A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 2016 / Multiple-Input Multiple-Output (MIMO) radar is a more general form of phased array radar, where each antenna in the array transmits linearly independent or mutually orthogonal signals. Sustained growth in computational power as well as the decline in the cost of integrated radio frequency (RF) components has made MIMO more viable than in the past. The potential emergence of practical MIMO radar has prompted an investigation into the detectability of MIMO radar signals using existing conventional Electronic warfare Support (ES) receivers such as the Crystal Video Receiver (CVR) and a specific type of superheterodyne receiver (superhet) known as the Zero IF Receiver (ZIFR). Literature on the detectability of MIMO radar signals is extremely scarce and this investigation aims to offer insights into the detectability of MIMO radar signals by means of computer simulations. The fundamental theory necessary for this research includes phased array radar theory, MIMO array radar theory and ES receiver signal detection theory. The detection of MIMO radar signals is compared to a reference phased array case to provide relative context. This investigation focusses on co-located Uniform Linear Arrays (ULA) based radar systems. The result of interest is the relative Signal-to-Noise Ratio (SNR) at which each type of radar can be detected by the ES receiver. Therefore, a lossless transmission, without loss of generality, is assumed. Constraints such as the equal transmit power over all antenna elements in the arrays, are used for a fair comparison. Many different array simulation setups are simulated. These setups are achieved by varying the number of elements in the array and the inter-element spacing. The phased array radar transmitted complex linear chirp signals, and the MIMO radar transmitted Hadamard sequences, interpolated using a Constant Envelope Linear-Route-of-Unity (CE-LRU) technique. The CVR and ZIFR detection thresholds were determined for a Probability of False Alarm (PFA) of 10-4. For all of the setups, the phased array radar was found to be more detectable than the MIMO radar at values of Probability of Detection (PD) below 0.6. The in phase coherent combination of phased array radar signals in its main beam resulted in a signal gain caused by the constructive addition of the signals. This gain thus increases with the number of antenna elements. In contrast, the MIMO signals also add coherently, but the instantaneous phase for each signal is a function of the transmitted signal as well as the direction of propagation relative to the array face. The set of orthogonal signals thus add constructively and destructively, resulting in the average signal power remaining approximately constant despite the number of antenna elements increasing. The difference in detectability of the phased array radar over MIMO radar therefore increases as the number of antenna elements is increased, due to the fact that each element is constrained to transmit a fixed power. Comparing the performance of the ZIFR and CVR, the ZIFR outperforms the CVR. This is due to the fact that the ZIFR implements a quadrature ES receiver, and was able to detect both types of radar signals at a lower SNR than the CVR. However, both ES receivers struggle to detect MIMO radar signals in comparison to detecting phased array radar signals and this performance margin widens as the number of transmitting elements is increased. This result suggests that research into dedicated techniques for the detection of MIMO radar signals using ES receivers may be necessary should the need arise to detect MIMO radar signals in future. This is the first quantitative analysis of the detectability of MIMO radar signals using conventional ES receivers that the author is aware of. / MT2017

