LTE uplink (Uplink), as it really is
A special feature of the E-UTRAN downlink is the use of SC-FDMA (Single Carrier - Frequency Division Multiple Access) Multiple Access technology with a single carrier frequency and average transmit power (Peak-to-Average Power Ratio) transmit power (PAPR). Eliminating the mutual influence of users is achieved by introducing cyclic prefixes and using effective equalizers in receiving devices. The transmission time interval TTI in the uplink of the E-UTRAN network corresponds to the downlink TTI and is equal to 0.5 ms. It is possible to use increased TTI for special types of connections (services). The basic configuration of uplink antennas using MIMO involves the use of two transmit antennas at the mobile terminal and two receive antennas at the base station.
SC-FDMA is a hybrid transmission scheme that combines low PAR values ​​inherent in single-carrier systems, such as GSM and CDMA, with a long symbol duration and flexible OFDM frequency distribution. The principles of SC-FDMA signal generation are shown in Figure 1, which is a fragment of one of the figures in the 3GPP TR 25.814 report on the study of the physical layer of LTE.
Fig. 1. SC-FDMA signal generation
Signal transmission in the uplink (Uplink).
Fig. 2. Comparison of transmission of a series of data symbols QPSK in OFDMA and SC-FDMA
In the left part of Figure 1, data symbols are represented in the time domain. The symbols are transformed into the frequency domain using the fast Fourier transform and then, in the frequency domain, they are distributed to the right places of the overall carrier spectrum. Then they need to be converted back into the time domain in order to add a cyclic prefix to them before transmission.
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Fig. 3. Creating the SC-FDMA symbol in the time domain
Fig. 4. Representation of the SC-FDMA symbol in the baseband and frequency offset.
In continuation of the graphical comparison of OFDMA and SC-FDMA, Figures 3 and 4 show the detailed process of generating the SC-FDMA signal. First, a representation of a data symbol sequence in the time domain is created, as shown in Fig. 3. In this example, with four subcarriers, four data symbols are required to generate one SC-FDMA symbol. Using the first four color QPSK symbols (see Figure 2), the process creates one SC-FDMA symbol in the time domain, calculating a path that moves from one QPSK data symbol to another. This is done at a rate that is M times faster than the SC-FDMA symbol rate, with the result that one SC-FDMA symbol contains M consecutive QPSK data symbols. In order not to complicate the consideration, we will not discuss the filtering of transitions between characters, although such filtering is necessarily present in any real scheme.
In the frequency and time domains, OFDMA and SC-FDMA show a sequence of eight QPSK symbols. In this simplified example, the number of subcarriers (M) has been reduced to four. For OFDMA, four (M) symbols are processed in parallel, each modulated with its own subcarrier with the corresponding QPSK phase. Each data symbol occupies a 15 kHz band for the duration of the transmission of one OFDMA symbol, which is equal to 66.7 ÎĽs. At the beginning of the next OFDMA symbol, a guard interval is inserted that contains a cyclic prefix (CP). A CP is a copy of the end of a character added to the beginning of a character. Due to parallel transmission, data symbols are the same length as OFDMA symbols.
In the case of SC-FDMA, data symbols are transmitted sequentially. Since in this example four subcarriers are used, four data symbols are transmitted per SC-FDMA symbol period. The SC-FDMA symbol period is the same length as the OFDMA symbol, i.e. 66.7 µs, but due to the sequential transmission, the data characters are shorter, i.e. equal to 66.7 / M µs. In connection with the increase in the speed of the following characters, a wider bandwidth is required for their transmission. As a result, each symbol occupies 60 kHz in the spectrum, rather than 15 kHz, as was the case with the slower symbols used in OFDMA. After transmitting four data characters, the CP is inserted.
Fig.5 Data conversion in SC-FDMA modulation.
The procedure for forming the SC-FDMA signal differs from the OFDMA scheme. After channel coding, scrambling, and the formation of modulation symbols, they are grouped into blocks of M symbols — SC-FDMA subsymbols. It is obvious that it is impossible to directly attribute them to subcarriers in 15 kHz steps - it is required in
N times the higher frequency, where N is the number of subcarriers available for transmission. Therefore, having formed groups of M modulation symbols (M <N), they are subjected to M-point discrete Fourier transform (DFT), i.e. form an analog signal. And then, using the standard procedure of the inverse N-point Fourier transform, a signal corresponding to the independent modulation of each subcarrier is synthesized, a cyclic prefix is ​​added and an RF output signal is generated.
After creating the IQ representation of one SC-FDMA symbol in the time domain, the next step is to represent it in the frequency domain using the discrete Fourier transform (DFT; see Fig. 4). The DFT sampling rate is chosen so that the shape of one SC-FDMA symbol in the time domain is fully represented by M DFT bins spaced 15 kHz apart, with each bin representing one subcarrier with constant amplitude and phase for one symbol period of SC-FDMA equal to 66.7 ÎĽs. At the same time, there is always a one-to-one correspondence between the number of data symbols transmitted in one period of the SC-FDMA symbol and the number of DFT bins created, which, in turn, is equal to the number of subcarriers occupied. This is quite logical: with an increase in the number of data symbols transmitted over one SC-FDMA period, the signal in the time domain changes faster, which leads to bandwidth expansion and, therefore, requires a larger number of DFT bins to fully represent the signal in the frequency domain.
