KR101747455B1 - System and method for controlling combined radio signals - Google Patents

Abstract

A method of controlling a combined waveform representing a combination of at least two signals having orthogonal frequency multiplexed signal components includes receiving information that each defines the at least two signals, Converting signals defining information into representations having orthogonal frequency multiplexed signal components, at least one signal having at least two alternate representations of the same information, and at least two alternating representations Combining the transformed information with each other in at least two different ways that define different bonds using each of the at least one criterion; analyzing the different bonds for at least one criterion; The converted information from each of the two signals And outputting information indicative of each combined waveform including a selected one of the combinations of the combinations.

Description

TECHNICAL FIELD [0001] The present invention relates to a combined waveform control method, a coupled waveform control method, a signal combining method,

Field of the Invention The present invention relates to the field of radio communication of radio frequency signals. In particular, the present invention relates to techniques for controlling a combined signal to reduce the PAPR (Peak to Average Power Ratio) of the combined signal or to reduce the estimated error in the receiver.

Typical signal formats for mobile wireless communications are the orthogonal frequency divisible multiplexing (OFDM) format and closely related formats, and one such closely related format is orthogonal frequency division multiple access (OFDMA). With respect to the characteristics of the signal carried on the OFDM signal channel, the frequency domain consists of a plurality of sub-channels adjacent to each other, and the time domain is a relatively slow sequence of OFDM symbols , And each OFDM symbol is spaced apart by a guard interval? T (see FIG. 1). In each guard interval, there is a cyclic prefix (CP) before each symbol period, and the cyclic prefix is composed of the same signal as the signal in the symbol period and is shifted periodically in time. Such a CP can be used to ensure that the received signal is less sensitive in terms of precise temporal synchronization, even in the presence of multipath, such as when radio frequency signals are reflected from a geographically large object, such as a tall building or hill Is designed. If a given symbol is received with a slight time delay (less than [Delta] T), this symbol will be received as before with no error. In addition to the data symbols associated with the OFDM payload, there is typically also a "preamble" that establishes timing and other criteria. This preamble may have its own unique CP which is not shown in FIG.

For OFDM, the subcarrier frequencies are chosen such that the subcarriers are orthogonal to one another. This means that the crosstalk between the subchannels is eliminated and no gurad band between the subcarriers is needed. Thus, the design of both the transmitter and the receiver can be greatly simplified. That is, unlike the conventional FDM scheme, individual filters are not required for each subchannel. Due to this orthogonality, the degree of spacing or spacing between subcarriers needs to be? F = k / (T _U ) hertz. Where T _U seconds is a useful symbol period (window size of the receiver side) and k is a positive integer and is usually equal to one. Therefore, when there are N subcarriers, the total passband bandwidth will satisfy the relation of B? N? F (Hz). Also, due to orthogonality, high spectral efficiency is possible and the total symbol rate approximates the Nyquist rate. The entire usable frequency band can be almost used. The OFDM scheme generally has a spectrum that is almost "white" so that OFDM has electromagnetic interference characteristics beneficial to other co-channel users.

When two OFDM signals are combined, the result is generally a non-orthogonal signal. A receiver limited to a single OFDM signal band will generally not be affected by out-of-channel signals, but when such off-channel signals pass through a common power amplifier, Interaction occurs due to nonlinearity.

In the OFDM scheme, very precise frequency synchronization between the receiver and the transmitter is required. That is, when a frequency deviation occurs, subcarriers are not orthogonal to each other, thereby causing inter-carrier interference (ICI). That is, crosstalk occurs between subcarriers. This frequency deviation is typically caused by misalignment between the transmitter oscillator and the receiver oscillator, or by a Doppler shift effect due to motion. The Doppler shift can be compensated by the receiver alone, but when the Doppler shift is combined with the multipath, the situation can get worse because wave reflections can occur in the case of various frequency deviations. This situation is very difficult to correct.

Further, due to the orthogonality, the modulator and the demodulator can be efficiently implemented using an FFT (Fast Fourier Transform) algorithm at the receiver side and an IFFT (Inverse Fast Fourier Transform) algorithm at the transmitter side. The FFT algorithm is relatively efficient, but it is a limiting factor because there is some computational complexity.

One key principle of the OFDM scheme is that instead of a single high-rate stream, a low symbol rate modulation scheme (i.e., symbol periods relatively steep compared to channel time characteristics) experiences less intersymbol interference caused by multipath propagation It is advantageous to transmit a plurality of low rate streams in parallel. Since each symbol period is long, a guard interval can be inserted between OFDM symbols, whereby inter-symbol interference can be eliminated. In addition, this guard interval eliminates the need for a pulse shaping filter and can be less sensitive to temporal synchronization problems.

The cyclic prefix (CP) transmitted during the guard interval consists of the end of the OFDM symbol replicated into this guard interval, the guard interval is transmitted and the OFDM symbol follows this guard interval. The reason that the cyclic prefix is composed of the end of the replicated OFDM symbol is that when the receiver performs OFDM demodulation using the FFT algorithm, the receiver can integrate for each of the multipaths over an integer number of sinusoid cycles .

For example, the effect of frequency selective channel conditions, such as fading caused by multipath propagation, is such that if the subchannels are sufficiently narrow, i. E. The number of subchannels is large enough, . ≪ / RTI > As a result, the equalization process in the receiver can be made very simple in OFDM as compared with the conventional single carrier modulation. The equalizer only multiplies each detected subcarrier (each Fourier coefficient) by a constant complex or nearly unchanged value. Thus, the receiver can generally allow such modifications of the signal without having to require explicit information to be transmitted.

