDM subcarrier, are summed together, and converted to the time domain in IFFT  25 . This signal is amplified and transmitted  30  as a high speed analog waveform through channel  35  as in  FIG. 1 . 
     The signal from channel  35  is received using receiver  40  and FFT  45  converts it back to parallel modulated subcarriers  150  in the frequency domain. Symbol demultiplexer  165  demultiplexes the symbols back to the same parallel order they had prior to being multiplexed by symbol multiplexer  130 . 
     Parallel M-QAM FEC demodulators  175  decodes these parallel QAM FEC symbol streams  170  back to parallel data streams which are serialized again in parallel to serial converter  55  thus completing the communication system link. 
     Symbol multiplexer  130  and symbol demultiplexer  165  can each be functionally implemented as a simple random access memory (RAM)  260  operating at the symbol period as illustrated in  FIG. 8 . For example, assume that 64 subcarriers (0 to 63) are implemented. Each symbol period, the 64 parallel symbols from M-QAM FEC modulators  120  are written to RAM  260  memory locations  265  using consecutive addresses from 0 to 63 generated by write address pointer  270 . On the first symbol period, 64 symbols are read from RAM  260  memory locations  265  using consecutive addresses from 0 to 63 generated by read address pointer  275 . At the second symbol period, read address pointer  275  uses consecutive addresses starting at address 1. At the third symbol period, read address pointer  275  starts at address 2 and so on in which the starting address is incremented by 1 for each consecutive symbol period. In this manner, consecutive data packet symbols from each channel of M-QAM FEC modulators  120  are spread across all subcarriers  70  illustrated in  FIG. 2 . 
     Symbol demultiplexer  165  operates in the same manner to reassemble parallel received symbols  150  from FFT  45  into the original M-QAM FEC modulators  120  parallel output order for M-QAM FEC demodulators  175 . Furthermore, symbol multiplexer  130  and symbol demultiplexer  165  can be implemented to commence at a random address and proceed using non-consecutive addresses, including a repeating sequence of random addresses, if desired. This would further randomize the symbols across subcarriers to combat situations in which consecutive subcarriers are faded. 
     Symbol multiplexer  130  and symbol demultiplexer  165  can be eliminated in communication system applications where frequency selective fading or isolated narrowband interference does not occur. Examples are line of sight satellite and microwave links, and wired communication links in which multipath or interference is not a problem. 
     Applying FEC coding to each subcarrier instead of the overall data input allows more errors to be corrected. The number of errors that must be corrected in  FIG. 1  is equal to the sum of errors that occur in each individual subcarrier. The number of errors that must be corrected in  FIG. 7  is equal to the errors that occur in a single subcarrier. 
     Although any type of M-QAM FEC coding could be implemented in the present invention, pseudo orthogonal QPSK coding provides the most benefit. As explained in U.S. Pat. Nos. 8,098,773 and 8,437,431, pseudo orthogonal QPSK coding is very different than traditional coding methods. It is a Shannon bound approaching code that is error free as long as the correct path through the decoding tree is not eliminated. Closeness to the bound is primarily a function of hardware resources implemented to retain all paths through the decoder tree until a path is determined to not be the correct path. 
     The code works with small data packets, unlike turbo codes that require very large blocks of data and long interleaver latencies to approach the Shannon bound. Since the code works with short data packets, coding can be applied to each individual data channel of the M-QAM FEC modulators  120  instead of having to be applied to the total data across all subcarriers as in current COFDM systems as illustrated by blocks  10  and  60  of  FIG. 1 . The pseudo orthogonal QPSK code is also self randomizing so the use of large interleavers along with their associated large latency is avoided. 
     Pseudo orthogonal QPSK coding is most susceptible to consecutive or nearly consecutive large symbol errors which can erroneously eliminate the correct path. It is not susceptible to the low amplitude errors illustrated in  FIG. 5  because multiple small errors (like noise) occur on all symbols, equally increases all path metrics, and will not eliminate the correct path. 
     To ensure that the correct path is identified, each parallel data stream out of serial to parallel converter  15  in  FIG. 7  can be divided into data packet bits  205  prior to sending them to the pseudo orthogonal QPSK M-QAM FEC modulator  212  as illustrated in  FIG. 9 . 
