Joint radar-communication waveform designs using signals from multiplexed users

Joint radar-communication designs are exploited in applications where radar and communications systems share the same frequency band or when both radar sensing and information communication functions are required in the same system. Finding a waveform that is suitable for both radar and communication is challenging due to the difference between radar and communication operations. In this paper, we propose a new method of designing dual-functional waveforms for both radar and communication using signals from multiplexed communications users. Specifically, signals from different communications users multiplexed in the time, code or frequency domains across different data bits are linearly combined to generate an overall radar waveform. Three typical radar waveforms are considered. The coefficients of the linear combination are optimized to minimize the mean squared error with or without a constraint on the signal-to-noise ratio (SNR) for the communications signals. Numerical results show that the optimization without SNR constraint can almost perfectly approximate the radar waveform in all the cases considered, giving good dual-functional waveforms for both radar and communication. Also, among different multiplexing techniques, time division multiple access is the best option to approximate the radar waveform, followed by code division multiple access and orthogonal frequency division multiple access

On SFBC schemes for enabling virtual array concept in monostatic ISAC scenarios

The integrated sensing and communication (ISAC) paradigm is being proposed for 6G as a new feature of the physical layer (PHY), for tackling dual-functional applications, i.e., demanding radio-sensing and communication functions, such as the Internet of Things (IoT) and autonomous driving systems. This work considers the integration of sensing and communications functionalities in a unique platform. To achieve this goal, the use of orthogonal space frequency block codes (SFBC) is proposed. SFBC code orthogonality enables both the separation of communications data streams at a user terminal and the estimation of target parameters. The SFBC enhances the communications link diversity without requiring channel state information knowledge at the transmitter and enable the virtual antenna array concept for enhancing the direction-finding resolution. The use of different SFBCs provides a tradeoff between achieved diversity and sensing resolution. For example, an Alamouti code, applicable for the case with two transmitting antennas, duplicates sensing resolution and achieves a diversity order of two while the use of a Tarokh code, applicable for a scenario with four transmitting antennas, provides a fourfold better resolution and diversity order of four. However, the code rate achieved with the Tarokh code is half of the one achieved with the Alamouti code. Furthermore, the unambiguous range is reduced since the bandwidth is divided to multiplex the different antenna signals. For its simplicity, good performance and reduced integration requirements, the method is promising for future ISAC systems.publishe

Codificação de bloco espaço-tempo na habilitação de sistemas MIMO-OFDM

The available bandwidth in the radio frequency spectrum is decreasing due to the growing number of applications and users. Therefore, in order to ensure a sustainable evolution in this area it is crucial to develop strategies to optimize the spectrum usage. Joining RADAR and communication functionalities in a single terminal represents exactly this same strategy. As such, the two functionalities, which usually compete for the same radio resources, can coexist through a cooperative relation in which they can thrive and cease to introduce interferences in between them. In this dissertation, the integration of both systems is achieved through the use of OFDM as the common waveform. Through the space time/frequency block codes, namely the Tarokh coding it is possible to introduce spatial diversity and orthogonality to the system, therefore increasing the system’s robustness and allowing to use the virtual antenna concept, which enables improved RADAR resolution and detection. In order to evaluate the system’s performance, a simulation platform was developed. In these simulations we start by firstly considering RADAR detection for single and multiple antenna systems and then integrate the radar and communication functionalities. We have verified the good performance levels of the proposed system, which thanks to its low complexity can be an interesting RadCom approach for future wireless systems.A largura de banda disponível no espectro de radio frequência enfrenta uma diminuição face ao crescente número de aplicações e utilizadores. Assim, por forma a assegurar uma evolução sustentável neste campo é fulcral desenvolver estratégias que otimizem o uso do espectro. A junção das funcionalidades RADAR e comunicação num só terminal faz parte dessa estratégia. Desta forma, duas funcionalidades usualmente concorrentes pelos mesmos recursos radio, podem coexistir em cooperação, sem interferência entre ambos. Nesta dissertação a integração dos dois sistemas é conseguida através do uso do OFDM como forma de onda comum. Através de códigos desenhados no espaço-tempo/frequência, nomeadamente a codificação de Tarokh, foi possível introduzir diversidade espacial e ortogonalidade no sistema, aumentando assim a sua robustez e permitindo o uso do conceito de antenas virtuais, que por sua vez possibilitam uma melhoria na resolução e deteção do RADAR. De forma a avaliar o desempenho do sistema desenvolveu-se uma plataforma de simulação. Nesta plataforma começou-se por considerar a deteção RADAR para sistemas com uma e múltiplas antenas, onde posteriormente se integraram as funcionalidades de comunicação. Os resultados obtidos mostraram um excelente desempenho do sistema, que devido à sua baixa complexidade, pode ser um sistema RadCom interessante para os futuros sistemas sem fios.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

OFDM Waveform Optimisation for Joint Communications and Sensing

Radar systems are radios to sense objects in their surrounding environment. These operate at a defined set of frequency ranges. Communication systems are used to transfer information between two points. In the present day, proliferation of mobile devices and the advancement of technology have led to communication systems being ubiquitous. This has made these systems to operate at the frequency bands already used by the radar systems. Thus, the communication signal interferes a radar receiver and vice versa, degrading performance of both systems. Different methods have been proposed to combat this phenomenon. One of the novel topics in this is the RF convergence, where a given bandwidth is used jointly by both systems. A differentiation criterion must be adopted between the two systems so that a receiver is able to separately extract radar and communication signals. The hardware convergence due to the emergence of software-defined radios also motivated a single system be used for both radar and communication. A joint waveform is adopted for both radar and communication systems, as the transmit signal. As orthogonal frequency-division multiplexing (OFDM) waveform is the most prominent in mobile communications, it is selected as the joint waveform. Considering practical cellular communication systems adopting OFDM, there often exist unused subcarriers within OFDM symbols. These can be filled up with arbitrary data to improve the performance of the radar system. This is the approach used, where the filling up is performed through an optimisation algorithm. The filled subcarriers are termed as radar subcarriers while the rest as communication subcarriers, throughout the thesis. The optimisation problem minimises the Cramer--Rao lower bounds of the delay and Doppler estimates made by the radar system subject to a set of constraints. It also outputs the indices of the radar and communication subcarriers within an OFDM symbol, which minimise the lower bounds. The first constraint allocates power between radar and communication subcarriers depending on their subcarrier ratio in an OFDM symbol. The second constraint ensures the peak-to-average power ratio (PAPR) of the joint waveform has an acceptable level of PAPR. The results show that the optimised waveform provides significant improvement in the Cramer--Rao lower bounds compared with the unoptimised waveform. In compensation for this, the power allocated to the communication subcarriers needs to be reduced. Thus, improving the performances of the radar and communication systems are a trade-off. It is also observed that for the minimum lower bounds, radar subcarriers need to be placed at the two edges of an OFDM symbol. Optimisation is also seen to improve the estimation performance of a maximum likelihood estimator, concluding that optimising the subcarriers to minimise a theoretical bound enables to achieve improvement for practical systems