认知无线电频谱感知干扰理论与方法研究
摘要
在传统的频谱资源固定分配的框架下,人们致力于调制、编码、多天线等技术的研究,以期能够提高频谱使用效率。然而这些技术均无法摆脱香农限容量的束缚。随着无线通信业务需求的日益增长,频谱资源紧缺已逐渐成为制约无线通信产业发展的瓶颈。另一方面,大量的实测结果显示：已分配的频谱资源闲置现象十分普遍,总体利用效率低下。正是基于这样的背景,具备频谱动态接入能力的认知无线电技术,为有效解决频谱需求与闲置之间的矛盾开辟出一条新的途径,被誉为下一代无线通信领域的“大事件”。
频谱感知技术是认知无线电的核心技术,是其走向实际应用的基石。它赋予了认知无线电设备在不影响授权频段内合法用户正常通信的前提下,发现并利用空闲频谱资源进行机会通信的能力。不过,这种“先感知,后接入”的频谱使用规范,在提高频谱效率的同时,也给认知无线电系统带来了新的安全隐患：频谱感知干扰。频谱感知干扰,指在频谱感知阶段,通过人为注入干扰信号,误导认知无线电对所处环境的频谱使用状态做出错误的判断,进而导致系统频谱使用效率的下降。
由此,如何认知这一新的干扰形式；它与哪些系统因素有关；如何量化它对认知无线电系统造成的影响,如最大可达性能损失等；这一系列问题的研究是设计安全的频谱感知技术的重要理论依据。目前,学术界对该课题的研究尚处于起步阶段,未有系统、理论的研究成果。
论文针对认知无线电频谱感知干扰理论与方法的研究,分两个部分：
首先,基于干扰功率受限的约束条件,研究了最优频谱感知干扰的理论与方法。包括,
(1)研究了加性高斯白噪声环境中的最优感知干扰问题。具体有：以最小化认知无线电系统可用带宽为目标,针对能量检测为频谱感知方式的认知无线电网络,推导了感知干扰机的最优干扰策略为,等功率、部分频段干扰；分析了最优感知干扰效果与频谱感知相关参数的关系,得知感知干扰效果随虚警概率和累积时间带宽积成正比例关系；通过理论分析与仿真验证得知：感知干扰会减少认知无线电系统可用带宽,在感知干扰机的功率足够大时,认知无线电系统甚至面临无法提供可用带宽进行通信,即系统瘫痪的危险；
(2)研究了大尺度衰落环境中的最优感知干扰问题。具体有：采用与加性高斯白噪声环境下类似的思路与方法,推导出大尺度衰落环境下的最优干扰策略仍为,等功率、部分频段干扰；通过理论分析与仿真验证,分析了在该衰落环境中的感知干扰效果随认知无线电系统相关参数变化的情况；
(3)研究了小尺度衰落环境中的最优感知干扰问题。具体有：推导了该环境下最优感知干扰模型仍为,干扰机总功率受限的约束条件下,最小化系统的可用带宽；考虑到模型中目标函数为干扰功率的非凸、非线性含参积分,无法得到最优解的闭合表达式,从目标函数和约束条件均具有可分离的特性出发,通过引入整数约束条件,提出一种通用(适用所有类型的小尺度衰落,如瑞利、莱斯、Nakagammi衰落等)全局最优的混合整数线性算法；基于该算法,首先分析了小尺度衰落环境下,最优感知干扰的性能上界,得到结论：对于衰减因子二阶矩为1的独立、同分布衰落信道,最优感知干扰性能上界(己知信道瞬时状态信息)渐进趋近于加性高斯白噪声环境中的最优感知干扰性能,并从物理机制上解释了该现象；其次得到独立、同分布瑞利衰落信道下最优感知干扰的可行性策略(仅己知信道统计状态信息)为,等功率、部分频段干扰,并对其性能进行了分析；
认知用户在完成频谱感知后,即可接入可用频段传输数据,此时可能面临传统的通信干扰(通信干扰,指在用户通信的频段上发射干扰信号,阻止目标用户信息的有效传输)。在忽略干扰能量／功率受限的约束,干扰机可以随意地在感知时段实施感知干扰,在数据传输时段实施通信干扰,以达到最优干扰效果。然而,个现实的问题是：实际环境中的干扰机总是能量有限的,如何将这有限的能量分配于两种干扰方式达到最优的干扰效果呢?显然这个问题对于是否有必要继续深入研究感知干扰理论具有举足轻重的意义。为此,论文第二部分对该问题进行了解答,包括,
(4)研究了基本认知网络环境中,能量受限的认知干扰机对实施频谱感知干扰与通信干扰如何抉择以达到最优干扰效果的问题。具体有：以最小化认知网络平均总吞吐量为目标,建立起吞吐量与感知干扰信号、通信干扰信号以及网络所需频段数等参数之间的联系,给出最优化数学模型；为简化求解复杂度,提出了两步骤子优化方案,即,先在感知时段实施最优频谱感知干扰,再在数据传输时段实施最优通信干扰,并通过数值仿真得到认知干扰机的最优攻击策略：基于感知干扰与通信干扰的战术联合,即部分能量用于频谱感知干扰,剩余能量用于通信干扰；
(5)研究了更为实际的CDMA-CR(使用码分多址的认知无线电)网络环境中,能量受限的认知干扰机的最优干扰策略与性能。具体有：以CDMA-CR系统平均总吞吐量为干扰机的性能指标,通过给出平均总吞吐量与感知干扰、通信干扰信号之间的关系,建立了能量受限认知干扰机的最优化数学模型；仍然采用两步骤子优化算法,通过数值仿真分析了不同场景下的最优干扰策略与性能；更进一步地,分析了感知干扰、通信干扰能力随系统参数的不同变化规律,由此给出最优干扰策略随系统参数变化的趋势。
论文关于认知无线电频谱感知干扰理论与方法的研究,填补了因频谱感知使用规范造成的新的安全隐患“频谱感知干扰”相关理论的空白,丰富了现有频谱感知技术理论与方法,为设计安全、稳定的认知无线电系统提供了必要的理论支撑,其研究成果可应用在基于认知无线电技术的下一代无线通信系统。
