05-02-2013, 02:57 PM
A comparative study of PAPR in SC-FDMA systems
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Abstract:
The rapid increasing demand on high data rates in wireless communications systems has arisen in order to support broadband services. The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) aims at very high peak data rates such as 1 Gbps in local areas and 100 Mbps in wide areas. LTE has adopted Orthogonal Frequency Division Multiple Access (OFDMA) as downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) as uplink for multiple access schemes. SC-FDMA is a modified form of OFDMA, and also a promising technique for higher data rate communication in future mobile scenario. The SC-FDMA is chosen for uplink in LTE due to its low Peak to Average Power Ratio (PAPR) compared to OFDMA. The high PAPR ratio causes reduction in power efficiency which causes significant burden on portable devices due to fast dry out of batteries. The SC-FDMA is almost similar to that of OFDMA systems in its complexity and it provides same throughput as that of OFDMA. In this project give an overview of SC-FDMA. Also we analyse the effect of two subcarrier mappings used in SC-FDMA i.e. Localised FDMA (LFDMA) and Interleaved FDMA (IFDMA) on PAPR of SC-FDMA and makes a comparison of the two subcarrier mapping techniques.
Introduction:
Wireless mobile communications are undergoing rapid progression towards fourth generation (4G). One common design approach in 4G systems is OFDMA. OFDMA is a multicarrier communication technique on the air interface. It is broadly accepted due of its high resistance to frequency selective fading channels. The immunity to multipath fading is basically from the fact that the OFDMA signal in the frequency domain consists of several orthogonal sub-carriers. Even though OFDMA system has many advantages, it has a major drawback: the high value of PAPR of the transmit signal. PAPR is defined as the ratio of the peak power of a transmit signal to the average power of the transmit signal. The lower PAPR is greatly beneficial in the uplink communications where the mobile terminal is the transmitter. Signals with a high PAPR requires highly linear power amplifiers to avoid excessive intermodulation distortion. To achieve this linearity, the amplifiers have to operate with a large backoff from their peak power. The result is low power efficiency (measured by the ratio of transmitted power to dc power dissipated). For fixed applications where the device is connected to the mains this is not a problem, but for small mobile devices running on their own batteries it creates more challenges. Another problem with OFDMA in cellular uplink transmissions derives from the inevitable offset in frequency references among the different terminals that transmit simultaneously. Frequency offset destroys the orthogonality of the transmissions, thus introducing multiple access interference.
System Configuration of Single Carrier FDMA:
Figure 1 shows an SC-FDMA transmitter sending one block of data to a receiver. The input of the transmitter and the output of the receiver are complex modulation symbols. Practical systems dynamically adapt the modulation technique to the channel quality. Generally Binary Phase Shift Keying (BPSK) is used in weak channels and up to 64-level Quadrature Amplitude Modulation (64-QAM) in strong channels. The data block consists of M complex modulation symbols generated at a rate Rsource symbols/second. Figure 2 provides details of the three important elements of transmitter in Figure 1.
Subcarrier Mapping:
Subcarrier mapping is the processing of mapping DFT of data symbols to a subset of subcarriers. The subcarrier mapping assigns DFT output complex values as the amplitudes of some of the selected subcarriers. Several approaches to map transmission symbols Xk to SC-FDMA subcarriers are currently under consideration. The subcarrier mapping is divided into two categories; distributed mapping and localized mapping.
Distributed Mapping (DFDMA):
In the distributed subcarrier mapping mode, DFT outputs of the input data are allocated over the entire bandwidth with zeros occupying the unused subcarriers resulting in a non-continuous comb-shaped spectrum. The case of N = Q×M for the distributed mode with equidistance between occupied subcarriers is called Interleaved FDMA (IFDMA). IFDMA is a special case of DFDMA and it is very efficient in that the transmitter can modulate the signal strictly in the time domain without the use of DFT and IDFT.
Localized Mapping:
The DFT outputs of the input data occupy consecutive subcarriers in the localized subcarrier mapping mode. We will refer to the localized subcarrier mapping mode of SC-FDMA as Localized FDMA (LFDMA).The data in the localized subcarrier mapping mode results in a continuous spectrum that occupies a fraction of the total available bandwidth.
Conclusion:
SC-FDMA is a promising technique for high data rate uplink communication in future cellular systems. There are many operational and design choices which affects the performance of SC-FDMA within a specific system configuration. In this paper, we mainly focused on the effects of subcarrier mapping on PAPR in SC-FDMA. Results show that in comparison with OFDMA the SC-FDMA has lower PAPR value for same system configurations. Among SC-FDMA techniques, IFDMA has the lowest PAPR compared to OFDMA, than LFDMA. The results indicate that SC-FDMA is more economic and useful in the LTE uplink schemes than OFDMA due to its low PAPR.
