Abstract:
The invention provides apparatuses and method for crest factor reduction. In one embodiment, an apparatus comprising: a plurality of peak detectors respectively corresponding to a plurality of paths, each peak detector detecting a peak from an input signal; a statistical multiplexer, statistically multiplexing the peaks from the plurality of peak detectors; an allocator, allocating one of the multiplexed peaks to one of a plurality of peak cancellation units; the plurality of peak cancellation units, each generating, for an allocated peak, a cancellation peak; a plurality of de-multiplexers respectively corresponding to the plurality of peak cancellation units, each de-multiplexing the cancellation peak onto a corresponding path; a plurality of summation units respectively corresponding to the plurality of paths, each summing the de-multiplexed cancellation peaks on one path; and a plurality of subtraction units, each subtracting the summed cancellation peaks from the input signal on the path to obtain a peak-cancelled signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a crest factor reduction technique in a communication field, and particularly to a method and an apparatus for peak cancellation crest factor reduction in a Radio Remote Unit (RRU) in a wireless communication system. 
         [0003]    2. Description of Prior Art 
         [0004]    Peak-to-average power ratio (PAPR) is one of the most critical problems of Orthogonal Frequency Division Modulation (OFDM). Generated by summation of a large number of subcarriers, an OFDM signal usually has a high PAPR. In a radio front end, high PAPR may lead to serious back-off of power amplifiers (PAs), and result in a low efficiency of the PA. 
         [0005]    To reduce the PAPR of OFDM, a series of Crest Factor Reduction (CFR) techniques have been introduced in reference document [1] (W. B. Wang, K. Zheng, “Wideband wireless communication OFDM technology”, pp. 93-110, China Posts and Telecom Press, 2007). In baseband processing, most PAPR reduction methods are based on redundancy employing such as coding, selective mapping and tone reservation. These techniques cause no signal distortion, but severe computation complexity and lower spectrum efficiency. Furthermore, for multicarrier application, baseband processing schemes can hardly predict the PAPR after interpolation and carrier combination. Therefore, it is better to reduce PAPR in a digital Intermediate Frequency (IF) domain after carrier combination. And the most efficient way to reduce PAPR in the digital IF is clipping. 
         [0006]    Clipping is a non-linear process that may cause severe in-band distortion and out-band noise. In-band distortion leads to Error Vector Magnitude (EVM), Signal-to-Noise Ratio (SNR) and Block Error Rate (BLER) degradation, while out-band noise may cause Adjacent Channel Leakage Ratio (ACLR) failure. Therefore, it is important to design the clipping algorithm deliberately to reduce PAPR with limited EVM and ACLR degradation. 
         [0007]    Noise Shaping Crest Factor Reduction (NS-CFR) and Peak Cancellation Crest Factor Reduction (PC-CFR) are two of the most popular clipping algorithms. As shown in  FIG. 1  ( a ), a NS-CFR algorithm  100   a  may extract an input signal above of a predetermined clipping threshold in a peak extraction block  110   a,  and reshape it by a filtering block  130   a.  Then the input signal delayed by a matched delay block  150   a  may be subtracted by this shaped peak signal. 
         [0008]    A PC-CFR algorithm may generate a pre-shaped peak signal when a peak is detected. Then this pre-shaped peak may be scaled and subtracted from the input signal. As seen from  FIG. 1(   b ), which shows a typical structure of the PC-CFR algorithm  100   b,  when a peak is detected by the peak detection block  110   b,  the PC-CFR algorithm  100   b  allocates the peak to one of the cancellation units inside the peak cancellation block  130   b,  so as to generate the pre-shaped and scaled cancellation peak to be subtracted from the input signal delayed by the match delay block  150   b.    
         [0009]    Generally, peaks of a Long-Term Evolution (LTE) signal arrive in low probabilities. This indicates a significant characteristic of signal peaks—sparsity. The PC-CFR algorithm may exploit the sparsity of signal peaks. A small number of peak cancellation units may be enough to do the clipping task, therefore hardware resource may be greatly reduced. As for clipping performance, in general, the NS-CFR algorithm has a comparable performance with the PC-CFR algorithm, because the intrinsic principles of these algorithms are similar. But specifically, the PC-CFR algorithm works a little better than the NS-CFR algorithm in metric of PAPR and EVM, as shown in reference document [2] (E. Hemphill, S. Summerfield, G. Wang, and D. Hawke, “Peak Cancellation Crest Factor Reduction Reference Design”, XAPP1033 (V1.0), Xilinx, Nov. 18, 2007). In reference document [3] (W. B. Wang, K. Zheng, “Wideband wireless communication OFDM technology”, pp. 93, China Posts and Telecom Press, 2007), simulations also show that the PC-CFR algorithm is better measured by PAPR reduction and SNR drop. 
         [0010]    Hereinafter, a detailed block diagram of a PC-CFR unit using the PC-CFR algorithm in the prior art may be described with reference to  FIG. 2 . 
         [0011]      FIG. 2  illustratively shows a detailed block diagram of a PC-CFR unit  200 . As shown in  FIG. 2 , the PC-CFR unit  200  may comprise a peak detector  210  including a peak extractor  211  and a peak indication unit  213 , an allocator  220 , a plurality of peak cancellation units  230   s  (here are four peak cancellation units  230   s  as an example), a summation unit  240 , a matched delay unit  250 , and a subtraction unit  260 . 
         [0012]    The peak detector  210  may detect a peak from an input signal. Generally, the detection of the peak may be carried out on magnitude of the input signal. 
