US 20030141938 A1 Abstract A quadrature vestigial sideband (QVSB) communication system provide bandwidth efficient data transmission using cross coupled data signaling during both transmit and receive having controlled intersymbol interference. The QVSB modem includes cross coupled arm transmit and receive data filtering on both of the I&Q channels providing a bandwidth efficient QVSB spectra. A quadrature crosstalk maximum likelihood sequence estimator implements a Viterbi decoding algorithm for providing estimated data sequence outputs. The receiver is a coherently aided demodulator synchronized by a synchronization loop providing time and phase references using the estimated data sequence outputs.
Claims(20) 1. A method of modulating a carrier for transmitting a quadrature vestigial single sideband (QVSB) signal for communicating I symbols and Q symbols, the method comprising the steps of,
cross arm filtering the I and Q symbols for generating i filter responses and i _{H }filter responses from the I symbols and for generating q_{H }filter responses and q filter responses from the Q symbols, combining the i and q _{H }filter responses into an I channel overlapping filter signal and the q and i_{H }filter responses into a Q channel overlapping filter signal, and quadrature modulating a carrier by the I and Q channel overlapping filter responses into a modulated inphase signal and a modulated quadrature signal, the Q channel overlapping filter response modulates the carrier shifted ninety degrees from the carrier modulating the I channel overlapping filter responses for modulating the carrier in quadrature, summing the modulated inphase signal and a modulated quadrature signal into the QVSB signal, and transmitting the QVSB signal for communicating the I and Q symbols over a single sideband. 2. The method of the i and i
_{H }filter responses are near Hilbert transform pair filtered responses, and the q and q
_{H }filter responses are near Hilbert transform paired filtered responses. 3. The method of the i and i
_{H }filter responses are near Hilbert transform pair filtered responses, the q and q
_{H }filter responses are near Hilbert transform paired filtered responses, the i and i
_{H }filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses, and the q and q
_{H }filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses. 4. The method of the I and Q symbols have a symbol time, and
the QVSB signal transmitted in quadrature has cross coupled intersymbol interference extending from the symbol time of one of the I and Q symbols into an adjacent symbol times of an adjacent one of the Q and I symbols, respectively.
5. The method of the mapping data into the I and Q symbols for providing the I and Q symbols as independent data sets having a constellation of detection levels.
6. The method of Hilbert transform filtering the I symbols into the i filter response,
inverse Hilbert transform filtering the Q symbol into the q
_{H }filter response, inverse Hilbert transform filtering the I symbols into the i
_{H }filter response, Hilbert transform filtering the Q symbols into the q filter response,
cross coupled summing the i and q
_{H }filter responses into the I channel overlapping filter responses as i+q_{H }I channel overlapping filter responses, and cross coupled subtracting the i
_{H }and q filter responses into the Q channel overlapping filter responses as q−i_{H }Q channel overlapping filter responses. 7. A method of demodulating a transmitted quadrature vestigial sideband (QVSB) signal modulating a carrier in quadrature by I and Q overlapping filter responses respectively from I and Q symbols, the method comprising the steps of,
receiving the transmitted QVSB signal as a received QVSB signal, splitting the received QVSB signal into an I channel QVSB signal and a Q channel QVSB signal, coherent demodulating the I channel QVSB signal by a replicated carrier and the Q channel QVSB signal by a ninety degree phase shifted replicated carrier for respectively generating the I and Q overlapping filter responses, cross arm filtering the I and Q overlapping filter responses for generating ĩ+{tilde over (q)} _{H }and −{tilde over (q)}+ĩ_{H }match filter responses from the I overlapping filter responses and for generating ĩ+{tilde over (q)}_{H }and {tilde over (q)}−ĩ_{H }matched filter responses from the Q overlapping filter responses, and combining both of the ĩ+{tilde over (q)} _{H }matched filter responses into an I channel response signal and the −{tilde over (q)}+ĩ_{H }and {tilde over (q)}−ĩ_{H }matched filter responses into a Q channel response signal. 8. The method of the ĩ+{tilde over (q)}
_{H }and −{tilde over (q)}+ĩ_{H }match filter responses are near Hilbert transform pair filtered responses, and the ĩ+{tilde over (q)}
_{H }and {tilde over (q)}−ĩ_{H }matched filter responses are near Hilbert transform pair filtered responses. 9. The method of the ĩ+{tilde over (q)}
_{H }and −{tilde over (q)}+ĩ_{H }match filter responses are near Hilbert transform pair filtered responses, the ĩ+{tilde over (q)}
_{H }and {tilde over (q)}−ĩ_{H }matched filter responses are near Hilbert transform pair filtered responses, the ĩ+{tilde over (q)}
_{H }and −{tilde over (q)}+ĩ_{H }match filter responses are filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses, and ĩ+{tilde over (q)}
