Orthogonal frequency division multiple access (OFDMA) is a key multiple access technique for the long term evolution (LTE) downlink. However, high peak-to-average power ratio (PAPR) can cause the degradation of power efficiency. The well-known PAPR reduction technique, dummy sequence insertion (DSI), can be a realistic solution because of its structural simplicity. However, the large usage of subcarriers for the dummy sequences may decrease the transmitted data rate in the DSI scheme. In this paper, a novel DSI scheme is applied to the LTE system. Firstly, we obtain the null subcarriers in single-input single-output (SISO) and multiple-input multiple-output (MIMO) systems, respectively; then, optimized dummy sequences are inserted into the obtained null subcarrier. Simulation results show that Walsh-Hadamard transform (WHT) sequence is the best for the dummy sequence and the ratio of 16 to 20 for the WHT and randomly generated sequences has the maximum PAPR reduction performance. The number of near optimal iteration is derived to prevent exhausted iterations. It is also shown that there is no bit error rate (BER) degradation with the proposed technique in LTE downlink system.
The fields of mobile communication techniques have been rapidly developed in recent decades. One of the development results is the 3rd generation partnership project (3GPP) long term evolution (LTE), which has been deployed all over the world. Downlink transmission of the LTE is based on the use of multiple access technology: orthogonal frequency division multiple access (OFDMA), which is a modification of orthogonal frequency division multiplexing (OFDM) for the multiple access [
However, together with its advantages, still some challenging issues remain for the OFDM access technology design. One of the major drawbacks is high peak-to-average power ratio (PAPR) of transmitted signals. Therefore, the detection efficiency of the OFDM receiver is very sensitive to the nonlinear devices such as digital-to-analog converter (DAC) and high power amplifier (HPA). That may severely diminish the system performance because of the detection efficiency degradation and induced spectral regrowth. Most of the transmitters of wireless communication systems employ the HPA to obtain sufficient transmit power. The HPA usually operates near the saturation region to achieve the maximum output power efficiency; thus the memoryless nonlinear distortions occur in the communication channels due to the high PAPR of the input signals. If the HPA does not operate within linear region with power back-off (PBO), it is difficult to keep the out-of-band power below the specified limits. This situation leads to very inefficient amplification and expensive transmitters [
To deal with the PAPR problem, various approaches have been proposed such as deliberate clipping [
In this paper, the null subcarriers for the dummy sequences are derived in LTE SISO,
This paper is organized as follows. Section
Several drawbacks arise in OFDM, the most severe of which is the highly nonconstant envelope of the transmitted signals, that is, the PAPR, making the OFDM very sensitive to nonlinear components in the transmission path. The use of HPA can be one of the solutions. However, owing to cost, design, and, most importantly, power efficiency considerations, the HPA cannot resolve the dynamics of the transmitted signal. A clipping method inevitably cuts off the signal at some point, which causes additional in-band distortion and adjacent channel interference. The power efficiency penalty is certainly the major obstacle in implementing OFDM systems in low-cost applications. Moreover, in power limited regimes determined by regulatory bodies, the average power is reduced in comparison to single-carrier systems. The main goal of peak power control is to diminish the influence of high peaks in transmit signals on the performance of the transmission system. The PAPR of the transmit signal can be defined as
The CCDF which denotes the probability that the PAPR of a data block exceeds a given threshold is one of the most frequently used performance measures for PAPR reduction techniques. If the number of subcarriers is large enough, magnitudes of real and imaginary parts of output signal have Gaussian distribution with mean of zero and variance of
There are two radio frame structures for LTE, that is, frame structure type 1 (FS1) for full and half duplex frequency division duplex (FDD) and frame structure type 2 (FS2) for time division duplex (TDD). This paper focuses on FDD. In FDD, because uplink and downlink transmissions are separated in the frequency domain, the frame structure is the same in the uplink and downlink in terms of frame, subframe, and slot duration. FS1 is shown in Figure
Frame structure type 1 [
The size of various fields in the time domain is expressed as a number of time units,
One symbol on one subcarrier is defined as the resource element, which is the smallest time-frequency unit used for downlink transmission. A group of twelve contiguous subcarriers in frequency and one slot in time is called a resource block (RB) [
Downlink resource grid [
A physical RB consists of
One downlink slot using the normal cyclic prefix (CP) length contains seven symbols. Variations on this configuration for FS1 are summarized in Table
Physical resource block parameters [
Configuration |
|
| |
---|---|---|---|
Normal cyclic prefix |
|
12 | 7 |
Extended cyclic prefix |
|
12 | 6 |
Extended cyclic prefix |
|
24 | 3 |
Firstly, the number of null subcarriers per frame,
Secondly, the number of null subcarriers per frame is obtained in the PDSCH of the LTE
Thirdly,
We have derived the null subcarriers for SISO,
In the proposed scheme, there is a trade-off between the type and pattern of dummy sequence and the iteration time for the cyclic shift. Therefore, consideration of these elements is an important aspect of PAPR reduction performance and suitable system complexity. In this section, we find the near optimum values for the DSI method, the ratio of dummy sequence for the null subcarriers, and the number of iterations by various simulation results. The simulations are performed under the 3GPP LTE physical layer standard [
Parameters of the computer simulations.