         [0013]    In particular, the peak extractor  211  may calculate the magnitude and a phase of the input signal using e.g. Coordinate Rotational Digital Computer (CORDIC) algorithm. Then, the peak extractor  211  may subtract the magnitude of the input signal by a predetermined clipping threshold to obtain a magnitude difference D Mag  between the magnitude and the threshold. The threshold may be determined based on PARP requirement and an average power of the input signal. 
         [0014]    The peak indication unit  213  may indicate a maximum value of the D Mag s as a peak indicator for a peak. The maximum value may be obtained by comparison with neighboring values. It can either be a local max value or a global max value in a given time window, as will be understood by the skilled in the art. 
         [0015]    Once an incoming peak is detected, the allocator  220  may allocate the peak to one of the peak cancellation units  230   s  for generating a cancellation peak. As well-known by the skilled in the art, the “peak” here is actually represented by peak information on the detected peak of the input signal, which includes the magnitude difference D Mag  and the phase of the detected peak. The term “peak” is used here and hereinafter for simplicity. 
         [0016]    In this example, it is assumed that there are four peak cancellation units  230   s . Each peak cancellation unit  230  may cancel only one peak at a time. The length of the cancellation peak together with the number of the peak cancellation units  230   s  may determine a rate at which the peaks may be cancelled. When a new peak is detected, the allocator  220  may assign an available peak cancellation unit  230  to cancel that peak. If all the peak cancellation units  230   s  are unavailable when a new peak is detected, the peak will not be cancelled. Multiple iterations of PC-CFR may be necessary to eliminate the peaks that were not cancelled during an earlier PC-CFR process. 
         [0017]    The magnitude of the cancellation peak may be scaled to the magnitude difference D Mag  between the corresponding peak magnitude and the clipping threshold. The cancellation peak may be designed to have a spectrum that matches that of the input signal and therefore may only introduce negligible out-of-band interference. The cancellation peak may be stored as cancellation peak coefficients in a RAM in advance. Generally, the peak cancellation unit  230  may include two blocks of RAM for storing the input signal (one for storing I coefficients and one for storing Q coefficients) and one complex multiplier which may include four DSP multipliers for scaling. As will be understood by the skilled in the art, the cancellation peak coefficients may be predetermined by carrier configuration including carrier number, carrier frequency and carrier bandwidth etc. For example, in the application of a single carrier with a central frequency of 0, the spectrum shape is rectangular, which corresponds to a set of coefficients like a sinc waveform in the time domain. 
         [0018]    Once the incoming peak is allocated to one of the peak cancellation units  230   s , these coefficients will be read out sequentially. Then the cancellation peak coefficients may be scaled by the magnitude difference D Mag  and the phase of the incoming peak in the peak cancellation unit  230 . Although there are four peak cancellation units  230   s  in this example, the number of the peak cancellation units may be varied depending on actual design demands in consideration of e.g. the carrier number, the carrier bandwidth, and statistic characteristics of the carrier data. 
         [0019]    Then, outputs of all the cancellation units  230   s  may be summed in the summation unit  240 . And the sum from the summation unit  240  may be subtracted from a delayed input signal output from the matched delay unit  250  by the subtraction unit  260 . 
         [0020]    Since the number of peak cancellation units is limited, it is always possible that when all peak cancellation units are occupied, it is unable to process new coming peaks. This will lead to peak leakage and bring a high risk to the PA. In order to mitigate the peak leakage problem and achieve better performance, iterative clipping may be used. For a single carrier LTE application with a bandwidth of 20 MHz, a simulation and on-board test shows that the PC-CFR with 2˜4 stages of clipping is suitable. 
         [0021]    Unfortunately, for various reasons, the current PC CFR algorithm may not be efficient any more for a multicarrier LTE application with carrier number no less than 2 and a signal bandwidth no less than 40 MHz. 
         [0022]    Firstly, the PAPR of the input signal grows along with the increase of carrier number, since for the OFDM signal, PAPR≈N, N is the total sub-carrier number (reference document [4] X. Li, L. J. Cimini Jr, “Effects of Clipping and Filtering on the Performance of OFDM” in Proc. of Vehicular Technology Conference, IEEE, pp. 1634, 1997 Invention Information).  FIG. 3  shows an example that the PAPR grows along with carrier number. This problem brings a serious challenge to the PC-CFR algorithm, since the arrival rate of peaks grows severely with the increase of PAPR, which may lead to a much larger number of peak cancellation units and clipping stages, also higher risk of peak leakage. 
         [0023]    Secondly, the sample rate in the digital IF must be high enough for interpolation and carrier combination. Implementation of digital pre-distortion (DPD) also requires a high oversampling rate. These result in a sample rate higher than 200 MHz in the digital IF. For PC CFR, a higher sample rate will lead to a higher order of coefficients and more difficult for time division multiplexing. Furthermore, peak re-growth after filtering and interpolation will compromise the reduction of PAPR for wideband signal after clipping. These challenges cause poor clipping performance. Iterative clipping may mitigate the degraded performance problem, but results in exponentially increase of clipping stages. Fractional filter between clipping stages may mitigate the effect of peak re-growth, but also needs extra filters for each stage. Therefore, the hardware implementation will be far more complicated and power consumption will be much higher. As an example, according to an on-board test, for a 2×20 MHz LTE application scenario, with 11 dB PAPR at 0.01% probability, the PC CFR algorithm works at a sample rate of 122.88 MHz after carrier combination. If one clipping stage contains four peak cancellation units, it will need 5˜6 stages of clipping at least to get PAPR≦8 dB and EVM≦5% for a transceiver (TRX) board output. 
         [0024]    For all these reasons, previous PC-CFR algorithm can hardly work efficiently in the multicarrier LTE application. There is a desire of simplifying design, in order to address at least some of the serious challenges brought by the multicarrier LTE application. 