_{H }and {tilde over (q)}−ĩ_{H }match filter responses are filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses. 10. The method of _{H }overlapping response signal and the Q channel overlapping response signal is a q+i_{H }overlapping response signal,
the cross arm filtering step and combining step comprises the steps of,
Hilbert transform filtering the i+q
_{H }overlapping response signal into an ĩ+{tilde over (q)}_{H }match filter response, inverse Hilbert transform filtering the q−i
_{H }overlapping response signal into an ĩ+{tilde over (q)}_{H }matched filter response, inverse Hilbert transform filtering the i+q
_{H }overlapping response signal into an −{tilde over (q)}+ĩ_{H }match filter response, Hilbert transform filtering the q−i
_{H }overlapping response signal into a {tilde over (q)}−ĩ_{H }match filter response, cross coupled summing the ĩ+{tilde over (q)}
_{H }matched filter responses into the I channel response signal as an {tilde over ({tilde over (i)})}=2[ĩ+{tilde over (q)}_{H}] I channel response signal, and cross coupled subtracting the −{tilde over (q)}+ĩ
_{H }and {tilde over (q)}−ĩ_{H }matched filter responses into the Q channel response signal as a {tilde over ({tilde over (q)})}=2[{tilde over (q)}−ĩ_{H}] Q channel response signal. 11. The method of detecting the I and Q channel responses for generating a synchronized timing signal for carrier tracking the replicated carrier and the ninety degree phase shifted carrier for coherent demodulation of the I and Q channel overlapping response signals.
12. The method of detecting from the I and Q symbols from the I and Q channel response signals and mapping the I and Q symbols into data.
13. A method of communicating I and Q symbols, the method comprising the steps of,
cross arm filtering the I and Q symbols for generating i filter responses and i _{H }filter responses from the I symbols and for generating q_{H }filter responses and q filter responses from the Q symbols, combining the i and i _{H }filter responses into an I channel overlapping filter signal and the q and q_{H }filter responses into a Q channel overlapping filter signal, and quadrature modulating a carrier by the I and Q channel overlapping filter responses into a modulated inphase signal and a modulated quadrature signal, the Q channel overlapping filter response modulates the carrier shifted ninety degrees from the carrier modulating the I channel overlapping filter responses for modulating the carrier in quadrature, combining the modulated inphase signal and a modulated quadrature signal into a quadrature vestigial single sideband (QVSB), transmitting the QVSB signal for communicating the I and Q symbols over a single sideband, receiving the transmitted QVSB signal as a received QVSB signal, splitting the received QVSB signal into an I channel QVSB signal and a Q channel QVSB signal, coherent demodulating the I channel QVSB signal by a replicated carrier and the Q channel QVSB signal by a ninety degree phase shifted replicated carrier for respectively generating the I and Q overlapping filter responses, cross arm filtering the I and Q overlapping filter responses for generating ĩ+{tilde over (q)} _{H }and −{tilde over (q)}+ĩ_{H }match filter responses from the I overlapping filter responses and for generating ĩ+{tilde over (q)}_{H }and {tilde over (q)}−ĩ_{H }matched filter responses from the Q overlapping filter responses, and combining the ĩ+{tilde over (q)} _{H }matched filter responses into an I channel response signal and −{tilde over (q)}+ĩ_{H }and {tilde over (q)}−ĩ_{H }matched filter responses into a Q channel response signal. 14. The method of the i and i
_{H }filter responses are near Hilbert transform pair filtered responses, the q and q
_{H }filter responses are near Hilbert transform paired filtered responses, the i and i
_{H }filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses, and the q and q
_{H }filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses, the ĩ+{tilde over (q)}
_{H }and −{tilde over (q)}+ĩ_{H }match filter responses are near Hilbert transform pair filtered responses, the ĩ+{tilde over (q)}
_{H }and {tilde over (q)}−ĩ_{H }matched filter responses, are near Hilbert transform pair filtered responses, the ĩ+{tilde over (q)}
_{H }and −{tilde over (q)}+ĩ_{H }match filter responses are filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses, and the ĩ+{tilde over (q)}
_{H }and {tilde over (q)}−ĩ_{H }match filter responses are filter responses are selected from the group consisting of raised cosine, jump, and smoothed jump filtered responses. 15. The method of the i and i
_{H }filter responses are near Hilbert transform pair filtered responses, the q and q
_{H }filter responses are near Hilbert transform paired filtered responses, the ĩ+{tilde over (q)}
_{H }and −{tilde over (q)}+ĩ_{H }match filter responses are near Hilbert transform pair filtered responses, the ĩ+{tilde over (q)}
_{H }and {tilde over (q)}−ĩ_{H }matched filter responses are near Hilbert transform pair filtered responses, and all of the near Hilbert transform pair filter responses are generated from identical cross arm filtering selected from the group consisting of raised cosine, jump, and smoothed jump filtering.
