Philippa A. Martin

Phase selection design for reconfigurable intelligent surfaces (RISs) is a significant research challenge, as a closed-form optimal solution for a multi-user (MU) system is believed to be intractable. While existing methods achieve strong near-optimal performance, they typically entail high computational complexity. In this work, we take a different approach and propose a practical method that achieves competitive performance while substantially reducing computational complexity. To do so, we consider a RIS divided into subsurfaces. Each subsurface is designed specifically for one user, who is served on their own frequency band. The other subsurfaces (those not designed for this user) provide additional uncontrolled scattering. We derive the exact closed-form expression for the mean signal-to-noise ratio (SNR) for the proposed subsurface design (SD) when all channels experience correlated Ricean fading. We simplify this to find the mean SNR for line-of-sight (LoS) channels and channels experiencing correlated Rayleigh fading. An iterative SD (ISD) process is proposed, where subsurfaces are designed sequentially, and the phases that are already set are used to enhance the design of the remaining subsurfaces. This is extended to a converged ISD (CISD), where the ISD process is repeated multiple times until the SNR increases by less than a specified tolerance. The ISD and CISD both provide a performance improvement over SD, which increases as the number of RIS elements increases. The SD is significantly simpler than the lowest complexity MU method we know of, and despite each user having less bandwidth, the SD outperforms the existing method in some key scenarios. The SD is more robust to strongly LoS channels and clustered users, as it does not rely on spatial multiplexing like other MU methods. Combined with the complexity reduction, this makes the SD an attractive phase selection method.

Deployment of Unmanned Aerial Vehicles (UAVs) in autonomous formations necessitates accurate and timely communication of safety information. A communication protocol that supports timely and successful transfer of safety information between UAVs is therefore needed. This paper presents Distributed Self-allocated Time slot Reuse (D-STR). Our D-STR protocol addresses the essential task of communicating safety information in rigid Unmanned Aerial Vehicle (UAV) formations with different network topologies, enabling collision-free deployment of the formation. This is an important step for improving the safety and practicality of UAV formations in application scenarios that span a range of industries.

A key challenge facing reconfigurable intelligent surfaces (RISs) is channel state information acquisition. Passive RISs cannot generate pilot signals or process data, making rapid temporal changes in the channel problematic. Additionally, the impact of spatial changes in RIS channels has not been thoroughly investigated. Therefore, in this work, we use second order statistics to investigate the spatio-temporal behaviour of a single-user (SU) RIS system. Assuming a line-of-sight (LoS) RIS to base station (BS) link, we derive an exact expression for the level crossing rate (LCR) of the RIS link (user equipment (UE)-RIS-BS path) and propose a numerically stable approximation for the LCR of the global UE-BS channel. Each LCR expression attained is then utilised to find the corresponding average fade duration (AFD). The temporal signal-to-noise ratio (SNR) correlation is also derived assuming an LoS RIS-BS link. Assuming a Ricean RIS-BS link, expressions for the spatial correlation matrix of the global channel and the mean SNR loss due to channel ageing are derived. All of the analyses are verified by simulation, and the impact of key system parameters is investigated. We show that the use of an RIS does not significantly amplify changes in the channel.

Two-hop relay networks for critical industries require high reliability within a limited spectrum. The lower hop between densely populated user equipment (UE) and the relay nodes (RNs) is assumed to have low spectral efficiency due to the inexpensive low-powered UEs equipped with omnidirectional antennas in non-line-of-sight (NLOS) conditions. Thus, spectrally efficient frequency reuse (FR) in the upper hop between RNs and base stations (BSs) is pivotal in enhancing the two-hop relay network’s uplink coverage and capacity. To achieve this objective, we propose the deployment of higher order sectorisation (HOS) at BS and directional antennas (DAs) at RNs in the upper hop and propose rotational (RFR), double (DFR), and triple (TFR) frequency reuse schemes. These schemes allow FR by exploiting the orientation of the DAs at RNs and are compared with conventional distance-based FR. Furthermore, an optimisation method is formulated to partition the limited bandwidth between the two hops to increase the average end-to-end uplink capacity while using the proposed FR schemes on the upper hop. When evaluating system performance with HOS at BSs (upper hop) and RNs (lower hop) in irregular cellular layout, the statistical evaluation of uplink interference becomes computationally expensive. To tackle this issue, we developed a semi-analytical model to efficiently compute the statistical distribution of uplink interference with reduced time and complexity. It is shown that this model is an accurate tool for performance assessments, which also validate the end-to-end two-hop capacity improvements up to 73% and 110% using the proposed DFR and TFR schemes, respectively.

