In the fifth generation (5G) mobile network system, user capacity should be improved compared with the current 4G network system. Orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiplexing (OFDMA), which have high spectral efficiency and resort to orthogonality between subcarriers, are the most widely used modulation technique in the current 4G network system. Despite its many advantages, GFDM also has disadvantages, such as a high peak-to-average power ratio (PAPR).
To reduce the PAPR, various PAPR reduction techniques have been studied on OFDM, such as clipping, selective mapping (SLM), partial transmission sequence (PTS), Tone Reservation (TR) and single carrier frequency division multiple access (SC-FDMA) for LTE uplink. Therefore, compared to the current 4G system, the importance of lower latency and resource efficiency is highly emphasized in the upcoming 5G mobile phone network system [5]. For this reason, OFDM has been used on various standards such as digital video broadcasting (DTV), digital audio broadcasting (DAB), worldwide interoperability for microwave access (WiMAX), Wi-Fi, LTE and LTE-A [14,16 ].
According to the 3GPP ITU-R document, the latency is approximately 10 ms, assuming connected transmission in the 4G LTE system. Therefore, methods to reduce PAPR have been proposed so far, such as clipping [22], SLM and partial transmission sequence (PTS). In current 4G LTE, OFDMA [36] and SC-FDMA [29] are universal modulations for uplink and downlink.
SLM does not change the design of the subsymbol carrier waveform, such as raised cosine (RC) or root raised cosine (RRC).
OFDM System Model
High PAPR is a performance factor related to the increase in circuit building costs and a distortion of signals due to nonlinearity. OFMDA is based on OFDM, but the difference is that the subcarriers of other users are taken into account at the same time. The main purpose of SC-FDMA is aimed at uplink scenarios with low PAPR signal transitions [2].
Therefore, these two modulation techniques are the standard for performance analysis of PAPR and spectral efficiency. Moreover, due to the similarity between OFDM SLM and GFDM SLM, SLM is suitable for application to GFDM. In this thesis, the theoretical expectation for GFDM SLM is estimated based on the estimate of OFDM SLM.
Finally, the PAPR and spectral efficiency are compared between the main current and GFDM SLM modulations. In Section II, the background details describe the basic concepts which the definition of PAPR, the basic concept and system models of OFDM, GFDM and SC-FDMA. Then (3) is the same as a multiplication of the IFFT matrix and the data symbol vector, (3) can be written by.
Assuming an LTI system and a multipath channel, the received signal is expanded by a multipath propagation. After removing the additional added parts, His change to a circulating matrix Hcpandy is represented by (8). 8) By multiplying an FFT matrix, the circular matrix Hcp is decomposed by singular value decomposition (SVD). The channel frequency response can be detected by pilot, based on detected channel response, the data can estimate.
In downlink, each user knows the allocated own frequency BW and timeslot from the resource block. In (8), a difference between OFDMA and OFDM is that among the total data symbol array, only assigned successive data symbols have data.
PAPR
Definition and general effect of PAPR in OFDM systems
Theoretical estimation of OFDM PAPR
OFDM PAPR Reduction Schemes
Clipping
As the name suggests, the technique repeats clipping and filtering until all excessive peaks are removed.
Selective mapping and partial transmit sequence
Tone reservation
To minimize the data loss, selection of optimal PAPR reduction tone rules or algorithms is investigated in [8,20].
SC-FDMA System Model
The main idea of SC-FDMA is that the FFT precoding matrix makes signals that are the same as symbols at the sampling point. IFDMA has higher PAPR reduction than LFDMA by using different roll-off factor filters as Fig.6. Even if you change the roll-off factor from 0 to 1, the difference between PAPR of LFDMA is less than 1dB.
GFDM
P0kΓRFMdk (18) R is the 2M x M recurrence matrix, Γ is the diagonalized waveform filter transformed into the frequency domain, and P0 is the mapping and upsampling matrix. If the modulation matrix GFDM A has orthogonally distributed subsymbol carriers in all available frequency bands, Hcp is diagonalized by the FFT matrix. Assuming the OFDM symbol time duration is T, the GFDM time duration is equal to the MT duration.
Because GFDM transmits KM symbols immediately, GFDM only needs one CP to transmit KM symbols. By the same token, the longMT rectangular window makes sharp power spectral density 1/MT zero for each subsymbol carrier. In [45], the sum rate is analyzed by comparing between GFDMA and SC-FDMA under AWGN asynchronous channels from Fig.