16. The method of the mapping data into the I and Q symbols for providing the I and Q symbols as independent data sets having a constellation of detection levels,
sampling the I and Q channel responses into I and Q discrete sample values at discrete times within a set of constellation amplitude values, and
estimating the data from the I and Q discrete sample values using the I and Q discrete sample values.
17. The method of the mapping original data into the I and Q symbols for providing the I and Q symbols as independent data set having a constellation of detection levels prior to modulation, and
quantizing the I and Q channel response signals for estimating the original data.
18. The method of carrier varying the replicated carrier and ninety degree phase shifted replicated carrier for synchronization to the symbol times for coherent demodulation of the received QVSB signal.
19. The method of amplitude varying the amplitude of the received signal for constant amplitude demodulation of the received QVSB signal.
20. The method of phase varying for phase synchronizing the replicated carrier and the ninety degree phase shifted carrier for coherently demodulating the received QVSB signal.
Description [0001] The invention relates to the field of quadrature modulation communication systems. More particularly, the present invention relates to vestigial sideband modulation communications systems using cross coupled independent data stream modulated on a common carrier in quadrature. [0002] The rapid, worldwide expansion of communications services underscores the importance of bandwidth conservation. With increased demands for cellular and personal communications services within a finite radio frequency spectrum, there is an ever-increasing contention for bandwidth. Cellular services are growing at a geometric rate. Microcellular sites are being advocated to handle the increased demand through localized frequency reuse, and hundreds of low earth orbit and medium earth orbit satellites will support the increasing demand for bandwidth over the next decade. In digital video communications, high definition television (HDTV) transmits at 21.5 Mbits/s with a greatly improved picture quality that must be compatible with the existing 6.0 MHz channel bandwidth allocation. This requires a bandwidth efficiency of greater than 3.0 bits/s/Hz. Additionally, data throughputs in communications have also been increasing at an exponential rate. Existing bandwidth allocations are typically shared among different services. A review of the current frequency allocations reveals that the majority of bands exhibit sharing of multiple services, such as, fixed and mobile satellite services and earth exploration satellites. A natural consequence of this sharing is increasing interference. With the bandwidth being a finite resource, there are increasing demands for this finite bandwidth resources creating a need to develop general purpose practical bandwidth efficiency communication techniques. [0003] Digital data has been transmitted using double sideband (DSB) or quadrature double sideband (QDSB) techniques. Occasionally, single sideband (SSB) formats have been used, and more recently two vestigial sideband (VSB) formats have been selected as the standards for off the air and cable HDTV. DSB signaling is the simplest and most straight forward means to transmit analog or digital information on a carrier, such as, when using AM and FM methods. SSB is employed when the bandwidth is at a premium, such as, when multiplexing terrestrial telephone channels. VSB is used when requiring a controlled component of energy at the carrier frequency, such as, in TV and HDTV communications. [0004] One of the most useful ways to assess bandwidth efficiency is to make use of the Shannon channel capacity bound that provides an upper limit on the signaling rate R [0005] The single sideband and quadrature single sideband (QSSB) modulation format data points have exactly twice the bandwidth efficiency of the corresponding double sideband counterparts where the effect of quadrature channel crosstalk can be rendered negligible. The quadrature channel crosstalk is inherent in QSSB transmission in which independent data is placed on quadrature carriers. The crosstalk degrades performance and has been a major problem in QSSB communications. The DSB techniques diverge from the bound as the number of bits/s/Hz or bandwidth efficiency is increased, whereas ideally transmitted QSSB formats run parallel to the bound. This divergence is due to the redundancy in transmitting two sideband replicas. As the bandwidth efficiency of the channel is increased, QSSB potentially offers a progressively larger advantage over DSB transmission. In particular, when a six bit/s/Hz efficiency is needed, a conventional phase shift keying (PSK) may be used, such as 64-PSK. The DSB scheme would be required as compared to an 8-PSK QSSB format. The DSB scheme requires 18 dB more signal power to achieve the same BER. In general, the number of signal levels needed with DSB techniques is the square of that required with an equivalent QSSB format. These large discrepancies in signal to noise ratio (SNR) and number of signal levels leave considerable margin for non-ideal SSB signaling due to crosstalk. SSB uses half the bandwidth of conventional DSB yielding twice the bandwidth efficiency. Because of the sharp cutoff characteristics at one of the SSB band edges, vestigial sideband method is often used to realize a more gradual rolloff. The VSB method is not as bandwidth efficient as the SSB method, but generally