Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2614—Peak power aspects
  • H04L27/2623—Reduction thereof by clipping
  • H04L27/2624—Reduction thereof by clipping by soft clipping

Definitions

• DAB Digital Audio Broadcasting
• IBOC FM In-Band On-Channel
• IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing FM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
• the advantages of digital transmission for audio include better signal quality with less noise and wider dynamic range than with existing FM radio channels.
• the hybrid format would be adopted allowing the existing receivers to continue to receive the analog FM signal while allowing new IBOC receivers to decode the digital signal.
• the goal of FM hybrid IBOC DAB is to provide virtual CD- quality stereo digital audio (plus data) while simultaneously transmitting the existing FM signal.
• the goal of FM all-digital IBOC DAB is to provide virtual CD-quality stereo audio along with a data channel with capacity of up to about 200 kbps, depending upon a particular station's interference environment.
• One proposed FM IBOC broadcasting system uses a plurality of orthogonal frequency division multiplexed (OFDM) carriers to transmit a digital signal.
• An OFDM signal consists of the sum of several carriers modulated at different equally spaced frequencies, which are orthogonal to each other. This ensures that different subcarriers do not interfere with each other.
• the magnitude of the transmitted signal in such a system occasionally has very high peaks.
• the linear power amplifiers used in IBOC DAB transmitters need to operate with large power back-offs so that the out-of-band power is below the imposed limits. This results in very expensive and inefficient amplifiers.
• PAR Peak to Average power Ratios
• This invention provides an efficient scheme for reducing the peak to average power ratio of electronic signals using orthogonal frequency division multiplexing, such as may be used in FM IBOC DAB systems.
• This invention provides a method for reducing peak to average power ratio in a radio frequency signal.
• the method comprises the steps of modulating a plurality of sub- carriers with a plurality of data symbol vectors to produce a first modulated signal; limiting the magnitude of the first modulated signal to produce a first limited modulated signal; demodulating the first limited modulated signal to recover the constellation points; predistorting the data symbol vectors to provide a minimum magnitude for in-phase and quadrature components thereof to produce predistorted data symbol vectors; modulating the plurality of carriers with the predistorted data symbol vectors to produce a second modulated signal; limiting the magnitude of the second modulated signal to produce a second limited modulated signal; and reducing intermodulation products in the second limited modulated signal.
• An alternative embodiment particularly applicable to an all digital IBOC DAB system additionally predistorts data symbol vectors of the central subcarriers while reducing intermodulation products in the second modulated limited signal.
• the invention also encompasses transmitters that perform the above method. BRIEF DESCRIPTION OF THE DRAWINGS
• Figure 1 is a schematic representation of the frequency allocations and relative power spectral density of the signal components for a hybrid FM IBOC DAB signal
• Figure 2 is a schematic representation of the frequency allocations and relative power spectral density of the signal components for an all-digital FM IBOC DAB signal;
• Figure 3 is a simplified block diagram of a radio transmitter that may incorporate the peak to average power ratio reduction method of the present invention
• Figure 4 is a graph illustration one type of limiting that may be employed in the method of this invention.
• Figure 5 is a schematic representation of the predistortion of the data symbols as applied in the invention.
• Figure 6 is a flow chart of the method of this invention as applied to a hybrid digital audio broadcasting system
• Figure 7 is a graph illustration another type of limiting that may be employed in the method of this invention
• Figure 8 is a graph of the results of a simulation of the power spectral densities of a modulated waveform processed according to the invention, using the limiting function of Figure 4;
• Figure 9 shows the bit error rates for the various scenarios illustrated in Figure 8.
• Figure 10 is a graph of the results of a simulation of the power spectral densities of a modulated waveform processed according to the invention, assuming a high power amplifier using the limiting function of Figure 7;
• FIG 11 shows the bit error rates for the various scenarios illustrated in Figure 10;
• Figure 12 is a flow chart of the method of this invention as applied to an all digital audio broadcasting transmitter
• Figure 13 is a graph of the results of a simulation of the power spectral densities of a modulated waveform processed according to the invention, using the limiting function of Figure 4;
• Figure 14 shows the bit error rates for the various scenarios illustrated in Figure 13;
• Figure 15 is a graph of the results of a simulation of the power spectral densities of a modulated waveform processed according to the invention, assuming a high power amplifier using the limiting function of Figure 7; and
• Figure 16 shows the bit error rates for the various scenarios illustrated in Figure 15. DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Figure 1 is a schematic representation of the frequency allocations (spectral placement) and relative power spectral density of the signal components for a hybrid FM IBOC DAB signal 10 in accordance with the present invention.
• the hybrid format includes the conventional FM stereo analog signal 12 having a power spectral density represented by the triangular shape 14 positioned in a central frequency band, 16 portion of the channel.
• the Power Spectral Density (PSD) of a typical analog FM broadcast signal is nearly triangular with a slope of about -0.35 dB/kHz from the center frequency.