认知无线电在处理频谱稀缺的问题上是一个非常有前途的解决方案。拥有多天线认知无线电的用戶通过发射波束成形技术可以和授权用在同一时刻同一频带共存，这样大大地增强了频谱效率。在实际系统中，最理想的情况是这些拥有多天线认知无线电用戶能够分布式地优化他们的发射波束形成向量以此达到系统的最优化。由于授权用戶受到的干扰是来自于所有认知无线电用戶的，为了实现分布式算法这些干扰必须被合理地规划以至于达到最优。也就是说，每个认知无线电用戶需要知道对授权用戶产生干扰的最佳约束上限。 / 从优化的角度处理这种解耦问题，最常用的方法是原始分解法和对偶分解法。然而这两种方法都需要用戶之间有大量的消息传递，这对于频谱效率来说是有害的。在对偶分解法中，指向授权用戶的耦合干扰被一协调者估测(通常是授权用戶本身)。协调者需要在每次迭代中更新和广播参数给认知无线电用戶。对于原始分解法，算法同样需要一协调者进行收集认知无线电用戶的目标函数信息以此计算每个用戶的最优干扰约束上限。协调者同样需要更新和广播大量消息给认知无线电用戶。这种大量的信息计算和传递在分布式系统中是不理想的，问题在认知无线电网络显得格外严重。因为授权用戶不希望担任这样的协调者除非他的计算参与降到最低。 / 在此论文中，我们提出了几种新型的基于认知无线电网络的分布式算法。目的是最小化授权用戶和认知无线电用戶的消息传递。通过研究半定规划中的最优分割法，我们指出不影响最优性条件下授权用戶和认知无线电用戶的大量消息传递是可以避免的。我们又提出了在多输入多数出认知无线电网络中一种基于对偶分解的鲁捧干扰控制。在此论文中提出的低消息传递算法大大地提高了多用戶多输入多数认知无线电网络的实用性。 / Cognitive radio (CR) is a promising solution to alleviate spectrum scarcity. In CR networks where mobile stations are equipped with multiple antennas, secondary users (SUs) can transmit at the same time as the primary users (PUs) by carefully controlling the interference through transmit beamforming, thus significantly enhancing the spectrum efficiency. In practical systems, it is desirable to have multiple SUs optimize their transmit beamforming vectors in a decentralized manner, and yet achieve an optimal system performance. In CR networks, the interference received by the PU is attributed to the transmission of all SUs. To facilitate distributed beamforming, the aggregate-interference constraint imposed by the PU must be decoupled, so that each individual SU knows the "fair share" of interference that is allowed to generate to the PU. / A commonly used technique for decoupling coupled constraintsin optimization problems is optimization decomposition, including dual and primal decompositions. Both the dual and primal decomposition methods require frequent message passing among users, which potentially offsets the spectrum benefit brought by cognitive radio techniques. Specifically, with dual decomposition, the aggregate interference generated to the PU must be measured by a coordinator,which is, naturally, the PU. The coordinator then updates and broadcasts the Lagrangian multiplier to all SUs. Likewise, the primal decomposition needs a coordinator, which can again be the PU, to gather the subgradient of the objective functions of each SUs for given interference partition. The coordinator then updates and broadcasts the permissible interference to all SUs. Whereas the large overhead incurred message computation and passing is undesirable in distributed systems, the problem is more acute in CR networks, because a typical PU would not be willing to take the coordinating role unless its involvement is minimized. / In this thesis, we propose several novel distributed optimization algorithms for CR networks with minimum message passing between the primary and secondary systems. By exploiting the theory of optimal partition (OP) for semi-definite programming (SDP), we show that most message passings between the primary and secondary systems can be eliminated without compromising the optimality of the solution. We also derive a robust interference control scheme based on the duality theory for MIMO CR network. The low message-passing distributed algorithms presented in this thesis greatly enhance the practicality of multiuser MIMO CR networks. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Yao, Leiyi. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 114-123). / Abstracts also in Chinese.

