无线网络中协作分集和中继选择机制研究
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摘要
无线信道遭受的衰落会严重降低检测性能。空间分集可以有效地抵抗衰落所带来的有害作用。空间分集一般是通过从不同位置传输信号使得目的节点可以收到相同信号独立衰落的不同副本来获得的。为了实现空间分集,无线网络的节点需要支持多根天线,如多输入多输出(multiple-input multiple-output, MIMO)系统。然而,因为大小,成本,硬件限制等原因,一个无线节点可能支持不了多根天线。
近来,人们提出协作通信(cooperative communication)技术来解决这个问题。其基本思想是无线网络中单天线的节点共享它们的天线并像虚拟MIMO系统一样协作传输,进而形成空间分集来有效抵抗多径衰落。在转发接收到的信号时,中继节点可以采用放大转发(amplify-and-forward, AF)和译码转发(decode-and-forward,DF)两种转发机制。在译码转发机制中,中继译码并重传接收的信号。然而,在放大转发机制中,中继仅仅简单地放大并重传接收到的含有噪音的信号。
协作通信中还有许多问题需要去解决。其中一个是协作分集机制,即什么时候协作并且怎样协作。另外一个重要的问题是多中继节点网络中的中继选择问题,换句话说,就是如果有多个可用的中继节点,和谁协作的问题。传统方法是邀请所有可用的中继节点都转发信号,目的节点采用最大比合并(maximum ratio combining, MRC)技术来结合收到的所有信号。然而,近来一种新的观点认为在每一次源节点与目的节点的传输中,仅一个最佳的节点应该被选出来作为中继。这种最佳中继选择机制的性能优于传统的协作网络(所有可用的中继都转发信号),并可以通过有限反馈以一种分布式的形式实现。试验结果表明,这种最佳中继选择机制可以和传统的协作网络在中断概率上达到相同的分集阶数。
基于这些问题,本论文着重对无线网络中协作分集和中继选择机制进行研究。论文的主要贡献及创新点有：
(1)结合增量DF中继策略和选择DF中继策略,提出了一种新的协作分集方案,命名为增量-选择DF中继。同时,推导了增量-选择DF中继和增量DF中继两种机制误比特率(bit error rate, BER)的闭式表达式。进而讨论了信噪比(signal to noise ratio, SNR)门限对BER的影响。
(2)针对独立不同分布的Rayleigh信道环境,分析了基于SNR的译码放大混合转发(hybrid decode-amplify-forward, HDAF)协作分集机制的性能,推导了HDAF机制误BER的闭式表达式。采用计算机仿真对理论分析进行了验证并调查了SNR门限和中继节点位置对BER性能的影响。
(3)分析了DF协作网络中机会中继(opportunistic relaying, OR)机制的平均误符号率(symbol error rate, SER)'性能。信道环境是独立不同分布的Rayleigh信道,接收端采用MRC技术,调制方式是M-PSK,推导出了固定译码转发(fixed DF, FDF)机会中继和自适应译码转发(adaptive DF, ADF)机会中继两种机制平均SER的近似闭合表达式。最终用仿真结果验证了理论推导的正确性。
(4)在采用DF机制和目的节点采用等增益合并(equal gain combinig, EGC)的情境下,研究了选择协作(selection cooperation, SC)中继选择机制的性能。假设信道经历独立不同分布的Rayleigh衰落信道,首先推导了目的节点采用等增益合并后总瞬时SNR的累积分布函数(cumulative distribution function, CDF)和概率密度函数(probability density function, PDF)。接着这些统计函数被用来推导选择协作机制中断概率和平均SER的闭式表达式。此外,在选择协作机制的基础上,为译码转发中继网络提出一种改进型的选择协作(ISC)机制。信道环境是独立不同分布的Rayleigh信道,接收端采用MRC技术,推导出了ISC机制的平均SER的闭合表达式。
论文最后总结了该领域尚待解决的问题以及下一步的研究重点。
The wireless channel suffers from fading, which can seriously degrade detection performance. Spatial diversity can effectively combat the deleterious effects of fading, which is generated by transmitting signals from different locations, allowing independently faded versions of the signal at the receiver. To realize spatial diversity, the nodes in wireless networks need to support more than one antenna (e.g., multiple-input multiple-output (MIMO) system). However, due to size, cost, or hardware limitations, a wireless node may not be able to support multiple transmit antennas.
Cooperative communication in wireless systems has recently emerged to overcome this problem. The basic idea is that single-antenna nodes in a wireless networks share their antennas and transmit cooperatively as a virtual MIMO system, thus realizing spatial diversity to mitigate fading without relying on multiple antennas. Based on the level of signal processing at the relay, cooperative relaying schemes can be classified as amplify-and-forward (AF) relaying and decode-and-forward (DF) relaying. In DF relaying, the relay detects and then retransmits the detected signal, whereas the relay just amplifies and retransmits the noisy signal in AF relaying.