       SUMMARY OF THE INVENTION 
       [0025]    Accordingly, a main object of the present invention is to provide a novel PC-CFR approach which employs statistical multiplexing. Since signal peaks always come by low probability (although higher in the multicarrier situation), randomicity is an inherent characteristic of peaks. Therefore, statistical multiplexing may use dynamic resource allocation technique to improve the efficiency of clipping. 
         [0026]    According to one aspect of the present invention, an apparatus for crest factor reduction may be provided. The apparatus may comprise: a plurality of peak detectors respectively corresponding to a plurality of paths, each peak detector being configured for detecting a peak from an input signal on a corresponding path; a statistical multiplexer, configured for statistically multiplexing the peaks from the plurality of peak detectors; an allocator, configured for allocating one of the multiplexed peaks to one of a plurality of peak cancellation units; the plurality of peak cancellation units, each configured for generating, for the allocated peak, a cancellation peak; a plurality of de-multiplexers respectively corresponding to the plurality of peak cancellation units, each de-multiplexer being configured for de-multiplexing the cancellation peak onto the corresponding path; a plurality of summation units respectively corresponding to the plurality of paths, each configured for summing the de-multiplexed cancellation peaks on one path; and a plurality of subtraction units respectively corresponding to the plurality of paths, each configured for subtracting the summed cancellation peaks from the input signal on the same path to obtain a peak-cancelled signal. 
         [0027]    Preferably, the apparatus may further comprise a plurality of matched delay units respectively corresponding to the plurality of paths, each configured for matched-delaying the input signal before being subtracted by the summed cancellation peaks on the same path. 
         [0028]    According to this aspect of the present invention, the efficiency of the apparatus may be improved due to the statistical multiplexing of the plurality of input signals. 
         [0029]    Preferably, the apparatus may further comprise an identifying unit connected to the plurality of peak detectors, configured for identifying which one of the plurality of paths the peak comes from, once the peak is detected. 
         [0030]    Preferably, the de-multiplexer may be configured to de-multiplex the cancellation peak onto the corresponding path according to a corresponding identifier output from the identifying unit. 
         [0031]    Preferably, the identifying unit is a priority encoder, so that one or more of the plurality of paths have a higher priority to be processed. 
         [0032]    Alternatively, the input signal may be a time division multiplexed (TDM) signal and the peak-cancelled signal is a peak-cancelled TDM signal. Accordingly, the apparatus may further comprise: a plurality of time division multiplexing units respectively connected to the plurality of peak detectors, each configured for multiplexing, according to a fixed order of time taps, signals from a plurality of ways into the TDM signal; and a plurality of time division de-multiplexing units respectively connected to the plurality of subtraction units, each configured for de-multiplexing, according to the fixed order of the time taps, the peak-cancelled TDM signal output from the subtraction unit onto the plurality of ways. 
         [0033]    Preferably, the plurality of ways are N ways and the plurality of paths are [N/M] paths, where M is a TDM factor which is defined as a ratio of a system clock frequency and a signal sample rate, N and M are integers, and “[ ]” represents a ceil operation. 
         [0034]    Preferably, the peak cancellation unit may generate the cancellation peak by scaling a plurality of cancellation peak coefficients pre-stored in the peak cancellation unit which are read out once every M time taps with a phase and a magnitude difference between a magnitude and a predetermined threshold of the peak. 
         [0035]    According to this aspect of the present invention, the apparatus may further improve M times of the circuit efficiency. 
         [0036]    In a case of N=M, the design of the apparatus may be greatly simplified, since the identifying unit, the statistical multiplexer and the de-multiplexers are not needed any more. 
         [0037]    Preferably, the apparatus according to the present invention may be included in either a base station or a user equipment. 
         [0038]    According to another aspect of the present invention, a base station comprising the apparatus may be provided. 
         [0039]    According to another aspect of the present invention, a user equipment comprising the apparatus may be provided. 
         [0040]    According to another aspect of the present invention, a method of crest factor reduction may be provided. The method may comprise steps of: detecting a peak from an input signal on a corresponding one of a plurality of paths; statistically multiplexing a plurality of peaks respectively from the plurality of paths; allocating one of the multiplexed peaks to one of a plurality of peak cancellation units; generating, for the allocated peak, a cancellation peak; de-multiplexing the cancellation peak onto the corresponding one of the plurality of paths; summing the de-multiplexed cancellation peaks on one path; and subtracting the summed cancellation peaks from the input signal on the same path to obtain a peak-cancelled signal. 
         [0041]    Preferably, the input signal is matched delayed before being subtracted by the summed cancellation peaks on the same path. 
         [0042]    According to this aspect of the present invention, the efficiency of the method may be improved due to the statistical multiplexing of the plurality of input signals. 
         [0043]    Preferably, the method may further comprise a step of: identifying which one of the plurality of paths the peak comes from, once the peak is detected. 
         [0044]    Preferably, de-multiplexing the cancellation peak onto the corresponding path is performed according to a corresponding identifier output from the identifying step. 
         [0045]    Preferably, the identifying step is a step of priority encoding, so that one or more of the plurality of paths have a higher priority to be processed. 
         [0046]    Alternatively, the input signal is a time division multiplexed (TDM) signal and the peak-cancelled signal is a peak-cancelled TDM signal, and the method may further comprise steps of: time division multiplexing, according to a fixed order of time taps, signals from a plurality of ways into the TDM signal; and time division de-multiplexing, according to the fixed order of time taps, the peak-cancelled TDM signal onto the plurality of ways. 
         [0047]    Preferably, the plurality of ways are N ways and the plurality of paths are [N/M] paths, where M is a TDM factor which is defined as a ratio of a system clock frequency and a signal sample rate, N and M are integers, and “[ ]” represents a ceil operation. 