OFDM is always used in conjunction with channel coding (forward error correction) and almost always uses frequency and / or time interleaving. Frequency (subcarrier) interleaving may increase the immunity to the effects of frequency selective channel conditions such as fading. For example, if a portion of the channel bandwidth is faded, by performing frequency interleaving, bit errors that can be caused from subcarriers in the faded portion of this bandwidth can be distributed rather than concentrated in the bitstream. Likewise, by performing time interleaving, the bits initially close to each other in the bit stream are transmitted at a distance from each other over time, thereby mitigating the extreme fading that may occur when moving at high speed. Thus, essentially similar to that in the equalization process, the receiver can somehow typically allow modifications of this type without increasing the error rate thereby generated.

The OFDM signal is generated from digital baseband data by an inverse fast Fourier transform (IFFT), which is complicated in IFFT computation and has a relatively high peak to average (PAPR) for a symbol set that includes a full range of symbols, power ratio). This high PAPR generally leads to an increase in the acquisition cost and operating cost of the power amplifier (PA), and a large non-linear distortion is typically encountered compared to systems designed for signals with low PAPR . This large nonlinear distortion causes, among other things, clipping distortion and IM (intermodulation) distortion that cause power loss, and also causes out-of-band interference and possibly in-band interference, error rate is increased.

In a typical type OFDM transmitter, the signal generator performs error correction encoding, interleaving, and symbol mapping on the input information bit sequence to generate transmission symbols. These transmit symbols are subjected to a serial-to-parallel conversion in a serial-to-parallel (S / P) converter and thereby converted into a plurality of parallel signal sequences. The serial to parallel converted signal is then subjected to an inverse fast Fourier transform in the IFFT unit. Then, this signal is parallel-to-serial converted in a parallel to serial (P / S) converter and converted into a single signal sequence. Then, guard intervals are added by a guard interval (GI) adding unit. The signal thus formed is then upconverted to a radio frequency, amplified in a power amplifier, and finally transmitted as an OFDM signal by a wireless antenna.

On the other hand, in a typical type OFDM receiver, the radio frequency signal is downconverted to a baseband or intermediate frequency and the guard interval is removed from the received signal by the guard interval removal unit. The received signal is then subjected to a serial-to-parallel conversion in an S / P converter, a fast Fourier transform in an FFT unit, and a parallel-to-serial conversion in a P / S converter. The decoded bit sequence is then output.

It is common for each OFDM channel to have its own transmission channel ending in the power amplifier (PA) and antenna element. However, in some cases, as shown in FIG. 2, one may want to transmit two or more individual OFDM channels using the same power amplifier (PA) and antenna element. Thereby, a system with added communication bandwidth on a limited number of base station towers may be possible. When the user is added and the data rate is also added, such a system is highly desirable. Two channels can be combined at an intermediate frequency by a two stage upconversion process as shown in FIG. Although amplification for real baseband signals is shown in FIG. 2, it is generally possible to have complex two-phase signals (not shown) with in-phase upconverting and quadrature upconverting . Figure 2 does not show the boundary between the digital signal and the analog signals. The baseband signals are normally digital and the RF transmission signal is normally analog, where there is a digital to analogue conversion stage between the two stages.

Each having an average power P ₀ And two similar channels with maximum instantaneous power P < _{1 &} gt ;. This corresponds to a peak to average power ratio (PAPR) = P ₁ / P ₀ , which is typically expressed in PAPR [dB] = 10 log (P ₁ / P ₀ ) in dB. In the case of a combined signal, the average power is 2 P ₀ (an increase of 3 dB), but the maximum instantaneous power can be as high as 4 P ₁ (an increase of 6 dB). Therefore, the PAPR for the combined signal can be increased by 3 dB. The maximum power value will occur when the signals from the two channels have peak values that are in phase. This is a difficult transient phenomenon, but in general, the linear dynamic range of all transmission components will not be free from this phenomenon. Nonlinearities will cause intermodulation products that can degrade the quality of the signal and cause the signal to spread into undesirable regions of the spectrum. As a result, filtering is required and in any case will reduce the power efficiency of the system.

The components with the increased linear dynamic range required to handle such a high PAPR include a digital-to-analog converter, which should have a larger number of significant bits to handle a larger linear dynamic range. However, a power amplifier (PA) is a more important component because a power amplifier is typically the largest and most power-intensive component in a transmitter. While it is possible to maintain components with excess dynamic range that are only occasionally used in a small amount of time, it is wasteful and inefficient and should be avoided as much as possible. Amplifiers with larger dynamic range are typically more expensive than amplifiers with smaller dynamic range and have lower current efficiency at comparable inputs and outputs and greater current consumption in quiescent state.

Since these channels in OFDM are made up of a number of closely spaced subchannels, the high PAPR problem described above is a common problem well known in OFDM and related waveforms. There are a number of conventional ways to reduce this high PAPR. These methods are described, for example, in Proc. 2006, pages 821-827 of IEEE International Symposium on Signal Processing and Information Technology, which is hereby incorporated by reference, such as in "Directions and Recent Advances in PAPR Reduction Methods" by Hanna Bogucka. Such PAPR mitigation schemes include a number of techniques such as an amplitude clipping and filtering scheme, a coding scheme, a tone reservation scheme, a tone injection scheme, an active constellation extension scheme, a partial transmit sequence (PTS) Signal representation techniques. Using these schemes significantly reduces the PAPR but increases the transmitted signal power, increases the bit error rate (BER), causes loss of data rate, and increases computational complexity. Further, in many of these schemes, additional sub-information (for signal conversion) must be transmitted in addition to the transmitted signal itself such that the received signal is properly decoded. These sub-information degrade the generality of the method, especially when it is desired to use simple mobile receivers to receive signals from various base station transmitters. The schemes disclosed by Bogucka above and other schemes known in the art may be used together to the extent that they are compatible with the schemes described below.

