M进制扩频OFDM通信：移位序列集设计与性能分析
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"M进制扩频OFDM通信中基于移位的序列集的构建"
在M进制扩频(MLS)与正交频分复用(OFDM)结合的通信系统中，有效地构建大量的MSS序列对于提高系统的频谱效率至关重要。本文提出了一种基于循环移位和选择操作的简单方法来生成这些序列。该方法的独特之处在于，通过牺牲序列之间的相关性来换取更多的序列数量，这使得MSS-OFDM系统能够支持更高的数据传输速率，从而提高通信性能。
传统的OFDM系统通常利用正交序列来避免子载波间的干扰，但这种方法可能限制了系统的容量。MSS-OFDM系统引入扩频技术，通过增加信号的带宽来分散能量，可以提高抗干扰能力和安全性。然而，序列的相关性是影响系统性能的关键因素，因为它直接影响到频谱利用率和误码率。
论文中提出的策略通过循环移位生成新的序列集合，这种方法允许从一个基础序列派生出大量不同的序列。每个序列都是通过对基础序列进行不同数量的循环移位得到的。然后，通过选择操作，可以从这些移位后的序列中挑选出满足特定条件的序列，如较低的互相关性。尽管这种方法导致序列之间的相关性降低，但它极大地增加了可用序列的数量，为实现高数据速率提供了可能性。
此外，由于MSS-OFDM系统采用了OFDM调制，因此序列的峰均功率比(PAPR)也是需要考虑的重要参数。PAPR过高会导致非线性失真，影响通信质量。论文中对生成的序列进行了PAPR计算和分析，讨论了其对系统性能的影响。结果显示，尽管牺牲了部分序列的相关性，但生成序列的PAPR仍然在可接受范围内，表明该方法在保持系统稳定性的同时，能有效提升系统容量。
这篇研究论文提出了一个创新的方法来构建MSS-OFDM通信系统中的序列集，以适应高数据速率的需求。通过循环移位和选择操作，能够在牺牲一定相关性的情况下获取更多序列，从而提高系统的频谱效率。同时，对PAPR的分析确保了系统的实际可行性。这些研究成果对于优化MSS-OFDM系统设计具有重要的理论和实践意义。
Construction of Shift-Based Sequence Sets for M-ary Spread Spectrum OFDM
Communications
Guixin Xuan
1, 2
Shibiao He
2
Lili Xiao
2
Zhenyu Zhang
1, 2
Fanxin Zeng
1, 2
1. College of Communication Engineering, Chongqing University, Chongqing, China;
2. Chongqing Communication Institute, Chongqing, China.
guixinxuan@hotmail.com, hdoctor@vip.sina.com, zhenyu.zhang@cqu.edu.cn
ABSTRACT
For M-ary spread spectrum (MSS) communication
systems with orthogonal frequency division multiplexing
(OFDM) technique, a simple scheme to generate lots of
MSS sequences is presented on the basis of circular shift
and selection operation. The number of the generated
sequences is far larger than sequence length at the cost of
the decreased correlation property, which is significantly
important to improve spectrum efficiency of MSS-OFDM
systems since more sequences may support higher data
rate. We provide the related simulations and analyses of
the decreased correlation property. Also, the
peak-to-average power ratio (PAPR) of the generated
sequences is calculated and discussed due to OFDM
modulation. The evaluation results show that the
proposed scheme may obtain a good trade-off between
spectrum efficiency and bit error rate performance.
Keywords: M-ary spread spectrum, Correlation
Property, Orthogonality, OFDM, PAPR
1. INTRODUCTION
Direct sequence spread spectrum (DSSS) will effectively
resist narrowband interference since it may obtain
processing gain (PG) by spreading one information bit
into multiple chips, which leads to its wide applications
[1]. To accommodate simultaneously more active users,
the DSSS-based code division multiple access (CDMA)
method were generally considered [2], where each user
was assigned one sequence with good auto-correlation
function (ACF) property and different sequences
possessed good cross-correlation function (CCF)
property. However, for point-to-point link
communications, DSSS only has low spectrum
efficiency [3-4]. As another spread spectrum technique
with high spectrum efficiency, M-ary spread spectrum
(MSS) may map multiple bits into one of a group of
spreading sequences, and then obtain higher data rate
than traditional DSSS method [5]. In order to further
obtain robust anti-jamming performance and higher
bandwidth efficiency, the combination scheme of MSS
and orthogonal frequency division multiplexing (OFDM),
namely MSS-OFDM system, was proposed [6-7]. The
idea of MSS-OFDM is similar to that of
multicarrier-CDMA (MC-CDMA) since both of them
not only can obtain PG from spread spectrum technique,
but also possess free inter-symbol interference ability
from OFDM modulation.
As a key point of MSS-OFDM or MC-CDMA, the
spreading sequence plays an important role. In addition
to Walsh-Hadamard (W-H) sequences used in
MSS-OFDM systems [6] and Gold sequences used in
high frequency MSS communications [4],
complementary sequences and quasi-complementary
sequences were widely employed in MC-CDMA
systems [8-12]. It is well known that complementary
sequence set only has small set size in spite of its ideal
aperiodic ACF and CCF [13]. In order to obtain larger
set size, cyclic shifted orthogonal complementary
(Cyclic-OC) sequences were constructed [14], where its
set size can achieve PG and it may obtain better bit error
rate (BER) performance than W-H sequences. However,
such construction omits peak-to-average power ratio
(PAPR) resistance which has an important effect on
OFDM modulation.
In our previous works [7, 15], several complementary
sequence-based MSS sets were proposed for
MSS-OFDM systems. However, these sets only
possessed the small number of sequences, that is, the set
size is not larger than sequence length. Since small set
size means low data rate, the previous sequence sets only
had relatively low spectrum efficiency. In this paper, we
propose a simple construction method based on shift
operation. By using the proposed method, the previous
sequence set with small set size may be expanded into
multiple sets, which will result in lots of available
sequences for MSS-OFDM communication systems. As
the cost of the increased number of MSS sequences, their
correlation properties and PAPR performance will
decrease to some extent.
2. PRELIMINARY KNOWLEDGE
2.1
Correlation properties of sequence set
Given two sequences
(0),(1),,( 1)
ii i i
CC CL C "
and
(0), (1), , ( 1)
jj j j
CC CL C "
with sequence
length
L , the aperiodic CCF
,
()
ij
\
W
CC
and periodic
CCF
,
()
ij
I
W
CC
between
i
C and
j
C
can be
respectively expressed as
2016 25th Wireless and Optical Communication Conference (WOCC)
978-1-4673-9958-6/16/$31.00 ©2016 IEEE
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