Many issues in cooperative communications still need to be addressed. An important question is cooperative diversity scheme, i.e., when and how to cooperate. Another important issue is relay selection method in multi-relay networks. In other words, who to cooperate if there more than one relays? Traditionally, all available nodes are requested to relay signals between the source and destination, and the destination will combine all received signal with maximum ratio combining (MRC) technique. It has recently been proposed, however, that for each source-destination transmission, only a single best node should be selected to act as a relay. The resulting scheme outperforms MRC schemes and can be implemented in a distributed fashion with limited feedback. The results showed that this scheme has the same diversity order in terms of the outage probability as the regular cooperative networks, where all the relays participate in sending the source signal to the destination.
Based on the aforementioned issues of cooperative networks, we concentrate our efforts on the research of cooperative diversity and relay selection schemes for wireless networks. The main contribution and innovation can be summarized as follows:
(1) A novel relaying scheme in conjunction with incremental DF relaying and selective DF relaying strategies, termed incremental-selective DF relaying, is proposed and analyzed. Closed-form expressions for bit error rate (BER) of both the incremental-selective DF relaying scheme and the incremental DF relaying scheme are derived. In addition, the effect of signal-to-noise ratio (SNR) threshold on the BER is discussed.
(2) We analyze the performance of SNR-based hybrid decode-amplify-forward (HDAF) relaying cooperative diversity networks over independent non-identical flat Rayleigh fading channels with MRC technique. Closed-form expression for the BER of the HDAF relaying scheme is derived. Computer simulations are carried out to illustrate and validate the correctness of analytical results. Besides, the impacts of the SNR threshold and relay location on the performances of BER are investigated.
(3) We analyze the symbol error rate (SER) performance of cooperative diversity networks with opportunistic DF relaying. Assuming that channels suffer from independent nonidentical Rayleigh fading and symbols are M-PSK modulated, the approximate closed-form SER expressions are derived for opportunistic relaying with adaptive DF (ADF) and fixed DF (FDF) protocols, respectively. Simulations are carried out to validate the theoretical analysis.
(4) The performances of selection cooperation (SC) are investigated in a scenario based on DF relaying and where equal gain combining (EGC) technique is adopted at the destination. Assuming that channels suffer from independent non-identical Rayleigh fading, we first derive the cumulative distribution function (CDF) and probability density function (PDF) for the total instantaneous SNR at the destination after EGC. Then, these statistical functions are used to derive closed-form expressions for outage probability and average SER of selection cooperation. Besides, an improved selection cooperation (ISC) scheme for DF relaying is proposed based on selection cooperation (SC) strategy. Assuming that channels suffer from independent non-identical Rayleigh fading, we derive closed-form expression for the exact SER of ISC scheme.
Finally, problems to be solved in this field and future research topics are summarized.
近来,人们提出协作通信(cooperative communication)技术来解决这个问题。其基本思想是无线网络中单天线的节点共享它们的天线并像虚拟MIMO系统一样协作传输,进而形成空间分集来有效抵抗多径衰落。在转发接收到的信号时,中继节点可以采用放大转发(amplify-and-forward, AF)和译码转发(decode-and-forward,DF)两种转发机制。在译码转发机制中,中继译码并重传接收的信号。然而,在放大转发机制中,中继仅仅简单地放大并重传接收到的含有噪音的信号。
协作通信中还有许多问题需要去解决。其中一个是协作分集机制,即什么时候协作并且怎样协作。另外一个重要的问题是多中继节点网络中的中继选择问题,换句话说,就是如果有多个可用的中继节点,和谁协作的问题。传统方法是邀请所有可用的中继节点都转发信号,目的节点采用最大比合并(maximum ratio combining, MRC)技术来结合收到的所有信号。然而,近来一种新的观点认为在每一次源节点与目的节点的传输中,仅一个最佳的节点应该被选出来作为中继。这种最佳中继选择机制的性能优于传统的协作网络(所有可用的中继都转发信号),并可以通过有限反馈以一种分布式的形式实现。试验结果表明,这种最佳中继选择机制可以和传统的协作网络在中断概率上达到相同的分集阶数。
基于这些问题,本论文着重对无线网络中协作分集和中继选择机制进行研究。论文的主要贡献及创新点有：
(1)结合增量DF中继策略和选择DF中继策略,提出了一种新的协作分集方案,命名为增量-选择DF中继。同时,推导了增量-选择DF中继和增量DF中继两种机制误比特率(bit error rate, BER)的闭式表达式。进而讨论了信噪比(signal to noise ratio, SNR)门限对BER的影响。
(2)针对独立不同分布的Rayleigh信道环境,分析了基于SNR的译码放大混合转发(hybrid decode-amplify-forward, HDAF)协作分集机制的性能,推导了HDAF机制误BER的闭式表达式。采用计算机仿真对理论分析进行了验证并调查了SNR门限和中继节点位置对BER性能的影响。
(3)分析了DF协作网络中机会中继(opportunistic relaying, OR)机制的平均误符号率(symbol error rate, SER)'性能。信道环境是独立不同分布的Rayleigh信道,接收端采用MRC技术,调制方式是M-PSK,推导出了固定译码转发(fixed DF, FDF)机会中继和自适应译码转发(adaptive DF, ADF)机会中继两种机制平均SER的近似闭合表达式。最终用仿真结果验证了理论推导的正确性。
(4)在采用DF机制和目的节点采用等增益合并(equal gain combinig, EGC)的情境下,研究了选择协作(selection cooperation, SC)中继选择机制的性能。假设信道经历独立不同分布的Rayleigh衰落信道,首先推导了目的节点采用等增益合并后总瞬时SNR的累积分布函数(cumulative distribution function, CDF)和概率密度函数(probability density function, PDF)。接着这些统计函数被用来推导选择协作机制中断概率和平均SER的闭式表达式。此外,在选择协作机制的基础上,为译码转发中继网络提出一种改进型的选择协作(ISC)机制。信道环境是独立不同分布的Rayleigh信道,接收端采用MRC技术,推导出了ISC机制的平均SER的闭合表达式。