Various efforts to solve the PAPR (Peak-to-Average Power Ratio) problem in the OFDM transmission technology have been studied using frequency domain interleaving, clipping filtering (for example, X. Li and LJ Cimini, " Effects of Clipping and Filtering on the Performance of OFDM (See, for example, L. J Cimini and NR Sollenberger, "Peak-to-Peak-to-Peak-to- Vol. 4, No. 3, pp. 86-88, March, 2000) and a cyclic shift sequence (CSS) method (see, for example, IEEE Std. , G. Hill and M. Faulkner, "Cyclic Shifting and Time Inversion of Partial Transmit Sequences to Reduce the Peak-to-Average Ratio in OFDM", PIMRC 2000, Vol.2, pp. 1256-1259, . In addition, a PTS method has been proposed that uses a minimum clipping power loss scheme (MCPLS) that minimizes the power loss clipped by the transmission amplifier in order to improve reception characteristics in OFDM transmission when a nonlinear transmission amplifier is used Lei, Youxi Tang, Shaoqian Li, "A Minimum Clipping Power Loss Scheme for Mitigation of Clipping Noise in OFDM ", GLOBECOM 2003, IEEE, Vol.1, pp. 6-9, Dec. 2003). MCPLS can also be applied to the CSS method.

In the PTS scheme, a predetermined set of suitable phase rotation values for each subcarrier is selected from a plurality of sets, and this selected set of phase rotation values is used to rotate the phase of each subcarrier prior to signal modulation. This reduces the peak to average power ratio (PAPR) (see, for example, SH Muller and JB Huber, "A Novel Peak Power Reduction Scheme for OFDM", Proc. Of PIMRC '97, pp. 1090-1094, 36, No. 6, 16 ^th March, 2000), which is incorporated herein by reference in its entirety).

A practical method for reducing the PAPR of the combined OFDM signals and what is needed in the related art is to not degrade the quality of the received signal or require no sub information transmission.

The following US patents relate to the Peak to Average Power Ratio (PAPR) problem, each of which is expressly incorporated herein by reference: US Patents 7,535,950, 7,499,496, 7,496,028, 7,467,338, 7,463,698, 7,443,904, 7,376,202, 7,376,074, 7,349,817, 7.34599 million, 7,342,978, 7,340,006, 7,321,629, 7.31558 million, 7,292,639, 7,002,904, 6,925,128, 7.53595 million, 7,499,496, 7,496,028, 7,467,338, 7,443,904, 7,376,074, 7,349,817, 7.34599 million, 7,342,978, 7,340,006, 7,339,884, 7,321,629, 7.31558 million, 7,301,891, 7,292,639, 7,002,904, 6,925,128, and 5,302,914. The following U.S. patent application publications relate to the Peak to Average Power Ratio (PAPR) problem, each of which is expressly incorporated herein by reference: US patent application publication numbers 20100142475, 20100124294, 20100002800, 20090303868, 20090238064, 20090147870, 20,090,135,949, 20,090,110,034, 20,090,110,033, 20090097579,20090086848, 20,090,080,500, 20,090,074,093, 20,090,067,318, 20,090,060,073, 20,090,060,070, 20,090,052,577, 20,090,052,561, 20,090,046,702, 20090034407,20090016464, 20,090,011,722, 20,090,003,308, 20,080,310,383, 20,080,298,490, 20,080,285,673, 20,080,285,432, 20,080,267,312, 20,080,232,235, 20,080,112,496, 20,080,049,602, 20,080,008,084, 20,070,291,860, 20,070,223,365, 20,070,217,329, 20,070,189,334, 20,070,140,367, 20,070,121,483, 20,070,098,094, 20,070,092,017, 20,070,089,015, 20,070,076,588, 20,070,019,537, 20,060,268,672, 20,060,247,898, 20,060,245,346, 20,060,215,732, 20,060,126,748, 20,060,120,269, 20,060,120,268, 20,060,115,010, 20,060,098,747, 20,060,078,066, 20,050,270,968, 20,050,265,468, 20,050,238,110, 20050100108, 20050089116, and 20050089109.

The following non-patent documents also relate to the Peak to Average Power Ratio (PAPR) problem, each of which is expressly incorporated by reference herein: Vijayarangan, et al., &Quot; An overview of techniques for reducing peak to average power ratio and its selection criteria for orthogonal frequency division multiplexing radio systems ", Journal of Theoretical and Applied Information Technology, vol 5, no. 5 (Feb. 2009). Hussain, et al., "Tone reservation's complexity reduction using fast calculation of maximal IDFT element ", IEEE, IWCMC 08, Greece (2008). Zhao, et al., "A study on PAPR reduction by hybrid algorithm based on PTS and GS technique ", The Journal of the Korean Institute of Communication Sciences, Vol. 31, No. 2A, p. 187, Feb. 2006. Marsalek, "On the reduced complexity interleaving method for OFDM PAPR reduction ", Radioengineering, vol. 1, no 3, September 2006. IEEE Int'l Conf. 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Sari, "OFDM peak power reduction techniques performance analysis for WiMAX Systems" Sequans Communications, 4th Annual Wireless Broadband Forum (2005). Lee et al., "Novel low-complexity SLM schemes for PAPR reduction in OFDM systems ", Proc. 2008 IEEE Global Telecommunications Conf. GLOBECOM 2008. Jimenez et al., "Study and Implementation of Complementary Golay sequences for PAR reduction in OFDM signals ", Proc. 11th Med. Electrotech. Conf. MELECON 2002, pp. 198-203. Choi, et al., "Peak power reduction scheme based on subcarrier scrambling for MC-CDMA systems ", IEE Proceedings on Communications, vol. 151, pp. 39-43, Feb. 2004. "Peak-to-average power ratio (PAPR)", Wireless Inf. Trans. System Lab., March 2, 2006. Wang, "PAPR reduction for OFDM ", circa 2007. Giannopoulos, et al., "Relationship among BER, power consumption and PAPR ", 2008 IEEE Int. Symp on Wireless Pervasive Computing, pp. 633-637.

