Research on modified selected mapping technique for PAPR reduction and reception performance improvement in 8QAM-OFDM optical access systems

Orthogonal frequency division multiplexing (OFDM) is the preferred technology in optical access networks since it offer resistance to chromatic dispersion (CD) and polarization mode dispersion (PMD), multiple services access and high spectral efficiency [1, 2]. However, high peak-to-average power ratio (PAPR) not only increases complexity of digital-to-analogue (D/A) and analogue-to-digital (A/D) converters, but also causes signals nonlinear distortion since it makes the power amplifier (PA) work in a non-linear area in optical access systems [2–8]. Hence, OFDM signals with low PAPR are desirable for optical signals access. In [9], there are many PAPR reduction techniques are introduced and analyzed. Amongst these techniques, selected mapping (SLM) is one of the promising methods for reducing PAPR influence because it is simple to achieve and not introduce any distortion [10]. Recently, SLM method is introduced into generation of (16 quadrature amplitude modulation orthogonal frequency division multiplexing) 16QAM-OFDM signals in millimeter wave optical access systems for solving high PAPR problem [11, 12]. In Ref. [13, 14], though high PAPR reduction of OFDM using recurring SLM with cyclic block codes scheme and a modified SLM scheme have been proposed, the BER performance has not been analyzed and their applications had not been carried out in communication systems. In this paper, a modified SLM technique is introduced for reducing the PAPR and enhancing reception performance of the downstream 8QAM-OFDM signals in optical access systems. Compared with 4QAM-OFDM signals, 8QAM-OFDM signals are selected as the downlink signals in this scheme, because 8QAM-OFDM signals can improve system capacity and spectrum efficiency. Moreover, generation and detection of 8QAM-OFDM downstream signals is easier to implement than 16QAM-OFDM signals. The complementary cumulative distribution function (CCDF) curves are measured for analyzing the PAPR influence, when traditional SLM and one modified SLM methods are adopted. Compared with the traditional SLM technique, the PAPR value of 8QAM-OFDM optical downstream signals with 128 subcarriers is reduced by almost 2 dB using modified SLM when the CCDF value is 10⁻². Moreover, the bit error rate (BER) performance will be improved, as 10Gbit/s 8QAM-OFDM optical downlink signals are transmitted over single mode fiber (SMF) with different access length using this scheme.

The paper is organized in different sections. Section II gives brief description about system configuration method. Section III discusses the comparison of both proposed method and existing method. Finally, conclusions are given in Section IV.

Where max[.] is the maximum value operation, and it is indication of maximal value. E[.] is the expectation operator and indication of average power. The above equation represents the ratio of the maximum power value to the average power. In the conventional SLM technique, to reduce the value of PAPR, the input sequence is multiplied by U phase rotation vectors [11]. After Inverse Fast Fourier Transform (IFFT), a few alternative sequences can be obtained (e.g., X _u  = {P ⁽¹⁾ a, P ⁽¹⁾ a, ⋯, P ^(N) a} ^T , 1 ≤ u ≤ N), as shown in Fig. 1a. X _k and X _m are two of the alternative sequences by using the conventional SLM method. When the modified SLM technique is adopted, the sequences after IFFT are arbitrarily selected out of two. One is multiplied bysin(π/4), and the other is multiplied byi ∗ cos(π/4), as shown in Fig. 1b. Since Fourier transform has linear property [14], the generated new sequence can be written as

$$ {X}_u= IFFT\left(a\ast \left(\sin \left(\pi /4\right)\ast {p}^{(k)}+i\ast \cos \left(\pi /4\right)\ast {p}^{(m)}\right)\right) $$

(3)

And then, they are input in the alternative sequence after adding the two together. If extra M alternative sequence is obtained, using this method, N + M alternative sequences will be generated. As shown in Fig. 1c and d, at the receiver, the OFDM data block can be restored by the reverse operation, which is similar to the used method in Ref. [8]. But, the number of reverse sequence is different.

For reducing PAPR influence and enhance signal reception performance in the 8QAM-OFDM optical access system, the modified SLM technique is introduced and compared with the traditional SLM method. In this scheme, 8QAM-OFDM optical downstream signals are transmitted successfully over SMF with different access distances at 10Gbit/s. The results demonstrated that, the PAPR probability of 8QAM-OFDM signals will be reduced adopting the modified SLM method. Moreover, the reception performance of 8QAM-OFDM signals using the modified SLM is better than the conventional SLM. Hence, the modified SLM method is a potential solution scheme for reducing PAPR influence and enhance the reception performance in future optical access OFDM systems.