         [0048]    Preferably, the step of generating comprises: scaling a plurality of cancellation peak coefficients pre-stored in the peak cancellation unit which are read out once every M time taps with a phase and a magnitude difference between a magnitude and a predetermined threshold of the peak. 
         [0049]    According to this aspect of the present invention, the method may further improve M times of the circuit efficiency. 
         [0050]    In a case of N=M, the method may be greatly simplified, since the steps of statistically multiplexing and de-multiplexing and identifying may be omitted. 
         [0051]    According to various aspect of the present invention, the statistical multiplexing solution as proposed in the present invention may overcome at least some of the disadvantages as previously discussed in the current PC-CFR techniques. The design of the circuit may be highly simplified, while the efficiency of the circuit may be highly improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    The objects, advantages and characteristics of the present invention will be more apparent, according to descriptions of preferred embodiments in connection with the drawings, wherein: 
           [0053]      FIG. 1(   a ) illustratively shows a block diagram of a NS-CFR algorithm in the prior art; 
           [0054]      FIG. 1(   b ) illustratively shows a block diagram of a PC-CFR algorithm in the prior art; 
           [0055]      FIG. 2  illustratively shows a detailed block diagram of a PC-CFR unit using the PC-CFR algorithm in the prior art; 
           [0056]      FIG. 3  illustratively shows an example that PAPR grows along with carrier number. 
           [0057]      FIG. 4  illustratively shows a concept of statistical multiplexing for random service request arrival; 
           [0058]      FIG. 5  illustratively shows a block diagram of an exemplary PC-CFR unit with statistical multiplexing according to an embodiment of the present invention; 
           [0059]      FIG. 6  shows an illustrative flowchart of an exemplary PC-CFR method with statistical multiplexing according to an embodiment of the present invention; 
           [0060]      FIG. 7  illustratively shows a block diagram of an exemplary general PC-CFR unit with a combination of statistical multiplexing and Time Division Multiplexing (TDM) according to an embodiment of the present invention; 
           [0061]      FIG. 8  illustratively shows a block diagram of an exemplary specific PC-CFR unit with a combination of statistical multiplexing and Time Division Multiplexing (TDM) according to an embodiment of the present invention; 
           [0062]      FIG. 9  shows an illustrative flowchart of an exemplary PC-CFR method with statistical multiplexing and Time Division Multiplexing (TDM) according to an embodiment of the present invention; and 
           [0063]      FIGS. 10 and 11  illustratively show block diagrams of CFR solutions in a multicarrier LTE system according to an embodiment of the present invention. 
       
    
    
       [0064]    It should be noted that various parts in the drawings are not drawn to scale, but only for an illustrative purpose, and thus should not be understood as any limitations and constraints on the scope of the present invention. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0065]    Hereinafter, the present invention will be further described in detail by referring to the drawings and exemplary embodiments in order to make the objects, technical scheme and advantages of the present invention more apparent. In the description, details and functions which are unnecessary to the present invention are omitted for clarity. Similar reference numbers may be used for similar components throughout the drawings. 
         [0066]    The following descriptions are based on a multicarrier LTE application scenario. However, it should be understood that the exemplary embodiments are only used for illustration but not for any limitation. The technical solutions of the present invention may also be used in any appropriate scenario, as will be appreciated by the skilled in the art. 
         [0067]    Considering characteristics of randomicity and independency of signal peaks in the multicarrier LTE application scenario, an embodiment of the present invention proposes a PC-CFR scheme with statistical multiplexing. 
         [0068]    In order to better illustrate the embodiment of the present invention, a concept of statistical multiplexing for random service request may be firstly introduced in connection with  FIG. 4 . Statistical multiplexing may be widely used in all kinds of communication systems and data networks. In some applications, statistical multiplexing may also be known as resource pooling. As shown in  FIG. 4 , each server may be used for either of two queues by dynamic allocation when it is idle. Comparing with the case that each server works only for one queue, the probability of each server being occupied is higher, therefore an the efficiency of the whole system may be improved. According to a statistical analysis, the efficiency may be improved 2˜4 times when a communication system is statistical multiplexed by multiple users. 
         [0069]    Hereinafter, a block diagram of an exemplary PC-CFR unit with statistical multiplexing according to the embodiment of the present invention will be described with reference to  FIG. 5 . 
         [0070]    The PC-CFR unit  500  in the embodiment of the present invention may comprise a plurality of peak detectors  510   s,  summation units  540   s,  matched delay units  550   s , subtraction units  560   s  same as those shown in  FIG. 2  for respectively processing a plurality of input signals from a plurality of paths. For simplicity, descriptions on some functions performed by the same units as those in  FIG. 2  are omitted. Furthermore, the PC-CFR unit  500  may comprise a pair of a multiplexer MUX unit  580  and multiple de-multiplexers DEMUX unit  590   s  as well as an identifying unit  570  as proposed in the present invention. Due to the statistical multiplexing, the PC-CFR unit  500  may need only one allocator  520  and corresponding multiple (in this example four) peak cancellation units  530   s  same as those shown in  FIG. 2 . The allocator  520 , the multiplexer  580 , and the de-multiplexers  590   s  may be connected to the identifying unit  570 . The multiple de-multiplexers  590   s  may be arranged for receiving outputs from the multiple (in this example four) peak cancellation units  530   s.    
         [0071]    In this example of  FIG. 5 , two input signals from two different paths are presented for illustration only but not for any limitation. As will be appreciated by the skilled in the art, other number of paths more than two may also be possible. 