When multiple radio signals with different carrier frequencies are combined to be transmitted, the PAPR (peak to average power ratio) of this combined signal may increase because peaks in phase are likely to be combined with each other. Thereby, a large radio frequency power amplifier (PA) operating at low average efficiency may be required. The PAPR when OFDM (Orthogonal Frequency Division Multiplexed) channels are digitally combined can be used to store time domain OFDM signals for a given symbol period in a memory buffer and to periodically time-shift for at least one OFDM signal. And selecting a time shift corresponding to the reduced PAPR of the combined multi-channel signal. This can be applied to signals on the baseband or upconverted signals. Simulation results show that PAPR of several decibels (dB) can be reduced without degrading system performance. Also, no sub information needs to be transmitted to the receiver.

A preferred embodiment of the system and method according to the present invention is characterized by storing time domain OFDM signals for a given symbol period in a memory buffer and performing periodic time shifts on at least one of said time domain OFDM signals, It is desirable to control the PAPR by selecting a time shift corresponding to the desired PAPR of the signal. In most cases, it is desirable to reduce PAPR to a minimum, but the present invention is not limited to this. Alternatively, the selected time shift may be based on another criterion.

Each of the OFDM signals is pre-processed according to a known manner so that each OFDM signal itself can be processed to reduce its intrinsic PAPR (Intrinsic PAPR), but in order to obtain optimal cost and benefit, Any pre-processing may be in cooperation with the processing for the combined signals. For example, when two individual signals each having a high PAPR are combined, a signal having a reduced PAPR can be obtained if the peaks are canceled by adding 180 degrees of phase difference to each other. Thus, even the initial singular efforts to modify the input OFDM signals are somewhat limited, but may have advantages.

A system according to the present invention is generally intended to combine independently formatted OFDM signals that are typically directed to different sets of receivers or receivers that are not in cooperation with each other. For example, in a cellular transceiver system, one base station may service hundreds or even thousands of cell phones, each monitoring a single OFDM broadcast channel. This base station can serve multiple OFDM channels. In particular, each OFDM subcarrier set is orthogonal but the individual OFDM signals and their subcarriers are generally not orthogonal to each other. The OFDM signals are present in adjacent or displaced channels of one another and therefore a relative phase change between the OFDM signals can occur during a single symbol period. Therefore, PAPR should be considered over the entire symbol period.

According to the method according to the invention, what is analyzed for optimization is the error inferred at the receiver, not the PAPR of the signal. If the PAPR of the combined signal is high only for a small fraction of the symbol period, the power amplifier can distort or clip the signal only in this small fraction of the time, but for most of the time the combined signal is in a good state Can be. Thus, the transmission may be an acceptable transmission and a low error probability may occur. In some cases, this low error probability may be lower than in the case of a signal having a low absolute peak. Thus, a particular communication channel impairment for particular receivers and a margin for Doppler shifts (which can be determined, for example, by determining return path characteristics) can be determined by itself as part of the transmitter signal processing path over a range of possible deviations By employing a receiver model that includes, better performance can be achieved in the case of simply minimizing the PAPR.

Other options are those that deviate from standard protocols such as IEEE-802 OFDM standard, WiFi, WiMax, DAB, DVB, cellular communication, LTE signals, etc. during all or part of a symbol period, Lt; RTI ID = 0.0 > OFDM < / RTI > For example, one or more subcarriers may be removed or modified during a portion of a symbol period such that the PAPR is high in a small portion of the symbol period such that the PAPR is allowed and the signal at the receiver uses the standard receiver The transmitter can implement the OFDM signal modification described above without the need to transmit sub-information identifying the above-mentioned OFDM signal modification needed for demodulation. Another possible modification is to frequency shift the signal (with a slight violation of orthogonality) within the tolerance range to operate within a range of frequency shift and equivalent Doppler shift.

Consider two OFDM signals that are combined as shown in FIG. For simplicity, we refer to signal 1 (S1) as the reference signal and signal 2 (S2) as the modified signal. During each OFDM symbol period, baseband digital data bits for each signal will be stored in memory. Suppose that the preamble has been removed, but a cyclic prefix (CP) remains. The bits of the reference signal S1 are stored in a first-in first-out (FIFO) shift register SR, as shown in Fig. 3 for one embodiment of the present invention. Corresponding bits of the modified signal S2 are stored in a circular shift register (CSR). The CSR is configured such that the data contained therein can be rotated under program control. As the digital format is maintained at an increased sampling frequency relative to the digital data rate, the data of the reference signal and the modified signal are upconverted to the intermediate frequency (IF) and then combined (added). The combined IF signals are then subjected to a PAPR test, thereby determining whether the peak power level is acceptable. In another embodiment, it is determined whether other criteria are met. The result can be equivalent to a PAPR of, for example, 9 dB. Once the test passes, the data bits of the combined OFDM symbols are read out, then recombined into the entire OFDM frame, upconverted to full RF, amplified by the power amplifier and transmitted. According to another embodiment, the combined OFDM representation of the combined data may itself be the source for the upconverting.

