Patent Publication Number: US-7916694-B2

Title: Method and system reducing peak to average power ratio (PAPR) in a communication network

Description:
RELATED APPLICATION DATA 
     This application claims priority to and incorporates by reference application Ser. No. 60/700,767 filed on Jul. 19, 2005, titled Method to Reduce Peak-to-Average Ratio in OFDM Communications Systems 
    
    
     BACKGROUND OF THE INVENTION 
     The invention generally relates to a communication network. More specifically, the invention relates to reducing Peak to Average Power Ratio (PAPR) in an Orthogonal Frequency Division Multiplex (OFDM)/Orthogonal Frequency Division Multiplex Access (OFDMA) communication network. 
     In an OFDM/OFDMA communication network, certain combinations of OFDM symbols may result in signals with a large PAPR. One or more signals that have a large PAPR may get clipped in the RF Power Amplifier (PA). Therefore, the RF PA has to operate linearly for a wide range of amplitudes, such that, clipping of one or more signals that have a large PAPR is prevented. Moreover, the signals that have large PAPR saturate the RF PA and a transmitter, as a result of which, data in the OFDM/OFDMA communication network is lost. Additionally, transmitting the signals that have large PAPR reduces the power efficiency of the transmitter. A reduction in the power efficiency, further, reduces the battery life of mobile devices in the OFDM/OFDMA communication network. Examples of the mobile devices may include, but are not limited to, cell phones, and Personal Digital Assistants (PDA). 
     In conventional applications, PAPR of a plurality of OFDM symbols is reduced by digitally clipping one or more of OFDM symbols to a predetermined level before transmission. The digital clipping of the signals reduces unintentional clipping of one or more OFDM symbols in the RF PA before transmission. However, the digital clipping before transmission results in spectral spreading, which has to be filtered to decode the data carried by the plurality of OFDM symbols. Moreover, if EVM requirement of constellations of the plurality of OFDM symbols is considerably strong, then PAPR of the plurality of OFDM symbols is not reduced. 
     Further, in some conventional applications, a predefined set of tones in a plurality of OFDM symbols is reserved for carrying data. Thereafter, data transmitted on the predefined set of tones is decoded at a receiver, such that, PAPR of the plurality of OFDM symbols is reduced. However, decoding of data on the predefined set of tones is computationally complex. 
     In some conventional applications, constellation manipulation is used at a transmitter to reduce PAPR of a plurality of OFDM symbols. In constellation manipulation, for example, a phase of the plurality of OFDM symbols is modulated to reduce PAPR. The phase of the plurality of OFDM symbols is modulated using Partial Transmit Sequences (PTS) and Selective Mapping (SLM) methods. In these methods, a phase of sub-blocks of the plurality of OFDM symbols is modulated. Additionally, in these methods, a data frame of the plurality of OFDM symbols is multiplied with random vectors to reduce PAPR. However, in these methods, a receiver is required to know the type of constellation manipulation carried out at the transmitter. Further, the receiver has to violate one or more standard specifications in the OFDM/OFDMA communication network to know the type of constellation manipulation carried out in the transmitter. 
     Additionally, some conventional applications use codes to reduce PAPR of the plurality of OFDM symbols. Applications may use block-coding methods. In the block-coding method, a polyphase sequence that has small PAPR is transmitted for each OFDM symbol constellation. Block-coding method requires the use of large look-up tables for encoding and decoding of the plurality of OFDM symbols. Additionally, block-coding method requires an exhaustive search to identify one or more OFDM symbols that have poly-phase sequence with small PAPR. Further, conventional applications using codes detect one or more OFDM symbols that have less PAPR in an unstructured way. 
     Further, specific codes, for example, co-sets of Reed-Muller codes, may be used by some conventional applications to reduce PAPR of a plurality of OFDM symbols. However, the use of the specific code is limited to a small number of OFDM symbols. 
     There is therefore a need for a method and system that reduces PAPR of a plurality of OFDM symbols in a computationally efficient way, which is easy to implement. Further, the method and system should be independent of one or more of source coding, employed modulation, pilot sub-carrier insertion, and equalization methods used by a receiver in the OFDM/OFDMA communication network. Therefore, the method and system should be applicable to existing implementations and standard specifications. Additionally, the method and system should support multiple degrees of freedom, which can be employed to reduce the number of computations and/or processing load. 
     SUMMARY 
     An embodiment provides a method and system to reduce Peak to Average Power Ratio (PAPR) of a plurality of OFDM symbols. 
     Another embodiment provides a method and system in which the plurality of OFDM symbols, after modulation of one or more of the phase and the magnitude, have positive modulus in the frequency domain. 
     Yet another embodiment provides a method and system to reduce PAPR in each communication network in which channel estimation is done at the receiver using pilot sub-carriers. 