leads to a more practical solution with controlled crosstalk. Conventional VSB filtering uses inphase and quadrature arm filters in both the transmit modulators and receive demodulators. The data stream is reinforced in the receiver by transmitting the data stream through both quadrature and inphase channels. The quadrature arm odd responses combine to yield a even response. The VSB is slightly less bandwidth efficient than SSB, but is more controlled and easier to implement. VSB or SSB frequency spectra transform to inphase and quadrature impulse response with even and odd time symmetry, respectively. [0006] In practice, during VSB modem transmit and receive filtering modeling, matched filtering is employed so that the square root of the Nyquist frequency response is apportioned equally to the transmitter and receiver as opposed to a full response. In general, impulse responses with equally spaced axis crossings will only occur after passing through matched sets of transmit and receive filters. The critical filtering for a Nyquist band limited VSB transmitters and receivers is employed in HDTV. In the HDTV system, a single data channel is communicated through both inphase and quadrature (I&Q) channels respectively having a h [0007] In the receiver, both the I&Q filter output responses are approximately the same with each having even time symmetry. The I-channel response is related to the Q-channel response. The Q-channel response is approximately the same at the I-Channel response because it is roughly the cascade of two Hilbert transform 90° phase shifted versions of the I-channel response that which is merely an inverted version of the I-channel response. The Q-channel output then is summed with the I-channel to improve the detection SNR by 3.0 dB. There is no ISI problem when Nyquist filtering is used in conjunction with VSB data transmission as is evidenced by I-channel responses in a 25% raised cosine VSB eye diagram. [0008] Vestigial sideband is defined such that when its spectrum is downconverted to baseband, the inner transition regions of its positive and negative frequency image bands overlap and are complementary so as to sum to unity with proper phasing. VSB is a good compromise between DSB and SSB because VSB approaches SSB in bandwidth efficiency, but does not require an infinitely sharp transition band. HDTV will be transmitted digitally using trellis coded 8-ary VSB and 16-ary VSB formats, for terrestrial and cable distribution, respectively. These VSB formats require 8-ary and 16-ary amplitude levels in their baseband modulating waveforms. To facilitate VSB signaling, a common digital data stream modulates two quadrature carriers where the impulse response pairs are orthogonal and correlated. Quadrature SSB (QSSB) and QVSB are more complex than SSB and VSB because each of the inphase (I) and quadrature (Q) baseband modulating channels contain the superposition of an independent pair of data streams having interference and crosstalk that must be controlled. The advantage of QSSB/QVSB over SSB/VSB is a doubling of the information carrying capacity. The disadvantages are greater implementation complexity, and a typically reduced noise margin due to crosstalk. [0009] There are fundamentally two methods of generating SSB/VSB, that is, the quadrature phase shift method and sideband filtering method. In this study, the phase shift method is favored over the filtering approach, because precise control over the modulating waveshapes will be necessary; and this precision is best achieved with digital signal processing techniques. The phase shift approach is shown analytically for an SSB modulator output, s(t) by an SSB modulator output equation s(t)=i(t)cos(ω [0010] To conserve bandwidth using SSB/VSB modem baseband filtering modeling, digital modulation techniques are filtered prior to transmission. To maximize the detection SNR, the receive filtering is matched to the transmit filter. The critical baseband filtering for a band limited SSB/VSB modem uses a single data stream with single arm filtering for VSB modulation. The modulated output signal is generated according to modulator output equation. To facilitate SSB transmission, the inphase filters with the i subscripts and the quadrature filters with the q subscripts must be a Hilbert transform pair h [0011] For memoryless Nyquist filtering, the Nyquist family of filters are evaluated for applicability in achieving bandwidth efficient transmission with minimal degradation in SNR performance due to ISI. Ideal rectangular and raised cosine filtering have been used for Nyquist filtering. Nyquist impulse responses are sinc based waveshapes with even time symmetry and equally spaced zero or axis crossings at integer multiples of the data symbol time. As a result, responses from adjacent data symbols do not interfere at the detection sampling instants. The impulse responses with equally spaced axis crossings are realized when the frequency response has vestigial symmetry about the half amplitude transmission points. The most concentrated distribution of signal bandwidth in the frequency domain is the ideal rectangular spectrum using ideal rectangular filtering. The magnitude for an SSB version of the ideal rectangular spectrum for the minimum Nyquist bandwidth R [0012] When the SSB spectrum is band limited to half the data symbol rate, the corresponding inphase impulse response will have equally spaced axis crossings at integer multiples of the symbol time T [0013] A widely used Nyquist filter realization is the raised cosine, which has a sinusoidally shaped