Su2A.22.pdf
Asia Communications and Photonics Conference (ACP) © OSA 2017
Non-Orthogonal Multiple Access for Visible Light Communications 1
Bangjiang Lin, 2Kaiwei Zhang, 2Yu Tian, 2Yuanxiang Chen, 1Xuan Tang, 1Min Zhang, 3Yi Wu, and 3Hui Li
1
Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, China; 2State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, China; 3 Key Laboratory of OptoElectronic Science and Technology for Medicine of Ministry of Education, Fujian Normal University, China
[email protected];
[email protected]
Abstract: We propose a NOMA scheme combined with OFDMA for visible light communications, which offers a high throughput, flexible bandwidth allocation and a higher system capacity for a larger number of users. OCIS codes: (060.2605) Free-space optical communication; (060.4510) Optics communications
1. Introduction Visible light communications (VLC) based on commercial white light-emitting diodes (LEDs) has attracted much attention from academic and industry, due to its advantages such as license free spectrum, free from electromagnetic interference and inherent high security. Multiple access (MA) support for VLC is essential to provide multi-user wireless services. The conventional MA techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA) together with some optical MA techniques such as wavelength division multiple access (WDMA) and space division multiple access (SDMA) have already been proposed for VLC [1]. Power domain multiple access, also known as non-orthogonal multiple access (NOMA) has recently been proposed as a promising solution to enhance the spectral efficiency for the 5th generation (5G) wireless networks [2-3]. NOMA superposes user messages in the power domain and uses successive interference cancellation (SIC) at the receivers to separate the users, so that all of the users can use the whole time-frequency (TF) resources. In this paper, we propose a NOMA-OFDMA scheme for VLC transmission, which offers a high throughput, high tolerance against multipath induced distortion, high spectral efficiency and a higher system capacity for a larger number of users. The feasibility of the NOMA-OFDMA VLC is verified with experiment demonstration. As shown in the experiment results, the optimum power allocation ratios (PAR) is about 0.25. We also investigate the effect of channel estimation on the bit error rate (BER) performance. Since intra symbol frequency averaging (ISFA) and minimum mean square error (MMSE) perform better channel estimation than least square (LS) [4] , they can eliminate the inter-user interference more effectively. 2. Technique principle User 1
Source Data
s '1
Source Data
QAM Modulation
sN
QAM Demodulation
QAM Modulation
Y1
QAM Modulation
s '2
QAM Demodulation
Y2
X1
XN
OFDM Modulation
OFDM Modulation
x1 xN
Preamble Insertion
Power Allocation
...
... User N
s1
Preamble Insertion
x +
Add DC
DAC
Power Allocation
Free Space Optical Channel
Y
Channel Equalization
H p1 X 1
-
LED
DFT
Channel Estimation
CP Remove
Frame Synchronization
y ADC
Optical Detector
H Channel Equalization
...
...
YN
'
sN QAM Demodulation
H pN 1 X N 1 Channel Equalization
Fig.1. Block diagram of downlink NOMA-OFDMA VLC (DFT: discrete Fourier transform, DAC: digital-to-analog converter, ADC: analog-todigital converter, DC: direct current, CP: cyclic prefix).
Su2A.22.pdf
Asia Communications and Photonics Conference (ACP) © OSA 2017
Figure 1 shows the schematic diagram of downlink NOMA-OFDMA VLC with N users. For simplicity, we assume each user uses the whole TF resources. In combination with OFDMA where each user uses a set of subcarriers to transmit or receive its data, more flexible bandwidth allocation can be achieved to support more users. In the transmitter (Tx), the source data for each user is mapped and encoded into OFDM symbol (x1, x2, . . ., xN) prior to power allocation, respectively. Then all the OFDM signals are combined with a total transmitted power of P. The final transmitted time-domain signal can be written as: N
x pi xi ,
(1)
i 1
where pi is the allocated power for user i, xi is the transmitted time-domain OFDM signal for user i. The combined digital OFDM signal is converted into analog signals using a digital-to-analog converter (DAC). Following the inclusion of the direct current (DC) bias, the DC- OFDM is used for intensity modulation (IM) of a LED. After the wireless optical channel, the received signal can be represented as: N
y h pi xi w,
(2)
i 1
the frequency-domain representation of which is given by: N
Y H pi X i W ,
(3)
i 1
where h and H are channel coefficients represented in time-domain and frequency-domain respectively, w and W are noises represented in time-domain and frequency-domain respectively. denotes the convolution operation. Xi is the frequency-domain representation of xi. We assume that p1 > p2 > p3…> pN. At the receiver (Rx), the optical signal is detected by a photo-detector and then converted into a digital format using an analog-to-digital converter (ADC). The output of the ADC is then passed through a frame synchronization module prior to removing the cyclic prefix (CP). After the discrete Fourier transform (DFT) operation, the received signal for user 1 can be obtained by dividing Y by H p1 , which can be written as: N
Y1 X 1 i 2
pi W Xi . p1 H p1
(4)
Note that H can be calculated from channel estimation using the training sequence inserted in the preamble. The transmitted signal of user 1 (i.e., s1) is recovered after demapping Y1. After removing the term of H p1 X 1 in (3), the received signal is divided by H p2 , which can be written as: N
Y2 X 2 i 3
pi W . Xi p2 H p2
(5)
The transmitted signal of user 2 (i.e., s2) can be recovered after demapping Y2. The decoding order of SIC is in the order of increasing channel gain (i.e., H pi ). Finally, the received signal for user N can be written as:
YN X N
W H pN
.