Under traditional licensed spectrum regulation, various technologies, such as modulation, coding, multiple antennas, have been attracting considerable attention as basic solutions for increasing spectral efficiency. However, with fixed spectral bandwidth, none of the above techniques can escape from the constraint of Shannon capacity. With the ever increasing demand of wireless services, limited spectral bandwidth arises as the bottleneck of wireless communications development. On the contrary, field measurements show that the actual spectral utilization by primary users (PU) is very low. Cognitive Radio (CR), with the ability of dynamically accessing unused spectral bands, is considered as a promising technique to solve the contradiction between low spectral utilization and the increasing spectral demand, and is termed "the Next Big Thing".
Spectrum sensing, which paves the way for the CR realization, is recognized as one heart technology of CR. It serves as the foundation of dynamic access capability embedded in CR without causing harmful interference to PUs. While increasing spectral efficiency, new security vulnerabilities are imposed by spectrum sensing: spectrum sensing disruption (termed spoofing for short). Spoofing, by emitting spoofing singals in unused bands during sensing periods, aims to mislead CR to reach false decisions on whether the observed band is occupied, such that the spectral efficiency is decreased.
Here arise a series of problems of how to understand this new form of attack, such as which factors of the CR system is vulnerable to spoofing, how to analyze its effect to the CR system, and what the worst-case performance would be in the presence of spoofing, etc. These unanswered questions are the fundamental issues toward designing secure spectrum sensing algorithms. However, it is in serious lack of systematic and theoretical investigations yet.
This dissertation carries out research focusing on the theory and methodology of spectrum sensing disruption in CR, which includes:
First, optimal sensing disruption strategies are derived and analyzed under the constraint of a power budget. Specifically,
(1) The problem of optimal sensing disruption under Additive White Gaussian Noise (AWGN) is investigated. By minimizing the available bandwidth for the CR system where energy detection is utilized for sensing, the optimal sensing disruption strategy is derived, which corresponds to equal-power, partial-band spoofing; The optimal spoofing performance with varying sensing parameters is analyzed, which turns out to be proportional to either false alarm probability or integration-time-bandwidth product; Theoretical along with numerical results indicates that:spoofing can effectively reduce the available bandwidth of the CR system. When the power budget of the adversary is large enough, the CR system could even be out of work since no available bandwidth can be found via sensing.