     Embodiments described below include methods and systems of transmitting a plurality of Orthogonal Frequency Division Multiple (OFDM) symbols in a block transmission system of a communication network. The block transmission system is a frequency reuse system for example but is not limited to a frequency reuse system. The methods include encoding the plurality of OFDM symbols. The methods further include, modulating at least one of a phase and a magnitude of the plurality of OFDM symbols. Thereafter, a plurality of modulated OFDM symbols is converted from a digital form to an analog form. The plurality of modulated OFDM symbols corresponds to the plurality of OFDM symbols. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A more complete appreciation of the present invention is provided by reference to the following detailed description when considered in conjunction with the accompanying drawings in which reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a block diagram showing an example environment in which various embodiments can function. 
         FIG. 2  is a block diagram showing various components of a transmitter for transmitting a plurality of Orthogonal Frequency Division Multiplex (OFDM) symbols in a block transmission system of a communication network, in accordance with an embodiment. 
         FIG. 3  is a flowchart for transmitting a plurality of OFDM symbols in a block transmission system of a communication network, in accordance with an embodiment. 
         FIG. 4  is a flowchart for transmitting a plurality of OFDM symbols in a block transmission system of a communication network, in accordance with another embodiment. 
         FIG. 5  is a flowchart for encoding the plurality of OFDM symbols in a block transmission system of a communication network, in accordance with an embodiment. 
         FIG. 6  is a flowchart for reducing Peak to Average Power Ratio (PAPR) of a plurality of OFDM symbols in a block transmission system of a communication network, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Various embodiments described herein provide methods and systems for reducing Peak to Average Power Ratio (PAPR) in an Orthogonal Frequency Division Multiplex (OFDM) communication network. It will be apparent to a person skilled in the art that the invention reduces PAPR in communication networks that make at least partial use OFDM tones. 
       FIG. 1  is a block diagram showing an example environment  100  in which various embodiments function. Environment  100  includes an encoder  102 , and an Inverse Fast Fourier Transform (IFFT) module  104 , a guard module  106 , and a Digital-to-Analog (D/A) converter  108 . Encoder  102  converts the data that has to be transmitted to a plurality of OFDM symbols. The plurality of OFDM symbols is then converted from a frequency domain to a time domain by IFFT module  104 . Thereafter, guard module  106  inserts a cyclic prefix to each OFDM symbol. The cyclic prefix is a guard interval in which the multi-path components of a symbol are absorbed to prevent the Inter Symbol Interference (ISI) in the plurality of OFDM symbols. The plurality of OFDM symbols is then converted from a digital form to an analog form by D/A converter  108 . Thereafter, the plurality of OFDM symbols is transmitted. 
       FIG. 2  is a block diagram showing various components of a transmitter  200  for transmitting a plurality of OFDM symbols, in accordance with an embodiment. Transmitter  200  transmits the plurality of OFDM symbols in a block transmission system of a communication network. The block transmission system is a frequency reuse system. Further, transmitter  200  is configured to transmit uplink signals. Additionally, transmitter  200  may be configured to transmit downlink signals. 
     Transmitter  200  includes an encoder  202 , a low-ripple-filter  204  and a D/A converter  206 . Encoder  202  encodes the plurality of OFDM symbols from the data that has to be transmitted. The plurality of OFDM symbols is then transformed from a frequency domain to a time domain by an IFFT module  208 . Thereafter, a guard module  210  inserts a cyclic prefix to each OFDM symbol. The cyclic prefix has a length greater than maximum-delay-spread of each symbol to prevent ISI. After inserting the cyclic prefix, the plurality of OFDM symbols is then passed through low-ripple-filter  204 . 
     Low-ripple-filter  204  modulates one or more of a phase and a magnitude of the plurality of OFDM symbols. Modulation of one or more of the phase and the magnitude reduces PAPR of the plurality of OFDM symbols. This is further explained in detail in conjunction with  FIG. 4  below. A modulation in the magnitude of the plurality of OFDM symbols is captured by pilot sub-carriers inserted in the plurality of OFDM symbols. Therefore, the data being transmitted is not altered. Further, to modulate one or more of the phase and the magnitude, the plurality of OFDM symbols undergo point wise multiplication of tones with low-ripple-filter  204 . Therefore, the phase and magnitude of each tone is modulated. 