(6)
The transmitted signal of user N (i.e., sN) can be obtained after demapping YN. 3. Experimental setup and results Figure 2 shows the experimental setup for downlink NOMA-OFDMA VLC with two users. At the Tx, two 1.7Mbaud baseband quadrature phase shift keying (QPSK) OFDM signals are three times up-sampled and then upconverted to 1.25 MHz by means of digital I-Q modulation. The two OFDM signals are combined after power allocation and then uploaded to an arbitrary waveform generator (AWG) operating at 5 MS/s. The DFT and CP sizes are 256 and 8, respectively. The generated waveform is converted into analog streams and then DC-level shifted using the bias Tee prior to IM of a commercially available phosphorescent white LED. At the Rx, a commercial optical Rx (THORLABS PDA10A) is used to convert the optical signal back into the electrical signal. The optical Rx output is passed through ADC and captured using a real-time digital oscilloscope for offline signal processing in order to recover the transmitted data. Figure 3 shows the BER as a function of the distance between the Tx and Rx. The PAR between the two users is
Su2A.22.pdf
Asia Communications and Photonics Conference (ACP) © OSA 2017
set to 0.09, 0.16, 0.25, 0.36, and 0.49, respectively. Each BER is calculated from the average of the two users, which is based on more than 1╳105 bits. As shown in Fig. 3, the best BER performance is achieved with a PAR of 0.25. Figure 4 shows the BER performance of the two users with LS, ISFA, and MMSE channel estimation methods. In our previous work4, we have shown that both MMSE and ISFA perform better channel estimation than LS. As shown in Fig. 4, both MMSE and ISFA can eliminate the inter-user interference more effectively than LS. At the Tx, user 1 is allocated with more power. At the Rx, the data of user 1 is decoded prior to decoding the data of user 2. If the data demodulation of user 1 cannot be accurately realized, the data of user 2 cannot be recovered with error free. As such, the BER performance of user 1 is better than that of user 2. Matlab
Matlab NOMA OFDMA Coder
Arbitrary Waveform Generator
NOMA OFDMA Decoder
Scope
LED
DC supply
Optical Receiver Bias Tee Tx
Rx
Fig. 2. Experimental setup for downlink NOMA-OFDMA VLC.
Fig. 3. BER performance for downlink NOMA-OFDMA VLC.
Fig. 4. BER performance with LS, ISFA and MMSE methods..
4. Conclusions We proposed an experimental demonstration of a NOMA-OFDMA scheme for VLC transmission, which provided a high throughput, flexible bandwidth allocation and a higher system capacity. The experimental results reveal that NOMA-OFDMA is a promising multiple access scheme for VLC networks. 5. Acknowledgments This work was supported by Chunmiao Project of Haixi Institutes, CAS, National Science Foundation of China under Grants 61501427, 61571128 and 61601439, External Cooperation Program of CAS under Grant 121835KYSB20160006, External Cooperation Program of Fujian Provincial Department of Science & Technology under Grant 2017I01010012, Fujian Science Foundation under Grant 2017J05111, Program of Quanzhou Science and Technology under Grant 2016G007 and Grant 2016T010. 6. References [1] H. Elgala, R. Mesleh, H. Haas, “Indoor optical wireless communication: potential and state-of-the-art,” IEEE Comm. Mag. 49 (9), 56-62 (2011). [2] uya Saito, Yoshihisa Kishiyama, Anass Benjebbour, Takehiro Nakamura, Anxin Li, and Kenichi Higuchi, “Non-Orthogonal Multiple Access (NOMA) for Cellular Future Radio Access,” in Proceedings of IEEE Conference on Vehicular Technology (IEEE, 2013), pp. 1-5. [3] Linglong Dai, B. Wang, Y. Yuan, S. Han, C. i, and Z. Wang, “Non-orthogonal multiple access for 5G: solutions, challenges, opportunities, and future research trends,” IEEE Commun. Mag. 53 (9), 74–81 (2015). [4] Bangjiang Lin, et al, “Efficient Frequency Domain Channel Equalization Methods for OFDM Visible Light Communications,” IET Commun. 11 (1), 25-29 (2017).