         [0072]    As shown in  FIG. 5 , the PC-CFR unit  500  may comprise two peak detectors  510 - 1  and  510 - 2  respectively corresponding to the two paths. Each peak detector  510  (here, the reference number “ 510 ” refers to any one of the two peak detectors  510 - 1  and  510 - 2  for easy description, similarly hereinafter) may be configured for detecting a peak from an input signal of a corresponding path. Same as the peak detector  210  of  FIG. 2 , the peak detector  510  may comprise a peak extractor  511  and a peak indication unit  513 . The peak extractor  511  may be configured for calculating a magnitude and a phase of the input signal from one of the two paths, and obtaining a magnitude difference D Mag  between the magnitude and the predetermined clipping threshold. As previously described, the threshold may be determined based on the PARP requirement and the average power of the input signal. The peak indication unit  513  may be configured for indicating a maximum value of the D Mag s as a peak indicator for a peak of the input signal on the corresponding path. 
         [0073]    Since the peaks from different two paths will be statically multiplexed, the identifying unit  570  is necessary for identifying, using a generated identifier, which path a peak comes from, according to the peak indicators on different paths respectively output from the peak indication units  513 - 1  and  513 - 2  to the identifying unit  570 . 
         [0074]    Preferably, the identifying unit  570  may be an encoder. When there comes a peak, the peak indication unit  513  may send a bit “ 1 ” as a peak indicator to the identifying unit  570 . The identifying unit  570  may identify which path the bit “ 1 ” comes from, using generated encoded bits. For example, if the input bit “ 1 ” comes from Path  2 , the encoder may encode it as 2, i.e., “10”. However, it will be appreciated by the skilled in the art, any other identifying method as appropriate may be possible. 
         [0075]    Then the identifying unit  570  may send the generated identifier to the MUX unit  580  for indicating the path on which the peak should be sent to the allocator  520  and peak cancellation units  530   s  and be processed. Also, the identifier may be sent to the DEMUX units  590   s  after each peak cancellation unit  530  to choose which path the processing result should be sent back to. The identifying unit  570  may be designed as a priority encoder, and then some of the signal paths may have a higher priority to be processed. The identifying unit  570  may also be designed as a round robin encoder, and then each path would have the same priority. 
         [0076]    Although the identifying unit  570  and the MUX unit  580  are separated components as shown in  FIG. 5 , the identifying unit  570  may be also incorporated into the MUX unit  580 , as will be appreciated by the skilled in the art. 
         [0077]    The detected peaks of the input signals from the two paths may be input to the MUX unit  580 . Then, the MUX unit  580  may statistically multiplexing the detected peaks from the peak detectors  510 - 1  and  510 - 2 , and output the multiplexed peaks to the allocator  520 . 
         [0078]    The allocator  520  may allocate each of the multiplexed peaks to one of the peak cancellation unit  530 , once it receives the peak from the MUX unit  580 . The allocator  520  may know which path the peak of the input signal comes from, according to the identifier output from the identifying unit  570 . The peak cancellation unit  530  may generate a corresponding cancellation peak for the allocated peak from the identified path. Same as the configuration of the PC-CFR unit  200 , there are four peak cancellation units  530   s  as an example for illustration only but not for any limitation. Each peak cancellation unit  530  may cancel only one peak at a time. The length of the cancellation peak combined with the number of the peak cancellation units  530   s  may determine a rate at which the multiplexed peaks may be cancelled. The number of the peak cancellation units  530   s  may depend on at least one of a carrier number, a carrier bandwidth, and statistic characteristics of carrier. 
         [0079]    In order to avoid introducing out-of-band interference, the cancellation peak may be designed to have a spectrum that matches that of the input signal. The spectrum of the cancellation peak in a frequency domain may correspond to cancellation peak coefficients in a time domain. The cancellation peak coefficients may be predetermined depending on a carrier configuration including the carrier number, the carrier frequency and the carrier bandwidth etc., and pre-stored in the RAM of the peak cancellation unit  530 . 
         [0080]    Once the multiplexed peak is allocated to the peak cancellation unit  530 , these cancellation peak coefficients will be read out sequentially for the multiplexed peak, as previously discussed. Thus, the cancellation peak may be generated by scaling the plurality of cancellation peak coefficients pre-stored in the peak cancellation unit  530  with the magnitude difference D Mag  and the phase of the allocated peak. 
         [0081]    Each DEMUX unit  590  may be connected to each peak cancellation unit  530 . The DEMUX unit  590  may de-multiplex the generated cancellation peak onto the corresponding path according to the identifier output from the identifying unit  570 , so as to choose which path the processing result should be sent back to. 
         [0082]    In this example, there are two outputs from each DEMUX unit  590 . Then, the outputs from all of the four DEMUX units  590   s  may be further output to the summation units  540 - 1  and  540 - 2  respectively. The summation unit  540  may sum the de-multiplexed cancellation peaks of the input signal on one and the same path together. And then the subtraction unit  560  may subtract the summed cancellation peaks from a delayed input signal which may be delayed by the corresponding matched delay unit  550 . 
         [0083]    Hereinafter, a flowchart of an exemplary PC-CFR method performed by the PC-CFR unit  500  according to the above embodiment of the present invention will be described with reference to  FIG. 6 . 
         [0084]      FIG. 6  shows an illustrative flowchart of the exemplary PC-CFR method  600 . 
         [0085]    In step S 602 , the peak detector  510  may detect a peak from an input signal on a corresponding path. In particular, the peak extractor  511  may calculate a magnitude and a phase of the input signal, and obtain a magnitude difference D Mag  between the magnitude and a predetermined clipping threshold. And the peak indication unit  513  may indicate a maximum value of the D Mag s as a peak indicator for a peak of the input signal on the corresponding path. 