     To flush out the correct path at the end of each data packet bits  205 , each encoder data packet  204  will include a few postamble bits prior to being encoded in pseudo orthogonal QPSK M-QAM FEC modulator  212  so as to end the encoded data packet  215  in the encoder zero state as illustrated in  FIG. 9 . 
     Encoded data packet  215  consists of encoded data packet symbols  220  created from data packet bits  205  and encoded postamble symbols  225  created from postamble bits  210  which consists of a number of “zero” encoder input bits, the number related to the constraint length of the Viterbi encoder, which returns pseudo orthogonal QPSK M-QAM FEC modulator  212  Viterbi encoder to its zero state. 
     The convolutional decoder described in U.S. Pat. Nos. 8,098,773 and 8,437,431 retains all paths through the decoder tree until it is certain a path is not the correct path. At the end of the data packet, in a noisy communication environment, the minimum metric path may not be the correct path. 
     When encoded data packet  215  is received, a number of noiseless locally generated zero symbols  230  are added to the end of received encoded data packet  215  prior to decoding. Since added local zero symbols  230  are noiseless, they quickly eliminate all incorrect paths thus defining the correct path in pseudo orthogonal QPSK M-QAM FEC demodulator  232  and thus the correct decoded data packet  235 . The decoded postamble bits  240  of decoded data packet  235  are discarded and the decoded data packet bits  238  (same as original transmitted data packet bits  205 ) are sent to parallel to serial converter  55 . 
     Currently, most COFDM systems are designed to increase data rate and combat fading in high speed wireless communication systems such as cellular, Wi-Fi, etc. For these applications, the rate ½ pseudo orthogonal 16-QAM constellation illustrated in  FIG. 10   a  provides 2 bits/symbol and the 64-QAM constellation illustrated in  FIG. 10   b  provides 3 bits/symbol. 
     Both of these constellations provide low BER at much lower SNR than conventional COFDM implementations. This allows lower transmitter power to be used for equivalent communications range. Operating the same transmitter linear amplifiers at lower power reduces their amplitude and phase distortion that causes loss of orthogonality among the subcarriers and introduces inter-carrier interference. 
     Another potential COFDM application of the present invention is for underwater RF where high increasing attenuation of water at increasing frequency is the most important problem. The attenuation of sea water with frequency is approximately 1 dB per meter at 1 KHz and 3.5 dB per meter at 10 KHz as illustrated in  FIG. 11 . Using multiple COFDM subcarriers as described in this application provides nearly flat attenuation across each subcarrier. Excessive attenuation with frequency across subcarriers will reduce the orthogonality of the subcarriers. Also, multipath fading and isolated narrowband interference is not a problem in most underwater RF applications because the longer reflected path signal is highly attenuated with respect to the direct path signal so fading does not occur. In those applications, symbol multiplexer  130  and symbol demultiplexer  165  in  FIG. 7  can be eliminated. 
     Another advantage of the convolutional decoder described in U.S. Pat. Nos. 8,098,773 and 8,437,431 is that parallel or serial processing can be used to calculate residuals on a path-by-path basis. For low data rate systems like underwater RF, very high order rate ½ pseudo orthogonal QPSK M-QAM modulators and demodulators can be implemented which increases the bits per symbol and allows more information to be transmitted per data packet without having to increase the data rate or bandwidth. With underwater RF, 256-QAM (4 bits/symbol), 1024-QAM (5 bits/symbol), and 4096-QAM (6 bits/symbol) can be implemented. This is possible because modern high speed application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and processors operate at rates of hundreds of thousands or even millions of operations per underwater RF data symbol period. Processing and memory resources can be time shared among all the decoder paths thus making it possible to implement very high order pseudo orthogonal QPSK M-QAM systems. 
     Although the examples presented herein were described for wireless OFDM applications like cellular, Wi-Fi, and underwater RF, this invention is applicable for numerous other wireless and wired communication applications such as satellite, digital subscriber line (DSL), etc. which require increased data rates without increased signal power or bandwidth. Many simple modifications to the described system are possible without departing from the spirit of the invention.