Doctor of Philosophy / The next generation mobile networks are expected to provide multimedia applications with a high quality of service. On the other hand, interference among multiple base stations (BS) that co-exist in the same location limits the capacity of wireless networks. In conventional wireless networks, the base stations do not cooperate with each other. The BSs transmit individually to their respective mobile stations (MS) and treat the transmission from other BSs as interference. An alternative to this structure is a network cooperation structure. Here, BSs cooperate with other BSs to simultaneously transmit to their respective MSs using the same frequency band at a given time slot. By doing this, we significantly increase the capacity of the networks. This thesis presents novel research results on a noncooperative transmission scheme and a cooperative transmission scheme for multi-user multiple-input-multiple-output orthogonal frequency division multiplexing (MIMO-OFDM). We first consider the performance limit of a noncooperative transmission scheme. Here, we propose a method to reduce the interference and increase the throughput of orthogonal frequency division multiplexing (OFDM) systems in co-working wireless local area networks (WLANs) by using joint adaptive multiple antennas(AMA) and adaptive modulation (AM) with acknowledgement (ACK) Eigen-steering. The calculation of AMA and AM are performed at the receiver. The AMA is used to suppress interference and to maximize the signal-to-interference-plus-noise ratio (SINR). The AM scheme is used to allocate OFDM sub-carriers, power, and modulation mode subject to the constraints of power, discrete modulation, and the bit error rate (BER). The transmit weights, the allocation of power, and the allocation of sub-carriers are obtained at the transmitter using ACK Eigen-steering. The derivations of AMA, AM, and ACK Eigen-steering are shown. The performance of joint AMA and AM for various AMA configurations is evaluated through the simulations of BER and spectral efficiency (SE) against SIR. To improve the performance of the system further, we propose a practical cooperative transmission scheme to mitigate against the interference in co-working WLANs. Here, we consider a network coordination among BSs. We employ Tomlinson Harashima precoding (THP), joint transmit-receive beamforming based on SINR (signal-to-interference-plus-noise-ratio) maximization, and an adaptive precoding order to eliminate co-working interference and achieve bit error rate (BER) fairness among different users. We also consider the design of the system when partial channel state information (CSI) (where each user only knows its own CSI) and full CSI (where each user knows CSI of all users) are available at the receiver respectively. We prove analytically and by simulation that the performance of our proposed scheme will not be degraded under partial CSI. The simulation results show that the proposed scheme considerably outperforms both the existing noncooperative and cooperative transmission schemes. A method to design a spectrally efficient cooperative downlink transmission scheme employing precoding and beamforming is also proposed. The algorithm eliminates the interference and achieves symbol error rate (SER) fairness among different users. To eliminate the interference, Tomlinson Harashima precoding (THP) is used to cancel part of the interference while the transmit-receive antenna weights cancel the remaining one. A new novel iterative method is applied to generate the transmit-receive antenna weights. To achieve SER fairness among different users and further improve the performance of MIMO systems, we develop algorithms that provide equal SINR across all users and order the users so that the minimum SINR for each user is maximized. The simulation results show that the proposed scheme considerably outperforms existing cooperative transmission schemes in terms of the SER performance and complexity and approaches an interference free performance under the same configuration. We could improve the performance of the proposed interference cancellation further. This is because the proposed interference cancellation does not consider receiver noise when calculating the transmit-receive weight antennas. In addition, the proposed scheme mentioned above is designed specifically for a single-stream multi-user transmission. Here, we employ THP precoding and an iterative method based on the uplink-downlink duality principle to generate the transmit-receive antenna weights. The algorithm provides an equal SINR across all users. A simpler method is then proposed by trading off the complexity with a slight performance degradation. The proposed methods are extended to also work when the receiver does not have complete Channel State Informations (CSIs). A new method of setting the user precoding order, which has a much lower complexity than the VBLAST type ordering scheme but with almost the same performance, is also proposed. The simulation results show that the proposed schemes considerably outperform existing cooperative transmission schemes in terms of SER performance and approach an interference free performance. In all the cooperative transmission schemes proposed above, we use THP to cancel part of the interference. In this thesis, we also consider an alternative approach that bypasses the use of THP. The task of cancelling the interference from other users now lies solely within the transmit-receive antenna weights. We consider multiuser Gaussian broadcast channels with multiple antennas at both transmitter and receivers. An iterative multiple beamforming (IMB) algorithm is proposed, which is flexible in the antenna configuration and performs well in low to moderate data rates. Its capacity and bit error rate performance are compared with the ones achieved by the traditional zero-forcing method.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 71-78). Also available in electronic version.

The accurate simulation of wireless channels is important since it permits the realistic and repeatable performance measurement of wireless systems. While software simulation is a flexible method for testing hardware models, its long-running simulation time can be prohibitive in many scenarios. Prior to the availability of accurate and standardized channel models, wireless products needed to be verified using extensive and expensive field testing. A far less costly approach is to model the behavior of radio channels on a hardware simulator. Different channel characteristics should be considered to ensure the faithful simulation of wireless propagation. Among the most important characteristics are the path-loss behavior, Doppler frequency, delay distribution, fading distribution, and time, frequency, and space correlation between fading samples across different antennas. Various fading channel models have been proposed for propagation modeling in different scenarios. A good homogeneous field programmable gate array (FPGA) fading simulator needs to accurately reproduce the propagation effects, yet it also needs to be compact and fast to be effectively used for rapid hardware prototyping and simulation. In this thesis, new channel models are proposed for the compact FPGA implementation of fading channel simulators with accurate statistics. Compact hardware implementations for physical and analytical fading channel models are proposed that can simulate fading channels with more than one thousand paths on a single FPGA. We also propose design techniques for accurate and compact statistical fading channel simulation of isotropic and non-isotropic scattering in Rayleigh, Rician, Nakagami-m, and Weibull fading channels. Compact FPGA implementations are presented for multiple-antenna fading simulators for geometric one-ring models, two-ring models, elliptical models, and analytical models including the i.i.d. model, and Kronecker, Weichselberger, and VCR channel models. Finally, a fading simulation and bit error performance evaluation platform is proposed for the rapid baseband prototyping and verification of single- and multiple-antenna wireless systems.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 99-111).