     Further, low-ripple-filter  204  includes an optimizer  212 . Optimizer  212  executes an optimization algorithm to modulate one or more of the phase and the magnitude of the plurality of OFDM symbols to generate a plurality of modulated OFDM symbols. This is further explained in detail in conjunction with  FIG. 4  below. The plurality of modulated OFDM symbols has less PAPR as compared to the plurality of OFDM symbols. Further, the plurality of modulated OFDM symbols is generated in the time domain. The plurality of modulated OFDM symbols is modulated such that a conversion of the plurality of modulated OFDM symbols from the time domain to the frequency domain gives positive modulus. In an embodiment, positive modulus for the plurality of modulated OFDM symbols in the frequency domain is ensured by constraining the number of taps of low-ripple-filter  204 , as the plurality of OFDM symbols undergo point wise multiplication of tones with low-ripple-filter  204 . The number of taps of low-ripple-filter  204  is constrained such that low-ripple-filter  204  does not have spectral nulls and deep notches in the frequency domain. For example, low-ripple-filter  204  should not have any sinusoidal frequency that has zero frequency at one or more points. If low-ripple-filter  204  has a sinusoidal frequency and an OFDM symbol passes through low-ripple-filter  204 , then at each point where the frequency of low-ripple-filter  204  is zero the data stored in the OFDM symbol will be lost. 
     In an embodiment, the number of taps of low-ripple-filter  204  is constrained to prevent ISI. The number of taps of low-ripple-filter  204  is constrained such that the sum of the number of taps and a maximum-delay-spread of each OFDM symbol does not exceed the cyclic prefix inserted by guard module  210 . 
       FIG. 3  is a flowchart for transmitting a plurality of OFDM symbols in a block transmission system of a communication network, in accordance with an embodiment. In another embodiment, a plurality of Orthogonal Frequency Division Multiplex Access (OFDMA) symbols is transmitted in the block transmission system of the communication network. The block transmission system is a frequency reuse system. The block transmission system may be a frequency reuse-1 system. Further, the communication network may be a Multiple Input Multiple Output (MIMO) communication network. 
     At  302 , the plurality of OFDM symbols is encoded by encoder  202 . The data, which has to be transmitted, is converted to the plurality of OFDM symbols. The encoding is further explained in detail in conjunction with  FIG. 5  below. Thereafter, at  304  a phase and a magnitude of the plurality of OFDM symbols is modulated by low-ripple-filter  204 . An optimization algorithm is used to modulate the phase and the magnitude of the plurality of OFDM symbols, such that, PAPR of the plurality of OFDM symbols is reduced. This is further explained in conjunction with  FIG. 4  below. After modulating the phase and magnitude of the plurality of OFDM symbols, a plurality of modulated OFDM symbols are converted from a digital form to an analog form by D/A converter  206 , at  306 . The plurality of modulated OFDM symbols corresponds to the plurality of OFDM symbols. 
       FIG. 4  is a flowchart for transmitting a plurality of OFDM symbols in a block transmission system of a communication network, in accordance with another embodiment. At  402 , the plurality of OFDM symbols is encoded by encoder  202 . The encoding is further explained in detail in conjunction with  FIG. 5  below. After encoding the plurality of OFDM symbols, an optimization algorithm is executed by optimizer  212 , at  404 . Thereafter, at  406 , one or more of the phase and the magnitude of the plurality of OFDM symbols is modulated by low-ripple-filter  204  using the optimization algorithm. The optimization algorithm modulates one or more of the phase and the magnitude of the plurality of OFDM symbols to generate the plurality of modulated OFDM symbols. The plurality of modulated OFDM symbols has less PAPR as compared to the plurality of OFDM symbols. The plurality of modulated OFDM symbols is generated in the time domain. Further, converting the plurality of modulated OFDM symbols from the time domain to the frequency domain gives positive modulus. 
     In an example embodiment, optimizer  212  executes an optimization algorithm to solve the optimization problem given by equation 1 below, which modulates one or more of a phase and a magnitude of OFDM symbol x: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     The optimization problem given by equation 1 can be solved by using convex optimization solvers. Examples of the convex optimization solvers may include, but are not limited to, interior point methods. 
     In another example embodiment, optimizer  212  executes an optimization algorithm to solve an optimization problem given by equation 2 below, which modulates one or more of a phase and a magnitude of the OFDM symbol x: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     A solution of the optimization problem given by equation 2 provides positive modulus for the plurality of modulated OFDM symbols in the frequency domain. The optimization problem given by equation 2 can be solved using constrained-optimization algorithms in the art. A global-constrained-optimization algorithm will yield a global optimum as a solution for the optimization problem. The global optimum is the best possible solution for the optimization problem. Further, the global optimum reduces PAPR most effectively. Additionally, a local-constrained-optimization algorithm will yield a local optimum as a solution for the optimization problem. The local optimum is more effective in reducing PAPR as compared to a plurality of solutions for the optimization problem. Additionally, the local optimum is less effective in reducing PAPR as compared to the global optimum. 
     In another example embodiment, a plurality of prototypes of low-ripple-filter  204  are computed using equation 3 as: 
     