论文最后总结了该领域尚待解决的问题以及下一步的研究重点。
The wireless channel suffers from fading, which can seriously degrade detection performance. Spatial diversity can effectively combat the deleterious effects of fading, which is generated by transmitting signals from different locations, allowing independently faded versions of the signal at the receiver. To realize spatial diversity, the nodes in wireless networks need to support more than one antenna (e.g., multiple-input multiple-output (MIMO) system). However, due to size, cost, or hardware limitations, a wireless node may not be able to support multiple transmit antennas.
Cooperative communication in wireless systems has recently emerged to overcome this problem. The basic idea is that single-antenna nodes in a wireless networks share their antennas and transmit cooperatively as a virtual MIMO system, thus realizing spatial diversity to mitigate fading without relying on multiple antennas. Based on the level of signal processing at the relay, cooperative relaying schemes can be classified as amplify-and-forward (AF) relaying and decode-and-forward (DF) relaying. In DF relaying, the relay detects and then retransmits the detected signal, whereas the relay just amplifies and retransmits the noisy signal in AF relaying.
Many issues in cooperative communications still need to be addressed. An important question is cooperative diversity scheme, i.e., when and how to cooperate. Another important issue is relay selection method in multi-relay networks. In other words, who to cooperate if there more than one relays? Traditionally, all available nodes are requested to relay signals between the source and destination, and the destination will combine all received signal with maximum ratio combining (MRC) technique. It has recently been proposed, however, that for each source-destination transmission, only a single best node should be selected to act as a relay. The resulting scheme outperforms MRC schemes and can be implemented in a distributed fashion with limited feedback. The results showed that this scheme has the same diversity order in terms of the outage probability as the regular cooperative networks, where all the relays participate in sending the source signal to the destination.
Based on the aforementioned issues of cooperative networks, we concentrate our efforts on the research of cooperative diversity and relay selection schemes for wireless networks. The main contribution and innovation can be summarized as follows:
(1) A novel relaying scheme in conjunction with incremental DF relaying and selective DF relaying strategies, termed incremental-selective DF relaying, is proposed and analyzed. Closed-form expressions for bit error rate (BER) of both the incremental-selective DF relaying scheme and the incremental DF relaying scheme are derived. In addition, the effect of signal-to-noise ratio (SNR) threshold on the BER is discussed.
(2) We analyze the performance of SNR-based hybrid decode-amplify-forward (HDAF) relaying cooperative diversity networks over independent non-identical flat Rayleigh fading channels with MRC technique. Closed-form expression for the BER of the HDAF relaying scheme is derived. Computer simulations are carried out to illustrate and validate the correctness of analytical results. Besides, the impacts of the SNR threshold and relay location on the performances of BER are investigated.
(3) We analyze the symbol error rate (SER) performance of cooperative diversity networks with opportunistic DF relaying. Assuming that channels suffer from independent nonidentical Rayleigh fading and symbols are M-PSK modulated, the approximate closed-form SER expressions are derived for opportunistic relaying with adaptive DF (ADF) and fixed DF (FDF) protocols, respectively. Simulations are carried out to validate the theoretical analysis.
(4) The performances of selection cooperation (SC) are investigated in a scenario based on DF relaying and where equal gain combining (EGC) technique is adopted at the destination. Assuming that channels suffer from independent non-identical Rayleigh fading, we first derive the cumulative distribution function (CDF) and probability density function (PDF) for the total instantaneous SNR at the destination after EGC. Then, these statistical functions are used to derive closed-form expressions for outage probability and average SER of selection cooperation. Besides, an improved selection cooperation (ISC) scheme for DF relaying is proposed based on selection cooperation (SC) strategy. Assuming that channels suffer from independent non-identical Rayleigh fading, we derive closed-form expression for the exact SER of ISC scheme.
Finally, problems to be solved in this field and future research topics are summarized.
引文
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