transition band. The frequency response for a raised cosine filter is defined by H(f). The raised cosine H(f) equation is a frequency response equation that defines the VSB magnitude response. The corresponding impulse response is defined as h(t). The rolloff factor is 0<r<1 and the half amplitude frequency is f [0014] A Nyquist frequency response is known for a 20% raised cosine filter in a VSB channel. In practice, this filter can be closely approximated, but not realized exactly because of the perfectly flat passband and stopband. In addition, the stopband also has infinite attenuation. The corresponding impulse response pair for the 20% square root raised cosine VSB response can be generated by means of an FFT, and the impulse responses are very similar to the ideal rectangular filter pair except ideal rectangular filter pair have more ringing due to an abrupt transition band. The even response has equally spaced axis crossings, and the odd response has zeros at ± odd multiples of the data symbol interval. [0015] Employing conventional raised cosine family filtering for QSSB or QVSB transmission would result in crosstalk that reduces the intersignal distances thereby degrading the BER performance. Partial response signaling has been used for SSB transmission. However, partial response signaling has not been extended to QSSB. The well known class-4 (1−D [0016] A restricted type of QVSB signaling has previously been disclosed in 1985. The QVSB system had two I&Q inphase and quadrature data channels modulated in quadrature by a carrier in the receiver and demodulated in quadrature in the receiver. There were no arm filters in the I&Q channels in the transmitter or receiver. At the output of the QVSB transmitter and at the input of the receiver were disposed bandpass raise cosine filters for VSB communication. The QVSB system operated only for very soft rolloff spectra of restricted bandwidth efficiency range with substantial degradations in signal to noise ratio (SNR) due to crosstalk. The QVSB data transmission used Nyquist filters from the raised cosine filtering to band limit the signal. Nyquist filtering is widely used to eliminate intersymbol interference in conventional digital data transmission schemes. However, in the QVSB system, Nyquist filtering exhibits quadrature crosstalk and ISI in both channels. The QVSB system has crosstalk between the inphase and quadrature (I&Q) channels in a controlled form similar to intersymbol interference in partial response systems. The QVSB system could use a maximum likelihood sequence estimator (MLSE) to remove the ISI based on a Viterbi algorithm. The QVSB system could employ digital data feedback in the synchronization loops. These techniques are taught in U.S. Pat. No. 4,419,759, entitled Concurrent Carrier and Clock Synchronization for Data Transmission Systems, and U.S. Pat. No. 4,472,817, entitled Non-PLL Concurrent Carrier and Clock Synchronization. The QVSB system can behave like partial response systems where precoding could be used to avoid error propagation. However, the QVSB system precoder did not exploit the correlation information in the received samples. Consequently, the Viterbi probabilistic MLSE decoder showed a marked improvement over preceding. The QVSB system achieved a bandwidth efficiency of 2.3 bits/s/Hz for a 75% raised cosine rolloff passband. This is double the rate of 1.14 bits/s/Hz for QPSK transmission with a corresponding rolloff passband. A digital SNR E [0017] Nyquist filtering during VSB data transmission for QVSB signaling can be analyzed using an eye diagram. An eye diagram is an overlay of the time response for all possible data sequences. The eye diagram highlights the effects of ISI. For the case of binary data, the Nyquist filtered waveforms that make up the eye diagram are typically bipolar. Hence, a threshold is set at zero and samples are taken in the center, at the maximum eye opening. Sample values above zero are detected as positive ones and samples below zero are detected as negative ones, that is, digital ones and zeros. Nyquist filtering does eliminate ISI at integer symbol time multiples. Hence, it is known that digital data may be transmitted without ISI when the channel filter response satisfies the Nyquist criterion. The best linear channel detection performance is obtained by matching the transmit and receive filter responses. The best known Nyquist filters are the raised cosine filters. For example, a VSB full raised cosine frequency response with a 25% rolloff rate would have corresponding inphase and quadrature impulse responses. These impulse responses correspond to the overall Nyquist channel response when a single data one is transmitted. Opposite polarity impulse responses would be used when a data zero is transmitted. To facilitate VSB, complementary impulse responses with even and odd time symmetry are needed in the quadrature channels. The impulse response horizontal axis marks are spaced such that adjacent symbol responses are centered at integer symbol time multiples. The tails from adjacent symbol impulse responses will overlap. However, for the inphase impulse response, there is no ISI at integer symbol time multiples. Therefore, data sequences can be symbol by symbol detected without any degradation in SNR performance. The quadrature impulse response has ISI only at odd symbol time multiples. The restricted QVSB system achieved a good BER performance using a 100% raised cosine filter. The performance for the 75% and 50% cases was substantially degraded, and solutions do not converge below 50% rolloff. These