Discrete Fourier transform on M-points.
Sub-carrier allocation methods in SC-FDMA
When forming a group signal in the “up” line, for each terminal it is decided which part of the subcarriers is used (filled with data) and which is not (filled with “zeros”) (see Fig. 12).
The distribution of resources, depending on the attenuation in the channels.
Assigns subcarriers for users is in accordance with the frequency response of the channels of each user.
Multipath Resistance
Now is the time to ask the question: “How can SC-FDMA maintain its resistance to multipath propagation with such short data symbols?”. In OFDMA, the modulating data symbols remain constant during the period of the OFDMA symbol of 66.7 μs, while the SC-FDMA symbol changes with time because it contains M short data symbols. The multipath resistance of the OFDMA demodulation process is due to the large length of data symbols that are superimposed directly on the individual subcarriers. Fortunately, resistance to delay spread is due precisely to the constant nature of each subcarrier, and not to the constancy of the data symbols. As shown above, the DFT of a time-varying SC-FDMA symbol creates a set of DFT bins that are constant for the SC-FDMA symbol, despite the fact that the modulating data symbols change. This is the main feature of the DFT process, that the time-varying SC-FDMA symbol, consisting of M consecutive data symbols, is represented in the frequency domain M by timeless subcarriers. Thus, even SC-FDMA with its inherent short data symbols is sufficiently resistant to multipath propagation. Now, to complete the generation of the SC-FDMA signal, the same operations are performed as for OFDMA. The inverse FFT transforms the frequency-shifted signal into the time domain, and then adding a CP provides fundamental OFDMA fundamental multipath resistance.
The relationship of modulation of OFDMA and SC-FDMA.
Figure 5 illustrates the close relationship between SC-FDMA and OFDMA. The orange blocks show the OFDMA processing, and the blue blocks represent the additional time domain processing required for SC-FDMA. The main thing to note is that the signal converted from the frequency domain back to the time domain is nothing more than a frequency-shifted version of the QPSK symbol sequence. This example demonstrates the main reason for the creation of SC-FDMA, namely, the PAR of the final signal does not exceed the PAR of the original data symbols, which in this case are QPSK symbols. This differs significantly from OFDMA, where the parallel transmission of the same QPSK symbols creates statistical peaks, very similar to Gaussian noise, which significantly exceed the PAR of the original data symbols. Limiting PAR using SC-FDMA significantly reduces the need for a mobile device to operate with high power peaks. This reduces both costs and power consumption.
SC-FDMA is a hybrid transmission scheme that combines low PAR values ​​inherent in single-carrier systems, such as GSM and CDMA, with a long symbol duration and flexible OFDM frequency distribution. The principles of SC-FDMA signal generation are shown in Figure 1, which is a fragment of one of the figures in the 3GPP TR 25.814 report on the study of the physical layer of LTE.
Fig. 1. SC-FDMA signal generation
Signal transmission in the uplink (Uplink).
Fig. 2. Comparison of transmission of a series of data symbols QPSK in OFDMA and SC-FDMA
In the left part of Figure 1, data symbols are represented in the time domain. The symbols are transformed into the frequency domain using the fast Fourier transform and then, in the frequency domain, they are distributed to the right places of the overall carrier spectrum. Then they need to be converted back into the time domain in order to add a cyclic prefix to them before transmission.
')
Fig. 3. Creating the SC-FDMA symbol in the time domain
Fig. 4. Representation of the SC-FDMA symbol in the baseband and frequency offset.
In continuation of the graphical comparison of OFDMA and SC-FDMA, Figures 3 and 4 show the detailed process of generating the SC-FDMA signal. First, a representation of a data symbol sequence in the time domain is created, as shown in Fig. 3. In this example, with four subcarriers, four data symbols are required to generate one SC-FDMA symbol. Using the first four color QPSK symbols (see Figure 2), the process creates one SC-FDMA symbol in the time domain, calculating a path that moves from one QPSK data symbol to another. This is done at a rate that is M times faster than the SC-FDMA symbol rate, with the result that one SC-FDMA symbol contains M consecutive QPSK data symbols. In order not to complicate the consideration, we will not discuss the filtering of transitions between characters, although such filtering is necessarily present in any real scheme.
In the frequency and time domains, OFDMA and SC-FDMA show a sequence of eight QPSK symbols. In this simplified example, the number of subcarriers (M) has been reduced to four. For OFDMA, four (M) symbols are processed in parallel, each modulated with its own subcarrier with the corresponding QPSK phase. Each data symbol occupies a 15 kHz band for the duration of the transmission of one OFDMA symbol, which is equal to 66.7 ÎĽs. At the beginning of the next OFDMA symbol, a guard interval is inserted that contains a cyclic prefix (CP). A CP is a copy of the end of a character added to the beginning of a character. Due to parallel transmission, data symbols are the same length as OFDMA symbols.