The result clearly shows that GFDM has higher spectral efficiency than OFDM and SC-FDMA. The exact definition of GFDMA is to distribute a few sets of subsymbol carriers in the same subcarrier to each user. This means that if GFDM is used with some subcarrier sets, such as empty other subcarriers for different users, it satisfies the condition of GFDMA.
The number of subsymbol carriers matches Kuser SC-FDMA, the number of subcarriers is Figure 12 shows that GFDMA has similar PAPR performance to IFDMA and similar OOB performance to LFDMA. However, if 4f between subsymbol carriers is assumed to be the same as SC-FDMA, GFDMA loses the advantages of GFDM such as low OOB and timing efficiency from CP.
As shown in Figure 13, GFDMA has a trade-off between loss of OOB and low PAPR performance compared to other techniques.
![Figure 8: Block diagram of GFDM from [12]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10529730.0/22.918.219.702.504.744/figure-8-block-diagram-of-gfdm-from-12.webp)
Precoded GFDM
Basically, a block diagram of cut GFDM is the same as Fig.2 except that GFDM modulation and demodulation are used in the processing. However, it is clear that the clipping method has disadvantages in accurate signal transmission, shown as lower bit error rate performance. If it is assumed that OFDM and GFDM use the same number of subcarriers, as mentioned in Section 2.2.1, the PAPR of GFDM is higher than OFDM.
In this paper, SLM for GFDM PAPR reduction is considered among several OFDM PAPR reduction techniques. An SLM technique does not make changes to basic data symbols and waveform of subsymbol carriers on GFDM. The simplest way to apply GFDM SLM is to apply KM phase-shifting arrays to the matched subsymbol carriers.
However, because the GFDM modulation is performed on the time axis, it is not easy to directly apply SLM in the frequency domain like OFDM SLM. The important note is that GFDM SLM should be avoided for overlapping subsymbol waveforms that have the same subcarrier index. The random phase-shifting set has too many possible cases and is not realistically applicable.
Just as OFDM SLM limits its specific phase shift set, the simplified phase shift set is proposed [41]. CCDF(γ) =P(PAPR> γ) = (1−(1−e−γ)αKM)U (21) where the compensation coefficient α is different fromβ of OFDM, U is the number of independent phase shift sets. Determines the OOB because it is the period MT of the GFDM signal, as it does not affect the period or frequency band of the signal.
2, the parameter setting of PAPR for OFDM SLM, GFDM SLM and SC-FDMA is defined as table. As the same as OFDM SLM, a is changed by changing the parameters of GFDM SLM. Due to a large number of subsymbol carriers, the minimum PAPR of GFDM SLM is larger than SC-FDMA by about 2.5 dB.
Spectrum Analysis
Assuming that the GFDM has KM sampling points in the time domain, each mK sampling point has the same amplitude with the data symbols. Compared to GFDM SLM, U=2 is enough for the same performance with DFT precoded GFDM and U=4 or 8 is enough for better performance of PAPR than BIDFT-N GFDM at CCDF (0.1%). It is assumed that GFDMA is the specific parameter for the lowest PAPR of GFDMA, the spectrum of GFDMA is higher than general GFDM and LFDMA.
Fig.
Spectral Efficiency
Considering noise improvement and effective BW of all methods, the simulation result of spectral efficiency can be represented by the Fig. In this paper, the GFDM SLM is proposed for reducing PAPR of GFDM without signal distortions. From a traditional OFDM SLM technique, the limited phase shift set is applied for a simplifying structure.
To compare performance, other PAPR reduction schemes such as SC-FDMA, low PAPR GFDMA and precoded GFDM are considered. GFDM SLM technique can be achieved a significant PAPR reduction compared to original GFDM, if increasing number of phase-shift sets. Compared with other GFDM PAPR reduction techniques, the proposed GFDM SLM technique has lower PAPR than precoded GFDM with some number of phase shift sets.
SC-FDMA and GFDMA cases with low PAPR have the lowest PAPR limit. Even if the complexity is reduced by the limited phase shift array, it has higher complexity than precoded GFDM or SC-FDMA. In OFDM SLM, there are several trials to select the optimal phase shift group for low complexity and optimal PAPR.
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