       
         
           
             
               
                 
                   
                     
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     Each OFDM symbol is passed through the plurality of prototypes of low-ripple-filter  204 . Thereafter, a prototype of low-ripple-filter  204  that gives the least PAPR is used to generate the plurality of modulated OFDM symbols. Further, as α is a positive number less than one, therefore, low-ripple-filter  204  has positive modulus in the frequency domain. As a result, the plurality of modulated OFDM symbols has positive modulus in the frequency domain. An exemplary value of α, i.e., 0.75 produces a plurality of prototypes of low-ripple-filter  204  that gives efficient PAPR reduction for the plurality of OFDM symbols. 
     In another exemplary embodiment, each OFDM symbol is passed through the total number of prototypes of low-ripple-filter  204 , i.e., M. Thereafter, a prototype of low-ripple-filter  204  that gives the least PAPR is selected for each OFDM symbol. 
     In another example embodiment, a user is assigned a predefined set of tones in the plurality of OFDM symbols. In this exemplary embodiment, low-ripple-filter  204  modulates the phase and the magnitude of the plurality of OFDM symbols, such that, only the tones in the predefined set of tones, which carry data of the user have positive modulus in frequency domain. In an embodiment, after one or more of the phase and the magnitude of the plurality of OFDM symbols have been modulated, tones in the predefined set of tones, which do not carry data of the user may have spectral nulls in the frequency domain. 
     After the optimization algorithm is executed by optimizer  212 , the plurality of modulated symbols is converted form a digital form to an analog form by D/A converter  206 , at  408 . 
       FIG. 5  is a flowchart for encoding the plurality of OFDM symbols in a block transmission system of a communication network, in accordance with another embodiment. At  502 , the plurality of OFDM symbols is transformed from a frequency domain to a time domain by IFFT module  208 . Thereafter, at  504 , a cyclic prefix is inserted to each OFDM symbol in the time domain by guard module  210 . The length of the cyclic prefix is larger than maximum-delay-spread of each symbol. After inserting the cyclic prefix, at  506 , one or more of the phase and the magnitude of the plurality of OFDM symbols is modified by low-ripple-filter  204 . This has been explained in detail in conjunction with  FIG. 3  and  FIG. 4  above. Thereafter, at  508 , a plurality of modulated OFDM symbols is converted from a digital form to an analog form by D/A converter  206 . The plurality of modulated OFDM symbols corresponds to the plurality of OFDM symbols. 
       FIG. 6  is a flowchart for reducing Peak to Average Power Ratio (PAPR) of the plurality of OFDM symbols in a block transmission system of a communication network, in accordance with an embodiment. The block transmission system is a frequency reuse system. At  602 , the plurality of OFDM symbols is transmitted in the block transmission system of the communication network. A transmitter is configured to transmit the plurality of OFDM symbols using encoder  202  to encode the plurality of OFDM symbols. Thereafter, one or more of the phase and the magnitude of the plurality of OFDM symbols is modulated by low-ripple-filter  204 . After modulating one or more of the phase and the magnitude, a plurality of modulated OFDM symbols are converted from a digital form to an analog form by D/A converter  206 . The plurality of modulated OFDM symbols corresponds to the plurality of OFDM symbols. The method of transmitting has been explained in detail in conjunction with  FIG. 3  and  FIG. 4  above. 
     Various embodiments of the invention provide methods and systems to reduce PAPR of the plurality of OFDM symbols. Further, reduction of PAPR of the plurality of OFDM symbols does not reduce data rate and the error correction capabilities of the plurality of OFDM symbols. Additionally, the plurality of OFDM symbols, after modulation of one or more of the phase and the magnitude, has positive modulus in the frequency domain. Further, a receiver in the communication network does not require any modification to decode the plurality of modulated OFDM symbols. Additionally, the receiver does not require data to encode the plurality of OFDM symbols. 
     Further, various embodiments of the invention provide methods and systems that can be used in conjunction with other PAPR reduction methods. Additionally, PAPR can be reduced in each communication network in which receiver estimates channel using pilot sub-carriers.