and other disadvantages are solved or reduced using the invention. [0018] An object of the invention is to provide bandwidth efficient communications using quadrature vestigial sideband signaling. [0019] Another object of the invention is to generate bandwidth efficient I&Q channel waveshapes that exhibit minimal intersymbol interference and crosstalk. [0020] Yet another object of the invention is to generate bandwidth efficient I&Q channel waveshapes that exhibit minimal intersymbol interference and crosstalk with reduced bit error rates. [0021] The present invention is a method for transmitting digital data in a bandwidth efficient manner using a quadrature vestigial sideband (QVSB) signaling. The method can be used in data communication systems. The QVSB method may double the capacity of comparable conventional formats by placing overlapping independent data on each of two carriers in phase quadrature using cross coupled arm filters. The data overlap is necessary to achieve QVSB spectral occupancy. [0022] The method eliminates as much of the crosstalk as desired in progressive steps. The method is realized by modulating transmit and demodulating receive hardware architectures, the later of which preferably including a quadrature crosstalk maximum likelihood sequence estimator (QCMLSE) specifically designed to support QVSB signaling within I and Q channel crosstalk. Using various combinations of filtering and higher level signaling constellations, the method can provide as high a bandwidth efficiency within signal processing technology permits with relatively little degradation in the signal to noise ratio (SNR). [0023] A normalized channel capacity versus SNR for the QVSB implementation can be derived from models of the QVSB structure within a linear additive white Gaussian noise channel at perfect synchronization. Over a very broad range of raised cosine filter rolloffs, 4-ary QVSB achieves the same capacity as conventional 16-ary quadrature double sideband (QDSB), with up to 2.0 dB less required SNR at a BER=10 [0024] Operation with a 4-ary rectangular constellation over the complete range of Nyquist spectral rolloff characteristics has been achieved up to and including the 25% raised cosine response with graceful SNR degradation. Thus, the method is robust with greater bandwidth efficiency that can be realized via sharper rolloff. M-ary QVSB signaling achieves twice the capacity of M-ary QDSB signaling that is equivalent to the capacity of M [0025] The method achieves more bandwidth efficient data transmission using QVSB signaling. Modulator and demodulator hardware structures implementing the method enable improved bandwidth efficient communications. These modem structures include SNR efficient synchronization loops that will substantially outperform brute force squaring circuitry. The method preferably relies upon transmit and receive data filtering, specialized QVSB spectra generation, the QCMLSE Viterbi decoding, and a coherently aiding demodulator synchronization loop. [0026] The symbol integer spaced zeros in the quadrature impulse response as well as the inphase response are preferably realized by jump filtering. The ISI removal at 6 (±3) symbols can be realized through multiplication of the impulse responses by a time domain cosine waveform with a 6±3 symbol period. When the time domain cosine multiplication is performed on a sin(x)/(x) response, the corresponding ideal rectangular single jump spectra is shifted up and down resulting in the double jump spectrum. The ideal rectangular bandwidth is expanded by ⅙th. Similarly, the ISI at ±5, ±7 symbols can be eliminated by further multiplication and spectral shifting, resulting in the quadruple jump and octal jump spectra, respectively. A less complex alternative, that also results in greatly improved ISI, can be realized by smoothing the transitions of the double jump spectrum. The ISI at ±1 symbols can not be removed because a doubling of the bandwidth would result. Hence, whenever a data pulse is transmitted, the pulse will be subjected to and dominated by controlled ISI from the adjacent symbols in the opposing quadrature channel and this ISI will be of approximate relative magnitude ±0.5. The QCMSLE provides for effective Viterbi decoding that minimizes the effect of the controlled ISI for improved bandwidth efficient communications. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. [0027]FIG. 1 is a schematic of a quadrature vestigial sideband (QVSB) modulator and demodulator system. [0028]FIG. 2A is a 25% raised cosine QVSB eye diagram of the filtered output of a quadrature channel. FIG. 2B is an octal jump QVSB eye diagram of the filtered output of a quadrature channel. [0029]FIG. 3A is a frequency domain plot of a double jump spectra of quadrature arm filters. [0030]FIG. 3B is a frequency domain plot of a quadruple jump spectra of quadrature arm filters. [0031]FIG. 3C is a frequency domain plot of an octal jump spectra of quadrature arm filters. [0032]FIG. 3D is a frequency domain plot of a smoothed double jump spectra of quadrature arm filters. [0033]FIG. 4 is a block diagram of a generalized QVSB receiver. [0034]FIG. 5 is a block diagram of an analog feedback QVSB receiver. [0035]FIG. 6 is a block diagram of a digital feedback QVSB receiver. [0036]FIG. 7A is a digital data feedback sequence diagram. [0037]FIG. 7B is a digital derivative data feedback sequence diagram. [0038]FIG. 7C is a digital Hilbert data feedback sequence diagram. [0039] An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIG. 1, a quadrature vestigial sideband (QVSB) modulator