         [0086]    In step S 604 , the MUX unit  580  may statistically multiplex the plurality of detected peaks respectively from the plurality of paths. The path from which the detected peak comes may be identified in step S 603  by the identifying unit  570  using a generated identifier, according to the peak indicators on different paths respectively output from the peak indication units  513   s  to the identifying unit  570 . 
         [0087]    In step S 606 , the allocator  520  may allocate one of the multiplexed peaks to one of the plurality of peak cancellation units  530 . 
         [0088]    In step S 608 , the peak cancellation unit  530  may generate the corresponding cancellation peak for the allocated peak from the identified path. The cancellation peak may be generated by scaling a plurality of cancellation peak coefficients pre-stored in the peak cancellation unit with the magnitude difference D Mag  and a phase of the allocated peak. 
         [0089]    In step S 610 , the DEMUX unit  590  may de-multiplex the corresponding cancellation peak onto the corresponding one of the plurality of paths according to the identifier output from the identifying unit  570 , so as to choose which path the processing result should be sent back to. 
         [0090]    In step S 612 , the summation unit  540  may sum the de-multiplexed cancellation peaks of the input signal on one and the same path together. 
         [0091]    In step S 614 , the subtraction unit  560  may subtract the summed cancellation peaks from a delayed input signal which may be delayed in step S 613  by the corresponding matched delay unit  550 . 
         [0092]    In another embodiment of the present invention, the PC-CFR unit  500  with statistical multiplexing as shown in  FIG. 5  may also be integrated with Time Division Multiplexing (TDM). 
         [0093]    Assuming a TDM factor is M which is defined as a ratio of a system clock frequency and a signal sample rate, and there are N ways of input signals, where M and N are integers. 
         [0094]    In this embodiment, all the N ways of input signals may be divided into [N/M] groups, where “[ ]” represents a ceil operation. The “group” here corresponds to the “path” in the embodiment of  FIG. 5 . That is, processings on the “group” here correspond to those on the “path” in the embodiment of  FIG. 5 . 
         [0095]    Generally, the number of ways N is no less than the TDM factor M (if N&lt;M, (M−N) ways will be filled with 0 for further processing). Inside each group, an order of TDM time taps may be used for differentiating the different ways of input signals, while between different groups, the identifying unit, the MUX unit and the DEMUX unit as shown in  FIG. 5  may be used for differentiating the different groups. From this point of view,  FIG. 5  may be regarded as a specific case of this embodiment, in which the TDM factor is 1, i.e. the data sample rate is equal to the system clock frequency. 
         [0096]    Hereinafter, a block diagram of the exemplary PC-CFR unit with a combination of statistical multiplexing and TDM according to the present embodiment will be described in detail in connection with an example as shown in  FIG. 7 . 
         [0097]    In the example of  FIG. 7 , the number of ways is four, and the TDM factor is two. As previously described, the four ways of input signals S 1 , S 2 , S 3  and S 4  may be divided into two groups. Group  1  includes two ways of input signals S 1  and S 2 , and Group  2  includes two ways of input signals S 3  and S 4 . 
         [0098]    The PC-CFR unit  700  in the present embodiment may comprise a plurality of peak detectors  710   s,  an allocator  720 , multiple peak cancellation units  730 , summation units  740   s,  matched delay units  750   s,  subtraction units  760   s,  an identifying unit  770 , a MUX unit  780  and multiple DEMUX units  790   s  same as those shown in  FIG. 5 . For simplicity, descriptions on some functions performed by the same units as those in  FIG. 5  are omitted. Furthermore, the PC-CFR unit  700  may comprise pairs of TDMs  701   s  and TDDs  702   s.    
         [0099]    Group  1  is taken as an example for illustration. Same description may apply to Group  2 , and thus is omitted for simplicity. A TDM unit  701 - 1  may combine sequentially the input signals S 1  and S 2  respectively from the two ways into a multiplexed input signal (hereinafter, a TDM signal) S 12  by time division multiplexing. A peak detector  710 - 1  may detect a peak from the input TDM signal S 12  from Group  1 . Due to a fixed order of TDM, the signals S 1  and S 2  in the TDM signal may be differentiated by the TDM time taps. For example, odd time taps may refer to peaks of the signal S 1 , and even time taps may refer to peaks of the signal S 2 . 
         [0100]    Since the peaks from Group  1  and Group  2  will be statically multiplexed, an identifying unit  770  is necessary for identifying, using a generated identifier, which group a peak comes from, according to the peak indicators for different TDM signals respectively output from the peak indication units  713 - 1  and  713 - 2  to the identifying unit  770 . 
         [0101]    Similar with the identifying unit  570 , the identifying unit  770  may preferably be an encoder. When there comes a peak, the peak indication unit  713  for the corresponding group may send a bit “ 1 ” as a peak indicator to the identifying unit  770 . The identifying unit  770  may identify which group the bit “ 1 ” comes from, using generated identifiers. For example, if the input bit “ 1 ” comes from Group  2 , the encoder may encode it as 2, i.e., “10”. However, it will be appreciated by the skilled in the art, any other identifying method as appropriate may be possible. 
         [0102]    Then the identifying unit  770  may send the generated identifier to the MUX unit  780  for indicating the group on which the peak should be sent to the allocator  720  and peak cancellation units  730   s  and be processed. Also, the identifier may be sent to the DEMUX units  790   s  after each peak cancellation unit  730  to choose which group the processing result should be sent back to. The identifying unit  770  may be designed as a priority encoder, and then some of the groups may have a higher priority to be processed. The identifying unit  570  may also be designed as a round robin encoder, and then each group would have the same priority. 