(2) The problem of optimal sensing disruption with path loss is investigated. Similar as the case for AWGN, the optimal spoofing strategy considering path loss is derived, which also corresponds to equal-power, partial-band spoofing; Theoretical along with numerical results further analyzed, for this path loss scenario, the optimal spoofing performances under different parameters of the CR system.
(3) The problem of optimal sensing disruption with fading is investigated. The mathematical model of optimal spoofing with fading is established:minimizing the available bandwidth of the CR system with the power constraint; Considering that the objective is non-convex with parameters involving integration of nonlinear functions, it is quite difficult to arrive at analytical solutions. However, by utilizing the separable characteristics of both objective and constraints, the non-convex, nonlinear optimization is transformed into mixed-integer linear programming via introducing additional integer variables. This numerical approach can obtain global optimum, and is applicable to various independent fading scenarios, such as Rayleigh, Rician and Nakagammi; Based on this approach, the worst-case (instantaneous channel state information is assumed to be known) performance of spoofing with fading is analyzed:for identical and independent Rayleigh fading whose second moments are normalized to unity, the worst-case performance of spoofing asymptotically approaches that under AWGN; Further, a feasible (only the statistical channel state information is known) optimal spoofing strategy is also given and analyzed:for identical and independent Rayleigh fading, the feasible optimal spoofing strategy turns out to be equal-power, partial-band.
When spectrum sensing is fulfilled, a cognitive user (CU) can access those bands that are determined to be vacant through sensing. This phase is called data transmission, where the CU could possibly face the traditional jamming (jamming signals are launched in those bands where the CU is in use, to degrade its performance such that effective transfer of information is denied). For the ideal case where the power/energy is unlimited, the adversary can choose to optimally spoof in the sensing slot and then optimally jam when the CU starts data transmission, in order to maximally degrade the performance of the CR system. However, for a practical matter, the energy of an adversary is usually limited. In this case, how to optimally distribute the energy budget into sensing slot and the data transmission slot so as to maximally attack the CR system remains unanswered. This issue is fundamental and critical to the development of the sensing disruption theory. Therefore, this problem is investigated in the latter part of this dissertation, including:
(4) For a basic CR network scenario, the problem of how to optimally combine both spoofing and jamming with an energy budget is investigated. The objective of the adversary is established as the average sum throughput of the CR system, based on which the relationship among the average sum throughput, spoofing signals, jamming signals, and the required bandwidth of the CR system is analytically given. Then the optimal attack to the CR system is mathematically
