27-11-2012, 05:33 PM
Comparison of Single-Carrier FDMA vs. OFDMA as 3GPP
Long-Term Evolution Uplink
[attachment=41379]
Introduction
In order to transition from today's 3rd generation (3G) communications systems to meet the needs of 4th generation (4G) systems, the 3rd Generation Partnership Project (3GPP) has released the Long Term Evolution (LTE) specification. Among the numerous differences between these generations are changes in the physical layer (PHY), specifically in the modulation and multiple access schemes. While its parent generation relied on variations of Code Division Multiple Access (CDMA), LTE implements Orthogonal Frequency Division Multiplexing (OFDM) for its downlink and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) for its uplink. The purpose of this project is to investigate the reasoning for this discord between uplink and downlink modulation schemes; specifically, why Orthogonal Frequency Division Multiple-Access (OFDMA) was not used as the uplink.
OFDMA and SC-FDMA are the multiple-access versions of OFDM and a similar modulation scheme, Single-Carrier Frequency-Domain Equalization (SC-FDE). In order to compare the differences between the multiple-access methods, it is important to first cover the differences between their underlying modulation schemes. Section 2 covers these differences, building the foundation for a comparison between the multiple-access methods themselves in Section 3.
The bases used for comparison will be capacity, outage probability and peak-to-average power ratio (PAPR). While the first two bases have always been traditionally used in analyses, PAPR is especially important for the uplink of mobile devices. Amplifiers used in circuits today have a linear region in which they must operate so as not to introduce signal distortion, and it is ideal to run with maximum amplification. However, if there is a high PAPR, the device is forced to run with lower amplification so the peak power does not lie in the non-linear gain region. The farther these amplifiers are operated from the peak, the less power efficient the devices become, leading to increased power consumption and while this might not be very important for a base station, it will reduce drain batteries on mobile devices more quickly. Therefore it is important to keep a low PAPR on the uplink.
Simulation setup
The simulations in this report are conducted with values similar to that in the LTE specifications. Some of the assumptions made are that the carrier is 1950MHz which is in Band I, the sample rate is 30.72MHz, and the FFTs/IFFTs will be 2048-point ones, with cyclic prefixes of 160 samples [1]. In the multi-user-case we assume that each user will use 12 subcarriers, requiring additional 12-point FFT/IFFTs as explained in Section 3.2. Also, 7 symbols will be sent at a time, corresponding to a single resource-block in the LTE specification [1]. The simulated channel follows the multipath propagation conditions specified by the 3GPP in [2].
Modulation Schemes
When facing a frequency selective channel, one well-known method of improving performance is to use multicarrier modulation. By transmitting at multiple carrier frequencies, it is possible to increase capacity by adapting the data rate to be higher for carriers with higher received SNR and lower for carriers with lower received SNR. A common multicarrier modulation scheme is OFDM, and it is the modulation scheme that OFDMA is based upon. A modulation scheme that is based on OFDM, but uses only a single carrier is SC-FDE, and it is the modulation scheme SC-FDMA uses.
Orthogonal Frequency Division Multiplexing (OFDM)
OFDM takes advantage of frequency diversity to achieve higher data rates. Figure 1 shows a discrete-time version of an OFDM system [5]. The input binary data is mapped to constellations in frequency domain and is converted to a time domain signal using an IFFT. Afterwards, the last part of the signal is appended to the beginning of the signal, known as the cyclic prefix (CP), in part to fight inter-symbol interference (ISI) and also to convert the signal’s linear convolution with the channel impulse response to a circular convolution. One important feature of OFDM is its ability to equalize in frequency domain. There are many different ways of equalizing, two of which are the zero-forcing (ZF) equalizer and the minimum mean square error (MMSE) equalizer. While they both correctly recover the phase of the signal, the ZF equalizer causes noise enhancement and the MMSE equalizer causes time distortion in the signal. One benefit of equalization using any frequency domain technique, is that applying it is simple – an element-wise multiplication in the frequency domain.
Single-Carrier Frequency-Domain Equalization(SC-FDE)
A variation of OFDM is SC-FDE, a modulation scheme that contains all the same blocks but moves the IFFT from the transmitter to the receiver. Figure 2 shows a discrete-time version of this system [5]. The difference between this system and OFDM is that the constellation mapping takes place in time domain, and the CP is the last time samples appended to the beginning of the signal. Equivalently, an SC-FDE symbol is a group of N+Δ modulated (MQAM, MPSK, etc.) sub-symbols, where N is the length of the FFT/IFFT used in the receiver, and Δ is the length of the cyclic prefix. This makes for a very simple transmitter. By moving the IFFT to the demodulator, equalization is done with a simple multiplication in frequency domain like in an OFDM demodulator.
Capacity and Outage Probability
Using the method described in [6], the plot shown below in Figure 3 was generated. The modulation scheme used mapping used was 16-QAM. It is clear that the capacity of OFDM is consistently higher than SC-FDE for all SNRs. This is mainly due to the fact that when the system experiences frequency selective fading, the bits that are transmitted on the subcarriers in deep fade result in lower instantaneous capacities, while the other subcarriers experience higher instantaneous capacity as they are not in a fade. SC-FDE however does not have this luxury, because the bits are mapped in time and transmitted across all subcarriers. It is also evident that at SC-FDE using an MMSE equalizer has consistently higher capacity than SC-FDE using a ZF equalizer. This difference, however, is quite minimal throughout the range of SNRs used, being less than 0.2 bps/Hz.