More generally, if parameter information (relative time shift) to achieve the desired criterion is determined, then the parameter information or the digital data bits of the baseband signal, or based on the last generated representation, which may be a transformed form of the baseband signal Thereby forming a final signal. In the latter case, i.e., if the final signal is formed based on the final generated representation, the system performs a series of transformations on the data. Some of these series of transforms may be extra transformations that have not been used or failed transformations. Perform these transformations to find what is acceptable or optimal. Once an acceptable or optimal one is found, it is not necessary to repeat this series of transformations again. Similarly, optionally repeating a series of transformations determined by returning to the original digital data, a somewhat different representation will be formed in the register. For example, this representation can be simplified or predistorted so that analog component performance problems can be considered in a coupling test.

More generally, in the present invention, a range of one or more allowable parameters, which may be incrementally, algorithmically, randomly or otherwise, is provided for each signal to be combined. Then, it is tested and / or analyzed as to whether at least some of the possible OFDM signal combinations meet one or more criteria. The combined OFDM signals are then implemented using parameters selected from a larger set of available parameters. These parameter changes and tests may be performed in a serial manner with high-speed digital circuits such as superconducting logic or, if necessary, using low-speed logic with parallelism. However, other technologies, including but not limited to, optical computers, programmable logic arrays, super parallel computers (e.g., graphics processors such as nVidia Tesla® GPU, ATI Radeon R66, R700) Can be used. For example, it may be advantageous to use superconducting digital circuits in the case of a very large number of complex calculations using a large number of specific high-speed processors, such as when a large number of independent receivers are modeled as part of the transmitter optimization.

In the preferred embodiments, in any state of the test over the parameter range, if the test is not passed, a control signal is provided to a register, such as a CSR, for example, which rotates the data bits of the modified signal S2 . The shifted data is then combined with the data initially stored from signal S1 as before and the PAPR is then retested. This is repeated until the PAPR test passes. The sequence of similar steps is shown in FIG. In Fig. 4, an operation of removing the preamble and reattaching the preamble to the end is explicitly shown. In some cases, the tests can be done in parallel, so that strict testing and repetitive testing is not required. This allows low-speed test logic to be used, although the complexity is high. Similarly, for each relative time shift, ancillary parameters may also be considered.

For example, ancillary considerations for optimal combining may be in-band (non-filterable) intermodulation distortion. Thus, at each basic parameter change, predicted intermodulation distortion, e.g., expressed in terms of power and / or inferred BER, can be computed. These considerations can be merged into the PAPR within an acceptable PAPR range, for example by thresholding or by optimizing a simple linear combination "cost function ".

There is some delay in this Shift-and-Add (SAA) process, but the total decision algorithm time, including the entire iteration, should not exceed the extended symbol period T + ΔT. The serial determination process is described in FIGS. 3 and 4. FIG. As described above, in some cases, it may be desirable to perform some of these processes in parallel using multiple CSRs and multiple parallel PAPR tests with different shifts to complete the process more quickly. Such a bar is shown in Fig. FIG. 5 shows parallel memories (shown as RAMs in this figure), each with a suitable time shift, and a minimum PAPR is selected for transmission to the RF subsystem. A preferable configuration can be determined in accordance with an optimal trade-off between the circuit speed and the complexity.

In some situations, a vast amount of computational resources is needed to search for the best combined signal. Indeed, an inductive method can be used to limit the amount of search while achieving an acceptable result. In the case of PAPR optimization, the goal is generally to test for a limited number of " worst case "symbol combinations with a low probability. When raw digital data is obtained, a lookup table is used to test for poor coupling. This poor combination can then be processed according to a predetermined modification. However, when complex symbols are combined in a multiplex manner, this lookup table can not be used. On the other hand, each individual OFDM waveform can be searched for a peak that is, for example, at least 6 dB above the average. It can be determined whether only this portion of the signal is analyzed and temporally aligned with the peaks of the other OFDM signals. If the peaks are not temporally synchronized with each other, it can be assumed that the unacceptable peak has not generated the final combined signal. This method makes assumptions that should be statistically acceptable. That is, it is assumed that only a part of the OFDM waveform, which itself is a relative peak, will contribute to a large peak of the combined OFDM signal. This method avoids the operation of serially testing the results of sequentially changing parameters and avoids superposition of binary critical conditions in the worst case.

It should be noted that the symbol data cyclically shifted for the modified signal represents exactly the same symbol set as the un-shifted data. In addition, due to the standard nature of OFDM signals, the shifted symbol set can be transmitted and received without any undue degradation of signal integrity without the need for any special sub information. Thus, the combined OFDM channels with reduced PAPR should exhibit essentially the same performance as the original unshifted version. A detailed set of simulations confirming this will be described in the following detailed description section.

Although the above figures focus on reducing PAPR when combining two OFDM channels, the present method is not limited to two channels. Three or more channels may be optimized by cyclical time shifting in a similar manner, followed by a PAPR test.