频谱感知技术是认知无线电的核心技术,是其走向实际应用的基石。它赋予了认知无线电设备在不影响授权频段内合法用户正常通信的前提下,发现并利用空闲频谱资源进行机会通信的能力。不过,这种“先感知,后接入”的频谱使用规范,在提高频谱效率的同时,也给认知无线电系统带来了新的安全隐患：频谱感知干扰。频谱感知干扰,指在频谱感知阶段,通过人为注入干扰信号,误导认知无线电对所处环境的频谱使用状态做出错误的判断,进而导致系统频谱使用效率的下降。
由此,如何认知这一新的干扰形式；它与哪些系统因素有关；如何量化它对认知无线电系统造成的影响,如最大可达性能损失等；这一系列问题的研究是设计安全的频谱感知技术的重要理论依据。目前,学术界对该课题的研究尚处于起步阶段,未有系统、理论的研究成果。
论文针对认知无线电频谱感知干扰理论与方法的研究,分两个部分：
首先,基于干扰功率受限的约束条件,研究了最优频谱感知干扰的理论与方法。包括,
(1)研究了加性高斯白噪声环境中的最优感知干扰问题。具体有：以最小化认知无线电系统可用带宽为目标,针对能量检测为频谱感知方式的认知无线电网络,推导了感知干扰机的最优干扰策略为,等功率、部分频段干扰；分析了最优感知干扰效果与频谱感知相关参数的关系,得知感知干扰效果随虚警概率和累积时间带宽积成正比例关系；通过理论分析与仿真验证得知：感知干扰会减少认知无线电系统可用带宽,在感知干扰机的功率足够大时,认知无线电系统甚至面临无法提供可用带宽进行通信,即系统瘫痪的危险；
(2)研究了大尺度衰落环境中的最优感知干扰问题。具体有：采用与加性高斯白噪声环境下类似的思路与方法,推导出大尺度衰落环境下的最优干扰策略仍为,等功率、部分频段干扰；通过理论分析与仿真验证,分析了在该衰落环境中的感知干扰效果随认知无线电系统相关参数变化的情况；
(3)研究了小尺度衰落环境中的最优感知干扰问题。具体有：推导了该环境下最优感知干扰模型仍为,干扰机总功率受限的约束条件下,最小化系统的可用带宽；考虑到模型中目标函数为干扰功率的非凸、非线性含参积分,无法得到最优解的闭合表达式,从目标函数和约束条件均具有可分离的特性出发,通过引入整数约束条件,提出一种通用(适用所有类型的小尺度衰落,如瑞利、莱斯、Nakagammi衰落等)全局最优的混合整数线性算法；基于该算法,首先分析了小尺度衰落环境下,最优感知干扰的性能上界,得到结论：对于衰减因子二阶矩为1的独立、同分布衰落信道,最优感知干扰性能上界(己知信道瞬时状态信息)渐进趋近于加性高斯白噪声环境中的最优感知干扰性能,并从物理机制上解释了该现象；其次得到独立、同分布瑞利衰落信道下最优感知干扰的可行性策略(仅己知信道统计状态信息)为,等功率、部分频段干扰,并对其性能进行了分析；
认知用户在完成频谱感知后,即可接入可用频段传输数据,此时可能面临传统的通信干扰(通信干扰,指在用户通信的频段上发射干扰信号,阻止目标用户信息的有效传输)。在忽略干扰能量／功率受限的约束,干扰机可以随意地在感知时段实施感知干扰,在数据传输时段实施通信干扰,以达到最优干扰效果。然而,个现实的问题是：实际环境中的干扰机总是能量有限的,如何将这有限的能量分配于两种干扰方式达到最优的干扰效果呢?显然这个问题对于是否有必要继续深入研究感知干扰理论具有举足轻重的意义。为此,论文第二部分对该问题进行了解答,包括,
(4)研究了基本认知网络环境中,能量受限的认知干扰机对实施频谱感知干扰与通信干扰如何抉择以达到最优干扰效果的问题。具体有：以最小化认知网络平均总吞吐量为目标,建立起吞吐量与感知干扰信号、通信干扰信号以及网络所需频段数等参数之间的联系,给出最优化数学模型；为简化求解复杂度,提出了两步骤子优化方案,即,先在感知时段实施最优频谱感知干扰,再在数据传输时段实施最优通信干扰,并通过数值仿真得到认知干扰机的最优攻击策略：基于感知干扰与通信干扰的战术联合,即部分能量用于频谱感知干扰,剩余能量用于通信干扰；
(5)研究了更为实际的CDMA-CR(使用码分多址的认知无线电)网络环境中,能量受限的认知干扰机的最优干扰策略与性能。具体有：以CDMA-CR系统平均总吞吐量为干扰机的性能指标,通过给出平均总吞吐量与感知干扰、通信干扰信号之间的关系,建立了能量受限认知干扰机的最优化数学模型；仍然采用两步骤子优化算法,通过数值仿真分析了不同场景下的最优干扰策略与性能；更进一步地,分析了感知干扰、通信干扰能力随系统参数的不同变化规律,由此给出最优干扰策略随系统参数变化的趋势。
论文关于认知无线电频谱感知干扰理论与方法的研究,填补了因频谱感知使用规范造成的新的安全隐患“频谱感知干扰”相关理论的空白,丰富了现有频谱感知技术理论与方法,为设计安全、稳定的认知无线电系统提供了必要的理论支撑,其研究成果可应用在基于认知无线电技术的下一代无线通信系统。
Under traditional licensed spectrum regulation, various technologies, such as modulation, coding, multiple antennas, have been attracting considerable attention as basic solutions for increasing spectral efficiency. However, with fixed spectral bandwidth, none of the above techniques can escape from the constraint of Shannon capacity. With the ever increasing demand of wireless services, limited spectral bandwidth arises as the bottleneck of wireless communications development. On the contrary, field measurements show that the actual spectral utilization by primary users (PU) is very low. Cognitive Radio (CR), with the ability of dynamically accessing unused spectral bands, is considered as a promising technique to solve the contradiction between low spectral utilization and the increasing spectral demand, and is termed "the Next Big Thing".