receives D [0040] The filter responses of filters [0041] Slightly less than one half of the signal spectrum of the transmitted signal is cancelled when the transform filter pairs [0042] The QVSB demodulator receives the transmitted signal [0043] The QCMLSE [0044] The QVSB modem, including the QVSB modulator of a QVSB transmitter and the QVSB demodulator of a QVSB receiver, is characterized by the cross coupled arm filters that are near Hilbert transform pairs in both the transmitting QVSB modulator and the receiving QVSB demodulator. The near Hilbert transform pairs are used for creating quadrature cross coupled vestigial sideband signaling and function to provide a sufficient demodulating detection distance for QCMLSE estimation of the data estimate {circumflex over (D)} [0045] The h [0046] The desired ĩ and {tilde over (q)} components can be respectively separated from the undesired crosstalk {tilde over (q)} [0047] The bandwidth efficiency of vestigial sideband (VSB) transmission is effectively doubled when independent data streams are placed on the I&Q filtering arms during QVSB signaling. The QVSB baseband filtering will result in crosstalk ISI in both channels because each I&Q channel has both a conventional even impulse response as well as near Hilbert odd impulse response component. Intersymbol interference is the superposition of time overlapping impulse responses, which last longer than a symbol interval, of adjacent symbols on one another. The arm filters are chosen so that at multiple symbol time detection points, the intersymbol interference is zero. The Hilbert components have ISI at odd symbol multiples. The minimum separation detection distance between the transmitted signals will be adversely affected by the crosstalk ISI. The arm filters are designed to minimize this crosstalk ISI. In the QVSB demodulator, contributions from both I&Q channels are summed so as to generate and increase the detection SNR by 3 dB. The cross coupling of the I [0048] Referring to FIGS. 1, 2A, [0049] Raised cosine filtering does not provide equal zero axis crossing resulting in small detection distances and hence poor Viterbi decoding. When specialized Nyquist filtering is used in the QVSB modem, crosstalk ISI results at odd integer multiple symbol time intervals. The superposition of impulse responses for all possible transmitted sequences can disadvantageously result in substantial eye closure as shown for the most raised cosine responses. The bandwidth efficiency for the 25% raised cosine filter response is 1.6 symbol/s/Hz. The raised cosine filter, for example, could use smooth rolloff filtering, such as with a 100% raised cosine filter for improved detection distances. However, the bandwidth efficiency for the 100% raised cosine filter is reduced to 1.0 symbol/s/Hz. [0050] A preferable way to minimize crosstalk ISI is the use of jump spectra filtering. For example, an SSB rectangular filter, that is effectively a 0% raised cosine filter, or a single jump spectra filter, exhibits crosstalk ISI in the quadrature response at odd symbol time multiples. The crosstalk at third symbol time can be eliminated by multiplying the impulse responses by a cosine signal of frequency R [0051] The effect of the crosstalk ISI can be minimized using smoothed jump spectra arm filtering to smooth the transitions of the double jump spectra. Crosstalk ISI minimization using smoothed double jump spectra arm filtering is simpler to implement than four or eight jump spectra filtering. The filter smoothing has the effect of substantially softening the ISI at the fifth and seventh symbol times. The eye openings for the spectra of octal jump filtering are wide reflecting large detection distances for improved Viterbi decoding with increased bandwidth efficiency, compared against the 25% raised cosine spectral filtering having small detection distances for the same bandwidth efficiency. The arm filtering selected should be optimized to increase the detection distances for improved Viterbi decoding while minimizing the excess bandwidth required for improved bandwidth efficiency. [0052] The octal jump QVSB eye diagram for octal jump filtering has nearly the same eye openings detection distances as does double jump filtering with 100% smoothing with about the same bit error rate (BER) and channel capacity bandwidth performance. For QVSB transmission, the choice of filtering can be optimized for the application. Regardless of the choice, the number of distinct amplitude levels of the I&Q channel responses {tilde over ({tilde over (i)})} and {tilde over ({tilde over (q)})} is greater than that required for quadrature double sideband transmission. The number of distinct amplitude levels that must be used is a function of the signal constellation. Any number of distinct amplitude levels may be used during QVSB transmission depending on the desired BER and channel capacity. As the channel capacity increases with improved bandwidth efficiency with raised cosine filtering, the distinct amplitude levels progressively smear together as with the 25% raised cosine spectra. As the smearing increases, the detection distance decreases resulting in poorer detection. For a given transmitted power, the more distinct amplitude signal levels for greater bandwidth efficiency, the less is the detection distance, and hence a trade off exists between bandwidth efficiency and BER. However, the crosstalk ISI that causes the additional levels in the QVSB modem is controlled under the crosstalk signal structure and the additional crosstalk amplitude levels can be to a large