摘要
多载波码分多址(MC-CDMA)技术是正交频分复用(OFDM)技术与码分多址(CDMA)技术相结合方案中的一种技术,该技术将OFDM技术应用到CDMA系统中,降低了CDMA系统的数据速率,增强了处理高速数据业务的能力。它继承了OFDM技术和CDMA技术的优良特性,在高速数据传输方面有独特的优势,具有抗频率选择性衰落,抗符号间干扰,频谱利用率高等特点。并且其多载波调制和解调可以利用离散傅立叶变换(DFT)来实现,能极大地降低系统的复杂度。
与任何多载波调制技术一样,MC-CDMA信号是许多独立子载波信号的叠加,其包络服从高斯分布。因此,其峰值平均功率比(PAPR)较大,导致对系统前端放大器的线性动态范围要求较高,这一要求严重限制了MC-CDMA的实际应用。因此,本文对如何降低系统的PAPR进行了研究并提出了一些解决方案。
本文首先,介绍了MC-CDMA通信系统、PAPR抑制技术的研究现状等,为以下的研究奠定理论基础并指出了研究方向。接着,分析了几种常用的扩频码及一些扩频码的PAPR特性,以避免使用具有较高PAPR的扩频序列作为扩频码。
选择应映射法(SLM)是一种降低MC-CDMA系统PAPR的方法。为了研究在SLM中,相位序列的选取对降低系统PAPR的影响。在对常见的几种序列进行介绍的基础上,利用它们作为系统的相位序列,并且进一步分析了同一相位序列取不同长度和系统取不同相位序列个数集时的情况。仿真结果表明,在实际应用中相比其它序列,可以用Gold伪随机序列作为实际的相位序列;系统的PAPR随着所用相位序列长度的增加而增大;随着所用相位序列的个数集的增大而减小。
本文提出了一种新的降低MC-CDMA系统PAPR和减少部分传输法(PTS)运算复杂度的方法,即非均匀分割—次优算法。仿真结果表明,该方法与传统的均匀分割—最优算法相比,在一定仿真参数下性能仅下降约0.4 dB,但计算复杂度却由指数级运算降为了线性级运算。
为了更好地降低MC-CDMA系统的PAPR和PTS运算的复杂度,提出了一种使最优二进制相位序列(OBPS)算法运算量减半的方法,并对双层相位序列(DLPS)算法提出了几种改进算法。仿真结果表明,改进算法既有效地减低了系统的PAPR,又使运算复杂度较原算法大大降低了。
Multi-carrier code division multiple access (MC-CDMA) technology is a kind of combination of OFDM and CDMA technology. The technology uses OFDM into the CDMA system, it reduces the data rate of the CDMA system, and enhances high speed data processing service ability. It is inherited from the OFDM and the CDMA technology. It has the unique superiority at the high speed data transmission, and has the anti-frequency selectivity ability, it also can against inter symbol interrupt. It has high spectrum utilization and, its modulation and demodulation could use discrete Fourier transform (DFT), it also can reduce the system complexity.
As the same of other multi-carrier technology, one of the major disadvantages of MC-CDMA is its high peak-to-average power ratio (PAPR), because the MC-CDMA signal is summed by many independent sub-signals and the signal envelope distribution is Gaussian distribution. So it requires a long linear range of the high power amplifier (HPA), otherwise a significant power efficiency penalty and the degradation of the bit error rate (BER) will be occured. To overcome this drawback, in this paper, we investigate the PAPR problem and propose some solutions about the problem.
We first discuss the construction of MC-CDMA system and the development of PAPR reduction methods,which provides the foundation of following discussion. Then analyze some commonly used spreading sequences and PAPR characteristics of some spreading codes, so as to avoid using the spreading code with high PAPR.
Selected mapping (SLM) is a method to reduce PAPR of MC-CDMA system .In order to study the impact of different phase sequences on the reduction of the PAPR in MC-CDMA system using SLM. Based on the introduction of the usually selected as the phase sequences, we point out that they can be used as the phase sequences of the MC-CDMA system, and then more deeply analyze the PAPR characteristics for a phase sequence when its length is different and the number of the phase sequence set is different. Simulation results show that, compared with other sequences, Gold sequence can be used for the phase sequence in the actual system.for the same phase sequence, the shorter phase sequence we used, the smaller PAPR value we will get; the number of the phase sequence set is bigger, the smaller PAPR value we can get.
In order to reduce the PAPR of MC-CDMA system and the computational complexity of partial transmit sequence(PTS) efficiently, a new method, un-uniform partition—suboptimal algorithm, is proposed in this paper. The simulation results show that , as compared with tradition method, uniform partition—optimal algorithm, the PAPR value increases only about 0.4 dB on some certain simulation parameters, while the computational complexity is reduced from exponential order operation to linear order operation.
In order to reduce the system PAPR and the computational complexity of PTS efficiently, a new method which can reduce half of the OBPS algorithm computational complexity is proposed in this paper, and then propose several improved methods of DLPS algorithm. The simulation results show that, the proposed improved methods not only reduce the system PAPR, but also reduce the computational complexities heavily as compared with the traditional algorithm.
引文
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