         [0103]    Then, the detected peaks of the TDM signals from Group  1  and Group  2  may be input to the MUX unit  780 . Then, the MUX unit  780  may statistically multiplexing the detected peaks from the peak detectors  710 - 1  and  710 - 2 , and output the multiplexed peaks to the allocator  720 . 
         [0104]    The allocator  720  may allocate one of the multiplexed peaks to one of the peak cancellation unit  530 , once it receives the peak from the MUX unit  780 . The allocator  720  may know which group the peak of the input signal comes from, according to the identifier output from the identifying unit  770  and the specific time tap for the peak. In particular, assuming that odd time taps refer to peaks of the signal S 1  in Group  1  and peaks of the signal S 3  in Group  2 , and even time taps refer to peaks of the signal S 2  in Group  1  and peaks of the signal S 4  in Group  2 , the allocator  720  may be aware that the odd time taps refer to the peaks of the signal S 1  or the peaks of the signal S 3  according to the identifier output from the identifying unit  770 . 
         [0105]    The peak cancellation unit  730  may generate a corresponding cancellation peak for the allocated peak from the identified group. Same as the configuration of the PC-CFR unit  200 , there are four peak cancellation units  730   s  as an example for illustration only but not for any limitation. Each peak cancellation unit  730  may cancel only one peak at a time. The only difference is that the peak cancellation coefficients stored in the RAMs inside the peak cancellation unit  730  are read out once every M time taps and properly delayed to align with the corresponding way of input signal. The length of the cancellation peak together with the number of the peak cancellation units  730   s  may determine the rate at which the multiplexed peaks may be cancelled. The number of the peak cancellation units  730   s  may depend on at least one of a carrier number, a carrier bandwidth, and statistic characteristics of carrier. 
         [0106]    Each DEMUX unit  790  may be connected to each peak cancellation unit  730 . The DEMUX unit  790  may de-multiplex the generated cancellation peak onto the corresponding group according to the identifier output from the identifying unit  770 , so as to choose which group the processing result should be sent back to. 
         [0107]    In this example, there are two outputs from each DEMUX unit  790 . Then, the outputs from all of the four DEMUX units  790   s  may be further output to the summation units  740 - 1  and  740 - 2  respectively. The summation unit  740  may sum the de-multiplexed cancellation peaks of the TDM signal on one and the same group together. And then the subtraction unit  760  may subtract the summed cancellation peaks from a delayed TDM signal which may be delayed by the corresponding matched delay unit  750 , so as to obtain a peak-cancelled TDM signal. 
         [0108]    Then, the peak-cancelled TDM signal from the subtraction unit  760  may be input to the TDD unit  702 . The TDD unit  702  may separate the peak-cancelled TDM signal onto the corresponding way according to the fixed order of time taps. 
         [0109]    Hereinafter, a flowchart of an exemplary PC-CFR method performed by the PC-CFR unit  700  according to the above embodiment of the present invention will be described with reference to  FIG. 8 . 
         [0110]      FIG. 8  shows an illustrative flowchart of the exemplary PC-CFR method  800 . 
         [0111]    In step S 801 , the TDM unit  701  may time division multiplexing, according to a fixed order of time taps, a plurality of input signals respectively from a plurality of ways into an TDM signal. 
         [0112]    In step S 802 , the peak detector  710  may detect a peak from the TDM signal on the corresponding group. In particular, the peak extractor  711  may calculate a magnitude and a phase of the TDM signal, and obtain a magnitude difference D Mag  between the magnitude and a predetermined clipping threshold. And the peak indication unit  713  may indicate a maximum value of the D Mag s as a peak indicator for a peak of the TDM signal on the corresponding group. 
         [0113]    In step S 804 , the MUX unit  880  may statistically multiplex the plurality of detected peaks respectively from the plurality of groups. The group from which the detected peak comes may be identified in step S 803  by the identifying unit  770  using a generated identifier, according to the peak indicators on different groups respectively output from the peak indication units  713   s  to the identifying unit  770 . 
         [0114]    In step S 806 , the allocator  820  may allocate one of the multiplexed plurality of peaks to one of the plurality of peak cancellation units  730 . 
         [0115]    In step S 808 , the peak cancellation unit  730  may generate the corresponding cancellation peak for the allocated peak from the identified group. The cancellation peak may be generated by scaling a plurality of cancellation peak coefficients pre-stored in the peak cancellation unit with the magnitude difference D Mag  and a phase of the allocated peak. The plurality of cancellation peak coefficients may be read out once every M time taps, in order to differentiate different ways in one group from which the peaks come from. 
         [0116]    In step S 810 , the DEMUX unit  790  may de-multiplex the corresponding cancellation peak onto the corresponding one of the plurality of groups according to the identifier output from the identifying unit  770 , so as to choose which group the processing result should be sent back to. 
         [0117]    In step S 812 , the summation unit  740  may sum the de-multiplexed cancellation peaks of the TDM signal from one and the same group together. 
         [0118]    In step S 814 , the subtraction unit  760  may subtract the summed cancellation peaks from a delayed TDM signal which may be delayed in step S 813  by the corresponding matched delay unit  750  to obtain a peak-cancelled TDM signal. 
         [0119]    In step S 815 , the TDD unit  702  may time division de-multiplexing, according to the fixed order of the time taps, the peak-cancelled TDM signal outputted from the subtraction unit  760  onto the plurality of ways. 
         [0120]    The PC-CFR unit  700  using the PC-CFR method  800  in the above embodiment of the present invention may improve M times of the circuit efficiency, compared with the PC-CFR unit  500  using the PC-CFR method  600  in the previous embodiment of the present invention. 