To allow faster and more reliable wireless communication a technique is to use multiple antennas in the transmitter and receiver. This technique is called MIMO. The usage of MIMO adds complexity to the receiver that must determine what the transmitter actually sent. This thesis focuses on hardware implementation suitable for an FPGA of a detection algorithm called SUMIS. A background to detection and SUMIS in particular is given as a theoretical aid for a better understanding of how an algorithm like this can be implemented. An introduction to hardware and digital design is also presented. A subset of the operations in the SUMIS algorithm such as matrix inversion and sum of logarithmic values are analyzed and suitable hardware architectures are presented. These operations are implemented in RTL hardware using VHDL targeted for an FPGA, Virtex-6 from Xilinx. The accuracy of the implemented operations is investigated showing promising results alongside of a presentation of the necessary resource usage. Finally other approaches to hardware implementation of detection algorithms are discussed and more suitable approaches for a future implementation of SUMIS are commented on. The key aspects are flexibility through software reprogrammability and area efficiency by designing a custom processor architecture.

This dissertation investigates the assumption of channel reciprocity in orthogonal frequency division multiplexing (OFDM) systems using time-division duplex (TDD) access. Within TDD systems uplink and downlink transmission share the same channel, and so channel state at the transmitter (CSIT) can be inherently obtained through uplink channel estimation assuming that the channel is reciprocal and static over a few packet transmissions. For both closed-loop SISO-OFDM (single-input single-output) and MIMO-OFDM (multiple-input multiple-output) systems, the availability of CSIT enables the transmitter to apply adaptive modulation and coding (AMC) to improve throughput or signal processing and precoding algorithms in order to obtain a spatial diversity and/or multiplexing gain. This results in improved performance as compared to open-loop MIMO systems in which the channel state is not known at the transmitter. However, signi cant deviations between transmitter and receiver channel state information may result in degradation of performance, as precoding at the transmitter will be based on erroneous channel state information. In this work, we observe the assumption of channel reciprocity using a real-time OFDM-PHY FPGA prototype wireless communications system and we look at possible factors that contribute to deviations between uplink and downlink channel estimates. We also look at common linear precoding schemes to compensate for channel non-reciprocity. Of all the possible factors that contribute to channel reciprocity deviations, we nd that the dominant factor comes from imperfections in the RF front-end components which result in signi cant channel phase response deviations across subcarriers between the uplink and downlink.

This dissertation investigates the assumption of channel reciprocity in orthogonal frequency division multiplexing (OFDM) systems using time-division duplex (TDD) access. Within TDD systems uplink and downlink transmission share the same channel, and so channel state at the transmitter (CSIT) can be inherently obtained through uplink channel estimation assuming that the channel is reciprocal and static over a few packet transmissions. For both closed-loop SISO-OFDM (single-input single-output) and MIMO-OFDM (multiple-input multiple-output) systems, the availability of CSIT enables the transmitter to apply adaptive modulation and coding (AMC) to improve throughput or signal processing and precoding algorithms in order to obtain a spatial diversity and/or multiplexing gain. This results in improved performance as compared to open-loop MIMO systems in which the channel state is not known at the transmitter. However, signi cant deviations between transmitter and receiver channel state information may result in degradation of performance, as precoding at the transmitter will be based on erroneous channel state information. In this work, we observe the assumption of channel reciprocity using a real-time OFDM-PHY FPGA prototype wireless communications system and we look at possible factors that contribute to deviations between uplink and downlink channel estimates. We also look at common linear precoding schemes to compensate for channel non-reciprocity. Of all the possible factors that contribute to channel reciprocity deviations, we nd that the dominant factor comes from imperfections in the RF front-end components which result in signi cant channel phase response deviations across subcarriers between the uplink and downlink.

It is know that the LORD (Layered Orthogonal Lattice Detector) and the sphere decoding achieve performance equaling that of ML. However, both detectors have different disadvantage. When the transmit antenna number is greater than three antennas, it is difficult to apply the LORD. Therefore, we consider using the sphere decoding together with the LORD. The complexity of the sphere decoding highly depends on the initial radius. In this thesis, we intend to reduce the sphere decoding complexity by using LORD.