[attachment=49715]
Abstract:
The rapid increasing demand on high data rates in wireless communications systems has arisen in order to support broadband services. The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) aims at very high peak data rates such as 1 Gbps in local areas and 100 Mbps in wide areas. LTE has adopted Orthogonal Frequency Division Multiple Access (OFDMA) as downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) as uplink for multiple access schemes. SC-FDMA is a modified form of OFDMA, and also a promising technique for higher data rate communication in future mobile scenario. The SC-FDMA is chosen for uplink in LTE due to its low Peak to Average Power Ratio (PAPR) compared to OFDMA. The high PAPR ratio causes reduction in power efficiency which causes significant burden on portable devices due to fast dry out of batteries. The SC-FDMA is almost similar to that of OFDMA systems in its complexity and it provides same throughput as that of OFDMA. In this project give an overview of SC-FDMA. Also we analyse the effect of two subcarrier mappings used in SC-FDMA i.e. Localised FDMA (LFDMA) and Interleaved FDMA (IFDMA) on PAPR of SC-FDMA and makes a comparison of the two subcarrier mapping techniques.
Introduction:
Wireless mobile communications are undergoing rapid progression towards fourth generation (4G). One common design approach in 4G systems is OFDMA. OFDMA is a multicarrier communication technique on the air interface. It is broadly accepted due of its high resistance to frequency selective fading channels. The immunity to multipath fading is basically from the fact that the OFDMA signal in the frequency domain consists of several orthogonal sub-carriers. Even though OFDMA system has many advantages, it has a major drawback: the high value of PAPR of the transmit signal. PAPR is defined as the ratio of the peak power of a transmit signal to the average power of the transmit signal. The lower PAPR is greatly beneficial in the uplink communications where the mobile terminal is the transmitter. Signals with a high PAPR requires highly linear power amplifiers to avoid excessive intermodulation distortion. To achieve this linearity, the amplifiers have to operate with a large backoff from their peak power. The result is low power efficiency (measured by the ratio of transmitted power to dc power dissipated). For fixed applications where the device is connected to the mains this is not a problem, but for small mobile devices running on their own batteries it creates more challenges. Another problem with OFDMA in cellular uplink transmissions derives from the inevitable offset in frequency references among the different terminals that transmit simultaneously. Frequency offset destroys the orthogonality of the transmissions, thus introducing multiple access interference.
System Configuration of Single Carrier FDMA:
Figure 1 shows an SC-FDMA transmitter sending one block of data to a receiver. The input of the transmitter and the output of the receiver are complex modulation symbols. Practical systems dynamically adapt the modulation technique to the channel quality. Generally Binary Phase Shift Keying (BPSK) is used in weak channels and up to 64-level Quadrature Amplitude Modulation (64-QAM) in strong channels. The data block consists of M complex modulation symbols generated at a rate Rsource symbols/second. Figure 2 provides details of the three important elements of transmitter in Figure 1.
Subcarrier Mapping:
Subcarrier mapping is the processing of mapping DFT of data symbols to a subset of subcarriers. The subcarrier mapping assigns DFT output complex values as the amplitudes of some of the selected subcarriers. Several approaches to map transmission symbols Xk to SC-FDMA subcarriers are currently under consideration. The subcarrier mapping is divided into two categories; distributed mapping and localized mapping.
Distributed Mapping (DFDMA):
In the distributed subcarrier mapping mode, DFT outputs of the input data are allocated over the entire bandwidth with zeros occupying the unused subcarriers resulting in a non-continuous comb-shaped spectrum. The case of N = Q×M for the distributed mode with equidistance between occupied subcarriers is called Interleaved FDMA (IFDMA). IFDMA is a special case of DFDMA and it is very efficient in that the transmitter can modulate the signal strictly in the time domain without the use of DFT and IDFT.
Localized Mapping:
The DFT outputs of the input data occupy consecutive subcarriers in the localized subcarrier mapping mode. We will refer to the localized subcarrier mapping mode of SC-FDMA as Localized FDMA (LFDMA).The data in the localized subcarrier mapping mode results in a continuous spectrum that occupies a fraction of the total available bandwidth.
Conclusion:
SC-FDMA is a promising technique for high data rate uplink communication in future cellular systems. There are many operational and design choices which affects the performance of SC-FDMA within a specific system configuration. In this paper, we mainly focused on the effects of subcarrier mapping on PAPR in SC-FDMA. Results show that in comparison with OFDMA the SC-FDMA has lower PAPR value for same system configurations. Among SC-FDMA techniques, IFDMA has the lowest PAPR compared to OFDMA, than LFDMA. The results indicate that SC-FDMA is more economic and useful in the LTE uplink schemes than OFDMA due to its low PAPR.