Figures 1A and 1B illustrate typical behavior of orthogonal frequency division multiplexed channels (OFDM channels) in the frequency domain and time domain.
FIG. 2 illustrates the coupling of two OFDM channels using a dual upconverting scheme in a transmitter.
FIG. 3 is a simplified block diagram illustrating the manner in which two OFDM channels are combined, in which data bits of one OFDM channel are cyclically shifted in order to reduce the PAPR (peak-to-average ratio).
4 shows the structure of two OFDM channels, in which data of one OFDM channel is periodically shifted in order to reduce the PAPR.
5 is a block diagram illustrating a method for storing a plurality of shifted replicated data of data from an OFDM channel in a memory and selecting one replicated data capable of minimizing PAPR.
6 is a block diagram of a simulated communication system including a shift and add (SAA) algorithm within the transmitter.
FIG. 7 shows the transfer function of the power amplifier included in the transmitter for the simulation shown in FIG.
FIG. 8 is a simulation graph showing the BER performance when the SAA algorithm is applied to the quadrature phase-shift keying (QPSK) modulation and the BER performance when the SAA algorithm is not applied as a function of SNR (signal-to-noise ratio).
9 is a simulation graph showing the BER performance when the SAA algorithm is applied to 16-QAM (16 quadrature amplitude modulation) and when the SAA algorithm is not applied as a function of SNR (signal-to-noise ratio).

The OFDM channels are composed of a plurality of subchannels each of which is a narrowband signal (see Figs. 1A and 1B). The OFDM channel itself has a time-varying envelope and can typically exhibit a substantial PAPR of 9 to 10 dB. However, if two separate OFDM channels are combined, the resulting signal will exhibit a PAPR of 12 to 13 dB in the case of 3 dB gain. This PAPR value is unacceptably large because it requires a power amplifier with a capacity as much as four times as large as the average to transmit a coupling signal that can be transmitted with a transmission capacity that is twice as large.

Thus, a preferred embodiment of the present invention provides a PAPR mitigation method that reduces the PAPR of the combined signal of two OFDM channels from 12 to 13 dB to 9 to 10 dB of the original components. This reduction of the PAPR by 3 dB is preferably achieved without degrading the quality of the signal and without requiring the receiver to transmit any particular sub information required to recover the OFDM symbols. In addition, the PAPR mitigation method is sufficiently simple to be implemented with any hardware technology as long as it is sufficiently fast.

Conventional PAPR mitigation methods focus on combining subchannels to create a single OFDM channel with no high PAPR. The present invention may in some aspects be thought of as a combination of partial transmit sequence (PTM) and selected mapping (SLM).

In a typical PTS, an input data block of N symbols is divided into separate sub-blocks. The subcarriers in each subblock are weighted by a phase factor for the subblock. The phase elements are selected such that the PAPR of the combined signal is minimized.

In the SLM scheme, the transmitter generates a set of candidate data blocks that are sufficiently different from each other, all of which represent the same information as the original data block, and transmit the best candidate data block (the lowest PAPR (I.e., a candidate data block having a plurality of candidate data blocks).

The hybrid scheme of the present invention combines a PTS element and an SLM element for combined carrier modulated signals. Various periodic time shifts of the oversampled OFDM waveform are searched and a time shift with the lowest PAPR is selected. In a manner similar to the PTS scheme, one OFDM signal is used as a reference and the other carrier modulated signal (s) is used to generate time shifts. The search window is determined by the cyclic prefix (CP) length and the oversampling rate.

The phase space of possible combinations of shifts increases significantly, but it will not be necessary to search all these combinations. In general, very high value PAPRs occur relatively infrequently, so most time shifts starting with a high PAPR state tend to lead to PAPR reduction. The shifts in the multiple channels may be implemented sequentially or in parallel or some combination thereof. This allows, for example, any combination with a PAPR that is within an acceptable range to be acceptable, any unacceptable PAPR condition occurs at 1% of the symbol period, and the search space searched for an acceptable PAPR is Generally less than 2% of the possible states. On the other hand, if other acceptability criteria are adopted, a larger search space will be necessary or appropriate. Assuming, for example, that it is more expensive to transmit a high PAPR signal, formal optimization may be appropriate, assuming that, for example, there is more power or interference cost. If no inductive method is available to predict the optimal state, it may be appropriate to search the entire parameter space so that the cost is minimized.

This scheme is different from the conventional scheme in which different OFDM channels are independent of one another and have separate transmission chains and are not synchronized with each other. In addition, the conventional method was directly performed on the baseband signal. In contrast, in the method according to the present invention, the PAPR is evaluated for the upconverted combined signal comprising two or more OFDM channels, and the symbol periods for each of these OFDM channels must be synchronized with each other. This causes no problems in receivers where each channel is independently received and clocked.

Some conventional schemes dealing with PAPR are based on clipping, but in this way distortion and out-of-band generation must occur. Some other conventional schemes can avoid this distortion, but require specific conversions that must be decoded at the receiving end. These conventional schemes may require sub-information transmission or may not follow the standard OFDM communication protocol. The scheme according to the invention has none of these disadvantages.

OFDM channels used in cellular communications may have a bandwidth of up to 10 MHz or 20 MHz. However, these channels may be located within a very wide frequency band, such as 2.5 GHz to 2.7 GHz. Thus, they can have a combination of two or more OFDM channels each having a bandwidth of 10 MHz and spaced apart from each other by 100 MHz. A 10 MHz digital baseband signal is sampled at a rate as low as 20 MS / s, but a combined digital signal covering 100 MHz should be sampled at a rate of at least 200 MS / 2.

In the preferred embodiment, the signal combining operation (including upconverting in FIG. 3) is performed in the digital domain with an increased sampling rate. The PAPR threshold test and CSR control are also implemented at higher rates. This rate should be fast enough so that multiple iterations can be performed within a single symbol period (microseconds).