         [0121]    If the TDM factor M is equal to the number of paths N, there is only one group including a plurality of ways of input signals. The identifying unit  770 , the MUX unit  780  and the DEMUX unit  790  as illustrated in  FIG. 7  are not needed any more for differentiating different groups. Thus, the design of the PC-CFR unit in this embodiment may be highly simplified. 
         [0122]      FIG. 9  illustratively shows a block diagram of the PC-CFR unit  900 , where the TDM factor M is equal to the number of ways of input signals N (which is two in this example for illustration only). 
         [0123]    The PC-CFR unit  900  may comprise a TDM unit  901  for combining incoming signals of two ways, and a TDD unit  902  for separating them onto the corresponding way respectively. According to the principle of TDM, different ways may be differentiated by the fixed order of time taps. As previously mentioned, the identifying unit  770 , the MUX unit  780  and the DEMUX unit  790  as illustrated in  FIG. 7  are not needed any more in this case for differentiating the different groups. However, to ensure that the peak cancellation waveform is aligned with the signal paths, the peak cancellation coefficients pre-stored in the peak cancellation unit  930  should be read out once every M (two in this example) clock cycle and delayed by a proper number of clock cycles. 
         [0124]    The PC-CFR method as performed by the PC-CFR unit  900  are similar with the PC-CFR method  800  as previously described, except that the steps S 803 , S 804  and S 810  may be omitted. Thus, the description on the PC-CFR method of the PC-CFR unit  900  are omitted for simplicity. 
         [0125]    As previously described, the PC-CFR unit  900  in this embodiment of the present invention may greatly simplify the design of the PC-CFR unit, compared with the PC-CFR unit  700  in the previous embodiment of the present invention. 
         [0126]      FIG. 10  shows a block diagram of a CFR solution in the multicarrier LTE system, which is an exemplary application of the above embodiments. 
         [0127]    In the example of  FIG. 10 , there are two paths of input signals S 1  and S 2  corresponding to two antennas, and each path has three stages of clipping. It should be noted that the exemplary solution as shown in  FIG. 10  is only described for illustration only but not for any limitation. The skilled in the art may use any appropriate combinations of the embodiments as proposed in the present invention for actual circuit designs. 
         [0128]    As shown in  FIG. 10 , the first two stages are working at the sample rate of 122.88 MHz, and the last stage is working at the sample rate of 245.76 MHz after an x2 interpolation. System clock is 245.76 MHz. 
         [0129]    The reason to put different clipping stages at different sample rates is to keep a balance between hardware resource consumption and clipping performance, because clipping at a lower sample rate can employ time division multiplexing and help to save hardware resource, while clipping at a higher sample rate can mitigate the peak re-growth after interpolation and get a better PAPR performance. 
         [0130]    For the first two stages of each path, a PC-CFR structure like the PC-CFR unit  900  as shown in  FIG. 9  may be employed. As shown in  FIG. 10 , the input signal S 1  and an peak-cancelled input signal S 1   cp  output from an TDD unit  1002  are combined in an TDM unit  1001 , so that the PC-CFR method same as what performed by the PC-CFR unit  900  is between different clipping stages instead of different paths. As such, the TDM factor is equal to the number of stages in this example, instead of the number of paths. It should be noted that before the peak-cancelled input signal S 1   cp  is generated, this input of the TDM unit  1001  is filled with 0 for initial processing. The PC-CFR operation performed in both of the first two clipping stages in  FIG. 10  is identical with that of the PC-CFR unit  900 . Thus, corresponding descriptions are omitted for simplicity. 
         [0131]    Alternatively, a PC-CFR structure like the PC-CRF unit  700  as shown in  FIG. 7  may be used for the first two stages of both two paths. Similarly, as shown in  FIG. 11 , the input signal S 1  and the peak-cancelled input signal S 1   cp  output from an TDD unit  1102  are combined in an TDM unit  1101 , so that the PC-CFR method same as the method  800  performed by the PC-CFR unit  700  is between different clipping stages instead of different paths. Again, before the peak-cancelled input signal S 1   cp  is generated, this input of the TDM unit  1101  is filled with 0 for initial processing. The PC-CFR operation performed in both of the first two clipping stages in  FIG. 11  is identical with that of the PC-CFR unit  700 . Thus, corresponding descriptions are omitted for simplicity. 
         [0132]    Obviously, the only difference between the above two approaches is that two PC-CFR units  900   s  are needed for the two paths of input signals as shown in  FIG. 10 , while only one PC-CFR unit is needed for the two paths of input signals as shown in  FIG. 11 . 
         [0133]    The two paths of input signals S 1  and S 2  after the first two clipping stage may be x2 interpolated for the last clipping stage working at the sample rate of 245.76 MHz, since the sample rate is equal to the system clock rate. A PC-CFR structure like the PC-CRF unit  500  as shown in  FIG. 5  may be employed. Statistical multiplexing may be used between two signal paths. Since structures and functions of this PC-CFR structure are identical with those of the PC-CFR unit  500 , the descriptions are omitted for simplicity. 
         [0134]    Furthermore, if needed, more clipping stages working at 122.88 MHz sample rate may be added, and fractional delay filter may also be employed between stages. Hard clipping, which is a hard magnitude limiter, is used to avoid peak leakage in extreme conditions, too. 
         [0135]    The PC-CFR unit as proposed in the present invention may be included in a transceiver of either a base station or a user equipment. That is, the base station or the user equipment may comprise such a PC-CFR unit according to the present invention in its transceiver. 
         [0136]    The above is only the preferred embodiments of the present invention and the present invention is not limited to the above embodiments. Therefore, any modifications, substitutions and improvements to the present invention are possible without departing from the spirit and scope of the present invention.