extent subsequently removed during data detection using Viterbi decoding in the sequence estimator [0053] Referring to FIGS. 1 through 4, and more particularly to FIG. 4, the adverse effect of the controlled intersymbol interference of the transmitted signal [0054] The QCMLSE [0055] For 4-ary QVSB {tilde over ({tilde over (i)})} and {tilde over ({tilde over (q)})} channel response signals with crosstalk ISI at the first symbol time, the QCMLSE [0056] The task of finding the best estimate of the I&Q sequence from the space of all possible sequences is equivalent to searching for the best path through the trellis based on the minimization of distance metrics. The Viterbi decoder algorithm provides a very efficient means for searching this trellis for the best path sequence using Euclidean distance minimization techniques that maximize the probability of receiving a particular sequence hypothesis given some observable inputs that has been obscured by additive Gaussian noise. This minimization is known as maximum likelihood sequence estimation and is based on conventional Viterbi decoding as is well known by those skilled in the art. [0057] At each symbol time k, the state of the QVSB MLSE corresponds to one of sixty four possible hypothesis or estimates of the last three inphase and quadrature symbols. That is, with I [0058] The function of the QMLSE [0059] At time n, the shift registers [0060] For a QVSB implementation, a local metric calculation is based on three consecutive i [0061] The sets of most likely symbols from minimum distance recursive calculator [0062] The best 64-paths from the selector [0063] The updated 64-path history is fed to updater [0064] The most probable paths from the updater [0065] The recursively calculated distance metrics from the calculator [0066] In the global processor [0067] Referring to FIGS. 1 through 6, and more particularly to FIGS. 5 and 6, an analog or a digital version of the QVSB demodulator offer enhanced operations to process the received signal [0068] In the analog demodulator, the estimate Î [0069] In both the analog version and the digital version, the demodulator I&Q channel overlapping filter response signals i(t)+q [0070] In the analog version, the estimated filter responses î(t)+{circumflex over (q)} [0071] In close loop operation, the estimate signals, {circumflex over (Q)} [0072] The demodulators provide enhanced amplitude tracking, carrier tracking, and symbol rate tracking under feedback control operation of the demodulator to a steady state condition. The purpose of the filters [0073] In the analog version, for amplitude synchronized feedback using the automatic gain control synchronization loop, the tracking loop error signal ΔA varies the amplitude of the input signal [0074] For phase recovery, the estimated outputs of summers [0075] For symbol rate recovery, the data feedback is different than the other two amplitude and phase tracking loops. The delayed mixer outputs of delays [0076] Referring to FIGS. 1 through 7, and more particularly to FIGS. 6 and 7, the digital version of the demodulator can be realized using digital equivalent components. The digital embodiment includes QVSB synchronization with digital data feedback that enables a digital signal processing embodiment of the QVSB modem. In many applications, all of the hardware except the mixers and phase shifters can be implemented with digital signal processing technology. On the transmit side, D/A converters, not shown, would typically be placed between the summed QVSB data filters and the mixers. On the receive side, A/D converters, not shown, would reside in the complementary position between the mixers and the cross coupled h [0077] Referring generally to all of the figures, there are several ways in which QVSB data demodulation and decoding may be realized in hardware including the data feedback structure. For example, the QCMLSE [0078] The digital version preferably uses the digitized Hilbert transforms [0079] To reduce the number of data filters in the modulator and demodulator in an equivalent manner, the even time filter responses of the h [0080] The preferred embodiments may be subject to bursts of symbol errors due to the QCMLSE processing. The QVSB modem implementation can be enhanced with concatenated or turbo coding with interleaving for improved performance. Similarly, a useful extension might also include trellis-coded modulation. The preferred embodiments use transmit and receive baseband filtering with overlaid quadrature signals to modulate and demodulate QVSB signals. The use of jump spectra minimizes crosstalk and ISI during QVSB transmission. Also, due to sensitivity to the channel transmission amplitude and phase dispersion, active or passive equalization circuitry may be employed. [0081] The system is general and could be used in various applications where bandwidth efficient digital data transmission is important. QVSB signaling can be readily implemented with currently available components at data rates below 100 Mbits/s. The 4-ary QVSB can be readily extended to 8-ary and 16-ary signaling having bandwidth efficiencies that are equivalent to 64-ary and 256-ary conventional modulation techniques, respectively. A 4-ary QVSB achieves the same bandwidth efficiency of conventional 16-ary QDSB data transmission, but it requires less SNR. In a linear channel, QVSB is substantially more efficient in bandwidth and SNR than conventional data transmission techniques. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims. Referenced by
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