VLC系统中OIM-DCO-OFDM的新方案：提高频谱效率与性能
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更新于2024-08-26
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本文探讨了一种创新的光索引调制辅助直流偏置光正交频分复用（OIM-DCO-OFDM）方案，旨在提升可见光通信（VLC）系统的性能。相比于传统的索引调制方法，这种新型方案利用索引位区分数据传输的两个通道：一个子载波承载原始信号，另一个子载波承载其共轭信号。这解决了传统相移键控（PSK）和正交幅度调制（QAM）技术中，由于星座图实轴对称导致无法区分原始信号与共轭信号的问题。
在OIM-DCO-OFDM中，采用单极性脉冲幅度调制（PAM）来调制星座位，以增强接收端的索引位识别能力。此外，文中提出了一种基于零强迫（ZF）检测器的设计，旨在提高信号的检测精度。通过对比，与双模索引调制辅助DCO-OFDM（DM-DCO-OFDM）方案相比，当频谱效率达到1.21 bits/s/Hz时，OIM-DCO-OFDM在BER为10^-4时显示出1 dB和2 dB的性能提升，而在2.18 bits/s/Hz的效率下，优势更为显著。特别是在M=4的调制阶数下，OIM-DCO-OFDM方案能够实现高达80%的频谱效率提升，这在保证数据传输速率的同时，显著提高了系统效率。
总体来说，这项研究对于 VLC 系统的信号处理和调制技术具有重要意义，不仅提高了频谱利用率，还通过解决共轭信号混淆问题，优化了系统的性能指标，为可见光通信系统的进一步发展提供了新的可能。
A Novel Optical IM Aided DCO-OFDM Scheme for VLC Systems 3
efficiency. Under the condition of same modulation order, the proposed scheme
has higher spectral efficiency improvement than DM-DCO-OFDM.
The rest of this paper is organized as follows. In Section 2, the system model
of DM-DCO-OFDM is reviewed. The OIM-DCO-OFDM system model is pre-
sented in Section 3. The simulation results and spectral efficiency analysis are
given in Section 4. Section 5 shows the conclusions.
2 Review of DM-DCO-OFDM
In DM-DCO-OFDM, N subcarriers are divided into G groups, each group con-
tains n subcarriers and p
DM
bits, p
DM
= p
1
+ p
2
. p
1
bits are fed into index selec-
tor to determine index pattern and p
2
bits are modulated by two different constel-
lations, M
A
-ary A and M
B
-ary B. For each group, k subcarriers out of n subcar-
riers are selected to transmit the constellation A signals, and other n − k subcar-
riers transmit the constellation B signals. The transmitted bits in each group can
be calculated by p
DM
= blog
2
(C (n, k))c + klog
2
(M
A
) + (n − k)log
2
(M
B
), where
bc is the floor function, C (n, l) is the binomial coefficient. Noted that the symbol
of constellation A and constellation B should be differentiated with each other.
Otherwise, the index bits cannot be recovered at the receiver, which will lead to
bad BER performance. After index modulation and constellation mapping, Her-
mitian symmetry operation is used to obtain real signals before IFFT operation.
The Hermitian symmetry property can be represented by
X
i
= X
∗
N−i
, 0 < i < N/2
X
0
= X
N/2
= 0
(1)
where X
i
∈ X, X= [X
0
,X
1
, . . . ,X
N−1
] is the frequency-domain complex signal-
s after constellation mapping. Then IFFT and parallel-to-serial operation are
employed to generate time-domain signal. Finally, a suitable DC-bias should be
added on the time-domain signals to create unipolar signals, i.e., only the time-
domain unipolar signals can be transmitted via LEDs. At the receiver side, the
reverse operations are used to obtain transmitted frequency-domain signals. The
maximum likelihood detector or the log-likelihood ratio detector can be selected
to recover index bits and constellation bits.
3 System Model of OIM-DCO-OFDM
Motivated by DM-DCO-OFDM, the proposed OIM-DCO-OFDM scheme also
uses all of subcarriers to transmit signals, which can achieve higher spectral
efficiency than O-OFDM-IM scheme. The block diagram of OIM-DCO-OFDM
transceiver is given in Fig. 1. For each OFDM block, m bits and N carriers
are divided into G groups, each group contains p bits and n subcarriers i.e.,
m = pG, N = nG. For each group, p bits are split into three parts, p
1
, p
2
and
p
3
, i.e., p = p
1
+ p
2
+ p
3
. The index bits p
1
are fed into index selector to choose
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