Parameter | Value |
---|---|
Carrier frequency |
2140 MHz |
Channel bandwidth | 2.5 MHz |
FFT size | 512 |
Duplex mode | FDD |
Cyclic shift | Normal |
Modulation type | QPSK |
Doppler frequency | 119 MHz (velocity = 60 Km/h) |
CRC | 24 bit |
Forward error correction (FEC) | 1/3 turbo coding |
Number of dummy bit | 36 |
For the suitable DSI, we compare the PAPR reduction performances of the well-known DSI methods. The DSI methods are briefly introduced as follows. Method 1: complementary sequences and correlation sequences corresponding to the first bits of each partitioned subblock are inserted as dummy sequences before the inverse fast Fourier transform (IFFT) stage [ Method 2: WHT is inserted as a dummy sequence before the IFFT stage [ Method 3: time-frequency domain swapping algorithm and flipping technique are used to optimize the phase of dummy sequences [ Method 4: every initial dummy sequence is “0” and employs bit flipping method to generate dummy sequences for next branch [ Method 5: the total sequences consist of Method 6: a partial DSI method is a combination of the DSI and the PTS. The original data sequences are partitioned and zero padded, and a “0” or “1” dummy sequence is inserted into each subblock. The time domain waveforms are summed after IFFT, and the sequence with the lowest PAPR is selected and transmitted [
Figure
CCDF comparison of DSI methods.
As we analyzed in Section
Flow chart of scheduling the null subcarriers for peak power reduction.
Since PAPR performance is affected by the patterns of the WHT, the PAPR performances of the proposed method are compared by the ratio of the WHT and randomly generated sequence. The five patterns of the WHT and randomly generated sequence can be in the ratio of
Figure
CCDF comparison as a function of WHT and randomly generated sequence ratio.
In order to approach a more efficient PAPR reduction within the limited null subcarriers, cyclic shifting is used with WHT sequences. Multiple iteration operations for the cyclic shifting, however, cause the high computational complexity of the LTE system. Therefore, we need to consider the cyclic shift loop times to approach the minimum computational complexity. The PAPR performances of various cyclic shift loop times are compared in Figure
CCDF comparison over the number of iterations.
In addition to the PAPR comparison, we examine the BER performance of the proposed method in the LTE downlink system. Simulation is performed under Rayleigh fading channel, and the turbo coding is used with a coding rate of
BER performance of the proposed scheme and LTE system.
This paper proposed a novel DSI scheme for the LTE downlink system. For the application of the DSI to LTE system, the null subcarriers were obtained in LTE SISO, 2 × 2, and 4 × 4 MIMO systems, respectively, and each transmission efficiency was calculated. The dummy sequence was designed by scheduling the ratio between WHT and random sequences. The number of near optimal iteration and BER performances were derived, which showed that exhausted iterations could be prevented and proposed DSI can reduce PAPR without BER degradation.
The future works will derive the number of subcarriers in LTE-Advanced and
The authors declare that there is no conflict of interests regarding the publication of this paper.