Long-Term Evolution Uplink
[attachment=41379]
Introduction
In order to transition from today's 3rd generation (3G) communications systems to meet the needs of 4th generation (4G) systems, the 3rd Generation Partnership Project (3GPP) has released the Long Term Evolution (LTE) specification. Among the numerous differences between these generations are changes in the physical layer (PHY), specifically in the modulation and multiple access schemes. While its parent generation relied on variations of Code Division Multiple Access (CDMA), LTE implements Orthogonal Frequency Division Multiplexing (OFDM) for its downlink and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) for its uplink. The purpose of this project is to investigate the reasoning for this discord between uplink and downlink modulation schemes; specifically, why Orthogonal Frequency Division Multiple-Access (OFDMA) was not used as the uplink.
OFDMA and SC-FDMA are the multiple-access versions of OFDM and a similar modulation scheme, Single-Carrier Frequency-Domain Equalization (SC-FDE). In order to compare the differences between the multiple-access methods, it is important to first cover the differences between their underlying modulation schemes. Section 2 covers these differences, building the foundation for a comparison between the multiple-access methods themselves in Section 3.
The bases used for comparison will be capacity, outage probability and peak-to-average power ratio (PAPR). While the first two bases have always been traditionally used in analyses, PAPR is especially important for the uplink of mobile devices. Amplifiers used in circuits today have a linear region in which they must operate so as not to introduce signal distortion, and it is ideal to run with maximum amplification. However, if there is a high PAPR, the device is forced to run with lower amplification so the peak power does not lie in the non-linear gain region. The farther these amplifiers are operated from the peak, the less power efficient the devices become, leading to increased power consumption and while this might not be very important for a base station, it will reduce drain batteries on mobile devices more quickly. Therefore it is important to keep a low PAPR on the uplink.
Simulation setup
The simulations in this report are conducted with values similar to that in the LTE specifications. Some of the assumptions made are that the carrier is 1950MHz which is in Band I, the sample rate is 30.72MHz, and the FFTs/IFFTs will be 2048-point ones, with cyclic prefixes of 160 samples [1]. In the multi-user-case we assume that each user will use 12 subcarriers, requiring additional 12-point FFT/IFFTs as explained in Section 3.2. Also, 7 symbols will be sent at a time, corresponding to a single resource-block in the LTE specification [1]. The simulated channel follows the multipath propagation conditions specified by the 3GPP in [2].
Modulation Schemes
When facing a frequency selective channel, one well-known method of improving performance is to use multicarrier modulation. By transmitting at multiple carrier frequencies, it is possible to increase capacity by adapting the data rate to be higher for carriers with higher received SNR and lower for carriers with lower received SNR. A common multicarrier modulation scheme is OFDM, and it is the modulation scheme that OFDMA is based upon. A modulation scheme that is based on OFDM, but uses only a single carrier is SC-FDE, and it is the modulation scheme SC-FDMA uses.
Orthogonal Frequency Division Multiplexing (OFDM)
OFDM takes advantage of frequency diversity to achieve higher data rates. Figure 1 shows a discrete-time version of an OFDM system [5]. The input binary data is mapped to constellations in frequency domain and is converted to a time domain signal using an IFFT. Afterwards, the last part of the signal is appended to the beginning of the signal, known as the cyclic prefix (CP), in part to fight inter-symbol interference (ISI) and also to convert the signal’s linear convolution with the channel impulse response to a circular convolution. One important feature of OFDM is its ability to equalize in frequency domain. There are many different ways of equalizing, two of which are the zero-forcing (ZF) equalizer and the minimum mean square error (MMSE) equalizer. While they both correctly recover the phase of the signal, the ZF equalizer causes noise enhancement and the MMSE equalizer causes time distortion in the signal. One benefit of equalization using any frequency domain technique, is that applying it is simple – an element-wise multiplication in the frequency domain.
Single-Carrier Frequency-Domain Equalization(SC-FDE)
A variation of OFDM is SC-FDE, a modulation scheme that contains all the same blocks but moves the IFFT from the transmitter to the receiver. Figure 2 shows a discrete-time version of this system [5]. The difference between this system and OFDM is that the constellation mapping takes place in time domain, and the CP is the last time samples appended to the beginning of the signal. Equivalently, an SC-FDE symbol is a group of N+Δ modulated (MQAM, MPSK, etc.) sub-symbols, where N is the length of the FFT/IFFT used in the receiver, and Δ is the length of the cyclic prefix. This makes for a very simple transmitter. By moving the IFFT to the demodulator, equalization is done with a simple multiplication in frequency domain like in an OFDM demodulator.
Capacity and Outage Probability
Using the method described in [6], the plot shown below in Figure 3 was generated. The modulation scheme used mapping used was 16-QAM. It is clear that the capacity of OFDM is consistently higher than SC-FDE for all SNRs. This is mainly due to the fact that when the system experiences frequency selective fading, the bits that are transmitted on the subcarriers in deep fade result in lower instantaneous capacities, while the other subcarriers experience higher instantaneous capacity as they are not in a fade. SC-FDE however does not have this luxury, because the bits are mapped in time and transmitted across all subcarriers. It is also evident that at SC-FDE using an MMSE equalizer has consistently higher capacity than SC-FDE using a ZF equalizer. This difference, however, is quite minimal throughout the range of SNRs used, being less than 0.2 bps/Hz.