• PSD Power Spectral Density
• a plurality of digitally modulated evenly spaced sub-carriers are positioned on either side of the analog FM signal, in an upper sideband 18 and a lower sideband 20, and are transmitted concurrently with the analog FM signal. All of the carriers are transmitted at a power level that falls within the United States Federal Communications Commission channel mask 22.
• the vertical axis in Figure 1 shows the peak power spectral density as opposed to a more conventional average power spectral density characterization.
• a plurality of evenly spaced orthogonal frequency division multiplexed (OFDM) sub-carriers are placed on each side of the host analog FM signal occupying the spectrum from about 129 kHz through about 199 kHz away from the host FM center frequency as illustrated by the upper sideband 18 and the lower sideband 20 in Figure 1.
• the total DAB power in the OFDM modulated sub-carriers in each sideband is set to about -25 dB relative to its host analog FM power.
• the DAB signal is transmitted on the OFDM subcarriers located on either side of the analog spectrum.
• the DAB system includes of 191 carriers above and 191 carriers below the host FM spectrum.
• Each DAB subcarrier is QPSK modulated at a symbol rate of 344.53125 Hz.
• the digitally modulated portion of the hybrid signal is a subset of the all- digital DAB signal that will be transmitted in the all-digital IBOC DAB format.
• the spectral placement and relative signal power density levels of the OFDM digital sub-carriers in a proposed all-digital FM DAB format illustrated by item number 24, is shown in Figure 2.
• the analog FM signal of Figure 1 has been replaced by an optional additional group of OFDM sub-carriers, referred to as the extended all-digital signal 26, located in the central frequency band 28.
• the extended all-digital signal 26 located in the central frequency band 28.
• the power spectral density level of the all-digital IBOC signal sidebands is set about 10 dB higher than that allowed in the hybrid IBOC sidebands. This provides the all-digital IBOC signal with a significant performance advantage. Furthermore the power spectral density of the extended all-digital signal is about 15 dB below that of the hybrid IBOC sidebands. This minimizes or eliminates any interference problems to adjacent hybrid or all-digital IBOC signal while providing additional capacity for other digital services.
• the all digital mode is a logical extension of the hybrid mode where the analog signal, which previously occupied the central +/-100kHz region is replaced with low level digital subcarriers.
• a proposed all digital DAB system includes 267 carriers in each sideband and 559 carriers in the center.
• Each DAB subcarrier is QPSK modulated.
• the power spectral density plots for the transmitted signal should be well within the All-Digital FM IBOC mask.
• FIG. 3 is a functional block diagram illustrating an implementation of the present invention in an IBOC DAB FM transmitter.
• TFFT Inverse Fast Fourier Transform
• Block 38 is the main block where the peak to average power ratio reduction is realized.
• the modulated output of DAB modulator 36 is passed as an input to this block.
• the output of block 38 is the signal to be transmitted with a reduced PAR.
• the modulated signal is limited in amplitude as illustrated by block 40, then it is demodulated as in block 42, and the symbol vectors obtained from the demodulator are predistorted, or constrained, to have a minimum in-phase and quadrature components in block 44.
• the constrained symbols are then modulated in block 46 to produce a second modulated signal that is subjected to further limiting in block 48. This limiting results in unwanted intermodulation products.
• the intermodulation products in the limited second modulated signal are then reduced or eliminated in block 50 prior to passing the signal to a high power amplifier 52 for broadcasting via antenna 54.
• Figure 4 is a graph that illustrates the operation of a limiter that may be used to perform the function of block 40.
• the limiter is set to a certain threshold, or limit value, Kl. At any instant in time if the input signal power exceeds Kl it is clipped to Kl. Since the input signal is normalized, this ensures that the PAR of signal at the output of the limiter is Kl.
• Kl a certain threshold
• the output of the limiter is set equal to -Kl; if the value of the input signal (X) is greater than Kl, then the output of the limiter is set equal to Kl; and if the input signal is between -Kl and Kl, then the output signal is equal to the input signal.
• a Kl of 1.58 implies that the peak to average is set to 4dB for this operation.
• the limited modulated signal is then passed to a DAB demodulator 42.
• a DAB demodulator In the DAB demodulator, an inverse cyclic prefix and windowing operation is first performed on the modulated samples. This is followed by the Fast Fourier Transform(FFT) to realize OFDM demodulation.
• FFT Fast Fourier Transform
• each OFDM symbol vector is in this step forced to lie in a certain region 54, 56, 58 or 60, as depicted in Figure 5, around the constellation point.
• constellation points 62, 64, 66 and 68 have an expected in-phase and quadrature magnitude of "A”.
• Some predetermined fraction of "A”, designated as "F" defines the region to which the data symbols are constrained.
• the in-phase and quadrature components of the symbol constellation points are forced to have at least a minimum magnitude equal to some predetermined fraction of the expected in-phase and quadrature magnitude.
• the constrained symbol vector is modulated through the DAB modulator 46 and the modulated output is passed through the limiter 48.