To demonstrate that the PAPR of the combined OFDM channels can be reduced without degrading the system performance by circular time shifts, full-band Monte-Carlo simulations have been performed for OFDM transmission and reception. A block diagram of this simulation is summarized in FIG. In Fig. 6, an " SAA evaluation test bench "is shown and the transmitter combines the OFDM signals of frequencies F ₁ and F ₂ through the SAA algorithm to reduce the PAPR. At the receiving end, this combined signal is downconverted and the signal at frequency F ₂ is recovered using a standard OFDM receiver. Further, an appropriate AWGN (Additive White Gaussian Noise) is added to this channel. This simulation also includes an actual transfer function for a nearly linear power amplifier (PA), which represents a deviation from the linear approximation to saturation (as shown in Figure 7).

The parameters in the PAPR BER (bit error rate) simulation are as follows. Each packet contains 800 bytes of information that is modulated over several OFDM symbol periods according to the modulation type used. Quadrature phase-shift keying (QPSK) modulation and 16-quadrature amplitude modulation (16-QAM) are both examined. Each SNR point is sustained until 250 packet errors occur. The cyclic prefix is set to 1/8 of the total symbol period. The carriers at frequencies F ₁ and F ₂ are sufficiently spaced such that their spectra do not overlap. The oversampling rate is 8 times. Finally, we have a very sharp rolloff and have a sampling frequency F _s = 160 MHz and a raised cosine filter with frequency cutoff F _c = 24 MHz was used. A PAPR threshold of about 9 dB was used for the combined OFDM channels.

FIG. 8 shows the BER performance when applying the SAA algorithm for QPSK modulation and when not applying it as a function of SNR (signal-to-noise ratio) (changing the AWGN power). Figure 9 shows the BER performance when applying the SAA algorithm for 16-QAM modulation and when not applied as a function of SNR (signal-to-noise ratio) (changing the AWGN power). In both cases, it can be seen that the BER is not substantially reduced from the zero-shift curve.

Quantitatively, the net performance improvement when applying the SAA algorithm is 2.35 dB for QPSK modulation and 2.9 dB for 16-QAM modulation, as can be deduced from the BER plot. For example, when the SAA (shift and add) algorithm is not applied, the BER shows an error floor of 0.03 at an input backoff of 8.5 dB (for the power amplifier), whereas a BER of 6.5 dB When SAA is applied, BER shows the same error floor of 0.03. Therefore, the performance improvement achieved by applying the SAA algorithm is 8.5 - 6.5 = 2 dB.

From this simulation, it can be seen that the SAA algorithm not only reduces the PAPR in the combined OFDM channel by 3 dB, but also this PAPR reduction can be achieved without including any specific sub information about the transform in the transmission signal without degrading the signal quality .

One preferred implementation of the present invention may be implemented using a high speed field programmable gate array (FPGA) with blocks for shift register memory, digital upconverting, and threshold testing. Alternatively, ultra high speed digital technology such as a rapid single flux quantum (RSFQ) superconducting circuit can be used. As the number of OFDM channels coupled increases, it is necessary to increase the speed of the algorithm, or to perform some of the processing in parallel.

The method according to the present invention can also be applied to a reconfigurable system following a wireless cognitive technology in which a plurality of channels to be transmitted are dynamically re-allocated according to user requirements and available bandwidth. The number of channels to be transmitted and the frequency assignments for them may vary entirely under software-controlled control. All channels follow the same general symbol protocol and timing, and a similar set of SAA (shift and add) algorithms can be applied to maintain PAPR for efficient transmission.

The foregoing description of the invention has been presented to enable any person skilled in the art to make or use the invention. Various modifications to this invention will become readily apparent to those skilled in the art, and the overall principles set forth herein may be applied to other modifications without departing from the spirit or scope of the invention. Accordingly, the invention is not limited to the design and illustrations described herein, but the widest scope consistent with the principles and novel features disclosed herein may be devoted to the invention. Accordingly, the scope of the present invention is defined by the following claims.

Claims (43)