To support the design of fifth-generation (5 G) beamforming and beamtracking algorithms, a spatially consistent channel propagation model is needed, namely a model whose geometrical properties – angle-of-departure, angle-of-arrival, and delay of its multipath components – evolve smoothly while the receiver is in motion. The 3 rd Generation Partnership Project (3GPP) has proposed two spatial consistency (SC) procedures – SC-A and SC-B – as part of its popular TR 38.901 model for 0.5 GHz to 100 GHz channels. In this paper, we perform a comparison of the two procedures with measurements to determine whether the procedures accurately fit our measurements. We use over 700 channel measurements collected with our high-resolution 28 GHz and 60 GHz directional channel sounders, across two indoor and three outdoor environments and for pedestrian and vehicular motion. We show that SC-A – a fully deterministic procedure – provides a much better approximation to the measured channel in all five environments than SC-B – a fully stochastic procedure. We also computed the correlation distance for both procedures – the distance over which they are valid – showing that SC-A's average correlation distance across all environments of 13.4 m is more than twice as long as SC-B's of 6.28 m, and that the channel is spatially consistent for significantly longer outdoors than indoors.

Millimeter-wave (mmWave) channel sparsity is widely exploited to reduce the complexity of compressed sensing and beamtracking algorithms for 5G, yet there are still many misconceptions about it: that it arises from pencilbeam antennas admitting just a few paths, that the channel in fact has just a few paths, that the channel is always sparse. What is missing to confirm these conjectures is measurement-based analysis, to our knowledge conducted by only two papers thus far, however both at microwave (5 GHz), in a single environment, and based on just a few ( $\leq$ 49) measurements. For conclusive analysis on mmWave sparsity, we have collected measurements with our high-resolution 3D double-directional 60 GHz channel sounder in three indoor and two outdoor environments, comprising a total of 750 measurements, while investigating both line-of-sight (LoS) and obstructed LoS (OLoS) conditions. We learned that in LoS the channel is always sparse, that sparsity drops notably from LoS to OLoS, and that mmWave sparsity is inherent to the channel itself – not to pencilbeam antennas – since our sounder has omnidirectional field-of-view. We also propose a sparsity metric that accounts for channels with just a few paths, which we found to occur in some environments.

We evaluate the performance of a low-power narrowband system utilising higher-order sectorisation (HOS) at the base stations (BS) and directional antennas (DA) at fixed user equipment (UE). We use a specific rotational frequency reuse scheme (RFR) for directional UEs and extend it for HOS (3, 6 and 12 sectors) by considering antenna orientations for interference avoidance. We propose two novel schemes for directional UEs with a cluster size less than one, named double frequency reuse (DFR) and triple frequency reuse (TFR). These schemes are tested and compared to conventional frequency reuse (CFR) through simulations in a realistic geographic terrain. The simulation results indicates that the proposed schemes are helpful to achieve higher capacity while using different BS and UE antenna beamwidths. It is shown that scheme RFR provides a performance enhancement compared to CFR only with wider UE antenna beamwidths. The implementation of DFR achieves reduced outage probability and offers a large increase in average spectral efficiency per cell (ASE) of up to 84.8-81.6% in 6 and 12 sectored system. The scheme TFR provides a maximum ASE of up to 149-153% in 6 and 12 sectored system. However, in a six-sectored system, TFR only meets the coverage criteria with a narrow beamwidth antenna of 20°.

Continuous sensing and transmission by secondary users (SUs) in cognitive radio (CR) allows efficient use of available spectrum. In this approach, the SU receiver continuously senses the spectrum while it transmits in the same frequency band. Leveraging this advantage, we study the performance of error vector magnitude (EVM) based detection to detect the reappearance of the primary user (PU) signal during ongoing SU data transmission. We exploit the pilot tones that are inherent to many OFDM based standards to measure the EVM. We consider two models of PU namely an unknown deterministic signal and a Gaussian random signal with additive white Gaussian noise (AWGN). We derive an analytical form of the probability density function (PDF) of the EVM based detector based on the exact representation of Chi-square distributions. We analyze the performance of our detector by deriving the type I (false positive) and type II (false negative) error probabilities. Simulation results show that the EVM based approach achieves better performance than energy detection.

We investigate multiple input multiple output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems that operate with insufficient cyclic prefix (CP). Using a CP shorter than the channel delay spread can enable a significant improvement in bandwidth utilization or range extension for OFDM networks at the cost of increased intersymbol interference (ISI) and inter-carrier interference (ICI). We first analyze the effect of ICI and ISI on the received signal. A bi-directional M-algorithm (BDMA) is then proposed for high performance trellis-based equalization to construct an iterative interference mitigation and detection process. Simulations show that, after only 2 iterations, the bit error rate (BER) of the proposed equalization scheme can converge to that of a sufficient-CP system even when the channel delay spread is 6 times longer than the insufficient CP.