• Limiter 48 uses a limiting function similar to that of Figure 4, but having a threshold value of K2. This ensures that the signal at the output of the limiter 48 has a PAR of K2 since the input signal is normalized.
• the signal is cleaned up in block 50 by zeroing out the non-data subcarriers. The distortion introduced due to this cleaning up operation is minimal.
• Block 70 shows that the DAB input OFDM symbol vector of subcarrier data is input to DAB modulator 72.
• the resulting first modulated signal on line 74 is limited in block 76 using a first threshold Kl. This produces a limited first modulated signal on line 78 that is subsequently demodulated in block 80 to recover the constellation points of the data symbol vectors on line 82.
• the recovered constellation points are predistorted in block 84 so that they are constrained to have a predetermined minimum magnitude in-phase and quadrature components as discussed above.
• DAB modulator 86 modulates the constrained symbol vectors to produce a second modulated signal on line 88.
• This second modulated signal is limited in limiter 90 having a second threshold K2. Since the limiting operation produces intermodulation products, these are reduced in the following steps.
• the second limited modulated signal on line 92 is passed to a demodulator in block 94.
• the demodulated output on line 96 is passed to a clean-up step in block 98 where the non-data subcarriers are clipped to zero.
• the resulting signal on line 100 is modulated in block 102 and the third modulated signal on line 104 is limited in block 106 using another limit threshold (K3).
• K3 another limit threshold
• the steps in blocks 94, 98, 102 and 106 are repeated two times, using threshold value K4 in limiter 106 in the first repetition. In the second repetition, limiter 106 is not used, but the signal is passed on line 108 to a high power amplifier for broadcast.
• Model 1 uses a "Z curve” limit function as described in Figure 4.
• the limiter is set to a certain threshold K5.
• K5 for a normalized input signal
• Model 2 uses an "S curve” limit function.
• a scaled error function 110 is used to model the HP A (as illustrated in Figure 7).
• the operating point is set by K5.
• a K5 of 6 dB implies that the signal rms is 6dB below the ldB compression point.
• the signal illustrated by line 112 represents clipping at 5.5+0.85 dB.
• Line 114 shows clipping at 5.0+0.85 dB
• line 116 shows clipping at 4.5+0.86 dB
• line 118 shows the results for clipping at 4.0+0.88 dB.
• Figure 9 shows the corresponding bit error rates for these scenarios, using primed numbers for corresponding results.
• Line 119 represents the undipped results.
• the undipped signal is illustrated by line 120.
• Figure 11 shows the corresponding bit error rates for these scenarios, using primed numbers for corresponding results.
• Figure 12 is a flow chart that illustrates the PAR reduction method of the invention for an all-digital signal.
• Block 124 shows that the DAB input OFDM symbol vector of subcarrier data is input to DAB modulator 126.
• the resulting first modulated signal on line 128 is limited in block 130 using a first threshold Kl.
• the recovered constellation points are predistorted in block 138 so that they are constrained to have a predetermined minimum magnitude in-phase and quadrature components as discussed above.
• unwanted non-data subcarriers are also clipped to zero in this step.
• DAB modulator 140 modulates the constrained symbol vectors to produce a second modulated signal on line 142. This second modulated signal is limited in limiter 144 having a second threshold K2.
• the second limited modulated signal on line 146 is passed to a demodulator in block 148.
• the demodulated output on line 150 is passed to block 152 where data symbols from the central carriers are predistorted and the non-data subcarriers are clipped to zero.
• the resulting signal on line 154 is modulated in block 156 and the third modulated signal on line 158 is limited in block 160 using another limit threshold (K3).
• K3 another limit threshold
• the steps in blocks 148, 152, 156 and 160 are repeated two times, using threshold value K4 in limiter 160 in the first repetition. In the second repetition, limiter 160 is not used, but the signal is passed on line 162 to a high power amplifier for broadcast.
• the undipped signal is illustrated by line 164.
• Line 166 shows clipping at 4.5+0.78 dB
• line 168 shows clipping at 5.0+0.77 dB
• line 170 shows the results for clipping at 5.5+0.77 dB.
• Figure 14 shows the corresponding bit error rates for these scenarios, using primed numbers for corresponding results.
• the undipped signal is illustrated by line 172.
• Figure 16 shows the corresponding bit error rates for these scenarios, using primed numbers for corresponding results.
• This invention describes a novel approach for Peak to Average Ratio (PAR) reduction in OFDM for FM IBOC DAB systems.
• Simulation results (using Z and S curves for power amplifier) for this approach show that the invention can achieve a PAR down to 4 - 7dB and still be within the FM mask.
• the distortion introduced due to this predistortion scheme is minimal.
• the distortion introduced with this particular set of values is minimal.
• This invention uses a combination of predistortion of the transmit signal along with clipping to minimize the PAR of the transmitted signal.
• the PAR reductions in the optimized transmit signal have been demonstrated with simulation results.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Transmitters (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

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