CLAIMS 1. A method of controlling a combined waveform representing a combination of at least two signals, each signal having orthogonal frequency multiplexed signal components, wherein the combined waveform comprises at least two The signal components are modulated in each communication channel according to a respective communication protocol,
Receiving information defining at least two signals,
Converting the information defining each signal into at least one representation having an orthogonal frequency multiplexed signal component, the information defining at least one of the respective signals having different orthogonal frequency multiplexed signal components representing the same information At least two alternate representations are converted, each alternate representation being compatible with the communication protocol of each signal,
Using at least two alternating representations having different orthogonal frequency multiplexed signal components representing the same information in at least two different ways to define respective different combinations of at least two signals representing the same information, Combining a transformed representation of information of at least two signals each having a frequency multiplexed signal component,
Analyzing each different combination representing the same information for at least one criterion,
Selecting each of the at least two alternate representations to be included in the association based on the analyzing step, and
(a) information identifying each of the coupled waveforms, (b) information defining each of the combined waveforms, and (c) a selected combination of transformed information from each of the at least two signals, And outputting at least one of information indicative of each coupled waveform included. The method of claim 1, wherein the first and second combinations of the transformed information comprise: (a) relative timing of modulation of the frequency components of the first signal with respect to modulation of the frequency components of the second signal; and (b) Wherein at least one of the relative phases of the frequency components is different from each other. 3. The method of claim 1 or claim 2, wherein the at least one criterion comprises a Peak to Average Power Ratio (PAPR), wherein the at least one signal is an IEEE 802.11 protocol, an IEEE 802.16 protocol, a 3GPP- And a DVB protocol, wherein the receiver compliant with the at least one protocol is capable of communicating with at least two of each of the at least two respective streams without requiring additional information to be transmitted outside the protocol. And demodulating the different combinations. 3. The method of claim 1 or 2, wherein the at least two alternate representations are cyclic time shifts in a modulation sequence comprising the entirety of information defining a respective signal representation with orthogonal frequency multiplexed signal components Wherein the at least one criterion comprises a peak to average power ratio (PAPR), and an alternate representation that produces a lowest PAPR is selected for combining. 3. The method of claim 1 or 2, wherein said analyzing comprises analyzing the clipping distortion of the coupled waveform in a model of the amplifier. 3. The method of claim 1 or claim 2, wherein the step of outputting comprises converting the selected combined signal to a radio frequency analog signal adapted to be transmitted without frequency modulation from a digital representation of the selected combined signal, Control method. 3. The method of claim 1 or 2, further comprising predistorting at least one of an intermediate frequency representation and a radio frequency representation of the selected combined signal. 8. The method of claim 7, wherein the predistorting further comprises generating at least one of a receiver characteristic of an analog wireless communication system that communicates using analog nonlinearity, a nonlinear distortion of a power amplifier amplifying a selected coupled signal, a transmission channel impairment, And compensates at least a portion of the combined waveform. 3. The method of claim 1 or claim 2, wherein each representation of at least two signals having orthogonal frequency multiplexed signal components of the same information comprises an orthogonal frequency domain multiplexed signal having a cyclic prefix, Wherein the two alternate representations are different in respective periodic time shifts. 3. The method of claim 1 or claim 2, wherein each of the at least two signals is received as an orthogonal frequency division multiplexed signal compliant with a communication protocol, and at least one of the signals comprises: (a) all of the first signal for the second signal The relative timing of the modulation of the frequency components, and (b) the relative phase of all frequency components of the signal are different, wherein at least one criterion is a PAPR (peak to average power ratio), and the selected combined signal is a combined signal indicative of the lowest PAPR. An apparatus for combining a plurality of signals in each of a plurality of channels, each signal comprising a set of phase and amplitude modulation, or phase modulation, or amplitude modulated orthogonal frequency components in one channel, ,
Receiving information defining each of the plurality of signals,
Wherein the information defining the respective signals of the plurality of signals is represented as a plurality of orthogonal frequency multiplexed signal components of the respective ones of the plurality of signals and information defining at least one of the plurality of signals defines the same information received together A plurality of alternating sets of orthogonal frequency-multiplexed signal components,
Analyzing a plurality of different combinations of each of a plurality of orthogonal frequency-multiplexed signal components representing a plurality of signals for at least one fitness criterion, each of the plurality of representations performing orthogonal frequency multiplexing of at least one signal of the plurality of signals Each comprising an alternating set of signal components,
A processor configured to select each of the different bonds based on the analysis;
An output port configured to present at least one of identification of the selected binding, selected binding, and information defining the selected binding
≪ / RTI > 12. The method of claim 11, wherein the first and second combinations of transformed information comprise: (a) relative timing of modulation of all frequency components of the first signal relative to the second signal; and (b) Wherein at least one of the at least one reference signal and the at least one reference signal is different from each other and at least one reference comprises a peak to average power ratio (PAPR). 13. The method of claim 11 or 12, wherein at least one of the plurality of signals comprises at least one protocol selected from the group consisting of an IEEE 802.11 protocol, an IEEE 802.16 protocol, a 3GPP-LTE downlink protocol, a DAB protocol, Wherein the at least one protocol compliant receiver is a compatible orthogonal frequency division multiplexed stream and wherein the receiver conforming to the at least one protocol is adapted to transmit each alternate frequency multiplexed signal component of at least one signal of the plurality of signals, And demodulating each of the plurality of sets of signals. 13. The method of claim 11 or 12, wherein each alternate set of orthogonal frequency multiplexed signal components of at least one of the plurality of signals is such that the cyclic time shift in the modulation sequence is different, Wherein the at least one criterion comprises a peak to average power ratio (PAPR), wherein an alternate representation that produces a lowest PAPR is selected for combining. 13. The apparatus of claim 11 or 12, wherein the output port is configured to output, in a digital representation of the combined signal, a direct conversion to a radio frequency analog signal for transmission without frequency modulation. 13. The apparatus of claim 11 or 12, wherein the processor is further configured to pre-warp at least one of an intermediate frequency and a radio frequency representation of a selected combined signal. 17. The receiver of claim 16, wherein the predistortion comprises at least a portion of at least one of an analog nonlinearity, a nonlinear distortion of the power amplifier amplifying the selected combined signal, a transmission channel impairment, and a receiver characteristic of an analog wireless communication system communicating using the selected combined signal Compensating a plurality of signals. 13. The method of claim 11 or 12, wherein each of the at least two signals is received as an orthogonal frequency division multiplexed signal that complies with a communication protocol, and wherein each of the alternating frequency multiplexed signal components of at least one of the plurality of signals Each of the sets being configured to be demodulated by a protocol compliant receiver without having to receive additional information outside the communication protocol, at least one criterion comprising a peak to average power ratio (PAPR) of the combined signal, Is a combined signal indicative of the lowest PAPR. 13. The apparatus of claim 11 or 12, wherein the processor analyzes the nonlinear distortion of the coupling waveform in the model of the amplifier and further pre-warp at least one component of the selected coupling.
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