In the case of SC-FDMA, data symbols are transmitted sequentially. Since in this example four subcarriers are used, four data symbols are transmitted per SC-FDMA symbol period. The SC-FDMA symbol period is the same length as the OFDMA symbol, i.e. 66.7 µs, but due to the sequential transmission, the data characters are shorter, i.e. equal to 66.7 / M µs. In connection with the increase in the speed of the following characters, a wider bandwidth is required for their transmission. As a result, each symbol occupies 60 kHz in the spectrum, rather than 15 kHz, as was the case with the slower symbols used in OFDMA. After transmitting four data characters, the CP is inserted.
Fig.5 Data conversion in SC-FDMA modulation.
The procedure for forming the SC-FDMA signal differs from the OFDMA scheme. After channel coding, scrambling, and the formation of modulation symbols, they are grouped into blocks of M symbols — SC-FDMA subsymbols. It is obvious that it is impossible to directly attribute them to subcarriers in 15 kHz steps - it is required in
N times the higher frequency, where N is the number of subcarriers available for transmission. Therefore, having formed groups of M modulation symbols (M <N), they are subjected to M-point discrete Fourier transform (DFT), i.e. form an analog signal. And then, using the standard procedure of the inverse N-point Fourier transform, a signal corresponding to the independent modulation of each subcarrier is synthesized, a cyclic prefix is ​​added and an RF output signal is generated.
After creating the IQ representation of one SC-FDMA symbol in the time domain, the next step is to represent it in the frequency domain using the discrete Fourier transform (DFT; see Fig. 4). The DFT sampling rate is chosen so that the shape of one SC-FDMA symbol in the time domain is fully represented by M DFT bins spaced 15 kHz apart, with each bin representing one subcarrier with constant amplitude and phase for one symbol period of SC-FDMA equal to 66.7 ÎĽs. At the same time, there is always a one-to-one correspondence between the number of data symbols transmitted in one period of the SC-FDMA symbol and the number of DFT bins created, which, in turn, is equal to the number of subcarriers occupied. This is quite logical: with an increase in the number of data symbols transmitted over one SC-FDMA period, the signal in the time domain changes faster, which leads to bandwidth expansion and, therefore, requires a larger number of DFT bins to fully represent the signal in the frequency domain.
Discrete Fourier transform on M-points.
Sub-carrier allocation methods in SC-FDMA
When forming a group signal in the “up” line, for each terminal it is decided which part of the subcarriers is used (filled with data) and which is not (filled with “zeros”) (see Fig. 12).
The distribution of resources, depending on the attenuation in the channels.
Assigns subcarriers for users is in accordance with the frequency response of the channels of each user.
Multipath Resistance
Now is the time to ask the question: “How can SC-FDMA maintain its resistance to multipath propagation with such short data symbols?”. In OFDMA, the modulating data symbols remain constant during the period of the OFDMA symbol of 66.7 μs, while the SC-FDMA symbol changes with time because it contains M short data symbols. The multipath resistance of the OFDMA demodulation process is due to the large length of data symbols that are superimposed directly on the individual subcarriers. Fortunately, resistance to delay spread is due precisely to the constant nature of each subcarrier, and not to the constancy of the data symbols. As shown above, the DFT of a time-varying SC-FDMA symbol creates a set of DFT bins that are constant for the SC-FDMA symbol, despite the fact that the modulating data symbols change. This is the main feature of the DFT process, that the time-varying SC-FDMA symbol, consisting of M consecutive data symbols, is represented in the frequency domain M by timeless subcarriers. Thus, even SC-FDMA with its inherent short data symbols is sufficiently resistant to multipath propagation. Now, to complete the generation of the SC-FDMA signal, the same operations are performed as for OFDMA. The inverse FFT transforms the frequency-shifted signal into the time domain, and then adding a CP provides fundamental OFDMA fundamental multipath resistance.
The relationship of modulation of OFDMA and SC-FDMA.
Figure 5 illustrates the close relationship between SC-FDMA and OFDMA. The orange blocks show the OFDMA processing, and the blue blocks represent the additional time domain processing required for SC-FDMA. The main thing to note is that the signal converted from the frequency domain back to the time domain is nothing more than a frequency-shifted version of the QPSK symbol sequence. This example demonstrates the main reason for the creation of SC-FDMA, namely, the PAR of the final signal does not exceed the PAR of the original data symbols, which in this case are QPSK symbols. This differs significantly from OFDMA, where the parallel transmission of the same QPSK symbols creates statistical peaks, very similar to Gaussian noise, which significantly exceed the PAR of the original data symbols. Limiting PAR using SC-FDMA significantly reduces the need for a mobile device to operate with high power peaks. This reduces both costs and power consumption.
Source: https://habr.com/ru/post/114401/
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