Spectrum sensing, which paves the way for the CR realization, is recognized as one heart technology of CR. It serves as the foundation of dynamic access capability embedded in CR without causing harmful interference to PUs. While increasing spectral efficiency, new security vulnerabilities are imposed by spectrum sensing: spectrum sensing disruption (termed spoofing for short). Spoofing, by emitting spoofing singals in unused bands during sensing periods, aims to mislead CR to reach false decisions on whether the observed band is occupied, such that the spectral efficiency is decreased.
Here arise a series of problems of how to understand this new form of attack, such as which factors of the CR system is vulnerable to spoofing, how to analyze its effect to the CR system, and what the worst-case performance would be in the presence of spoofing, etc. These unanswered questions are the fundamental issues toward designing secure spectrum sensing algorithms. However, it is in serious lack of systematic and theoretical investigations yet.
This dissertation carries out research focusing on the theory and methodology of spectrum sensing disruption in CR, which includes:
First, optimal sensing disruption strategies are derived and analyzed under the constraint of a power budget. Specifically,
(1) The problem of optimal sensing disruption under Additive White Gaussian Noise (AWGN) is investigated. By minimizing the available bandwidth for the CR system where energy detection is utilized for sensing, the optimal sensing disruption strategy is derived, which corresponds to equal-power, partial-band spoofing; The optimal spoofing performance with varying sensing parameters is analyzed, which turns out to be proportional to either false alarm probability or integration-time-bandwidth product; Theoretical along with numerical results indicates that:spoofing can effectively reduce the available bandwidth of the CR system. When the power budget of the adversary is large enough, the CR system could even be out of work since no available bandwidth can be found via sensing.
(2) The problem of optimal sensing disruption with path loss is investigated. Similar as the case for AWGN, the optimal spoofing strategy considering path loss is derived, which also corresponds to equal-power, partial-band spoofing; Theoretical along with numerical results further analyzed, for this path loss scenario, the optimal spoofing performances under different parameters of the CR system.
(3) The problem of optimal sensing disruption with fading is investigated. The mathematical model of optimal spoofing with fading is established:minimizing the available bandwidth of the CR system with the power constraint; Considering that the objective is non-convex with parameters involving integration of nonlinear functions, it is quite difficult to arrive at analytical solutions. However, by utilizing the separable characteristics of both objective and constraints, the non-convex, nonlinear optimization is transformed into mixed-integer linear programming via introducing additional integer variables. This numerical approach can obtain global optimum, and is applicable to various independent fading scenarios, such as Rayleigh, Rician and Nakagammi; Based on this approach, the worst-case (instantaneous channel state information is assumed to be known) performance of spoofing with fading is analyzed:for identical and independent Rayleigh fading whose second moments are normalized to unity, the worst-case performance of spoofing asymptotically approaches that under AWGN; Further, a feasible (only the statistical channel state information is known) optimal spoofing strategy is also given and analyzed:for identical and independent Rayleigh fading, the feasible optimal spoofing strategy turns out to be equal-power, partial-band.
When spectrum sensing is fulfilled, a cognitive user (CU) can access those bands that are determined to be vacant through sensing. This phase is called data transmission, where the CU could possibly face the traditional jamming (jamming signals are launched in those bands where the CU is in use, to degrade its performance such that effective transfer of information is denied). For the ideal case where the power/energy is unlimited, the adversary can choose to optimally spoof in the sensing slot and then optimally jam when the CU starts data transmission, in order to maximally degrade the performance of the CR system. However, for a practical matter, the energy of an adversary is usually limited. In this case, how to optimally distribute the energy budget into sensing slot and the data transmission slot so as to maximally attack the CR system remains unanswered. This issue is fundamental and critical to the development of the sensing disruption theory. Therefore, this problem is investigated in the latter part of this dissertation, including:
(4) For a basic CR network scenario, the problem of how to optimally combine both spoofing and jamming with an energy budget is investigated. The objective of the adversary is established as the average sum throughput of the CR system, based on which the relationship among the average sum throughput, spoofing signals, jamming signals, and the required bandwidth of the CR system is analytically given. Then the optimal attack to the CR system is mathematically
引文
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